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==============================
LLVM Language Reference Manual
==============================
.. contents::
:local:
:depth: 3
Abstract
========
This document is a reference manual for the LLVM assembly language. LLVM
is a Static Single Assignment (SSA) based representation that provides
type safety, low-level operations, flexibility, and the capability of
representing 'all' high-level languages cleanly. It is the common code
representation used throughout all phases of the LLVM compilation
strategy.
Introduction
============
The LLVM code representation is designed to be used in three different
forms: as an in-memory compiler IR, as an on-disk bitcode representation
(suitable for fast loading by a Just-In-Time compiler), and as a human
readable assembly language representation. This allows LLVM to provide a
powerful intermediate representation for efficient compiler
transformations and analysis, while providing a natural means to debug
and visualize the transformations. The three different forms of LLVM are
all equivalent. This document describes the human readable
representation and notation.
The LLVM representation aims to be light-weight and low-level while
being expressive, typed, and extensible at the same time. It aims to be
a "universal IR" of sorts, by being at a low enough level that
high-level ideas may be cleanly mapped to it (similar to how
microprocessors are "universal IR's", allowing many source languages to
be mapped to them). By providing type information, LLVM can be used as
the target of optimizations: for example, through pointer analysis, it
can be proven that a C automatic variable is never accessed outside of
the current function, allowing it to be promoted to a simple SSA value
instead of a memory location.
.. _wellformed:
Well-Formedness
---------------
It is important to note that this document describes 'well formed' LLVM
assembly language. There is a difference between what the parser accepts
and what is considered 'well formed'. For example, the following
instruction is syntactically okay, but not well formed:
.. code-block:: llvm
%x = add i32 1, %x
because the definition of ``%x`` does not dominate all of its uses. The
LLVM infrastructure provides a verification pass that may be used to
verify that an LLVM module is well formed. This pass is automatically
run by the parser after parsing input assembly and by the optimizer
before it outputs bitcode. The violations pointed out by the verifier
pass indicate bugs in transformation passes or input to the parser.
.. _identifiers:
Identifiers
===========
LLVM identifiers come in two basic types: global and local. Global
identifiers (functions, global variables) begin with the ``'@'``
character. Local identifiers (register names, types) begin with the
``'%'`` character. Additionally, there are three different formats for
identifiers, for different purposes:
#. Named values are represented as a string of characters with their
prefix. For example, ``%foo``, ``@DivisionByZero``,
``%a.really.long.identifier``. The actual regular expression used is
'``[%@][a-zA-Z$._][a-zA-Z$._0-9]*``'. Identifiers which require other
characters in their names can be surrounded with quotes. Special
characters may be escaped using ``"\xx"`` where ``xx`` is the ASCII
code for the character in hexadecimal. In this way, any character can
be used in a name value, even quotes themselves.
#. Unnamed values are represented as an unsigned numeric value with
their prefix. For example, ``%12``, ``@2``, ``%44``.
#. Constants, which are described in the section Constants_ below.
LLVM requires that values start with a prefix for two reasons: Compilers
don't need to worry about name clashes with reserved words, and the set
of reserved words may be expanded in the future without penalty.
Additionally, unnamed identifiers allow a compiler to quickly come up
with a temporary variable without having to avoid symbol table
conflicts.
Reserved words in LLVM are very similar to reserved words in other
languages. There are keywords for different opcodes ('``add``',
'``bitcast``', '``ret``', etc...), for primitive type names ('``void``',
'``i32``', etc...), and others. These reserved words cannot conflict
with variable names, because none of them start with a prefix character
(``'%'`` or ``'@'``).
Here is an example of LLVM code to multiply the integer variable
'``%X``' by 8:
The easy way:
.. code-block:: llvm
%result = mul i32 %X, 8
After strength reduction:
.. code-block:: llvm
%result = shl i32 %X, 3
And the hard way:
.. code-block:: llvm
%0 = add i32 %X, %X ; yields {i32}:%0
%1 = add i32 %0, %0 ; yields {i32}:%1
%result = add i32 %1, %1
This last way of multiplying ``%X`` by 8 illustrates several important
lexical features of LLVM:
#. Comments are delimited with a '``;``' and go until the end of line.
#. Unnamed temporaries are created when the result of a computation is
not assigned to a named value.
#. Unnamed temporaries are numbered sequentially (using a per-function
incrementing counter, starting with 0). Note that basic blocks are
included in this numbering. For example, if the entry basic block is not
given a label name, then it will get number 0.
It also shows a convention that we follow in this document. When
demonstrating instructions, we will follow an instruction with a comment
that defines the type and name of value produced.
High Level Structure
====================
Module Structure
----------------
LLVM programs are composed of ``Module``'s, each of which is a
translation unit of the input programs. Each module consists of
functions, global variables, and symbol table entries. Modules may be
combined together with the LLVM linker, which merges function (and
global variable) definitions, resolves forward declarations, and merges
symbol table entries. Here is an example of the "hello world" module:
.. code-block:: llvm
; Declare the string constant as a global constant.
@.str = private unnamed_addr constant [13 x i8] c"hello world\0A\00"
; External declaration of the puts function
declare i32 @puts(i8* nocapture) nounwind
; Definition of main function
define i32 @main() { ; i32()*
; Convert [13 x i8]* to i8 *...
%cast210 = getelementptr [13 x i8]* @.str, i64 0, i64 0
; Call puts function to write out the string to stdout.
call i32 @puts(i8* %cast210)
ret i32 0
}
; Named metadata
!1 = metadata !{i32 42}
!foo = !{!1, null}
This example is made up of a :ref:`global variable <globalvars>` named
"``.str``", an external declaration of the "``puts``" function, a
:ref:`function definition <functionstructure>` for "``main``" and
:ref:`named metadata <namedmetadatastructure>` "``foo``".
In general, a module is made up of a list of global values (where both
functions and global variables are global values). Global values are
represented by a pointer to a memory location (in this case, a pointer
to an array of char, and a pointer to a function), and have one of the
following :ref:`linkage types <linkage>`.
.. _linkage:
Linkage Types
-------------
All Global Variables and Functions have one of the following types of
linkage:
``private``
Global values with "``private``" linkage are only directly
accessible by objects in the current module. In particular, linking
code into a module with an private global value may cause the
private to be renamed as necessary to avoid collisions. Because the
symbol is private to the module, all references can be updated. This
doesn't show up in any symbol table in the object file.
``linker_private``
Similar to ``private``, but the symbol is passed through the
assembler and evaluated by the linker. Unlike normal strong symbols,
they are removed by the linker from the final linked image
(executable or dynamic library).
``linker_private_weak``
Similar to "``linker_private``", but the symbol is weak. Note that
``linker_private_weak`` symbols are subject to coalescing by the
linker. The symbols are removed by the linker from the final linked
image (executable or dynamic library).
``internal``
Similar to private, but the value shows as a local symbol
(``STB_LOCAL`` in the case of ELF) in the object file. This
corresponds to the notion of the '``static``' keyword in C.
``available_externally``
Globals with "``available_externally``" linkage are never emitted
into the object file corresponding to the LLVM module. They exist to
allow inlining and other optimizations to take place given knowledge
of the definition of the global, which is known to be somewhere
outside the module. Globals with ``available_externally`` linkage
are allowed to be discarded at will, and are otherwise the same as
``linkonce_odr``. This linkage type is only allowed on definitions,
not declarations.
``linkonce``
Globals with "``linkonce``" linkage are merged with other globals of
the same name when linkage occurs. This can be used to implement
some forms of inline functions, templates, or other code which must
be generated in each translation unit that uses it, but where the
body may be overridden with a more definitive definition later.
Unreferenced ``linkonce`` globals are allowed to be discarded. Note
that ``linkonce`` linkage does not actually allow the optimizer to
inline the body of this function into callers because it doesn't
know if this definition of the function is the definitive definition
within the program or whether it will be overridden by a stronger
definition. To enable inlining and other optimizations, use
"``linkonce_odr``" linkage.
``weak``
"``weak``" linkage has the same merging semantics as ``linkonce``
linkage, except that unreferenced globals with ``weak`` linkage may
not be discarded. This is used for globals that are declared "weak"
in C source code.
``common``
"``common``" linkage is most similar to "``weak``" linkage, but they
are used for tentative definitions in C, such as "``int X;``" at
global scope. Symbols with "``common``" linkage are merged in the
same way as ``weak symbols``, and they may not be deleted if
unreferenced. ``common`` symbols may not have an explicit section,
must have a zero initializer, and may not be marked
':ref:`constant <globalvars>`'. Functions and aliases may not have
common linkage.
.. _linkage_appending:
``appending``
"``appending``" linkage may only be applied to global variables of
pointer to array type. When two global variables with appending
linkage are linked together, the two global arrays are appended
together. This is the LLVM, typesafe, equivalent of having the
system linker append together "sections" with identical names when
.o files are linked.
``extern_weak``
The semantics of this linkage follow the ELF object file model: the
symbol is weak until linked, if not linked, the symbol becomes null
instead of being an undefined reference.
``linkonce_odr``, ``weak_odr``
Some languages allow differing globals to be merged, such as two
functions with different semantics. Other languages, such as
``C++``, ensure that only equivalent globals are ever merged (the
"one definition rule" --- "ODR"). Such languages can use the
``linkonce_odr`` and ``weak_odr`` linkage types to indicate that the
global will only be merged with equivalent globals. These linkage
types are otherwise the same as their non-``odr`` versions.
``external``
If none of the above identifiers are used, the global is externally
visible, meaning that it participates in linkage and can be used to
resolve external symbol references.
The next two types of linkage are targeted for Microsoft Windows
platform only. They are designed to support importing (exporting)
symbols from (to) DLLs (Dynamic Link Libraries).
``dllimport``
"``dllimport``" linkage causes the compiler to reference a function
or variable via a global pointer to a pointer that is set up by the
DLL exporting the symbol. On Microsoft Windows targets, the pointer
name is formed by combining ``__imp_`` and the function or variable
name.
``dllexport``
"``dllexport``" linkage causes the compiler to provide a global
pointer to a pointer in a DLL, so that it can be referenced with the
``dllimport`` attribute. On Microsoft Windows targets, the pointer
name is formed by combining ``__imp_`` and the function or variable
name.
For example, since the "``.LC0``" variable is defined to be internal, if
another module defined a "``.LC0``" variable and was linked with this
one, one of the two would be renamed, preventing a collision. Since
"``main``" and "``puts``" are external (i.e., lacking any linkage
declarations), they are accessible outside of the current module.
It is illegal for a function *declaration* to have any linkage type
other than ``external``, ``dllimport`` or ``extern_weak``.
.. _callingconv:
Calling Conventions
-------------------
LLVM :ref:`functions <functionstructure>`, :ref:`calls <i_call>` and
:ref:`invokes <i_invoke>` can all have an optional calling convention
specified for the call. The calling convention of any pair of dynamic
caller/callee must match, or the behavior of the program is undefined.
The following calling conventions are supported by LLVM, and more may be
added in the future:
"``ccc``" - The C calling convention
This calling convention (the default if no other calling convention
is specified) matches the target C calling conventions. This calling
convention supports varargs function calls and tolerates some
mismatch in the declared prototype and implemented declaration of
the function (as does normal C).
"``fastcc``" - The fast calling convention
This calling convention attempts to make calls as fast as possible
(e.g. by passing things in registers). This calling convention
allows the target to use whatever tricks it wants to produce fast
code for the target, without having to conform to an externally
specified ABI (Application Binary Interface). `Tail calls can only
be optimized when this, the GHC or the HiPE convention is
used. <CodeGenerator.html#id80>`_ This calling convention does not
support varargs and requires the prototype of all callees to exactly
match the prototype of the function definition.
"``coldcc``" - The cold calling convention
This calling convention attempts to make code in the caller as
efficient as possible under the assumption that the call is not
commonly executed. As such, these calls often preserve all registers
so that the call does not break any live ranges in the caller side.
This calling convention does not support varargs and requires the
prototype of all callees to exactly match the prototype of the
function definition.
"``cc 10``" - GHC convention
This calling convention has been implemented specifically for use by
the `Glasgow Haskell Compiler (GHC) <http://www.haskell.org/ghc>`_.
It passes everything in registers, going to extremes to achieve this
by disabling callee save registers. This calling convention should
not be used lightly but only for specific situations such as an
alternative to the *register pinning* performance technique often
used when implementing functional programming languages. At the
moment only X86 supports this convention and it has the following
limitations:
- On *X86-32* only supports up to 4 bit type parameters. No
floating point types are supported.
- On *X86-64* only supports up to 10 bit type parameters and 6
floating point parameters.
This calling convention supports `tail call
optimization <CodeGenerator.html#id80>`_ but requires both the
caller and callee are using it.
"``cc 11``" - The HiPE calling convention
This calling convention has been implemented specifically for use by
the `High-Performance Erlang
(HiPE) <http://www.it.uu.se/research/group/hipe/>`_ compiler, *the*
native code compiler of the `Ericsson's Open Source Erlang/OTP
system <http://www.erlang.org/download.shtml>`_. It uses more
registers for argument passing than the ordinary C calling
convention and defines no callee-saved registers. The calling
convention properly supports `tail call
optimization <CodeGenerator.html#id80>`_ but requires that both the
caller and the callee use it. It uses a *register pinning*
mechanism, similar to GHC's convention, for keeping frequently
accessed runtime components pinned to specific hardware registers.
At the moment only X86 supports this convention (both 32 and 64
bit).
"``cc <n>``" - Numbered convention
Any calling convention may be specified by number, allowing
target-specific calling conventions to be used. Target specific
calling conventions start at 64.
More calling conventions can be added/defined on an as-needed basis, to
support Pascal conventions or any other well-known target-independent
convention.
.. _visibilitystyles:
Visibility Styles
-----------------
All Global Variables and Functions have one of the following visibility
styles:
"``default``" - Default style
On targets that use the ELF object file format, default visibility
means that the declaration is visible to other modules and, in
shared libraries, means that the declared entity may be overridden.
On Darwin, default visibility means that the declaration is visible
to other modules. Default visibility corresponds to "external
linkage" in the language.
"``hidden``" - Hidden style
Two declarations of an object with hidden visibility refer to the
same object if they are in the same shared object. Usually, hidden
visibility indicates that the symbol will not be placed into the
dynamic symbol table, so no other module (executable or shared
library) can reference it directly.
"``protected``" - Protected style
On ELF, protected visibility indicates that the symbol will be
placed in the dynamic symbol table, but that references within the
defining module will bind to the local symbol. That is, the symbol
cannot be overridden by another module.
.. _namedtypes:
Named Types
-----------
LLVM IR allows you to specify name aliases for certain types. This can
make it easier to read the IR and make the IR more condensed
(particularly when recursive types are involved). An example of a name
specification is:
.. code-block:: llvm
%mytype = type { %mytype*, i32 }
You may give a name to any :ref:`type <typesystem>` except
":ref:`void <t_void>`". Type name aliases may be used anywhere a type is
expected with the syntax "%mytype".
