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<div class="doc_title"> LLVM Language Reference Manual </div>
<li><a href="#abstract">Abstract</a></li>
<li><a href="#introduction">Introduction</a></li>
<li><a href="#identifiers">Identifiers</a></li>
<li><a href="#t_primitive">Primitive Types</a>
<ol>
<li><a href="#t_classifications">Type Classifications</a></li>
<li><a href="#t_function">Function Type</a></li>
<li><a href="#t_pointer">Pointer Type</a></li>
<li><a href="#t_struct">Structure Type</a></li>
<!-- <li><a href="#t_packed" >Packed Type</a> -->
</ol>
</li>
</ol>
</li>
<li><a href="#highlevel">High Level Structure</a>
<ol>
<li><a href="#modulestructure">Module Structure</a></li>
<li><a href="#globalvars">Global Variables</a></li>
<li><a href="#functionstructure">Function Structure</a></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_invoke">'<tt>invoke</tt>' Instruction</a></li>
<li><a href="#i_unwind">'<tt>unwind</tt>' Instruction</a></li>
</ol>
</li>
<li><a href="#i_add">'<tt>add</tt>' Instruction</a></li>
<li><a href="#i_sub">'<tt>sub</tt>' Instruction</a></li>
<li><a href="#i_mul">'<tt>mul</tt>' Instruction</a></li>
<li><a href="#i_div">'<tt>div</tt>' Instruction</a></li>
<li><a href="#i_rem">'<tt>rem</tt>' Instruction</a></li>
<li><a href="#i_setcc">'<tt>set<i>cc</i></tt>' Instructions</a></li>
<li><a href="#bitwiseops">Bitwise Binary Operations</a>
<ol>
<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>
<li><a href="#i_shl">'<tt>shl</tt>' Instruction</a></li>
<li><a href="#i_shr">'<tt>shr</tt>' Instruction</a></li>
<li><a href="#memoryops">Memory Access Operations</a>
<ol>
<li><a href="#i_malloc">'<tt>malloc</tt>' Instruction</a></li>
<li><a href="#i_free">'<tt>free</tt>' Instruction</a></li>
<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_getelementptr">'<tt>getelementptr</tt>' Instruction</a></li>
</ol>
</li>
<li><a href="#i_phi">'<tt>phi</tt>' Instruction</a></li>
<li><a href="#i_cast">'<tt>cast .. to</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_vanext">'<tt>vanext</tt>' Instruction</a></li>
<li><a href="#i_vaarg">'<tt>vaarg</tt>' Instruction</a></li>
<li><a href="#intrinsics">Intrinsic Functions</a>
<ol>
<li><a href="#int_varargs">Variable Argument Handling Intrinsics</a>
<ol>
<li><a href="#i_va_start">'<tt>llvm.va_start</tt>' Intrinsic</a></li>
<li><a href="#i_va_end">'<tt>llvm.va_end</tt>' Intrinsic</a></li>
<li><a href="#i_va_copy">'<tt>llvm.va_copy</tt>' Intrinsic</a></li>
</ol>
</li>
<li><a href="#int_codegen">Code Generator Intrinsics</a>
<ol>
<li><a href="#i_returnaddress">'<tt>llvm.returnaddress</tt>' Intrinsic</a></li>
<li><a href="#i_frameaddress">'<tt>llvm.frameaddress</tt>' Intrinsic</a></li>
</ol>
</li>
<li><a href="#int_os">Operating System Intrinsics</a>
<ol>
<li><a href="#i_readport">'<tt>llvm.readport</tt>' Intrinsic</a></li>
<li><a href="#i_writeport">'<tt>llvm.writeport</tt>' Intrinsic</a></li>
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<li><a href="#int_libc">Standard C Library Intrinsics</a>
<ol>
<li><a href="#i_memcpy">'<tt>llvm.memcpy</tt>' Intrinsic</a></li>
<li><a href="#i_memmove">'<tt>llvm.memmove</tt>' Intrinsic</a></li>
<li><a href="#i_memset">'<tt>llvm.memset</tt>' Intrinsic</a></li>
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</ol>
</li>
<li><a href="#int_debugger">Debugger intrinsics</a>
<div class="doc_text">
<p><b>Written by <a href="mailto:sabre@nondot.org">Chris Lattner</a>
and <a href="mailto:vadve@cs.uiuc.edu">Vikram Adve</a></b></p>
<p> </p>
</div>
<!-- *********************************************************************** -->
<div class="doc_section"> <a name="abstract">Abstract </a></div>
<!-- *********************************************************************** -->
<div class="doc_text">
<p>This document is a reference manual for the LLVM assembly language.
