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==============================
LLVM Language Reference Manual
==============================
.. contents::
:local:
:depth: 4
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 that 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. The ``"\01"`` prefix
can be used on global values to suppress mangling.
#. 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 and unnamed
function parameters are included in this numbering. For example, if the
entry basic block is not given a label name and all function parameters are
named, 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], [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
!0 = !{i32 42, null, !"string"}
!foo = !{!0}
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 a 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.
``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. From the linker's
perspective, an ``available_externally`` global is equivalent to
an external declaration. 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 allow inlining and other optimizations. 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.
Unfortunately this doesn't correspond to any feature in .o files, so it
can only be used for variables like ``llvm.global_ctors`` which llvm
interprets specially.
``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.
It is illegal for a global variable or function *declaration* to have any
linkage type other than ``external`` 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 tailcc, 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. Furthermore the inliner doesn't consider such function
calls for inlining.
"``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).
"``webkit_jscc``" - WebKit's JavaScript calling convention
This calling convention has been implemented for `WebKit FTL JIT
<https://trac.webkit.org/wiki/FTLJIT>`_. It passes arguments on the
stack right to left (as cdecl does), and returns a value in the
platform's customary return register.
"``anyregcc``" - Dynamic calling convention for code patching
This is a special convention that supports patching an arbitrary code
sequence in place of a call site. This convention forces the call
arguments into registers but allows them to be dynamically
allocated. This can currently only be used with calls to
llvm.experimental.patchpoint because only this intrinsic records
the location of its arguments in a side table. See :doc:`StackMaps`.
"``preserve_mostcc``" - The `PreserveMost` calling convention
This calling convention attempts to make the code in the caller as
unintrusive as possible. This convention behaves identically to the `C`
calling convention on how arguments and return values are passed, but it
uses a different set of caller/callee-saved registers. This alleviates the
burden of saving and recovering a large register set before and after the
call in the caller. If the arguments are passed in callee-saved registers,
then they will be preserved by the callee across the call. This doesn't
apply for values returned in callee-saved registers.
- On X86-64 the callee preserves all general purpose registers, except for
R11. R11 can be used as a scratch register. Floating-point registers
(XMMs/YMMs) are not preserved and need to be saved by the caller.
The idea behind this convention is to support calls to runtime functions
that have a hot path and a cold path. The hot path is usually a small piece
of code that doesn't use many registers. The cold path might need to call out to
another function and therefore only needs to preserve the caller-saved
registers, which haven't already been saved by the caller. The
`PreserveMost` calling convention is very similar to the `cold` calling
convention in terms of caller/callee-saved registers, but they are used for
different types of function calls. `coldcc` is for function calls that are
rarely executed, whereas `preserve_mostcc` function calls are intended to be
on the hot path and definitely executed a lot. Furthermore `preserve_mostcc`
doesn't prevent the inliner from inlining the function call.
This calling convention will be used by a future version of the ObjectiveC
runtime and should therefore still be considered experimental at this time.
Although this convention was created to optimize certain runtime calls to
the ObjectiveC runtime, it is not limited to this runtime and might be used
by other runtimes in the future too. The current implementation only
supports X86-64, but the intention is to support more architectures in the
future.
"``preserve_allcc``" - The `PreserveAll` calling convention
This calling convention attempts to make the code in the caller even less
intrusive than the `PreserveMost` calling convention. This calling
convention also behaves identical to the `C` calling convention on how
arguments and return values are passed, but it uses a different set of
caller/callee-saved registers. This removes the burden of saving and
recovering a large register set before and after the call in the caller. If
the arguments are passed in callee-saved registers, then they will be
preserved by the callee across the call. This doesn't apply for values
returned in callee-saved registers.
- On X86-64 the callee preserves all general purpose registers, except for
R11. R11 can be used as a scratch register. Furthermore it also preserves
all floating-point registers (XMMs/YMMs).
The idea behind this convention is to support calls to runtime functions
that don't need to call out to any other functions.
This calling convention, like the `PreserveMost` calling convention, will be
used by a future version of the ObjectiveC runtime and should be considered
experimental at this time.
"``cxx_fast_tlscc``" - The `CXX_FAST_TLS` calling convention for access functions
Clang generates an access function to access C++-style TLS. The access
function generally has an entry block, an exit block and an initialization
block that is run at the first time. The entry and exit blocks can access
a few TLS IR variables, each access will be lowered to a platform-specific
sequence.
This calling convention aims to minimize overhead in the caller by
preserving as many registers as possible (all the registers that are
preserved on the fast path, composed of the entry and exit blocks).
This calling convention behaves identical to the `C` calling convention on
how arguments and return values are passed, but it uses a different set of
caller/callee-saved registers.
Given that each platform has its own lowering sequence, hence its own set
of preserved registers, we can't use the existing `PreserveMost`.
- On X86-64 the callee preserves all general purpose registers, except for
RDI and RAX.
"``swiftcc``" - This calling convention is used for Swift language.
- On X86-64 RCX and R8 are available for additional integer returns, and
XMM2 and XMM3 are available for additional FP/vector returns.
- On iOS platforms, we use AAPCS-VFP calling convention.
"``tailcc``" - Tail callable calling convention
This calling convention ensures that calls in tail position will always be
tail call optimized. This calling convention is equivalent to fastcc,
except for an additional guarantee that tail calls will be produced
whenever possible. `Tail calls can only be optimized when this, the fastcc,
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.
"``cfguard_checkcc``" - Windows Control Flow Guard (Check mechanism)
This calling convention is used for the Control Flow Guard check function,
calls to which can be inserted before indirect calls to check that the call
target is a valid function address. The check function has no return value,
but it will trigger an OS-level error if the address is not a valid target.
The set of registers preserved by the check function, and the register
containing the target address are architecture-specific.
- On X86 the target address is passed in ECX.
- On ARM the target address is passed in R0.
- On AArch64 the target address is passed in X15.
"``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.
A symbol with ``internal`` or ``private`` linkage must have ``default``
visibility.
.. _dllstorageclass:
DLL Storage Classes
-------------------
All Global Variables, Functions and Aliases can have one of the following
DLL storage class:
``dllimport``
"``dllimport``" 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``" 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. Since this storage class
exists for defining a dll interface, the compiler, assembler and linker know
it is externally referenced and must refrain from deleting the symbol.
.. _tls_model:
Thread Local Storage Models
---------------------------
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.
If no explicit model is given, the "general dynamic" model is used.
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 model can also be specified in an alias, but then it only governs how
the alias is accessed. It will not have any effect in the aliasee.
For platforms without linker support of ELF TLS model, the -femulated-tls
flag can be used to generate GCC compatible emulated TLS code.
.. _runtime_preemption_model:
Runtime Preemption Specifiers
-----------------------------
Global variables, functions and aliases may have an optional runtime preemption
specifier. If a preemption specifier isn't given explicitly, then a
symbol is assumed to be ``dso_preemptable``.
``dso_preemptable``
Indicates that the function or variable may be replaced by a symbol from
outside the linkage unit at runtime.
``dso_local``
The compiler may assume that a function or variable marked as ``dso_local``
will resolve to a symbol within the same linkage unit. Direct access will
be generated even if the definition is not within this compilation unit.
.. _namedtypes:
Structure Types
---------------
LLVM IR allows you to specify both "identified" and "literal" :ref:`structure
types <t_struct>`. Literal types are uniqued structurally, but identified types
are never uniqued. An :ref:`opaque structural type <t_opaque>` can also be used
to forward declare a type that is not yet available.
An example of an identified structure specification is:
.. code-block:: llvm
%mytype = type { %mytype*, i32 }
Prior to the LLVM 3.0 release, identified types were structurally uniqued. Only
literal types are uniqued in recent versions of LLVM.
.. _nointptrtype:
Non-Integral Pointer Type
-------------------------
Note: non-integral pointer types are a work in progress, and they should be
considered experimental at this time.
LLVM IR optionally allows the frontend to denote pointers in certain address
spaces as "non-integral" via the :ref:`datalayout string<langref_datalayout>`.
Non-integral pointer types represent pointers that have an *unspecified* bitwise
representation; that is, the integral representation may be target dependent or
unstable (not backed by a fixed integer).
``inttoptr`` instructions converting integers to non-integral pointer types are
ill-typed, and so are ``ptrtoint`` instructions converting values of
non-integral pointer types to integers. Vector versions of said instructions
are ill-typed as well.
.. _globalvars:
Global Variables
----------------
Global variables define regions of memory allocated at compilation time
instead of run-time.
Global variable definitions must be initialized.
Global variables in other translation units can also be declared, in which
case they don't have an initializer.
Global variables can optionally specify a :ref:`linkage type <linkage>`.
Either global variable definitions or declarations may have an explicit section
to be placed in and may have an optional explicit alignment specified. If there
is a mismatch between the explicit or inferred section information for the
variable declaration and its definition the resulting behavior is undefined.
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.
If the ``local_unnamed_addr`` attribute is given, the address is known to
not be significant within the module.
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.
Additionally, the global can placed in a comdat if the target has the necessary
support.
External declarations may have an explicit section specified. Section
information is retained in LLVM IR for targets that make use of this
information. Attaching section information to an external declaration is an
assertion that its definition is located in the specified section. If the
definition is located in a different section, the behavior is undefined.
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`` or dllexported variables. 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. The maximum alignment is ``1 << 29``.
For global variables declarations, as well as definitions that may be
replaced at link time (``linkonce``, ``weak``, ``extern_weak`` and ``common``
linkage types), LLVM makes no assumptions about the allocation size of the
variables, except that they may not overlap. The alignment of a global variable
declaration or replaceable definition must not be greater than the alignment of
the definition it resolves to.