Note that type names are aliases for the structural type that they
indicate, and that you can therefore specify multiple names for the same
type. This often leads to confusing behavior when dumping out a .ll
file. Since LLVM IR uses structural typing, the name is not part of the
type. When printing out LLVM IR, the printer will pick *one name* to
render all types of a particular shape. This means that if you have code
where two different source types end up having the same LLVM type, that
the dumper will sometimes print the "wrong" or unexpected type. This is
an important design point and isn't going to change.
.. _globalvars:
Global Variables
----------------
Global variables define regions of memory allocated at compilation time
instead of run-time.
Global variables definitions must be initialized, may have an explicit section
to be placed in, and may have an optional explicit alignment specified.
Global variables in other translation units can also be declared, in which
case they don't have an initializer.
A variable may be defined as ``thread_local``, which means that it will
not be shared by threads (each thread will have a separated copy of the
variable). Not all targets support thread-local variables. Optionally, a
TLS model may be specified:
``localdynamic``
For variables that are only used within the current shared library.
``initialexec``
For variables in modules that will not be loaded dynamically.
``localexec``
For variables defined in the executable and only used within it.
The models correspond to the ELF TLS models; see `ELF Handling For
Thread-Local Storage <http://people.redhat.com/drepper/tls.pdf>`_ for
more information on under which circumstances the different models may
be used. The target may choose a different TLS model if the specified
model is not supported, or if a better choice of model can be made.
A variable may be defined as a global ``constant``, which indicates that
the contents of the variable will **never** be modified (enabling better
optimization, allowing the global data to be placed in the read-only
section of an executable, etc). Note that variables that need runtime
initialization cannot be marked ``constant`` as there is a store to the
variable.
LLVM explicitly allows *declarations* of global variables to be marked
constant, even if the final definition of the global is not. This
capability can be used to enable slightly better optimization of the
program, but requires the language definition to guarantee that
optimizations based on the 'constantness' are valid for the translation
units that do not include the definition.
As SSA values, global variables define pointer values that are in scope
(i.e. they dominate) all basic blocks in the program. Global variables
always define a pointer to their "content" type because they describe a
region of memory, and all memory objects in LLVM are accessed through
pointers.
Global variables can be marked with ``unnamed_addr`` which indicates
that the address is not significant, only the content. Constants marked
like this can be merged with other constants if they have the same
initializer. Note that a constant with significant address *can* be
merged with a ``unnamed_addr`` constant, the result being a constant
whose address is significant.
A global variable may be declared to reside in a target-specific
numbered address space. For targets that support them, address spaces
may affect how optimizations are performed and/or what target
instructions are used to access the variable. The default address space
is zero. The address space qualifier must precede any other attributes.
LLVM allows an explicit section to be specified for globals. If the
target supports it, it will emit globals to the section specified.
By default, global initializers are optimized by assuming that global
variables defined within the module are not modified from their
initial values before the start of the global initializer. This is
true even for variables potentially accessible from outside the
module, including those with external linkage or appearing in
``@llvm.used``. This assumption may be suppressed by marking the
variable with ``externally_initialized``.
An explicit alignment may be specified for a global, which must be a
power of 2. If not present, or if the alignment is set to zero, the
alignment of the global is set by the target to whatever it feels
convenient. If an explicit alignment is specified, the global is forced
to have exactly that alignment. Targets and optimizers are not allowed
to over-align the global if the global has an assigned section. In this
case, the extra alignment could be observable: for example, code could
assume that the globals are densely packed in their section and try to
iterate over them as an array, alignment padding would break this
iteration.
For example, the following defines a global in a numbered address space
with an initializer, section, and alignment:
.. code-block:: llvm
@G = addrspace(5) constant float 1.0, section "foo", align 4
The following example just declares a global variable
.. code-block:: llvm
@G = external global i32
The following example defines a thread-local global with the
``initialexec`` TLS model:
.. code-block:: llvm
@G = thread_local(initialexec) global i32 0, align 4
.. _functionstructure:
Functions
---------
LLVM function definitions consist of the "``define``" keyword, an
optional :ref:`linkage type <linkage>`, an optional :ref:`visibility
style <visibility>`, an optional :ref:`calling convention <callingconv>`,
an optional ``unnamed_addr`` attribute, a return type, an optional
:ref:`parameter attribute <paramattrs>` for the return type, a function
name, a (possibly empty) argument list (each with optional :ref:`parameter
attributes <paramattrs>`), optional :ref:`function attributes <fnattrs>`,
an optional section, an optional alignment, an optional :ref:`garbage
collector name <gc>`, an optional :ref:`prefix <prefixdata>`, an opening
curly brace, a list of basic blocks, and a closing curly brace.
LLVM function declarations consist of the "``declare``" keyword, an
optional :ref:`linkage type <linkage>`, an optional :ref:`visibility
style <visibility>`, an optional :ref:`calling convention <callingconv>`,
an optional ``unnamed_addr`` attribute, a return type, an optional
:ref:`parameter attribute <paramattrs>` for the return type, a function
name, a possibly empty list of arguments, an optional alignment, an optional
:ref:`garbage collector name <gc>` and an optional :ref:`prefix <prefixdata>`.
A function definition contains a list of basic blocks, forming the CFG (Control
Flow Graph) for the function. Each basic block may optionally start with a label
(giving the basic block a symbol table entry), contains a list of instructions,
and ends with a :ref:`terminator <terminators>` instruction (such as a branch or
function return). If an explicit label is not provided, a block is assigned an
implicit numbered label, using the next value from the same counter as used for
unnamed temporaries (:ref:`see above<identifiers>`). For example, if a function
entry block does not have an explicit label, it will be assigned label "%0",
then the first unnamed temporary in that block will be "%1", etc.
The first basic block in a function is special in two ways: it is
immediately executed on entrance to the function, and it is not allowed
to have predecessor basic blocks (i.e. there can not be any branches to
the entry block of a function). Because the block can have no
predecessors, it also cannot have any :ref:`PHI nodes <i_phi>`.
LLVM allows an explicit section to be specified for functions. If the
target supports it, it will emit functions to the section specified.
An explicit alignment may be specified for a function. If not present,
or if the alignment is set to zero, the alignment of the function is set
by the target to whatever it feels convenient. If an explicit alignment
is specified, the function is forced to have at least that much
alignment. All alignments must be a power of 2.
If the ``unnamed_addr`` attribute is given, the address is know to not
be significant and two identical functions can be merged.
Syntax::
define [linkage] [visibility]
[cconv] [ret attrs]
<ResultType> @<FunctionName> ([argument list])
[fn Attrs] [section "name"] [align N]
[gc] [prefix Constant] { ... }
.. _langref_aliases:
Aliases
-------
Aliases act as "second name" for the aliasee value (which can be either
function, global variable, another alias or bitcast of global value).
Aliases may have an optional :ref:`linkage type <linkage>`, and an optional
:ref:`visibility style <visibility>`.
Syntax::
@<Name> = alias [Linkage] [Visibility] <AliaseeTy> @<Aliasee>
The linkage must be one of ``private``, ``linker_private``,
``linker_private_weak``, ``internal``, ``linkonce``, ``weak``,
``linkonce_odr``, ``weak_odr``, ``external``. Note that some system linkers
might not correctly handle dropping a weak symbol that is aliased by a non weak
alias.
.. _namedmetadatastructure:
Named Metadata
--------------
Named metadata is a collection of metadata. :ref:`Metadata
nodes <metadata>` (but not metadata strings) are the only valid
operands for a named metadata.
Syntax::
; Some unnamed metadata nodes, which are referenced by the named metadata.
!0 = metadata !{metadata !"zero"}
!1 = metadata !{metadata !"one"}
!2 = metadata !{metadata !"two"}
; A named metadata.
!name = !{!0, !1, !2}
.. _paramattrs:
Parameter Attributes
--------------------
The return type and each parameter of a function type may have a set of
*parameter attributes* associated with them. Parameter attributes are
used to communicate additional information about the result or
parameters of a function. Parameter attributes are considered to be part
of the function, not of the function type, so functions with different
parameter attributes can have the same function type.
Parameter attributes are simple keywords that follow the type specified.
If multiple parameter attributes are needed, they are space separated.
For example:
.. code-block:: llvm
declare i32 @printf(i8* noalias nocapture, ...)
declare i32 @atoi(i8 zeroext)
declare signext i8 @returns_signed_char()
Note that any attributes for the function result (``nounwind``,
``readonly``) come immediately after the argument list.
Currently, only the following parameter attributes are defined:
``zeroext``
This indicates to the code generator that the parameter or return
value should be zero-extended to the extent required by the target's
ABI (which is usually 32-bits, but is 8-bits for a i1 on x86-64) by
the caller (for a parameter) or the callee (for a return value).
``signext``
This indicates to the code generator that the parameter or return
value should be sign-extended to the extent required by the target's
ABI (which is usually 32-bits) by the caller (for a parameter) or
the callee (for a return value).
``inreg``
This indicates that this parameter or return value should be treated
in a special target-dependent fashion during while emitting code for
a function call or return (usually, by putting it in a register as
opposed to memory, though some targets use it to distinguish between
two different kinds of registers). Use of this attribute is
target-specific.
``byval``
This indicates that the pointer parameter should really be passed by
value to the function. The attribute implies that a hidden copy of
the pointee is made between the caller and the callee, so the callee
is unable to modify the value in the caller. This attribute is only
valid on LLVM pointer arguments. It is generally used to pass
structs and arrays by value, but is also valid on pointers to
scalars. The copy is considered to belong to the caller not the
callee (for example, ``readonly`` functions should not write to
``byval`` parameters). This is not a valid attribute for return
values.
The byval attribute also supports specifying an alignment with the
align attribute. It indicates the alignment of the stack slot to
form and the known alignment of the pointer specified to the call
site. If the alignment is not specified, then the code generator
makes a target-specific assumption.
``sret``
This indicates that the pointer parameter specifies the address of a
structure that is the return value of the function in the source
program. This pointer must be guaranteed by the caller to be valid:
loads and stores to the structure may be assumed by the callee
not to trap and to be properly aligned. This may only be applied to
the first parameter. This is not a valid attribute for return
values.
``noalias``
This indicates that pointer values :ref:`based <pointeraliasing>` on
the argument or return value do not alias pointer values which are
not *based* on it, ignoring certain "irrelevant" dependencies. For a
call to the parent function, dependencies between memory references
from before or after the call and from those during the call are
"irrelevant" to the ``noalias`` keyword for the arguments and return
value used in that call. The caller shares the responsibility with
the callee for ensuring that these requirements are met. For further
details, please see the discussion of the NoAlias response in `alias
analysis <AliasAnalysis.html#MustMayNo>`_.
Note that this definition of ``noalias`` is intentionally similar
to the definition of ``restrict`` in C99 for function arguments,
though it is slightly weaker.
For function return values, C99's ``restrict`` is not meaningful,
while LLVM's ``noalias`` is.
``nocapture``
This indicates that the callee does not make any copies of the
pointer that outlive the callee itself. This is not a valid
attribute for return values.
.. _nest:
``nest``
This indicates that the pointer parameter can be excised using the
:ref:`trampoline intrinsics <int_trampoline>`. This is not a valid
attribute for return values and can only be applied to one parameter.
``returned``
This indicates that the function always returns the argument as its return
value. This is an optimization hint to the code generator when generating
the caller, allowing tail call optimization and omission of register saves
and restores in some cases; it is not checked or enforced when generating
the callee. The parameter and the function return type must be valid
operands for the :ref:`bitcast instruction <i_bitcast>`. This is not a
valid attribute for return values and can only be applied to one parameter.
.. _gc:
Garbage Collector Names
-----------------------
Each function may specify a garbage collector name, which is simply a
string:
.. code-block:: llvm
define void @f() gc "name" { ... }
The compiler declares the supported values of *name*. Specifying a
collector which will cause the compiler to alter its output in order to
support the named garbage collection algorithm.
.. _prefixdata:
Prefix Data
-----------
Prefix data is data associated with a function which the code generator
will emit immediately before the function body. The purpose of this feature
is to allow frontends to associate language-specific runtime metadata with
specific functions and make it available through the function pointer while
still allowing the function pointer to be called. To access the data for a
given function, a program may bitcast the function pointer to a pointer to
the constant's type. This implies that the IR symbol points to the start
of the prefix data.
To maintain the semantics of ordinary function calls, the prefix data must
have a particular format. Specifically, it must begin with a sequence of
bytes which decode to a sequence of machine instructions, valid for the
module's target, which transfer control to the point immediately succeeding
the prefix data, without performing any other visible action. This allows
the inliner and other passes to reason about the semantics of the function
definition without needing to reason about the prefix data. Obviously this
makes the format of the prefix data highly target dependent.
Prefix data is laid out as if it were an initializer for a global variable
of the prefix data's type. No padding is automatically placed between the
prefix data and the function body. If padding is required, it must be part
of the prefix data.
A trivial example of valid prefix data for the x86 architecture is ``i8 144``,
which encodes the ``nop`` instruction:
.. code-block:: llvm
define void @f() prefix i8 144 { ... }
Generally prefix data can be formed by encoding a relative branch instruction
which skips the metadata, as in this example of valid prefix data for the
x86_64 architecture, where the first two bytes encode ``jmp .+10``:
.. code-block:: llvm
%0 = type <{ i8, i8, i8* }>
define void @f() prefix %0 <{ i8 235, i8 8, i8* @md}> { ... }
A function may have prefix data but no body. This has similar semantics
to the ``available_externally`` linkage in that the data may be used by the
optimizers but will not be emitted in the object file.
.. _attrgrp:
Attribute Groups
----------------
Attribute groups are groups of attributes that are referenced by objects within
the IR. They are important for keeping ``.ll`` files readable, because a lot of
functions will use the same set of attributes. In the degenerative case of a
``.ll`` file that corresponds to a single ``.c`` file, the single attribute
group will capture the important command line flags used to build that file.
An attribute group is a module-level object. To use an attribute group, an
object references the attribute group's ID (e.g. ``#37``). An object may refer
to more than one attribute group. In that situation, the attributes from the
different groups are merged.
Here is an example of attribute groups for a function that should always be
inlined, has a stack alignment of 4, and which shouldn't use SSE instructions:
.. code-block:: llvm
; Target-independent attributes:
attributes #0 = { alwaysinline alignstack=4 }
; Target-dependent attributes:
attributes #1 = { "no-sse" }
; Function @f has attributes: alwaysinline, alignstack=4, and "no-sse".
define void @f() #0 #1 { ... }
.. _fnattrs:
Function Attributes
-------------------
Function attributes are set to communicate additional information about
a function. Function attributes are considered to be part of the
function, not of the function type, so functions with different function
attributes can have the same function type.
Function attributes are simple keywords that follow the type specified.
If multiple attributes are needed, they are space separated. For
example:
.. code-block:: llvm
define void @f() noinline { ... }
define void @f() alwaysinline { ... }
define void @f() alwaysinline optsize { ... }
define void @f() optsize { ... }
``alignstack(<n>)``
This attribute indicates that, when emitting the prologue and
epilogue, the backend should forcibly align the stack pointer.
Specify the desired alignment, which must be a power of two, in
parentheses.
