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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
"http://www.w3.org/TR/html4/strict.dtd">
<html>
<head>
<title>LLVM Assembly Language Reference Manual</title>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<meta name="author" content="Chris Lattner">
<meta name="description"
content="LLVM Assembly Language Reference Manual.">
<link rel="stylesheet" href="llvm.css" type="text/css">
</head>
<body>
<h1>LLVM Language Reference Manual</h1>
<ol>
<li><a href="#abstract">Abstract</a></li>
<li><a href="#introduction">Introduction</a></li>
<li><a href="#identifiers">Identifiers</a></li>
<li><a href="#highlevel">High Level Structure</a>
<ol>
<li><a href="#modulestructure">Module Structure</a></li>
<li><a href="#linkage">Linkage Types</a>
<ol>
<li><a href="#linkage_private">'<tt>private</tt>' Linkage</a></li>
<li><a href="#linkage_linker_private">'<tt>linker_private</tt>' Linkage</a></li>
<li><a href="#linkage_linker_private_weak">'<tt>linker_private_weak</tt>' Linkage</a></li>
<li><a href="#linkage_linker_private_weak_def_auto">'<tt>linker_private_weak_def_auto</tt>' Linkage</a></li>
<li><a href="#linkage_internal">'<tt>internal</tt>' Linkage</a></li>
<li><a href="#linkage_available_externally">'<tt>available_externally</tt>' Linkage</a></li>
<li><a href="#linkage_linkonce">'<tt>linkonce</tt>' Linkage</a></li>
<li><a href="#linkage_common">'<tt>common</tt>' Linkage</a></li>
<li><a href="#linkage_weak">'<tt>weak</tt>' Linkage</a></li>
<li><a href="#linkage_appending">'<tt>appending</tt>' Linkage</a></li>
<li><a href="#linkage_externweak">'<tt>extern_weak</tt>' Linkage</a></li>
<li><a href="#linkage_linkonce_odr">'<tt>linkonce_odr</tt>' Linkage</a></li>
<li><a href="#linkage_weak">'<tt>weak_odr</tt>' Linkage</a></li>
<li><a href="#linkage_external">'<tt>external</tt>' Linkage</a></li>
<li><a href="#linkage_dllimport">'<tt>dllimport</tt>' Linkage</a></li>
<li><a href="#linkage_dllexport">'<tt>dllexport</tt>' Linkage</a></li>
</ol>
</li>
<li><a href="#callingconv">Calling Conventions</a></li>
<li><a href="#namedtypes">Named Types</a></li>
<li><a href="#globalvars">Global Variables</a></li>
<li><a href="#functionstructure">Functions</a></li>
<li><a href="#aliasstructure">Aliases</a></li>
<li><a href="#namedmetadatastructure">Named Metadata</a></li>
<li><a href="#paramattrs">Parameter Attributes</a></li>
<li><a href="#fnattrs">Function Attributes</a></li>
<li><a href="#gc">Garbage Collector Names</a></li>
<li><a href="#moduleasm">Module-Level Inline Assembly</a></li>
<li><a href="#datalayout">Data Layout</a></li>
<li><a href="#pointeraliasing">Pointer Aliasing Rules</a></li>
<li><a href="#volatile">Volatile Memory Accesses</a></li>
<li><a href="#memmodel">Memory Model for Concurrent Operations</a></li>
<li><a href="#ordering">Atomic Memory Ordering Constraints</a></li>
</ol>
</li>
<li><a href="#typesystem">Type System</a>
<ol>
<li><a href="#t_classifications">Type Classifications</a></li>
<li><a href="#t_primitive">Primitive Types</a>
<ol>
<li><a href="#t_integer">Integer Type</a></li>
<li><a href="#t_floating">Floating Point Types</a></li>
<li><a href="#t_x86mmx">X86mmx Type</a></li>
<li><a href="#t_void">Void Type</a></li>
<li><a href="#t_label">Label Type</a></li>
<li><a href="#t_metadata">Metadata Type</a></li>
</ol>
</li>
<li><a href="#t_derived">Derived Types</a>
<ol>
<li><a href="#t_aggregate">Aggregate Types</a>
<ol>
<li><a href="#t_array">Array Type</a></li>
<li><a href="#t_struct">Structure Type</a></li>
<li><a href="#t_opaque">Opaque Structure Types</a></li>
<li><a href="#t_vector">Vector Type</a></li>
</ol>
</li>
<li><a href="#t_function">Function Type</a></li>
<li><a href="#t_pointer">Pointer Type</a></li>
</ol>
</li>
</ol>
</li>
<li><a href="#constants">Constants</a>
<ol>
<li><a href="#simpleconstants">Simple Constants</a></li>
<li><a href="#complexconstants">Complex Constants</a></li>
<li><a href="#globalconstants">Global Variable and Function Addresses</a></li>
<li><a href="#undefvalues">Undefined Values</a></li>
<li><a href="#poisonvalues">Poison Values</a></li>
<li><a href="#blockaddress">Addresses of Basic Blocks</a></li>
<li><a href="#constantexprs">Constant Expressions</a></li>
</ol>
</li>
<li><a href="#othervalues">Other Values</a>
<ol>
<li><a href="#inlineasm">Inline Assembler Expressions</a></li>
<li><a href="#metadata">Metadata Nodes and Metadata Strings</a>
<ol>
<li><a href="#tbaa">'<tt>tbaa</tt>' Metadata</a></li>
<li><a href="#fpaccuracy">'<tt>fpaccuracy</tt>' Metadata</a></li>
</ol>
</li>
</ol>
</li>
<li><a href="#intrinsic_globals">Intrinsic Global Variables</a>
<ol>
<li><a href="#intg_used">The '<tt>llvm.used</tt>' Global Variable</a></li>
<li><a href="#intg_compiler_used">The '<tt>llvm.compiler.used</tt>'
Global Variable</a></li>
<li><a href="#intg_global_ctors">The '<tt>llvm.global_ctors</tt>'
Global Variable</a></li>
<li><a href="#intg_global_dtors">The '<tt>llvm.global_dtors</tt>'
Global Variable</a></li>
</ol>
</li>
<li><a href="#instref">Instruction Reference</a>
<ol>
<li><a href="#terminators">Terminator Instructions</a>
<ol>
<li><a href="#i_ret">'<tt>ret</tt>' Instruction</a></li>
<li><a href="#i_br">'<tt>br</tt>' Instruction</a></li>
<li><a href="#i_switch">'<tt>switch</tt>' Instruction</a></li>
<li><a href="#i_indirectbr">'<tt>indirectbr</tt>' Instruction</a></li>
<li><a href="#i_invoke">'<tt>invoke</tt>' Instruction</a></li>
<li><a href="#i_unwind">'<tt>unwind</tt>' Instruction</a></li>
<li><a href="#i_resume">'<tt>resume</tt>' Instruction</a></li>
<li><a href="#i_unreachable">'<tt>unreachable</tt>' Instruction</a></li>
</ol>
</li>
<li><a href="#binaryops">Binary Operations</a>
<ol>
<li><a href="#i_add">'<tt>add</tt>' Instruction</a></li>
<li><a href="#i_fadd">'<tt>fadd</tt>' Instruction</a></li>
<li><a href="#i_sub">'<tt>sub</tt>' Instruction</a></li>
<li><a href="#i_fsub">'<tt>fsub</tt>' Instruction</a></li>
<li><a href="#i_mul">'<tt>mul</tt>' Instruction</a></li>
<li><a href="#i_fmul">'<tt>fmul</tt>' Instruction</a></li>
<li><a href="#i_udiv">'<tt>udiv</tt>' Instruction</a></li>
<li><a href="#i_sdiv">'<tt>sdiv</tt>' Instruction</a></li>
<li><a href="#i_fdiv">'<tt>fdiv</tt>' Instruction</a></li>
<li><a href="#i_urem">'<tt>urem</tt>' Instruction</a></li>
<li><a href="#i_srem">'<tt>srem</tt>' Instruction</a></li>
<li><a href="#i_frem">'<tt>frem</tt>' Instruction</a></li>
</ol>
</li>
<li><a href="#bitwiseops">Bitwise Binary Operations</a>
<ol>
<li><a href="#i_shl">'<tt>shl</tt>' Instruction</a></li>
<li><a href="#i_lshr">'<tt>lshr</tt>' Instruction</a></li>
<li><a href="#i_ashr">'<tt>ashr</tt>' Instruction</a></li>
<li><a href="#i_and">'<tt>and</tt>' Instruction</a></li>
<li><a href="#i_or">'<tt>or</tt>' Instruction</a></li>
<li><a href="#i_xor">'<tt>xor</tt>' Instruction</a></li>
</ol>
</li>
<li><a href="#vectorops">Vector Operations</a>
<ol>
<li><a href="#i_extractelement">'<tt>extractelement</tt>' Instruction</a></li>
<li><a href="#i_insertelement">'<tt>insertelement</tt>' Instruction</a></li>
<li><a href="#i_shufflevector">'<tt>shufflevector</tt>' Instruction</a></li>
</ol>
</li>
<li><a href="#aggregateops">Aggregate Operations</a>
<ol>
<li><a href="#i_extractvalue">'<tt>extractvalue</tt>' Instruction</a></li>
<li><a href="#i_insertvalue">'<tt>insertvalue</tt>' Instruction</a></li>
</ol>
</li>
<li><a href="#memoryops">Memory Access and Addressing Operations</a>
<ol>
<li><a href="#i_alloca">'<tt>alloca</tt>' Instruction</a></li>
<li><a href="#i_load">'<tt>load</tt>' Instruction</a></li>
<li><a href="#i_store">'<tt>store</tt>' Instruction</a></li>
<li><a href="#i_fence">'<tt>fence</tt>' Instruction</a></li>
<li><a href="#i_cmpxchg">'<tt>cmpxchg</tt>' Instruction</a></li>
<li><a href="#i_atomicrmw">'<tt>atomicrmw</tt>' Instruction</a></li>
<li><a href="#i_getelementptr">'<tt>getelementptr</tt>' Instruction</a></li>
</ol>
</li>
<li><a href="#convertops">Conversion Operations</a>
<ol>
<li><a href="#i_trunc">'<tt>trunc .. to</tt>' Instruction</a></li>
<li><a href="#i_zext">'<tt>zext .. to</tt>' Instruction</a></li>
<li><a href="#i_sext">'<tt>sext .. to</tt>' Instruction</a></li>
<li><a href="#i_fptrunc">'<tt>fptrunc .. to</tt>' Instruction</a></li>
<li><a href="#i_fpext">'<tt>fpext .. to</tt>' Instruction</a></li>
<li><a href="#i_fptoui">'<tt>fptoui .. to</tt>' Instruction</a></li>
<li><a href="#i_fptosi">'<tt>fptosi .. to</tt>' Instruction</a></li>
<li><a href="#i_uitofp">'<tt>uitofp .. to</tt>' Instruction</a></li>
<li><a href="#i_sitofp">'<tt>sitofp .. to</tt>' Instruction</a></li>
<li><a href="#i_ptrtoint">'<tt>ptrtoint .. to</tt>' Instruction</a></li>
<li><a href="#i_inttoptr">'<tt>inttoptr .. to</tt>' Instruction</a></li>
<li><a href="#i_bitcast">'<tt>bitcast .. to</tt>' Instruction</a></li>
</ol>
</li>
<li><a href="#otherops">Other Operations</a>
<ol>
<li><a href="#i_icmp">'<tt>icmp</tt>' Instruction</a></li>
<li><a href="#i_fcmp">'<tt>fcmp</tt>' Instruction</a></li>
<li><a href="#i_phi">'<tt>phi</tt>' Instruction</a></li>
<li><a href="#i_select">'<tt>select</tt>' Instruction</a></li>
<li><a href="#i_call">'<tt>call</tt>' Instruction</a></li>
<li><a href="#i_va_arg">'<tt>va_arg</tt>' Instruction</a></li>
<li><a href="#i_landingpad">'<tt>landingpad</tt>' Instruction</a></li>
</ol>
</li>
</ol>
</li>
<li><a href="#intrinsics">Intrinsic Functions</a>
<ol>
<li><a href="#int_varargs">Variable Argument Handling Intrinsics</a>
<ol>
<li><a href="#int_va_start">'<tt>llvm.va_start</tt>' Intrinsic</a></li>
<li><a href="#int_va_end">'<tt>llvm.va_end</tt>' Intrinsic</a></li>
<li><a href="#int_va_copy">'<tt>llvm.va_copy</tt>' Intrinsic</a></li>
</ol>
</li>
<li><a href="#int_gc">Accurate Garbage Collection Intrinsics</a>
<ol>
<li><a href="#int_gcroot">'<tt>llvm.gcroot</tt>' Intrinsic</a></li>
<li><a href="#int_gcread">'<tt>llvm.gcread</tt>' Intrinsic</a></li>
<li><a href="#int_gcwrite">'<tt>llvm.gcwrite</tt>' Intrinsic</a></li>
</ol>
</li>
<li><a href="#int_codegen">Code Generator Intrinsics</a>
<ol>
<li><a href="#int_returnaddress">'<tt>llvm.returnaddress</tt>' Intrinsic</a></li>
<li><a href="#int_frameaddress">'<tt>llvm.frameaddress</tt>' Intrinsic</a></li>
<li><a href="#int_stacksave">'<tt>llvm.stacksave</tt>' Intrinsic</a></li>
<li><a href="#int_stackrestore">'<tt>llvm.stackrestore</tt>' Intrinsic</a></li>
<li><a href="#int_prefetch">'<tt>llvm.prefetch</tt>' Intrinsic</a></li>
<li><a href="#int_pcmarker">'<tt>llvm.pcmarker</tt>' Intrinsic</a></li>
<li><a href="#int_readcyclecounter">'<tt>llvm.readcyclecounter</tt>' Intrinsic</a></li>
</ol>
</li>
<li><a href="#int_libc">Standard C Library Intrinsics</a>
<ol>
<li><a href="#int_memcpy">'<tt>llvm.memcpy.*</tt>' Intrinsic</a></li>
<li><a href="#int_memmove">'<tt>llvm.memmove.*</tt>' Intrinsic</a></li>
<li><a href="#int_memset">'<tt>llvm.memset.*</tt>' Intrinsic</a></li>
<li><a href="#int_sqrt">'<tt>llvm.sqrt.*</tt>' Intrinsic</a></li>
<li><a href="#int_powi">'<tt>llvm.powi.*</tt>' Intrinsic</a></li>
<li><a href="#int_sin">'<tt>llvm.sin.*</tt>' Intrinsic</a></li>
<li><a href="#int_cos">'<tt>llvm.cos.*</tt>' Intrinsic</a></li>
<li><a href="#int_pow">'<tt>llvm.pow.*</tt>' Intrinsic</a></li>
<li><a href="#int_exp">'<tt>llvm.exp.*</tt>' Intrinsic</a></li>
<li><a href="#int_log">'<tt>llvm.log.*</tt>' Intrinsic</a></li>
<li><a href="#int_fma">'<tt>llvm.fma.*</tt>' Intrinsic</a></li>
</ol>
</li>
<li><a href="#int_manip">Bit Manipulation Intrinsics</a>
<ol>
<li><a href="#int_bswap">'<tt>llvm.bswap.*</tt>' Intrinsics</a></li>
<li><a href="#int_ctpop">'<tt>llvm.ctpop.*</tt>' Intrinsic </a></li>
<li><a href="#int_ctlz">'<tt>llvm.ctlz.*</tt>' Intrinsic </a></li>
<li><a href="#int_cttz">'<tt>llvm.cttz.*</tt>' Intrinsic </a></li>
</ol>
</li>
<li><a href="#int_overflow">Arithmetic with Overflow Intrinsics</a>
<ol>
<li><a href="#int_sadd_overflow">'<tt>llvm.sadd.with.overflow.*</tt> Intrinsics</a></li>
<li><a href="#int_uadd_overflow">'<tt>llvm.uadd.with.overflow.*</tt> Intrinsics</a></li>
<li><a href="#int_ssub_overflow">'<tt>llvm.ssub.with.overflow.*</tt> Intrinsics</a></li>
<li><a href="#int_usub_overflow">'<tt>llvm.usub.with.overflow.*</tt> Intrinsics</a></li>
<li><a href="#int_smul_overflow">'<tt>llvm.smul.with.overflow.*</tt> Intrinsics</a></li>
<li><a href="#int_umul_overflow">'<tt>llvm.umul.with.overflow.*</tt> Intrinsics</a></li>
</ol>
</li>
<li><a href="#int_fp16">Half Precision Floating Point Intrinsics</a>
<ol>
<li><a href="#int_convert_to_fp16">'<tt>llvm.convert.to.fp16</tt>' Intrinsic</a></li>
<li><a href="#int_convert_from_fp16">'<tt>llvm.convert.from.fp16</tt>' Intrinsic</a></li>
</ol>
</li>
<li><a href="#int_debugger">Debugger intrinsics</a></li>
<li><a href="#int_eh">Exception Handling intrinsics</a></li>
<li><a href="#int_trampoline">Trampoline Intrinsics</a>
<ol>
<li><a href="#int_it">'<tt>llvm.init.trampoline</tt>' Intrinsic</a></li>
<li><a href="#int_at">'<tt>llvm.adjust.trampoline</tt>' Intrinsic</a></li>
</ol>
</li>
<li><a href="#int_memorymarkers">Memory Use Markers</a>
<ol>
<li><a href="#int_lifetime_start">'<tt>llvm.lifetime.start</tt>' Intrinsic</a></li>
<li><a href="#int_lifetime_end">'<tt>llvm.lifetime.end</tt>' Intrinsic</a></li>
<li><a href="#int_invariant_start">'<tt>llvm.invariant.start</tt>' Intrinsic</a></li>
<li><a href="#int_invariant_end">'<tt>llvm.invariant.end</tt>' Intrinsic</a></li>
</ol>
</li>
<li><a href="#int_general">General intrinsics</a>
<ol>
<li><a href="#int_var_annotation">
'<tt>llvm.var.annotation</tt>' Intrinsic</a></li>
<li><a href="#int_annotation">
'<tt>llvm.annotation.*</tt>' Intrinsic</a></li>
<li><a href="#int_trap">
'<tt>llvm.trap</tt>' Intrinsic</a></li>
<li><a href="#int_stackprotector">
'<tt>llvm.stackprotector</tt>' Intrinsic</a></li>
<li><a href="#int_objectsize">
'<tt>llvm.objectsize</tt>' Intrinsic</a></li>
<li><a href="#int_expect">
'<tt>llvm.expect</tt>' Intrinsic</a></li>
</ol>
</li>
</ol>
</li>
</ol>
<div class="doc_author">
<p>Written by <a href="mailto:[email protected]">Chris Lattner</a>
and <a href="mailto:[email protected]">Vikram Adve</a></p>
</div>
<!-- *********************************************************************** -->
<h2><a name="abstract">Abstract</a></h2>
<!-- *********************************************************************** -->
<div>
<p>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.</p>
</div>
<!-- *********************************************************************** -->
<h2><a name="introduction">Introduction</a></h2>
<!-- *********************************************************************** -->
<div>
<p>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.</p>
<p>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.</p>
<!-- _______________________________________________________________________ -->
<h4>
<a name="wellformed">Well-Formedness</a>
</h4>
<div>
<p>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:</p>
<pre class="doc_code">
%x = <a href="#i_add">add</a> i32 1, %x
</pre>
<p>because the definition of <tt>%x</tt> 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.</p>
</div>
</div>
<!-- Describe the typesetting conventions here. -->
<!-- *********************************************************************** -->
<h2><a name="identifiers">Identifiers</a></h2>
<!-- *********************************************************************** -->
<div>
<p>LLVM identifiers come in two basic types: global and local. Global
identifiers (functions, global variables) begin with the <tt>'@'</tt>
character. Local identifiers (register names, types) begin with
the <tt>'%'</tt> character. Additionally, there are three different formats
for identifiers, for different purposes:</p>
<ol>
<li>Named values are represented as a string of characters with their prefix.
For example, <tt>%foo</tt>, <tt>@DivisionByZero</tt>,
<tt>%a.really.long.identifier</tt>. The actual regular expression used is
'<tt>[%@][a-zA-Z$._][a-zA-Z$._0-9]*</tt>'. Identifiers which require
other characters in their names can be surrounded with quotes. Special
characters may be escaped using <tt>"\xx"</tt> where <tt>xx</tt> is the
ASCII code for the character in hexadecimal. In this way, any character
can be used in a name value, even quotes themselves.</li>
<li>Unnamed values are represented as an unsigned numeric value with their
prefix. For example, <tt>%12</tt>, <tt>@2</tt>, <tt>%44</tt>.</li>
<li>Constants, which are described in a <a href="#constants">section about
constants</a>, below.</li>
</ol>
<p>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.</p>
<p>Reserved words in LLVM are very similar to reserved words in other
languages. There are keywords for different opcodes
('<tt><a href="#i_add">add</a></tt>',
'<tt><a href="#i_bitcast">bitcast</a></tt>',
'<tt><a href="#i_ret">ret</a></tt>', etc...), for primitive type names
('<tt><a href="#t_void">void</a></tt>',
'<tt><a href="#t_primitive">i32</a></tt>', etc...), and others. These
reserved words cannot conflict with variable names, because none of them
start with a prefix character (<tt>'%'</tt> or <tt>'@'</tt>).</p>
<p>Here is an example of LLVM code to multiply the integer variable
'<tt>%X</tt>' by 8:</p>
<p>The easy way:</p>
<pre class="doc_code">
%result = <a href="#i_mul">mul</a> i32 %X, 8
</pre>
<p>After strength reduction:</p>
<pre class="doc_code">
%result = <a href="#i_shl">shl</a> i32 %X, i8 3
</pre>
<p>And the hard way:</p>
<pre class="doc_code">
%0 = <a href="#i_add">add</a> i32 %X, %X <i>; yields {i32}:%0</i>
%1 = <a href="#i_add">add</a> i32 %0, %0 <i>; yields {i32}:%1</i>
%result = <a href="#i_add">add</a> i32 %1, %1
</pre>
<p>This last way of multiplying <tt>%X</tt> by 8 illustrates several important
lexical features of LLVM:</p>
<ol>
<li>Comments are delimited with a '<tt>;</tt>' and go until the end of
line.</li>
<li>Unnamed temporaries are created when the result of a computation is not
assigned to a named value.</li>
<li>Unnamed temporaries are numbered sequentially</li>
</ol>
<p>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. Comments are shown in italic
text.</p>
</div>
<!-- *********************************************************************** -->
<h2><a name="highlevel">High Level Structure</a></h2>
<!-- *********************************************************************** -->
<div>
<!-- ======================================================================= -->
<h3>
<a name="modulestructure">Module Structure</a>
</h3>
<div>
<p>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:</p>
<pre class="doc_code">
<i>; Declare the string constant as a global constant.</i>
<a href="#identifiers">@.LC0</a> = <a href="#linkage_internal">internal</a> <a href="#globalvars">constant</a> <a href="#t_array">[13 x i8]</a> c"hello world\0A\00" <i>; [13 x i8]*</i>
<i>; External declaration of the puts function</i>
<a href="#functionstructure">declare</a> i32 @puts(i8*) <i>; i32 (i8*)* </i>
<i>; Definition of main function</i>
define i32 @main() { <i>; i32()* </i>
<i>; Convert [13 x i8]* to i8 *...</i>
%cast210 = <a href="#i_getelementptr">getelementptr</a> [13 x i8]* @.LC0, i64 0, i64 0 <i>; i8*</i>
<i>; Call puts function to write out the string to stdout.</i>
<a href="#i_call">call</a> i32 @puts(i8* %cast210) <i>; i32</i>
<a href="#i_ret">ret</a> i32 0
}
<i>; Named metadata</i>
!1 = metadata !{i32 41}
!foo = !{!1, null}
</pre>
<p>This example is made up of a <a href="#globalvars">global variable</a> named
"<tt>.LC0</tt>", an external declaration of the "<tt>puts</tt>" function,
a <a href="#functionstructure">function definition</a> for
"<tt>main</tt>" and <a href="#namedmetadatastructure">named metadata</a>
"<tt>foo"</tt>.</p>
<p>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 <a href="#linkage">linkage types</a>.</p>
</div>
<!-- ======================================================================= -->
<h3>
<a name="linkage">Linkage Types</a>
</h3>
<div>
<p>All Global Variables and Functions have one of the following types of
linkage:</p>
<dl>
<dt><tt><b><a name="linkage_private">private</a></b></tt></dt>
<dd>Global values with "<tt>private</tt>" linkage are only directly accessible
by objects in the current module. In particular, linking code into a
module with an private global value may cause the private to be renamed as
necessary to avoid collisions. Because the symbol is private to the
module, all references can be updated. This doesn't show up in any symbol
table in the object file.</dd>
<dt><tt><b><a name="linkage_linker_private">linker_private</a></b></tt></dt>
<dd>Similar to <tt>private</tt>, but the symbol is passed through the
assembler and evaluated by the linker. Unlike normal strong symbols, they
are removed by the linker from the final linked image (executable or
dynamic library).</dd>
<dt><tt><b><a name="linkage_linker_private_weak">linker_private_weak</a></b></tt></dt>
<dd>Similar to "<tt>linker_private</tt>", but the symbol is weak. Note that
<tt>linker_private_weak</tt> symbols are subject to coalescing by the
linker. The symbols are removed by the linker from the final linked image
(executable or dynamic library).</dd>
<dt><tt><b><a name="linkage_linker_private_weak_def_auto">linker_private_weak_def_auto</a></b></tt></dt>
<dd>Similar to "<tt>linker_private_weak</tt>", but it's known that the address
of the object is not taken. For instance, functions that had an inline
definition, but the compiler decided not to inline it. Note,
unlike <tt>linker_private</tt> and <tt>linker_private_weak</tt>,
<tt>linker_private_weak_def_auto</tt> may have only <tt>default</tt>
visibility. The symbols are removed by the linker from the final linked
image (executable or dynamic library).</dd>
<dt><tt><b><a name="linkage_internal">internal</a></b></tt></dt>
<dd>Similar to private, but the value shows as a local symbol
(<tt>STB_LOCAL</tt> in the case of ELF) in the object file. This
corresponds to the notion of the '<tt>static</tt>' keyword in C.</dd>
<dt><tt><b><a name="linkage_available_externally">available_externally</a></b></tt></dt>
<dd>Globals with "<tt>available_externally</tt>" linkage are never emitted
into the object file corresponding to the LLVM module. They exist to
allow inlining and other optimizations to take place given knowledge of
the definition of the global, which is known to be somewhere outside the
module. Globals with <tt>available_externally</tt> linkage are allowed to
be discarded at will, and are otherwise the same as <tt>linkonce_odr</tt>.
This linkage type is only allowed on definitions, not declarations.</dd>
<dt><tt><b><a name="linkage_linkonce">linkonce</a></b></tt></dt>
<dd>Globals with "<tt>linkonce</tt>" 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
<tt>linkonce</tt> globals are allowed to be discarded. Note that
<tt>linkonce</tt> 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 "<tt>linkonce_odr</tt>"
linkage.</dd>
<dt><tt><b><a name="linkage_weak">weak</a></b></tt></dt>
<dd>"<tt>weak</tt>" linkage has the same merging semantics as
<tt>linkonce</tt> linkage, except that unreferenced globals with
<tt>weak</tt> linkage may not be discarded. This is used for globals that
are declared "weak" in C source code.</dd>
<dt><tt><b><a name="linkage_common">common</a></b></tt></dt>
<dd>"<tt>common</tt>" linkage is most similar to "<tt>weak</tt>" linkage, but
they are used for tentative definitions in C, such as "<tt>int X;</tt>" at
global scope.
Symbols with "<tt>common</tt>" linkage are merged in the same way as
<tt>weak symbols</tt>, and they may not be deleted if unreferenced.
<tt>common</tt> symbols may not have an explicit section,
must have a zero initializer, and may not be marked '<a
href="#globalvars"><tt>constant</tt></a>'. Functions and aliases may not
have common linkage.</dd>
<dt><tt><b><a name="linkage_appending">appending</a></b></tt></dt>
<dd>"<tt>appending</tt>" 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.</dd>
<dt><tt><b><a name="linkage_externweak">extern_weak</a></b></tt></dt>
<dd>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.</dd>
<dt><tt><b><a name="linkage_linkonce_odr">linkonce_odr</a></b></tt></dt>
<dt><tt><b><a name="linkage_weak_odr">weak_odr</a></b></tt></dt>
<dd>Some languages allow differing globals to be merged, such as two functions
with different semantics. Other languages, such as <tt>C++</tt>, ensure
that only equivalent globals are ever merged (the "one definition rule"
— "ODR"). Such languages can use the <tt>linkonce_odr</tt>
and <tt>weak_odr</tt> linkage types to indicate that the global will only
be merged with equivalent globals. These linkage types are otherwise the
same as their non-<tt>odr</tt> versions.</dd>
<dt><tt><b><a name="linkage_external">external</a></b></tt></dt>
<dd>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.</dd>
</dl>
<p>The next two types of linkage are targeted for Microsoft Windows platform
only. They are designed to support importing (exporting) symbols from (to)
DLLs (Dynamic Link Libraries).</p>
<dl>
<dt><tt><b><a name="linkage_dllimport">dllimport</a></b></tt></dt>
<dd>"<tt>dllimport</tt>" linkage causes the compiler to reference a function
or variable via a global pointer to a pointer that is set up by the DLL
exporting the symbol. On Microsoft Windows targets, the pointer name is
formed by combining <code>__imp_</code> and the function or variable
name.</dd>
<dt><tt><b><a name="linkage_dllexport">dllexport</a></b></tt></dt>
<dd>"<tt>dllexport</tt>" linkage causes the compiler to provide a global
pointer to a pointer in a DLL, so that it can be referenced with the
<tt>dllimport</tt> attribute. On Microsoft Windows targets, the pointer
name is formed by combining <code>__imp_</code> and the function or
variable name.</dd>
</dl>
<p>For example, since the "<tt>.LC0</tt>" variable is defined to be internal, if
another module defined a "<tt>.LC0</tt>" variable and was linked with this
one, one of the two would be renamed, preventing a collision. Since
"<tt>main</tt>" and "<tt>puts</tt>" are external (i.e., lacking any linkage
declarations), they are accessible outside of the current module.</p>
<p>It is illegal for a function <i>declaration</i> to have any linkage type
other than <tt>external</tt>, <tt>dllimport</tt>
or <tt>extern_weak</tt>.</p>
<p>Aliases can have only <tt>external</tt>, <tt>internal</tt>, <tt>weak</tt>
or <tt>weak_odr</tt> linkages.</p>
</div>
<!-- ======================================================================= -->
<h3>
<a name="callingconv">Calling Conventions</a>
</h3>
<div>
<p>LLVM <a href="#functionstructure">functions</a>, <a href="#i_call">calls</a>
and <a href="#i_invoke">invokes</a> 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:</p>
<dl>
<dt><b>"<tt>ccc</tt>" - The C calling convention</b>:</dt>
<dd>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).</dd>
<dt><b>"<tt>fastcc</tt>" - The fast calling convention</b>:</dt>
<dd>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).
<a href="CodeGenerator.html#tailcallopt">Tail calls can only be optimized
when this or the GHC convention is used.</a> This calling convention
does not support varargs and requires the prototype of all callees to
exactly match the prototype of the function definition.</dd>
<dt><b>"<tt>coldcc</tt>" - The cold calling convention</b>:</dt>
<dd>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.</dd>
<dt><b>"<tt>cc <em>10</em></tt>" - GHC convention</b>:</dt>
<dd>This calling convention has been implemented specifically for use by the
<a href="http://www.haskell.org/ghc">Glasgow Haskell Compiler (GHC)</a>.
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 <em>register pinning</em> performance technique often used when
implementing functional programming languages.At the moment only X86
supports this convention and it has the following limitations:
<ul>
<li>On <em>X86-32</em> only supports up to 4 bit type parameters. No
floating point types are supported.</li>
<li>On <em>X86-64</em> only supports up to 10 bit type parameters and
6 floating point parameters.</li>
</ul>
This calling convention supports
<a href="CodeGenerator.html#tailcallopt">tail call optimization</a> but
requires both the caller and callee are using it.
</dd>
<dt><b>"<tt>cc <<em>n</em>></tt>" - Numbered convention</b>:</dt>
<dd>Any calling convention may be specified by number, allowing
target-specific calling conventions to be used. Target specific calling
conventions start at 64.</dd>
</dl>
<p>More calling conventions can be added/defined on an as-needed basis, to
support Pascal conventions or any other well-known target-independent
convention.</p>
</div>
<!-- ======================================================================= -->
<h3>
<a name="visibility">Visibility Styles</a>
</h3>
<div>
<p>All Global Variables and Functions have one of the following visibility
styles:</p>
<dl>
<dt><b>"<tt>default</tt>" - Default style</b>:</dt>
<dd>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.</dd>
<dt><b>"<tt>hidden</tt>" - Hidden style</b>:</dt>
<dd>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.</dd>
<dt><b>"<tt>protected</tt>" - Protected style</b>:</dt>
<dd>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.</dd>
</dl>
</div>
<!-- ======================================================================= -->
<h3>
<a name="namedtypes">Named Types</a>
</h3>
<div>
<p>LLVM IR allows you to specify name aliases for certain types. This can make
it easier to read the IR and make the IR more condensed (particularly when
recursive types are involved). An example of a name specification is:</p>
<pre class="doc_code">
%mytype = type { %mytype*, i32 }
</pre>
<p>You may give a name to any <a href="#typesystem">type</a> except
"<a href="#t_void">void</a>". Type name aliases may be used anywhere a type
is expected with the syntax "%mytype".</p>
<p>Note that type names are aliases for the structural type that they indicate,
and that you can therefore specify multiple names for the same type. This
often leads to confusing behavior when dumping out a .ll file. Since LLVM IR
uses structural typing, the name is not part of the type. When printing out
LLVM IR, the printer will pick <em>one name</em> to render all types of a
particular shape. This means that if you have code where two different
source types end up having the same LLVM type, that the dumper will sometimes
print the "wrong" or unexpected type. This is an important design point and
isn't going to change.</p>
</div>
<!-- ======================================================================= -->
<h3>
<a name="globalvars">Global Variables</a>
</h3>
<div>
<p>Global variables define regions of memory allocated at compilation time
instead of run-time. Global variables may optionally be initialized, may
have an explicit section to be placed in, and may have an optional explicit
alignment specified. 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). A variable may be defined as a global
"constant," which indicates that the contents of the variable
will <b>never</b> 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.</p>
<p>LLVM explicitly allows <em>declarations</em> 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.</p>
<p>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.</p>
<p>Global variables can be marked with <tt>unnamed_addr</tt> 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 <em>can</em>
be merged with a <tt>unnamed_addr</tt> constant, the result being a
constant whose address is significant.</p>
<p>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.</p>
<p>LLVM allows an explicit section to be specified for globals. If the target
supports it, it will emit globals to the section specified.</p>
<p>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.</p>
<p>For example, the following defines a global in a numbered address space with
an initializer, section, and alignment:</p>
<pre class="doc_code">
@G = addrspace(5) constant float 1.0, section "foo", align 4
</pre>
</div>
<!-- ======================================================================= -->
<h3>
<a name="functionstructure">Functions</a>
</h3>
<div>
<p>LLVM function definitions consist of the "<tt>define</tt>" keyword, an
optional <a href="#linkage">linkage type</a>, an optional
<a href="#visibility">visibility style</a>, an optional
<a href="#callingconv">calling convention</a>,
an optional <tt>unnamed_addr</tt> attribute, a return type, an optional
<a href="#paramattrs">parameter attribute</a> for the return type, a function
name, a (possibly empty) argument list (each with optional
<a href="#paramattrs">parameter attributes</a>), optional
<a href="#fnattrs">function attributes</a>, an optional section, an optional
alignment, an optional <a href="#gc">garbage collector name</a>, an opening
curly brace, a list of basic blocks, and a closing curly brace.</p>
<p>LLVM function declarations consist of the "<tt>declare</tt>" keyword, an
optional <a href="#linkage">linkage type</a>, an optional
<a href="#visibility">visibility style</a>, an optional
<a href="#callingconv">calling convention</a>,
an optional <tt>unnamed_addr</tt> attribute, a return type, an optional
<a href="#paramattrs">parameter attribute</a> for the return type, a function
name, a possibly empty list of arguments, an optional alignment, and an
optional <a href="#gc">garbage collector name</a>.</p>
<p>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 <a href="#terminators">terminator</a>
instruction (such as a branch or function return).</p>
<p>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 <a href="#i_phi">PHI nodes</a>.</p>
<p>LLVM allows an explicit section to be specified for functions. If the target
supports it, it will emit functions to the section specified.</p>
<p>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.</p>
<p>If the <tt>unnamed_addr</tt> attribute is given, the address is know to not
be significant and two identical functions can be merged.</p>
<h5>Syntax:</h5>
<pre class="doc_code">
define [<a href="#linkage">linkage</a>] [<a href="#visibility">visibility</a>]
[<a href="#callingconv">cconv</a>] [<a href="#paramattrs">ret attrs</a>]
<ResultType> @<FunctionName> ([argument list])
[<a href="#fnattrs">fn Attrs</a>] [section "name"] [align N]
[<a href="#gc">gc</a>] { ... }
</pre>
</div>
<!-- ======================================================================= -->
<h3>
<a name="aliasstructure">Aliases</a>
</h3>
<div>
<p>Aliases act as "second name" for the aliasee value (which can be either
function, global variable, another alias or bitcast of global value). Aliases
may have an optional <a href="#linkage">linkage type</a>, and an
optional <a href="#visibility">visibility style</a>.</p>
<h5>Syntax:</h5>
<pre class="doc_code">
@<Name> = alias [Linkage] [Visibility] <AliaseeTy> @<Aliasee>
</pre>
</div>
<!-- ======================================================================= -->
<h3>
<a name="namedmetadatastructure">Named Metadata</a>
</h3>
<div>
<p>Named metadata is a collection of metadata. <a href="#metadata">Metadata
nodes</a> (but not metadata strings) are the only valid operands for
a named metadata.</p>
<h5>Syntax:</h5>
<pre class="doc_code">
; Some unnamed metadata nodes, which are referenced by the named metadata.
!0 = metadata !{metadata !"zero"}
!1 = metadata !{metadata !"one"}
!2 = metadata !{metadata !"two"}
; A named metadata.
!name = !{!0, !1, !2}
</pre>
</div>
<!-- ======================================================================= -->
<h3>
<a name="paramattrs">Parameter Attributes</a>
</h3>
<div>
<p>The return type and each parameter of a function type may have a set of
<i>parameter attributes</i> associated with them. Parameter attributes are