LLVM is an 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>
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<div class="doc_section"> <a name="introduction">Introduction</a> </div>
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<div class="doc_text">
<p>The LLVM code representation is designed to be used in three
different forms: as an in-memory compiler IR, as an on-disk bytecode
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 a 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>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection"> <a name="wellformed">Well-Formedness</a> </div>
<div class="doc_text">
<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> %x = <a href="#i_add">add</a> int 1, %x<br></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 bytecode. The violations pointed out
by the verifier pass indicate bugs in transformation passes or input to
the parser.</p>
<!-- Describe the typesetting conventions here. --> </div>
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<div class="doc_section"> <a name="identifiers">Identifiers</a> </div>
<!-- *********************************************************************** -->
<div class="doc_text">
<p>LLVM uses three different forms of identifiers, for different
purposes:</p>
<li>Numeric constants are represented as you would expect: 12, -3
123.421, etc. Floating point constants have an optional hexidecimal
notation.</li>
<li>Named values are represented as a string of characters with a '%'
prefix. For example, %foo, %DivisionByZero,
%a.really.long.identifier. 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. In this way, anything except a <tt>"</tt>
character can be used in a name.</li>
<li>Unnamed values are represented as an unsigned numeric value with
a '%' prefix. For example, %12, %2, %44.</li>
</ol>
<p>LLVM requires that values start with a '%' sign 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_cast">cast</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_uint">uint</a></tt>',
etc...), and others. These reserved words cannot conflict with
variable names, because none of them start with a '%' character.</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> %result = <a href="#i_mul">mul</a> uint %X, 8<br></pre>
<p>After strength reduction:</p>
<pre> %result = <a href="#i_shl">shl</a> uint %X, ubyte 3<br></pre>
<p>And the hard way:</p>
<pre> <a href="#i_add">add</a> uint %X, %X <i>; yields {uint}:%0</i>
<a
href="#i_add">add</a> uint %0, %0 <i>; yields {uint}:%1</i>
%result = <a
href="#i_add">add</a> uint %1, %1<br></pre>
<p>This last way of multiplying <tt>%X</tt> by 8 illustrates several
important lexical features of LLVM:</p>
<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>...and it also show 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>
<p>The one non-intuitive notation for constants is the optional
hexidecimal form of floating point constants. For example, the form '<tt>double
0x432ff973cafa8000</tt>' is equivalent to (but harder to read than) '<tt>double
4.5e+15</tt>' which is also supported by the parser. The only time
hexadecimal floating point constants are useful (and the only time that
they are generated by the disassembler) is when an FP constant has to
be emitted that is not representable as a decimal floating point number
exactly. For example, NaN's, infinities, and other special cases are
represented in their IEEE hexadecimal format so that assembly and
disassembly do not cause any bits to change in the constants.</p>
</div>
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<div class="doc_section"> <a name="typesystem">Type System</a> </div>
<!-- *********************************************************************** -->
<div class="doc_text">
<p>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 IR 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.</p>
<!-- The written form for the type system was heavily influenced by the
syntactic problems with types in the C language<sup><a
href="#rw_stroustrup">1</a></sup>.<p> --> </div>
<!-- ======================================================================= -->
<div class="doc_subsection"> <a name="t_primitive">Primitive Types</a> </div>
<div class="doc_text">
<p>The primitive types are the fundemental building blocks of the LLVM
system. The current set of primitive types are as follows:</p>