``alwaysinline``
This attribute indicates that the inliner should attempt to inline
this function into callers whenever possible, ignoring any active
inlining size threshold for this caller.
``builtin``
This indicates that the callee function at a call site should be
recognized as a built-in function, even though the function's declaration
uses the ``nobuiltin`` attribute. This is only valid at call sites for
direct calls to functions which are declared with the ``nobuiltin``
attribute.
``cold``
This attribute indicates that this function is rarely called. When
computing edge weights, basic blocks post-dominated by a cold
function call are also considered to be cold; and, thus, given low
weight.
``inlinehint``
This attribute indicates that the source code contained a hint that
inlining this function is desirable (such as the "inline" keyword in
C/C++). It is just a hint; it imposes no requirements on the
inliner.
``minsize``
This attribute suggests that optimization passes and code generator
passes make choices that keep the code size of this function as small
as possible and perform optimizations that may sacrifice runtime
performance in order to minimize the size of the generated code.
``naked``
This attribute disables prologue / epilogue emission for the
function. This can have very system-specific consequences.
``nobuiltin``
This indicates that the callee function at a call site is not recognized as
a built-in function. LLVM will retain the original call and not replace it
with equivalent code based on the semantics of the built-in function, unless
the call site uses the ``builtin`` attribute. This is valid at call sites
and on function declarations and definitions.
``noduplicate``
This attribute indicates that calls to the function cannot be
duplicated. A call to a ``noduplicate`` function may be moved
within its parent function, but may not be duplicated within
its parent function.
A function containing a ``noduplicate`` call may still
be an inlining candidate, provided that the call is not
duplicated by inlining. That implies that the function has
internal linkage and only has one call site, so the original
call is dead after inlining.
``noimplicitfloat``
This attributes disables implicit floating point instructions.
``noinline``
This attribute indicates that the inliner should never inline this
function in any situation. This attribute may not be used together
with the ``alwaysinline`` attribute.
``nonlazybind``
This attribute suppresses lazy symbol binding for the function. This
may make calls to the function faster, at the cost of extra program
startup time if the function is not called during program startup.
``noredzone``
This attribute indicates that the code generator should not use a
red zone, even if the target-specific ABI normally permits it.
``noreturn``
This function attribute indicates that the function never returns
normally. This produces undefined behavior at runtime if the
function ever does dynamically return.
``nounwind``
This function attribute indicates that the function never returns
with an unwind or exceptional control flow. If the function does
unwind, its runtime behavior is undefined.
``optnone``
This function attribute indicates that the function is not optimized
by any optimization or code generator passes with the
exception of interprocedural optimization passes.
This attribute cannot be used together with the ``alwaysinline``
attribute; this attribute is also incompatible
with the ``minsize`` attribute and the ``optsize`` attribute.
This attribute requires the ``noinline`` attribute to be specified on
the function as well, so the function is never inlined into any caller.
Only functions with the ``alwaysinline`` attribute are valid
candidates for inlining into the body of this function.
``optsize``
This attribute suggests that optimization passes and code generator
passes make choices that keep the code size of this function low,
and otherwise do optimizations specifically to reduce code size as
long as they do not significantly impact runtime performance.
``readnone``
On a function, this attribute indicates that the function computes its
result (or decides to unwind an exception) based strictly on its arguments,
without dereferencing any pointer arguments or otherwise accessing
any mutable state (e.g. memory, control registers, etc) visible to
caller functions. It does not write through any pointer arguments
(including ``byval`` arguments) and never changes any state visible
to callers. This means that it cannot unwind exceptions by calling
the ``C++`` exception throwing methods.
On an argument, this attribute indicates that the function does not
dereference that pointer argument, even though it may read or write the
memory that the pointer points to if accessed through other pointers.
``readonly``
On a function, this attribute indicates that the function does not write
through any pointer arguments (including ``byval`` arguments) or otherwise
modify any state (e.g. memory, control registers, etc) visible to
caller functions. It may dereference pointer arguments and read
state that may be set in the caller. A readonly function always
returns the same value (or unwinds an exception identically) when
called with the same set of arguments and global state. It cannot
unwind an exception by calling the ``C++`` exception throwing
methods.
On an argument, this attribute indicates that the function does not write
through this pointer argument, even though it may write to the memory that
the pointer points to.
``returns_twice``
This attribute indicates that this function can return twice. The C
``setjmp`` is an example of such a function. The compiler disables
some optimizations (like tail calls) in the caller of these
functions.
``sanitize_address``
This attribute indicates that AddressSanitizer checks
(dynamic address safety analysis) are enabled for this function.
``sanitize_memory``
This attribute indicates that MemorySanitizer checks (dynamic detection
of accesses to uninitialized memory) are enabled for this function.
``sanitize_thread``
This attribute indicates that ThreadSanitizer checks
(dynamic thread safety analysis) are enabled for this function.
``ssp``
This attribute indicates that the function should emit a stack
smashing protector. It is in the form of a "canary" --- a random value
placed on the stack before the local variables that's checked upon
return from the function to see if it has been overwritten. A
heuristic is used to determine if a function needs stack protectors
or not. The heuristic used will enable protectors for functions with:
- Character arrays larger than ``ssp-buffer-size`` (default 8).
- Aggregates containing character arrays larger than ``ssp-buffer-size``.
- Calls to alloca() with variable sizes or constant sizes greater than
``ssp-buffer-size``.
If a function that has an ``ssp`` attribute is inlined into a
function that doesn't have an ``ssp`` attribute, then the resulting
function will have an ``ssp`` attribute.
``sspreq``
This attribute indicates that the function should *always* emit a
stack smashing protector. This overrides the ``ssp`` function
attribute.
If a function that has an ``sspreq`` attribute is inlined into a
function that doesn't have an ``sspreq`` attribute or which has an
``ssp`` or ``sspstrong`` attribute, then the resulting function will have
an ``sspreq`` attribute.
``sspstrong``
This attribute indicates that the function should emit a stack smashing
protector. This attribute causes a strong heuristic to be used when
determining if a function needs stack protectors. The strong heuristic
will enable protectors for functions with:
- Arrays of any size and type
- Aggregates containing an array of any size and type.
- Calls to alloca().
- Local variables that have had their address taken.
This overrides the ``ssp`` function attribute.
If a function that has an ``sspstrong`` attribute is inlined into a
function that doesn't have an ``sspstrong`` attribute, then the
resulting function will have an ``sspstrong`` attribute.
``uwtable``
This attribute indicates that the ABI being targeted requires that
an unwind table entry be produce for this function even if we can
show that no exceptions passes by it. This is normally the case for
the ELF x86-64 abi, but it can be disabled for some compilation
units.
.. _moduleasm:
Module-Level Inline Assembly
----------------------------
Modules may contain "module-level inline asm" blocks, which corresponds
to the GCC "file scope inline asm" blocks. These blocks are internally
concatenated by LLVM and treated as a single unit, but may be separated
in the ``.ll`` file if desired. The syntax is very simple:
.. code-block:: llvm
module asm "inline asm code goes here"
module asm "more can go here"
The strings can contain any character by escaping non-printable
characters. The escape sequence used is simply "\\xx" where "xx" is the
two digit hex code for the number.
The inline asm code is simply printed to the machine code .s file when
assembly code is generated.
.. _langref_datalayout:
Data Layout
-----------
A module may specify a target specific data layout string that specifies
how data is to be laid out in memory. The syntax for the data layout is
simply:
.. code-block:: llvm
target datalayout = "layout specification"
The *layout specification* consists of a list of specifications
separated by the minus sign character ('-'). Each specification starts
with a letter and may include other information after the letter to
define some aspect of the data layout. The specifications accepted are
as follows:
``E``
Specifies that the target lays out data in big-endian form. That is,
the bits with the most significance have the lowest address
location.
``e``
Specifies that the target lays out data in little-endian form. That
is, the bits with the least significance have the lowest address
location.
``S<size>``
Specifies the natural alignment of the stack in bits. Alignment
promotion of stack variables is limited to the natural stack
alignment to avoid dynamic stack realignment. The stack alignment
must be a multiple of 8-bits. If omitted, the natural stack
alignment defaults to "unspecified", which does not prevent any
alignment promotions.
``p[n]:<size>:<abi>:<pref>``
This specifies the *size* of a pointer and its ``<abi>`` and
``<pref>``\erred alignments for address space ``n``. All sizes are in
bits. Specifying the ``<pref>`` alignment is optional. If omitted, the
preceding ``:`` should be omitted too. The address space, ``n`` is
optional, and if not specified, denotes the default address space 0.
The value of ``n`` must be in the range [1,2^23).
``i<size>:<abi>:<pref>``
This specifies the alignment for an integer type of a given bit
``<size>``. The value of ``<size>`` must be in the range [1,2^23).
``v<size>:<abi>:<pref>``
This specifies the alignment for a vector type of a given bit
``<size>``.
``f<size>:<abi>:<pref>``
This specifies the alignment for a floating point type of a given bit
``<size>``. Only values of ``<size>`` that are supported by the target
will work. 32 (float) and 64 (double) are supported on all targets; 80
or 128 (different flavors of long double) are also supported on some
targets.
``a<size>:<abi>:<pref>``
This specifies the alignment for an aggregate type of a given bit
``<size>``.
``s<size>:<abi>:<pref>``
This specifies the alignment for a stack object of a given bit
``<size>``.
``n<size1>:<size2>:<size3>...``
This specifies a set of native integer widths for the target CPU in
bits. For example, it might contain ``n32`` for 32-bit PowerPC,
``n32:64`` for PowerPC 64, or ``n8:16:32:64`` for X86-64. Elements of
this set are considered to support most general arithmetic operations
efficiently.
When constructing the data layout for a given target, LLVM starts with a
default set of specifications which are then (possibly) overridden by
the specifications in the ``datalayout`` keyword. The default
specifications are given in this list:
- ``E`` - big endian
- ``p:64:64:64`` - 64-bit pointers with 64-bit alignment.
- ``p[n]:64:64:64`` - Other address spaces are assumed to be the
same as the default address space.
- ``S0`` - natural stack alignment is unspecified
- ``i1:8:8`` - i1 is 8-bit (byte) aligned
- ``i8:8:8`` - i8 is 8-bit (byte) aligned
- ``i16:16:16`` - i16 is 16-bit aligned
- ``i32:32:32`` - i32 is 32-bit aligned
- ``i64:32:64`` - i64 has ABI alignment of 32-bits but preferred
alignment of 64-bits
- ``f16:16:16`` - half is 16-bit aligned
- ``f32:32:32`` - float is 32-bit aligned
- ``f64:64:64`` - double is 64-bit aligned
- ``f128:128:128`` - quad is 128-bit aligned
- ``v64:64:64`` - 64-bit vector is 64-bit aligned
- ``v128:128:128`` - 128-bit vector is 128-bit aligned
- ``a0:0:64`` - aggregates are 64-bit aligned
When LLVM is determining the alignment for a given type, it uses the
following rules:
#. If the type sought is an exact match for one of the specifications,
that specification is used.
#. If no match is found, and the type sought is an integer type, then
the smallest integer type that is larger than the bitwidth of the
sought type is used. If none of the specifications are larger than
the bitwidth then the largest integer type is used. For example,
given the default specifications above, the i7 type will use the
alignment of i8 (next largest) while both i65 and i256 will use the
alignment of i64 (largest specified).
#. If no match is found, and the type sought is a vector type, then the
largest vector type that is smaller than the sought vector type will
be used as a fall back. This happens because <128 x double> can be
implemented in terms of 64 <2 x double>, for example.
The function of the data layout string may not be what you expect.
Notably, this is not a specification from the frontend of what alignment
the code generator should use.
Instead, if specified, the target data layout is required to match what
the ultimate *code generator* expects. This string is used by the
mid-level optimizers to improve code, and this only works if it matches
what the ultimate code generator uses. If you would like to generate IR
that does not embed this target-specific detail into the IR, then you
don't have to specify the string. This will disable some optimizations
that require precise layout information, but this also prevents those
optimizations from introducing target specificity into the IR.
.. _langref_triple:
Target Triple
-------------
A module may specify a target triple string that describes the target
host. The syntax for the target triple is simply:
.. code-block:: llvm
target triple = "x86_64-apple-macosx10.7.0"
The *target triple* string consists of a series of identifiers delimited
by the minus sign character ('-'). The canonical forms are:
::
ARCHITECTURE-VENDOR-OPERATING_SYSTEM
ARCHITECTURE-VENDOR-OPERATING_SYSTEM-ENVIRONMENT
This information is passed along to the backend so that it generates
code for the proper architecture. It's possible to override this on the
command line with the ``-mtriple`` command line option.
.. _pointeraliasing:
Pointer Aliasing Rules
----------------------
Any memory access must be done through a pointer value associated with
an address range of the memory access, otherwise the behavior is
undefined. Pointer values are associated with address ranges according
to the following rules:
- A pointer value is associated with the addresses associated with any
value it is *based* on.
- An address of a global variable is associated with the address range
of the variable's storage.
- The result value of an allocation instruction is associated with the
address range of the allocated storage.
- A null pointer in the default address-space is associated with no
address.
- An integer constant other than zero or a pointer value returned from
a function not defined within LLVM may be associated with address
ranges allocated through mechanisms other than those provided by
LLVM. Such ranges shall not overlap with any ranges of addresses
allocated by mechanisms provided by LLVM.
A pointer value is *based* on another pointer value according to the
following rules:
- A pointer value formed from a ``getelementptr`` operation is *based*
on the first operand of the ``getelementptr``.
- The result value of a ``bitcast`` is *based* on the operand of the
``bitcast``.
- A pointer value formed by an ``inttoptr`` is *based* on all pointer
values that contribute (directly or indirectly) to the computation of
the pointer's value.
- The "*based* on" relationship is transitive.
Note that this definition of *"based"* is intentionally similar to the
definition of *"based"* in C99, though it is slightly weaker.
LLVM IR does not associate types with memory. The result type of a
``load`` merely indicates the size and alignment of the memory from
which to load, as well as the interpretation of the value. The first
operand type of a ``store`` similarly only indicates the size and
alignment of the store.
Consequently, type-based alias analysis, aka TBAA, aka
``-fstrict-aliasing``, is not applicable to general unadorned LLVM IR.
:ref:`Metadata <metadata>` may be used to encode additional information
which specialized optimization passes may use to implement type-based
alias analysis.
.. _volatile:
Volatile Memory Accesses
------------------------
Certain memory accesses, such as :ref:`load <i_load>`'s,
:ref:`store <i_store>`'s, and :ref:`llvm.memcpy <int_memcpy>`'s may be
marked ``volatile``. The optimizers must not change the number of
volatile operations or change their order of execution relative to other
volatile operations. The optimizers *may* change the order of volatile
operations relative to non-volatile operations. This is not Java's
"volatile" and has no cross-thread synchronization behavior.
IR-level volatile loads and stores cannot safely be optimized into
llvm.memcpy or llvm.memmove intrinsics even when those intrinsics are
flagged volatile. Likewise, the backend should never split or merge
target-legal volatile load/store instructions.
