docs/SourceLevelDebugging.html - llvm - Git at Google


            
            <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"

            
                                  "http://www.w3.org/TR/html4/strict.dtd">

            
            <html>

            
            <head>

            
              <meta http-equiv="Content-Type" content="text/html; charset=utf-8">

            
              <title>Source Level Debugging with LLVM</title>

            
              <link rel="stylesheet" href="llvm.css" type="text/css">

            
            </head>

            
            <body>


            <h1>Source Level Debugging with LLVM</h1>


            <table class="layout" style="width:100%">

            
              <tr class="layout">

            
                <td class="left">

            
            <ul>

            
              <li><a href="#introduction">Introduction</a>

            
              <ol>

            
                <li><a href="#phil">Philosophy behind LLVM debugging information</a></li>

            
                <li><a href="#consumers">Debug information consumers</a></li>

            
                <li><a href="#debugopt">Debugging optimized code</a></li>

            
              </ol></li>

            
              <li><a href="#format">Debugging information format</a>

            
              <ol>

            
                <li><a href="#debug_info_descriptors">Debug information descriptors</a>

            
                <ul>

            
                  <li><a href="#format_compile_units">Compile unit descriptors</a></li>

            
                  <li><a href="#format_files">File descriptors</a></li>

            
                  <li><a href="#format_global_variables">Global variable descriptors</a></li>

            
                  <li><a href="#format_subprograms">Subprogram descriptors</a></li>

            
                  <li><a href="#format_blocks">Block descriptors</a></li>

            
                  <li><a href="#format_basic_type">Basic type descriptors</a></li>

            
                  <li><a href="#format_derived_type">Derived type descriptors</a></li>

            
                  <li><a href="#format_composite_type">Composite type descriptors</a></li>

            
                  <li><a href="#format_subrange">Subrange descriptors</a></li>

            
                  <li><a href="#format_enumeration">Enumerator descriptors</a></li>

            
                  <li><a href="#format_variables">Local variables</a></li>

            
                </ul></li>

            
                <li><a href="#format_common_intrinsics">Debugger intrinsic functions</a>

            
                  <ul>

            
                  <li><a href="#format_common_declare">llvm.dbg.declare</a></li>

            
                  <li><a href="#format_common_value">llvm.dbg.value</a></li>

            
                </ul></li>

            
              </ol></li>

            
              <li><a href="#format_common_lifetime">Object lifetimes and scoping</a></li>

            
              <li><a href="#ccxx_frontend">C/C++ front-end specific debug information</a>

            
              <ol>

            
                <li><a href="#ccxx_compile_units">C/C++ source file information</a></li>

            
                <li><a href="#ccxx_global_variable">C/C++ global variable information</a></li>

            
                <li><a href="#ccxx_subprogram">C/C++ function information</a></li>

            
                <li><a href="#ccxx_basic_types">C/C++ basic types</a></li>

            
                <li><a href="#ccxx_derived_types">C/C++ derived types</a></li>

            
                <li><a href="#ccxx_composite_types">C/C++ struct/union types</a></li>

            
                <li><a href="#ccxx_enumeration_types">C/C++ enumeration types</a></li>

            
              </ol></li>

            
              <li><a href="#llvmdwarfextension">LLVM Dwarf Extensions</a>

            
                <ol>

            
                  <li><a href="#objcproperty">Debugging Information Extension

            
            	  for Objective C Properties</a>

            
                    <ul>

            
            	  <li><a href="#objcpropertyintroduction">Introduction</a></li>

            
            	  <li><a href="#objcpropertyproposal">Proposal</a></li>

            
            	  <li><a href="#objcpropertynewattributes">New DWARF Attributes</a></li>

            
            	  <li><a href="#objcpropertynewconstants">New DWARF Constants</a></li>

            
                    </ul>

            
                  </li>

            
                  <li><a href="#acceltable">Name Accelerator Tables</a>

            
                    <ul>

            
                      <li><a href="#acceltableintroduction">Introduction</a></li>

            
                      <li><a href="#acceltablehashes">Hash Tables</a></li>

            
                      <li><a href="#acceltabledetails">Details</a></li>

            
                      <li><a href="#acceltablecontents">Contents</a></li>

            
                      <li><a href="#acceltableextensions">Language Extensions and File Format Changes</a></li>

            
                    </ul>

            
                  </li>

            
                </ol>

            
              </li>

            
            </ul>

            
            </td>

            
            <td class="right">

            
            <img src="img/venusflytrap.jpg" alt="A leafy and green bug eater" width="247"

            
            height="369">

            
            </td>

            
            </tr></table>


            <div class="doc_author">

            
              <p>Written by <a href="mailto:sabre@nondot.org">Chris Lattner</a>

            
                        and <a href="mailto:jlaskey@mac.com">Jim Laskey</a></p>

            
            </div>


            <!-- *********************************************************************** -->

            
            <h2><a name="introduction">Introduction</a></h2>

            
            <!-- *********************************************************************** -->


            <div>


            <p>This document is the central repository for all information pertaining to

            
               debug information in LLVM.  It describes the <a href="#format">actual format

            
               that the LLVM debug information</a> takes, which is useful for those

            
               interested in creating front-ends or dealing directly with the information.

            
               Further, this document provides specific examples of what debug information

            
               for C/C++ looks like.</p>


            <!-- ======================================================================= -->

            
            <h3>

            
              <a name="phil">Philosophy behind LLVM debugging information</a>

            
            </h3>


            <div>


            <p>The idea of the LLVM debugging information is to capture how the important

            
               pieces of the source-language's Abstract Syntax Tree map onto LLVM code.

            
               Several design aspects have shaped the solution that appears here.  The

            
               important ones are:</p>


            <ul>

            
              <li>Debugging information should have very little impact on the rest of the

            
                  compiler.  No transformations, analyses, or code generators should need to

            
                  be modified because of debugging information.</li>


              <li>LLVM optimizations should interact in <a href="#debugopt">well-defined and

            
                  easily described ways</a> with the debugging information.</li>


              <li>Because LLVM is designed to support arbitrary programming languages,

            
                  LLVM-to-LLVM tools should not need to know anything about the semantics of

            
                  the source-level-language.</li>


              <li>Source-level languages are often <b>widely</b> different from one another.

            
                  LLVM should not put any restrictions of the flavor of the source-language,

            
                  and the debugging information should work with any language.</li>


              <li>With code generator support, it should be possible to use an LLVM compiler

            
                  to compile a program to native machine code and standard debugging

            
                  formats.  This allows compatibility with traditional machine-code level

            
                  debuggers, like GDB or DBX.</li>

            
            </ul>


            <p>The approach used by the LLVM implementation is to use a small set

            
               of <a href="#format_common_intrinsics">intrinsic functions</a> to define a

            
               mapping between LLVM program objects and the source-level objects.  The

            
               description of the source-level program is maintained in LLVM metadata

            
               in an <a href="#ccxx_frontend">implementation-defined format</a>

            
               (the C/C++ front-end currently uses working draft 7 of

            
               the <a href="http://www.eagercon.com/dwarf/dwarf3std.htm">DWARF 3

            
               standard</a>).</p>


            <p>When a program is being debugged, a debugger interacts with the user and

            
               turns the stored debug information into source-language specific information.

            
               As such, a debugger must be aware of the source-language, and is thus tied to

            
               a specific language or family of languages.</p>


            </div>


            <!-- ======================================================================= -->

            
            <h3>

            
              <a name="consumers">Debug information consumers</a>

            
            </h3>


            <div>


            <p>The role of debug information is to provide meta information normally

            
               stripped away during the compilation process.  This meta information provides

            
               an LLVM user a relationship between generated code and the original program

            
               source code.</p>


            <p>Currently, debug information is consumed by DwarfDebug to produce dwarf

            
               information used by the gdb debugger.  Other targets could use the same

            
               information to produce stabs or other debug forms.</p>


            <p>It would also be reasonable to use debug information to feed profiling tools

            
               for analysis of generated code, or, tools for reconstructing the original

            
               source from generated code.</p>


            <p>TODO - expound a bit more.</p>


            </div>


            <!-- ======================================================================= -->

            
            <h3>

            
              <a name="debugopt">Debugging optimized code</a>

            
            </h3>


            <div>


            <p>An extremely high priority of LLVM debugging information is to make it

            
               interact well with optimizations and analysis.  In particular, the LLVM debug

            
               information provides the following guarantees:</p>


            <ul>

            
              <li>LLVM debug information <b>always provides information to accurately read

            
                  the source-level state of the program</b>, regardless of which LLVM

            
                  optimizations have been run, and without any modification to the

            
                  optimizations themselves.  However, some optimizations may impact the

            
                  ability to modify the current state of the program with a debugger, such

            
                  as setting program variables, or calling functions that have been

            
                  deleted.</li>


              <li>As desired, LLVM optimizations can be upgraded to be aware of the LLVM

            
                  debugging information, allowing them to update the debugging information

            
                  as they perform aggressive optimizations.  This means that, with effort,

            
                  the LLVM optimizers could optimize debug code just as well as non-debug

            
                  code.</li>


              <li>LLVM debug information does not prevent optimizations from

            
                  happening (for example inlining, basic block reordering/merging/cleanup,

            
                  tail duplication, etc).</li>


              <li>LLVM debug information is automatically optimized along with the rest of

            
                  the program, using existing facilities.  For example, duplicate

            
                  information is automatically merged by the linker, and unused information

            
                  is automatically removed.</li>

            
            </ul>


            <p>Basically, the debug information allows you to compile a program with

            
               "<tt>-O0 -g</tt>" and get full debug information, allowing you to arbitrarily

            
               modify the program as it executes from a debugger.  Compiling a program with

            
               "<tt>-O3 -g</tt>" gives you full debug information that is always available

            
               and accurate for reading (e.g., you get accurate stack traces despite tail

            
               call elimination and inlining), but you might lose the ability to modify the

            
               program and call functions where were optimized out of the program, or

            
               inlined away completely.</p>


            <p><a href="TestingGuide.html#quicktestsuite">LLVM test suite</a> provides a

            
               framework to test optimizer's handling of debugging information. It can be

            
               run like this:</p>


            <div class="doc_code">

            
            <pre>

            
            % cd llvm/projects/test-suite/MultiSource/Benchmarks  # or some other level

            
            % make TEST=dbgopt

            
            </pre>

            
            </div>


            <p>This will test impact of debugging information on optimization passes. If

            
               debugging information influences optimization passes then it will be reported

            
               as a failure. See <a href="TestingGuide.html">TestingGuide</a> for more

            
               information on LLVM test infrastructure and how to run various tests.</p>


            </div>


            </div>


            <!-- *********************************************************************** -->

            
            <h2>

            
              <a name="format">Debugging information format</a>

            
            </h2>

            
            <!-- *********************************************************************** -->


            <div>


            <p>LLVM debugging information has been carefully designed to make it possible

            
               for the optimizer to optimize the program and debugging information without

            
               necessarily having to know anything about debugging information.  In

            
               particular, the use of metadata avoids duplicated debugging information from

            
               the beginning, and the global dead code elimination pass automatically

            
               deletes debugging information for a function if it decides to delete the

            
               function. </p>


            <p>To do this, most of the debugging information (descriptors for types,

            
               variables, functions, source files, etc) is inserted by the language

            
               front-end in the form of LLVM metadata. </p>


            <p>Debug information is designed to be agnostic about the target debugger and

            
               debugging information representation (e.g. DWARF/Stabs/etc).  It uses a

            
               generic pass to decode the information that represents variables, types,

            
               functions, namespaces, etc: this allows for arbitrary source-language

            
               semantics and type-systems to be used, as long as there is a module

            
               written for the target debugger to interpret the information. </p>


            <p>To provide basic functionality, the LLVM debugger does have to make some

            
               assumptions about the source-level language being debugged, though it keeps

            
               these to a minimum.  The only common features that the LLVM debugger assumes

            
               exist are <a href="#format_files">source files</a>,

            
               and <a href="#format_global_variables">program objects</a>.  These abstract

            
               objects are used by a debugger to form stack traces, show information about

            
               local variables, etc.</p>


            <p>This section of the documentation first describes the representation aspects

            
               common to any source-language.  The <a href="#ccxx_frontend">next section</a>

            
               describes the data layout conventions used by the C and C++ front-ends.</p>


            <!-- ======================================================================= -->

            
            <h3>

            
              <a name="debug_info_descriptors">Debug information descriptors</a>

            
            </h3>


            <div>


            <p>In consideration of the complexity and volume of debug information, LLVM

            
               provides a specification for well formed debug descriptors. </p>


            <p>Consumers of LLVM debug information expect the descriptors for program

            
               objects to start in a canonical format, but the descriptors can include

            
               additional information appended at the end that is source-language

            
               specific. All LLVM debugging information is versioned, allowing backwards

            
               compatibility in the case that the core structures need to change in some

            
               way.  Also, all debugging information objects start with a tag to indicate

            
               what type of object it is.  The source-language is allowed to define its own

            
               objects, by using unreserved tag numbers.  We recommend using with tags in

            
               the range 0x1000 through 0x2000 (there is a defined enum DW_TAG_user_base =

            
               0x1000.)</p>


            <p>The fields of debug descriptors used internally by LLVM

            
               are restricted to only the simple data types <tt>i32</tt>, <tt>i1</tt>,

            
               <tt>float</tt>, <tt>double</tt>, <tt>mdstring</tt> and <tt>mdnode</tt>. </p>


            <div class="doc_code">

            
            <pre>

            
            !1 = metadata !{

            
              i32,   ;; A tag

            
              ...

            
            }

            
            </pre>

            
            </div>


            <p><a name="LLVMDebugVersion">The first field of a descriptor is always an

            
               <tt>i32</tt> containing a tag value identifying the content of the

            
               descriptor.  The remaining fields are specific to the descriptor.  The values

            
               of tags are loosely bound to the tag values of DWARF information entries.