<table border="0" style="align: center">
<table border="1" cellspacing="0" cellpadding="4" style="align: center">
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<tbody>
<tr>
<td><tt>void</tt></td>
<td>No value</td>
</tr>
<tr>
<td><tt>ubyte</tt></td>
<td>Unsigned 8 bit value</td>
</tr>
<tr>
<td><tt>ushort</tt></td>
<td>Unsigned 16 bit value</td>
</tr>
<tr>
<td><tt>uint</tt></td>
<td>Unsigned 32 bit value</td>
</tr>
<tr>
<td><tt>ulong</tt></td>
<td>Unsigned 64 bit value</td>
</tr>
<tr>
<td><tt>float</tt></td>
<td>32 bit floating point value</td>
</tr>
<tr>
<td><tt>label</tt></td>
<td>Branch destination</td>
</tr>
</tbody>
</table>
</td>
<td valign="top">
<table border="1" cellspacing="0" cellpadding="4">
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<tbody>
<tr>
<td><tt>bool</tt></td>
<td>True or False value</td>
</tr>
<tr>
<td><tt>sbyte</tt></td>
<td>Signed 8 bit value</td>
</tr>
<tr>
<td><tt>short</tt></td>
<td>Signed 16 bit value</td>
</tr>
<tr>
<td><tt>int</tt></td>
<td>Signed 32 bit value</td>
</tr>
<tr>
<td><tt>long</tt></td>
<td>Signed 64 bit value</td>
</tr>
<tr>
<td><tt>double</tt></td>
<td>64 bit floating point value</td>
</tr>
</tbody>
</table>
</td>
</tr>
</tbody>
</table>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection"> <a name="t_classifications">Type
Classifications</a> </div>
<div class="doc_text">
<p>These different primitive types fall into a few useful
classifications:</p>
<table border="1" cellspacing="0" cellpadding="4">
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<tbody>
<tr>
<td><a name="t_signed">signed</a></td>
<td><tt>sbyte, short, int, long, float, double</tt></td>
</tr>
<tr>
<td><a name="t_unsigned">unsigned</a></td>
<td><tt>ubyte, ushort, uint, ulong</tt></td>
</tr>
<tr>
<td><a name="t_integer">integer</a></td>
<td><tt>ubyte, sbyte, ushort, short, uint, int, ulong, long</tt></td>
</tr>
<tr>
<td><a name="t_integral">integral</a></td>
<td><tt>bool, ubyte, sbyte, ushort, short, uint, int, ulong, long</tt></td>
</tr>
<tr>
<td><a name="t_floating">floating point</a></td>
<td><tt>float, double</tt></td>
</tr>
<tr>
<td><a name="t_firstclass">first class</a></td>
<td><tt>bool, ubyte, sbyte, ushort, short,<br>
uint, int, ulong, long, float, double, <a href="#t_pointer">pointer</a></tt></td>
</tr>
</tbody>
</table>
<p>The <a href="#t_firstclass">first class</a> types are perhaps the
most important. Values of these types are the only ones which can be
produced by instructions, passed as arguments, or used as operands to
instructions. This means that all structures and arrays must be
manipulated either by pointer or by component.</p>
</div>
<!-- ======================================================================= -->
<div class="doc_subsection"> <a name="t_derived">Derived Types</a> </div>
<div class="doc_text">
<p>The real power in LLVM comes from the derived types in the system.
This is what allows a programmer to represent arrays, functions,
pointers, and other useful types. Note that these derived types may be
recursive: For example, it is possible to have a two dimensional array.</p>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection"> <a name="t_array">Array Type</a> </div>
<div class="doc_text">
<p>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.</p>
<h5>Syntax:</h5>
<pre> [<# elements> x <elementtype>]<br></pre>
<p>The number of elements is a constant integer value, elementtype may
be any type with a size.</p>
<h5>Examples:</h5>
<p> <tt>[40 x int ]</tt>: Array of 40 integer values.<br>
<tt>[41 x int ]</tt>: Array of 41 integer values.<br>
<tt>[40 x uint]</tt>: Array of 40 unsigned integer values.</p>
<p> </p>
<p>Here are some examples of multidimensional arrays:</p>
<table border="0" cellpadding="0" cellspacing="0">
<tbody>
<tr>
<td><tt>[3 x [4 x int]]</tt></td>
<td>: 3x4 array integer values.</td>
</tr>
<tr>
<td><tt>[12 x [10 x float]]</tt></td>
<td>: 12x10 array of single precision floating point values.</td>
</tr>
<tr>
<td><tt>[2 x [3 x [4 x uint]]]</tt></td>
<td>: 2x3x4 array of unsigned integer values.</td>
</tr>
</tbody>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection"> <a name="t_function">Function Type</a> </div>
<div class="doc_text">
<p>The function type can be thought of as a function signature. It
consists of a return type and a list of formal parameter types.