Globals can also have a :ref:`DLL storage class <dllstorageclass>`,
an optional :ref:`runtime preemption specifier <runtime_preemption_model>`,
an optional :ref:`global attributes <glattrs>` and
an optional list of attached :ref:`metadata <metadata>`.
Variables and aliases can have a
:ref:`Thread Local Storage Model <tls_model>`.
:ref:`Scalable vectors <t_vector>` cannot be global variables or members of
arrays because their size is unknown at compile time. They are allowed in
structs to facilitate intrinsics returning multiple values. Structs containing
scalable vectors cannot be used in loads, stores, allocas, or GEPs.
Syntax::
@<GlobalVarName> = [Linkage] [PreemptionSpecifier] [Visibility]
[DLLStorageClass] [ThreadLocal]
[(unnamed_addr|local_unnamed_addr)] [AddrSpace]
[ExternallyInitialized]
<global | constant> <Type> [<InitializerConstant>]
[, section "name"] [, comdat [($name)]]
[, align <Alignment>] (, !name !N)*
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:`runtime preemption
specifier <runtime_preemption_model>`, an optional :ref:`visibility
style <visibility>`, an optional :ref:`DLL storage class <dllstorageclass>`,
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 address space, an optional section, an optional alignment,
an optional :ref:`comdat <langref_comdats>`,
an optional :ref:`garbage collector name <gc>`, an optional :ref:`prefix <prefixdata>`,
an optional :ref:`prologue <prologuedata>`,
an optional :ref:`personality <personalityfn>`,
an optional list of attached :ref:`metadata <metadata>`,
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:`DLL storage class <dllstorageclass>`, an
optional :ref:`calling convention <callingconv>`, an optional ``unnamed_addr``
or ``local_unnamed_addr`` attribute, an optional address space, 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>`, an optional :ref:`prefix <prefixdata>`, and an optional
:ref:`prologue <prologuedata>`.
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 name 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. If a
numeric label is explicitly specified, it must match the numeric label that
would be used implicitly.
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.
Additionally, the function can be placed in a COMDAT.
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 known to not
be significant and two identical functions can be merged.
If the ``local_unnamed_addr`` attribute is given, the address is known to
not be significant within the module.
If an explicit address space is not given, it will default to the program
address space from the :ref:`datalayout string<langref_datalayout>`.
Syntax::
define [linkage] [PreemptionSpecifier] [visibility] [DLLStorageClass]
[cconv] [ret attrs]
<ResultType> @<FunctionName> ([argument list])
[(unnamed_addr|local_unnamed_addr)] [AddrSpace] [fn Attrs]
[section "name"] [comdat [($name)]] [align N] [gc] [prefix Constant]
[prologue Constant] [personality Constant] (!name !N)* { ... }
The argument list is a comma separated sequence of arguments where each
argument is of the following form:
Syntax::
<type> [parameter Attrs] [name]
.. _langref_aliases:
Aliases
-------
Aliases, unlike function or variables, don't create any new data. They
are just a new symbol and metadata for an existing position.
Aliases have a name and an aliasee that is either a global value or a
constant expression.
Aliases may have an optional :ref:`linkage type <linkage>`, an optional
:ref:`runtime preemption specifier <runtime_preemption_model>`, an optional
:ref:`visibility style <visibility>`, an optional :ref:`DLL storage class
<dllstorageclass>` and an optional :ref:`tls model <tls_model>`.
Syntax::
@<Name> = [Linkage] [PreemptionSpecifier] [Visibility] [DLLStorageClass] [ThreadLocal] [(unnamed_addr|local_unnamed_addr)] alias <AliaseeTy>, <AliaseeTy>* @<Aliasee>
The linkage must be one of ``private``, ``internal``, ``linkonce``, ``weak``,
``linkonce_odr``, ``weak_odr``, ``external``. Note that some system linkers
might not correctly handle dropping a weak symbol that is aliased.
Aliases that are not ``unnamed_addr`` are guaranteed to have the same address as
the aliasee expression. ``unnamed_addr`` ones are only guaranteed to point
to the same content.
If the ``local_unnamed_addr`` attribute is given, the address is known to
not be significant within the module.
Since aliases are only a second name, some restrictions apply, of which
some can only be checked when producing an object file:
* The expression defining the aliasee must be computable at assembly
time. Since it is just a name, no relocations can be used.
* No alias in the expression can be weak as the possibility of the
intermediate alias being overridden cannot be represented in an
object file.
* No global value in the expression can be a declaration, since that
would require a relocation, which is not possible.
.. _langref_ifunc:
IFuncs
-------
IFuncs, like as aliases, don't create any new data or func. They are just a new
symbol that dynamic linker resolves at runtime by calling a resolver function.
IFuncs have a name and a resolver that is a function called by dynamic linker
that returns address of another function associated with the name.
IFunc may have an optional :ref:`linkage type <linkage>` and an optional
:ref:`visibility style <visibility>`.
Syntax::
@<Name> = [Linkage] [Visibility] ifunc <IFuncTy>, <ResolverTy>* @<Resolver>
.. _langref_comdats:
Comdats
-------
Comdat IR provides access to COFF and ELF object file COMDAT functionality.
Comdats have a name which represents the COMDAT key. All global objects that
specify this key will only end up in the final object file if the linker chooses
that key over some other key. Aliases are placed in the same COMDAT that their
aliasee computes to, if any.
Comdats have a selection kind to provide input on how the linker should
choose between keys in two different object files.
Syntax::
$<Name> = comdat SelectionKind
The selection kind must be one of the following:
``any``
The linker may choose any COMDAT key, the choice is arbitrary.
``exactmatch``
The linker may choose any COMDAT key but the sections must contain the
same data.
``largest``
The linker will choose the section containing the largest COMDAT key.
``noduplicates``
The linker requires that only section with this COMDAT key exist.
``samesize``
The linker may choose any COMDAT key but the sections must contain the
same amount of data.
Note that XCOFF and the Mach-O platform don't support COMDATs, and ELF and
WebAssembly only support ``any`` as a selection kind.
Here is an example of a COMDAT group where a function will only be selected if
the COMDAT key's section is the largest:
.. code-block:: text
$foo = comdat largest
@foo = global i32 2, comdat($foo)
define void @bar() comdat($foo) {
ret void
}
As a syntactic sugar the ``$name`` can be omitted if the name is the same as
the global name:
.. code-block:: llvm
$foo = comdat any
@foo = global i32 2, comdat
In a COFF object file, this will create a COMDAT section with selection kind
``IMAGE_COMDAT_SELECT_LARGEST`` containing the contents of the ``@foo`` symbol
and another COMDAT section with selection kind
``IMAGE_COMDAT_SELECT_ASSOCIATIVE`` which is associated with the first COMDAT
section and contains the contents of the ``@bar`` symbol.
There are some restrictions on the properties of the global object.
It, or an alias to it, must have the same name as the COMDAT group when
targeting COFF.
The contents and size of this object may be used during link-time to determine
which COMDAT groups get selected depending on the selection kind.
Because the name of the object must match the name of the COMDAT group, the
linkage of the global object must not be local; local symbols can get renamed
if a collision occurs in the symbol table.
The combined use of COMDATS and section attributes may yield surprising results.
For example:
.. code-block:: llvm
$foo = comdat any
$bar = comdat any
@g1 = global i32 42, section "sec", comdat($foo)
@g2 = global i32 42, section "sec", comdat($bar)
From the object file perspective, this requires the creation of two sections
with the same name. This is necessary because both globals belong to different
COMDAT groups and COMDATs, at the object file level, are represented by
sections.
Note that certain IR constructs like global variables and functions may
create COMDATs in the object file in addition to any which are specified using
COMDAT IR. This arises when the code generator is configured to emit globals
in individual sections (e.g. when `-data-sections` or `-function-sections`
is supplied to `llc`).
.. _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.
#. Named metadata are represented as a string of characters with the
metadata prefix. The rules for metadata names are the same as for
identifiers, but quoted names are not allowed. ``"\xx"`` type escapes
are still valid, which allows any character to be part of a name.
Syntax::
; Some unnamed metadata nodes, which are referenced by the named metadata.
!0 = !{!"zero"}
!1 = !{!"one"}
!2 = !{!"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 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 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(<ty>)``
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 type argument indicates the in-memory value type, and
must be the same as the pointee type of the argument.
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.
.. _attr_byref:
``byref(<ty>)``
The ``byref`` argument attribute allows specifying the pointee
memory type of an argument. This is similar to ``byval``, but does
not imply a copy is made anywhere, or that the argument is passed
on the stack. This implies the pointer is dereferenceable up to
the storage size of the type.
It is not generally permissible to introduce a write to an
``byref`` pointer. The pointer may have any address space and may
be read only.
This is not a valid attribute for return values.
The alignment for an ``byref`` parameter can be explicitly
specified by combining it with the ``align`` attribute, similar to
``byval``. If the alignment is not specified, then the code generator
makes a target-specific assumption.
This is intended for representing ABI constraints, and is not
intended to be inferred for optimization use.
.. _attr_preallocated:
``preallocated(<ty>)``
This indicates that the pointer parameter should really be passed by
value to the function, and that the pointer parameter's pointee has
already been initialized before the call instruction. This attribute
is only valid on LLVM pointer arguments. The argument must be the value
returned by the appropriate
:ref:`llvm.call.preallocated.arg<int_call_preallocated_arg>` on non
``musttail`` calls, or the corresponding caller parameter in ``musttail``
calls, although it is ignored during codegen.
A non ``musttail`` function call with a ``preallocated`` attribute in
any parameter must have a ``"preallocated"`` operand bundle. A ``musttail``
function call cannot have a ``"preallocated"`` operand bundle.