            
               However, that does not restrict the use of the information supplied to DWARF

            
               targets.  To facilitate versioning of debug information, the tag is augmented

            
               with the current debug version (LLVMDebugVersion = 8 &lt;&lt; 16 or

            
               0x80000 or 524288.)</a></p>


            <p>The details of the various descriptors follow.</p>


            <!-- ======================================================================= -->

            
            <h4>

            
              <a name="format_compile_units">Compile unit descriptors</a>

            
            </h4>


            <div>


            <div class="doc_code">

            
            <pre>

            
            !0 = metadata !{

            
              i32,       ;; Tag = 17 + <a href="#LLVMDebugVersion">LLVMDebugVersion</a>

            
                         ;; (DW_TAG_compile_unit)

            
              i32,       ;; Unused field.

            
              i32,       ;; DWARF language identifier (ex. DW_LANG_C89)

            
              metadata,  ;; Source file name

            
              metadata,  ;; Source file directory (includes trailing slash)

            
              metadata   ;; Producer (ex. "4.0.1 LLVM (LLVM research group)")

            
              i1,        ;; True if this is a main compile unit.

            
              i1,        ;; True if this is optimized.

            
              metadata,  ;; Flags

            
              i32        ;; Runtime version

            
              metadata   ;; List of enums types

            
              metadata   ;; List of retained types

            
              metadata   ;; List of subprograms

            
              metadata   ;; List of global variables

            
            }

            
            </pre>

            
            </div>


            <p>These descriptors contain a source language ID for the file (we use the DWARF

            
               3.0 ID numbers, such as <tt>DW_LANG_C89</tt>, <tt>DW_LANG_C_plus_plus</tt>,

            
               <tt>DW_LANG_Cobol74</tt>, etc), three strings describing the filename,

            
               working directory of the compiler, and an identifier string for the compiler

            
               that produced it.</p>


            <p>Compile unit descriptors provide the root context for objects declared in a

            
               specific compilation unit. File descriptors are defined using this context.

            
               These descriptors are collected by a named metadata

            
               <tt>!llvm.dbg.cu</tt>. Compile unit descriptor keeps track of subprograms,

            
               global variables and type information.


            </div>


            <!-- ======================================================================= -->

            
            <h4>

            
              <a name="format_files">File descriptors</a>

            
            </h4>


            <div>


            <div class="doc_code">

            
            <pre>

            
            !0 = metadata !{

            
              i32,       ;; Tag = 41 + <a href="#LLVMDebugVersion">LLVMDebugVersion</a>

            
                         ;; (DW_TAG_file_type)

            
              metadata,  ;; Source file name

            
              metadata,  ;; Source file directory (includes trailing slash)

            
              metadata   ;; Unused

            
            }

            
            </pre>

            
            </div>


            <p>These descriptors contain information for a file. Global variables and top

            
               level functions would be defined using this context.k File descriptors also

            
               provide context for source line correspondence. </p>


            <p>Each input file is encoded as a separate file descriptor in LLVM debugging

            
               information output. </p>


            </div>


            <!-- ======================================================================= -->

            
            <h4>

            
              <a name="format_global_variables">Global variable descriptors</a>

            
            </h4>


            <div>


            <div class="doc_code">

            
            <pre>

            
            !1 = metadata !{

            
              i32,      ;; Tag = 52 + <a href="#LLVMDebugVersion">LLVMDebugVersion</a>

            
                        ;; (DW_TAG_variable)

            
              i32,      ;; Unused field.

            
              metadata, ;; Reference to context descriptor

            
              metadata, ;; Name

            
              metadata, ;; Display name (fully qualified C++ name)

            
              metadata, ;; MIPS linkage name (for C++)

            
              metadata, ;; Reference to file where defined

            
              i32,      ;; Line number where defined

            
              metadata, ;; Reference to type descriptor

            
              i1,       ;; True if the global is local to compile unit (static)

            
              i1,       ;; True if the global is defined in the compile unit (not extern)

            
              {}*       ;; Reference to the global variable

            
            }

            
            </pre>

            
            </div>


            <p>These descriptors provide debug information about globals variables.  The

            
            provide details such as name, type and where the variable is defined. All

            
            global variables are collected inside the named metadata

            
            <tt>!llvm.dbg.cu</tt>.</p>


            </div>


            <!-- ======================================================================= -->

            
            <h4>

            
              <a name="format_subprograms">Subprogram descriptors</a>

            
            </h4>


            <div>


            <div class="doc_code">

            
            <pre>

            
            !2 = metadata !{

            
              i32,      ;; Tag = 46 + <a href="#LLVMDebugVersion">LLVMDebugVersion</a>

            
                        ;; (DW_TAG_subprogram)

            
              i32,      ;; Unused field.

            
              metadata, ;; Reference to context descriptor

            
              metadata, ;; Name

            
              metadata, ;; Display name (fully qualified C++ name)

            
              metadata, ;; MIPS linkage name (for C++)

            
              metadata, ;; Reference to file where defined

            
              i32,      ;; Line number where defined

            
              metadata, ;; Reference to type descriptor

            
              i1,       ;; True if the global is local to compile unit (static)

            
              i1,       ;; True if the global is defined in the compile unit (not extern)

            
              i32,      ;; Line number where the scope of the subprogram begins

            
              i32,      ;; Virtuality, e.g. dwarf::DW_VIRTUALITY__virtual

            
              i32,      ;; Index into a virtual function

            
              metadata, ;; indicates which base type contains the vtable pointer for the

            
                        ;; derived class

            
              i32,      ;; Flags - Artifical, Private, Protected, Explicit, Prototyped.

            
              i1,       ;; isOptimized

            
              Function *,;; Pointer to LLVM function

            
              metadata, ;; Lists function template parameters

            
              metadata  ;; Function declaration descriptor

            
              metadata  ;; List of function variables

            
            }

            
            </pre>

            
            </div>


            <p>These descriptors provide debug information about functions, methods and

            
               subprograms.  They provide details such as name, return types and the source

            
               location where the subprogram is defined.

            
            </p>


            </div>


            <!-- ======================================================================= -->

            
            <h4>

            
              <a name="format_blocks">Block descriptors</a>

            
            </h4>


            <div>


            <div class="doc_code">

            
            <pre>

            
            !3 = metadata !{

            
              i32,     ;; Tag = 11 + <a href="#LLVMDebugVersion">LLVMDebugVersion</a> (DW_TAG_lexical_block)

            
              metadata,;; Reference to context descriptor

            
              i32,     ;; Line number

            
              i32,     ;; Column number

            
              metadata,;; Reference to source file

            
              i32      ;; Unique ID to identify blocks from a template function

            
            }

            
            </pre>

            
            </div>


            <p>This descriptor provides debug information about nested blocks within a

            
               subprogram. The line number and column numbers are used to dinstinguish

            
               two lexical blocks at same depth. </p>


            <div class="doc_code">

            
            <pre>

            
            !3 = metadata !{

            
              i32,     ;; Tag = 11 + <a href="#LLVMDebugVersion">LLVMDebugVersion</a> (DW_TAG_lexical_block)

            
              metadata ;; Reference to the scope we're annotating with a file change

            
              metadata,;; Reference to the file the scope is enclosed in.

            
            }

            
            </pre>

            
            </div>


            <p>This descriptor provides a wrapper around a lexical scope to handle file

            
               changes in the middle of a lexical block.</p>


            </div>


            <!-- ======================================================================= -->

            
            <h4>

            
              <a name="format_basic_type">Basic type descriptors</a>

            
            </h4>


            <div>


            <div class="doc_code">

            
            <pre>

            
            !4 = metadata !{

            
              i32,      ;; Tag = 36 + <a href="#LLVMDebugVersion">LLVMDebugVersion</a>

            
                        ;; (DW_TAG_base_type)

            
              metadata, ;; Reference to context

            
              metadata, ;; Name (may be "" for anonymous types)

            
              metadata, ;; Reference to file where defined (may be NULL)

            
              i32,      ;; Line number where defined (may be 0)

            
              i64,      ;; Size in bits

            
              i64,      ;; Alignment in bits

            
              i64,      ;; Offset in bits

            
              i32,      ;; Flags

            
              i32       ;; DWARF type encoding

            
            }

            
            </pre>

            
            </div>


            <p>These descriptors define primitive types used in the code. Example int, bool

            
               and float.  The context provides the scope of the type, which is usually the

            
               top level.  Since basic types are not usually user defined the context

            
               and line number can be left as NULL and 0.  The size, alignment and offset

            
               are expressed in bits and can be 64 bit values.  The alignment is used to

            
               round the offset when embedded in a

            
               <a href="#format_composite_type">composite type</a> (example to keep float

            
               doubles on 64 bit boundaries.) The offset is the bit offset if embedded in

            
               a <a href="#format_composite_type">composite type</a>.</p>


            <p>The type encoding provides the details of the type.  The values are typically

            
               one of the following:</p>


            <div class="doc_code">

            
            <pre>

            
            DW_ATE_address       = 1

            
            DW_ATE_boolean       = 2

            
            DW_ATE_float         = 4

            
            DW_ATE_signed        = 5

            
            DW_ATE_signed_char   = 6

            
            DW_ATE_unsigned      = 7

            
            DW_ATE_unsigned_char = 8

            
            </pre>

            
            </div>


            </div>


            <!-- ======================================================================= -->

            
            <h4>

            
              <a name="format_derived_type">Derived type descriptors</a>

            
            </h4>


            <div>


            <div class="doc_code">

            
            <pre>

            
            !5 = metadata !{

            
              i32,      ;; Tag (see below)

            
              metadata, ;; Reference to context

            
              metadata, ;; Name (may be "" for anonymous types)

            
              metadata, ;; Reference to file where defined (may be NULL)

            
              i32,      ;; Line number where defined (may be 0)

            
              i64,      ;; Size in bits

            
              i64,      ;; Alignment in bits

            
              i64,      ;; Offset in bits

            
              i32,      ;; Flags to encode attributes, e.g. private

            
              metadata, ;; Reference to type derived from

            
              metadata, ;; (optional) Name of the Objective C property associated with

            
                        ;; Objective-C an ivar

            
              metadata, ;; (optional) Name of the Objective C property getter selector.

            
              metadata, ;; (optional) Name of the Objective C property setter selector.

            
              i32       ;; (optional) Objective C property attributes.

            
            }

            
            </pre>

            
            </div>


            <p>These descriptors are used to define types derived from other types.  The

            
            value of the tag varies depending on the meaning.  The following are possible

            
            tag values:</p>


            <div class="doc_code">

            
            <pre>

            
            DW_TAG_formal_parameter = 5

            
            DW_TAG_member           = 13

            
            DW_TAG_pointer_type     = 15

            
            DW_TAG_reference_type   = 16

            
            DW_TAG_typedef          = 22

            
            DW_TAG_const_type       = 38

            
            DW_TAG_volatile_type    = 53

            
            DW_TAG_restrict_type    = 55

            
            </pre>

            
            </div>


            <p><tt>DW_TAG_member</tt> is used to define a member of

            
               a <a href="#format_composite_type">composite type</a>

            
               or <a href="#format_subprograms">subprogram</a>.  The type of the member is

            
               the <a href="#format_derived_type">derived

            
               type</a>. <tt>DW_TAG_formal_parameter</tt> is used to define a member which

            
               is a formal argument of a subprogram.</p>


            <p><tt>DW_TAG_typedef</tt> is used to provide a name for the derived type.</p>


            <p><tt>DW_TAG_pointer_type</tt>, <tt>DW_TAG_reference_type</tt>,

            
               <tt>DW_TAG_const_type</tt>, <tt>DW_TAG_volatile_type</tt> and

            
               <tt>DW_TAG_restrict_type</tt> are used to qualify

            
               the <a href="#format_derived_type">derived type</a>. </p>


            <p><a href="#format_derived_type">Derived type</a> location can be determined

            
               from the context and line number.  The size, alignment and offset are

            
               expressed in bits and can be 64 bit values.  The alignment is used to round

            
               the offset when embedded in a <a href="#format_composite_type">composite

            
               type</a> (example to keep float doubles on 64 bit boundaries.) The offset is

            
               the bit offset if embedded in a <a href="#format_composite_type">composite

            
               type</a>.</p>


            <p>Note that the <tt>void *</tt> type is expressed as a type derived from NULL.