Function types are usually used to build virtual function tables
(which are structures of pointers to functions), for indirect function
calls, and when defining a function.</p>
<p>
The return type of a function type cannot be an aggregate type.
</p>
<pre> <returntype> (<parameter list>)<br></pre>
<p>Where '<tt><parameter list></tt>' is a comma-separated list of
type specifiers. Optionally, the parameter list may include a type <tt>...</tt>,
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which indicates that the function takes a variable number of arguments.
Variable argument functions can access their arguments with the <a
href="#int_varargs">variable argument handling intrinsic</a> functions.</p>
<table border="0" cellpadding="0" cellspacing="0">
<tbody>
<tr>
<td><tt>int (int)</tt></td>
<td>: function taking an <tt>int</tt>, returning an <tt>int</tt></td>
</tr>
<tr>
<td><tt>float (int, int *) *</tt></td>
<td>: <a href="#t_pointer">Pointer</a> to a function that takes
an <tt>int</tt> and a <a href="#t_pointer">pointer</a> to <tt>int</tt>,
returning <tt>float</tt>.</td>
</tr>
<tr>
<td><tt>int (sbyte *, ...)</tt></td>
<td>: A vararg function that takes at least one <a
href="#t_pointer">pointer</a> to <tt>sbyte</tt> (signed char in C),
which returns an integer. This is the signature for <tt>printf</tt>
in LLVM.</td>
</tr>
</tbody>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection"> <a name="t_struct">Structure Type</a> </div>
<div class="doc_text">
<p>The structure type is used to represent a collection of data members
together in memory. The packing of the field types is defined to match
the ABI of the underlying processor. The elements of a structure may
be any type that has a size.</p>
<p>Structures are accessed using '<tt><a href="#i_load">load</a></tt>
and '<tt><a href="#i_store">store</a></tt>' by getting a pointer to a
field with the '<tt><a href="#i_getelementptr">getelementptr</a></tt>'
instruction.</p>
<table border="0" cellpadding="0" cellspacing="0">
<tbody>
<tr>
<td><tt>{ int, int, int }</tt></td>
<td>: a triple of three <tt>int</tt> values</td>
</tr>
<tr>
<td><tt>{ float, int (int) * }</tt></td>
<td>: A pair, where the first element is a <tt>float</tt> and the
second element is a <a href="#t_pointer">pointer</a> to a <a
href="t_function">function</a> that takes an <tt>int</tt>, returning
an <tt>int</tt>.</td>
</tr>
</tbody>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection"> <a name="t_pointer">Pointer Type</a> </div>
<div class="doc_text">
<h5>Overview:</h5>
<p>As in many languages, the pointer type represents a pointer or
reference to another object, which must live in memory.</p>
<h5>Syntax:</h5>
<h5>Examples:</h5>
<table border="0" cellpadding="0" cellspacing="0">
<tbody>
<tr>
<td><tt>[4x int]*</tt></td>
<td>: <a href="#t_pointer">pointer</a> to <a href="#t_array">array</a>
of four <tt>int</tt> values</td>
</tr>
<tr>
<td><tt>int (int *) *</tt></td>
<td>: A <a href="#t_pointer">pointer</a> to a <a
href="t_function">function</a> that takes an <tt>int</tt>, returning
an <tt>int</tt>.</td>
</tr>
</tbody>
</table>
</div>
<!-- _______________________________________________________________________ --><!--
<div class="doc_subsubsection">
<a name="t_packed">Packed Type</a>
</div>
<div class="doc_text">
Mention/decide that packed types work with saturation or not. Maybe have a packed+saturated type in addition to just a packed type.<p>
Packed types should be 'nonsaturated' because standard data types are not saturated. Maybe have a saturated packed type?<p>
</div>
--><!-- *********************************************************************** -->
<div class="doc_section"> <a name="highlevel">High Level Structure</a> </div>
<!-- *********************************************************************** --><!-- ======================================================================= -->
<div class="doc_subsection"> <a name="modulestructure">Module Structure</a> </div>
<div class="doc_text">
<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><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 sbyte]</a> c"hello world\0A\00" <i>; [13 x sbyte]*</i>
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<i>; External declaration of the puts function</i>
<a href="#functionstructure">declare</a> int %puts(sbyte*) <i>; int(sbyte*)* </i>
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int %main() { <i>; int()* </i>
<i>; Convert [13x sbyte]* to sbyte *...</i>
%cast210 = <a
href="#i_getelementptr">getelementptr</a> [13 x sbyte]* %.LC0, long 0, long 0 <i>; sbyte*</i>
<i>; Call puts function to write out the string to stdout...</i>
<a
href="#i_call">call</a> int %puts(sbyte* %cast210) <i>; int</i>
<a
href="#i_ret">ret</a> int 0<br>}<br></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, and a <a href="#functionstructure">function definition</a>
for "<tt>main</tt>".</p>
<a name="linkage"> 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 linkage types:</a>
<p> </p>
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<dl>
<dt><tt><b><a name="linkage_internal">internal</a></b></tt> </dt>
<dd>Global values with internal linkage are only directly accessible
by objects in the current module. In particular, linking code into a
module with an internal global value may cause the internal to be
renamed as necessary to avoid collisions. Because the symbol is
internal to the module, all references can be updated. This
corresponds to the notion of the '<tt>static</tt>' keyword in C, or the
idea of "anonymous namespaces" in C++.
<p> </p>
</dd>
<dt><tt><b><a name="linkage_linkonce">linkonce</a></b></tt>: </dt>
<dd>"<tt>linkonce</tt>" linkage is similar to <tt>internal</tt>
linkage, with the twist that linking together two modules defining the
same <tt>linkonce</tt> globals will cause one of the globals to be
discarded. This is typically used to implement inline functions.
Unreferenced <tt>linkonce</tt> globals are allowed to be discarded.
<p> </p>
</dd>
<dt><tt><b><a name="linkage_weak">weak</a></b></tt>: </dt>
<dd>"<tt>weak</tt>" linkage is exactly the same as <tt>linkonce</tt>
linkage, except that unreferenced <tt>weak</tt> globals may not be
discarded. This is used to implement constructs in C such as "<tt>int
X;</tt>" at global scope.
<p> </p>
</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.
<p> </p>
</dd>
<dt><tt><b><a name="linkage_external">externally visible</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.
<p> </p>
</dd>
</dl>
<p> </p>
<p><a name="linkage_external">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. It is illegal for a function <i>declaration</i>
to have any linkage type other than "externally visible".</a></p>
</div>
<!-- ======================================================================= -->
<div class="doc_subsection">
<a name="globalvars">Global Variables</a>
</div>
<div class="doc_text">
<p>Global variables define regions of memory allocated at compilation
time instead of run-time. Global variables may optionally be
initialized. A variable may be defined as a global "constant", which
indicates that the contents of the variable will never be modified
(opening options for optimization).</p>
<p>As SSA values, global variables define pointer values that are in
scope (i.e. they dominate) for 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>
</div>
<!-- ======================================================================= -->
<div class="doc_subsection">
<a name="functionstructure">Functions</a>
</div>
<div class="doc_text">
<p>LLVM function definitions are composed of a (possibly empty) argument list,
an opening curly brace, a list of basic blocks, and a closing curly brace. LLVM
function declarations are defined with the "<tt>declare</tt>" keyword, a
function name, and a function signature.</p>
<p>A function definition contains a list of basic blocks, forming the CFG 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 program 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 functions are identified by their name and type signature. Hence, two
functions with the same name but different parameter lists or return values are
considered different functions, and LLVM will resolves references to each
appropriately.</p>
</div>
<!-- *********************************************************************** -->
<div class="doc_section"> <a name="instref">Instruction Reference</a> </div>
<!-- *********************************************************************** -->
<div class="doc_text">
<p>The LLVM instruction set consists of several different
classifications of instructions: <a href="#terminators">terminator
instructions</a>, <a href="#binaryops">binary instructions</a>, <a
href="#memoryops">memory instructions</a>, and <a href="#otherops">other
instructions</a>.</p>
</div>
<!-- ======================================================================= -->
<div class="doc_subsection"> <a name="terminators">Terminator
Instructions</a> </div>
<div class="doc_text">
<p>As mentioned <a href="#functionstructure">previously</a>, every
basic block in a program ends with a "Terminator" instruction, which
indicates which block should be executed after the current block is
finished. These terminator instructions typically yield a '<tt>void</tt>'
value: they produce control flow, not values (the one exception being
the '<a href="#i_invoke"><tt>invoke</tt></a>' instruction).</p>
<p>There are five different terminator instructions: the '<a
href="#i_ret"><tt>ret</tt></a>' instruction, the '<a href="#i_br"><tt>br</tt></a>'
instruction, the '<a href="#i_switch"><tt>switch</tt></a>' instruction,
the '<a href="#i_invoke"><tt>invoke</tt></a>' instruction, and the '<a
href="#i_unwind"><tt>unwind</tt></a>' instruction.</p>
</div>
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<div class="doc_subsubsection"> <a name="i_ret">'<tt>ret</tt>'
Instruction</a> </div>
<div class="doc_text">
<pre> ret <type> <value> <i>; Return a value from a non-void function</i>
ret void <i>; Return from void function</i>
<p>The '<tt>ret</tt>' instruction is used to return control flow (and a
value) from a function, back to the caller.</p>
<p>There are two forms of the '<tt>ret</tt>' instructruction: one that
returns a value and then causes control flow, and one that just causes
control flow to occur.</p>
<p>The '<tt>ret</tt>' instruction may return any '<a
href="#t_firstclass">first class</a>' type. Notice that a function is
not <a href="#wellformed">well formed</a> if there exists a '<tt>ret</tt>'
instruction inside of the function that returns a value that does not
match the return type of the function.</p>
<p>When the '<tt>ret</tt>' instruction is executed, control flow
returns back to the calling function's context. If the caller is a "<a
href="#i_call"><tt>call</tt></a> instruction, execution continues at
the instruction after the call. If the caller was an "<a
href="#i_invoke"><tt>invoke</tt></a>" instruction, execution continues
at the beginning "normal" of the destination block. If the instruction
returns a value, that value shall set the call or invoke instruction's
return value.</p>
<pre> ret int 5 <i>; Return an integer value of 5</i>
ret void <i>; Return from a void function</i>
</div>
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<div class="doc_subsubsection"> <a name="i_br">'<tt>br</tt>' Instruction</a> </div>
<div class="doc_text">
<pre> br bool <cond>, label <iftrue>, label <iffalse><br> br label <dest> <i>; Unconditional branch</i>
<p>The '<tt>br</tt>' instruction is used to cause control flow to
transfer to a different basic block in the current function. There are
two forms of this instruction, corresponding to a conditional branch
and an unconditional branch.</p>
<p>The conditional branch form of the '<tt>br</tt>' instruction takes a
single '<tt>bool</tt>' value and two '<tt>label</tt>' values. The
unconditional form of the '<tt>br</tt>' instruction takes a single '<tt>label</tt>'
value as a target.</p>
<p>Upon execution of a conditional '<tt>br</tt>' instruction, the '<tt>bool</tt>'
argument is evaluated. If the value is <tt>true</tt>, control flows
to the '<tt>iftrue</tt>' <tt>label</tt> argument. If "cond" is <tt>false</tt>,
control flows to the '<tt>iffalse</tt>' <tt>label</tt> argument.</p>
<pre>Test:<br> %cond = <a href="#i_setcc">seteq</a> int %a, %b<br> br bool %cond, label %IfEqual, label %IfUnequal<br>IfEqual:<br> <a
href="#i_ret">ret</a> int 1<br>IfUnequal:<br> <a href="#i_ret">ret</a> int 0<br></pre>
</div>
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<div class="doc_subsubsection">
<a name="i_switch">'<tt>switch</tt>' Instruction</a>
</div>
<div class="doc_text">
<pre>
switch <intty> <value>, label <defaultdest> [ <intty> <val>, label <dest> ... ]
</pre>
<p>The '<tt>switch</tt>' instruction is used to transfer control flow to one of