.. admonition:: Rationale
Platforms may rely on volatile loads and stores of natively supported
data width to be executed as single instruction. For example, in C
this holds for an l-value of volatile primitive type with native
hardware support, but not necessarily for aggregate types. The
frontend upholds these expectations, which are intentionally
unspecified in the IR. The rules above ensure that IR transformation
do not violate the frontend's contract with the language.
.. _memmodel:
Memory Model for Concurrent Operations
--------------------------------------
The LLVM IR does not define any way to start parallel threads of
execution or to register signal handlers. Nonetheless, there are
platform-specific ways to create them, and we define LLVM IR's behavior
in their presence. This model is inspired by the C++0x memory model.
For a more informal introduction to this model, see the :doc:`Atomics`.
We define a *happens-before* partial order as the least partial order
that
- Is a superset of single-thread program order, and
- When a *synchronizes-with* ``b``, includes an edge from ``a`` to
``b``. *Synchronizes-with* pairs are introduced by platform-specific
techniques, like pthread locks, thread creation, thread joining,
etc., and by atomic instructions. (See also :ref:`Atomic Memory Ordering
Constraints <ordering>`).
Note that program order does not introduce *happens-before* edges
between a thread and signals executing inside that thread.
Every (defined) read operation (load instructions, memcpy, atomic
loads/read-modify-writes, etc.) R reads a series of bytes written by
(defined) write operations (store instructions, atomic
stores/read-modify-writes, memcpy, etc.). For the purposes of this
section, initialized globals are considered to have a write of the
initializer which is atomic and happens before any other read or write
of the memory in question. For each byte of a read R, R\ :sub:`byte`
may see any write to the same byte, except:
- If write\ :sub:`1` happens before write\ :sub:`2`, and
write\ :sub:`2` happens before R\ :sub:`byte`, then
R\ :sub:`byte` does not see write\ :sub:`1`.
- If R\ :sub:`byte` happens before write\ :sub:`3`, then
R\ :sub:`byte` does not see write\ :sub:`3`.
Given that definition, R\ :sub:`byte` is defined as follows:
- If R is volatile, the result is target-dependent. (Volatile is
supposed to give guarantees which can support ``sig_atomic_t`` in
C/C++, and may be used for accesses to addresses which do not behave
like normal memory. It does not generally provide cross-thread
synchronization.)
- Otherwise, if there is no write to the same byte that happens before
R\ :sub:`byte`, R\ :sub:`byte` returns ``undef`` for that byte.
- Otherwise, if R\ :sub:`byte` may see exactly one write,
R\ :sub:`byte` returns the value written by that write.
- Otherwise, if R is atomic, and all the writes R\ :sub:`byte` may
see are atomic, it chooses one of the values written. See the :ref:`Atomic
Memory Ordering Constraints <ordering>` section for additional
constraints on how the choice is made.
- Otherwise R\ :sub:`byte` returns ``undef``.
R returns the value composed of the series of bytes it read. This
implies that some bytes within the value may be ``undef`` **without**
the entire value being ``undef``. Note that this only defines the
semantics of the operation; it doesn't mean that targets will emit more
than one instruction to read the series of bytes.
Note that in cases where none of the atomic intrinsics are used, this
model places only one restriction on IR transformations on top of what
is required for single-threaded execution: introducing a store to a byte
which might not otherwise be stored is not allowed in general.
(Specifically, in the case where another thread might write to and read
from an address, introducing a store can change a load that may see
exactly one write into a load that may see multiple writes.)
.. _ordering:
Atomic Memory Ordering Constraints
----------------------------------
Atomic instructions (:ref:`cmpxchg <i_cmpxchg>`,
:ref:`atomicrmw <i_atomicrmw>`, :ref:`fence <i_fence>`,
:ref:`atomic load <i_load>`, and :ref:`atomic store <i_store>`) take
an ordering parameter that determines which other atomic instructions on
the same address they *synchronize with*. These semantics are borrowed
from Java and C++0x, but are somewhat more colloquial. If these
descriptions aren't precise enough, check those specs (see spec
references in the :doc:`atomics guide <Atomics>`).
:ref:`fence <i_fence>` instructions treat these orderings somewhat
differently since they don't take an address. See that instruction's
documentation for details.
For a simpler introduction to the ordering constraints, see the
:doc:`Atomics`.
``unordered``
The set of values that can be read is governed by the happens-before
partial order. A value cannot be read unless some operation wrote
it. This is intended to provide a guarantee strong enough to model
Java's non-volatile shared variables. This ordering cannot be
specified for read-modify-write operations; it is not strong enough
to make them atomic in any interesting way.
``monotonic``
In addition to the guarantees of ``unordered``, there is a single
total order for modifications by ``monotonic`` operations on each
address. All modification orders must be compatible with the
happens-before order. There is no guarantee that the modification
orders can be combined to a global total order for the whole program
(and this often will not be possible). The read in an atomic
read-modify-write operation (:ref:`cmpxchg <i_cmpxchg>` and
:ref:`atomicrmw <i_atomicrmw>`) reads the value in the modification
order immediately before the value it writes. If one atomic read
happens before another atomic read of the same address, the later
read must see the same value or a later value in the address's
modification order. This disallows reordering of ``monotonic`` (or
stronger) operations on the same address. If an address is written
``monotonic``-ally by one thread, and other threads ``monotonic``-ally
read that address repeatedly, the other threads must eventually see
the write. This corresponds to the C++0x/C1x
``memory_order_relaxed``.
``acquire``
In addition to the guarantees of ``monotonic``, a
*synchronizes-with* edge may be formed with a ``release`` operation.
This is intended to model C++'s ``memory_order_acquire``.
``release``
In addition to the guarantees of ``monotonic``, if this operation
writes a value which is subsequently read by an ``acquire``
operation, it *synchronizes-with* that operation. (This isn't a
complete description; see the C++0x definition of a release
sequence.) This corresponds to the C++0x/C1x
``memory_order_release``.
``acq_rel`` (acquire+release)
Acts as both an ``acquire`` and ``release`` operation on its
address. This corresponds to the C++0x/C1x ``memory_order_acq_rel``.
``seq_cst`` (sequentially consistent)
In addition to the guarantees of ``acq_rel`` (``acquire`` for an
operation which only reads, ``release`` for an operation which only
writes), there is a global total order on all
sequentially-consistent operations on all addresses, which is
consistent with the *happens-before* partial order and with the
modification orders of all the affected addresses. Each
sequentially-consistent read sees the last preceding write to the
same address in this global order. This corresponds to the C++0x/C1x
``memory_order_seq_cst`` and Java volatile.
.. _singlethread:
If an atomic operation is marked ``singlethread``, it only *synchronizes
with* or participates in modification and seq\_cst total orderings with
other operations running in the same thread (for example, in signal
handlers).
.. _fastmath:
Fast-Math Flags
---------------
LLVM IR floating-point binary ops (:ref:`fadd <i_fadd>`,
:ref:`fsub <i_fsub>`, :ref:`fmul <i_fmul>`, :ref:`fdiv <i_fdiv>`,
:ref:`frem <i_frem>`) have the following flags that can set to enable
otherwise unsafe floating point operations
``nnan``
No NaNs - Allow optimizations to assume the arguments and result are not
NaN. Such optimizations are required to retain defined behavior over
NaNs, but the value of the result is undefined.
``ninf``
No Infs - Allow optimizations to assume the arguments and result are not
+/-Inf. Such optimizations are required to retain defined behavior over
+/-Inf, but the value of the result is undefined.
``nsz``
No Signed Zeros - Allow optimizations to treat the sign of a zero
argument or result as insignificant.
``arcp``
Allow Reciprocal - Allow optimizations to use the reciprocal of an
argument rather than perform division.
``fast``
Fast - Allow algebraically equivalent transformations that may
dramatically change results in floating point (e.g. reassociate). This
flag implies all the others.
.. _typesystem:
Type System
===========
The LLVM type system is one of the most important features of the
intermediate representation. Being typed enables a number of
optimizations to be performed on the intermediate representation
directly, without having to do extra analyses on the side before the
transformation. A strong type system makes it easier to read the
generated code and enables novel analyses and transformations that are
not feasible to perform on normal three address code representations.
.. _typeclassifications:
Type Classifications
--------------------
The types fall into a few useful classifications:
.. list-table::
:header-rows: 1
* - Classification
- Types
* - :ref:`integer <t_integer>`
- ``i1``, ``i2``, ``i3``, ... ``i8``, ... ``i16``, ... ``i32``, ...
``i64``, ...
* - :ref:`floating point <t_floating>`
- ``half``, ``float``, ``double``, ``x86_fp80``, ``fp128``,
``ppc_fp128``
* - first class
.. _t_firstclass:
- :ref:`integer <t_integer>`, :ref:`floating point <t_floating>`,
:ref:`pointer <t_pointer>`, :ref:`vector <t_vector>`,
:ref:`structure <t_struct>`, :ref:`array <t_array>`,
:ref:`label <t_label>`, :ref:`metadata <t_metadata>`.
* - :ref:`primitive <t_primitive>`
- :ref:`label <t_label>`,
:ref:`void <t_void>`,
:ref:`integer <t_integer>`,
:ref:`floating point <t_floating>`,
:ref:`x86mmx <t_x86mmx>`,
:ref:`metadata <t_metadata>`.
* - :ref:`derived <t_derived>`
- :ref:`array <t_array>`,
:ref:`function <t_function>`,
:ref:`pointer <t_pointer>`,
:ref:`structure <t_struct>`,
:ref:`vector <t_vector>`,
:ref:`opaque <t_opaque>`.
The :ref:`first class <t_firstclass>` types are perhaps the most important.
Values of these types are the only ones which can be produced by
instructions.
.. _t_primitive:
Primitive Types
---------------
The primitive types are the fundamental building blocks of the LLVM
system.
.. _t_integer:
Integer Type
^^^^^^^^^^^^
Overview:
"""""""""
The integer type is a very simple type that simply specifies an
arbitrary bit width for the integer type desired. Any bit width from 1
bit to 2\ :sup:`23`\ -1 (about 8 million) can be specified.
Syntax:
"""""""
::
iN
The number of bits the integer will occupy is specified by the ``N``
value.
Examples:
"""""""""
+----------------+------------------------------------------------+
| ``i1`` | a single-bit integer. |
+----------------+------------------------------------------------+
| ``i32`` | a 32-bit integer. |
+----------------+------------------------------------------------+
| ``i1942652`` | a really big integer of over 1 million bits. |
+----------------+------------------------------------------------+
.. _t_floating:
Floating Point Types
^^^^^^^^^^^^^^^^^^^^
.. list-table::
:header-rows: 1
* - Type
- Description
* - ``half``
- 16-bit floating point value
* - ``float``
- 32-bit floating point value
* - ``double``
- 64-bit floating point value
* - ``fp128``
- 128-bit floating point value (112-bit mantissa)
* - ``x86_fp80``
- 80-bit floating point value (X87)
* - ``ppc_fp128``
- 128-bit floating point value (two 64-bits)
.. _t_x86mmx:
X86mmx Type
^^^^^^^^^^^
Overview:
"""""""""
The x86mmx type represents a value held in an MMX register on an x86
machine. The operations allowed on it are quite limited: parameters and
return values, load and store, and bitcast. User-specified MMX
instructions are represented as intrinsic or asm calls with arguments
and/or results of this type. There are no arrays, vectors or constants
of this type.
Syntax:
"""""""
::
x86mmx
.. _t_void:
Void Type
^^^^^^^^^
Overview:
"""""""""
The void type does not represent any value and has no size.
Syntax:
"""""""
::
void
.. _t_label:
Label Type
^^^^^^^^^^
Overview:
"""""""""
The label type represents code labels.
Syntax:
"""""""
::
label
.. _t_metadata:
Metadata Type
^^^^^^^^^^^^^
Overview:
"""""""""
The metadata type represents embedded metadata. No derived types may be
created from metadata except for :ref:`function <t_function>` arguments.
Syntax:
"""""""
::
metadata
.. _t_derived:
Derived Types
-------------
The real power in LLVM comes from the derived types in the system. This
is what allows a programmer to represent arrays, functions, pointers,
and other useful types. Each of these types contain one or more element
types which may be a primitive type, or another derived type. For
example, it is possible to have a two dimensional array, using an array
as the element type of another array.
.. _t_aggregate:
Aggregate Types
^^^^^^^^^^^^^^^
Aggregate Types are a subset of derived types that can contain multiple
member types. :ref:`Arrays <t_array>` and :ref:`structs <t_struct>` are
aggregate types. :ref:`Vectors <t_vector>` are not considered to be
aggregate types.
.. _t_array:
Array Type
^^^^^^^^^^
Overview:
"""""""""
The array type is a very simple derived type that arranges elements
sequentially in memory. The array type requires a size (number of
elements) and an underlying data type.
Syntax:
"""""""
::
[<# elements> x <elementtype>]
The number of elements is a constant integer value; ``elementtype`` may
be any type with a size.
Examples:
"""""""""
+------------------+--------------------------------------+
| ``[40 x i32]`` | Array of 40 32-bit integer values. |
+------------------+--------------------------------------+
| ``[41 x i32]`` | Array of 41 32-bit integer values. |
+------------------+--------------------------------------+
| ``[4 x i8]`` | Array of 4 8-bit integer values. |
+------------------+--------------------------------------+
Here are some examples of multidimensional arrays:
+-----------------------------+----------------------------------------------------------+
| ``[3 x [4 x i32]]`` | 3x4 array of 32-bit integer values. |
+-----------------------------+----------------------------------------------------------+
| ``[12 x [10 x float]]`` | 12x10 array of single precision floating point values. |
+-----------------------------+----------------------------------------------------------+
| ``[2 x [3 x [4 x i16]]]`` | 2x3x4 array of 16-bit integer values. |
+-----------------------------+----------------------------------------------------------+
There is no restriction on indexing beyond the end of the array implied
by a static type (though there are restrictions on indexing beyond the
bounds of an allocated object in some cases). This means that
single-dimension 'variable sized array' addressing can be implemented in
LLVM with a zero length array type. An implementation of 'pascal style
arrays' in LLVM could use the type "``{ i32, [0 x float]}``", for
example.
.. _t_function:
Function Type
^^^^^^^^^^^^^
Overview:
"""""""""
The function type can be thought of as a function signature. It consists of a
return type and a list of formal parameter types. The return type of a function
type is a void type or first class type --- except for :ref:`label <t_label>`
and :ref:`metadata <t_metadata>` types.
Syntax:
"""""""
::
<returntype> (<parameter list>)
...where '``<parameter list>``' is a comma-separated list of type
specifiers. Optionally, the parameter list may include a type ``...``, which
indicates that the function takes a variable number of arguments. Variable
argument functions can access their arguments with the :ref:`variable argument
handling intrinsic <int_varargs>` functions. '``<returntype>``' is any type
except :ref:`label <t_label>` and :ref:`metadata <t_metadata>`.