            
            </p>


            </div>


            <!-- ======================================================================= -->

            
            <h4>

            
              <a name="format_composite_type">Composite type descriptors</a>

            
            </h4>


            <div>


            <div class="doc_code">

            
            <pre>

            
            !6 = metadata !{

            
              i32,      ;; Tag (see below)

            
              metadata, ;; Reference to context

            
              metadata, ;; Name (may be "" for anonymous types)

            
              metadata, ;; Reference to file where defined (may be NULL)

            
              i32,      ;; Line number where defined (may be 0)

            
              i64,      ;; Size in bits

            
              i64,      ;; Alignment in bits

            
              i64,      ;; Offset in bits

            
              i32,      ;; Flags

            
              metadata, ;; Reference to type derived from

            
              metadata, ;; Reference to array of member descriptors

            
              i32       ;; Runtime languages

            
            }

            
            </pre>

            
            </div>


            <p>These descriptors are used to define types that are composed of 0 or more

            
            elements.  The value of the tag varies depending on the meaning.  The following

            
            are possible tag values:</p>


            <div class="doc_code">

            
            <pre>

            
            DW_TAG_array_type       = 1

            
            DW_TAG_enumeration_type = 4

            
            DW_TAG_structure_type   = 19

            
            DW_TAG_union_type       = 23

            
            DW_TAG_vector_type      = 259

            
            DW_TAG_subroutine_type  = 21

            
            DW_TAG_inheritance      = 28

            
            </pre>

            
            </div>


            <p>The vector flag indicates that an array type is a native packed vector.</p>


            <p>The members of array types (tag = <tt>DW_TAG_array_type</tt>) or vector types

            
               (tag = <tt>DW_TAG_vector_type</tt>) are <a href="#format_subrange">subrange

            
               descriptors</a>, each representing the range of subscripts at that level of

            
               indexing.</p>


            <p>The members of enumeration types (tag = <tt>DW_TAG_enumeration_type</tt>) are

            
               <a href="#format_enumeration">enumerator descriptors</a>, each representing

            
               the definition of enumeration value for the set. All enumeration type

            
               descriptors are collected inside the named metadata

            
               <tt>!llvm.dbg.cu</tt>.</p>


            <p>The members of structure (tag = <tt>DW_TAG_structure_type</tt>) or union (tag

            
               = <tt>DW_TAG_union_type</tt>) types are any one of

            
               the <a href="#format_basic_type">basic</a>,

            
               <a href="#format_derived_type">derived</a>

            
               or <a href="#format_composite_type">composite</a> type descriptors, each

            
               representing a field member of the structure or union.</p>


            <p>For C++ classes (tag = <tt>DW_TAG_structure_type</tt>), member descriptors

            
               provide information about base classes, static members and member

            
               functions. If a member is a <a href="#format_derived_type">derived type

            
               descriptor</a> and has a tag of <tt>DW_TAG_inheritance</tt>, then the type

            
               represents a base class. If the member of is

            
               a <a href="#format_global_variables">global variable descriptor</a> then it

            
               represents a static member.  And, if the member is

            
               a <a href="#format_subprograms">subprogram descriptor</a> then it represents

            
               a member function.  For static members and member

            
               functions, <tt>getName()</tt> returns the members link or the C++ mangled

            
               name.  <tt>getDisplayName()</tt> the simplied version of the name.</p>


            <p>The first member of subroutine (tag = <tt>DW_TAG_subroutine_type</tt>) type

            
               elements is the return type for the subroutine.  The remaining elements are

            
               the formal arguments to the subroutine.</p>


            <p><a href="#format_composite_type">Composite type</a> location can be

            
               determined from the context and line number.  The size, alignment and

            
               offset are expressed in bits and can be 64 bit values.  The alignment is used

            
               to round the offset when embedded in

            
               a <a href="#format_composite_type">composite type</a> (as an example, to keep

            
               float doubles on 64 bit boundaries.) The offset is the bit offset if embedded

            
               in a <a href="#format_composite_type">composite type</a>.</p>


            </div>


            <!-- ======================================================================= -->

            
            <h4>

            
              <a name="format_subrange">Subrange descriptors</a>

            
            </h4>


            <div>


            <div class="doc_code">

            
            <pre>

            
            !42 = metadata !{

            
              i32,    ;; Tag = 33 + <a href="#LLVMDebugVersion">LLVMDebugVersion</a> (DW_TAG_subrange_type)

            
              i64,    ;; Low value

            
              i64     ;; High value

            
            }

            
            </pre>

            
            </div>


            <p>These descriptors are used to define ranges of array subscripts for an array

            
               <a href="#format_composite_type">composite type</a>.  The low value defines

            
               the lower bounds typically zero for C/C++.  The high value is the upper

            
               bounds.  Values are 64 bit.  High - low + 1 is the size of the array.  If low

            
               > high the array bounds are not included in generated debugging information.

            
            </p>


            </div>


            <!-- ======================================================================= -->

            
            <h4>

            
              <a name="format_enumeration">Enumerator descriptors</a>

            
            </h4>


            <div>


            <div class="doc_code">

            
            <pre>

            
            !6 = metadata !{

            
              i32,      ;; Tag = 40 + <a href="#LLVMDebugVersion">LLVMDebugVersion</a>

            
                        ;; (DW_TAG_enumerator)

            
              metadata, ;; Name

            
              i64       ;; Value

            
            }

            
            </pre>

            
            </div>


            <p>These descriptors are used to define members of an

            
               enumeration <a href="#format_composite_type">composite type</a>, it

            
               associates the name to the value.</p>


            </div>


            <!-- ======================================================================= -->

            
            <h4>

            
              <a name="format_variables">Local variables</a>

            
            </h4>


            <div>


            <div class="doc_code">

            
            <pre>

            
            !7 = metadata !{

            
              i32,      ;; Tag (see below)

            
              metadata, ;; Context

            
              metadata, ;; Name

            
              metadata, ;; Reference to file where defined

            
              i32,      ;; 24 bit - Line number where defined

            
                        ;; 8 bit - Argument number. 1 indicates 1st argument.

            
              metadata, ;; Type descriptor

            
              i32,      ;; flags

            
              metadata  ;; (optional) Reference to inline location

            
            }

            
            </pre>

            
            </div>


            <p>These descriptors are used to define variables local to a sub program.  The

            
               value of the tag depends on the usage of the variable:</p>


            <div class="doc_code">

            
            <pre>

            
            DW_TAG_auto_variable   = 256

            
            DW_TAG_arg_variable    = 257

            
            DW_TAG_return_variable = 258

            
            </pre>

            
            </div>


            <p>An auto variable is any variable declared in the body of the function.  An

            
               argument variable is any variable that appears as a formal argument to the

            
               function.  A return variable is used to track the result of a function and

            
               has no source correspondent.</p>


            <p>The context is either the subprogram or block where the variable is defined.

            
               Name the source variable name.  Context and line indicate where the

            
               variable was defined. Type descriptor defines the declared type of the

            
               variable.</p>


            </div>


            </div>


            <!-- ======================================================================= -->

            
            <h3>

            
              <a name="format_common_intrinsics">Debugger intrinsic functions</a>

            
            </h3>


            <div>


            <p>LLVM uses several intrinsic functions (name prefixed with "llvm.dbg") to

            
               provide debug information at various points in generated code.</p>


            <!-- ======================================================================= -->

            
            <h4>

            
              <a name="format_common_declare">llvm.dbg.declare</a>

            
            </h4>


            <div>

            
            <pre>

            
              void %<a href="#format_common_declare">llvm.dbg.declare</a>(metadata, metadata)

            
            </pre>


            <p>This intrinsic provides information about a local element (e.g., variable). The

            
               first argument is metadata holding the alloca for the variable. The

            
               second argument is metadata containing a description of the variable.</p>

            
            </div>


            <!-- ======================================================================= -->

            
            <h4>

            
              <a name="format_common_value">llvm.dbg.value</a>

            
            </h4>


            <div>

            
            <pre>

            
              void %<a href="#format_common_value">llvm.dbg.value</a>(metadata, i64, metadata)

            
            </pre>


            <p>This intrinsic provides information when a user source variable is set to a

            
               new value.  The first argument is the new value (wrapped as metadata).  The

            
               second argument is the offset in the user source variable where the new value

            
               is written.  The third argument is metadata containing a description of the

            
               user source variable.</p>

            
            </div>


            </div>


            <!-- ======================================================================= -->

            
            <h3>

            
              <a name="format_common_lifetime">Object lifetimes and scoping</a>

            
            </h3>


            <div>

            
            <p>In many languages, the local variables in functions can have their lifetimes

            
               or scopes limited to a subset of a function.  In the C family of languages,

            
               for example, variables are only live (readable and writable) within the

            
               source block that they are defined in.  In functional languages, values are

            
               only readable after they have been defined.  Though this is a very obvious

            
               concept, it is non-trivial to model in LLVM, because it has no notion of

            
               scoping in this sense, and does not want to be tied to a language's scoping

            
               rules.</p>


            <p>In order to handle this, the LLVM debug format uses the metadata attached to

            
               llvm instructions to encode line number and scoping information. Consider

            
               the following C fragment, for example:</p>


            <div class="doc_code">

            
            <pre>

            
            1.  void foo() {

            
            2.    int X = 21;

            
            3.    int Y = 22;

            
            4.    {

            
            5.      int Z = 23;

            
            6.      Z = X;

            
            7.    }

            
            8.    X = Y;

            
            9.  }

            
            </pre>

            
            </div>


            <p>Compiled to LLVM, this function would be represented like this:</p>


            <div class="doc_code">

            
            <pre>

            
            define void @foo() nounwind ssp {

            
            entry:

            
              %X = alloca i32, align 4                        ; &lt;i32*&gt; [#uses=4]

            
              %Y = alloca i32, align 4                        ; &lt;i32*&gt; [#uses=4]

            
              %Z = alloca i32, align 4                        ; &lt;i32*&gt; [#uses=3]

            
              %0 = bitcast i32* %X to {}*                     ; &lt;{}*&gt; [#uses=1]

            
              call void @llvm.dbg.declare(metadata !{i32 * %X}, metadata !0), !dbg !7

            
              store i32 21, i32* %X, !dbg !8

            
              %1 = bitcast i32* %Y to {}*                     ; &lt;{}*&gt; [#uses=1]

            
              call void @llvm.dbg.declare(metadata !{i32 * %Y}, metadata !9), !dbg !10

            
              store i32 22, i32* %Y, !dbg !11

            
              %2 = bitcast i32* %Z to {}*                     ; &lt;{}*&gt; [#uses=1]

            
              call void @llvm.dbg.declare(metadata !{i32 * %Z}, metadata !12), !dbg !14

            
              store i32 23, i32* %Z, !dbg !15

            
              %tmp = load i32* %X, !dbg !16                   ; &lt;i32&gt; [#uses=1]

            
              %tmp1 = load i32* %Y, !dbg !16                  ; &lt;i32&gt; [#uses=1]

            
              %add = add nsw i32 %tmp, %tmp1, !dbg !16        ; &lt;i32&gt; [#uses=1]

            
              store i32 %add, i32* %Z, !dbg !16

            
              %tmp2 = load i32* %Y, !dbg !17                  ; &lt;i32&gt; [#uses=1]

            
              store i32 %tmp2, i32* %X, !dbg !17

            
              ret void, !dbg !18

            
            }


            declare void @llvm.dbg.declare(metadata, metadata) nounwind readnone


            !0 = metadata !{i32 459008, metadata !1, metadata !"X",

            
                            metadata !3, i32 2, metadata !6}; [ DW_TAG_auto_variable ]

            
            !1 = metadata !{i32 458763, metadata !2}; [DW_TAG_lexical_block ]

            
            !2 = metadata !{i32 458798, i32 0, metadata !3, metadata !"foo", metadata !"foo",

            
                           metadata !"foo", metadata !3, i32 1, metadata !4,

            
                           i1 false, i1 true}; [DW_TAG_subprogram ]

            
            !3 = metadata !{i32 458769, i32 0, i32 12, metadata !"foo.c",

            
                            metadata !"/private/tmp", metadata !"clang 1.1", i1 true,

            
                            i1 false, metadata !"", i32 0}; [DW_TAG_compile_unit ]

            
            !4 = metadata !{i32 458773, metadata !3, metadata !"", null, i32 0, i64 0, i64 0,

            
                            i64 0, i32 0, null, metadata !5, i32 0}; [DW_TAG_subroutine_type ]

            
            !5 = metadata !{null}

            
            !6 = metadata !{i32 458788, metadata !3, metadata !"int", metadata !3, i32 0,

            
                            i64 32, i64 32, i64 0, i32 0, i32 5}; [DW_TAG_base_type ]

            
            !7 = metadata !{i32 2, i32 7, metadata !1, null}

            
            !8 = metadata !{i32 2, i32 3, metadata !1, null}

            
            !9 = metadata !{i32 459008, metadata !1, metadata !"Y", metadata !3, i32 3,

            
                            metadata !6}; [ DW_TAG_auto_variable ]

            
            !10 = metadata !{i32 3, i32 7, metadata !1, null}

            
            !11 = metadata !{i32 3, i32 3, metadata !1, null}

            
            !12 = metadata !{i32 459008, metadata !13, metadata !"Z", metadata !3, i32 5,

            
                             metadata !6}; [ DW_TAG_auto_variable ]

            
            !13 = metadata !{i32 458763, metadata !1}; [DW_TAG_lexical_block ]

            
            !14 = metadata !{i32 5, i32 9, metadata !13, null}

            
            !15 = metadata !{i32 5, i32 5, metadata !13, null}

            
            !16 = metadata !{i32 6, i32 5, metadata !13, null}

            
            !17 = metadata !{i32 8, i32 3, metadata !1, null}

            
            !18 = metadata !{i32 9, i32 1, metadata !2, null}

            
            </pre>

            
            </div>


            <p>This example illustrates a few important details about LLVM debugging

            
               information. In particular, it shows how the <tt>llvm.dbg.declare</tt>

            
               intrinsic and location information, which are attached to an instruction,

            
               are applied together to allow a debugger to analyze the relationship between

            
               statements, variable definitions, and the code used to implement the

            
               function.</p>


            <div class="doc_code">

            
            <pre>

            
            call void @llvm.dbg.declare(metadata, metadata !0), !dbg !7

            
            </pre>

            
            </div>


            <p>The first intrinsic

            
               <tt>%<a href="#format_common_declare">llvm.dbg.declare</a></tt>

            
               encodes debugging information for the variable <tt>X</tt>. The metadata

            
               <tt>!dbg !7</tt> attached to the intrinsic provides scope information for the

            
               variable <tt>X</tt>.</p>


            <div class="doc_code">

            
            <pre>

            
            !7 = metadata !{i32 2, i32 7, metadata !1, null}

            
            !1 = metadata !{i32 458763, metadata !2}; [DW_TAG_lexical_block ]

            
            !2 = metadata !{i32 458798, i32 0, metadata !3, metadata !"foo",

            
                            metadata !"foo", metadata !"foo", metadata !3, i32 1,

            
                            metadata !4, i1 false, i1 true}; [DW_TAG_subprogram ]