several different places. It is a generalization of the '<tt>br</tt>'
instruction, allowing a branch to occur to one of many possible
destinations.</p>
<p>The '<tt>switch</tt>' instruction uses three parameters: an integer
comparison value '<tt>value</tt>', a default '<tt>label</tt>' destination, and
an array of pairs of comparison value constants and '<tt>label</tt>'s. The
table is not allowed to contain duplicate constant entries.</p>
<p>The <tt>switch</tt> instruction specifies a table of values and
destinations. When the '<tt>switch</tt>' instruction is executed, this
table is searched for the given value. If the value is found, the
corresponding destination is branched to, otherwise the default value
it transfered to.</p>
<h5>Implementation:</h5>
<p>Depending on properties of the target machine and the particular
<tt>switch</tt> instruction, this instruction may be code generated in different
ways, for example as a series of chained conditional branches, or with a lookup
table.</p>
<pre>
<i>; Emulate a conditional br instruction</i>
%Val = <a href="#i_cast">cast</a> bool %value to int
switch int %Val, label %truedest [int 0, label %falsedest ]
<i>; Emulate an unconditional br instruction</i>
switch uint 0, label %dest [ ]
<i>; Implement a jump table:</i>
switch uint %val, label %otherwise [ uint 0, label %onzero
uint 1, label %onone
uint 2, label %ontwo ]
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection"> <a name="i_invoke">'<tt>invoke</tt>'
Instruction</a> </div>
<div class="doc_text">
<pre> <result> = invoke <ptr to function ty> %<function ptr val>(<function args>)<br> to label <normal label> except label <exception label><br></pre>
<p>The '<tt>invoke</tt>' instruction causes control to transfer to a
specified function, with the possibility of control flow transfer to
either the '<tt>normal</tt>' <tt>label</tt> label or the '<tt>exception</tt>'<tt>label</tt>.
If the callee function returns with the "<tt><a href="#i_ret">ret</a></tt>"
instruction, control flow will return to the "normal" label. If the
callee (or any indirect callees) returns with the "<a href="#i_unwind"><tt>unwind</tt></a>"
instruction, control is interrupted, and continued at the dynamically
nearest "except" label.</p>
<p>This instruction requires several arguments:</p>
<li>'<tt>ptr to function ty</tt>': shall be the signature of the
pointer to function value being invoked. In most cases, this is a
direct function invocation, but indirect <tt>invoke</tt>s are just as
possible, branching off an arbitrary pointer to function value. </li>
<li>'<tt>function ptr val</tt>': An LLVM value containing a pointer
to a function to be invoked. </li>
<li>'<tt>function args</tt>': argument list whose types match the
function signature argument types. If the function signature indicates
the function accepts a variable number of arguments, the extra
arguments can be specified. </li>
<li>'<tt>normal label</tt>': the label reached when the called
function executes a '<tt><a href="#i_ret">ret</a></tt>' instruction. </li>
<li>'<tt>exception label</tt>': the label reached when a callee
returns with the <a href="#i_unwind"><tt>unwind</tt></a> instruction. </li>
<p>This instruction is designed to operate as a standard '<tt><a
href="#i_call">call</a></tt>' instruction in most regards. The
primary difference is that it establishes an association with a label,
which is used by the runtime library to unwind the stack.</p>
<p>This instruction is used in languages with destructors to ensure
that proper cleanup is performed in the case of either a <tt>longjmp</tt>
or a thrown exception. Additionally, this is important for
implementation of '<tt>catch</tt>' clauses in high-level languages that
support them.</p>
<pre> %retval = invoke int %Test(int 15)<br> to label %Continue<br> except label %TestCleanup <i>; {int}:retval set</i>
</div>
Chris Lattner
committed
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection"> <a name="i_unwind">'<tt>unwind</tt>'
Instruction</a> </div>
<div class="doc_text">
Chris Lattner
committed
<h5>Syntax:</h5>
Chris Lattner
committed
<h5>Overview:</h5>
<p>The '<tt>unwind</tt>' instruction unwinds the stack, continuing
control flow at the first callee in the dynamic call stack which used
an <a href="#i_invoke"><tt>invoke</tt></a> instruction to perform the
call. This is primarily used to implement exception handling.</p>
Chris Lattner
committed
<h5>Semantics:</h5>
<p>The '<tt>unwind</tt>' intrinsic causes execution of the current
function to immediately halt. The dynamic call stack is then searched
for the first <a href="#i_invoke"><tt>invoke</tt></a> instruction on
the call stack. Once found, execution continues at the "exceptional"
destination block specified by the <tt>invoke</tt> instruction. If
there is no <tt>invoke</tt> instruction in the dynamic call chain,
undefined behavior results.</p>
</div>
<!-- ======================================================================= -->
<div class="doc_subsection"> <a name="binaryops">Binary Operations</a> </div>
<div class="doc_text">
<p>Binary operators are used to do most of the computation in a
program. They require two operands, execute an operation on them, and
produce a single value. The result value of a binary operator is not
necessarily the same type as its operands.</p>
<p>There are several different binary operators:</p>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection"> <a name="i_add">'<tt>add</tt>'
Instruction</a> </div>
<div class="doc_text">
<pre> <result> = add <ty> <var1>, <var2> <i>; yields {ty}:result</i>
<p>The '<tt>add</tt>' instruction returns the sum of its two operands.</p>
<p>The two arguments to the '<tt>add</tt>' instruction must be either <a
href="#t_integer">integer</a> or <a href="#t_floating">floating point</a>
values. Both arguments must have identical types.</p>
<p>The value produced is the integer or floating point sum of the two
operands.</p>
<pre> <result> = add int 4, %var <i>; yields {int}:result = 4 + %var</i>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection"> <a name="i_sub">'<tt>sub</tt>'
Instruction</a> </div>
<div class="doc_text">
<pre> <result> = sub <ty> <var1>, <var2> <i>; yields {ty}:result</i>
<p>The '<tt>sub</tt>' instruction returns the difference of its two
operands.</p>
<p>Note that the '<tt>sub</tt>' instruction is used to represent the '<tt>neg</tt>'
instruction present in most other intermediate representations.</p>
<p>The two arguments to the '<tt>sub</tt>' instruction must be either <a
href="#t_integer">integer</a> or <a href="#t_floating">floating point</a>
values. Both arguments must have identical types.</p>
<p>The value produced is the integer or floating point difference of
the two operands.</p>
<pre> <result> = sub int 4, %var <i>; yields {int}:result = 4 - %var</i>
<result> = sub int 0, %val <i>; yields {int}:result = -%var</i>
</pre>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection"> <a name="i_mul">'<tt>mul</tt>'
Instruction</a> </div>
<div class="doc_text">
<pre> <result> = mul <ty> <var1>, <var2> <i>; yields {ty}:result</i>
<p>The '<tt>mul</tt>' instruction returns the product of its two
operands.</p>
<p>The two arguments to the '<tt>mul</tt>' instruction must be either <a
href="#t_integer">integer</a> or <a href="#t_floating">floating point</a>
values. Both arguments must have identical types.</p>
<h5>Semantics:</h5>
<p>The value produced is the integer or floating point product of the
two operands.</p>
<p>There is no signed vs unsigned multiplication. The appropriate
action is taken based on the type of the operand.</p>
<pre> <result> = mul int 4, %var <i>; yields {int}:result = 4 * %var</i>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection"> <a name="i_div">'<tt>div</tt>'
Instruction</a> </div>
<div class="doc_text">
<pre> <result> = div <ty> <var1>, <var2> <i>; yields {ty}:result</i>
<p>The '<tt>div</tt>' instruction returns the quotient of its two
operands.</p>
<p>The two arguments to the '<tt>div</tt>' instruction must be either <a
href="#t_integer">integer</a> or <a href="#t_floating">floating point</a>
values. Both arguments must have identical types.</p>
<p>The value produced is the integer or floating point quotient of the
two operands.</p>
<pre> <result> = div int 4, %var <i>; yields {int}:result = 4 / %var</i>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection"> <a name="i_rem">'<tt>rem</tt>'
Instruction</a> </div>
<div class="doc_text">
<pre> <result> = rem <ty> <var1>, <var2> <i>; yields {ty}:result</i>