Examples:
"""""""""
+---------------------------------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| ``i32 (i32)`` | function taking an ``i32``, returning an ``i32`` |
+---------------------------------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| ``float (i16, i32 *) *`` | :ref:`Pointer <t_pointer>` to a function that takes an ``i16`` and a :ref:`pointer <t_pointer>` to ``i32``, returning ``float``. |
+---------------------------------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| ``i32 (i8*, ...)`` | A vararg function that takes at least one :ref:`pointer <t_pointer>` to ``i8`` (char in C), which returns an integer. This is the signature for ``printf`` in LLVM. |
+---------------------------------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| ``{i32, i32} (i32)`` | A function taking an ``i32``, returning a :ref:`structure <t_struct>` containing two ``i32`` values |
+---------------------------------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------+
.. _t_struct:
Structure Type
^^^^^^^^^^^^^^
Overview:
"""""""""
The structure type is used to represent a collection of data members
together in memory. The elements of a structure may be any type that has
a size.
Structures in memory are accessed using '``load``' and '``store``' by
getting a pointer to a field with the '``getelementptr``' instruction.
Structures in registers are accessed using the '``extractvalue``' and
'``insertvalue``' instructions.
Structures may optionally be "packed" structures, which indicate that
the alignment of the struct is one byte, and that there is no padding
between the elements. In non-packed structs, padding between field types
is inserted as defined by the DataLayout string in the module, which is
required to match what the underlying code generator expects.
Structures can either be "literal" or "identified". A literal structure
is defined inline with other types (e.g. ``{i32, i32}*``) whereas
identified types are always defined at the top level with a name.
Literal types are uniqued by their contents and can never be recursive
or opaque since there is no way to write one. Identified types can be
recursive, can be opaqued, and are never uniqued.
Syntax:
"""""""
::
%T1 = type { <type list> } ; Identified normal struct type
%T2 = type <{ <type list> }> ; Identified packed struct type
Examples:
"""""""""
+------------------------------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| ``{ i32, i32, i32 }`` | A triple of three ``i32`` values |
+------------------------------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| ``{ float, i32 (i32) * }`` | A pair, where the first element is a ``float`` and the second element is a :ref:`pointer <t_pointer>` to a :ref:`function <t_function>` that takes an ``i32``, returning an ``i32``. |
+------------------------------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| ``<{ i8, i32 }>`` | A packed struct known to be 5 bytes in size. |
+------------------------------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
.. _t_opaque:
Opaque Structure Types
^^^^^^^^^^^^^^^^^^^^^^
Overview:
"""""""""
Opaque structure types are used to represent named structure types that
do not have a body specified. This corresponds (for example) to the C
notion of a forward declared structure.
Syntax:
"""""""
::
%X = type opaque
%52 = type opaque
Examples:
"""""""""
+--------------+-------------------+
| ``opaque`` | An opaque type. |
+--------------+-------------------+
.. _t_pointer:
Pointer Type
^^^^^^^^^^^^
Overview:
"""""""""
The pointer type is used to specify memory locations. Pointers are
commonly used to reference objects in memory.
Pointer types may have an optional address space attribute defining the
numbered address space where the pointed-to object resides. The default
address space is number zero. The semantics of non-zero address spaces
are target-specific.
Note that LLVM does not permit pointers to void (``void*``) nor does it
permit pointers to labels (``label*``). Use ``i8*`` instead.
Syntax:
"""""""
::
<type> *
Examples:
"""""""""
+-------------------------+--------------------------------------------------------------------------------------------------------------+
| ``[4 x i32]*`` | A :ref:`pointer <t_pointer>` to :ref:`array <t_array>` of four ``i32`` values. |
+-------------------------+--------------------------------------------------------------------------------------------------------------+
| ``i32 (i32*) *`` | A :ref:`pointer <t_pointer>` to a :ref:`function <t_function>` that takes an ``i32*``, returning an ``i32``. |
+-------------------------+--------------------------------------------------------------------------------------------------------------+
| ``i32 addrspace(5)*`` | A :ref:`pointer <t_pointer>` to an ``i32`` value that resides in address space #5. |
+-------------------------+--------------------------------------------------------------------------------------------------------------+
.. _t_vector:
Vector Type
^^^^^^^^^^^
Overview:
"""""""""
A vector type is a simple derived type that represents a vector of
elements. Vector types are used when multiple primitive data are
operated in parallel using a single instruction (SIMD). A vector type
requires a size (number of elements) and an underlying primitive data
type. Vector types are considered :ref:`first class <t_firstclass>`.
Syntax:
"""""""
::
< <# elements> x <elementtype> >
The number of elements is a constant integer value larger than 0;
elementtype may be any integer or floating point type, or a pointer to
these types. Vectors of size zero are not allowed.
Examples:
"""""""""
+-------------------+--------------------------------------------------+
| ``<4 x i32>`` | Vector of 4 32-bit integer values. |
+-------------------+--------------------------------------------------+
| ``<8 x float>`` | Vector of 8 32-bit floating-point values. |
+-------------------+--------------------------------------------------+
| ``<2 x i64>`` | Vector of 2 64-bit integer values. |
+-------------------+--------------------------------------------------+
| ``<4 x i64*>`` | Vector of 4 pointers to 64-bit integer values. |
+-------------------+--------------------------------------------------+
Constants
=========
LLVM has several different basic types of constants. This section
describes them all and their syntax.
Simple Constants
----------------
**Boolean constants**
The two strings '``true``' and '``false``' are both valid constants
of the ``i1`` type.
**Integer constants**
Standard integers (such as '4') are constants of the
:ref:`integer <t_integer>` type. Negative numbers may be used with
integer types.
**Floating point constants**
Floating point constants use standard decimal notation (e.g.
123.421), exponential notation (e.g. 1.23421e+2), or a more precise
hexadecimal notation (see below). The assembler requires the exact
decimal value of a floating-point constant. For example, the
assembler accepts 1.25 but rejects 1.3 because 1.3 is a repeating
decimal in binary. Floating point constants must have a :ref:`floating
point <t_floating>` type.
**Null pointer constants**
The identifier '``null``' is recognized as a null pointer constant
and must be of :ref:`pointer type <t_pointer>`.
The one non-intuitive notation for constants is the hexadecimal form of
floating point constants. For example, the form
'``double 0x432ff973cafa8000``' is equivalent to (but harder to read
than) '``double 4.5e+15``'. The only time hexadecimal floating point
constants are required (and the only time that they are generated by the
disassembler) is when a floating point constant must be emitted but it
cannot be represented as a decimal floating point number in a reasonable
number of digits. For example, NaN's, infinities, and other special
values are represented in their IEEE hexadecimal format so that assembly
and disassembly do not cause any bits to change in the constants.
When using the hexadecimal form, constants of types half, float, and
double are represented using the 16-digit form shown above (which
matches the IEEE754 representation for double); half and float values
must, however, be exactly representable as IEEE 754 half and single
precision, respectively. Hexadecimal format is always used for long
double, and there are three forms of long double. The 80-bit format used
by x86 is represented as ``0xK`` followed by 20 hexadecimal digits. The
128-bit format used by PowerPC (two adjacent doubles) is represented by
``0xM`` followed by 32 hexadecimal digits. The IEEE 128-bit format is
represented by ``0xL`` followed by 32 hexadecimal digits. Long doubles
will only work if they match the long double format on your target.
The IEEE 16-bit format (half precision) is represented by ``0xH``
followed by 4 hexadecimal digits. All hexadecimal formats are big-endian
(sign bit at the left).
There are no constants of type x86mmx.
.. _complexconstants:
Complex Constants
-----------------
Complex constants are a (potentially recursive) combination of simple
constants and smaller complex constants.
**Structure constants**
Structure constants are represented with notation similar to
structure type definitions (a comma separated list of elements,
surrounded by braces (``{}``)). For example:
"``{ i32 4, float 17.0, i32* @G }``", where "``@G``" is declared as
"``@G = external global i32``". Structure constants must have
:ref:`structure type <t_struct>`, and the number and types of elements
must match those specified by the type.
**Array constants**
Array constants are represented with notation similar to array type
definitions (a comma separated list of elements, surrounded by
square brackets (``[]``)). For example:
"``[ i32 42, i32 11, i32 74 ]``". Array constants must have
:ref:`array type <t_array>`, and the number and types of elements must
match those specified by the type.
**Vector constants**
Vector constants are represented with notation similar to vector
type definitions (a comma separated list of elements, surrounded by
less-than/greater-than's (``<>``)). For example:
"``< i32 42, i32 11, i32 74, i32 100 >``". Vector constants
must have :ref:`vector type <t_vector>`, and the number and types of
elements must match those specified by the type.
**Zero initialization**
The string '``zeroinitializer``' can be used to zero initialize a
value to zero of *any* type, including scalar and
:ref:`aggregate <t_aggregate>` types. This is often used to avoid
having to print large zero initializers (e.g. for large arrays) and
is always exactly equivalent to using explicit zero initializers.
**Metadata node**
A metadata node is a structure-like constant with :ref:`metadata
type <t_metadata>`. For example:
"``metadata !{ i32 0, metadata !"test" }``". Unlike other
constants that are meant to be interpreted as part of the
instruction stream, metadata is a place to attach additional
information such as debug info.
Global Variable and Function Addresses
--------------------------------------
The addresses of :ref:`global variables <globalvars>` and
:ref:`functions <functionstructure>` are always implicitly valid
(link-time) constants. These constants are explicitly referenced when
the :ref:`identifier for the global <identifiers>` is used and always have
:ref:`pointer <t_pointer>` type. For example, the following is a legal LLVM
file:
.. code-block:: llvm
@X = global i32 17
@Y = global i32 42
@Z = global [2 x i32*] [ i32* @X, i32* @Y ]
.. _undefvalues:
Undefined Values
----------------
The string '``undef``' can be used anywhere a constant is expected, and
indicates that the user of the value may receive an unspecified
bit-pattern. Undefined values may be of any type (other than '``label``'
or '``void``') and be used anywhere a constant is permitted.
Undefined values are useful because they indicate to the compiler that
the program is well defined no matter what value is used. This gives the
compiler more freedom to optimize. Here are some examples of
(potentially surprising) transformations that are valid (in pseudo IR):
.. code-block:: llvm
%A = add %X, undef
%B = sub %X, undef
%C = xor %X, undef
Safe:
%A = undef
%B = undef
%C = undef
This is safe because all of the output bits are affected by the undef
bits. Any output bit can have a zero or one depending on the input bits.
.. code-block:: llvm
%A = or %X, undef
%B = and %X, undef
Safe:
%A = -1
%B = 0
Unsafe:
%A = undef
%B = undef
These logical operations have bits that are not always affected by the
input. For example, if ``%X`` has a zero bit, then the output of the
'``and``' operation will always be a zero for that bit, no matter what
the corresponding bit from the '``undef``' is. As such, it is unsafe to
optimize or assume that the result of the '``and``' is '``undef``'.
However, it is safe to assume that all bits of the '``undef``' could be
0, and optimize the '``and``' to 0. Likewise, it is safe to assume that
all the bits of the '``undef``' operand to the '``or``' could be set,
allowing the '``or``' to be folded to -1.
.. code-block:: llvm
%A = select undef, %X, %Y
%B = select undef, 42, %Y
%C = select %X, %Y, undef
Safe:
%A = %X (or %Y)
%B = 42 (or %Y)
%C = %Y
Unsafe:
%A = undef
%B = undef
%C = undef
This set of examples shows that undefined '``select``' (and conditional
branch) conditions can go *either way*, but they have to come from one
of the two operands. In the ``%A`` example, if ``%X`` and ``%Y`` were
both known to have a clear low bit, then ``%A`` would have to have a
cleared low bit. However, in the ``%C`` example, the optimizer is
allowed to assume that the '``undef``' operand could be the same as
``%Y``, allowing the whole '``select``' to be eliminated.
.. code-block:: llvm
%A = xor undef, undef
%B = undef
%C = xor %B, %B
%D = undef
%E = icmp lt %D, 4
%F = icmp gte %D, 4
Safe:
%A = undef
%B = undef
%C = undef
%D = undef
%E = undef
%F = undef
This example points out that two '``undef``' operands are not
necessarily the same. This can be surprising to people (and also matches
C semantics) where they assume that "``X^X``" is always zero, even if
``X`` is undefined. This isn't true for a number of reasons, but the
short answer is that an '``undef``' "variable" can arbitrarily change
its value over its "live range". This is true because the variable
doesn't actually *have a live range*. Instead, the value is logically
read from arbitrary registers that happen to be around when needed, so
the value is not necessarily consistent over time. In fact, ``%A`` and
``%C`` need to have the same semantics or the core LLVM "replace all
uses with" concept would not hold.
.. code-block:: llvm
%A = fdiv undef, %X
%B = fdiv %X, undef
Safe:
%A = undef
b: unreachable
These examples show the crucial difference between an *undefined value*
and *undefined behavior*. An undefined value (like '``undef``') is
allowed to have an arbitrary bit-pattern. This means that the ``%A``
operation can be constant folded to '``undef``', because the '``undef``'
could be an SNaN, and ``fdiv`` is not (currently) defined on SNaN's.
However, in the second example, we can make a more aggressive
assumption: because the ``undef`` is allowed to be an arbitrary value,
we are allowed to assume that it could be zero. Since a divide by zero
has *undefined behavior*, we are allowed to assume that the operation
does not execute at all. This allows us to delete the divide and all
code after it. Because the undefined operation "can't happen", the
optimizer can assume that it occurs in dead code.
.. code-block:: llvm
a: store undef -> %X
b: store %X -> undef
Safe:
a: <deleted>
b: unreachable
These examples reiterate the ``fdiv`` example: a store *of* an undefined
value can be assumed to not have any effect; we can assume that the
value is overwritten with bits that happen to match what was already
there. However, a store *to* an undefined location could clobber
arbitrary memory, therefore, it has undefined behavior.
.. _poisonvalues:
Poison Values
-------------
Poison values are similar to :ref:`undef values <undefvalues>`, however
they also represent the fact that an instruction or constant expression
which cannot evoke side effects has nevertheless detected a condition
which results in undefined behavior.
There is currently no way of representing a poison value in the IR; they
only exist when produced by operations such as :ref:`add <i_add>` with
the ``nsw`` flag.
Poison value behavior is defined in terms of value *dependence*:
- Values other than :ref:`phi <i_phi>` nodes depend on their operands.
- :ref:`Phi <i_phi>` nodes depend on the operand corresponding to
their dynamic predecessor basic block.
- Function arguments depend on the corresponding actual argument values
in the dynamic callers of their functions.
- :ref:`Call <i_call>` instructions depend on the :ref:`ret <i_ret>`
instructions that dynamically transfer control back to them.
- :ref:`Invoke <i_invoke>` instructions depend on the
:ref:`ret <i_ret>`, :ref:`resume <i_resume>`, or exception-throwing
call instructions that dynamically transfer control back to them.
- Non-volatile loads and stores depend on the most recent stores to all
of the referenced memory addresses, following the order in the IR
(including loads and stores implied by intrinsics such as
:ref:`@llvm.memcpy <int_memcpy>`.)
- An instruction with externally visible side effects depends on the
most recent preceding instruction with externally visible side
effects, following the order in the IR. (This includes :ref:`volatile
operations <volatile>`.)
- An instruction *control-depends* on a :ref:`terminator
instruction <terminators>` if the terminator instruction has
multiple successors and the instruction is always executed when
control transfers to one of the successors, and may not be executed
when control is transferred to another.
- Additionally, an instruction also *control-depends* on a terminator
instruction if the set of instructions it otherwise depends on would
be different if the terminator had transferred control to a different
successor.
- Dependence is transitive.
Poison Values have the same behavior as :ref:`undef values <undefvalues>`,
with the additional affect that any instruction which has a *dependence*
on a poison value has undefined behavior.
Here are some examples:
.. code-block:: llvm
entry:
%poison = sub nuw i32 0, 1 ; Results in a poison value.
%still_poison = and i32 %poison, 0 ; 0, but also poison.
%poison_yet_again = getelementptr i32* @h, i32 %still_poison
store i32 0, i32* %poison_yet_again ; memory at @h[0] is poisoned
store i32 %poison, i32* @g ; Poison value stored to memory.
%poison2 = load i32* @g ; Poison value loaded back from memory.
store volatile i32 %poison, i32* @g ; External observation; undefined behavior.
%narrowaddr = bitcast i32* @g to i16*
%wideaddr = bitcast i32* @g to i64*
%poison3 = load i16* %narrowaddr ; Returns a poison value.
%poison4 = load i64* %wideaddr ; Returns a poison value.
%cmp = icmp slt i32 %poison, 0 ; Returns a poison value.
br i1 %cmp, label %true, label %end ; Branch to either destination.
true:
store volatile i32 0, i32* @g ; This is control-dependent on %cmp, so
; it has undefined behavior.
br label %end
end:
%p = phi i32 [ 0, %entry ], [ 1, %true ]
; Both edges into this PHI are
; control-dependent on %cmp, so this
; always results in a poison value.
store volatile i32 0, i32* @g ; This would depend on the store in %true
; if %cmp is true, or the store in %entry
; otherwise, so this is undefined behavior.
br i1 %cmp, label %second_true, label %second_end
; The same branch again, but this time the
; true block doesn't have side effects.
second_true:
; No side effects!
ret void
second_end:
store volatile i32 0, i32* @g ; This time, the instruction always depends
; on the store in %end. Also, it is
; control-equivalent to %end, so this is
; well-defined (ignoring earlier undefined
; behavior in this example).
.. _blockaddress:
Addresses of Basic Blocks
-------------------------
``blockaddress(@function, %block)``
The '``blockaddress``' constant computes the address of the specified
basic block in the specified function, and always has an ``i8*`` type.
Taking the address of the entry block is illegal.
This value only has defined behavior when used as an operand to the
':ref:`indirectbr <i_indirectbr>`' instruction, or for comparisons
against null. Pointer equality tests between labels addresses results in
undefined behavior --- though, again, comparison against null is ok, and
no label is equal to the null pointer. This may be passed around as an
opaque pointer sized value as long as the bits are not inspected. This
allows ``ptrtoint`` and arithmetic to be performed on these values so
long as the original value is reconstituted before the ``indirectbr``
instruction.
Finally, some targets may provide defined semantics when using the value
as the operand to an inline assembly, but that is target specific.
.. _constantexprs:
Constant Expressions
--------------------
Constant expressions are used to allow expressions involving other
constants to be used as constants. Constant expressions may be of any
:ref:`first class <t_firstclass>` type and may involve any LLVM operation
that does not have side effects (e.g. load and call are not supported).
The following is the syntax for constant expressions:
``trunc (CST to TYPE)``
Truncate a constant to another type. The bit size of CST must be
larger than the bit size of TYPE. Both types must be integers.
``zext (CST to TYPE)``
Zero extend a constant to another type. The bit size of CST must be
smaller than the bit size of TYPE. Both types must be integers.
``sext (CST to TYPE)``
Sign extend a constant to another type. The bit size of CST must be
smaller than the bit size of TYPE. Both types must be integers.
``fptrunc (CST to TYPE)``
Truncate a floating point constant to another floating point type.
The size of CST must be larger than the size of TYPE. Both types
must be floating point.
``fpext (CST to TYPE)``
Floating point extend a constant to another type. The size of CST
must be smaller or equal to the size of TYPE. Both types must be
floating point.
``fptoui (CST to TYPE)``
Convert a floating point constant to the corresponding unsigned
integer constant. TYPE must be a scalar or vector integer type. CST
must be of scalar or vector floating point type. Both CST and TYPE
must be scalars, or vectors of the same number of elements. If the
value won't fit in the integer type, the results are undefined.
``fptosi (CST to TYPE)``
Convert a floating point constant to the corresponding signed
integer constant. TYPE must be a scalar or vector integer type. CST
must be of scalar or vector floating point type. Both CST and TYPE
must be scalars, or vectors of the same number of elements. If the
value won't fit in the integer type, the results are undefined.
``uitofp (CST to TYPE)``
Convert an unsigned integer constant to the corresponding floating
point constant. TYPE must be a scalar or vector floating point type.
CST must be of scalar or vector integer type. Both CST and TYPE must
be scalars, or vectors of the same number of elements. If the value
won't fit in the floating point type, the results are undefined.
``sitofp (CST to TYPE)``
Convert a signed integer constant to the corresponding floating
point constant. TYPE must be a scalar or vector floating point type.
CST must be of scalar or vector integer type. Both CST and TYPE must
be scalars, or vectors of the same number of elements. If the value
won't fit in the floating point type, the results are undefined.
``ptrtoint (CST to TYPE)``
Convert a pointer typed constant to the corresponding integer
constant. ``TYPE`` must be an integer type. ``CST`` must be of
pointer type. The ``CST`` value is zero extended, truncated, or
unchanged to make it fit in ``TYPE``.
``inttoptr (CST to TYPE)``
Convert an integer constant to a pointer constant. TYPE must be a
pointer type. CST must be of integer type. The CST value is zero
extended, truncated, or unchanged to make it fit in a pointer size.
This one is *really* dangerous!
``bitcast (CST to TYPE)``
Convert a constant, CST, to another TYPE. The constraints of the
operands are the same as those for the :ref:`bitcast
instruction <i_bitcast>`.
``addrspacecast (CST to TYPE)``
Convert a constant pointer or constant vector of pointer, CST, to another
TYPE in a different address space. The constraints of the operands are the
same as those for the :ref:`addrspacecast instruction <i_addrspacecast>`.
``getelementptr (CSTPTR, IDX0, IDX1, ...)``, ``getelementptr inbounds (CSTPTR, IDX0, IDX1, ...)``
Perform the :ref:`getelementptr operation <i_getelementptr>` on
constants. As with the :ref:`getelementptr <i_getelementptr>`
instruction, the index list may have zero or more indexes, which are
required to make sense for the type of "CSTPTR".
``select (COND, VAL1, VAL2)``
Perform the :ref:`select operation <i_select>` on constants.
``icmp COND (VAL1, VAL2)``
Performs the :ref:`icmp operation <i_icmp>` on constants.
``fcmp COND (VAL1, VAL2)``
Performs the :ref:`fcmp operation <i_fcmp>` on constants.
``extractelement (VAL, IDX)``
Perform the :ref:`extractelement operation <i_extractelement>` on
constants.
``insertelement (VAL, ELT, IDX)``
Perform the :ref:`insertelement operation <i_insertelement>` on
constants.
``shufflevector (VEC1, VEC2, IDXMASK)``
Perform the :ref:`shufflevector operation <i_shufflevector>` on
constants.
``extractvalue (VAL, IDX0, IDX1, ...)``
Perform the :ref:`extractvalue operation <i_extractvalue>` on
constants. The index list is interpreted in a similar manner as
indices in a ':ref:`getelementptr <i_getelementptr>`' operation. At
least one index value must be specified.
``insertvalue (VAL, ELT, IDX0, IDX1, ...)``
Perform the :ref:`insertvalue operation <i_insertvalue>` on constants.
The index list is interpreted in a similar manner as indices in a
':ref:`getelementptr <i_getelementptr>`' operation. At least one index
value must be specified.
``OPCODE (LHS, RHS)``
Perform the specified operation of the LHS and RHS constants. OPCODE
may be any of the :ref:`binary <binaryops>` or :ref:`bitwise
binary <bitwiseops>` operations. The constraints on operands are
the same as those for the corresponding instruction (e.g. no bitwise
operations on floating point values are allowed).
Other Values
============
.. _inlineasmexprs:
Inline Assembler Expressions
----------------------------
LLVM supports inline assembler expressions (as opposed to :ref:`Module-Level
Inline Assembly <moduleasm>`) through the use of a special value. This
value represents the inline assembler as a string (containing the
instructions to emit), a list of operand constraints (stored as a
string), a flag that indicates whether or not the inline asm expression
has side effects, and a flag indicating whether the function containing
the asm needs to align its stack conservatively. An example inline
assembler expression is:
.. code-block:: llvm
i32 (i32) asm "bswap $0", "=r,r"
Inline assembler expressions may **only** be used as the callee operand
of a :ref:`call <i_call>` or an :ref:`invoke <i_invoke>` instruction.
Thus, typically we have:
.. code-block:: llvm
%X = call i32 asm "bswap $0", "=r,r"(i32 %Y)
Inline asms with side effects not visible in the constraint list must be
marked as having side effects. This is done through the use of the
'``sideeffect``' keyword, like so:
.. code-block:: llvm
call void asm sideeffect "eieio", ""()
In some cases inline asms will contain code that will not work unless
the stack is aligned in some way, such as calls or SSE instructions on
x86, yet will not contain code that does that alignment within the asm.
The compiler should make conservative assumptions about what the asm
might contain and should generate its usual stack alignment code in the
prologue if the '``alignstack``' keyword is present:
.. code-block:: llvm
call void asm alignstack "eieio", ""()
Inline asms also support using non-standard assembly dialects. The
assumed dialect is ATT. When the '``inteldialect``' keyword is present,
the inline asm is using the Intel dialect. Currently, ATT and Intel are
the only supported dialects. An example is:
.. code-block:: llvm
call void asm inteldialect "eieio", ""()
If multiple keywords appear the '``sideeffect``' keyword must come
first, the '``alignstack``' keyword second and the '``inteldialect``'
keyword last.
Inline Asm Metadata
^^^^^^^^^^^^^^^^^^^
The call instructions that wrap inline asm nodes may have a
"``!srcloc``" MDNode attached to it that contains a list of constant
integers. If present, the code generator will use the integer as the
location cookie value when report errors through the ``LLVMContext``
error reporting mechanisms. This allows a front-end to correlate backend
errors that occur with inline asm back to the source code that produced
it. For example:
.. code-block:: llvm
call void asm sideeffect "something bad", ""(), !srcloc !42
...
!42 = !{ i32 1234567 }
It is up to the front-end to make sense of the magic numbers it places
in the IR. If the MDNode contains multiple constants, the code generator
will use the one that corresponds to the line of the asm that the error
occurs on.
.. _metadata:
Metadata Nodes and Metadata Strings
-----------------------------------
LLVM IR allows metadata to be attached to instructions in the program
that can convey extra information about the code to the optimizers and
code generator. One example application of metadata is source-level
debug information. There are two metadata primitives: strings and nodes.
All metadata has the ``metadata`` type and is identified in syntax by a
preceding exclamation point ('``!``').
A metadata string is a string surrounded by double quotes. It can
contain any character by escaping non-printable characters with
"``\xx``" where "``xx``" is the two digit hex code. For example:
"``!"test\00"``".
Metadata nodes are represented with notation similar to structure
constants (a comma separated list of elements, surrounded by braces and
preceded by an exclamation point). Metadata nodes can have any values as
their operand. For example:
.. code-block:: llvm
!{ metadata !"test\00", i32 10}
A :ref:`named metadata <namedmetadatastructure>` is a collection of
metadata nodes, which can be looked up in the module symbol table. For
example:
.. code-block:: llvm
!foo = metadata !{!4, !3}
Metadata can be used as function arguments. Here ``llvm.dbg.value``
function is using two metadata arguments:
.. code-block:: llvm
call void @llvm.dbg.value(metadata !24, i64 0, metadata !25)
Metadata can be attached with an instruction. Here metadata ``!21`` is
attached to the ``add`` instruction using the ``!dbg`` identifier:
.. code-block:: llvm
%indvar.next = add i64 %indvar, 1, !dbg !21
More information about specific metadata nodes recognized by the
optimizers and code generator is found below.
'``tbaa``' Metadata
^^^^^^^^^^^^^^^^^^^
In LLVM IR, memory does not have types, so LLVM's own type system is not
suitable for doing TBAA. Instead, metadata is added to the IR to
describe a type system of a higher level language. This can be used to
implement typical C/C++ TBAA, but it can also be used to implement
custom alias analysis behavior for other languages.
The current metadata format is very simple. TBAA metadata nodes have up
to three fields, e.g.:
.. code-block:: llvm
!0 = metadata !{ metadata !"an example type tree" }
!1 = metadata !{ metadata !"int", metadata !0 }
!2 = metadata !{ metadata !"float", metadata !0 }
!3 = metadata !{ metadata !"const float", metadata !2, i64 1 }
The first field is an identity field. It can be any value, usually a
metadata string, which uniquely identifies the type. The most important
name in the tree is the name of the root node. Two trees with different
root node names are entirely disjoint, even if they have leaves with
common names.
The second field identifies the type's parent node in the tree, or is
null or omitted for a root node. A type is considered to alias all of
its descendants and all of its ancestors in the tree. Also, a type is
considered to alias all types in other trees, so that bitcode produced
from multiple front-ends is handled conservatively.
If the third field is present, it's an integer which if equal to 1
indicates that the type is "constant" (meaning
``pointsToConstantMemory`` should return true; see `other useful
AliasAnalysis methods <AliasAnalysis.html#OtherItfs>`_).
'``tbaa.struct``' Metadata
^^^^^^^^^^^^^^^^^^^^^^^^^^
The :ref:`llvm.memcpy <int_memcpy>` is often used to implement
aggregate assignment operations in C and similar languages, however it
is defined to copy a contiguous region of memory, which is more than
strictly necessary for aggregate types which contain holes due to
padding. Also, it doesn't contain any TBAA information about the fields
of the aggregate.
``!tbaa.struct`` metadata can describe which memory subregions in a
memcpy are padding and what the TBAA tags of the struct are.
The current metadata format is very simple. ``!tbaa.struct`` metadata
nodes are a list of operands which are in conceptual groups of three.
For each group of three, the first operand gives the byte offset of a
field in bytes, the second gives its size in bytes, and the third gives
its tbaa tag. e.g.:
.. code-block:: llvm
!4 = metadata !{ i64 0, i64 4, metadata !1, i64 8, i64 4, metadata !2 }
This describes a struct with two fields. The first is at offset 0 bytes
with size 4 bytes, and has tbaa tag !1. The second is at offset 8 bytes
and has size 4 bytes and has tbaa tag !2.
Note that the fields need not be contiguous. In this example, there is a
4 byte gap between the two fields. This gap represents padding which
does not carry useful data and need not be preserved.
'``fpmath``' Metadata
^^^^^^^^^^^^^^^^^^^^^
``fpmath`` metadata may be attached to any instruction of floating point
type. It can be used to express the maximum acceptable error in the
result of that instruction, in ULPs, thus potentially allowing the
compiler to use a more efficient but less accurate method of computing
it. ULP is defined as follows:
If ``x`` is a real number that lies between two finite consecutive
floating-point numbers ``a`` and ``b``, without being equal to one
of them, then ``ulp(x) = |b - a|``, otherwise ``ulp(x)`` is the
distance between the two non-equal finite floating-point numbers
nearest ``x``. Moreover, ``ulp(NaN)`` is ``NaN``.
The metadata node shall consist of a single positive floating point
number representing the maximum relative error, for example:
.. code-block:: llvm
!0 = metadata !{ float 2.5 } ; maximum acceptable inaccuracy is 2.5 ULPs
'``range``' Metadata
^^^^^^^^^^^^^^^^^^^^
``range`` metadata may be attached only to loads of integer types. It
expresses the possible ranges the loaded value is in. The ranges are
represented with a flattened list of integers. The loaded value is known
to be in the union of the ranges defined by each consecutive pair. Each
pair has the following properties:
- The type must match the type loaded by the instruction.
- The pair ``a,b`` represents the range ``[a,b)``.
- Both ``a`` and ``b`` are constants.
- The range is allowed to wrap.
- The range should not represent the full or empty set. That is,
``a!=b``.
In addition, the pairs must be in signed order of the lower bound and
they must be non-contiguous.
Examples:
.. code-block:: llvm
%a = load i8* %x, align 1, !range !0 ; Can only be 0 or 1
%b = load i8* %y, align 1, !range !1 ; Can only be 255 (-1), 0 or 1
%c = load i8* %z, align 1, !range !2 ; Can only be 0, 1, 3, 4 or 5
%d = load i8* %z, align 1, !range !3 ; Can only be -2, -1, 3, 4 or 5
...
!0 = metadata !{ i8 0, i8 2 }
!1 = metadata !{ i8 255, i8 2 }
!2 = metadata !{ i8 0, i8 2, i8 3, i8 6 }
!3 = metadata !{ i8 -2, i8 0, i8 3, i8 6 }
'``llvm.loop``'
^^^^^^^^^^^^^^^
It is sometimes useful to attach information to loop constructs. Currently,
loop metadata is implemented as metadata attached to the branch instruction
in the loop latch block. This type of metadata refer to a metadata node that is
guaranteed to be separate for each loop. The loop identifier metadata is
specified with the name ``llvm.loop``.
The loop identifier metadata is implemented using a metadata that refers to
itself to avoid merging it with any other identifier metadata, e.g.,
during module linkage or function inlining. That is, each loop should refer
to their own identification metadata even if they reside in separate functions.
The following example contains loop identifier metadata for two separate loop
constructs:
.. code-block:: llvm
!0 = metadata !{ metadata !0 }
!1 = metadata !{ metadata !1 }
The loop identifier metadata can be used to specify additional per-loop
metadata. Any operands after the first operand can be treated as user-defined
metadata. For example the ``llvm.vectorizer.unroll`` metadata is understood
by the loop vectorizer to indicate how many times to unroll the loop:
.. code-block:: llvm
br i1 %exitcond, label %._crit_edge, label %.lr.ph, !llvm.loop !0
...
!0 = metadata !{ metadata !0, metadata !1 }
!1 = metadata !{ metadata !"llvm.vectorizer.unroll", i32 2 }
'``llvm.mem``'
^^^^^^^^^^^^^^^
Metadata types used to annotate memory accesses with information helpful
for optimizations are prefixed with ``llvm.mem``.
'``llvm.mem.parallel_loop_access``' Metadata
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
For a loop to be parallel, in addition to using
the ``llvm.loop`` metadata to mark the loop latch branch instruction,
also all of the memory accessing instructions in the loop body need to be
marked with the ``llvm.mem.parallel_loop_access`` metadata. If there
is at least one memory accessing instruction not marked with the metadata,
the loop must be considered a sequential loop. This causes parallel loops to be
converted to sequential loops due to optimization passes that are unaware of
the parallel semantics and that insert new memory instructions to the loop
body.
Example of a loop that is considered parallel due to its correct use of
both ``llvm.loop`` and ``llvm.mem.parallel_loop_access``
metadata types that refer to the same loop identifier metadata.
.. code-block:: llvm
for.body:
...
%0 = load i32* %arrayidx, align 4, !llvm.mem.parallel_loop_access !0
...
store i32 %0, i32* %arrayidx4, align 4, !llvm.mem.parallel_loop_access !0
...
br i1 %exitcond, label %for.end, label %for.body, !llvm.loop !0
for.end:
...
!0 = metadata !{ metadata !0 }
It is also possible to have nested parallel loops. In that case the
memory accesses refer to a list of loop identifier metadata nodes instead of
the loop identifier metadata node directly:
.. code-block:: llvm
outer.for.body:
...
inner.for.body:
...
%0 = load i32* %arrayidx, align 4, !llvm.mem.parallel_loop_access !0
...
store i32 %0, i32* %arrayidx4, align 4, !llvm.mem.parallel_loop_access !0
...
br i1 %exitcond, label %inner.for.end, label %inner.for.body, !llvm.loop !1
inner.for.end:
...
%0 = load i32* %arrayidx, align 4, !llvm.mem.parallel_loop_access !0
...
store i32 %0, i32* %arrayidx4, align 4, !llvm.mem.parallel_loop_access !0
...
br i1 %exitcond, label %outer.for.end, label %outer.for.body, !llvm.loop !2
outer.for.end: ; preds = %for.body
...
!0 = metadata !{ metadata !1, metadata !2 } ; a list of loop identifiers
!1 = metadata !{ metadata !1 } ; an identifier for the inner loop
!2 = metadata !{ metadata !2 } ; an identifier for the outer loop
'``llvm.vectorizer``'
^^^^^^^^^^^^^^^^^^^^^
Metadata prefixed with ``llvm.vectorizer`` is used to control per-loop
vectorization parameters such as vectorization factor and unroll factor.
``llvm.vectorizer`` metadata should be used in conjunction with ``llvm.loop``
loop identification metadata.
'``llvm.vectorizer.unroll``' Metadata
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
This metadata instructs the loop vectorizer to unroll the specified
loop exactly ``N`` times.
The first operand is the string ``llvm.vectorizer.unroll`` and the second
operand is an integer specifying the unroll factor. For example:
.. code-block:: llvm
!0 = metadata !{ metadata !"llvm.vectorizer.unroll", i32 4 }
Note that setting ``llvm.vectorizer.unroll`` to 1 disables unrolling of the
loop.
If ``llvm.vectorizer.unroll`` is set to 0 then the amount of unrolling will be
determined automatically.
'``llvm.vectorizer.width``' Metadata
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
This metadata sets the target width of the vectorizer to ``N``. Without
this metadata, the vectorizer will choose a width automatically.
Regardless of this metadata, the vectorizer will only vectorize loops if
it believes it is valid to do so.
The first operand is the string ``llvm.vectorizer.width`` and the second
operand is an integer specifying the width. For example:
.. code-block:: llvm
!0 = metadata !{ metadata !"llvm.vectorizer.width", i32 4 }
Note that setting ``llvm.vectorizer.width`` to 1 disables vectorization of the
loop.
If ``llvm.vectorizer.width`` is set to 0 then the width will be determined
automatically.
Module Flags Metadata
=====================
Information about the module as a whole is difficult to convey to LLVM's
subsystems. The LLVM IR isn't sufficient to transmit this information.
The ``llvm.module.flags`` named metadata exists in order to facilitate
this. These flags are in the form of key / value pairs --- much like a
dictionary --- making it easy for any subsystem who cares about a flag to
look it up.
The ``llvm.module.flags`` metadata contains a list of metadata triplets.
Each triplet has the following form:
- The first element is a *behavior* flag, which specifies the behavior
when two (or more) modules are merged together, and it encounters two
(or more) metadata with the same ID. The supported behaviors are
described below.
- The second element is a metadata string that is a unique ID for the
metadata. Each module may only have one flag entry for each unique ID (not
including entries with the **Require** behavior).
- The third element is the value of the flag.
When two (or more) modules are merged together, the resulting
``llvm.module.flags`` metadata is the union of the modules' flags. That is, for
each unique metadata ID string, there will be exactly one entry in the merged
modules ``llvm.module.flags`` metadata table, and the value for that entry will
be determined by the merge behavior flag, as described below. The only exception
is that entries with the *Require* behavior are always preserved.
The following behaviors are supported:
.. list-table::
:header-rows: 1
:widths: 10 90
* - Value
- Behavior
* - 1
- **Error**
Emits an error if two values disagree, otherwise the resulting value
is that of the operands.
* - 2
- **Warning**
Emits a warning if two values disagree. The result value will be the
operand for the flag from the first module being linked.
* - 3
- **Require**
Adds a requirement that another module flag be present and have a
specified value after linking is performed. The value must be a
metadata pair, where the first element of the pair is the ID of the
module flag to be restricted, and the second element of the pair is
the value the module flag should be restricted to. This behavior can
be used to restrict the allowable results (via triggering of an
error) of linking IDs with the **Override** behavior.
* - 4
- **Override**
Uses the specified value, regardless of the behavior or value of the
other module. If both modules specify **Override**, but the values
differ, an error will be emitted.
* - 5
- **Append**
Appends the two values, which are required to be metadata nodes.
* - 6
- **AppendUnique**
Appends the two values, which are required to be metadata
nodes. However, duplicate entries in the second list are dropped
during the append operation.
It is an error for a particular unique flag ID to have multiple behaviors,
except in the case of **Require** (which adds restrictions on another metadata
value) or **Override**.
An example of module flags:
.. code-block:: llvm
!0 = metadata !{ i32 1, metadata !"foo", i32 1 }
!1 = metadata !{ i32 4, metadata !"bar", i32 37 }
!2 = metadata !{ i32 2, metadata !"qux", i32 42 }
!3 = metadata !{ i32 3, metadata !"qux",
metadata !{
metadata !"foo", i32 1
}
}
!llvm.module.flags = !{ !0, !1, !2, !3 }
- Metadata ``!0`` has the ID ``!"foo"`` and the value '1'. The behavior
if two or more ``!"foo"`` flags are seen is to emit an error if their
values are not equal.
- Metadata ``!1`` has the ID ``!"bar"`` and the value '37'. The
behavior if two or more ``!"bar"`` flags are seen is to use the value
'37'.
- Metadata ``!2`` has the ID ``!"qux"`` and the value '42'. The
behavior if two or more ``!"qux"`` flags are seen is to emit a
warning if their values are not equal.
- Metadata ``!3`` has the ID ``!"qux"`` and the value:
::
metadata !{ metadata !"foo", i32 1 }
The behavior is to emit an error if the ``llvm.module.flags`` does not
contain a flag with the ID ``!"foo"`` that has the value '1' after linking is
performed.
Objective-C Garbage Collection Module Flags Metadata
----------------------------------------------------
On the Mach-O platform, Objective-C stores metadata about garbage
collection in a special section called "image info". The metadata
consists of a version number and a bitmask specifying what types of
garbage collection are supported (if any) by the file. If two or more
modules are linked together their garbage collection metadata needs to
be merged rather than appended together.
The Objective-C garbage collection module flags metadata consists of the
following key-value pairs:
.. list-table::
:header-rows: 1
:widths: 30 70
* - Key
- Value
* - ``Objective-C Version``
- **[Required]** --- The Objective-C ABI version. Valid values are 1 and 2.
* - ``Objective-C Image Info Version``
- **[Required]** --- The version of the image info section. Currently
always 0.
* - ``Objective-C Image Info Section``
- **[Required]** --- The section to place the metadata. Valid values are
``"__OBJC, __image_info, regular"`` for Objective-C ABI version 1, and
``"__DATA,__objc_imageinfo, regular, no_dead_strip"`` for
Objective-C ABI version 2.
* - ``Objective-C Garbage Collection``
- **[Required]** --- Specifies whether garbage collection is supported or
not. Valid values are 0, for no garbage collection, and 2, for garbage
collection supported.
* - ``Objective-C GC Only``
- **[Optional]** --- Specifies that only garbage collection is supported.
If present, its value must be 6. This flag requires that the
``Objective-C Garbage Collection`` flag have the value 2.
Some important flag interactions:
- If a module with ``Objective-C Garbage Collection`` set to 0 is
merged with a module with ``Objective-C Garbage Collection`` set to
2, then the resulting module has the
``Objective-C Garbage Collection`` flag set to 0.
- A module with ``Objective-C Garbage Collection`` set to 0 cannot be
merged with a module with ``Objective-C GC Only`` set to 6.
Automatic Linker Flags Module Flags Metadata
--------------------------------------------
Some targets support embedding flags to the linker inside individual object
files. Typically this is used in conjunction with language extensions which
allow source files to explicitly declare the libraries they depend on, and have
these automatically be transmitted to the linker via object files.
These flags are encoded in the IR using metadata in the module flags section,
using the ``Linker Options`` key. The merge behavior for this flag is required
to be ``AppendUnique``, and the value for the key is expected to be a metadata
node which should be a list of other metadata nodes, each of which should be a
list of metadata strings defining linker options.
For example, the following metadata section specifies two separate sets of
linker options, presumably to link against ``libz`` and the ``Cocoa``
framework::
!0 = metadata !{ i32 6, metadata !"Linker Options",
metadata !{
metadata !{ metadata !"-lz" },
metadata !{ metadata !"-framework", metadata !"Cocoa" } } }
!llvm.module.flags = !{ !0 }
The metadata encoding as lists of lists of options, as opposed to a collapsed
list of options, is chosen so that the IR encoding can use multiple option
strings to specify e.g., a single library, while still having that specifier be
preserved as an atomic element that can be recognized by a target specific
assembly writer or object file emitter.
Each individual option is required to be either a valid option for the target's
linker, or an option that is reserved by the target specific assembly writer or
object file emitter. No other aspect of these options is defined by the IR.
.. _intrinsicglobalvariables:
Intrinsic Global Variables
==========================
LLVM has a number of "magic" global variables that contain data that
affect code generation or other IR semantics. These are documented here.
All globals of this sort should have a section specified as
"``llvm.metadata``". This section and all globals that start with
"``llvm.``" are reserved for use by LLVM.
.. _gv_llvmused:
The '``llvm.used``' Global Variable
-----------------------------------
The ``@llvm.used`` global is an array which has
:ref:`appending linkage <linkage_appending>`. This array contains a list of
pointers to named global variables, functions and aliases which may optionally
have a pointer cast formed of bitcast or getelementptr. For example, a legal
use of it is:
.. code-block:: llvm
@X = global i8 4
@Y = global i32 123
@llvm.used = appending global [2 x i8*] [
i8* @X,
i8* bitcast (i32* @Y to i8*)
], section "llvm.metadata"
If a symbol appears in the ``@llvm.used`` list, then the compiler, assembler,
and linker are required to treat the symbol as if there is a reference to the
symbol that it cannot see (which is why they have to be named). For example, if
a variable has internal linkage and no references other than that from the
``@llvm.used`` list, it cannot be deleted. This is commonly used to represent
references from inline asms and other things the compiler cannot "see", and
corresponds to "``attribute((used))``" in GNU C.