The preallocated attribute requires a type argument, which must be
the same as the pointee type of the argument.
The preallocated 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.
.. _attr_inalloca:
``inalloca(<ty>)``
The ``inalloca`` argument attribute allows the caller to take the
address of outgoing stack arguments. An ``inalloca`` argument must
be a pointer to stack memory produced by an ``alloca`` instruction.
The alloca, or argument allocation, must also be tagged with the
inalloca keyword. Only the last argument may have the ``inalloca``
attribute, and that argument is guaranteed to be passed in memory.
An argument allocation may be used by a call at most once because
the call may deallocate it. The ``inalloca`` attribute cannot be
used in conjunction with other attributes that affect argument
storage, like ``inreg``, ``nest``, ``sret``, or ``byval``. The
``inalloca`` attribute also disables LLVM's implicit lowering of
large aggregate return values, which means that frontend authors
must lower them with ``sret`` pointers.
When the call site is reached, the argument allocation must have
been the most recent stack allocation that is still live, or the
behavior is undefined. It is possible to allocate additional stack
space after an argument allocation and before its call site, but it
must be cleared off with :ref:`llvm.stackrestore
<int_stackrestore>`.
The inalloca attribute requires a type argument, which must be the
same as the pointee type of the argument.
See :doc:`InAlloca` for more information on how to use this
attribute.
``sret(<ty>)``
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 is not a valid attribute
for return values.
The sret type argument specifies the in memory type, which must be
the same as the pointee type of the argument.
.. _attr_align:
``align <n>`` or ``align(<n>)``
This indicates that the pointer value has the specified alignment.
If the pointer value does not have the specified alignment,
:ref:`poison value <poisonvalues>` is returned or passed instead. The
``align`` attribute should be combined with the ``noundef`` attribute to
ensure a pointer is aligned, or otherwise the behavior is undefined. Note
that ``align 1`` has no effect on non-byval, non-preallocated arguments.
Note that this attribute has additional semantics when combined with the
``byval`` or ``preallocated`` attribute, which are documented there.
.. _noalias:
``noalias``
This indicates that memory locations accessed via pointer values
:ref:`based <pointeraliasing>` on the argument or return value are not also
accessed, during the execution of the function, via pointer values not
*based* on the argument or return value. This guarantee only holds for
memory locations that are *modified*, by any means, during the execution of
the function. The attribute on a return value also has additional semantics
described below. 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 :ref:`alias analysis <Must, May,
or No>`.
Note that this definition of ``noalias`` is intentionally similar
to the definition of ``restrict`` in C99 for function arguments.
For function return values, C99's ``restrict`` is not meaningful,
while LLVM's ``noalias`` is. Furthermore, the semantics of the ``noalias``
attribute on return values are stronger than the semantics of the attribute
when used on function arguments. On function return values, the ``noalias``
attribute indicates that the function acts like a system memory allocation
function, returning a pointer to allocated storage disjoint from the
storage for any other object accessible to the caller.
.. _nocapture:
``nocapture``
This indicates that the callee does not :ref:`capture <pointercapture>` the
pointer. This is not a valid attribute for return values.
This attribute applies only to the particular copy of the pointer passed in
this argument. A caller could pass two copies of the same pointer with one
being annotated nocapture and the other not, and the callee could validly
capture through the non annotated parameter.
.. code-block:: llvm
define void @f(i8* nocapture %a, i8* %b) {
; (capture %b)
}
call void @f(i8* @glb, i8* @glb) ; well-defined
``nofree``
This indicates that callee does not free the pointer argument. 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 a hint to the optimizer and code generator used when
generating the caller, allowing value propagation, 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.
``nonnull``
This indicates that the parameter or return pointer is not null. This
attribute may only be applied to pointer typed parameters. This is not
checked or enforced by LLVM; if the parameter or return pointer is null,
:ref:`poison value <poisonvalues>` is returned or passed instead.
The ``nonnull`` attribute should be combined with the ``noundef`` attribute
to ensure a pointer is not null or otherwise the behavior is undefined.
``dereferenceable(<n>)``
This indicates that the parameter or return pointer is dereferenceable. This
attribute may only be applied to pointer typed parameters. A pointer that
is dereferenceable can be loaded from speculatively without a risk of
trapping. The number of bytes known to be dereferenceable must be provided
in parentheses. It is legal for the number of bytes to be less than the
size of the pointee type. The ``nonnull`` attribute does not imply
dereferenceability (consider a pointer to one element past the end of an
array), however ``dereferenceable(<n>)`` does imply ``nonnull`` in
``addrspace(0)`` (which is the default address space), except if the
``null_pointer_is_valid`` function attribute is present.
``n`` should be a positive number. The pointer should be well defined,
otherwise it is undefined behavior. This means ``dereferenceable(<n>)``
implies ``noundef``.
``dereferenceable_or_null(<n>)``
This indicates that the parameter or return value isn't both
non-null and non-dereferenceable (up to ``<n>`` bytes) at the same
time. All non-null pointers tagged with
``dereferenceable_or_null(<n>)`` are ``dereferenceable(<n>)``.
For address space 0 ``dereferenceable_or_null(<n>)`` implies that
a pointer is exactly one of ``dereferenceable(<n>)`` or ``null``,
and in other address spaces ``dereferenceable_or_null(<n>)``
implies that a pointer is at least one of ``dereferenceable(<n>)``
or ``null`` (i.e. it may be both ``null`` and
``dereferenceable(<n>)``). This attribute may only be applied to
pointer typed parameters.
``swiftself``
This indicates that the parameter is the self/context parameter. This is not
a valid attribute for return values and can only be applied to one
parameter.
``swifterror``
This attribute is motivated to model and optimize Swift error handling. It
can be applied to a parameter with pointer to pointer type or a
pointer-sized alloca. At the call site, the actual argument that corresponds
to a ``swifterror`` parameter has to come from a ``swifterror`` alloca or
the ``swifterror`` parameter of the caller. A ``swifterror`` value (either
the parameter or the alloca) can only be loaded and stored from, or used as
a ``swifterror`` argument. This is not a valid attribute for return values
and can only be applied to one parameter.
These constraints allow the calling convention to optimize access to
``swifterror`` variables by associating them with a specific register at
call boundaries rather than placing them in memory. Since this does change
the calling convention, a function which uses the ``swifterror`` attribute
on a parameter is not ABI-compatible with one which does not.
These constraints also allow LLVM to assume that a ``swifterror`` argument
does not alias any other memory visible within a function and that a
``swifterror`` alloca passed as an argument does not escape.
``immarg``
This indicates the parameter is required to be an immediate
value. This must be a trivial immediate integer or floating-point
constant. Undef or constant expressions are not valid. This is
only valid on intrinsic declarations and cannot be applied to a
call site or arbitrary function.
``noundef``
This attribute applies to parameters and return values. If the value
representation contains any undefined or poison bits, the behavior is
undefined. Note that this does not refer to padding introduced by the
type's storage representation.
``alignstack(<n>)``
This indicates the alignment that should be considered by the backend when
assigning this parameter to a stack slot during calling convention
lowering. The enforcement of the specified alignment is target-dependent,
as target-specific calling convention rules may override this value. This
attribute serves the purpose of carrying language specific alignment
information that is not mapped to base types in the backend (for example,
over-alignment specification through language attributes).
.. _gc:
Garbage Collector Strategy Names
--------------------------------
Each function may specify a garbage collector strategy name, which is simply a
string:
.. code-block:: llvm
define void @f() gc "name" { ... }
The supported values of *name* includes those :ref:`built in to LLVM
<builtin-gc-strategies>` and any provided by loaded plugins. Specifying a GC
strategy will cause the compiler to alter its output in order to support the
named garbage collection algorithm. Note that LLVM itself does not contain a
garbage collector, this functionality is restricted to generating machine code
which can interoperate with a collector provided externally.
.. _prefixdata:
Prefix Data
-----------
Prefix data is data associated with a function which the code
generator will emit immediately before the function's entrypoint.
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 and dereference
index -1. This implies that the IR symbol points just past the end of
the prefix data. For instance, take the example of a function annotated
with a single ``i32``,
.. code-block:: llvm
define void @f() prefix i32 123 { ... }
The prefix data can be referenced as,
.. code-block:: llvm
%0 = bitcast void* () @f to i32*
%a = getelementptr inbounds i32, i32* %0, i32 -1
%b = load i32, i32* %a
Prefix data is laid out as if it were an initializer for a global variable
of the prefix data's type. The function will be placed such that the
beginning of the prefix data is aligned. This means that if the size
of the prefix data is not a multiple of the alignment size, the
function's entrypoint will not be aligned. If alignment of the
function's entrypoint is desired, padding must be added to the prefix
data.
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.
.. _prologuedata:
Prologue Data
-------------
The ``prologue`` attribute allows arbitrary code (encoded as bytes) to
be inserted prior to the function body. This can be used for enabling
function hot-patching and instrumentation.
To maintain the semantics of ordinary function calls, the prologue 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 prologue 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 prologue data. Obviously this
makes the format of the prologue data highly target dependent.
A trivial example of valid prologue data for the x86 architecture is ``i8 144``,
which encodes the ``nop`` instruction:
.. code-block:: text
define void @f() prologue i8 144 { ... }
Generally prologue data can be formed by encoding a relative branch instruction
which skips the metadata, as in this example of valid prologue data for the
x86_64 architecture, where the first two bytes encode ``jmp .+10``:
.. code-block:: text
%0 = type <{ i8, i8, i8* }>
define void @f() prologue %0 <{ i8 235, i8 8, i8* @md}> { ... }
A function may have prologue 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.
