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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="#typesystem">Type System</a>
<ol>
<li><a href="#t_primitive">Primitive Types</a>
<ol>
<li><a href="#t_classifications">Type Classifications</a></li>
</ol></li>
<li><a href="#t_array" >Array Type</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>
</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_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>
</ol></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>
</ol></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_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>
</ol></li>
<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>
</div>
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<div class="doc_section">
<a name="abstract">Abstract
</div>
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<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
</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>
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<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 the 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>
%result = <a href="#i_mul">mul</a> uint %X, 8
<p>After strength reduction:</p>
%result = <a href="#i_shl">shl</a> uint %X, ubyte 3
<p>And the hard way:</p>
<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
<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>
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<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>
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<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>
<p>
<table border="0" align="center">
<tr>
<td>
<table border="1" cellspacing="0" cellpadding="4" align="center">
<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>
</table>
</td><td valign=top>
<table border="1" cellspacing="0" cellpadding="4" align=center">
<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>
</table>
</td>
</tr>
</table>
</p>
</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>
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<p>
<table border="1" cellspacing="0" cellpadding="4" align="center">
<tr>
<td><a name="t_signed">signed</td>
<td><tt>sbyte, short, int, long, float, double</tt></td>
</tr>
<tr>
<td><a name="t_unsigned">unsigned</td>
<td><tt>ubyte, ushort, uint, ulong</tt></td>
</tr>
<tr>
<td><a name="t_integer">integer</td>
<td><tt>ubyte, sbyte, ushort, short, uint, int, ulong, long</tt></td>
</tr>
<tr>
<td><a name="t_integral">integral</td>
<td><tt>bool, ubyte, sbyte, ushort, short, uint, int, ulong, long</tt></td>
</tr>
<tr>
<td><a name="t_floating">floating point</td>
<td><tt>float, double</tt></td>
</tr>
<tr>
<td><a name="t_firstclass">first class</td>
<td><tt>bool, ubyte, sbyte, ushort, short,<br>
uint, int, ulong, long, float, double,
<a href="#t_pointer">pointer</a></tt></td>
</tr>
</table>
</p>
<p>The <a href="#t_firstclass">first class</a> types are perhaps the most
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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>]
</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>Here are some examples of multidimensional arrays:</p>
<p>
<table border="0" cellpadding="0" cellspacing="0">
<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>
</p>
</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 when to build virtual function tables (which are structures of pointers to
functions), for indirect function calls, and when defining a function.</p>
<pre>
<returntype> (<parameter list>)
</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>
<p>
<table border="0" cellpadding="0" cellspacing="0">
<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>
</p>
</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>
<pre>
{ <type list> }
</pre>
<h5>Examples:</h5>
<p>
<table border="0" cellpadding="0" cellspacing="0">
<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>
</p>
</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>
<pre>
<type> *
</pre>
<h5>Examples:</h5>
<p>
<table border="0" cellpadding="0" cellspacing="0">
<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>
</table>
</p>
</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>
-->
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<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>
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<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
}
</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>
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<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
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pointer to a function), and have one of the following linkage types:<p>
<dl>
<a name="linkage_internal">
<dt><tt><b>internal</b></tt>
<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>
<a name="linkage_linkonce">
<dt><tt><b>linkonce</b></tt>:
<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>
<a name="linkage_weak">
<dt><tt><b>weak</b></tt>:
<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>
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<a name="linkage_appending">
<dt><tt><b>appending</b></tt>:
<dd>"<tt>appending</tt>" linkage may only be applied to global variables of
pointer
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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>
<a name="linkage_external">
<dt><tt><b>externally visible</b></tt>:
<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>
</dl><p>
<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>"
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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".</p>
</div>
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<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). Constants must always have an initial value.</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>
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<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
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function). Because the block can have no predecessors, it also cannot have any
<a href="#i_phi">PHI nodes</a>.</p>
</div>
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<div class="doc_section">
<a name="instref">Instruction Reference</a>
</div>
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<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
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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">
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
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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>
<h5>Example:</h5>
<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 label <dest> <i>; Unconditional branch</i>
</pre>
<h5>Overview:</h5>
<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:
%cond = <a href="#i_setcc">seteq</a> int %a, %b
br bool %cond, label %IfEqual, label %IfUnequal
IfEqual:
</div>
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<div class="doc_subsubsection">
<a name="i_switch">'<tt>switch</tt>' Instruction</a>
</div>
<div class="doc_text">
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switch uint <value>, label <defaultdest> [ int <val>, label &dest>, ... ]
<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: a '<tt>uint</tt>'
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.</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 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 uint
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switch uint %Val, label %truedest [int 0, label %falsedest ]
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switch uint 0, label %dest [ ]
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switch uint %val, label %otherwise [ int 0, label %onzero,
int 1, label %onone,
int 2, label %ontwo ]
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
<a name="i_invoke">'<tt>invoke</tt>' Instruction</a>
</div>
<div class="doc_text">
<result> = invoke <ptr to function ty> %<function ptr val>(<function args>)
to label <normal label> except label <exception label>
<p>The '<tt>invoke</tt>' instruction causes control to transfer to a specified
function, with the possibility of control flow transfer to either the
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'<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>'<tt>function ptr val</tt>': An LLVM value containing a pointer to a
function to be invoked.
<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>'<tt>normal label</tt>': the label reached when the called function executes
a '<tt><a href="#i_ret">ret</a></tt>' instruction.
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<li>'<tt>exception label</tt>': the label reached when a callee returns with the
<a href="#i_unwind"><tt>unwind</tt></a> instruction.
<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>