On some targets, the code generator must emit a directive to the
assembler or object file to prevent the assembler and linker from
molesting the symbol.
.. _gv_llvmcompilerused:
The '``llvm.compiler.used``' Global Variable
--------------------------------------------
The ``@llvm.compiler.used`` directive is the same as the ``@llvm.used``
directive, except that it only prevents the compiler from touching the
symbol. On targets that support it, this allows an intelligent linker to
optimize references to the symbol without being impeded as it would be
by ``@llvm.used``.
This is a rare construct that should only be used in rare circumstances,
and should not be exposed to source languages.
.. _gv_llvmglobalctors:
The '``llvm.global_ctors``' Global Variable
-------------------------------------------
.. code-block:: llvm
%0 = type { i32, void ()* }
@llvm.global_ctors = appending global [1 x %0] [%0 { i32 65535, void ()* @ctor }]
The ``@llvm.global_ctors`` array contains a list of constructor
functions and associated priorities. The functions referenced by this
array will be called in ascending order of priority (i.e. lowest first)
when the module is loaded. The order of functions with the same priority
is not defined.
.. _llvmglobaldtors:
The '``llvm.global_dtors``' Global Variable
-------------------------------------------
.. code-block:: llvm
%0 = type { i32, void ()* }
@llvm.global_dtors = appending global [1 x %0] [%0 { i32 65535, void ()* @dtor }]
The ``@llvm.global_dtors`` array contains a list of destructor functions
and associated priorities. The functions referenced by this array will
be called in descending order of priority (i.e. highest first) when the
module is loaded. The order of functions with the same priority is not
defined.
Instruction Reference
=====================
The LLVM instruction set consists of several different classifications
of instructions: :ref:`terminator instructions <terminators>`, :ref:`binary
instructions <binaryops>`, :ref:`bitwise binary
instructions <bitwiseops>`, :ref:`memory instructions <memoryops>`, and
:ref:`other instructions <otherops>`.
.. _terminators:
Terminator Instructions
-----------------------
As mentioned :ref:`previously <functionstructure>`, every basic block in a
program ends with a "Terminator" instruction, which indicates which
block should be executed after the current block is finished. These
terminator instructions typically yield a '``void``' value: they produce
control flow, not values (the one exception being the
':ref:`invoke <i_invoke>`' instruction).
The terminator instructions are: ':ref:`ret <i_ret>`',
':ref:`br <i_br>`', ':ref:`switch <i_switch>`',
':ref:`indirectbr <i_indirectbr>`', ':ref:`invoke <i_invoke>`',
':ref:`resume <i_resume>`', and ':ref:`unreachable <i_unreachable>`'.
.. _i_ret:
'``ret``' Instruction
^^^^^^^^^^^^^^^^^^^^^
Syntax:
"""""""
::
ret <type> <value> ; Return a value from a non-void function
ret void ; Return from void function
Overview:
"""""""""
The '``ret``' instruction is used to return control flow (and optionally
a value) from a function back to the caller.
There are two forms of the '``ret``' instruction: one that returns a
value and then causes control flow, and one that just causes control
flow to occur.
Arguments:
""""""""""
The '``ret``' instruction optionally accepts a single argument, the
return value. The type of the return value must be a ':ref:`first
class <t_firstclass>`' type.
A function is not :ref:`well formed <wellformed>` if it it has a non-void
return type and contains a '``ret``' instruction with no return value or
a return value with a type that does not match its type, or if it has a
void return type and contains a '``ret``' instruction with a return
value.
Semantics:
""""""""""
When the '``ret``' instruction is executed, control flow returns back to
the calling function's context. If the caller is a
":ref:`call <i_call>`" instruction, execution continues at the
instruction after the call. If the caller was an
":ref:`invoke <i_invoke>`" instruction, execution continues at the
beginning of the "normal" destination block. If the instruction returns
a value, that value shall set the call or invoke instruction's return
value.
Example:
""""""""
.. code-block:: llvm
ret i32 5 ; Return an integer value of 5
ret void ; Return from a void function
ret { i32, i8 } { i32 4, i8 2 } ; Return a struct of values 4 and 2
.. _i_br:
'``br``' Instruction
^^^^^^^^^^^^^^^^^^^^
Syntax:
"""""""
::
br i1 <cond>, label <iftrue>, label <iffalse>
br label <dest> ; Unconditional branch
Overview:
"""""""""
The '``br``' instruction is used to cause control flow to transfer to a
different basic block in the current function. There are two forms of
this instruction, corresponding to a conditional branch and an
unconditional branch.
Arguments:
""""""""""
The conditional branch form of the '``br``' instruction takes a single
'``i1``' value and two '``label``' values. The unconditional form of the
'``br``' instruction takes a single '``label``' value as a target.
Semantics:
""""""""""
Upon execution of a conditional '``br``' instruction, the '``i1``'
argument is evaluated. If the value is ``true``, control flows to the
'``iftrue``' ``label`` argument. If "cond" is ``false``, control flows
to the '``iffalse``' ``label`` argument.
Example:
""""""""
.. code-block:: llvm
Test:
%cond = icmp eq i32 %a, %b
br i1 %cond, label %IfEqual, label %IfUnequal
IfEqual:
ret i32 1
IfUnequal:
ret i32 0
.. _i_switch:
'``switch``' Instruction
^^^^^^^^^^^^^^^^^^^^^^^^
Syntax:
"""""""
::
switch <intty> <value>, label <defaultdest> [ <intty> <val>, label <dest> ... ]
Overview:
"""""""""
The '``switch``' instruction is used to transfer control flow to one of
several different places. It is a generalization of the '``br``'
instruction, allowing a branch to occur to one of many possible
destinations.
Arguments:
""""""""""
The '``switch``' instruction uses three parameters: an integer
comparison value '``value``', a default '``label``' destination, and an
array of pairs of comparison value constants and '``label``'s. The table
is not allowed to contain duplicate constant entries.
Semantics:
""""""""""
The ``switch`` instruction specifies a table of values and destinations.
When the '``switch``' instruction is executed, this table is searched
for the given value. If the value is found, control flow is transferred
to the corresponding destination; otherwise, control flow is transferred
to the default destination.
Implementation:
"""""""""""""""
Depending on properties of the target machine and the particular
``switch`` instruction, this instruction may be code generated in
different ways. For example, it could be generated as a series of
chained conditional branches or with a lookup table.
Example:
""""""""
.. code-block:: llvm
; Emulate a conditional br instruction
%Val = zext i1 %value to i32
switch i32 %Val, label %truedest [ i32 0, label %falsedest ]
; Emulate an unconditional br instruction
switch i32 0, label %dest [ ]
; Implement a jump table:
switch i32 %val, label %otherwise [ i32 0, label %onzero
i32 1, label %onone
i32 2, label %ontwo ]
.. _i_indirectbr:
'``indirectbr``' Instruction
^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Syntax:
"""""""
::
indirectbr <somety>* <address>, [ label <dest1>, label <dest2>, ... ]
Overview:
"""""""""
The '``indirectbr``' instruction implements an indirect branch to a
label within the current function, whose address is specified by
"``address``". Address must be derived from a
:ref:`blockaddress <blockaddress>` constant.
Arguments:
""""""""""
The '``address``' argument is the address of the label to jump to. The
rest of the arguments indicate the full set of possible destinations
that the address may point to. Blocks are allowed to occur multiple
times in the destination list, though this isn't particularly useful.
This destination list is required so that dataflow analysis has an
accurate understanding of the CFG.
Semantics:
""""""""""
Control transfers to the block specified in the address argument. All
possible destination blocks must be listed in the label list, otherwise
this instruction has undefined behavior. This implies that jumps to
labels defined in other functions have undefined behavior as well.
Implementation:
"""""""""""""""
This is typically implemented with a jump through a register.
Example:
""""""""
.. code-block:: llvm
indirectbr i8* %Addr, [ label %bb1, label %bb2, label %bb3 ]
.. _i_invoke:
'``invoke``' Instruction
^^^^^^^^^^^^^^^^^^^^^^^^
Syntax:
"""""""
::
<result> = invoke [cconv] [ret attrs] <ptr to function ty> <function ptr val>(<function args>) [fn attrs]
to label <normal label> unwind label <exception label>
Overview:
"""""""""
The '``invoke``' instruction causes control to transfer to a specified
function, with the possibility of control flow transfer to either the
'``normal``' label or the '``exception``' label. If the callee function
returns with the "``ret``" instruction, control flow will return to the
"normal" label. If the callee (or any indirect callees) returns via the
":ref:`resume <i_resume>`" instruction or other exception handling
mechanism, control is interrupted and continued at the dynamically
nearest "exception" label.
The '``exception``' label is a `landing
pad <ExceptionHandling.html#overview>`_ for the exception. As such,
'``exception``' label is required to have the
":ref:`landingpad <i_landingpad>`" instruction, which contains the
information about the behavior of the program after unwinding happens,
as its first non-PHI instruction. The restrictions on the
"``landingpad``" instruction's tightly couples it to the "``invoke``"
instruction, so that the important information contained within the
"``landingpad``" instruction can't be lost through normal code motion.
Arguments:
""""""""""
This instruction requires several arguments:
#. The optional "cconv" marker indicates which :ref:`calling
convention <callingconv>` the call should use. If none is
specified, the call defaults to using C calling conventions.
#. The optional :ref:`Parameter Attributes <paramattrs>` list for return
values. Only '``zeroext``', '``signext``', and '``inreg``' attributes
are valid here.
#. '``ptr to function ty``': shall be the signature of the pointer to
function value being invoked. In most cases, this is a direct
function invocation, but indirect ``invoke``'s are just as possible,
branching off an arbitrary pointer to function value.
#. '``function ptr val``': An LLVM value containing a pointer to a
function to be invoked.
#. '``function args``': argument list whose types match the function
signature argument types and parameter attributes. All arguments must
be of :ref:`first class <t_firstclass>` type. If the function signature
indicates the function accepts a variable number of arguments, the
extra arguments can be specified.
#. '``normal label``': the label reached when the called function
executes a '``ret``' instruction.
#. '``exception label``': the label reached when a callee returns via
the :ref:`resume <i_resume>` instruction or other exception handling
mechanism.
#. The optional :ref:`function attributes <fnattrs>` list. Only
'``noreturn``', '``nounwind``', '``readonly``' and '``readnone``'
attributes are valid here.
Semantics:
""""""""""
This instruction is designed to operate as a standard '``call``'
instruction in most regards. The primary difference is that it
establishes an association with a label, which is used by the runtime
library to unwind the stack.
This instruction is used in languages with destructors to ensure that
proper cleanup is performed in the case of either a ``longjmp`` or a
thrown exception. Additionally, this is important for implementation of
'``catch``' clauses in high-level languages that support them.
For the purposes of the SSA form, the definition of the value returned
by the '``invoke``' instruction is deemed to occur on the edge from the
current block to the "normal" label. If the callee unwinds then no
return value is available.
Example:
""""""""
.. code-block:: llvm
%retval = invoke i32 @Test(i32 15) to label %Continue
unwind label %TestCleanup ; {i32}:retval set
%retval = invoke coldcc i32 %Testfnptr(i32 15) to label %Continue
unwind label %TestCleanup ; {i32}:retval set
.. _i_resume:
'``resume``' Instruction
^^^^^^^^^^^^^^^^^^^^^^^^
Syntax:
"""""""
::
resume <type> <value>
Overview:
"""""""""
The '``resume``' instruction is a terminator instruction that has no
successors.
Arguments:
""""""""""
The '``resume``' instruction requires one argument, which must have the
same type as the result of any '``landingpad``' instruction in the same
function.
Semantics:
""""""""""
The '``resume``' instruction resumes propagation of an existing
(in-flight) exception whose unwinding was interrupted with a
:ref:`landingpad <i_landingpad>` instruction.
Example:
""""""""
.. code-block:: llvm
resume { i8*, i32 } %exn
.. _i_unreachable:
'``unreachable``' Instruction
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Syntax:
"""""""
::
unreachable
Overview:
"""""""""
The '``unreachable``' instruction has no defined semantics. This
instruction is used to inform the optimizer that a particular portion of
the code is not reachable. This can be used to indicate that the code
after a no-return function cannot be reached, and other facts.
Semantics:
""""""""""
The '``unreachable``' instruction has no defined semantics.
.. _binaryops:
Binary Operations
-----------------
Binary operators are used to do most of the computation in a program.
They require two operands of the same type, execute an operation on
them, and produce a single value. The operands might represent multiple
data, as is the case with the :ref:`vector <t_vector>` data type. The
result value has the same type as its operands.
There are several different binary operators:
.. _i_add:
'``add``' Instruction
^^^^^^^^^^^^^^^^^^^^^
Syntax:
"""""""
::
<result> = add <ty> <op1>, <op2> ; yields {ty}:result
<result> = add nuw <ty> <op1>, <op2> ; yields {ty}:result
<result> = add nsw <ty> <op1>, <op2> ; yields {ty}:result
<result> = add nuw nsw <ty> <op1>, <op2> ; yields {ty}:result
Overview:
"""""""""
The '``add``' instruction returns the sum of its two operands.
Arguments:
""""""""""
The two arguments to the '``add``' instruction must be
:ref:`integer <t_integer>` or :ref:`vector <t_vector>` of integer values. Both
arguments must have identical types.
Semantics:
""""""""""
The value produced is the integer sum of the two operands.
If the sum has unsigned overflow, the result returned is the
mathematical result modulo 2\ :sup:`n`\ , where n is the bit width of
the result.
Because LLVM integers use a two's complement representation, this
instruction is appropriate for both signed and unsigned integers.
``nuw`` and ``nsw`` stand for "No Unsigned Wrap" and "No Signed Wrap",
respectively. If the ``nuw`` and/or ``nsw`` keywords are present, the
result value of the ``add`` is a :ref:`poison value <poisonvalues>` if
unsigned and/or signed overflow, respectively, occurs.
Example:
""""""""
.. code-block:: llvm
<result> = add i32 4, %var ; yields {i32}:result = 4 + %var
.. _i_fadd:
'``fadd``' Instruction
^^^^^^^^^^^^^^^^^^^^^^
Syntax:
"""""""
::
<result> = fadd [fast-math flags]* <ty> <op1>, <op2> ; yields {ty}:result
Overview:
"""""""""
The '``fadd``' instruction returns the sum of its two operands.
Arguments:
""""""""""
The two arguments to the '``fadd``' instruction must be :ref:`floating
point <t_floating>` or :ref:`vector <t_vector>` of floating point values.
Both arguments must have identical types.
Semantics:
""""""""""
The value produced is the floating point sum of the two operands. This
instruction can also take any number of :ref:`fast-math flags <fastmath>`,
which are optimization hints to enable otherwise unsafe floating point
optimizations:
Example:
""""""""
.. code-block:: llvm
<result> = fadd float 4.0, %var ; yields {float}:result = 4.0 + %var
'``sub``' Instruction
^^^^^^^^^^^^^^^^^^^^^
Syntax:
"""""""