.. _personalityfn:
Personality Function
--------------------
The ``personality`` attribute permits functions to specify what function
to use for exception handling.
.. _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.
``allocsize(<EltSizeParam>[, <NumEltsParam>])``
This attribute indicates that the annotated function will always return at
least a given number of bytes (or null). Its arguments are zero-indexed
parameter numbers; if one argument is provided, then it's assumed that at
least ``CallSite.Args[EltSizeParam]`` bytes will be available at the
returned pointer. If two are provided, then it's assumed that
``CallSite.Args[EltSizeParam] * CallSite.Args[NumEltsParam]`` bytes are
available. The referenced parameters must be integer types. No assumptions
are made about the contents of the returned block of memory.
``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 that 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.
``convergent``
In some parallel execution models, there exist operations that cannot be
made control-dependent on any additional values. We call such operations
``convergent``, and mark them with this attribute.
The ``convergent`` attribute may appear on functions or call/invoke
instructions. When it appears on a function, it indicates that calls to
this function should not be made control-dependent on additional values.
For example, the intrinsic ``llvm.nvvm.barrier0`` is ``convergent``, so
calls to this intrinsic cannot be made control-dependent on additional
values.
When it appears on a call/invoke, the ``convergent`` attribute indicates
that we should treat the call as though we're calling a convergent
function. This is particularly useful on indirect calls; without this we
may treat such calls as though the target is non-convergent.
The optimizer may remove the ``convergent`` attribute on functions when it
can prove that the function does not execute any convergent operations.
Similarly, the optimizer may remove ``convergent`` on calls/invokes when it
can prove that the call/invoke cannot call a convergent function.
``hot``
This attribute indicates that this function is a hot spot of the program
execution. The function will be optimized more aggressively and will be
placed into special subsection of the text section to improving locality.
When profile feedback is enabled, this attribute has the precedence over
the profile information. By marking a function ``hot``, users can work
around the cases where the training input does not have good coverage
on all the hot functions.
``inaccessiblememonly``
This attribute indicates that the function may only access memory that
is not accessible by the module being compiled. This is a weaker form
of ``readnone``. If the function reads or writes other memory, the
behavior is undefined.
``inaccessiblemem_or_argmemonly``
This attribute indicates that the function may only access memory that is
either not accessible by the module being compiled, or is pointed to
by its pointer arguments. This is a weaker form of ``argmemonly``. If the
function reads or writes other memory, the behavior is undefined.
``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.
``jumptable``
This attribute indicates that the function should be added to a
jump-instruction table at code-generation time, and that all address-taken
references to this function should be replaced with a reference to the
appropriate jump-instruction-table function pointer. Note that this creates
a new pointer for the original function, which means that code that depends
on function-pointer identity can break. So, any function annotated with
``jumptable`` must also be ``unnamed_addr``.
``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.
``"no-inline-line-tables"``
When this attribute is set to true, the inliner discards source locations
when inlining code and instead uses the source location of the call site.
Breakpoints set on code that was inlined into the current function will
not fire during the execution of the inlined call sites. If the debugger
stops inside an inlined call site, it will appear to be stopped at the
outermost inlined call site.
``no-jump-tables``
When this attribute is set to true, the jump tables and lookup tables that
can be generated from a switch case lowering are disabled.
``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.
``nofree``
This function attribute indicates that the function does not, directly or
transitively, call a memory-deallocation function (``free``, for example)
on a memory allocation which existed before the call.
As a result, uncaptured pointers that are known to be dereferenceable
prior to a call to a function with the ``nofree`` attribute are still
known to be dereferenceable after the call. The capturing condition is
necessary in environments where the function might communicate the
pointer to another thread which then deallocates the memory. Alternatively,
``nosync`` would ensure such communication cannot happen and even captured
pointers cannot be freed by the function.
A ``nofree`` function is explicitly allowed to free memory which it
allocated or (if not ``nosync``) arrange for another thread to free
memory on it's behalf. As a result, perhaps surprisingly, a ``nofree``
function can return a pointer to a previously deallocated memory object.
``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.
``nomerge``
This attribute indicates that calls to this function should never be merged
during optimization. For example, it will prevent tail merging otherwise
identical code sequences that raise an exception or terminate the program.
Tail merging normally reduces the precision of source location information,
making stack traces less useful for debugging. This attribute gives the
user control over the tradeoff between code size and debug information
precision.
``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.
``indirect-tls-seg-refs``
This attribute indicates that the code generator should not use
direct TLS access through segment registers, even if the
target-specific ABI normally permits it.
``noreturn``
This function attribute indicates that the function never returns
normally, hence through a return instruction. This produces undefined
behavior at runtime if the function ever does dynamically return. Annotated
functions may still raise an exception, i.a., ``nounwind`` is not implied.
``norecurse``
This function attribute indicates that the function does not call itself
either directly or indirectly down any possible call path. This produces
undefined behavior at runtime if the function ever does recurse.
``willreturn``
This function attribute indicates that a call of this function will
either exhibit undefined behavior or comes back and continues execution
at a point in the existing call stack that includes the current invocation.
Annotated functions may still raise an exception, i.a., ``nounwind`` is not implied.
If an invocation of an annotated function does not return control back
to a point in the call stack, the behavior is undefined.
``nosync``
This function attribute indicates that the function does not communicate
(synchronize) with another thread through memory or other well-defined means.
Synchronization is considered possible in the presence of `atomic` accesses
that enforce an order, thus not "unordered" and "monotonic", `volatile` accesses,
as well as `convergent` function calls. Note that through `convergent` function calls
non-memory communication, e.g., cross-lane operations, are possible and are also
considered synchronization. However `convergent` does not contradict `nosync`.
If an annotated function does ever synchronize with another thread,
the behavior is undefined.
``nounwind``
This function attribute indicates that the function never raises an
exception. If the function does raise an exception, its runtime
behavior is undefined. However, functions marked nounwind may still
trap or generate asynchronous exceptions. Exception handling schemes
that are recognized by LLVM to handle asynchronous exceptions, such
as SEH, will still provide their implementation defined semantics.
``null_pointer_is_valid``
If ``null_pointer_is_valid`` is set, then the ``null`` address
in address-space 0 is considered to be a valid address for memory loads and
stores. Any analysis or optimization should not treat dereferencing a
pointer to ``null`` as undefined behavior in this function.
Note: Comparing address of a global variable to ``null`` may still
evaluate to false because of a limitation in querying this attribute inside
constant expressions.
``optforfuzzing``
This attribute indicates that this function should be optimized
for maximum fuzzing signal.
``optnone``
This function attribute indicates that most optimization passes will skip
this function, with the exception of interprocedural optimization passes.
Code generation defaults to the "fast" instruction selector.
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.
``"patchable-function"``
This attribute tells the code generator that the code
generated for this function needs to follow certain conventions that
make it possible for a runtime function to patch over it later.
The exact effect of this attribute depends on its string value,
for which there currently is one legal possibility:
* ``"prologue-short-redirect"`` - This style of patchable
function is intended to support patching a function prologue to
redirect control away from the function in a thread safe
manner. It guarantees that the first instruction of the
function will be large enough to accommodate a short jump
instruction, and will be sufficiently aligned to allow being
fully changed via an atomic compare-and-swap instruction.
While the first requirement can be satisfied by inserting large
enough NOP, LLVM can and will try to re-purpose an existing
instruction (i.e. one that would have to be emitted anyway) as
the patchable instruction larger than a short jump.
``"prologue-short-redirect"`` is currently only supported on
x86-64.
This attribute by itself does not imply restrictions on
inter-procedural optimizations. All of the semantic effects the
patching may have to be separately conveyed via the linkage type.
``"probe-stack"``
This attribute indicates that the function will trigger a guard region
in the end of the stack. It ensures that accesses to the stack must be
no further apart than the size of the guard region to a previous
access of the stack. It takes one required string value, the name of
the stack probing function that will be called.
If a function that has a ``"probe-stack"`` attribute is inlined into
a function with another ``"probe-stack"`` attribute, the resulting
function has the ``"probe-stack"`` attribute of the caller. If a
function that has a ``"probe-stack"`` attribute is inlined into a
function that has no ``"probe-stack"`` attribute at all, the resulting
function has the ``"probe-stack"`` attribute of the callee.
``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 while it cannot unwind exceptions by calling
the ``C++`` exception throwing methods (since they write to memory), there may
be non-``C++`` mechanisms that throw exceptions without writing to LLVM
visible memory.
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.
If a readnone function reads or writes memory visible to the program, or
has other side-effects, the behavior is undefined. If a function reads from
or writes to a readnone pointer argument, the behavior is undefined.
``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. This means while it
cannot unwind exceptions by calling the ``C++`` exception throwing methods
(since they write to memory), there may be non-``C++`` mechanisms that throw
exceptions without writing to LLVM visible memory.
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.
If a readonly function writes memory visible to the program, or
has other side-effects, the behavior is undefined. If a function writes to
a readonly pointer argument, the behavior is undefined.
``"stack-probe-size"``
This attribute controls the behavior of stack probes: either
the ``"probe-stack"`` attribute, or ABI-required stack probes, if any.
It defines the size of the guard region. It ensures that if the function
may use more stack space than the size of the guard region, stack probing
sequence will be emitted. It takes one required integer value, which
is 4096 by default.
If a function that has a ``"stack-probe-size"`` attribute is inlined into
a function with another ``"stack-probe-size"`` attribute, the resulting
function has the ``"stack-probe-size"`` attribute that has the lower
numeric value. If a function that has a ``"stack-probe-size"`` attribute is
inlined into a function that has no ``"stack-probe-size"`` attribute
at all, the resulting function has the ``"stack-probe-size"`` attribute
of the callee.