            
            </pre>

            
            </div>


            <p>Here <tt>!7</tt> is metadata providing location information. It has four

            
               fields: line number, column number, scope, and original scope. The original

            
               scope represents inline location if this instruction is inlined inside a

            
               caller, and is null otherwise. In this example, scope is encoded by

            
               <tt>!1</tt>. <tt>!1</tt> represents a lexical block inside the scope

            
               <tt>!2</tt>, where <tt>!2</tt> is a

            
               <a href="#format_subprograms">subprogram descriptor</a>. This way the

            
               location information attached to the intrinsics indicates that the

            
               variable <tt>X</tt> is declared at line number 2 at a function level scope in

            
               function <tt>foo</tt>.</p>


            <p>Now lets take another example.</p>


            <div class="doc_code">

            
            <pre>

            
            call void @llvm.dbg.declare(metadata, metadata !12), !dbg !14

            
            </pre>

            
            </div>


            <p>The second intrinsic

            
               <tt>%<a href="#format_common_declare">llvm.dbg.declare</a></tt>

            
               encodes debugging information for variable <tt>Z</tt>. The metadata

            
               <tt>!dbg !14</tt> attached to the intrinsic provides scope information for

            
               the variable <tt>Z</tt>.</p>


            <div class="doc_code">

            
            <pre>

            
            !13 = metadata !{i32 458763, metadata !1}; [DW_TAG_lexical_block ]

            
            !14 = metadata !{i32 5, i32 9, metadata !13, null}

            
            </pre>

            
            </div>


            <p>Here <tt>!14</tt> indicates that <tt>Z</tt> is declared at line number 5 and

            
               column number 9 inside of lexical scope <tt>!13</tt>. The lexical scope

            
               itself resides inside of lexical scope <tt>!1</tt> described above.</p>


            <p>The scope information attached with each instruction provides a

            
               straightforward way to find instructions covered by a scope.</p>


            </div>


            </div>


            <!-- *********************************************************************** -->

            
            <h2>

            
              <a name="ccxx_frontend">C/C++ front-end specific debug information</a>

            
            </h2>

            
            <!-- *********************************************************************** -->


            <div>


            <p>The C and C++ front-ends represent information about the program in a format

            
               that is effectively identical

            
               to <a href="http://www.eagercon.com/dwarf/dwarf3std.htm">DWARF 3.0</a> in

            
               terms of information content.  This allows code generators to trivially

            
               support native debuggers by generating standard dwarf information, and

            
               contains enough information for non-dwarf targets to translate it as

            
               needed.</p>


            <p>This section describes the forms used to represent C and C++ programs. Other

            
               languages could pattern themselves after this (which itself is tuned to

            
               representing programs in the same way that DWARF 3 does), or they could

            
               choose to provide completely different forms if they don't fit into the DWARF

            
               model.  As support for debugging information gets added to the various LLVM

            
               source-language front-ends, the information used should be documented

            
               here.</p>


            <p>The following sections provide examples of various C/C++ constructs and the

            
               debug information that would best describe those constructs.</p>


            <!-- ======================================================================= -->

            
            <h3>

            
              <a name="ccxx_compile_units">C/C++ source file information</a>

            
            </h3>


            <div>


            <p>Given the source files <tt>MySource.cpp</tt> and <tt>MyHeader.h</tt> located

            
               in the directory <tt>/Users/mine/sources</tt>, the following code:</p>


            <div class="doc_code">

            
            <pre>

            
            #include "MyHeader.h"


            int main(int argc, char *argv[]) {

            
              return 0;

            
            }

            
            </pre>

            
            </div>


            <p>a C/C++ front-end would generate the following descriptors:</p>


            <div class="doc_code">

            
            <pre>

            
            ...

            
            ;;

            
            ;; Define the compile unit for the main source file "/Users/mine/sources/MySource.cpp".

            
            ;;

            
            !2 = metadata !{

            
              i32 524305,    ;; Tag

            
              i32 0,         ;; Unused

            
              i32 4,         ;; Language Id

            
              metadata !"MySource.cpp",

            
              metadata !"/Users/mine/sources",

            
              metadata !"4.2.1 (Based on Apple Inc. build 5649) (LLVM build 00)",

            
              i1 true,       ;; Main Compile Unit

            
              i1 false,      ;; Optimized compile unit

            
              metadata !"",  ;; Compiler flags

            
              i32 0}         ;; Runtime version


            ;;

            
            ;; Define the file for the file "/Users/mine/sources/MySource.cpp".

            
            ;;

            
            !1 = metadata !{

            
              i32 524329,    ;; Tag

            
              metadata !"MySource.cpp",

            
              metadata !"/Users/mine/sources",

            
              metadata !2    ;; Compile unit

            
            }


            ;;

            
            ;; Define the file for the file "/Users/mine/sources/Myheader.h"

            
            ;;

            
            !3 = metadata !{

            
              i32 524329,    ;; Tag

            
              metadata !"Myheader.h"

            
              metadata !"/Users/mine/sources",

            
              metadata !2    ;; Compile unit

            
            }


            ...

            
            </pre>

            
            </div>


            <p>llvm::Instruction provides easy access to metadata attached with an

            
            instruction. One can extract line number information encoded in LLVM IR

            
            using <tt>Instruction::getMetadata()</tt> and

            
            <tt>DILocation::getLineNumber()</tt>.

            
            <pre>

            
             if (MDNode *N = I->getMetadata("dbg")) {  // Here I is an LLVM instruction

            
               DILocation Loc(N);                      // DILocation is in DebugInfo.h

            
               unsigned Line = Loc.getLineNumber();

            
               StringRef File = Loc.getFilename();

            
               StringRef Dir = Loc.getDirectory();

            
             }

            
            </pre>

            
            </div>


            <!-- ======================================================================= -->

            
            <h3>

            
              <a name="ccxx_global_variable">C/C++ global variable information</a>

            
            </h3>


            <div>


            <p>Given an integer global variable declared as follows:</p>


            <div class="doc_code">

            
            <pre>

            
            int MyGlobal = 100;

            
            </pre>

            
            </div>


            <p>a C/C++ front-end would generate the following descriptors:</p>


            <div class="doc_code">

            
            <pre>

            
            ;;

            
            ;; Define the global itself.

            
            ;;

            
            %MyGlobal = global int 100

            
            ...

            
            ;;

            
            ;; List of debug info of globals

            
            ;;

            
            !llvm.dbg.cu = !{!0}


            ;; Define the compile unit.

            
            !0 = metadata !{

            
              i32 786449,                       ;; Tag

            
              i32 0,                            ;; Context

            
              i32 4,                            ;; Language

            
              metadata !"foo.cpp",              ;; File

            
              metadata !"/Volumes/Data/tmp",    ;; Directory

            
              metadata !"clang version 3.1 ",   ;; Producer

            
              i1 true,                          ;; Deprecated field

            
              i1 false,                         ;; "isOptimized"?

            
              metadata !"",                     ;; Flags

            
              i32 0,                            ;; Runtime Version

            
              metadata !1,                      ;; Enum Types

            
              metadata !1,                      ;; Retained Types

            
              metadata !1,                      ;; Subprograms

            
              metadata !3                       ;; Global Variables

            
            } ; [ DW_TAG_compile_unit ]


            ;; The Array of Global Variables

            
            !3 = metadata !{

            
              metadata !4

            
            }


            !4 = metadata !{

            
              metadata !5

            
            }


            ;;

            
            ;; Define the global variable itself.

            
            ;;

            
            !5 = metadata !{

            
              i32 786484,                        ;; Tag

            
              i32 0,                             ;; Unused

            
              null,                              ;; Unused

            
              metadata !"MyGlobal",              ;; Name

            
              metadata !"MyGlobal",              ;; Display Name

            
              metadata !"",                      ;; Linkage Name

            
              metadata !6,                       ;; File

            
              i32 1,                             ;; Line

            
              metadata !7,                       ;; Type

            
              i32 0,                             ;; IsLocalToUnit

            
              i32 1,                             ;; IsDefinition

            
              i32* @MyGlobal                     ;; LLVM-IR Value

            
            } ; [ DW_TAG_variable ]


            ;;

            
            ;; Define the file

            
            ;;

            
            !6 = metadata !{

            
              i32 786473,                        ;; Tag

            
              metadata !"foo.cpp",               ;; File

            
              metadata !"/Volumes/Data/tmp",     ;; Directory

            
              null                               ;; Unused

            
            } ; [ DW_TAG_file_type ]


            ;;

            
            ;; Define the type

            
            ;;

            
            !7 = metadata !{

            
              i32 786468,                         ;; Tag

            
              null,                               ;; Unused

            
              metadata !"int",                    ;; Name

            
              null,                               ;; Unused

            
              i32 0,                              ;; Line

            
              i64 32,                             ;; Size in Bits

            
              i64 32,                             ;; Align in Bits

            
              i64 0,                              ;; Offset

            
              i32 0,                              ;; Flags

            
              i32 5                               ;; Encoding

            
            } ; [ DW_TAG_base_type ]


            </pre>

            
            </div>


            </div>


            <!-- ======================================================================= -->

            
            <h3>

            
              <a name="ccxx_subprogram">C/C++ function information</a>

            
            </h3>


            <div>


            <p>Given a function declared as follows:</p>


            <div class="doc_code">

            
            <pre>

            
            int main(int argc, char *argv[]) {

            
              return 0;

            
            }

            
            </pre>

            
            </div>


            <p>a C/C++ front-end would generate the following descriptors:</p>


            <div class="doc_code">

            
            <pre>

            
            ;;

            
            ;; Define the anchor for subprograms.  Note that the second field of the

            
            ;; anchor is 46, which is the same as the tag for subprograms

            
            ;; (46 = DW_TAG_subprogram.)

            
            ;;

            
            !6 = metadata !{

            
              i32 524334,        ;; Tag

            
              i32 0,             ;; Unused

            
              metadata !1,       ;; Context

            
              metadata !"main",  ;; Name

            
              metadata !"main",  ;; Display name

            
              metadata !"main",  ;; Linkage name

            
              metadata !1,       ;; File

            
              i32 1,             ;; Line number

            
              metadata !4,       ;; Type

            
              i1 false,          ;; Is local

            
              i1 true,           ;; Is definition

            
              i32 0,             ;; Virtuality attribute, e.g. pure virtual function

            
              i32 0,             ;; Index into virtual table for C++ methods

            
              i32 0,             ;; Type that holds virtual table.

            
              i32 0,             ;; Flags

            
              i1 false,          ;; True if this function is optimized

            
              Function *,        ;; Pointer to llvm::Function

            
              null               ;; Function template parameters

            
            }

            
            ;;

            
            ;; Define the subprogram itself.

            
            ;;

            
            define i32 @main(i32 %argc, i8** %argv) {

            
            ...