``"no-stack-arg-probe"``
This attribute disables ABI-required stack probes, if any.
``writeonly``
On a function, this attribute indicates that the function may write to but
does not read from memory.
On an argument, this attribute indicates that the function may write to but
does not read through this pointer argument (even though it may read from
the memory that the pointer points to).
If a writeonly function reads memory visible to the program, or
has other side-effects, the behavior is undefined. If a function reads
from a writeonly pointer argument, the behavior is undefined.
``argmemonly``
This attribute indicates that the only memory accesses inside function are
loads and stores from objects pointed to by its pointer-typed arguments,
with arbitrary offsets. Or in other words, all memory operations in the
function can refer to memory only using pointers based on its function
arguments.
Note that ``argmemonly`` can be used together with ``readonly`` attribute
in order to specify that function reads only from its arguments.
If an argmemonly function reads or writes memory other than the pointer
arguments, or has other side-effects, the behavior is undefined.
``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.
``safestack``
This attribute indicates that
`SafeStack <https://clang.llvm.org/docs/SafeStack.html>`_
protection is enabled for this function.
If a function that has a ``safestack`` attribute is inlined into a
function that doesn't have a ``safestack`` attribute or which has an
``ssp``, ``sspstrong`` or ``sspreq`` attribute, then the resulting
function will have a ``safestack`` attribute.
``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.
``sanitize_hwaddress``
This attribute indicates that HWAddressSanitizer checks
(dynamic address safety analysis based on tagged pointers) are enabled for
this function.
``sanitize_memtag``
This attribute indicates that MemTagSanitizer checks
(dynamic address safety analysis based on Armv8 MTE) are enabled for
this function.
``speculative_load_hardening``
This attribute indicates that
`Speculative Load Hardening <https://llvm.org/docs/SpeculativeLoadHardening.html>`_
should be enabled for the function body.
Speculative Load Hardening is a best-effort mitigation against
information leak attacks that make use of control flow
miss-speculation - specifically miss-speculation of whether a branch
is taken or not. Typically vulnerabilities enabling such attacks are
classified as "Spectre variant #1". Notably, this does not attempt to
mitigate against miss-speculation of branch target, classified as
"Spectre variant #2" vulnerabilities.
When inlining, the attribute is sticky. Inlining a function that carries
this attribute will cause the caller to gain the attribute. This is intended
to provide a maximally conservative model where the code in a function
annotated with this attribute will always (even after inlining) end up
hardened.
``speculatable``
This function attribute indicates that the function does not have any
effects besides calculating its result and does not have undefined behavior.
Note that ``speculatable`` is not enough to conclude that along any
particular execution path the number of calls to this function will not be
externally observable. This attribute is only valid on functions
and declarations, not on individual call sites. If a function is
incorrectly marked as speculatable and really does exhibit
undefined behavior, the undefined behavior may be observed even
if the call site is dead code.
``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``.
Variables that are identified as requiring a protector will be arranged
on the stack such that they are adjacent to the stack protector guard.
A function with the ``ssp`` attribute but without the ``alwaysinline``
attribute cannot be inlined into a function without a
``ssp/sspreq/sspstrong`` attribute. If inlined, the caller will get the
``ssp`` 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.
Variables that are identified as requiring a protector will be arranged
on the stack such that they are adjacent to the stack protector guard.
The specific layout rules are:
#. Large arrays and structures containing large arrays
(``>= ssp-buffer-size``) are closest to the stack protector.
#. Small arrays and structures containing small arrays
(``< ssp-buffer-size``) are 2nd closest to the protector.
#. Variables that have had their address taken are 3rd closest to the
protector.
This overrides the ``ssp`` function attribute.
A function with the ``sspstrong`` attribute but without the
``alwaysinline`` attribute cannot be inlined into a function without a
``ssp/sspstrong/sspreq`` attribute. If inlined, the caller will get the
``sspstrong`` attribute unless the ``sspreq`` attribute exists.
``sspreq``
This attribute indicates that the function should *always* emit a stack
smashing protector. This overrides the ``ssp`` and ``sspstrong`` function
attributes.
Variables that are identified as requiring a protector will be arranged
on the stack such that they are adjacent to the stack protector guard.
The specific layout rules are:
#. Large arrays and structures containing large arrays
(``>= ssp-buffer-size``) are closest to the stack protector.
#. Small arrays and structures containing small arrays
(``< ssp-buffer-size``) are 2nd closest to the protector.
#. Variables that have had their address taken are 3rd closest to the
protector.
A function with the ``sspreq`` attribute but without the ``alwaysinline``
attribute cannot be inlined into a function without a
``ssp/sspstrong/sspreq`` attribute. If inlined, the caller will get the
``sspreq`` attribute.
``strictfp``
This attribute indicates that the function was called from a scope that
requires strict floating-point semantics. LLVM will not attempt any
optimizations that require assumptions about the floating-point rounding
mode or that might alter the state of floating-point status flags that
might otherwise be set or cleared by calling this function. LLVM will
not introduce any new floating-point instructions that may trap.
``"denormal-fp-math"``
This indicates the denormal (subnormal) handling that may be
assumed for the default floating-point environment. This is a
comma separated pair. The elements may be one of ``"ieee"``,
``"preserve-sign"``, or ``"positive-zero"``. The first entry
indicates the flushing mode for the result of floating point
operations. The second indicates the handling of denormal inputs
to floating point instructions. For compatibility with older
bitcode, if the second value is omitted, both input and output
modes will assume the same mode.
If this is attribute is not specified, the default is
``"ieee,ieee"``.
If the output mode is ``"preserve-sign"``, or ``"positive-zero"``,
denormal outputs may be flushed to zero by standard floating-point
operations. It is not mandated that flushing to zero occurs, but if
a denormal output is flushed to zero, it must respect the sign
mode. Not all targets support all modes. While this indicates the
expected floating point mode the function will be executed with,
this does not make any attempt to ensure the mode is
consistent. User or platform code is expected to set the floating
point mode appropriately before function entry.
If the input mode is ``"preserve-sign"``, or ``"positive-zero"``, a
floating-point operation must treat any input denormal value as
zero. In some situations, if an instruction does not respect this
mode, the input may need to be converted to 0 as if by
``@llvm.canonicalize`` during lowering for correctness.
``"denormal-fp-math-f32"``
Same as ``"denormal-fp-math"``, but only controls the behavior of
the 32-bit float type (or vectors of 32-bit floats). If both are
are present, this overrides ``"denormal-fp-math"``. Not all targets
support separately setting the denormal mode per type, and no
attempt is made to diagnose unsupported uses. Currently this
attribute is respected by the AMDGPU and NVPTX backends.
``"thunk"``
This attribute indicates that the function will delegate to some other
function with a tail call. The prototype of a thunk should not be used for
optimization purposes. The caller is expected to cast the thunk prototype to
match the thunk target prototype.
``uwtable``
This attribute indicates that the ABI being targeted requires that
an unwind table entry be produced 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.
``nocf_check``
This attribute indicates that no control-flow check will be performed on
the attributed entity. It disables -fcf-protection=<> for a specific
entity to fine grain the HW control flow protection mechanism. The flag
is target independent and currently appertains to a function or function
pointer.
``shadowcallstack``
This attribute indicates that the ShadowCallStack checks are enabled for
the function. The instrumentation checks that the return address for the
function has not changed between the function prolog and epilog. It is
currently x86_64-specific.
``mustprogress``
This attribute indicates that the function is required to return, unwind,
or interact with the environment in an observable way e.g. via a volatile
memory access, I/O, or other synchronization. The ``mustprogress``
attribute is intended to model the requirements of the first section of
[intro.progress] of the C++ Standard. As a consequence, a loop in a
function with the `mustprogress` attribute can be assumed to terminate if
it does not interact with the environment in an observable way, and
terminating loops without side-effects can be removed. If a `mustprogress`
function does not satisfy this contract, the behavior is undefined. This
attribute does not apply transitively to callees, but does apply to call
sites within the function. Note that `willreturn` implies `mustprogress`.
``vscale_range(<min>[, <max>])``
This attribute indicates the minimum and maximum vscale value for the given
function. A value of 0 means unbounded. If the optional max value is omitted
then max is set to the value of min. If the attribute is not present, no
assumptions are made about the range of vscale.
Call Site Attributes
----------------------
In addition to function attributes the following call site only
attributes are supported:
``vector-function-abi-variant``
This attribute can be attached to a :ref:`call <i_call>` to list
the vector functions associated to the function. Notice that the
attribute cannot be attached to a :ref:`invoke <i_invoke>` or a
:ref:`callbr <i_callbr>` instruction. The attribute consists of a
comma separated list of mangled names. The order of the list does
not imply preference (it is logically a set). The compiler is free
to pick any listed vector function of its choosing.
The syntax for the mangled names is as follows:::
_ZGV<isa><mask><vlen><parameters>_<scalar_name>[(<vector_redirection>)]
When present, the attribute informs the compiler that the function
``<scalar_name>`` has a corresponding vector variant that can be
used to perform the concurrent invocation of ``<scalar_name>`` on
vectors. The shape of the vector function is described by the
tokens between the prefix ``_ZGV`` and the ``<scalar_name>``
token. The standard name of the vector function is
``_ZGV<isa><mask><vlen><parameters>_<scalar_name>``. When present,
the optional token ``(<vector_redirection>)`` informs the compiler
that a custom name is provided in addition to the standard one
(custom names can be provided for example via the use of ``declare
variant`` in OpenMP 5.0). The declaration of the variant must be
present in the IR Module. The signature of the vector variant is
determined by the rules of the Vector Function ABI (VFABI)
specifications of the target. For Arm and X86, the VFABI can be
found at https://github.com/ARM-software/abi-aa and
https://software.intel.com/en-us/articles/vector-simd-function-abi,
respectively.