            
            }

            
            </pre>

            
            </div>


            </div>


            <!-- ======================================================================= -->

            
            <h3>

            
              <a name="ccxx_basic_types">C/C++ basic types</a>

            
            </h3>


            <div>


            <p>The following are the basic type descriptors for C/C++ core types:</p>


            <!-- ======================================================================= -->

            
            <h4>

            
              <a name="ccxx_basic_type_bool">bool</a>

            
            </h4>


            <div>


            <div class="doc_code">

            
            <pre>

            
            !2 = metadata !{

            
              i32 524324,        ;; Tag

            
              metadata !1,       ;; Context

            
              metadata !"bool",  ;; Name

            
              metadata !1,       ;; File

            
              i32 0,             ;; Line number

            
              i64 8,             ;; Size in Bits

            
              i64 8,             ;; Align in Bits

            
              i64 0,             ;; Offset in Bits

            
              i32 0,             ;; Flags

            
              i32 2              ;; Encoding

            
            }

            
            </pre>

            
            </div>


            </div>


            <!-- ======================================================================= -->

            
            <h4>

            
              <a name="ccxx_basic_char">char</a>

            
            </h4>


            <div>


            <div class="doc_code">

            
            <pre>

            
            !2 = metadata !{

            
              i32 524324,        ;; Tag

            
              metadata !1,       ;; Context

            
              metadata !"char",  ;; Name

            
              metadata !1,       ;; File

            
              i32 0,             ;; Line number

            
              i64 8,             ;; Size in Bits

            
              i64 8,             ;; Align in Bits

            
              i64 0,             ;; Offset in Bits

            
              i32 0,             ;; Flags

            
              i32 6              ;; Encoding

            
            }

            
            </pre>

            
            </div>


            </div>


            <!-- ======================================================================= -->

            
            <h4>

            
              <a name="ccxx_basic_unsigned_char">unsigned char</a>

            
            </h4>


            <div>


            <div class="doc_code">

            
            <pre>

            
            !2 = metadata !{

            
              i32 524324,        ;; Tag

            
              metadata !1,       ;; Context

            
              metadata !"unsigned char",

            
              metadata !1,       ;; File

            
              i32 0,             ;; Line number

            
              i64 8,             ;; Size in Bits

            
              i64 8,             ;; Align in Bits

            
              i64 0,             ;; Offset in Bits

            
              i32 0,             ;; Flags

            
              i32 8              ;; Encoding

            
            }

            
            </pre>

            
            </div>


            </div>


            <!-- ======================================================================= -->

            
            <h4>

            
              <a name="ccxx_basic_short">short</a>

            
            </h4>


            <div>


            <div class="doc_code">

            
            <pre>

            
            !2 = metadata !{

            
              i32 524324,        ;; Tag

            
              metadata !1,       ;; Context

            
              metadata !"short int",

            
              metadata !1,       ;; File

            
              i32 0,             ;; Line number

            
              i64 16,            ;; Size in Bits

            
              i64 16,            ;; Align in Bits

            
              i64 0,             ;; Offset in Bits

            
              i32 0,             ;; Flags

            
              i32 5              ;; Encoding

            
            }

            
            </pre>

            
            </div>


            </div>


            <!-- ======================================================================= -->

            
            <h4>

            
              <a name="ccxx_basic_unsigned_short">unsigned short</a>

            
            </h4>


            <div>


            <div class="doc_code">

            
            <pre>

            
            !2 = metadata !{

            
              i32 524324,        ;; Tag

            
              metadata !1,       ;; Context

            
              metadata !"short unsigned int",

            
              metadata !1,       ;; File

            
              i32 0,             ;; Line number

            
              i64 16,            ;; Size in Bits

            
              i64 16,            ;; Align in Bits

            
              i64 0,             ;; Offset in Bits

            
              i32 0,             ;; Flags

            
              i32 7              ;; Encoding

            
            }

            
            </pre>

            
            </div>


            </div>


            <!-- ======================================================================= -->

            
            <h4>

            
              <a name="ccxx_basic_int">int</a>

            
            </h4>


            <div>


            <div class="doc_code">

            
            <pre>

            
            !2 = metadata !{

            
              i32 524324,        ;; Tag

            
              metadata !1,       ;; Context

            
              metadata !"int",   ;; Name

            
              metadata !1,       ;; File

            
              i32 0,             ;; Line number

            
              i64 32,            ;; Size in Bits

            
              i64 32,            ;; Align in Bits

            
              i64 0,             ;; Offset in Bits

            
              i32 0,             ;; Flags

            
              i32 5              ;; Encoding

            
            }

            
            </pre></div>


            </div>


            <!-- ======================================================================= -->

            
            <h4>

            
              <a name="ccxx_basic_unsigned_int">unsigned int</a>

            
            </h4>


            <div>


            <div class="doc_code">

            
            <pre>

            
            !2 = metadata !{

            
              i32 524324,        ;; Tag

            
              metadata !1,       ;; Context

            
              metadata !"unsigned int",

            
              metadata !1,       ;; File

            
              i32 0,             ;; Line number

            
              i64 32,            ;; Size in Bits

            
              i64 32,            ;; Align in Bits

            
              i64 0,             ;; Offset in Bits

            
              i32 0,             ;; Flags

            
              i32 7              ;; Encoding

            
            }

            
            </pre>

            
            </div>


            </div>


            <!-- ======================================================================= -->

            
            <h4>

            
              <a name="ccxx_basic_long_long">long long</a>

            
            </h4>


            <div>


            <div class="doc_code">

            
            <pre>

            
            !2 = metadata !{

            
              i32 524324,        ;; Tag

            
              metadata !1,       ;; Context

            
              metadata !"long long int",

            
              metadata !1,       ;; File

            
              i32 0,             ;; Line number

            
              i64 64,            ;; Size in Bits

            
              i64 64,            ;; Align in Bits

            
              i64 0,             ;; Offset in Bits

            
              i32 0,             ;; Flags

            
              i32 5              ;; Encoding

            
            }

            
            </pre>

            
            </div>


            </div>


            <!-- ======================================================================= -->

            
            <h4>

            
              <a name="ccxx_basic_unsigned_long_long">unsigned long long</a>

            
            </h4>


            <div>


            <div class="doc_code">

            
            <pre>

            
            !2 = metadata !{

            
              i32 524324,        ;; Tag

            
              metadata !1,       ;; Context

            
              metadata !"long long unsigned int",

            
              metadata !1,       ;; File

            
              i32 0,             ;; Line number

            
              i64 64,            ;; Size in Bits

            
              i64 64,            ;; Align in Bits

            
              i64 0,             ;; Offset in Bits

            
              i32 0,             ;; Flags

            
              i32 7              ;; Encoding

            
            }

            
            </pre>

            
            </div>


            </div>


            <!-- ======================================================================= -->

            
            <h4>

            
              <a name="ccxx_basic_float">float</a>

            
            </h4>


            <div>


            <div class="doc_code">

            
            <pre>

            
            !2 = metadata !{

            
              i32 524324,        ;; Tag

            
              metadata !1,       ;; Context

            
              metadata !"float",

            
              metadata !1,       ;; File

            
              i32 0,             ;; Line number

            
              i64 32,            ;; Size in Bits

            
              i64 32,            ;; Align in Bits

            
              i64 0,             ;; Offset in Bits

            
              i32 0,             ;; Flags

            
              i32 4              ;; Encoding

            
            }

            
            </pre>

            
            </div>


            </div>


            <!-- ======================================================================= -->

            
            <h4>

            
              <a name="ccxx_basic_double">double</a>

            
            </h4>


            <div>


            <div class="doc_code">

            
            <pre>

            
            !2 = metadata !{

            
              i32 524324,        ;; Tag

            
              metadata !1,       ;; Context

            
              metadata !"double",;; Name

            
              metadata !1,       ;; File

            
              i32 0,             ;; Line number

            
              i64 64,            ;; Size in Bits

            
              i64 64,            ;; Align in Bits

            
              i64 0,             ;; Offset in Bits

            
              i32 0,             ;; Flags

            
              i32 4              ;; Encoding

            
            }

            
            </pre>

            
            </div>


            </div>


            </div>


            <!-- ======================================================================= -->

            
            <h3>

            
              <a name="ccxx_derived_types">C/C++ derived types</a>

            
            </h3>


            <div>


            <p>Given the following as an example of C/C++ derived type:</p>


            <div class="doc_code">

            
            <pre>

            
            typedef const int *IntPtr;

            
            </pre>

            
            </div>


            <p>a C/C++ front-end would generate the following descriptors:</p>


            <div class="doc_code">

            
            <pre>

            
            ;;

            
            ;; Define the typedef "IntPtr".

            
            ;;

            
            !2 = metadata !{

            
              i32 524310,          ;; Tag

            
              metadata !1,         ;; Context

            
              metadata !"IntPtr",  ;; Name

            
              metadata !3,         ;; File

            
              i32 0,               ;; Line number

            
              i64 0,               ;; Size in bits

            
              i64 0,               ;; Align in bits

            
              i64 0,               ;; Offset in bits

            
              i32 0,               ;; Flags

            
              metadata !4          ;; Derived From type

            
            }


            ;;

            
            ;; Define the pointer type.

            
            ;;

            
            !4 = metadata !{

            
              i32 524303,          ;; Tag

            
              metadata !1,         ;; Context

            
              metadata !"",        ;; Name

            
              metadata !1,         ;; File

            
              i32 0,               ;; Line number

            
              i64 64,              ;; Size in bits

            
              i64 64,              ;; Align in bits

            
              i64 0,               ;; Offset in bits

            
              i32 0,               ;; Flags

            
              metadata !5          ;; Derived From type

            
            }

            
            ;;

            
            ;; Define the const type.

            
            ;;

            
            !5 = metadata !{

            
              i32 524326,          ;; Tag

            
              metadata !1,         ;; Context

            
              metadata !"",        ;; Name

            
              metadata !1,         ;; File

            
              i32 0,               ;; Line number

            
              i64 32,              ;; Size in bits

            
              i64 32,              ;; Align in bits

            
              i64 0,               ;; Offset in bits

            
              i32 0,               ;; Flags

            
              metadata !6          ;; Derived From type

            
            }

            
            ;;

            
            ;; Define the int type.

            
            ;;

            
            !6 = metadata !{

            
              i32 524324,          ;; Tag

            
              metadata !1,         ;; Context

            
              metadata !"int",     ;; Name

            
              metadata !1,         ;; File

            
              i32 0,               ;; Line number

            
              i64 32,              ;; Size in bits

            
              i64 32,              ;; Align in bits

            
              i64 0,               ;; Offset in bits

            
              i32 0,               ;; Flags

            
              5                    ;; Encoding

            
            }

            
            </pre>

            
            </div>


            </div>


            <!-- ======================================================================= -->

            
            <h3>

            
              <a name="ccxx_composite_types">C/C++ struct/union types</a>

            
            </h3>


            <div>


            <p>Given the following as an example of C/C++ struct type:</p>


            <div class="doc_code">

            
            <pre>

            
            struct Color {

            
              unsigned Red;

            
              unsigned Green;

            
              unsigned Blue;

            
            };

            
            </pre>

            
            </div>


            <p>a C/C++ front-end would generate the following descriptors:</p>


            <div class="doc_code">

            
            <pre>

            
            ;;

            
            ;; Define basic type for unsigned int.

            
            ;;

            
            !5 = metadata !{

            
              i32 524324,        ;; Tag

            
              metadata !1,       ;; Context

            
              metadata !"unsigned int",

            
              metadata !1,       ;; File

            
              i32 0,             ;; Line number

            
              i64 32,            ;; Size in Bits

            
              i64 32,            ;; Align in Bits

            
              i64 0,             ;; Offset in Bits

            
              i32 0,             ;; Flags

            
              i32 7              ;; Encoding

            
            }

            
            ;;

            
            ;; Define composite type for struct Color.

            
            ;;

            
            !2 = metadata !{

            
              i32 524307,        ;; Tag

            
              metadata !1,       ;; Context

            
              metadata !"Color", ;; Name

            
              metadata !1,       ;; Compile unit

            
              i32 1,             ;; Line number

            
              i64 96,            ;; Size in bits

            
              i64 32,            ;; Align in bits

            
              i64 0,             ;; Offset in bits

            
              i32 0,             ;; Flags

            
              null,              ;; Derived From

            
              metadata !3,       ;; Elements

            
              i32 0              ;; Runtime Language

            
            }


            ;;

            
            ;; Define the Red field.

            
            ;;

            
            !4 = metadata !{

            
              i32 524301,        ;; Tag

            
              metadata !1,       ;; Context

            
              metadata !"Red",   ;; Name

            
              metadata !1,       ;; File

            
              i32 2,             ;; Line number

            
              i64 32,            ;; Size in bits

            
              i64 32,            ;; Align in bits

            
              i64 0,             ;; Offset in bits

            
              i32 0,             ;; Flags

            
              metadata !5        ;; Derived From type

            
            }


            ;;

            
            ;; Define the Green field.

            
            ;;

            
            !6 = metadata !{

            
              i32 524301,        ;; Tag

            
              metadata !1,       ;; Context

            
              metadata !"Green", ;; Name

            
              metadata !1,       ;; File

            
              i32 3,             ;; Line number

            
              i64 32,            ;; Size in bits

            
              i64 32,            ;; Align in bits

            
              i64 32,             ;; Offset in bits

            
              i32 0,             ;; Flags

            
              metadata !5        ;; Derived From type

            
            }


            ;;

            
            ;; Define the Blue field.

            
            ;;

            
            !7 = metadata !{

            
              i32 524301,        ;; Tag

            
              metadata !1,       ;; Context

            
              metadata !"Blue",  ;; Name

            
              metadata !1,       ;; File

            
              i32 4,             ;; Line number

            
              i64 32,            ;; Size in bits

            
              i64 32,            ;; Align in bits

            
              i64 64,             ;; Offset in bits

            
              i32 0,             ;; Flags

            
              metadata !5        ;; Derived From type

            
            }


            ;;

            
            ;; Define the array of fields used by the composite type Color.

            
            ;;

            
            !3 = metadata !{metadata !4, metadata !6, metadata !7}

            
            </pre>

            
            </div>


            </div>


            <!-- ======================================================================= -->

            
            <h3>

            
              <a name="ccxx_enumeration_types">C/C++ enumeration types</a>

            
            </h3>


            <div>


            <p>Given the following as an example of C/C++ enumeration type:</p>


            <div class="doc_code">

            
            <pre>

            
            enum Trees {

            
              Spruce = 100,

            
              Oak = 200,

            
              Maple = 300

            
            };

            
            </pre>

            
            </div>


            <p>a C/C++ front-end would generate the following descriptors:</p>


            <div class="doc_code">

            
            <pre>

            
            ;;

            
            ;; Define composite type for enum Trees

            
            ;;

            
            !2 = metadata !{

            
              i32 524292,        ;; Tag

            
              metadata !1,       ;; Context

            
              metadata !"Trees", ;; Name

            
              metadata !1,       ;; File

            
              i32 1,             ;; Line number

            
              i64 32,            ;; Size in bits

            
              i64 32,            ;; Align in bits

            
              i64 0,             ;; Offset in bits

            
              i32 0,             ;; Flags

            
              null,              ;; Derived From type

            
              metadata !3,       ;; Elements

            
              i32 0              ;; Runtime language

            
            }


            ;;

            
            ;; Define the array of enumerators used by composite type Trees.

            
            ;;

            
            !3 = metadata !{metadata !4, metadata !5, metadata !6}


            ;;

            
            ;; Define Spruce enumerator.

            
            ;;

            
            !4 = metadata !{i32 524328, metadata !"Spruce", i64 100}


            ;;

            
            ;; Define Oak enumerator.

            
            ;;

            
            !5 = metadata !{i32 524328, metadata !"Oak", i64 200}


            ;;

            
            ;; Define Maple enumerator.

            
            ;;

            
            !6 = metadata !{i32 524328, metadata !"Maple", i64 300}


            </pre>

            
            </div>


            </div>


            </div>


            <!-- *********************************************************************** -->

            
            <h2>

            
              <a name="llvmdwarfextension">Debugging information format</a>

            
            </h2>

            
            <!-- *********************************************************************** -->

            
            <div>

            
            <!-- ======================================================================= -->

            
            <h3>

            
              <a name="objcproperty">Debugging Information Extension for Objective C Properties</a>

            
            </h3>

            
            <div>

            
            <!-- *********************************************************************** -->

            
            <h4>

            
              <a name="objcpropertyintroduction">Introduction</a>

            
            </h4>

            
            <!-- *********************************************************************** -->


            <div>

            
            <p>Objective C provides a simpler way to declare and define accessor methods

            
            using declared properties. The language provides features to declare a

            
            property and to let compiler synthesize accessor methods.

            
            </p>


            <p>The debugger lets developer inspect Objective C interfaces and their

            
            instance variables and class variables. However, the debugger does not know

            
            anything about the properties defined in Objective C interfaces. The debugger

            
            consumes information generated by compiler in DWARF format. The format does

            
            not support encoding of Objective C properties. This proposal describes DWARF

            
            extensions to encode Objective C properties, which the debugger can use to let

            
            developers inspect Objective C properties.

            
            </p>


            </div>


            <!-- *********************************************************************** -->

            
            <h4>

            
              <a name="objcpropertyproposal">Proposal</a>

            
            </h4>

            
            <!-- *********************************************************************** -->


            <div>

            
            <p>Objective C properties exist separately from class members. A property

            
            can be defined only by &quot;setter&quot; and &quot;getter&quot; selectors, and

            
            be calculated anew on each access.  Or a property can just be a direct access

            
            to some declared ivar.  Finally it can have an ivar &quot;automatically

            
            synthesized&quot; for it by the compiler, in which case the property can be

            
            referred to in user code directly using the standard C dereference syntax as

            
            well as through the property &quot;dot&quot; syntax, but there is no entry in

            
            the @interface declaration corresponding to this ivar.

            
            </p>

            
            <p>

            
            To facilitate debugging, these properties we will add a new DWARF TAG into the

            
            DW_TAG_structure_type definition for the class to hold the description of a

            
            given property, and a set of DWARF attributes that provide said description.

            
            The property tag will also contain the name and declared type of the property.

            
            </p>

            
            <p>

            
            If there is a related ivar, there will also be a DWARF property attribute placed

            
            in the DW_TAG_member DIE for that ivar referring back to the property TAG for

            
            that property. And in the case where the compiler synthesizes the ivar directly,

            
            the compiler is expected to generate a DW_TAG_member for that ivar (with the

            
            DW_AT_artificial set to 1), whose name will be the name used to access this

            
            ivar directly in code, and with the property attribute pointing back to the

            
            property it is backing.

            
            </p>

            
            <p>

            
            The following examples will serve as illustration for our discussion:

            
            </p>


            <div class="doc_code">

            
            <pre>

            
            @interface I1 {

            
              int n2;

            
            }


            @property int p1;

            
            @property int p2;

            
            @end


            @implementation I1

            
            @synthesize p1;

            
            @synthesize p2 = n2;

            
            @end

            
            </pre>

            
            </div>


            <p>

            
            This produces the following DWARF (this is a &quot;pseudo dwarfdump&quot; output):

            
            </p>

            
            <div class="doc_code">

            
            <pre>

            
            0x00000100:  TAG_structure_type [7] *

            
                           AT_APPLE_runtime_class( 0x10 )

            
                           AT_name( "I1" )

            
                           AT_decl_file( "Objc_Property.m" )

            
                           AT_decl_line( 3 )


            0x00000110    TAG_APPLE_property

            
                            AT_name ( "p1" )

            
                            AT_type ( {0x00000150} ( int ) )


            0x00000120:   TAG_APPLE_property

            
                            AT_name ( "p2" )

            
                            AT_type ( {0x00000150} ( int ) )


            0x00000130:   TAG_member [8]

            
                            AT_name( "_p1" )

            
                            AT_APPLE_property ( {0x00000110} "p1" )

            
                            AT_type( {0x00000150} ( int ) )

            
                            AT_artificial ( 0x1 )


            0x00000140:    TAG_member [8]

            
                             AT_name( "n2" )

            
                             AT_APPLE_property ( {0x00000120} "p2" )

            
                             AT_type( {0x00000150} ( int ) )


            0x00000150:  AT_type( ( int ) )

            
            </pre>

            
            </div>


            <p> Note, the current convention is that the name of the ivar for an

            
            auto-synthesized property is the name of the property from which it derives with

            
            an underscore prepended, as is shown in the example.

            
            But we actually don't need to know this convention, since we are given the name

            
            of the ivar directly.

            
            </p>


            <p>

            
            Also, it is common practice in ObjC to have different property declarations in

            
            the @interface and @implementation - e.g. to provide a read-only property in

            
            the interface,and a read-write interface in the implementation.  In that case,

            
            the compiler should emit whichever property declaration will be in force in the

            
            current translation unit.

            
            </p>


            <p> Developers can decorate a property with attributes which are encoded using

            
            DW_AT_APPLE_property_attribute.

            
            </p>


            <div class="doc_code">

            
            <pre>

            
            @property (readonly, nonatomic) int pr;

            
            </pre>

            
            </div>

            
            <p>

            
            Which produces a property tag:

            
            <p>

            
            <div class="doc_code">

            
            <pre>

            
            TAG_APPLE_property [8]

            
              AT_name( "pr" )

            
              AT_type ( {0x00000147} (int) )

            
              AT_APPLE_property_attribute (DW_APPLE_PROPERTY_readonly, DW_APPLE_PROPERTY_nonatomic)

            
            </pre>

            
            </div>


            <p> The setter and getter method names are attached to the property using

            
            DW_AT_APPLE_property_setter and DW_AT_APPLE_property_getter attributes.

            
            </p>

            
            <div class="doc_code">

            
            <pre>

            
            @interface I1

            
            @property (setter=myOwnP3Setter:) int p3;

            
            -(void)myOwnP3Setter:(int)a;

            
            @end


            @implementation I1

            
            @synthesize p3;

            
            -(void)myOwnP3Setter:(int)a{ }

            
            @end

            
            </pre>

            
            </div>


            <p>

            
            The DWARF for this would be:

            
            </p>

            
            <div class="doc_code">

            
            <pre>

            
            0x000003bd: TAG_structure_type [7] *

            
                          AT_APPLE_runtime_class( 0x10 )

            
                          AT_name( "I1" )

            
                          AT_decl_file( "Objc_Property.m" )

            
                          AT_decl_line( 3 )


            0x000003cd      TAG_APPLE_property

            
                              AT_name ( "p3" )

            
                              AT_APPLE_property_setter ( "myOwnP3Setter:" )

            
                              AT_type( {0x00000147} ( int ) )


            0x000003f3:     TAG_member [8]

            
                              AT_name( "_p3" )

            
                              AT_type ( {0x00000147} ( int ) )

            
                              AT_APPLE_property ( {0x000003cd} )

            
                              AT_artificial ( 0x1 )

            
            </pre>

            
            </div>


            </div>


            <!-- *********************************************************************** -->

            
            <h4>

            
              <a name="objcpropertynewtags">New DWARF Tags</a>

            
            </h4>

            
            <!-- *********************************************************************** -->


            <div>

            
            <table border="1" cellspacing="0">

            
              <col width="200">

            
              <col width="200">

            
              <tr>

            
                <th>TAG</th>

            
                <th>Value</th>

            
              </tr>

            
              <tr>

            
                <td>DW_TAG_APPLE_property</td>

            
                <td>0x4200</td>

            
              </tr>

            
            </table>


            </div>


            <!-- *********************************************************************** -->

            
            <h4>

            
              <a name="objcpropertynewattributes">New DWARF Attributes</a>

            
            </h4>

            
            <!-- *********************************************************************** -->


            <div>

            
            <table border="1" cellspacing="0">

            
              <col width="200">

            
              <col width="200">

            
              <col width="200">

            
              <tr>

            
                <th>Attribute</th>

            
                <th>Value</th>

            
                <th>Classes</th>

            
              </tr>

            
              <tr>

            
                <td>DW_AT_APPLE_property</td>

            
                <td>0x3fed</td>

            
                <td>Reference</td>

            
              </tr>

            
              <tr>

            
                <td>DW_AT_APPLE_property_getter</td>

            
                <td>0x3fe9</td>

            
                <td>String</td>

            
              </tr>

            
              <tr>

            
                <td>DW_AT_APPLE_property_setter</td>

            
                <td>0x3fea</td>

            
                <td>String</td>

            
              </tr>

            
              <tr>

            
                <td>DW_AT_APPLE_property_attribute</td>

            
                <td>0x3feb</td>

            
                <td>Constant</td>

            
              </tr>

            
            </table>


            </div>


            <!-- *********************************************************************** -->

            
            <h4>

            
              <a name="objcpropertynewconstants">New DWARF Constants</a>

            
            </h4>

            
            <!-- *********************************************************************** -->


            <div>

            
            <table border="1" cellspacing="0">

            
              <col width="200">

            
              <col width="200">

            
              <tr>

            
                <th>Name</th>

            
                <th>Value</th>

            
              </tr>

            
              <tr>

            
                <td>DW_AT_APPLE_PROPERTY_readonly</td>

            
                <td>0x1</td>

            
              </tr>

            
              <tr>

            
                <td>DW_AT_APPLE_PROPERTY_readwrite</td>

            
                <td>0x2</td>

            
              </tr>

            
              <tr>

            
                <td>DW_AT_APPLE_PROPERTY_assign</td>

            
                <td>0x4</td>

            
              </tr>

            
              <tr>

            
                <td>DW_AT_APPLE_PROPERTY_retain</td>

            
                <td>0x8</td>

            
              </tr>

            
              <tr>

            
                <td>DW_AT_APPLE_PROPERTY_copy</td>

            
                <td>0x10</td>

            
              </tr>

            
              <tr>

            
                <td>DW_AT_APPLE_PROPERTY_nonatomic</td>

            
                <td>0x20</td>

            
              </tr>

            
            </table>


            </div>

            
            </div>


            <!-- ======================================================================= -->

            
            <h3>

            
              <a name="acceltable">Name Accelerator Tables</a>

            
            </h3>

            
            <!-- ======================================================================= -->

            
            <div>

            
            <!-- ======================================================================= -->

            
            <h4>

            
              <a name="acceltableintroduction">Introduction</a>

            
            </h4>

            
            <!-- ======================================================================= -->

            
            <div>

            
            <p>The .debug_pubnames and .debug_pubtypes formats are not what a debugger

            
              needs. The "pub" in the section name indicates that the entries in the

            
              table are publicly visible names only. This means no static or hidden

            
              functions show up in the .debug_pubnames. No static variables or private class

            
              variables are in the .debug_pubtypes. Many compilers add different things to

            
              these tables, so we can't rely upon the contents between gcc, icc, or clang.</p>


            <p>The typical query given by users tends not to match up with the contents of

            
              these tables. For example, the DWARF spec states that "In the case of the

            
              name of a function member or static data member of a C++ structure, class or

            
              union, the name presented in the .debug_pubnames section is not the simple

            
              name given by the DW_AT_name attribute of the referenced debugging information

            
              entry, but rather the fully qualified name of the data or function member."

            
              So the only names in these tables for complex C++ entries is a fully

            
              qualified name.  Debugger users tend not to enter their search strings as

            
              "a::b::c(int,const Foo&) const", but rather as "c", "b::c" , or "a::b::c".  So

            
              the name entered in the name table must be demangled in order to chop it up

            
              appropriately and additional names must be manually entered into the table

            
              to make it effective as a name lookup table for debuggers to use.</p>


            <p>All debuggers currently ignore the .debug_pubnames table as a result of

            
              its inconsistent and useless public-only name content making it a waste of

            
              space in the object file. These tables, when they are written to disk, are

            
              not sorted in any way, leaving every debugger to do its own parsing

            
              and sorting. These tables also include an inlined copy of the string values

            
              in the table itself making the tables much larger than they need to be on

            
              disk, especially for large C++ programs.</p>


            <p>Can't we just fix the sections by adding all of the names we need to this

            
              table? No, because that is not what the tables are defined to contain and we

            
              won't know the difference between the old bad tables and the new good tables.

            
              At best we could make our own renamed sections that contain all of the data

            
              we need.</p>


            <p>These tables are also insufficient for what a debugger like LLDB needs.

            
              LLDB uses clang for its expression parsing where LLDB acts as a PCH. LLDB is

            
              then often asked to look for type "foo" or namespace "bar", or list items in

            
              namespace "baz". Namespaces are not included in the pubnames or pubtypes

            
              tables. Since clang asks a lot of questions when it is parsing an expression,

            
              we need to be very fast when looking up names, as it happens a lot. Having new

            
              accelerator tables that are optimized for very quick lookups will benefit

            
              this type of debugging experience greatly.</p>


            <p>We would like to generate name lookup tables that can be mapped into

            
              memory from disk, and used as is, with little or no up-front parsing. We would

            
              also be able to control the exact content of these different tables so they

            
              contain exactly what we need. The Name Accelerator Tables were designed

            
              to fix these issues. In order to solve these issues we need to:</p>


            <ul>

            
              <li>Have a format that can be mapped into memory from disk and used as is</li>

            
              <li>Lookups should be very fast</li>

            
              <li>Extensible table format so these tables can be made by many producers</li>

            
              <li>Contain all of the names needed for typical lookups out of the box</li>

            
              <li>Strict rules for the contents of tables</li>

            
            </ul>


            <p>Table size is important and the accelerator table format should allow the

            
              reuse of strings from common string tables so the strings for the names are

            
              not duplicated. We also want to make sure the table is ready to be used as-is

            
              by simply mapping the table into memory with minimal header parsing.</p>


            <p>The name lookups need to be fast and optimized for the kinds of lookups

            
              that debuggers tend to do. Optimally we would like to touch as few parts of

            
              the mapped table as possible when doing a name lookup and be able to quickly

            
              find the name entry we are looking for, or discover there are no matches. In

            
              the case of debuggers we optimized for lookups that fail most of the time.</p>


            <p>Each table that is defined should have strict rules on exactly what is in

            
              the accelerator tables and documented so clients can rely on the content.</p>


            </div>


            <!-- ======================================================================= -->

            
            <h4>

            
              <a name="acceltablehashes">Hash Tables</a>

            
            </h4>

            
            <!-- ======================================================================= -->


            <div>

            
            <h5>Standard Hash Tables</h5>


            <p>Typical hash tables have a header, buckets, and each bucket points to the

            
            bucket contents:

            
            </p>


            <div class="doc_code">

            
            <pre>

            
            .------------.

            
            |  HEADER    |

            
            |------------|

            
            |  BUCKETS   |

            
            |------------|

            
            |  DATA      |

            
            `------------'

            
            </pre>

            
            </div>


            <p>The BUCKETS are an array of offsets to DATA for each hash:</p>


            <div class="doc_code">

            
            <pre>

            
            .------------.

            
            | 0x00001000 | BUCKETS[0]

            
            | 0x00002000 | BUCKETS[1]

            
            | 0x00002200 | BUCKETS[2]

            
            | 0x000034f0 | BUCKETS[3]

            
            |            | ...

            
            | 0xXXXXXXXX | BUCKETS[n_buckets]

            
            '------------'

            
            </pre>

            
            </div>


            <p>So for bucket[3] in the example above, we have an offset into the table

            
              0x000034f0 which points to a chain of entries for the bucket. Each bucket

            
              must contain a next pointer, full 32 bit hash value, the string itself,

            
              and the data for the current string value.</p>


            <div class="doc_code">

            
            <pre>

            
                        .------------.

            
            0x000034f0: | 0x00003500 | next pointer

            
                        | 0x12345678 | 32 bit hash

            
                        | "erase"    | string value

            
                        | data[n]    | HashData for this bucket

            
                        |------------|

            
            0x00003500: | 0x00003550 | next pointer

            
                        | 0x29273623 | 32 bit hash

            
                        | "dump"     | string value

            
                        | data[n]    | HashData for this bucket

            
                        |------------|

            
            0x00003550: | 0x00000000 | next pointer

            
                        | 0x82638293 | 32 bit hash

            
                        | "main"     | string value

            
                        | data[n]    | HashData for this bucket

            
                        `------------'

            
            </pre>

            
            </div>


            <p>The problem with this layout for debuggers is that we need to optimize for

            
              the negative lookup case where the symbol we're searching for is not present.

            
              So if we were to lookup "printf" in the table above, we would make a 32 hash

            
              for "printf", it might match bucket[3]. We would need to go to the offset

            
              0x000034f0 and start looking to see if our 32 bit hash matches. To do so, we

            
              need to read the next pointer, then read the hash, compare it, and skip to

            
              the next bucket. Each time we are skipping many bytes in memory and touching

            
              new cache pages just to do the compare on the full 32 bit hash. All of these

            
              accesses then tell us that we didn't have a match.</p>


            <h5>Name Hash Tables</h5>


            <p>To solve the issues mentioned above we have structured the hash tables

            
              a bit differently: a header, buckets, an array of all unique 32 bit hash

            
              values, followed by an array of hash value data offsets, one for each hash

            
              value, then the data for all hash values:</p>


            <div class="doc_code">

            
            <pre>

            
            .-------------.

            
            |  HEADER     |

            
            |-------------|

            
            |  BUCKETS    |

            
            |-------------|

            
            |  HASHES     |

            
            |-------------|

            
            |  OFFSETS    |

            
            |-------------|

            
            |  DATA       |

            
            `-------------'

            
            </pre>

            
            </div>


            <p>The BUCKETS in the name tables are an index into the HASHES array. By

            
              making all of the full 32 bit hash values contiguous in memory, we allow

            
              ourselves to efficiently check for a match while touching as little

            
              memory as possible. Most often checking the 32 bit hash values is as far as

            
              the lookup goes. If it does match, it usually is a match with no collisions.

            
              So for a table with "n_buckets" buckets, and "n_hashes" unique 32 bit hash

            
              values, we can clarify the contents of the BUCKETS, HASHES and OFFSETS as:</p>


            <div class="doc_code">

            
            <pre>

            
            .-------------------------.

            
            |  HEADER.magic           | uint32_t

            
            |  HEADER.version         | uint16_t

            
            |  HEADER.hash_function   | uint16_t

            
            |  HEADER.bucket_count    | uint32_t

            
            |  HEADER.hashes_count    | uint32_t

            
            |  HEADER.header_data_len | uint32_t

            
            |  HEADER_DATA            | HeaderData

            
            |-------------------------|

            
            |  BUCKETS                | uint32_t[n_buckets] // 32 bit hash indexes

            
            |-------------------------|

            
            |  HASHES                 | uint32_t[n_buckets] // 32 bit hash values

            
            |-------------------------|

            
            |  OFFSETS                | uint32_t[n_buckets] // 32 bit offsets to hash value data

            
            |-------------------------|

            
            |  ALL HASH DATA          |

            
            `-------------------------'

            
            </pre>

            
            </div>


            <p>So taking the exact same data from the standard hash example above we end up

            
              with:</p>


            <div class="doc_code">

            
            <pre>

            
                        .------------.

            
                        | HEADER     |

            
                        |------------|

            
                        |          0 | BUCKETS[0]

            
                        |          2 | BUCKETS[1]

            
                        |          5 | BUCKETS[2]

            
                        |          6 | BUCKETS[3]

            
                        |            | ...

            
                        |        ... | BUCKETS[n_buckets]

            
                        |------------|

            
                        | 0x........ | HASHES[0]

            
                        | 0x........ | HASHES[1]

            
                        | 0x........ | HASHES[2]

            
                        | 0x........ | HASHES[3]

            
                        | 0x........ | HASHES[4]

            
                        | 0x........ | HASHES[5]

            
                        | 0x12345678 | HASHES[6]    hash for BUCKETS[3]

            
                        | 0x29273623 | HASHES[7]    hash for BUCKETS[3]

            
                        | 0x82638293 | HASHES[8]    hash for BUCKETS[3]

            
                        | 0x........ | HASHES[9]

            
                        | 0x........ | HASHES[10]

            
                        | 0x........ | HASHES[11]

            
                        | 0x........ | HASHES[12]

            
                        | 0x........ | HASHES[13]

            
                        | 0x........ | HASHES[n_hashes]

            
                        |------------|

            
                        | 0x........ | OFFSETS[0]

            
                        | 0x........ | OFFSETS[1]

            
                        | 0x........ | OFFSETS[2]

            
                        | 0x........ | OFFSETS[3]

            
                        | 0x........ | OFFSETS[4]

            
                        | 0x........ | OFFSETS[5]

            
                        | 0x000034f0 | OFFSETS[6]   offset for BUCKETS[3]

            
                        | 0x00003500 | OFFSETS[7]   offset for BUCKETS[3]

            
                        | 0x00003550 | OFFSETS[8]   offset for BUCKETS[3]

            
                        | 0x........ | OFFSETS[9]

            
                        | 0x........ | OFFSETS[10]

            
                        | 0x........ | OFFSETS[11]

            
                        | 0x........ | OFFSETS[12]

            
                        | 0x........ | OFFSETS[13]

            
                        | 0x........ | OFFSETS[n_hashes]

            
                        |------------|

            
                        |            |

            
                        |            |

            
                        |            |

            
                        |            |

            
                        |            |

            
                        |------------|

            
            0x000034f0: | 0x00001203 | .debug_str ("erase")

            
                        | 0x00000004 | A 32 bit array count - number of HashData with name "erase"

            
                        | 0x........ | HashData[0]

            
                        | 0x........ | HashData[1]

            
                        | 0x........ | HashData[2]

            
                        | 0x........ | HashData[3]

            
                        | 0x00000000 | String offset into .debug_str (terminate data for hash)

            
                        |------------|

            
            0x00003500: | 0x00001203 | String offset into .debug_str ("collision")

            
                        | 0x00000002 | A 32 bit array count - number of HashData with name "collision"

            
                        | 0x........ | HashData[0]

            
                        | 0x........ | HashData[1]

            
                        | 0x00001203 | String offset into .debug_str ("dump")

            
                        | 0x00000003 | A 32 bit array count - number of HashData with name "dump"

            
                        | 0x........ | HashData[0]

            
                        | 0x........ | HashData[1]

            
                        | 0x........ | HashData[2]

            
                        | 0x00000000 | String offset into .debug_str (terminate data for hash)

            
                        |------------|

            
            0x00003550: | 0x00001203 | String offset into .debug_str ("main")

            
                        | 0x00000009 | A 32 bit array count - number of HashData with name "main"

            
                        | 0x........ | HashData[0]

            
                        | 0x........ | HashData[1]

            
                        | 0x........ | HashData[2]

            
                        | 0x........ | HashData[3]

            
                        | 0x........ | HashData[4]

            
                        | 0x........ | HashData[5]

            
                        | 0x........ | HashData[6]

            
                        | 0x........ | HashData[7]

            
                        | 0x........ | HashData[8]

            
                        | 0x00000000 | String offset into .debug_str (terminate data for hash)

            
                        `------------'

            
            </pre>

            
            </div>


            <p>So we still have all of the same data, we just organize it more efficiently

            
              for debugger lookup. If we repeat the same "printf" lookup from above, we

            
              would hash "printf" and find it matches BUCKETS[3] by taking the 32 bit hash

            
              value and modulo it by n_buckets. BUCKETS[3] contains "6" which is the index

            
              into the HASHES table. We would then compare any consecutive 32 bit hashes

            
              values in the HASHES array as long as the hashes would be in BUCKETS[3]. We

            
              do this by verifying that each subsequent hash value modulo n_buckets is still

            
              3. In the case of a failed lookup we would access the memory for BUCKETS[3], and

            
              then compare a few consecutive 32 bit hashes before we know that we have no match.

            
              We don't end up marching through multiple words of memory and we really keep the

            
              number of processor data cache lines being accessed as small as possible.</p>


            <p>The string hash that is used for these lookup tables is the Daniel J.

            
              Bernstein hash which is also used in the ELF GNU_HASH sections. It is a very

            
              good hash for all kinds of names in programs with very few hash collisions.</p>


            <p>Empty buckets are designated by using an invalid hash index of UINT32_MAX.</p>

            
            </div>


            <!-- ======================================================================= -->

            
            <h4>

            
              <a name="acceltabledetails">Details</a>

            
            </h4>

            
            <!-- ======================================================================= -->

            
            <div>

            
            <p>These name hash tables are designed to be generic where specializations of

            
              the table get to define additional data that goes into the header

            
              ("HeaderData"), how the string value is stored ("KeyType") and the content

            
              of the data for each hash value.</p>


            <h5>Header Layout</h5>

            
            <p>The header has a fixed part, and the specialized part. The exact format of

            
              the header is:</p>

            
            <div class="doc_code">

            
            <pre>

            
            struct Header

            
            {

            
              uint32_t   magic;           // 'HASH' magic value to allow endian detection

            
              uint16_t   version;         // Version number

            
              uint16_t   hash_function;   // The hash function enumeration that was used

            
              uint32_t   bucket_count;    // The number of buckets in this hash table

            
              uint32_t   hashes_count;    // The total number of unique hash values and hash data offsets in this table

            
              uint32_t   header_data_len; // The bytes to skip to get to the hash indexes (buckets) for correct alignment

            
                                          // Specifically the length of the following HeaderData field - this does not

            
                                          // include the size of the preceding fields

            
              HeaderData header_data;     // Implementation specific header data

            
            };

            
            </pre>

            
            </div>

            
            <p>The header starts with a 32 bit "magic" value which must be 'HASH' encoded as

            
              an ASCII integer. This allows the detection of the start of the hash table and

            
              also allows the table's byte order to be determined so the table can be

            
              correctly extracted. The "magic" value is followed by a 16 bit version number

            
              which allows the table to be revised and modified in the future. The current

            
              version number is 1. "hash_function" is a uint16_t enumeration that specifies

            
              which hash function was used to produce this table. The current values for the

            
              hash function enumerations include:</p>

            
            <div class="doc_code">

            
            <pre>

            
            enum HashFunctionType

            
            {

            
              eHashFunctionDJB = 0u, // Daniel J Bernstein hash function

            
            };

            
            </pre>

            
            </div>

            
            <p>"bucket_count" is a 32 bit unsigned integer that represents how many buckets

            
              are in the BUCKETS array. "hashes_count" is the number of unique 32 bit hash

            
              values that are in the HASHES array, and is the same number of offsets are

            
              contained in the OFFSETS array. "header_data_len" specifies the size in

            
              bytes of the HeaderData that is filled in by specialized versions of this

            
              table.</p>


            <h5>Fixed Lookup</h5>

            
            <p>The header is followed by the buckets, hashes, offsets, and hash value

            
              data.

            
            <div class="doc_code">

            
            <pre>

            
            struct FixedTable

            
            {

            
              uint32_t buckets[Header.bucket_count];  // An array of hash indexes into the "hashes[]" array below

            
              uint32_t hashes [Header.hashes_count];  // Every unique 32 bit hash for the entire table is in this table

            
              uint32_t offsets[Header.hashes_count];  // An offset that corresponds to each item in the "hashes[]" array above

            
            };

            
            </pre>

            
            </div>

            
            <p>"buckets" is an array of 32 bit indexes into the "hashes" array. The

            
              "hashes" array contains all of the 32 bit hash values for all names in the

            
              hash table. Each hash in the "hashes" table has an offset in the "offsets"

            
              array that points to the data for the hash value.</p>


            <p>This table setup makes it very easy to repurpose these tables to contain

            
              different data, while keeping the lookup mechanism the same for all tables.

            
              This layout also makes it possible to save the table to disk and map it in

            
              later and do very efficient name lookups with little or no parsing.</p>


            <p>DWARF lookup tables can be implemented in a variety of ways and can store

            
              a lot of information for each name. We want to make the DWARF tables

            
              extensible and able to store the data efficiently so we have used some of the

            
              DWARF features that enable efficient data storage to define exactly what kind

            
              of data we store for each name.</p>


            <p>The "HeaderData" contains a definition of the contents of each HashData

            
              chunk. We might want to store an offset to all of the debug information

            
              entries (DIEs) for each name. To keep things extensible, we create a list of

            
              items, or Atoms, that are contained in the data for each name. First comes the

            
              type of the data in each atom:</p>

            
            <div class="doc_code">

            
            <pre>

            
            enum AtomType

            
            {

            
              eAtomTypeNULL       = 0u,

            
              eAtomTypeDIEOffset  = 1u,   // DIE offset, check form for encoding

            
              eAtomTypeCUOffset   = 2u,   // DIE offset of the compiler unit header that contains the item in question

            
              eAtomTypeTag        = 3u,   // DW_TAG_xxx value, should be encoded as DW_FORM_data1 (if no tags exceed 255) or DW_FORM_data2

            
              eAtomTypeNameFlags  = 4u,   // Flags from enum NameFlags

            
              eAtomTypeTypeFlags  = 5u,   // Flags from enum TypeFlags

            
            };

            
            </pre>

            
            </div>

            
            <p>The enumeration values and their meanings are:</p>

            
            <div class="doc_code">

            
            <pre>

            
              eAtomTypeNULL       - a termination atom that specifies the end of the atom list

            
              eAtomTypeDIEOffset  - an offset into the .debug_info section for the DWARF DIE for this name

            
              eAtomTypeCUOffset   - an offset into the .debug_info section for the CU that contains the DIE

            
              eAtomTypeDIETag     - The DW_TAG_XXX enumeration value so you don't have to parse the DWARF to see what it is

            
              eAtomTypeNameFlags  - Flags for functions and global variables (isFunction, isInlined, isExternal...)

            
              eAtomTypeTypeFlags  - Flags for types (isCXXClass, isObjCClass, ...)

            
            </pre>

            
            </div>

            
            <p>Then we allow each atom type to define the atom type and how the data for

            
              each atom type data is encoded:</p>

            
            <div class="doc_code">

            
            <pre>

            
            struct Atom

            
            {

            
              uint16_t type;  // AtomType enum value

            
              uint16_t form;  // DWARF DW_FORM_XXX defines

            
            };

            
            </pre>

            
            </div>

            
            <p>The "form" type above is from the DWARF specification and defines the

            
              exact encoding of the data for the Atom type. See the DWARF specification for

            
              the DW_FORM_ definitions.</p>

            
            <div class="doc_code">

            
            <pre>

            
            struct HeaderData

            
            {

            
              uint32_t die_offset_base;

            
              uint32_t atom_count;

            
              Atoms    atoms[atom_count0];

            
            };

            
            </pre>

            
            </div>

            
            <p>"HeaderData" defines the base DIE offset that should be added to any atoms

            
              that are encoded using the DW_FORM_ref1, DW_FORM_ref2, DW_FORM_ref4,

            
              DW_FORM_ref8 or DW_FORM_ref_udata. It also defines what is contained in

            
              each "HashData" object -- Atom.form tells us how large each field will be in

            
              the HashData and the Atom.type tells us how this data should be interpreted.</p>


            <p>For the current implementations of the ".apple_names" (all functions + globals),

            
              the ".apple_types" (names of all types that are defined), and the

            
              ".apple_namespaces" (all namespaces), we currently set the Atom array to be:</p>

            
            <div class="doc_code">

            
            <pre>

            
            HeaderData.atom_count = 1;

            
            HeaderData.atoms[0].type = eAtomTypeDIEOffset;

            
            HeaderData.atoms[0].form = DW_FORM_data4;

            
            </pre>

            
            </div>

            
            <p>This defines the contents to be the DIE offset (eAtomTypeDIEOffset) that is

            
              encoded as a 32 bit value (DW_FORM_data4). This allows a single name to have

            
              multiple matching DIEs in a single file, which could come up with an inlined

            
              function for instance. Future tables could include more information about the

            
              DIE such as flags indicating if the DIE is a function, method, block,

            
              or inlined.</p>


            <p>The KeyType for the DWARF table is a 32 bit string table offset into the

            
              ".debug_str" table. The ".debug_str" is the string table for the DWARF which

            
              may already contain copies of all of the strings. This helps make sure, with

            
              help from the compiler, that we reuse the strings between all of the DWARF

            
              sections and keeps the hash table size down. Another benefit to having the

            
              compiler generate all strings as DW_FORM_strp in the debug info, is that

            
              DWARF parsing can be made much faster.</p>


            <p>After a lookup is made, we get an offset into the hash data. The hash data

            
              needs to be able to deal with 32 bit hash collisions, so the chunk of data

            
              at the offset in the hash data consists of a triple:</p>

            
            <div class="doc_code">

            
            <pre>

            
            uint32_t str_offset

            
            uint32_t hash_data_count

            
            HashData[hash_data_count]

            
            </pre>

            
            </div>

            
            <p>If "str_offset" is zero, then the bucket contents are done. 99.9% of the

            
              hash data chunks contain a single item (no 32 bit hash collision):</p>

            
            <div class="doc_code">

            
            <pre>

            
            .------------.

            
            | 0x00001023 | uint32_t KeyType (.debug_str[0x0001023] => "main")

            
            | 0x00000004 | uint32_t HashData count

            
            | 0x........ | uint32_t HashData[0] DIE offset

            
            | 0x........ | uint32_t HashData[1] DIE offset

            
            | 0x........ | uint32_t HashData[2] DIE offset

            
            | 0x........ | uint32_t HashData[3] DIE offset

            
            | 0x00000000 | uint32_t KeyType (end of hash chain)

            
            `------------'

            
            </pre>

            
            </div>

            
            <p>If there are collisions, you will have multiple valid string offsets:</p>

            
            <div class="doc_code">

            
            <pre>

            
            .------------.

            
            | 0x00001023 | uint32_t KeyType (.debug_str[0x0001023] => "main")

            
            | 0x00000004 | uint32_t HashData count

            
            | 0x........ | uint32_t HashData[0] DIE offset

            
            | 0x........ | uint32_t HashData[1] DIE offset

            
            | 0x........ | uint32_t HashData[2] DIE offset

            
            | 0x........ | uint32_t HashData[3] DIE offset

            
            | 0x00002023 | uint32_t KeyType (.debug_str[0x0002023] => "print")

            
            | 0x00000002 | uint32_t HashData count

            
            | 0x........ | uint32_t HashData[0] DIE offset

            
            | 0x........ | uint32_t HashData[1] DIE offset

            
            | 0x00000000 | uint32_t KeyType (end of hash chain)

            
            `------------'

            
            </pre>

            
            </div>

            
            <p>Current testing with real world C++ binaries has shown that there is around 1

            
              32 bit hash collision per 100,000 name entries.</p>

            
            </div>

            
            <!-- ======================================================================= -->

            
            <h4>

            
              <a name="acceltablecontents">Contents</a>

            
            </h4>

            
            <!-- ======================================================================= -->

            
            <div>

            
            <p>As we said, we want to strictly define exactly what is included in the

            
              different tables. For DWARF, we have 3 tables: ".apple_names", ".apple_types",

            
              and ".apple_namespaces".</p>


            <p>".apple_names" sections should contain an entry for each DWARF DIE whose

            
              DW_TAG is a DW_TAG_label, DW_TAG_inlined_subroutine, or DW_TAG_subprogram that

            
              has address attributes: DW_AT_low_pc, DW_AT_high_pc, DW_AT_ranges or

            
              DW_AT_entry_pc. It also contains DW_TAG_variable DIEs that have a DW_OP_addr

            
              in the location (global and static variables). All global and static variables

            
              should be included, including those scoped withing functions and classes. For

            
              example using the following code:</p>

            
            <div class="doc_code">

            
            <pre>

            
            static int var = 0;


            void f ()

            
            {

            
              static int var = 0;

            
            }

            
            </pre>

            
            </div>

            
            <p>Both of the static "var" variables would be included in the table. All

            
              functions should emit both their full names and their basenames. For C or C++,

            
              the full name is the mangled name (if available) which is usually in the

            
              DW_AT_MIPS_linkage_name attribute, and the DW_AT_name contains the function

            
              basename. If global or static variables have a mangled name in a

            
              DW_AT_MIPS_linkage_name attribute, this should be emitted along with the

            
              simple name found in the DW_AT_name attribute.</p>


            <p>".apple_types" sections should contain an entry for each DWARF DIE whose

            
              tag is one of:</p>

            
            <ul>

            
              <li>DW_TAG_array_type</li>

            
              <li>DW_TAG_class_type</li>

            
              <li>DW_TAG_enumeration_type</li>

            
              <li>DW_TAG_pointer_type</li>

            
              <li>DW_TAG_reference_type</li>

            
              <li>DW_TAG_string_type</li>

            
              <li>DW_TAG_structure_type</li>

            
              <li>DW_TAG_subroutine_type</li>

            
              <li>DW_TAG_typedef</li>

            
              <li>DW_TAG_union_type</li>

            
              <li>DW_TAG_ptr_to_member_type</li>

            
              <li>DW_TAG_set_type</li>

            
              <li>DW_TAG_subrange_type</li>

            
              <li>DW_TAG_base_type</li>

            
              <li>DW_TAG_const_type</li>

            
              <li>DW_TAG_constant</li>

            
              <li>DW_TAG_file_type</li>

            
              <li>DW_TAG_namelist</li>

            
              <li>DW_TAG_packed_type</li>

            
              <li>DW_TAG_volatile_type</li>

            
              <li>DW_TAG_restrict_type</li>

            
              <li>DW_TAG_interface_type</li>

            
              <li>DW_TAG_unspecified_type</li>

            
              <li>DW_TAG_shared_type</li>

            
            </ul>

            
            <p>Only entries with a DW_AT_name attribute are included, and the entry must

            
              not be a forward declaration (DW_AT_declaration attribute with a non-zero value).

            
              For example, using the following code:</p>

            
            <div class="doc_code">

            
            <pre>

            
            int main ()

            
            {

            
              int *b = 0;

            
              return *b;

            
            }

            
            </pre>

            
            </div>

            
            <p>We get a few type DIEs:</p>

            
            <div class="doc_code">

            
            <pre>

            
            0x00000067:     TAG_base_type [5]

            
                            AT_encoding( DW_ATE_signed )

            
                            AT_name( "int" )

            
                            AT_byte_size( 0x04 )


            0x0000006e:     TAG_pointer_type [6]

            
                            AT_type( {0x00000067} ( int ) )

            
                            AT_byte_size( 0x08 )

            
            </pre>

            
            </div>

            
            <p>The DW_TAG_pointer_type is not included because it does not have a DW_AT_name.</p>


            <p>".apple_namespaces" section should contain all DW_TAG_namespace DIEs. If

            
              we run into a namespace that has no name this is an anonymous namespace,

            
              and the name should be output as "(anonymous namespace)" (without the quotes).

            
              Why? This matches the output of the abi::cxa_demangle() that is in the standard

            
              C++ library that demangles mangled names.</p>

            
            </div>


            <!-- ======================================================================= -->

            
            <h4>

            
              <a name="acceltableextensions">Language Extensions and File Format Changes</a>

            
            </h4>

            
            <!-- ======================================================================= -->

            
            <div>

            
            <h5>Objective-C Extensions</h5>

            
            <p>".apple_objc" section should contain all DW_TAG_subprogram DIEs for an

            
              Objective-C class. The name used in the hash table is the name of the

            
              Objective-C class itself. If the Objective-C class has a category, then an

            
              entry is made for both the class name without the category, and for the class

            
              name with the category. So if we have a DIE at offset 0x1234 with a name

            
              of method "-[NSString(my_additions) stringWithSpecialString:]", we would add

            
              an entry for "NSString" that points to DIE 0x1234, and an entry for

            
              "NSString(my_additions)" that points to 0x1234. This allows us to quickly

            
              track down all Objective-C methods for an Objective-C class when doing

            
              expressions. It is needed because of the dynamic nature of Objective-C where

            
              anyone can add methods to a class. The DWARF for Objective-C methods is also

            
              emitted differently from C++ classes where the methods are not usually

            
              contained in the class definition, they are scattered about across one or more

            
              compile units. Categories can also be defined in different shared libraries.

            
              So we need to be able to quickly find all of the methods and class functions

            
              given the Objective-C class name, or quickly find all methods and class

            
              functions for a class + category name. This table does not contain any selector

            
              names, it just maps Objective-C class names (or class names + category) to all

            
              of the methods and class functions. The selectors are added as function

            
              basenames in the .debug_names section.</p>


            <p>In the ".apple_names" section for Objective-C functions, the full name is the

            
              entire function name with the brackets ("-[NSString stringWithCString:]") and the

            
              basename is the selector only ("stringWithCString:").</p>


            <h5>Mach-O Changes</h5>

            
            <p>The sections names for the apple hash tables are for non mach-o files. For

            
              mach-o files, the sections should be contained in the "__DWARF" segment with

            
              names as follows:</p>

            
            <ul>

            
              <li>".apple_names" -> "__apple_names"</li>

            
              <li>".apple_types" -> "__apple_types"</li>

            
              <li>".apple_namespaces" -> "__apple_namespac" (16 character limit)</li>

            
              <li> ".apple_objc" -> "__apple_objc"</li>

            
            </ul>

            
            </div>

            
            </div>

            
            </div>


            <!-- *********************************************************************** -->


            <hr>

            
            <address>

            
              <a href="http://jigsaw.w3.org/css-validator/check/referer"><img

            
              src="http://jigsaw.w3.org/css-validator/images/vcss-blue" alt="Valid CSS"></a>

            
              <a href="http://validator.w3.org/check/referer"><img

            
              src="http://www.w3.org/Icons/valid-html401-blue" alt="Valid HTML 4.01"></a>


              <a href="mailto:sabre@nondot.org">Chris Lattner</a><br>

            
              <a href="http://llvm.org/">LLVM Compiler Infrastructure</a><br>

            
              Last modified: $Date$

            
            </address>


            </body>

            
            </html>