For X86 and Arm targets, the values of the tokens in the standard
name are those that are defined in the VFABI. LLVM has an internal
``<isa>`` token that can be used to create scalar-to-vector
mappings for functions that are not directly associated to any of
the target ISAs (for example, some of the mappings stored in the
TargetLibraryInfo). Valid values for the ``<isa>`` token are:::
<isa>:= b | c | d | e -> X86 SSE, AVX, AVX2, AVX512
| n | s -> Armv8 Advanced SIMD, SVE
| __LLVM__ -> Internal LLVM Vector ISA
For all targets currently supported (x86, Arm and Internal LLVM),
the remaining tokens can have the following values:::
<mask>:= M | N -> mask | no mask
<vlen>:= number -> number of lanes
| x -> VLA (Vector Length Agnostic)
<parameters>:= v -> vector
| l | l <number> -> linear
| R | R <number> -> linear with ref modifier
| L | L <number> -> linear with val modifier
| U | U <number> -> linear with uval modifier
| ls <pos> -> runtime linear
| Rs <pos> -> runtime linear with ref modifier
| Ls <pos> -> runtime linear with val modifier
| Us <pos> -> runtime linear with uval modifier
| u -> uniform
<scalar_name>:= name of the scalar function
<vector_redirection>:= optional, custom name of the vector function
``preallocated(<ty>)``
This attribute is required on calls to ``llvm.call.preallocated.arg``
and cannot be used on any other call. See
:ref:`llvm.call.preallocated.arg<int_call_preallocated_arg>` for more
details.
.. _glattrs:
Global Attributes
-----------------
Attributes may be set to communicate additional information about a global variable.
Unlike :ref:`function attributes <fnattrs>`, attributes on a global variable
are grouped into a single :ref:`attribute group <attrgrp>`.
.. _opbundles:
Operand Bundles
---------------
Operand bundles are tagged sets of SSA values that can be associated
with certain LLVM instructions (currently only ``call`` s and
``invoke`` s). In a way they are like metadata, but dropping them is
incorrect and will change program semantics.
Syntax::
operand bundle set ::= '[' operand bundle (, operand bundle )* ']'
operand bundle ::= tag '(' [ bundle operand ] (, bundle operand )* ')'
bundle operand ::= SSA value
tag ::= string constant
Operand bundles are **not** part of a function's signature, and a
given function may be called from multiple places with different kinds
of operand bundles. This reflects the fact that the operand bundles
are conceptually a part of the ``call`` (or ``invoke``), not the
callee being dispatched to.
Operand bundles are a generic mechanism intended to support
runtime-introspection-like functionality for managed languages. While
the exact semantics of an operand bundle depend on the bundle tag,
there are certain limitations to how much the presence of an operand
bundle can influence the semantics of a program. These restrictions
are described as the semantics of an "unknown" operand bundle. As
long as the behavior of an operand bundle is describable within these
restrictions, LLVM does not need to have special knowledge of the
operand bundle to not miscompile programs containing it.
- The bundle operands for an unknown operand bundle escape in unknown
ways before control is transferred to the callee or invokee.
- Calls and invokes with operand bundles have unknown read / write
effect on the heap on entry and exit (even if the call target is
``readnone`` or ``readonly``), unless they're overridden with
callsite specific attributes.
- An operand bundle at a call site cannot change the implementation
of the called function. Inter-procedural optimizations work as
usual as long as they take into account the first two properties.
More specific types of operand bundles are described below.
.. _deopt_opbundles:
Deoptimization Operand Bundles
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Deoptimization operand bundles are characterized by the ``"deopt"``
operand bundle tag. These operand bundles represent an alternate
"safe" continuation for the call site they're attached to, and can be
used by a suitable runtime to deoptimize the compiled frame at the
specified call site. There can be at most one ``"deopt"`` operand
bundle attached to a call site. Exact details of deoptimization is
out of scope for the language reference, but it usually involves
rewriting a compiled frame into a set of interpreted frames.
From the compiler's perspective, deoptimization operand bundles make
the call sites they're attached to at least ``readonly``. They read
through all of their pointer typed operands (even if they're not
otherwise escaped) and the entire visible heap. Deoptimization
operand bundles do not capture their operands except during
deoptimization, in which case control will not be returned to the
compiled frame.
The inliner knows how to inline through calls that have deoptimization
operand bundles. Just like inlining through a normal call site
involves composing the normal and exceptional continuations, inlining
through a call site with a deoptimization operand bundle needs to
appropriately compose the "safe" deoptimization continuation. The
inliner does this by prepending the parent's deoptimization
continuation to every deoptimization continuation in the inlined body.
E.g. inlining ``@f`` into ``@g`` in the following example
.. code-block:: llvm
define void @f() {
call void @x() ;; no deopt state
call void @y() [ "deopt"(i32 10) ]
call void @y() [ "deopt"(i32 10), "unknown"(i8* null) ]
ret void
}
define void @g() {
call void @f() [ "deopt"(i32 20) ]
ret void
}
will result in
.. code-block:: llvm
define void @g() {
call void @x() ;; still no deopt state
call void @y() [ "deopt"(i32 20, i32 10) ]
call void @y() [ "deopt"(i32 20, i32 10), "unknown"(i8* null) ]
ret void
}
It is the frontend's responsibility to structure or encode the
deoptimization state in a way that syntactically prepending the
caller's deoptimization state to the callee's deoptimization state is
semantically equivalent to composing the caller's deoptimization
continuation after the callee's deoptimization continuation.
.. _ob_funclet:
Funclet Operand Bundles
^^^^^^^^^^^^^^^^^^^^^^^
Funclet operand bundles are characterized by the ``"funclet"``
operand bundle tag. These operand bundles indicate that a call site
is within a particular funclet. There can be at most one
``"funclet"`` operand bundle attached to a call site and it must have
exactly one bundle operand.
If any funclet EH pads have been "entered" but not "exited" (per the
`description in the EH doc\ <ExceptionHandling.html#wineh-constraints>`_),
it is undefined behavior to execute a ``call`` or ``invoke`` which:
* does not have a ``"funclet"`` bundle and is not a ``call`` to a nounwind
intrinsic, or
* has a ``"funclet"`` bundle whose operand is not the most-recently-entered
not-yet-exited funclet EH pad.
Similarly, if no funclet EH pads have been entered-but-not-yet-exited,
executing a ``call`` or ``invoke`` with a ``"funclet"`` bundle is undefined behavior.
GC Transition Operand Bundles
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
GC transition operand bundles are characterized by the
``"gc-transition"`` operand bundle tag. These operand bundles mark a
call as a transition between a function with one GC strategy to a
function with a different GC strategy. If coordinating the transition
between GC strategies requires additional code generation at the call
site, these bundles may contain any values that are needed by the
generated code. For more details, see :ref:`GC Transitions
<gc_transition_args>`.
The bundle contain an arbitrary list of Values which need to be passed
to GC transition code. They will be lowered and passed as operands to
the appropriate GC_TRANSITION nodes in the selection DAG. It is assumed
that these arguments must be available before and after (but not
necessarily during) the execution of the callee.
.. _assume_opbundles:
Assume Operand Bundles
^^^^^^^^^^^^^^^^^^^^^^
Operand bundles on an :ref:`llvm.assume <int_assume>` allows representing
assumptions that a :ref:`parameter attribute <paramattrs>` or a
:ref:`function attribute <fnattrs>` holds for a certain value at a certain
location. Operand bundles enable assumptions that are either hard or impossible
to represent as a boolean argument of an :ref:`llvm.assume <int_assume>`.
An assume operand bundle has the form:
::
"<tag>"([ <holds for value> [, <attribute argument>] ])
* The tag of the operand bundle is usually the name of attribute that can be
assumed to hold. It can also be `ignore`, this tag doesn't contain any
information and should be ignored.
* The first argument if present is the value for which the attribute hold.
* The second argument if present is an argument of the attribute.
If there are no arguments the attribute is a property of the call location.
If the represented attribute expects a constant argument, the argument provided
to the operand bundle should be a constant as well.
For example:
.. code-block:: llvm
call void @llvm.assume(i1 true) ["align"(i32* %val, i32 8)]
allows the optimizer to assume that at location of call to
:ref:`llvm.assume <int_assume>` ``%val`` has an alignment of at least 8.
.. code-block:: llvm
call void @llvm.assume(i1 %cond) ["cold"(), "nonnull"(i64* %val)]
allows the optimizer to assume that the :ref:`llvm.assume <int_assume>`
call location is cold and that ``%val`` may not be null.
Just like for the argument of :ref:`llvm.assume <int_assume>`, if any of the
provided guarantees are violated at runtime the behavior is undefined.
Even if the assumed property can be encoded as a boolean value, like
``nonnull``, using operand bundles to express the property can still have
benefits:
* Attributes that can be expressed via operand bundles are directly the
property that the optimizer uses and cares about. Encoding attributes as
operand bundles removes the need for an instruction sequence that represents
the property (e.g., `icmp ne i32* %p, null` for `nonnull`) and for the
optimizer to deduce the property from that instruction sequence.
* Expressing the property using operand bundles makes it easy to identify the
use of the value as a use in an :ref:`llvm.assume <int_assume>`. This then
simplifies and improves heuristics, e.g., for use "use-sensitive"
optimizations.
.. _ob_preallocated:
Preallocated Operand Bundles
^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Preallocated operand bundles are characterized by the ``"preallocated"``
operand bundle tag. These operand bundles allow separation of the allocation
of the call argument memory from the call site. This is necessary to pass
non-trivially copyable objects by value in a way that is compatible with MSVC
on some targets. There can be at most one ``"preallocated"`` operand bundle
attached to a call site and it must have exactly one bundle operand, which is
a token generated by ``@llvm.call.preallocated.setup``. A call with this
operand bundle should not adjust the stack before entering the function, as
that will have been done by one of the ``@llvm.call.preallocated.*`` intrinsics.
.. code-block:: llvm
%foo = type { i64, i32 }
...
%t = call token @llvm.call.preallocated.setup(i32 1)
%a = call i8* @llvm.call.preallocated.arg(token %t, i32 0) preallocated(%foo)
%b = bitcast i8* %a to %foo*
; initialize %b
call void @bar(i32 42, %foo* preallocated(%foo) %b) ["preallocated"(token %t)]
.. _ob_gc_live:
GC Live Operand Bundles
^^^^^^^^^^^^^^^^^^^^^^^^^^^^
A "gc-live" operand bundle is only valid on a :ref:`gc.statepoint <gc_statepoint>`
intrinsic. The operand bundle must contain every pointer to a garbage collected
object which potentially needs to be updated by the garbage collector.
When lowered, any relocated value will be recorded in the corresponding
:ref:`stackmap entry <statepoint-stackmap-format>`. See the intrinsic description
for further details.
ObjC ARC Attached Call Operand Bundles
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
A ``"clang.arc.attachedcall`` operand bundle on a call indicates the call is
implicitly followed by a marker instruction and a call to an ObjC runtime
function that uses the result of the call. If the argument passed to the operand
bundle is 0, ``@objc_retainAutoreleasedReturnValue`` is called. If 1 is passed,
``@objc_unsafeClaimAutoreleasedReturnValue`` is called. A call with this bundle
implicitly uses its return value.
The operand bundle is needed to ensure the call is immediately followed by the
marker instruction or the ObjC runtime call in the final output.
.. _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.
Note that the assembly string *must* be parseable by LLVM's integrated assembler
(unless it is disabled), even when emitting a ``.s`` file.
.. _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<address space>``
Specifies the address space that corresponds to program memory.
Harvard architectures can use this to specify what space LLVM
should place things such as functions into. If omitted, the
program memory space defaults to the default address space of 0,
which corresponds to a Von Neumann architecture that has code
and data in the same space.
``G<address space>``
Specifies the address space to be used by default when creating global
variables. If omitted, the globals address space defaults to the default
address space 0.
Note: variable declarations without an address space are always created in
address space 0, this property only affects the default value to be used
when creating globals without additional contextual information (e.g. in
LLVM passes).
``A<address space>``
Specifies the address space of objects created by '``alloca``'.
Defaults to the default address space of 0.
``p[n]:<size>:<abi>:<pref>:<idx>``
This specifies the *size* of a pointer and its ``<abi>`` and
``<pref>``\erred alignments for address space ``n``. The fourth parameter
``<idx>`` is a size of index that used for address calculation. If not
specified, the default index size is equal to the pointer size. All sizes
are in bits. 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:<abi>:<pref>``
This specifies the alignment for an object of aggregate type.
``F<type><abi>``
This specifies the alignment for function pointers.
The options for ``<type>`` are:
* ``i``: The alignment of function pointers is independent of the alignment
of functions, and is a multiple of ``<abi>``.
* ``n``: The alignment of function pointers is a multiple of the explicit
alignment specified on the function, and is a multiple of ``<abi>``.
``m:<mangling>``
If present, specifies that llvm names are mangled in the output. Symbols
prefixed with the mangling escape character ``\01`` are passed through
directly to the assembler without the escape character. The mangling style
options are
* ``e``: ELF mangling: Private symbols get a ``.L`` prefix.
* ``m``: Mips mangling: Private symbols get a ``$`` prefix.
* ``o``: Mach-O mangling: Private symbols get ``L`` prefix. Other
symbols get a ``_`` prefix.
* ``x``: Windows x86 COFF mangling: Private symbols get the usual prefix.
Regular C symbols get a ``_`` prefix. Functions with ``__stdcall``,
``__fastcall``, and ``__vectorcall`` have custom mangling that appends
``@N`` where N is the number of bytes used to pass parameters. C++ symbols
starting with ``?`` are not mangled in any way.
* ``w``: Windows COFF mangling: Similar to ``x``, except that normal C
symbols do not receive a ``_`` prefix.
* ``a``: XCOFF mangling: Private symbols get a ``L..`` prefix.
``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.
``ni:<address space0>:<address space1>:<address space2>...``
This specifies pointer types with the specified address spaces
as :ref:`Non-Integral Pointer Type <nointptrtype>` s. The ``0``
address space cannot be specified as non-integral.
On every specification that takes a ``<abi>:<pref>``, specifying the
``<pref>`` alignment is optional. If omitted, the preceding ``:``
should be omitted too and ``<pref>`` will be equal to ``<abi>``.
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
- ``a: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. There is no way to generate IR
that does not embed this target-specific detail into the IR. If you
don't specify the string, the default specifications will be used to
generate a Data Layout and the optimization phases will operate
accordingly and introduce target specificity into the IR with respect to
these default specifications.
.. _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.
.. _objectlifetime:
Object Lifetime
----------------------
A memory object, or simply object, is a region of a memory space that is
reserved by a memory allocation such as :ref:`alloca <i_alloca>`, heap
allocation calls, and global variable definitions.
Once it is allocated, the bytes stored in the region can only be read or written
through a pointer that is :ref:`based on <pointeraliasing>` the allocation
value.
If a pointer that is not based on the object tries to read or write to the
object, it is undefined behavior.
A lifetime of a memory object is a property that decides its accessibility.
Unless stated otherwise, a memory object is alive since its allocation, and
dead after its deallocation.
It is undefined behavior to access a memory object that isn't alive, but
operations that don't dereference it such as
:ref:`getelementptr <i_getelementptr>`, :ref:`ptrtoint <i_ptrtoint>` and
:ref:`icmp <i_icmp>` return a valid result.
This explains code motion of these instructions across operations that
impact the object's lifetime.
A stack object's lifetime can be explicitly specified using
:ref:`llvm.lifetime.start <int_lifestart>` and
:ref:`llvm.lifetime.end <int_lifeend>` intrinsic function calls.
.. _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 :ref:`undef value <undefvalues>` in *any* 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 scalar ``getelementptr`` operation is *based* on
the pointer-typed operand of the ``getelementptr``.
- The pointer in lane *l* of the result of a vector ``getelementptr`` operation
is *based* on the pointer in lane *l* of the vector-of-pointers-typed 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.
.. _pointercapture:
Pointer Capture
---------------
Given a function call and a pointer that is passed as an argument or stored in
the memory before the call, a pointer is *captured* by the call if it makes a
copy of any part of the pointer that outlives the call.
To be precise, a pointer is captured if one or more of the following conditions
hold:
1. The call stores any bit of the pointer carrying information into a place,
and the stored bits can be read from the place by the caller after this call
exits.
.. code-block:: llvm
@glb = global i8* null
@glb2 = global i8* null
@glb3 = global i8* null
@glbi = global i32 0
define i8* @f(i8* %a, i8* %b, i8* %c, i8* %d, i8* %e) {
store i8* %a, i8** @glb ; %a is captured by this call
store i8* %b, i8** @glb2 ; %b isn't captured because the stored value is overwritten by the store below
store i8* null, i8** @glb2
store i8* %c, i8** @glb3
call void @g() ; If @g makes a copy of %c that outlives this call (@f), %c is captured
store i8* null, i8** @glb3
%i = ptrtoint i8* %d to i64
%j = trunc i64 %i to i32
store i32 %j, i32* @glbi ; %d is captured
ret i8* %e ; %e is captured
}
2. The call stores any bit of the pointer carrying information into a place,
and the stored bits can be safely read from the place by another thread via
synchronization.
.. code-block:: llvm
@lock = global i1 true
define void @f(i8* %a) {
store i8* %a, i8** @glb
store atomic i1 false, i1* @lock release ; %a is captured because another thread can safely read @glb
store i8* null, i8** @glb
ret void
}
3. The call's behavior depends on any bit of the pointer carrying information.
.. code-block:: llvm
@glb = global i8 0
define void @f(i8* %a) {
%c = icmp eq i8* %a, @glb
br i1 %c, label %BB_EXIT, label %BB_CONTINUE ; escapes %a
BB_EXIT:
call void @exit()
unreachable
BB_CONTINUE:
ret void
}
4. The pointer is used in a volatile access as its address.
.. _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.
A volatile load or store may have additional target-specific semantics.
Any volatile operation can have side effects, and any volatile operation
can read and/or modify state which is not accessible via a regular load
or store in this module. Volatile operations may use addresses which do
not point to memory (like MMIO registers). This means the compiler may
not use a volatile operation to prove a non-volatile access to that
address has defined behavior.
The allowed side-effects for volatile accesses are limited. If a
non-volatile store to a given address would be legal, a volatile
operation may modify the memory at that address. A volatile operation
may not modify any other memory accessible by the module being compiled.
A volatile operation may not call any code in the current module.
The compiler may assume execution will continue after a volatile operation,
so operations which modify memory or may have undefined behavior can be
hoisted past a volatile operation.
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. Similarly, IR-level volatile loads and stores cannot
change from integer to floating-point or vice versa.
.. 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 transformations
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 that 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
ordering parameters that determine 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 that only reads, ``release`` for an operation that 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.
.. _syncscope:
If an atomic operation is marked ``syncscope("singlethread")``, it only
*synchronizes with* and only participates in the seq\_cst total orderings of
other operations running in the same thread (for example, in signal handlers).
If an atomic operation is marked ``syncscope("<target-scope>")``, where
``<target-scope>`` is a target specific synchronization scope, then it is target
dependent if it *synchronizes with* and participates in the seq\_cst total
orderings of other operations.
Otherwise, an atomic operation that is not marked ``syncscope("singlethread")``
or ``syncscope("<target-scope>")`` *synchronizes with* and participates in the
seq\_cst total orderings of other operations that are not marked
``syncscope("singlethread")`` or ``syncscope("<target-scope>")``.
.. _floatenv:
Floating-Point Environment
--------------------------
The default LLVM floating-point environment assumes that floating-point
instructions do not have side effects. Results assume the round-to-nearest
rounding mode. No floating-point exception state is maintained in this
environment. Therefore, there is no attempt to create or preserve invalid
operation (SNaN) or division-by-zero exceptions.
The benefit of this exception-free assumption is that floating-point
operations may be speculated freely without any other fast-math relaxations
to the floating-point model.
Code that requires different behavior than this should use the
:ref:`Constrained Floating-Point Intrinsics <constrainedfp>`.
.. _fastmath:
Fast-Math Flags
---------------
LLVM IR floating-point operations (:ref:`fneg <i_fneg>`, :ref:`fadd <i_fadd>`,
:ref:`fsub <i_fsub>`, :ref:`fmul <i_fmul>`, :ref:`fdiv <i_fdiv>`,
:ref:`frem <i_frem>`, :ref:`fcmp <i_fcmp>`), :ref:`phi <i_phi>`,
:ref:`select <i_select>` and :ref:`call <i_call>`
may use the following flags to enable otherwise unsafe
floating-point transformations.
``nnan``
No NaNs - Allow optimizations to assume the arguments and result are not
NaN. If an argument is a nan, or the result would be a nan, it produces
a :ref:`poison value <poisonvalues>` instead.
``ninf``
No Infs - Allow optimizations to assume the arguments and result are not
+/-Inf. If an argument is +/-Inf, or the result would be +/-Inf, it
produces a :ref:`poison value <poisonvalues>` instead.
``nsz``
No Signed Zeros - Allow optimizations to treat the sign of a zero
argument or result as insignificant. This does not imply that -0.0
is poison and/or guaranteed to not exist in the operation.
``arcp``
Allow Reciprocal - Allow optimizations to use the reciprocal of an
argument rather than perform division.
``contract``
Allow floating-point contraction (e.g. fusing a multiply followed by an
addition into a fused multiply-and-add). This does not enable reassociating
to form arbitrary contractions. For example, ``(a*b) + (c*d) + e`` can not
be transformed into ``(a*b) + ((c*d) + e)`` to create two fma operations.
``afn``
Approximate functions - Allow substitution of approximate calculations for
functions (sin, log, sqrt, etc). See floating-point intrinsic definitions
for places where this can apply to LLVM's intrinsic math functions.
``reassoc``
Allow reassociation transformations for floating-point instructions.
This may dramatically change results in floating-point.
``fast``
This flag implies all of the others.
.. _uselistorder:
Use-list Order Directives
-------------------------
Use-list directives encode the in-memory order of each use-list, allowing the
order to be recreated. ``<order-indexes>`` is a comma-separated list of
indexes that are assigned to the referenced value's uses. The referenced
value's use-list is immediately sorted by these indexes.
Use-list directives may appear at function scope or global scope. They are not
instructions, and have no effect on the semantics of the IR. When they're at
function scope, they must appear after the terminator of the final basic block.
If basic blocks have their address taken via ``blockaddress()`` expressions,
``uselistorder_bb`` can be used to reorder their use-lists from outside their
function's scope.
:Syntax:
::
uselistorder <ty> <value>, { <order-indexes> }
uselistorder_bb @function, %block { <order-indexes> }
:Examples:
::
define void @foo(i32 %arg1, i32 %arg2) {
entry:
; ... instructions ...
bb:
; ... instructions ...
; At function scope.
uselistorder i32 %arg1, { 1, 0, 2 }
uselistorder label %bb, { 1, 0 }
}
; At global scope.
uselistorder i32* @global, { 1, 2, 0 }
uselistorder i32 7, { 1, 0 }
uselistorder i32 (i32) @bar, { 1, 0 }
uselistorder_bb @foo, %bb, { 5, 1, 3, 2, 0, 4 }
.. _source_filename:
Source Filename
---------------
The *source filename* string is set to the original module identifier,
which will be the name of the compiled source file when compiling from
source through the clang front end, for example. It is then preserved through
the IR and bitcode.
This is currently necessary to generate a consistent unique global
identifier for local functions used in profile data, which prepends the
source file name to the local function name.
The syntax for the source file name is simply:
.. code-block:: text
source_filename = "/path/to/source.c"
.. _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.
.. _t_void:
Void Type
---------
:Overview:
The void type does not represent any value and has no size.
:Syntax:
::
void
.. _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_firstclass:
First Class Types
-----------------
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_single_value:
Single Value Types
^^^^^^^^^^^^^^^^^^
These are the types that are valid in registers from CodeGen's perspective.
.. _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
* - ``bfloat``
- 16-bit "brain" floating-point value (7-bit significand). Provides the
same number of exponent bits as ``float``, so that it matches its dynamic
range, but with greatly reduced precision. Used in Intel's AVX-512 BF16
extensions and Arm's ARMv8.6-A extensions, among others.
* - ``float``
- 32-bit floating-point value
* - ``double``
- 64-bit floating-point value
* - ``fp128``
- 128-bit floating-point value (113-bit significand)
* - ``x86_fp80``
- 80-bit floating-point value (X87)
* - ``ppc_fp128``
- 128-bit floating-point value (two 64-bits)
The binary format of half, float, double, and fp128 correspond to the
IEEE-754-2008 specifications for binary16, binary32, binary64, and binary128
respectively.
X86_mmx Type
""""""""""""
:Overview:
The x86_mmx 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:
::
x86_mmx
.. _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), an underlying primitive data type,
and a scalable property to represent vectors where the exact hardware
vector length is unknown at compile time. Vector types are considered
:ref:`first class <t_firstclass>`.
:Memory Layout:
In general vector elements are laid out in memory in the same way as
:ref:`array types <t_array>`. Such an anology works fine as long as the vector
elements are byte sized. However, when the elements of the vector aren't byte
sized it gets a bit more complicated. One way to describe the layout is by
describing what happens when a vector such as <N x iM> is bitcasted to an
integer type with N*M bits, and then following the rules for storing such an
integer to memory.
A bitcast from a vector type to a scalar integer type will see the elements
being packed together (without padding). The order in which elements are
inserted in the integer depends on endianess. For little endian element zero
is put in the least significant bits of the integer, and for big endian
element zero is put in the most significant bits.
Using a vector such as ``<i4 1, i4 2, i4 3, i4 5>`` as an example, together
with the analogy that we can replace a vector store by a bitcast followed by
an integer store, we get this for big endian:
.. code-block:: llvm
%val = bitcast <4 x i4> <i4 1, i4 2, i4 3, i4 5> to i16
; Bitcasting from a vector to an integral type can be seen as
; concatenating the values:
; %val now has the hexadecimal value 0x1235.
store i16 %val, i16* %ptr
; In memory the content will be (8-bit addressing):
;
; [%ptr + 0]: 00010010 (0x12)
; [%ptr + 1]: 00110101 (0x35)
The same example for little endian:
.. code-block:: llvm
%val = bitcast <4 x i4> <i4 1, i4 2, i4 3, i4 5> to i16
; Bitcasting from a vector to an integral type can be seen as
; concatenating the values:
; %val now has the hexadecimal value 0x5321.
store i16 %val, i16* %ptr
; In memory the content will be (8-bit addressing):
;
; [%ptr + 0]: 01010011 (0x53)
; [%ptr + 1]: 00100001 (0x21)
When ``<N*M>`` isn't evenly divisible by the byte size the exact memory layout
is unspecified (just like it is for an integral type of the same size). This
is because different targets could put the padding at different positions when
the type size is smaller than the type's store size.
:Syntax:
::
< <# elements> x <elementtype> > ; Fixed-length vector
< vscale x <# elements> x <elementtype> > ; Scalable vector
The number of elements is a constant integer value larger than 0;
elementtype may be any integer, floating-point or pointer type. Vectors
of size zero are not allowed. For scalable vectors, the total number of
elements is a constant multiple (called vscale) of the specified number
of elements; vscale is a positive integer that is unknown at compile time
and the same hardware-dependent constant for all scalable vectors at run
time. The size of a specific scalable vector type is thus constant within
IR, even if the exact size in bytes cannot be determined until run time.
: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. |
+------------------------+----------------------------------------------------+
| ``<vscale x 4 x i32>`` | Vector with a multiple of 4 32-bit integer values. |
+------------------------+----------------------------------------------------+
.. _t_label:
Label Type
^^^^^^^^^^
:Overview:
The label type represents code labels.
:Syntax:
::
label
.. _t_token:
Token Type
^^^^^^^^^^
:Overview:
The token type is used when a value is associated with an instruction
but all uses of the value must not attempt to introspect or obscure it.
As such, it is not appropriate to have a :ref:`phi <i_phi>` or
:ref:`select <i_select>` of type token.
:Syntax:
::
token
.. _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_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_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 structure types that
do not have a body specified. This corresponds (for example) to the C
notion of a forward declared structure. They can be named (``%X``) or
unnamed (``%52``).
:Syntax:
::
%X = type opaque
%52 = type opaque
:Examples:
+--------------+-------------------+
| ``opaque`` | An opaque type. |
+--------------+-------------------+
.. _constants: