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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 Programmer's Manual</title>
<link rel="stylesheet" href="llvm.css" type="text/css">
</head>
<body>
<div class="doc_title">
LLVM Programmer's Manual
</div>
<ol>
<li><a href="#introduction">Introduction</a></li>
<li><a href="#general">General Information</a>
<ul>
<li><a href="#stl">The C++ Standard Template Library</a></li>
<!--
<li>The <tt>-time-passes</tt> option</li>
<li>How to use the LLVM Makefile system</li>
<li>How to write a regression test</li>
-->
</ul>
</li>
<li><a href="#apis">Important and useful LLVM APIs</a>
<ul>
<li><a href="#isa">The <tt>isa&lt;&gt;</tt>, <tt>cast&lt;&gt;</tt>
and <tt>dyn_cast&lt;&gt;</tt> templates</a> </li>
<li><a href="#DEBUG">The <tt>DEBUG()</tt> macro and <tt>-debug</tt>
option</a>
<ul>
<li><a href="#DEBUG_TYPE">Fine grained debug info with <tt>DEBUG_TYPE</tt>
and the <tt>-debug-only</tt> option</a> </li>
</ul>
</li>
<li><a href="#Statistic">The <tt>Statistic</tt> class &amp; <tt>-stats</tt>
option</a></li>
<!--
<li>The <tt>InstVisitor</tt> template
<li>The general graph API
-->
<li><a href="#ViewGraph">Viewing graphs while debugging code</a></li>
</ul>
</li>
<li><a href="#datastructure">Picking the Right Data Structure for a Task</a>
<ul>
<li><a href="#ds_sequential">Sequential Containers (std::vector, std::list, etc)</a>
<ul>
<li><a href="#dss_fixedarrays">Fixed Size Arrays</a></li>
<li><a href="#dss_heaparrays">Heap Allocated Arrays</a></li>
<li><a href="#dss_smallvector">"llvm/ADT/SmallVector.h"</a></li>
<li><a href="#dss_vector">&lt;vector&gt;</a></li>
<li><a href="#dss_deque">&lt;deque&gt;</a></li>
<li><a href="#dss_list">&lt;list&gt;</a></li>
<li><a href="#dss_ilist">llvm/ADT/ilist</a></li>
<li><a href="#dss_other">Other Sequential Container Options</a></li>
</ul></li>
<li><a href="#ds_set">Set-Like Containers (std::set, SmallSet, SetVector, etc)</a>
<ul>
<li><a href="#dss_sortedvectorset">A sorted 'vector'</a></li>
<li><a href="#dss_smallset">"llvm/ADT/SmallSet.h"</a></li>
<li><a href="#dss_smallptrset">"llvm/ADT/SmallPtrSet.h"</a></li>
<li><a href="#dss_denseset">"llvm/ADT/DenseSet.h"</a></li>
<li><a href="#dss_FoldingSet">"llvm/ADT/FoldingSet.h"</a></li>
<li><a href="#dss_set">&lt;set&gt;</a></li>
<li><a href="#dss_setvector">"llvm/ADT/SetVector.h"</a></li>
<li><a href="#dss_uniquevector">"llvm/ADT/UniqueVector.h"</a></li>
<li><a href="#dss_otherset">Other Set-Like ContainerOptions</a></li>
</ul></li>
<li><a href="#ds_map">Map-Like Containers (std::map, DenseMap, etc)</a>
<ul>
<li><a href="#dss_sortedvectormap">A sorted 'vector'</a></li>
<li><a href="#dss_stringmap">"llvm/ADT/StringMap.h"</a></li>
<li><a href="#dss_indexedmap">"llvm/ADT/IndexedMap.h"</a></li>
<li><a href="#dss_densemap">"llvm/ADT/DenseMap.h"</a></li>
<li><a href="#dss_map">&lt;map&gt;</a></li>
<li><a href="#dss_othermap">Other Map-Like Container Options</a></li>
</ul></li>
<li><a href="#ds_bit">BitVector-like containers</a>
<ul>
<li><a href="#dss_bitvector">A dense bitvector</a></li>
<li><a href="#dss_sparsebitvector">A sparse bitvector</a></li>
</ul></li>
</ul>
</li>
<li><a href="#common">Helpful Hints for Common Operations</a>
<ul>
<li><a href="#inspection">Basic Inspection and Traversal Routines</a>
<ul>
<li><a href="#iterate_function">Iterating over the <tt>BasicBlock</tt>s
in a <tt>Function</tt></a> </li>
<li><a href="#iterate_basicblock">Iterating over the <tt>Instruction</tt>s
in a <tt>BasicBlock</tt></a> </li>
<li><a href="#iterate_institer">Iterating over the <tt>Instruction</tt>s
in a <tt>Function</tt></a> </li>
<li><a href="#iterate_convert">Turning an iterator into a
class pointer</a> </li>
<li><a href="#iterate_complex">Finding call sites: a more
complex example</a> </li>
<li><a href="#calls_and_invokes">Treating calls and invokes
the same way</a> </li>
<li><a href="#iterate_chains">Iterating over def-use &amp;
use-def chains</a> </li>
</ul>
</li>
<li><a href="#simplechanges">Making simple changes</a>
<ul>
<li><a href="#schanges_creating">Creating and inserting new
<tt>Instruction</tt>s</a> </li>
<li><a href="#schanges_deleting">Deleting <tt>Instruction</tt>s</a> </li>
<li><a href="#schanges_replacing">Replacing an <tt>Instruction</tt>
with another <tt>Value</tt></a> </li>
<li><a href="#schanges_deletingGV">Deleting <tt>GlobalVariable</tt>s</a> </li>
</ul>
</li>
<!--
<li>Working with the Control Flow Graph
<ul>
<li>Accessing predecessors and successors of a <tt>BasicBlock</tt>
<li>
<li>
</ul>
-->
</ul>
</li>
<li><a href="#advanced">Advanced Topics</a>
<ul>
<li><a href="#TypeResolve">LLVM Type Resolution</a>
<ul>
<li><a href="#BuildRecType">Basic Recursive Type Construction</a></li>
<li><a href="#refineAbstractTypeTo">The <tt>refineAbstractTypeTo</tt> method</a></li>
<li><a href="#PATypeHolder">The PATypeHolder Class</a></li>
<li><a href="#AbstractTypeUser">The AbstractTypeUser Class</a></li>
</ul></li>
<li><a href="#SymbolTable">The <tt>ValueSymbolTable</tt> and <tt>TypeSymbolTable</tt> classes </a></li>
</ul></li>
<li><a href="#coreclasses">The Core LLVM Class Hierarchy Reference</a>
<ul>
<li><a href="#Type">The <tt>Type</tt> class</a> </li>
<li><a href="#Module">The <tt>Module</tt> class</a></li>
<li><a href="#Value">The <tt>Value</tt> class</a>
<ul>
<li><a href="#User">The <tt>User</tt> class</a>
<ul>
<li><a href="#Instruction">The <tt>Instruction</tt> class</a></li>
<li><a href="#Constant">The <tt>Constant</tt> class</a>
<ul>
<li><a href="#GlobalValue">The <tt>GlobalValue</tt> class</a>
<ul>
<li><a href="#Function">The <tt>Function</tt> class</a></li>
<li><a href="#GlobalVariable">The <tt>GlobalVariable</tt> class</a></li>
</ul>
</li>
</ul>
</li>
</ul>
</li>
<li><a href="#BasicBlock">The <tt>BasicBlock</tt> class</a></li>
<li><a href="#Argument">The <tt>Argument</tt> class</a></li>
</ul>
</li>
</ul>
</li>
</ol>
<div class="doc_author">
<p>Written by <a href="mailto:sabre@nondot.org">Chris Lattner</a>,
<a href="mailto:dhurjati@cs.uiuc.edu">Dinakar Dhurjati</a>,
<a href="mailto:jstanley@cs.uiuc.edu">Joel Stanley</a>, and
<a href="mailto:rspencer@x10sys.com">Reid Spencer</a></p>
</div>
<!-- *********************************************************************** -->
<div class="doc_section">
<a name="introduction">Introduction </a>
</div>
<!-- *********************************************************************** -->
<div class="doc_text">
<p>This document is meant to highlight some of the important classes and
interfaces available in the LLVM source-base. This manual is not
intended to explain what LLVM is, how it works, and what LLVM code looks
like. It assumes that you know the basics of LLVM and are interested
in writing transformations or otherwise analyzing or manipulating the
code.</p>
<p>This document should get you oriented so that you can find your
way in the continuously growing source code that makes up the LLVM
infrastructure. Note that this manual is not intended to serve as a
replacement for reading the source code, so if you think there should be
a method in one of these classes to do something, but it's not listed,
check the source. Links to the <a href="/doxygen/">doxygen</a> sources
are provided to make this as easy as possible.</p>
<p>The first section of this document describes general information that is
useful to know when working in the LLVM infrastructure, and the second describes
the Core LLVM classes. In the future this manual will be extended with
information describing how to use extension libraries, such as dominator
information, CFG traversal routines, and useful utilities like the <tt><a
href="/doxygen/InstVisitor_8h-source.html">InstVisitor</a></tt> template.</p>
</div>
<!-- *********************************************************************** -->
<div class="doc_section">
<a name="general">General Information</a>
</div>
<!-- *********************************************************************** -->
<div class="doc_text">
<p>This section contains general information that is useful if you are working
in the LLVM source-base, but that isn't specific to any particular API.</p>
</div>
<!-- ======================================================================= -->
<div class="doc_subsection">
<a name="stl">The C++ Standard Template Library</a>
</div>
<div class="doc_text">
<p>LLVM makes heavy use of the C++ Standard Template Library (STL),
perhaps much more than you are used to, or have seen before. Because of
this, you might want to do a little background reading in the
techniques used and capabilities of the library. There are many good
pages that discuss the STL, and several books on the subject that you
can get, so it will not be discussed in this document.</p>
<p>Here are some useful links:</p>
<ol>
<li><a href="http://www.dinkumware.com/refxcpp.html">Dinkumware C++ Library
reference</a> - an excellent reference for the STL and other parts of the
standard C++ library.</li>
<li><a href="http://www.tempest-sw.com/cpp/">C++ In a Nutshell</a> - This is an
O'Reilly book in the making. It has a decent
Standard Library
Reference that rivals Dinkumware's, and is unfortunately no longer free since the book has been
published.</li>
<li><a href="http://www.parashift.com/c++-faq-lite/">C++ Frequently Asked
Questions</a></li>
<li><a href="http://www.sgi.com/tech/stl/">SGI's STL Programmer's Guide</a> -
Contains a useful <a
href="http://www.sgi.com/tech/stl/stl_introduction.html">Introduction to the
STL</a>.</li>
<li><a href="http://www.research.att.com/%7Ebs/C++.html">Bjarne Stroustrup's C++
Page</a></li>
<li><a href="http://64.78.49.204/">
Bruce Eckel's Thinking in C++, 2nd ed. Volume 2 Revision 4.0 (even better, get
the book).</a></li>
</ol>
<p>You are also encouraged to take a look at the <a
href="CodingStandards.html">LLVM Coding Standards</a> guide which focuses on how
to write maintainable code more than where to put your curly braces.</p>
</div>
<!-- ======================================================================= -->
<div class="doc_subsection">
<a name="stl">Other useful references</a>
</div>
<div class="doc_text">
<ol>
<li><a href="http://www.psc.edu/%7Esemke/cvs_branches.html">CVS
Branch and Tag Primer</a></li>
<li><a href="http://www.fortran-2000.com/ArnaudRecipes/sharedlib.html">Using
static and shared libraries across platforms</a></li>
</ol>
</div>
<!-- *********************************************************************** -->
<div class="doc_section">
<a name="apis">Important and useful LLVM APIs</a>
</div>
<!-- *********************************************************************** -->
<div class="doc_text">
<p>Here we highlight some LLVM APIs that are generally useful and good to
know about when writing transformations.</p>
</div>
<!-- ======================================================================= -->
<div class="doc_subsection">
<a name="isa">The <tt>isa&lt;&gt;</tt>, <tt>cast&lt;&gt;</tt> and
<tt>dyn_cast&lt;&gt;</tt> templates</a>
</div>
<div class="doc_text">
<p>The LLVM source-base makes extensive use of a custom form of RTTI.
These templates have many similarities to the C++ <tt>dynamic_cast&lt;&gt;</tt>
operator, but they don't have some drawbacks (primarily stemming from
the fact that <tt>dynamic_cast&lt;&gt;</tt> only works on classes that
have a v-table). Because they are used so often, you must know what they
do and how they work. All of these templates are defined in the <a
href="/doxygen/Casting_8h-source.html"><tt>llvm/Support/Casting.h</tt></a>
file (note that you very rarely have to include this file directly).</p>
<dl>
<dt><tt>isa&lt;&gt;</tt>: </dt>
<dd><p>The <tt>isa&lt;&gt;</tt> operator works exactly like the Java
"<tt>instanceof</tt>" operator. It returns true or false depending on whether
a reference or pointer points to an instance of the specified class. This can
be very useful for constraint checking of various sorts (example below).</p>
</dd>
<dt><tt>cast&lt;&gt;</tt>: </dt>
<dd><p>The <tt>cast&lt;&gt;</tt> operator is a "checked cast" operation. It
converts a pointer or reference from a base class to a derived cast, causing
an assertion failure if it is not really an instance of the right type. This
should be used in cases where you have some information that makes you believe
that something is of the right type. An example of the <tt>isa&lt;&gt;</tt>
and <tt>cast&lt;&gt;</tt> template is:</p>
<div class="doc_code">
<pre>
static bool isLoopInvariant(const <a href="#Value">Value</a> *V, const Loop *L) {
if (isa&lt;<a href="#Constant">Constant</a>&gt;(V) || isa&lt;<a href="#Argument">Argument</a>&gt;(V) || isa&lt;<a href="#GlobalValue">GlobalValue</a>&gt;(V))
return true;
// <i>Otherwise, it must be an instruction...</i>
return !L-&gt;contains(cast&lt;<a href="#Instruction">Instruction</a>&gt;(V)-&gt;getParent());
}
</pre>
</div>
<p>Note that you should <b>not</b> use an <tt>isa&lt;&gt;</tt> test followed
by a <tt>cast&lt;&gt;</tt>, for that use the <tt>dyn_cast&lt;&gt;</tt>
operator.</p>
</dd>
<dt><tt>dyn_cast&lt;&gt;</tt>:</dt>
<dd><p>The <tt>dyn_cast&lt;&gt;</tt> operator is a "checking cast" operation.
It checks to see if the operand is of the specified type, and if so, returns a
pointer to it (this operator does not work with references). If the operand is
not of the correct type, a null pointer is returned. Thus, this works very
much like the <tt>dynamic_cast&lt;&gt;</tt> operator in C++, and should be
used in the same circumstances. Typically, the <tt>dyn_cast&lt;&gt;</tt>
operator is used in an <tt>if</tt> statement or some other flow control
statement like this:</p>
<div class="doc_code">
<pre>
if (<a href="#AllocationInst">AllocationInst</a> *AI = dyn_cast&lt;<a href="#AllocationInst">AllocationInst</a>&gt;(Val)) {
// <i>...</i>
}
</pre>
</div>
<p>This form of the <tt>if</tt> statement effectively combines together a call
to <tt>isa&lt;&gt;</tt> and a call to <tt>cast&lt;&gt;</tt> into one
statement, which is very convenient.</p>
<p>Note that the <tt>dyn_cast&lt;&gt;</tt> operator, like C++'s
<tt>dynamic_cast&lt;&gt;</tt> or Java's <tt>instanceof</tt> operator, can be
abused. In particular, you should not use big chained <tt>if/then/else</tt>
blocks to check for lots of different variants of classes. If you find
yourself wanting to do this, it is much cleaner and more efficient to use the
<tt>InstVisitor</tt> class to dispatch over the instruction type directly.</p>
</dd>
<dt><tt>cast_or_null&lt;&gt;</tt>: </dt>
<dd><p>The <tt>cast_or_null&lt;&gt;</tt> operator works just like the
<tt>cast&lt;&gt;</tt> operator, except that it allows for a null pointer as an
argument (which it then propagates). This can sometimes be useful, allowing
you to combine several null checks into one.</p></dd>
<dt><tt>dyn_cast_or_null&lt;&gt;</tt>: </dt>
<dd><p>The <tt>dyn_cast_or_null&lt;&gt;</tt> operator works just like the
<tt>dyn_cast&lt;&gt;</tt> operator, except that it allows for a null pointer
as an argument (which it then propagates). This can sometimes be useful,
allowing you to combine several null checks into one.</p></dd>
</dl>
<p>These five templates can be used with any classes, whether they have a
v-table or not. To add support for these templates, you simply need to add
<tt>classof</tt> static methods to the class you are interested casting
to. Describing this is currently outside the scope of this document, but there
are lots of examples in the LLVM source base.</p>
</div>
<!-- ======================================================================= -->
<div class="doc_subsection">
<a name="DEBUG">The <tt>DEBUG()</tt> macro and <tt>-debug</tt> option</a>
</div>
<div class="doc_text">
<p>Often when working on your pass you will put a bunch of debugging printouts
and other code into your pass. After you get it working, you want to remove
it, but you may need it again in the future (to work out new bugs that you run
across).</p>
<p> Naturally, because of this, you don't want to delete the debug printouts,
but you don't want them to always be noisy. A standard compromise is to comment
them out, allowing you to enable them if you need them in the future.</p>
<p>The "<tt><a href="/doxygen/Debug_8h-source.html">llvm/Support/Debug.h</a></tt>"
file provides a macro named <tt>DEBUG()</tt> that is a much nicer solution to
this problem. Basically, you can put arbitrary code into the argument of the
<tt>DEBUG</tt> macro, and it is only executed if '<tt>opt</tt>' (or any other
tool) is run with the '<tt>-debug</tt>' command line argument:</p>
<div class="doc_code">
<pre>
DOUT &lt;&lt; "I am here!\n";
</pre>
</div>
<p>Then you can run your pass like this:</p>
<div class="doc_code">
<pre>
$ opt &lt; a.bc &gt; /dev/null -mypass
<i>&lt;no output&gt;</i>
$ opt &lt; a.bc &gt; /dev/null -mypass -debug
I am here!
</pre>
</div>
<p>Using the <tt>DEBUG()</tt> macro instead of a home-brewed solution allows you
to not have to create "yet another" command line option for the debug output for
your pass. Note that <tt>DEBUG()</tt> macros are disabled for optimized builds,
so they do not cause a performance impact at all (for the same reason, they
should also not contain side-effects!).</p>
<p>One additional nice thing about the <tt>DEBUG()</tt> macro is that you can
enable or disable it directly in gdb. Just use "<tt>set DebugFlag=0</tt>" or
"<tt>set DebugFlag=1</tt>" from the gdb if the program is running. If the
program hasn't been started yet, you can always just run it with
<tt>-debug</tt>.</p>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
<a name="DEBUG_TYPE">Fine grained debug info with <tt>DEBUG_TYPE</tt> and
the <tt>-debug-only</tt> option</a>
</div>
<div class="doc_text">
<p>Sometimes you may find yourself in a situation where enabling <tt>-debug</tt>
just turns on <b>too much</b> information (such as when working on the code
generator). If you want to enable debug information with more fine-grained
control, you define the <tt>DEBUG_TYPE</tt> macro and the <tt>-debug</tt> only
option as follows:</p>
<div class="doc_code">
<pre>
DOUT &lt;&lt; "No debug type\n";
#undef DEBUG_TYPE
#define DEBUG_TYPE "foo"
DOUT &lt;&lt; "'foo' debug type\n";
#undef DEBUG_TYPE
#define DEBUG_TYPE "bar"
DOUT &lt;&lt; "'bar' debug type\n";
#undef DEBUG_TYPE
#define DEBUG_TYPE ""
DOUT &lt;&lt; "No debug type (2)\n";
</pre>
</div>
<p>Then you can run your pass like this:</p>
<div class="doc_code">
<pre>
$ opt &lt; a.bc &gt; /dev/null -mypass
<i>&lt;no output&gt;</i>
$ opt &lt; a.bc &gt; /dev/null -mypass -debug
No debug type
'foo' debug type
'bar' debug type
No debug type (2)
$ opt &lt; a.bc &gt; /dev/null -mypass -debug-only=foo
'foo' debug type
$ opt &lt; a.bc &gt; /dev/null -mypass -debug-only=bar
'bar' debug type
</pre>
</div>
<p>Of course, in practice, you should only set <tt>DEBUG_TYPE</tt> at the top of
a file, to specify the debug type for the entire module (if you do this before
you <tt>#include "llvm/Support/Debug.h"</tt>, you don't have to insert the ugly
<tt>#undef</tt>'s). Also, you should use names more meaningful than "foo" and
"bar", because there is no system in place to ensure that names do not
conflict. If two different modules use the same string, they will all be turned
on when the name is specified. This allows, for example, all debug information
for instruction scheduling to be enabled with <tt>-debug-type=InstrSched</tt>,
even if the source lives in multiple files.</p>
</div>
<!-- ======================================================================= -->
<div class="doc_subsection">
<a name="Statistic">The <tt>Statistic</tt> class &amp; <tt>-stats</tt>
option</a>
</div>
<div class="doc_text">
<p>The "<tt><a
href="/doxygen/Statistic_8h-source.html">llvm/ADT/Statistic.h</a></tt>" file
provides a class named <tt>Statistic</tt> that is used as a unified way to
keep track of what the LLVM compiler is doing and how effective various
optimizations are. It is useful to see what optimizations are contributing to
making a particular program run faster.</p>
<p>Often you may run your pass on some big program, and you're interested to see
how many times it makes a certain transformation. Although you can do this with
hand inspection, or some ad-hoc method, this is a real pain and not very useful
for big programs. Using the <tt>Statistic</tt> class makes it very easy to
keep track of this information, and the calculated information is presented in a
uniform manner with the rest of the passes being executed.</p>
<p>There are many examples of <tt>Statistic</tt> uses, but the basics of using
it are as follows:</p>
<ol>
<li><p>Define your statistic like this:</p>
<div class="doc_code">
<pre>
#define <a href="#DEBUG_TYPE">DEBUG_TYPE</a> "mypassname" <i>// This goes before any #includes.</i>
STATISTIC(NumXForms, "The # of times I did stuff");
</pre>
</div>
<p>The <tt>STATISTIC</tt> macro defines a static variable, whose name is
specified by the first argument. The pass name is taken from the DEBUG_TYPE
macro, and the description is taken from the second argument. The variable
defined ("NumXForms" in this case) acts like an unsigned integer.</p></li>
<li><p>Whenever you make a transformation, bump the counter:</p>
<div class="doc_code">
<pre>
++NumXForms; // <i>I did stuff!</i>
</pre>
</div>
</li>
</ol>
<p>That's all you have to do. To get '<tt>opt</tt>' to print out the
statistics gathered, use the '<tt>-stats</tt>' option:</p>
<div class="doc_code">
<pre>
$ opt -stats -mypassname &lt; program.bc &gt; /dev/null
<i>... statistics output ...</i>
</pre>
</div>
<p> When running <tt>opt</tt> on a C file from the SPEC benchmark
suite, it gives a report that looks like this:</p>
<div class="doc_code">
<pre>
7646 bitcodewriter - Number of normal instructions
725 bitcodewriter - Number of oversized instructions
129996 bitcodewriter - Number of bitcode bytes written
2817 raise - Number of insts DCEd or constprop'd
3213 raise - Number of cast-of-self removed
5046 raise - Number of expression trees converted
75 raise - Number of other getelementptr's formed
138 raise - Number of load/store peepholes
42 deadtypeelim - Number of unused typenames removed from symtab
392 funcresolve - Number of varargs functions resolved
27 globaldce - Number of global variables removed
2 adce - Number of basic blocks removed
134 cee - Number of branches revectored
49 cee - Number of setcc instruction eliminated
532 gcse - Number of loads removed
2919 gcse - Number of instructions removed
86 indvars - Number of canonical indvars added
87 indvars - Number of aux indvars removed
25 instcombine - Number of dead inst eliminate
434 instcombine - Number of insts combined
248 licm - Number of load insts hoisted
1298 licm - Number of insts hoisted to a loop pre-header
3 licm - Number of insts hoisted to multiple loop preds (bad, no loop pre-header)
75 mem2reg - Number of alloca's promoted
1444 cfgsimplify - Number of blocks simplified
</pre>
</div>
<p>Obviously, with so many optimizations, having a unified framework for this
stuff is very nice. Making your pass fit well into the framework makes it more
maintainable and useful.</p>
</div>
<!-- ======================================================================= -->
<div class="doc_subsection">
<a name="ViewGraph">Viewing graphs while debugging code</a>
</div>
<div class="doc_text">
<p>Several of the important data structures in LLVM are graphs: for example
CFGs made out of LLVM <a href="#BasicBlock">BasicBlock</a>s, CFGs made out of
LLVM <a href="CodeGenerator.html#machinebasicblock">MachineBasicBlock</a>s, and
<a href="CodeGenerator.html#selectiondag_intro">Instruction Selection
DAGs</a>. In many cases, while debugging various parts of the compiler, it is
nice to instantly visualize these graphs.</p>
<p>LLVM provides several callbacks that are available in a debug build to do
exactly that. If you call the <tt>Function::viewCFG()</tt> method, for example,
the current LLVM tool will pop up a window containing the CFG for the function
where each basic block is a node in the graph, and each node contains the
instructions in the block. Similarly, there also exists
<tt>Function::viewCFGOnly()</tt> (does not include the instructions), the
<tt>MachineFunction::viewCFG()</tt> and <tt>MachineFunction::viewCFGOnly()</tt>,
and the <tt>SelectionDAG::viewGraph()</tt> methods. Within GDB, for example,
you can usually use something like <tt>call DAG.viewGraph()</tt> to pop
up a window. Alternatively, you can sprinkle calls to these functions in your
code in places you want to debug.</p>
<p>Getting this to work requires a small amount of configuration. On Unix
systems with X11, install the <a href="http://www.graphviz.org">graphviz</a>
toolkit, and make sure 'dot' and 'gv' are in your path. If you are running on
Mac OS/X, download and install the Mac OS/X <a
href="http://www.pixelglow.com/graphviz/">Graphviz program</a>, and add
<tt>/Applications/Graphviz.app/Contents/MacOS/</tt> (or wherever you install
it) to your path. Once in your system and path are set up, rerun the LLVM
configure script and rebuild LLVM to enable this functionality.</p>
<p><tt>SelectionDAG</tt> has been extended to make it easier to locate
<i>interesting</i> nodes in large complex graphs. From gdb, if you
<tt>call DAG.setGraphColor(<i>node</i>, "<i>color</i>")</tt>, then the
next <tt>call DAG.viewGraph()</tt> would highlight the node in the
specified color (choices of colors can be found at <a
href="http://www.graphviz.org/doc/info/colors.html">colors</a>.) More
complex node attributes can be provided with <tt>call
DAG.setGraphAttrs(<i>node</i>, "<i>attributes</i>")</tt> (choices can be
found at <a href="http://www.graphviz.org/doc/info/attrs.html">Graph
Attributes</a>.) If you want to restart and clear all the current graph
attributes, then you can <tt>call DAG.clearGraphAttrs()</tt>. </p>
</div>
<!-- *********************************************************************** -->
<div class="doc_section">
<a name="datastructure">Picking the Right Data Structure for a Task</a>
</div>
<!-- *********************************************************************** -->
<div class="doc_text">
<p>LLVM has a plethora of data structures in the <tt>llvm/ADT/</tt> directory,
and we commonly use STL data structures. This section describes the trade-offs
you should consider when you pick one.</p>
<p>
The first step is a choose your own adventure: do you want a sequential
container, a set-like container, or a map-like container? The most important
thing when choosing a container is the algorithmic properties of how you plan to
access the container. Based on that, you should use:</p>
<ul>
<li>a <a href="#ds_map">map-like</a> container if you need efficient look-up
of an value based on another value. Map-like containers also support
efficient queries for containment (whether a key is in the map). Map-like
containers generally do not support efficient reverse mapping (values to
keys). If you need that, use two maps. Some map-like containers also
support efficient iteration through the keys in sorted order. Map-like
containers are the most expensive sort, only use them if you need one of
these capabilities.</li>
<li>a <a href="#ds_set">set-like</a> container if you need to put a bunch of
stuff into a container that automatically eliminates duplicates. Some
set-like containers support efficient iteration through the elements in
sorted order. Set-like containers are more expensive than sequential
containers.
</li>
<li>a <a href="#ds_sequential">sequential</a> container provides
the most efficient way to add elements and keeps track of the order they are
added to the collection. They permit duplicates and support efficient
iteration, but do not support efficient look-up based on a key.
</li>
<li>a <a href="#ds_bit">bit</a> container provides an efficient way to store and
perform set operations on sets of numeric id's, while automatically
eliminating duplicates. Bit containers require a maximum of 1 bit for each
identifier you want to store.
</li>
</ul>
<p>
Once the proper category of container is determined, you can fine tune the
memory use, constant factors, and cache behaviors of access by intelligently
picking a member of the category. Note that constant factors and cache behavior
can be a big deal. If you have a vector that usually only contains a few
elements (but could contain many), for example, it's much better to use
<a href="#dss_smallvector">SmallVector</a> than <a href="#dss_vector">vector</a>
. Doing so avoids (relatively) expensive malloc/free calls, which dwarf the
cost of adding the elements to the container. </p>
</div>
<!-- ======================================================================= -->
<div class="doc_subsection">
<a name="ds_sequential">Sequential Containers (std::vector, std::list, etc)</a>
</div>
<div class="doc_text">
There are a variety of sequential containers available for you, based on your
needs. Pick the first in this section that will do what you want.
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
<a name="dss_fixedarrays">Fixed Size Arrays</a>
</div>
<div class="doc_text">
<p>Fixed size arrays are very simple and very fast. They are good if you know
exactly how many elements you have, or you have a (low) upper bound on how many
you have.</p>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
<a name="dss_heaparrays">Heap Allocated Arrays</a>
</div>
<div class="doc_text">
<p>Heap allocated arrays (new[] + delete[]) are also simple. They are good if
the number of elements is variable, if you know how many elements you will need
before the array is allocated, and if the array is usually large (if not,
consider a <a href="#dss_smallvector">SmallVector</a>). The cost of a heap
allocated array is the cost of the new/delete (aka malloc/free). Also note that
if you are allocating an array of a type with a constructor, the constructor and
destructors will be run for every element in the array (re-sizable vectors only
construct those elements actually used).</p>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
<a name="dss_smallvector">"llvm/ADT/SmallVector.h"</a>
</div>
<div class="doc_text">
<p><tt>SmallVector&lt;Type, N&gt;</tt> is a simple class that looks and smells
just like <tt>vector&lt;Type&gt;</tt>:
it supports efficient iteration, lays out elements in memory order (so you can
do pointer arithmetic between elements), supports efficient push_back/pop_back
operations, supports efficient random access to its elements, etc.</p>
<p>The advantage of SmallVector is that it allocates space for
some number of elements (N) <b>in the object itself</b>. Because of this, if
the SmallVector is dynamically smaller than N, no malloc is performed. This can
be a big win in cases where the malloc/free call is far more expensive than the
code that fiddles around with the elements.</p>
<p>This is good for vectors that are "usually small" (e.g. the number of
predecessors/successors of a block is usually less than 8). On the other hand,
this makes the size of the SmallVector itself large, so you don't want to
allocate lots of them (doing so will waste a lot of space). As such,
SmallVectors are most useful when on the stack.</p>
<p>SmallVector also provides a nice portable and efficient replacement for
<tt>alloca</tt>.</p>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
<a name="dss_vector">&lt;vector&gt;</a>
</div>
<div class="doc_text">
<p>
std::vector is well loved and respected. It is useful when SmallVector isn't:
when the size of the vector is often large (thus the small optimization will
rarely be a benefit) or if you will be allocating many instances of the vector
itself (which would waste space for elements that aren't in the container).
vector is also useful when interfacing with code that expects vectors :).
</p>
<p>One worthwhile note about std::vector: avoid code like this:</p>
<div class="doc_code">
<pre>
for ( ... ) {
std::vector&lt;foo&gt; V;
use V;
}
</pre>
</div>
<p>Instead, write this as:</p>
<div class="doc_code">
<pre>
std::vector&lt;foo&gt; V;
for ( ... ) {
use V;
V.clear();
}
</pre>
</div>
<p>Doing so will save (at least) one heap allocation and free per iteration of
the loop.</p>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
<a name="dss_deque">&lt;deque&gt;</a>
</div>
<div class="doc_text">
<p>std::deque is, in some senses, a generalized version of std::vector. Like
std::vector, it provides constant time random access and other similar
properties, but it also provides efficient access to the front of the list. It
does not guarantee continuity of elements within memory.</p>
<p>In exchange for this extra flexibility, std::deque has significantly higher
constant factor costs than std::vector. If possible, use std::vector or
something cheaper.</p>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
<a name="dss_list">&lt;list&gt;</a>
</div>
<div class="doc_text">
<p>std::list is an extremely inefficient class that is rarely useful.
It performs a heap allocation for every element inserted into it, thus having an
extremely high constant factor, particularly for small data types. std::list
also only supports bidirectional iteration, not random access iteration.</p>
<p>In exchange for this high cost, std::list supports efficient access to both
ends of the list (like std::deque, but unlike std::vector or SmallVector). In
addition, the iterator invalidation characteristics of std::list are stronger
than that of a vector class: inserting or removing an element into the list does
not invalidate iterator or pointers to other elements in the list.</p>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
<a name="dss_ilist">llvm/ADT/ilist</a>
</div>
<div class="doc_text">
<p><tt>ilist&lt;T&gt;</tt> implements an 'intrusive' doubly-linked list. It is
intrusive, because it requires the element to store and provide access to the
prev/next pointers for the list.</p>
<p>ilist has the same drawbacks as std::list, and additionally requires an
ilist_traits implementation for the element type, but it provides some novel
characteristics. In particular, it can efficiently store polymorphic objects,
the traits class is informed when an element is inserted or removed from the
list, and ilists are guaranteed to support a constant-time splice operation.
</p>
<p>These properties are exactly what we want for things like Instructions and
basic blocks, which is why these are implemented with ilists.</p>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
<a name="dss_other">Other Sequential Container options</a>
</div>
<div class="doc_text">
<p>Other STL containers are available, such as std::string.</p>
<p>There are also various STL adapter classes such as std::queue,
std::priority_queue, std::stack, etc. These provide simplified access to an
underlying container but don't affect the cost of the container itself.</p>
</div>
<!-- ======================================================================= -->
<div class="doc_subsection">
<a name="ds_set">Set-Like Containers (std::set, SmallSet, SetVector, etc)</a>
</div>
<div class="doc_text">
<p>Set-like containers are useful when you need to canonicalize multiple values
into a single representation. There are several different choices for how to do
this, providing various trade-offs.</p>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
<a name="dss_sortedvectorset">A sorted 'vector'</a>
</div>
<div class="doc_text">
<p>If you intend to insert a lot of elements, then do a lot of queries, a
great approach is to use a vector (or other sequential container) with
std::sort+std::unique to remove duplicates. This approach works really well if
your usage pattern has these two distinct phases (insert then query), and can be
coupled with a good choice of <a href="#ds_sequential">sequential container</a>.
</p>
<p>
This combination provides the several nice properties: the result data is
contiguous in memory (good for cache locality), has few allocations, is easy to
address (iterators in the final vector are just indices or pointers), and can be
efficiently queried with a standard binary or radix search.</p>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
<a name="dss_smallset">"llvm/ADT/SmallSet.h"</a>
</div>
<div class="doc_text">
<p>If you have a set-like data structure that is usually small and whose elements
are reasonably small, a <tt>SmallSet&lt;Type, N&gt;</tt> is a good choice. This set
has space for N elements in place (thus, if the set is dynamically smaller than
N, no malloc traffic is required) and accesses them with a simple linear search.
When the set grows beyond 'N' elements, it allocates a more expensive representation that
guarantees efficient access (for most types, it falls back to std::set, but for
pointers it uses something far better, <a
href="#dss_smallptrset">SmallPtrSet</a>).</p>
<p>The magic of this class is that it handles small sets extremely efficiently,
but gracefully handles extremely large sets without loss of efficiency. The
drawback is that the interface is quite small: it supports insertion, queries
and erasing, but does not support iteration.</p>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
<a name="dss_smallptrset">"llvm/ADT/SmallPtrSet.h"</a>
</div>
<div class="doc_text">
<p>SmallPtrSet has all the advantages of SmallSet (and a SmallSet of pointers is
transparently implemented with a SmallPtrSet), but also supports iterators. If
more than 'N' insertions are performed, a single quadratically
probed hash table is allocated and grows as needed, providing extremely
efficient access (constant time insertion/deleting/queries with low constant
factors) and is very stingy with malloc traffic.</p>
<p>Note that, unlike std::set, the iterators of SmallPtrSet are invalidated
whenever an insertion occurs. Also, the values visited by the iterators are not
visited in sorted order.</p>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
<a name="dss_denseset">"llvm/ADT/DenseSet.h"</a>
</div>
<div class="doc_text">
<p>
DenseSet is a simple quadratically probed hash table. It excels at supporting
small values: it uses a single allocation to hold all of the pairs that
are currently inserted in the set. DenseSet is a great way to unique small
values that are not simple pointers (use <a
href="#dss_smallptrset">SmallPtrSet</a> for pointers). Note that DenseSet has
the same requirements for the value type that <a
href="#dss_densemap">DenseMap</a> has.
</p>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
<a name="dss_FoldingSet">"llvm/ADT/FoldingSet.h"</a>
</div>
<div class="doc_text">
<p>
FoldingSet is an aggregate class that is really good at uniquing
expensive-to-create or polymorphic objects. It is a combination of a chained
hash table with intrusive links (uniqued objects are required to inherit from
FoldingSetNode) that uses <a href="#dss_smallvector">SmallVector</a> as part of
its ID process.</p>
<p>Consider a case where you want to implement a "getOrCreateFoo" method for
a complex object (for example, a node in the code generator). The client has a
description of *what* it wants to generate (it knows the opcode and all the
operands), but we don't want to 'new' a node, then try inserting it into a set
only to find out it already exists, at which point we would have to delete it
and return the node that already exists.
</p>
<p>To support this style of client, FoldingSet perform a query with a
FoldingSetNodeID (which wraps SmallVector) that can be used to describe the
element that we want to query for. The query either returns the element
matching the ID or it returns an opaque ID that indicates where insertion should
take place. Construction of the ID usually does not require heap traffic.</p>
<p>Because FoldingSet uses intrusive links, it can support polymorphic objects
in the set (for example, you can have SDNode instances mixed with LoadSDNodes).
Because the elements are individually allocated, pointers to the elements are
stable: inserting or removing elements does not invalidate any pointers to other
elements.
</p>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
<a name="dss_set">&lt;set&gt;</a>
</div>
<div class="doc_text">
<p><tt>std::set</tt> is a reasonable all-around set class, which is decent at
many things but great at nothing. std::set allocates memory for each element
inserted (thus it is very malloc intensive) and typically stores three pointers
per element in the set (thus adding a large amount of per-element space
overhead). It offers guaranteed log(n) performance, which is not particularly
fast from a complexity standpoint (particularly if the elements of the set are
expensive to compare, like strings), and has extremely high constant factors for
lookup, insertion and removal.</p>
<p>The advantages of std::set are that its iterators are stable (deleting or
inserting an element from the set does not affect iterators or pointers to other
elements) and that iteration over the set is guaranteed to be in sorted order.
If the elements in the set are large, then the relative overhead of the pointers
and malloc traffic is not a big deal, but if the elements of the set are small,
std::set is almost never a good choice.</p>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
<a name="dss_setvector">"llvm/ADT/SetVector.h"</a>
</div>
<div class="doc_text">
<p>LLVM's SetVector&lt;Type&gt; is an adapter class that combines your choice of
a set-like container along with a <a href="#ds_sequential">Sequential
Container</a>. The important property
that this provides is efficient insertion with uniquing (duplicate elements are
ignored) with iteration support. It implements this by inserting elements into
both a set-like container and the sequential container, using the set-like
container for uniquing and the sequential container for iteration.
</p>
<p>The difference between SetVector and other sets is that the order of
iteration is guaranteed to match the order of insertion into the SetVector.
This property is really important for things like sets of pointers. Because
pointer values are non-deterministic (e.g. vary across runs of the program on
different machines), iterating over the pointers in the set will
not be in a well-defined order.</p>
<p>
The drawback of SetVector is that it requires twice as much space as a normal
set and has the sum of constant factors from the set-like container and the
sequential container that it uses. Use it *only* if you need to iterate over
the elements in a deterministic order. SetVector is also expensive to delete
elements out of (linear time), unless you use it's "pop_back" method, which is
faster.
</p>
<p>SetVector is an adapter class that defaults to using std::vector and std::set
for the underlying containers, so it is quite expensive. However,
<tt>"llvm/ADT/SetVector.h"</tt> also provides a SmallSetVector class, which
defaults to using a SmallVector and SmallSet of a specified size. If you use
this, and if your sets are dynamically smaller than N, you will save a lot of
heap traffic.</p>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
<a name="dss_uniquevector">"llvm/ADT/UniqueVector.h"</a>
</div>
<div class="doc_text">
<p>
UniqueVector is similar to <a href="#dss_setvector">SetVector</a>, but it
retains a unique ID for each element inserted into the set. It internally
contains a map and a vector, and it assigns a unique ID for each value inserted
into the set.</p>
<p>UniqueVector is very expensive: its cost is the sum of the cost of
maintaining both the map and vector, it has high complexity, high constant
factors, and produces a lot of malloc traffic. It should be avoided.</p>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
<a name="dss_otherset">Other Set-Like Container Options</a>
</div>
<div class="doc_text">
<p>
The STL provides several other options, such as std::multiset and the various
"hash_set" like containers (whether from C++ TR1 or from the SGI library).</p>
<p>std::multiset is useful if you're not interested in elimination of
duplicates, but has all the drawbacks of std::set. A sorted vector (where you
don't delete duplicate entries) or some other approach is almost always
better.</p>
<p>The various hash_set implementations (exposed portably by
"llvm/ADT/hash_set") is a simple chained hashtable. This algorithm is as malloc
intensive as std::set (performing an allocation for each element inserted,
thus having really high constant factors) but (usually) provides O(1)
insertion/deletion of elements. This can be useful if your elements are large
(thus making the constant-factor cost relatively low) or if comparisons are
expensive. Element iteration does not visit elements in a useful order.</p>
</div>
<!-- ======================================================================= -->
<div class="doc_subsection">
<a name="ds_map">Map-Like Containers (std::map, DenseMap, etc)</a>
</div>
<div class="doc_text">
Map-like containers are useful when you want to associate data to a key. As
usual, there are a lot of different ways to do this. :)
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
<a name="dss_sortedvectormap">A sorted 'vector'</a>
</div>
<div class="doc_text">
<p>
If your usage pattern follows a strict insert-then-query approach, you can
trivially use the same approach as <a href="#dss_sortedvectorset">sorted vectors
for set-like containers</a>. The only difference is that your query function
(which uses std::lower_bound to get efficient log(n) lookup) should only compare
the key, not both the key and value. This yields the same advantages as sorted
vectors for sets.
</p>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
<a name="dss_stringmap">"llvm/ADT/StringMap.h"</a>
</div>
<div class="doc_text">
<p>
Strings are commonly used as keys in maps, and they are difficult to support
efficiently: they are variable length, inefficient to hash and compare when
long, expensive to copy, etc. StringMap is a specialized container designed to
cope with these issues. It supports mapping an arbitrary range of bytes to an
arbitrary other object.</p>
<p>The StringMap implementation uses a quadratically-probed hash table, where
the buckets store a pointer to the heap allocated entries (and some other
stuff). The entries in the map must be heap allocated because the strings are
variable length. The string data (key) and the element object (value) are
stored in the same allocation with the string data immediately after the element
object. This container guarantees the "<tt>(char*)(&amp;Value+1)</tt>" points
to the key string for a value.</p>
<p>The StringMap is very fast for several reasons: quadratic probing is very
cache efficient for lookups, the hash value of strings in buckets is not
recomputed when lookup up an element, StringMap rarely has to touch the
memory for unrelated objects when looking up a value (even when hash collisions
happen), hash table growth does not recompute the hash values for strings
already in the table, and each pair in the map is store in a single allocation
(the string data is stored in the same allocation as the Value of a pair).</p>
<p>StringMap also provides query methods that take byte ranges, so it only ever
copies a string if a value is inserted into the table.</p>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
<a name="dss_indexedmap">"llvm/ADT/IndexedMap.h"</a>
</div>
<div class="doc_text">
<p>
IndexedMap is a specialized container for mapping small dense integers (or
values that can be mapped to small dense integers) to some other type. It is
internally implemented as a vector with a mapping function that maps the keys to
the dense integer range.
</p>
<p>
This is useful for cases like virtual registers in the LLVM code generator: they
have a dense mapping that is offset by a compile-time constant (the first
virtual register ID).</p>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
<a name="dss_densemap">"llvm/ADT/DenseMap.h"</a>
</div>
<div class="doc_text">
<p>
DenseMap is a simple quadratically probed hash table. It excels at supporting
small keys and values: it uses a single allocation to hold all of the pairs that
are currently inserted in the map. DenseMap is a great way to map pointers to
pointers, or map other small types to each other.
</p>
<p>
There are several aspects of DenseMap that you should be aware of, however. The
iterators in a densemap are invalidated whenever an insertion occurs, unlike
map. Also, because DenseMap allocates space for a large number of key/value
pairs (it starts with 64 by default), it will waste a lot of space if your keys
or values are large. Finally, you must implement a partial specialization of
DenseMapInfo for the key that you want, if it isn't already supported. This
is required to tell DenseMap about two special marker values (which can never be
inserted into the map) that it needs internally.</p>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
<a name="dss_map">&lt;map&gt;</a>
</div>
<div class="doc_text">
<p>
std::map has similar characteristics to <a href="#dss_set">std::set</a>: it uses
a single allocation per pair inserted into the map, it offers log(n) lookup with
an extremely large constant factor, imposes a space penalty of 3 pointers per
pair in the map, etc.</p>
<p>std::map is most useful when your keys or values are very large, if you need
to iterate over the collection in sorted order, or if you need stable iterators
into the map (i.e. they don't get invalidated if an insertion or deletion of
another element takes place).</p>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
<a name="dss_othermap">Other Map-Like Container Options</a>
</div>
<div class="doc_text">
<p>
The STL provides several other options, such as std::multimap and the various
"hash_map" like containers (whether from C++ TR1 or from the SGI library).</p>
<p>std::multimap is useful if you want to map a key to multiple values, but has
all the drawbacks of std::map. A sorted vector or some other approach is almost
always better.</p>
<p>The various hash_map implementations (exposed portably by
"llvm/ADT/hash_map") are simple chained hash tables. This algorithm is as
malloc intensive as std::map (performing an allocation for each element
inserted, thus having really high constant factors) but (usually) provides O(1)
insertion/deletion of elements. This can be useful if your elements are large
(thus making the constant-factor cost relatively low) or if comparisons are
expensive. Element iteration does not visit elements in a useful order.</p>
</div>
<!-- ======================================================================= -->
<div class="doc_subsection">
<a name="ds_bit">Bit storage containers (BitVector, SparseBitVector)</a>
</div>
<div class="doc_text">
<p>Unlike the other containers, there are only two bit storage containers, and
choosing when to use each is relatively straightforward.</p>
<p>One additional option is
<tt>std::vector&lt;bool&gt;</tt>: we discourage its use for two reasons 1) the
implementation in many common compilers (e.g. commonly available versions of
GCC) is extremely inefficient and 2) the C++ standards committee is likely to
deprecate this container and/or change it significantly somehow. In any case,
please don't use it.</p>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
<a name="dss_bitvector">BitVector</a>
</div>
<div class="doc_text">
<p> The BitVector container provides a fixed size set of bits for manipulation.
It supports individual bit setting/testing, as well as set operations. The set
operations take time O(size of bitvector), but operations are performed one word
at a time, instead of one bit at a time. This makes the BitVector very fast for
set operations compared to other containers. Use the BitVector when you expect
the number of set bits to be high (IE a dense set).
</p>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
<a name="dss_sparsebitvector">SparseBitVector</a>
</div>
<div class="doc_text">
<p> The SparseBitVector container is much like BitVector, with one major
difference: Only the bits that are set, are stored. This makes the
SparseBitVector much more space efficient than BitVector when the set is sparse,
as well as making set operations O(number of set bits) instead of O(size of
universe). The downside to the SparseBitVector is that setting and testing of random bits is O(N), and on large SparseBitVectors, this can be slower than BitVector. In our implementation, setting or testing bits in sorted order
(either forwards or reverse) is O(1) worst case. Testing and setting bits within 128 bits (depends on size) of the current bit is also O(1). As a general statement, testing/setting bits in a SparseBitVector is O(distance away from last set bit).
</p>
</div>
<!-- *********************************************************************** -->
<div class="doc_section">
<a name="common">Helpful Hints for Common Operations</a>
</div>
<!-- *********************************************************************** -->
<div class="doc_text">
<p>This section describes how to perform some very simple transformations of
LLVM code. This is meant to give examples of common idioms used, showing the
practical side of LLVM transformations. <p> Because this is a "how-to" section,
you should also read about the main classes that you will be working with. The
<a href="#coreclasses">Core LLVM Class Hierarchy Reference</a> contains details
and descriptions of the main classes that you should know about.</p>
</div>
<!-- NOTE: this section should be heavy on example code -->
<!-- ======================================================================= -->
<div class="doc_subsection">
<a name="inspection">Basic Inspection and Traversal Routines</a>
</div>
<div class="doc_text">
<p>The LLVM compiler infrastructure have many different data structures that may
be traversed. Following the example of the C++ standard template library, the
techniques used to traverse these various data structures are all basically the
same. For a enumerable sequence of values, the <tt>XXXbegin()</tt> function (or
method) returns an iterator to the start of the sequence, the <tt>XXXend()</tt>
function returns an iterator pointing to one past the last valid element of the
sequence, and there is some <tt>XXXiterator</tt> data type that is common
between the two operations.</p>
<p>Because the pattern for iteration is common across many different aspects of
the program representation, the standard template library algorithms may be used
on them, and it is easier to remember how to iterate. First we show a few common
examples of the data structures that need to be traversed. Other data
structures are traversed in very similar ways.</p>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
<a name="iterate_function">Iterating over the </a><a
href="#BasicBlock"><tt>BasicBlock</tt></a>s in a <a
href="#Function"><tt>Function</tt></a>
</div>
<div class="doc_text">
<p>It's quite common to have a <tt>Function</tt> instance that you'd like to
transform in some way; in particular, you'd like to manipulate its
<tt>BasicBlock</tt>s. To facilitate this, you'll need to iterate over all of
the <tt>BasicBlock</tt>s that constitute the <tt>Function</tt>. The following is
an example that prints the name of a <tt>BasicBlock</tt> and the number of
<tt>Instruction</tt>s it contains:</p>
<div class="doc_code">
<pre>
// <i>func is a pointer to a Function instance</i>
for (Function::iterator i = func-&gt;begin(), e = func-&gt;end(); i != e; ++i)
// <i>Print out the name of the basic block if it has one, and then the</i>
// <i>number of instructions that it contains</i>
llvm::cerr &lt;&lt; "Basic block (name=" &lt;&lt; i-&gt;getName() &lt;&lt; ") has "
&lt;&lt; i-&gt;size() &lt;&lt; " instructions.\n";
</pre>
</div>
<p>Note that i can be used as if it were a pointer for the purposes of
invoking member functions of the <tt>Instruction</tt> class. This is
because the indirection operator is overloaded for the iterator
classes. In the above code, the expression <tt>i-&gt;size()</tt> is
exactly equivalent to <tt>(*i).size()</tt> just like you'd expect.</p>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
<a name="iterate_basicblock">Iterating over the </a><a
href="#Instruction"><tt>Instruction</tt></a>s in a <a
href="#BasicBlock"><tt>BasicBlock</tt></a>
</div>
<div class="doc_text">
<p>Just like when dealing with <tt>BasicBlock</tt>s in <tt>Function</tt>s, it's
easy to iterate over the individual instructions that make up
<tt>BasicBlock</tt>s. Here's a code snippet that prints out each instruction in
a <tt>BasicBlock</tt>:</p>
<div class="doc_code">
<pre>
// <i>blk is a pointer to a BasicBlock instance</i>
for (BasicBlock::iterator i = blk-&gt;begin(), e = blk-&gt;end(); i != e; ++i)
// <i>The next statement works since operator&lt;&lt;(ostream&amp;,...)</i>
// <i>is overloaded for Instruction&amp;</i>
llvm::cerr &lt;&lt; *i &lt;&lt; "\n";
</pre>
</div>
<p>However, this isn't really the best way to print out the contents of a
<tt>BasicBlock</tt>! Since the ostream operators are overloaded for virtually
anything you'll care about, you could have just invoked the print routine on the
basic block itself: <tt>llvm::cerr &lt;&lt; *blk &lt;&lt; "\n";</tt>.</p>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
<a name="iterate_institer">Iterating over the </a><a
href="#Instruction"><tt>Instruction</tt></a>s in a <a
href="#Function"><tt>Function</tt></a>
</div>
<div class="doc_text">
<p>If you're finding that you commonly iterate over a <tt>Function</tt>'s
<tt>BasicBlock</tt>s and then that <tt>BasicBlock</tt>'s <tt>Instruction</tt>s,
<tt>InstIterator</tt> should be used instead. You'll need to include <a
href="/doxygen/InstIterator_8h-source.html"><tt>llvm/Support/InstIterator.h</tt></a>,
and then instantiate <tt>InstIterator</tt>s explicitly in your code. Here's a
small example that shows how to dump all instructions in a function to the standard error stream:<p>
<div class="doc_code">
<pre>
#include "<a href="/doxygen/InstIterator_8h-source.html">llvm/Support/InstIterator.h</a>"
// <i>F is a pointer to a Function instance</i>
for (inst_iterator i = inst_begin(F), e = inst_end(F); i != e; ++i)
llvm::cerr &lt;&lt; *i &lt;&lt; "\n";
</pre>
</div>
<p>Easy, isn't it? You can also use <tt>InstIterator</tt>s to fill a
work list with its initial contents. For example, if you wanted to
initialize a work list to contain all instructions in a <tt>Function</tt>
F, all you would need to do is something like:</p>
<div class="doc_code">
<pre>
std::set&lt;Instruction*&gt; worklist;
worklist.insert(inst_begin(F), inst_end(F));
</pre>
</div>
<p>The STL set <tt>worklist</tt> would now contain all instructions in the
<tt>Function</tt> pointed to by F.</p>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
<a name="iterate_convert">Turning an iterator into a class pointer (and
vice-versa)</a>
</div>
<div class="doc_text">
<p>Sometimes, it'll be useful to grab a reference (or pointer) to a class
instance when all you've got at hand is an iterator. Well, extracting
a reference or a pointer from an iterator is very straight-forward.
Assuming that <tt>i</tt> is a <tt>BasicBlock::iterator</tt> and <tt>j</tt>
is a <tt>BasicBlock::const_iterator</tt>:</p>
<div class="doc_code">
<pre>
Instruction&amp; inst = *i; // <i>Grab reference to instruction reference</i>
Instruction* pinst = &amp;*i; // <i>Grab pointer to instruction reference</i>
const Instruction&amp; inst = *j;
</pre>
</div>
<p>However, the iterators you'll be working with in the LLVM framework are
special: they will automatically convert to a ptr-to-instance type whenever they
need to. Instead of dereferencing the iterator and then taking the address of
the result, you can simply assign the iterator to the proper pointer type and
you get the dereference and address-of operation as a result of the assignment
(behind the scenes, this is a result of overloading casting mechanisms). Thus
the last line of the last example,</p>
<div class="doc_code">
<pre>
Instruction* pinst = &amp;*i;
</pre>
</div>
<p>is semantically equivalent to</p>
<div class="doc_code">
<pre>
Instruction* pinst = i;
</pre>
</div>
<p>It's also possible to turn a class pointer into the corresponding iterator,
and this is a constant time operation (very efficient). The following code
snippet illustrates use of the conversion constructors provided by LLVM
iterators. By using these, you can explicitly grab the iterator of something
without actually obtaining it via iteration over some structure:</p>
<div class="doc_code">
<pre>
void printNextInstruction(Instruction* inst) {
BasicBlock::iterator it(inst);
++it; // <i>After this line, it refers to the instruction after *inst</i>
if (it != inst-&gt;getParent()-&gt;end()) llvm::cerr &lt;&lt; *it &lt;&lt; "\n";
}
</pre>
</div>
</div>
<!--_______________________________________________________________________-->
<div class="doc_subsubsection">
<a name="iterate_complex">Finding call sites: a slightly more complex
example</a>
</div>
<div class="doc_text">
<p>Say that you're writing a FunctionPass and would like to count all the
locations in the entire module (that is, across every <tt>Function</tt>) where a
certain function (i.e., some <tt>Function</tt>*) is already in scope. As you'll
learn later, you may want to use an <tt>InstVisitor</tt> to accomplish this in a
much more straight-forward manner, but this example will allow us to explore how
you'd do it if you didn't have <tt>InstVisitor</tt> around. In pseudo-code, this
is what we want to do:</p>
<div class="doc_code">
<pre>
initialize callCounter to zero
for each Function f in the Module
for each BasicBlock b in f
for each Instruction i in b
if (i is a CallInst and calls the given function)
increment callCounter
</pre>
</div>
<p>And the actual code is (remember, because we're writing a
<tt>FunctionPass</tt>, our <tt>FunctionPass</tt>-derived class simply has to
override the <tt>runOnFunction</tt> method):</p>
<div class="doc_code">
<pre>
Function* targetFunc = ...;
class OurFunctionPass : public FunctionPass {
public:
OurFunctionPass(): callCounter(0) { }
virtual runOnFunction(Function&amp; F) {
for (Function::iterator b = F.begin(), be = F.end(); b != be; ++b) {
for (BasicBlock::iterator i = b-&gt;begin(); ie = b-&gt;end(); i != ie; ++i) {
if (<a href="#CallInst">CallInst</a>* callInst = <a href="#isa">dyn_cast</a>&lt;<a
href="#CallInst">CallInst</a>&gt;(&amp;*i)) {
// <i>We know we've encountered a call instruction, so we</i>
// <i>need to determine if it's a call to the</i>
// <i>function pointed to by m_func or not</i>
if (callInst-&gt;getCalledFunction() == targetFunc)
++callCounter;
}
}
}
}
private:
unsigned callCounter;
};
</pre>
</div>
</div>
<!--_______________________________________________________________________-->
<div class="doc_subsubsection">
<a name="calls_and_invokes">Treating calls and invokes the same way</a>
</div>
<div class="doc_text">
<p>You may have noticed that the previous example was a bit oversimplified in
that it did not deal with call sites generated by 'invoke' instructions. In
this, and in other situations, you may find that you want to treat
<tt>CallInst</tt>s and <tt>InvokeInst</tt>s the same way, even though their
most-specific common base class is <tt>Instruction</tt>, which includes lots of
less closely-related things. For these cases, LLVM provides a handy wrapper
class called <a
href="http://llvm.org/doxygen/classllvm_1_1CallSite.html"><tt>CallSite</tt></a>.
It is essentially a wrapper around an <tt>Instruction</tt> pointer, with some
methods that provide functionality common to <tt>CallInst</tt>s and
<tt>InvokeInst</tt>s.</p>
<p>This class has "value semantics": it should be passed by value, not by
reference and it should not be dynamically allocated or deallocated using
<tt>operator new</tt> or <tt>operator delete</tt>. It is efficiently copyable,
assignable and constructable, with costs equivalents to that of a bare pointer.
If you look at its definition, it has only a single pointer member.</p>
</div>
<!--_______________________________________________________________________-->
<div class="doc_subsubsection">
<a name="iterate_chains">Iterating over def-use &amp; use-def chains</a>
</div>
<div class="doc_text">
<p>Frequently, we might have an instance of the <a
href="/doxygen/classllvm_1_1Value.html">Value Class</a> and we want to
determine which <tt>User</tt>s use the <tt>Value</tt>. The list of all
<tt>User</tt>s of a particular <tt>Value</tt> is called a <i>def-use</i> chain.
For example, let's say we have a <tt>Function*</tt> named <tt>F</tt> to a
particular function <tt>foo</tt>. Finding all of the instructions that
<i>use</i> <tt>foo</tt> is as simple as iterating over the <i>def-use</i> chain
of <tt>F</tt>:</p>
<div class="doc_code">
<pre>
Function* F = ...;
for (Value::use_iterator i = F-&gt;use_begin(), e = F-&gt;use_end(); i != e; ++i)
if (Instruction *Inst = dyn_cast&lt;Instruction&gt;(*i)) {
llvm::cerr &lt;&lt; "F is used in instruction:\n";
llvm::cerr &lt;&lt; *Inst &lt;&lt; "\n";
}
</pre>
</div>
<p>Alternately, it's common to have an instance of the <a
href="/doxygen/classllvm_1_1User.html">User Class</a> and need to know what
<tt>Value</tt>s are used by it. The list of all <tt>Value</tt>s used by a
<tt>User</tt> is known as a <i>use-def</i> chain. Instances of class
<tt>Instruction</tt> are common <tt>User</tt>s, so we might want to iterate over
all of the values that a particular instruction uses (that is, the operands of
the particular <tt>Instruction</tt>):</p>
<div class="doc_code">
<pre>
Instruction* pi = ...;
for (User::op_iterator i = pi-&gt;op_begin(), e = pi-&gt;op_end(); i != e; ++i) {
Value* v = *i;
// <i>...</i>
}
</pre>
</div>
<!--
def-use chains ("finding all users of"): Value::use_begin/use_end
use-def chains ("finding all values used"): User::op_begin/op_end [op=operand]
-->
</div>
<!-- ======================================================================= -->
<div class="doc_subsection">
<a name="simplechanges">Making simple changes</a>
</div>
<div class="doc_text">
<p>There are some primitive transformation operations present in the LLVM
infrastructure that are worth knowing about. When performing
transformations, it's fairly common to manipulate the contents of basic
blocks. This section describes some of the common methods for doing so
and gives example code.</p>
</div>
<!--_______________________________________________________________________-->
<div class="doc_subsubsection">
<a name="schanges_creating">Creating and inserting new
<tt>Instruction</tt>s</a>
</div>
<div class="doc_text">
<p><i>Instantiating Instructions</i></p>
<p>Creation of <tt>Instruction</tt>s is straight-forward: simply call the
constructor for the kind of instruction to instantiate and provide the necessary
parameters. For example, an <tt>AllocaInst</tt> only <i>requires</i> a
(const-ptr-to) <tt>Type</tt>. Thus:</p>
<div class="doc_code">
<pre>
AllocaInst* ai = new AllocaInst(Type::IntTy);
</pre>
</div>
<p>will create an <tt>AllocaInst</tt> instance that represents the allocation of
one integer in the current stack frame, at run time. Each <tt>Instruction</tt>
subclass is likely to have varying default parameters which change the semantics
of the instruction, so refer to the <a
href="/doxygen/classllvm_1_1Instruction.html">doxygen documentation for the subclass of
Instruction</a> that you're interested in instantiating.</p>
<p><i>Naming values</i></p>
<p>It is very useful to name the values of instructions when you're able to, as
this facilitates the debugging of your transformations. If you end up looking
at generated LLVM machine code, you definitely want to have logical names
associated with the results of instructions! By supplying a value for the
<tt>Name</tt> (default) parameter of the <tt>Instruction</tt> constructor, you
associate a logical name with the result of the instruction's execution at
run time. For example, say that I'm writing a transformation that dynamically
allocates space for an integer on the stack, and that integer is going to be
used as some kind of index by some other code. To accomplish this, I place an
<tt>AllocaInst</tt> at the first point in the first <tt>BasicBlock</tt> of some
<tt>Function</tt>, and I'm intending to use it within the same
<tt>Function</tt>. I might do:</p>
<div class="doc_code">
<pre>
AllocaInst* pa = new AllocaInst(Type::IntTy, 0, "indexLoc");
</pre>
</div>
<p>where <tt>indexLoc</tt> is now the logical name of the instruction's
execution value, which is a pointer to an integer on the run time stack.</p>
<p><i>Inserting instructions</i></p>
<p>There are essentially two ways to insert an <tt>Instruction</tt>
into an existing sequence of instructions that form a <tt>BasicBlock</tt>:</p>
<ul>
<li>Insertion into an explicit instruction list
<p>Given a <tt>BasicBlock* pb</tt>, an <tt>Instruction* pi</tt> within that
<tt>BasicBlock</tt>, and a newly-created instruction we wish to insert
before <tt>*pi</tt>, we do the following: </p>
<div class="doc_code">
<pre>
BasicBlock *pb = ...;
Instruction *pi = ...;
Instruction *newInst = new Instruction(...);
pb-&gt;getInstList().insert(pi, newInst); // <i>Inserts newInst before pi in pb</i>
</pre>
</div>
<p>Appending to the end of a <tt>BasicBlock</tt> is so common that
the <tt>Instruction</tt> class and <tt>Instruction</tt>-derived
classes provide constructors which take a pointer to a
<tt>BasicBlock</tt> to be appended to. For example code that
looked like: </p>
<div class="doc_code">
<pre>
BasicBlock *pb = ...;
Instruction *newInst = new Instruction(...);
pb-&gt;getInstList().push_back(newInst); // <i>Appends newInst to pb</i>
</pre>
</div>
<p>becomes: </p>
<div class="doc_code">
<pre>
BasicBlock *pb = ...;
Instruction *newInst = new Instruction(..., pb);
</pre>
</div>
<p>which is much cleaner, especially if you are creating
long instruction streams.</p></li>
<li>Insertion into an implicit instruction list
<p><tt>Instruction</tt> instances that are already in <tt>BasicBlock</tt>s
are implicitly associated with an existing instruction list: the instruction
list of the enclosing basic block. Thus, we could have accomplished the same
thing as the above code without being given a <tt>BasicBlock</tt> by doing:
</p>
<div class="doc_code">
<pre>
Instruction *pi = ...;
Instruction *newInst = new Instruction(...);
pi-&gt;getParent()-&gt;getInstList().insert(pi, newInst);
</pre>
</div>
<p>In fact, this sequence of steps occurs so frequently that the
<tt>Instruction</tt> class and <tt>Instruction</tt>-derived classes provide
constructors which take (as a default parameter) a pointer to an
<tt>Instruction</tt> which the newly-created <tt>Instruction</tt> should
precede. That is, <tt>Instruction</tt> constructors are capable of
inserting the newly-created instance into the <tt>BasicBlock</tt> of a
provided instruction, immediately before that instruction. Using an
<tt>Instruction</tt> constructor with a <tt>insertBefore</tt> (default)
parameter, the above code becomes:</p>
<div class="doc_code">
<pre>
Instruction* pi = ...;
Instruction* newInst = new Instruction(..., pi);
</pre>
</div>
<p>which is much cleaner, especially if you're creating a lot of
instructions and adding them to <tt>BasicBlock</tt>s.</p></li>
</ul>
</div>
<!--_______________________________________________________________________-->
<div class="doc_subsubsection">
<a name="schanges_deleting">Deleting <tt>Instruction</tt>s</a>
</div>
<div class="doc_text">
<p>Deleting an instruction from an existing sequence of instructions that form a
<a href="#BasicBlock"><tt>BasicBlock</tt></a> is very straight-forward. First,
you must have a pointer to the instruction that you wish to delete. Second, you
need to obtain the pointer to that instruction's basic block. You use the
pointer to the basic block to get its list of instructions and then use the
erase function to remove your instruction. For example:</p>
<div class="doc_code">
<pre>
<a href="#Instruction">Instruction</a> *I = .. ;
<a href="#BasicBlock">BasicBlock</a> *BB = I-&gt;getParent();
BB-&gt;getInstList().erase(I);
</pre>
</div>
</div>
<!--_______________________________________________________________________-->
<div class="doc_subsubsection">
<a name="schanges_replacing">Replacing an <tt>Instruction</tt> with another
<tt>Value</tt></a>
</div>
<div class="doc_text">
<p><i>Replacing individual instructions</i></p>
<p>Including "<a href="/doxygen/BasicBlockUtils_8h-source.html">llvm/Transforms/Utils/BasicBlockUtils.h</a>"
permits use of two very useful replace functions: <tt>ReplaceInstWithValue</tt>
and <tt>ReplaceInstWithInst</tt>.</p>
<h4><a name="schanges_deleting">Deleting <tt>Instruction</tt>s</a></h4>
<ul>
<li><tt>ReplaceInstWithValue</tt>
<p>This function replaces all uses (within a basic block) of a given
instruction with a value, and then removes the original instruction. The
following example illustrates the replacement of the result of a particular
<tt>AllocaInst</tt> that allocates memory for a single integer with a null
pointer to an integer.</p>
<div class="doc_code">
<pre>
AllocaInst* instToReplace = ...;
BasicBlock::iterator ii(instToReplace);
ReplaceInstWithValue(instToReplace-&gt;getParent()-&gt;getInstList(), ii,
Constant::getNullValue(PointerType::get(Type::IntTy)));
</pre></div></li>
<li><tt>ReplaceInstWithInst</tt>
<p>This function replaces a particular instruction with another
instruction. The following example illustrates the replacement of one
<tt>AllocaInst</tt> with another.</p>
<div class="doc_code">
<pre>
AllocaInst* instToReplace = ...;
BasicBlock::iterator ii(instToReplace);
ReplaceInstWithInst(instToReplace-&gt;getParent()-&gt;getInstList(), ii,
new AllocaInst(Type::IntTy, 0, "ptrToReplacedInt"));
</pre></div></li>
</ul>
<p><i>Replacing multiple uses of <tt>User</tt>s and <tt>Value</tt>s</i></p>
<p>You can use <tt>Value::replaceAllUsesWith</tt> and
<tt>User::replaceUsesOfWith</tt> to change more than one use at a time. See the
doxygen documentation for the <a href="/doxygen/classllvm_1_1Value.html">Value Class</a>
and <a href="/doxygen/classllvm_1_1User.html">User Class</a>, respectively, for more
information.</p>
<!-- Value::replaceAllUsesWith User::replaceUsesOfWith Point out:
include/llvm/Transforms/Utils/ especially BasicBlockUtils.h with:
ReplaceInstWithValue, ReplaceInstWithInst -->
</div>
<!--_______________________________________________________________________-->
<div class="doc_subsubsection">
<a name="schanges_deletingGV">Deleting <tt>GlobalVariable</tt>s</a>
</div>
<div class="doc_text">
<p>Deleting a global variable from a module is just as easy as deleting an
Instruction. First, you must have a pointer to the global variable that you wish
to delete. You use this pointer to erase it from its parent, the module.
For example:</p>
<div class="doc_code">
<pre>
<a href="#GlobalVariable">GlobalVariable</a> *GV = .. ;
GV-&gt;eraseFromParent();
</pre>
</div>
</div>
<!-- *********************************************************************** -->
<div class="doc_section">
<a name="advanced">Advanced Topics</a>
</div>
<!-- *********************************************************************** -->
<div class="doc_text">
<p>
This section describes some of the advanced or obscure API's that most clients
do not need to be aware of. These API's tend manage the inner workings of the
LLVM system, and only need to be accessed in unusual circumstances.
</p>
</div>
<!-- ======================================================================= -->
<div class="doc_subsection">
<a name="TypeResolve">LLVM Type Resolution</a>
</div>
<div class="doc_text">
<p>
The LLVM type system has a very simple goal: allow clients to compare types for
structural equality with a simple pointer comparison (aka a shallow compare).
This goal makes clients much simpler and faster, and is used throughout the LLVM
system.
</p>
<p>
Unfortunately achieving this goal is not a simple matter. In particular,
recursive types and late resolution of opaque types makes the situation very
difficult to handle. Fortunately, for the most part, our implementation makes
most clients able to be completely unaware of the nasty internal details. The
primary case where clients are exposed to the inner workings of it are when
building a recursive type. In addition to this case, the LLVM bitcode reader,
assembly parser, and linker also have to be aware of the inner workings of this
system.
</p>
<p>
For our purposes below, we need three concepts. First, an "Opaque Type" is
exactly as defined in the <a href="LangRef.html#t_opaque">language
reference</a>. Second an "Abstract Type" is any type which includes an
opaque type as part of its type graph (for example "<tt>{ opaque, i32 }</tt>").
Third, a concrete type is a type that is not an abstract type (e.g. "<tt>{ i32,
float }</tt>").
</p>
</div>
<!-- ______________________________________________________________________ -->
<div class="doc_subsubsection">
<a name="BuildRecType">Basic Recursive Type Construction</a>
</div>
<div class="doc_text">
<p>
Because the most common question is "how do I build a recursive type with LLVM",
we answer it now and explain it as we go. Here we include enough to cause this
to be emitted to an output .ll file:
</p>
<div class="doc_code">
<pre>
%mylist = type { %mylist*, i32 }
</pre>
</div>
<p>
To build this, use the following LLVM APIs:
</p>
<div class="doc_code">
<pre>
// <i>Create the initial outer struct</i>
<a href="#PATypeHolder">PATypeHolder</a> StructTy = OpaqueType::get();
std::vector&lt;const Type*&gt; Elts;
Elts.push_back(PointerType::get(StructTy));
Elts.push_back(Type::IntTy);
StructType *NewSTy = StructType::get(Elts);
// <i>At this point, NewSTy = "{ opaque*, i32 }". Tell VMCore that</i>
// <i>the struct and the opaque type are actually the same.</i>
cast&lt;OpaqueType&gt;(StructTy.get())-&gt;<a href="#refineAbstractTypeTo">refineAbstractTypeTo</a>(NewSTy);
// <i>NewSTy is potentially invalidated, but StructTy (a <a href="#PATypeHolder">PATypeHolder</a>) is</i>
// <i>kept up-to-date</i>
NewSTy = cast&lt;StructType&gt;(StructTy.get());
// <i>Add a name for the type to the module symbol table (optional)</i>
MyModule-&gt;addTypeName("mylist", NewSTy);
</pre>
</div>
<p>
This code shows the basic approach used to build recursive types: build a
non-recursive type using 'opaque', then use type unification to close the cycle.
The type unification step is performed by the <tt><a
href="#refineAbstractTypeTo">refineAbstractTypeTo</a></tt> method, which is
described next. After that, we describe the <a
href="#PATypeHolder">PATypeHolder class</a>.
</p>
</div>
<!-- ______________________________________________________________________ -->
<div class="doc_subsubsection">
<a name="refineAbstractTypeTo">The <tt>refineAbstractTypeTo</tt> method</a>
</div>
<div class="doc_text">
<p>
The <tt>refineAbstractTypeTo</tt> method starts the type unification process.
While this method is actually a member of the DerivedType class, it is most
often used on OpaqueType instances. Type unification is actually a recursive
process. After unification, types can become structurally isomorphic to
existing types, and all duplicates are deleted (to preserve pointer equality).
</p>
<p>
In the example above, the OpaqueType object is definitely deleted.
Additionally, if there is an "{ \2*, i32}" type already created in the system,
the pointer and struct type created are <b>also</b> deleted. Obviously whenever
a type is deleted, any "Type*" pointers in the program are invalidated. As
such, it is safest to avoid having <i>any</i> "Type*" pointers to abstract types
live across a call to <tt>refineAbstractTypeTo</tt> (note that non-abstract
types can never move or be deleted). To deal with this, the <a
href="#PATypeHolder">PATypeHolder</a> class is used to maintain a stable
reference to a possibly refined type, and the <a
href="#AbstractTypeUser">AbstractTypeUser</a> class is used to update more
complex datastructures.
</p>
</div>
<!-- ______________________________________________________________________ -->
<div class="doc_subsubsection">
<a name="PATypeHolder">The PATypeHolder Class</a>
</div>
<div class="doc_text">
<p>
PATypeHolder is a form of a "smart pointer" for Type objects. When VMCore
happily goes about nuking types that become isomorphic to existing types, it
automatically updates all PATypeHolder objects to point to the new type. In the
example above, this allows the code to maintain a pointer to the resultant
resolved recursive type, even though the Type*'s are potentially invalidated.
</p>
<p>
PATypeHolder is an extremely light-weight object that uses a lazy union-find
implementation to update pointers. For example the pointer from a Value to its
Type is maintained by PATypeHolder objects.
</p>
</div>
<!-- ______________________________________________________________________ -->
<div class="doc_subsubsection">
<a name="AbstractTypeUser">The AbstractTypeUser Class</a>
</div>
<div class="doc_text">
<p>
Some data structures need more to perform more complex updates when types get
resolved. To support this, a class can derive from the AbstractTypeUser class.
This class
allows it to get callbacks when certain types are resolved. To register to get
callbacks for a particular type, the DerivedType::{add/remove}AbstractTypeUser
methods can be called on a type. Note that these methods only work for <i>
abstract</i> types. Concrete types (those that do not include any opaque
objects) can never be refined.
</p>
</div>
<!-- ======================================================================= -->
<div class="doc_subsection">
<a name="SymbolTable">The <tt>ValueSymbolTable</tt> and
<tt>TypeSymbolTable</tt> classes</a>
</div>
<div class="doc_text">
<p>The <tt><a href="http://llvm.org/doxygen/classllvm_1_1ValueSymbolTable.html">
ValueSymbolTable</a></tt> class provides a symbol table that the <a
href="#Function"><tt>Function</tt></a> and <a href="#Module">
<tt>Module</tt></a> classes use for naming value definitions. The symbol table
can provide a name for any <a href="#Value"><tt>Value</tt></a>.
The <tt><a href="http://llvm.org/doxygen/classllvm_1_1TypeSymbolTable.html">
TypeSymbolTable</a></tt> class is used by the <tt>Module</tt> class to store
names for types.</p>
<p>Note that the <tt>SymbolTable</tt> class should not be directly accessed
by most clients. It should only be used when iteration over the symbol table
names themselves are required, which is very special purpose. Note that not
all LLVM
<a href="#Value">Value</a>s have names, and those without names (i.e. they have
an empty name) do not exist in the symbol table.
</p>
<p>These symbol tables support iteration over the values/types in the symbol
table with <tt>begin/end/iterator</tt> and supports querying to see if a
specific name is in the symbol table (with <tt>lookup</tt>). The
<tt>ValueSymbolTable</tt> class exposes no public mutator methods, instead,
simply call <tt>setName</tt> on a value, which will autoinsert it into the
appropriate symbol table. For types, use the Module::addTypeName method to
insert entries into the symbol table.</p>
</div>
<!-- *********************************************************************** -->
<div class="doc_section">
<a name="coreclasses">The Core LLVM Class Hierarchy Reference </a>
</div>
<!-- *********************************************************************** -->
<div class="doc_text">
<p><tt>#include "<a href="/doxygen/Type_8h-source.html">llvm/Type.h</a>"</tt>
<br>doxygen info: <a href="/doxygen/classllvm_1_1Type.html">Type Class</a></p>
<p>The Core LLVM classes are the primary means of representing the program
being inspected or transformed. The core LLVM classes are defined in
header files in the <tt>include/llvm/</tt> directory, and implemented in
the <tt>lib/VMCore</tt> directory.</p>
</div>
<!-- ======================================================================= -->
<div class="doc_subsection">
<a name="Type">The <tt>Type</tt> class and Derived Types</a>
</div>
<div class="doc_text">
<p><tt>Type</tt> is a superclass of all type classes. Every <tt>Value</tt> has
a <tt>Type</tt>. <tt>Type</tt> cannot be instantiated directly but only
through its subclasses. Certain primitive types (<tt>VoidType</tt>,
<tt>LabelType</tt>, <tt>FloatType</tt> and <tt>DoubleType</tt>) have hidden
subclasses. They are hidden because they offer no useful functionality beyond
what the <tt>Type</tt> class offers except to distinguish themselves from
other subclasses of <tt>Type</tt>.</p>
<p>All other types are subclasses of <tt>DerivedType</tt>. Types can be
named, but this is not a requirement. There exists exactly
one instance of a given shape at any one time. This allows type equality to
be performed with address equality of the Type Instance. That is, given two
<tt>Type*</tt> values, the types are identical if the pointers are identical.
</p>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
<a name="m_Value">Important Public Methods</a>
</div>
<div class="doc_text">
<ul>
<li><tt>bool isInteger() const</tt>: Returns true for any integer type.</li>
<li><tt>bool isFloatingPoint()</tt>: Return true if this is one of the two
floating point types.</li>
<li><tt>bool isAbstract()</tt>: Return true if the type is abstract (contains
an OpaqueType anywhere in its definition).</li>
<li><tt>bool isSized()</tt>: Return true if the type has known size. Things
that don't have a size are abstract types, labels and void.</li>
</ul>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
<a name="m_Value">Important Derived Types</a>
</div>
<div class="doc_text">
<dl>
<dt><tt>IntegerType</tt></dt>
<dd>Subclass of DerivedType that represents integer types of any bit width.
Any bit width between <tt>IntegerType::MIN_INT_BITS</tt> (1) and
<tt>IntegerType::MAX_INT_BITS</tt> (~8 million) can be represented.
<ul>
<li><tt>static const IntegerType* get(unsigned NumBits)</tt>: get an integer
type of a specific bit width.</li>
<li><tt>unsigned getBitWidth() const</tt>: Get the bit width of an integer
type.</li>
</ul>
</dd>
<dt><tt>SequentialType</tt></dt>
<dd>This is subclassed by ArrayType and PointerType
<ul>
<li><tt>const Type * getElementType() const</tt>: Returns the type of each
of the elements in the sequential type. </li>
</ul>
</dd>
<dt><tt>ArrayType</tt></dt>
<dd>This is a subclass of SequentialType and defines the interface for array
types.
<ul>
<li><tt>unsigned getNumElements() const</tt>: Returns the number of
elements in the array. </li>
</ul>
</dd>
<dt><tt>PointerType</tt></dt>
<dd>Subclass of SequentialType for pointer types.</dd>
<dt><tt>VectorType</tt></dt>
<dd>Subclass of SequentialType for vector types. A
vector type is similar to an ArrayType but is distinguished because it is
a first class type wherease ArrayType is not. Vector types are used for
vector operations and are usually small vectors of of an integer or floating
point type.</dd>
<dt><tt>StructType</tt></dt>
<dd>Subclass of DerivedTypes for struct types.</dd>
<dt><tt><a name="FunctionType">FunctionType</a></tt></dt>
<dd>Subclass of DerivedTypes for function types.
<ul>
<li><tt>bool isVarArg() const</tt>: Returns true if its a vararg
function</li>
<li><tt> const Type * getReturnType() const</tt>: Returns the
return type of the function.</li>
<li><tt>const Type * getParamType (unsigned i)</tt>: Returns
the type of the ith parameter.</li>
<li><tt> const unsigned getNumParams() const</tt>: Returns the
number of formal parameters.</li>
</ul>
</dd>
<dt><tt>OpaqueType</tt></dt>
<dd>Sublcass of DerivedType for abstract types. This class
defines no content and is used as a placeholder for some other type. Note
that OpaqueType is used (temporarily) during type resolution for forward
references of types. Once the referenced type is resolved, the OpaqueType
is replaced with the actual type. OpaqueType can also be used for data
abstraction. At link time opaque types can be resolved to actual types
of the same name.</dd>
</dl>
</div>
<!-- ======================================================================= -->
<div class="doc_subsection">
<a name="Module">The <tt>Module</tt> class</a>
</div>
<div class="doc_text">
<p><tt>#include "<a
href="/doxygen/Module_8h-source.html">llvm/Module.h</a>"</tt><br> doxygen info:
<a href="/doxygen/classllvm_1_1Module.html">Module Class</a></p>
<p>The <tt>Module</tt> class represents the top level structure present in LLVM
programs. An LLVM module is effectively either a translation unit of the
original program or a combination of several translation units merged by the
linker. The <tt>Module</tt> class keeps track of a list of <a
href="#Function"><tt>Function</tt></a>s, a list of <a
href="#GlobalVariable"><tt>GlobalVariable</tt></a>s, and a <a
href="#SymbolTable"><tt>SymbolTable</tt></a>. Additionally, it contains a few
helpful member functions that try to make common operations easy.</p>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
<a name="m_Module">Important Public Members of the <tt>Module</tt> class</a>
</div>
<div class="doc_text">
<ul>
<li><tt>Module::Module(std::string name = "")</tt></li>
</ul>
<p>Constructing a <a href="#Module">Module</a> is easy. You can optionally
provide a name for it (probably based on the name of the translation unit).</p>
<ul>
<li><tt>Module::iterator</tt> - Typedef for function list iterator<br>
<tt>Module::const_iterator</tt> - Typedef for const_iterator.<br>
<tt>begin()</tt>, <tt>end()</tt>
<tt>size()</tt>, <tt>empty()</tt>
<p>These are forwarding methods that make it easy to access the contents of
a <tt>Module</tt> object's <a href="#Function"><tt>Function</tt></a>
list.</p></li>
<li><tt>Module::FunctionListType &amp;getFunctionList()</tt>
<p> Returns the list of <a href="#Function"><tt>Function</tt></a>s. This is
necessary to use when you need to update the list or perform a complex
action that doesn't have a forwarding method.</p>
<p><!-- Global Variable --></p></li>
</ul>
<hr>
<ul>
<li><tt>Module::global_iterator</tt> - Typedef for global variable list iterator<br>
<tt>Module::const_global_iterator</tt> - Typedef for const_iterator.<br>
<tt>global_begin()</tt>, <tt>global_end()</tt>
<tt>global_size()</tt>, <tt>global_empty()</tt>
<p> These are forwarding methods that make it easy to access the contents of
a <tt>Module</tt> object's <a
href="#GlobalVariable"><tt>GlobalVariable</tt></a> list.</p></li>
<li><tt>Module::GlobalListType &amp;getGlobalList()</tt>
<p>Returns the list of <a
href="#GlobalVariable"><tt>GlobalVariable</tt></a>s. This is necessary to
use when you need to update the list or perform a complex action that
doesn't have a forwarding method.</p>
<p><!-- Symbol table stuff --> </p></li>
</ul>
<hr>
<ul>
<li><tt><a href="#SymbolTable">SymbolTable</a> *getSymbolTable()</tt>
<p>Return a reference to the <a href="#SymbolTable"><tt>SymbolTable</tt></a>
for this <tt>Module</tt>.</p>
<p><!-- Convenience methods --></p></li>
</ul>
<hr>
<ul>
<li><tt><a href="#Function">Function</a> *getFunction(const std::string
&amp;Name, const <a href="#FunctionType">FunctionType</a> *Ty)</tt>
<p>Look up the specified function in the <tt>Module</tt> <a
href="#SymbolTable"><tt>SymbolTable</tt></a>. If it does not exist, return
<tt>null</tt>.</p></li>
<li><tt><a href="#Function">Function</a> *getOrInsertFunction(const
std::string &amp;Name, const <a href="#FunctionType">FunctionType</a> *T)</tt>
<p>Look up the specified function in the <tt>Module</tt> <a
href="#SymbolTable"><tt>SymbolTable</tt></a>. If it does not exist, add an
external declaration for the function and return it.</p></li>
<li><tt>std::string getTypeName(const <a href="#Type">Type</a> *Ty)</tt>
<p>If there is at least one entry in the <a
href="#SymbolTable"><tt>SymbolTable</tt></a> for the specified <a
href="#Type"><tt>Type</tt></a>, return it. Otherwise return the empty
string.</p></li>
<li><tt>bool addTypeName(const std::string &amp;Name, const <a
href="#Type">Type</a> *Ty)</tt>
<p>Insert an entry in the <a href="#SymbolTable"><tt>SymbolTable</tt></a>
mapping <tt>Name</tt> to <tt>Ty</tt>. If there is already an entry for this
name, true is returned and the <a
href="#SymbolTable"><tt>SymbolTable</tt></a> is not modified.</p></li>
</ul>
</div>
<!-- ======================================================================= -->
<div class="doc_subsection">
<a name="Value">The <tt>Value</tt> class</a>
</div>
<div class="doc_text">
<p><tt>#include "<a href="/doxygen/Value_8h-source.html">llvm/Value.h</a>"</tt>
<br>
doxygen info: <a href="/doxygen/classllvm_1_1Value.html">Value Class</a></p>
<p>The <tt>Value</tt> class is the most important class in the LLVM Source
base. It represents a typed value that may be used (among other things) as an
operand to an instruction. There are many different types of <tt>Value</tt>s,
such as <a href="#Constant"><tt>Constant</tt></a>s,<a
href="#Argument"><tt>Argument</tt></a>s. Even <a
href="#Instruction"><tt>Instruction</tt></a>s and <a
href="#Function"><tt>Function</tt></a>s are <tt>Value</tt>s.</p>
<p>A particular <tt>Value</tt> may be used many times in the LLVM representation
for a program. For example, an incoming argument to a function (represented
with an instance of the <a href="#Argument">Argument</a> class) is "used" by
every instruction in the function that references the argument. To keep track
of this relationship, the <tt>Value</tt> class keeps a list of all of the <a
href="#User"><tt>User</tt></a>s that is using it (the <a
href="#User"><tt>User</tt></a> class is a base class for all nodes in the LLVM
graph that can refer to <tt>Value</tt>s). This use list is how LLVM represents
def-use information in the program, and is accessible through the <tt>use_</tt>*
methods, shown below.</p>
<p>Because LLVM is a typed representation, every LLVM <tt>Value</tt> is typed,
and this <a href="#Type">Type</a> is available through the <tt>getType()</tt>
method. In addition, all LLVM values can be named. The "name" of the
<tt>Value</tt> is a symbolic string printed in the LLVM code:</p>
<div class="doc_code">
<pre>
%<b>foo</b> = add i32 1, 2
</pre>
</div>
<p><a name="nameWarning">The name of this instruction is "foo".</a> <b>NOTE</b>
that the name of any value may be missing (an empty string), so names should
<b>ONLY</b> be used for debugging (making the source code easier to read,
debugging printouts), they should not be used to keep track of values or map
between them. For this purpose, use a <tt>std::map</tt> of pointers to the
<tt>Value</tt> itself instead.</p>
<p>One important aspect of LLVM is that there is no distinction between an SSA
variable and the operation that produces it. Because of this, any reference to
the value produced by an instruction (or the value available as an incoming
argument, for example) is represented as a direct pointer to the instance of
the class that
represents this value. Although this may take some getting used to, it
simplifies the representation and makes it easier to manipulate.</p>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
<a name="m_Value">Important Public Members of the <tt>Value</tt> class</a>
</div>
<div class="doc_text">
<ul>
<li><tt>Value::use_iterator</tt> - Typedef for iterator over the
use-list<br>
<tt>Value::use_const_iterator</tt> - Typedef for const_iterator over
the use-list<br>
<tt>unsigned use_size()</tt> - Returns the number of users of the
value.<br>
<tt>bool use_empty()</tt> - Returns true if there are no users.<br>
<tt>use_iterator use_begin()</tt> - Get an iterator to the start of
the use-list.<br>
<tt>use_iterator use_end()</tt> - Get an iterator to the end of the
use-list.<br>
<tt><a href="#User">User</a> *use_back()</tt> - Returns the last
element in the list.
<p> These methods are the interface to access the def-use
information in LLVM. As with all other iterators in LLVM, the naming
conventions follow the conventions defined by the <a href="#stl">STL</a>.</p>
</li>
<li><tt><a href="#Type">Type</a> *getType() const</tt>
<p>This method returns the Type of the Value.</p>
</li>
<li><tt>bool hasName() const</tt><br>
<tt>std::string getName() const</tt><br>
<tt>void setName(const std::string &amp;Name)</tt>
<p> This family of methods is used to access and assign a name to a <tt>Value</tt>,
be aware of the <a href="#nameWarning">precaution above</a>.</p>
</li>
<li><tt>void replaceAllUsesWith(Value *V)</tt>
<p>This method traverses the use list of a <tt>Value</tt> changing all <a
href="#User"><tt>User</tt>s</a> of the current value to refer to
"<tt>V</tt>" instead. For example, if you detect that an instruction always
produces a constant value (for example through constant folding), you can
replace all uses of the instruction with the constant like this:</p>
<div class="doc_code">
<pre>
Inst-&gt;replaceAllUsesWith(ConstVal);
</pre>
</div>
</ul>
</div>
<!-- ======================================================================= -->
<div class="doc_subsection">
<a name="User">The <tt>User</tt> class</a>
</div>
<div class="doc_text">
<p>
<tt>#include "<a href="/doxygen/User_8h-source.html">llvm/User.h</a>"</tt><br>
doxygen info: <a href="/doxygen/classllvm_1_1User.html">User Class</a><br>
Superclass: <a href="#Value"><tt>Value</tt></a></p>
<p>The <tt>User</tt> class is the common base class of all LLVM nodes that may
refer to <a href="#Value"><tt>Value</tt></a>s. It exposes a list of "Operands"
that are all of the <a href="#Value"><tt>Value</tt></a>s that the User is
referring to. The <tt>User</tt> class itself is a subclass of
<tt>Value</tt>.</p>
<p>The operands of a <tt>User</tt> point directly to the LLVM <a
href="#Value"><tt>Value</tt></a> that it refers to. Because LLVM uses Static
Single Assignment (SSA) form, there can only be one definition referred to,
allowing this direct connection. This connection provides the use-def
information in LLVM.</p>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
<a name="m_User">Important Public Members of the <tt>User</tt> class</a>
</div>
<div class="doc_text">
<p>The <tt>User</tt> class exposes the operand list in two ways: through
an index access interface and through an iterator based interface.</p>
<ul>
<li><tt>Value *getOperand(unsigned i)</tt><br>
<tt>unsigned getNumOperands()</tt>
<p> These two methods expose the operands of the <tt>User</tt> in a
convenient form for direct access.</p></li>
<li><tt>User::op_iterator</tt> - Typedef for iterator over the operand
list<br>
<tt>op_iterator op_begin()</tt> - Get an iterator to the start of
the operand list.<br>
<tt>op_iterator op_end()</tt> - Get an iterator to the end of the
operand list.
<p> Together, these methods make up the iterator based interface to
the operands of a <tt>User</tt>.</p></li>
</ul>
</div>
<!-- ======================================================================= -->
<div class="doc_subsection">
<a name="Instruction">The <tt>Instruction</tt> class</a>
</div>
<div class="doc_text">
<p><tt>#include "</tt><tt><a
href="/doxygen/Instruction_8h-source.html">llvm/Instruction.h</a>"</tt><br>
doxygen info: <a href="/doxygen/classllvm_1_1Instruction.html">Instruction Class</a><br>
Superclasses: <a href="#User"><tt>User</tt></a>, <a
href="#Value"><tt>Value</tt></a></p>
<p>The <tt>Instruction</tt> class is the common base class for all LLVM
instructions. It provides only a few methods, but is a very commonly used
class. The primary data tracked by the <tt>Instruction</tt> class itself is the
opcode (instruction type) and the parent <a
href="#BasicBlock"><tt>BasicBlock</tt></a> the <tt>Instruction</tt> is embedded
into. To represent a specific type of instruction, one of many subclasses of
<tt>Instruction</tt> are used.</p>
<p> Because the <tt>Instruction</tt> class subclasses the <a
href="#User"><tt>User</tt></a> class, its operands can be accessed in the same
way as for other <a href="#User"><tt>User</tt></a>s (with the
<tt>getOperand()</tt>/<tt>getNumOperands()</tt> and
<tt>op_begin()</tt>/<tt>op_end()</tt> methods).</p> <p> An important file for
the <tt>Instruction</tt> class is the <tt>llvm/Instruction.def</tt> file. This
file contains some meta-data about the various different types of instructions
in LLVM. It describes the enum values that are used as opcodes (for example
<tt>Instruction::Add</tt> and <tt>Instruction::ICmp</tt>), as well as the
concrete sub-classes of <tt>Instruction</tt> that implement the instruction (for
example <tt><a href="#BinaryOperator">BinaryOperator</a></tt> and <tt><a
href="#CmpInst">CmpInst</a></tt>). Unfortunately, the use of macros in
this file confuses doxygen, so these enum values don't show up correctly in the
<a href="/doxygen/classllvm_1_1Instruction.html">doxygen output</a>.</p>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
<a name="s_Instruction">Important Subclasses of the <tt>Instruction</tt>
class</a>
</div>
<div class="doc_text">
<ul>
<li><tt><a name="BinaryOperator">BinaryOperator</a></tt>
<p>This subclasses represents all two operand instructions whose operands
must be the same type, except for the comparison instructions.</p></li>
<li><tt><a name="CastInst">CastInst</a></tt>
<p>This subclass is the parent of the 12 casting instructions. It provides
common operations on cast instructions.</p>
<li><tt><a name="CmpInst">CmpInst</a></tt>
<p>This subclass respresents the two comparison instructions,
<a href="LangRef.html#i_icmp">ICmpInst</a> (integer opreands), and
<a href="LangRef.html#i_fcmp">FCmpInst</a> (floating point operands).</p>
<li><tt><a name="TerminatorInst">TerminatorInst</a></tt>
<p>This subclass is the parent of all terminator instructions (those which
can terminate a block).</p>
</ul>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
<a name="m_Instruction">Important Public Members of the <tt>Instruction</tt>
class</a>
</div>
<div class="doc_text">
<ul>
<li><tt><a href="#BasicBlock">BasicBlock</a> *getParent()</tt>
<p>Returns the <a href="#BasicBlock"><tt>BasicBlock</tt></a> that
this <tt>Instruction</tt> is embedded into.</p></li>
<li><tt>bool mayWriteToMemory()</tt>
<p>Returns true if the instruction writes to memory, i.e. it is a
<tt>call</tt>,<tt>free</tt>,<tt>invoke</tt>, or <tt>store</tt>.</p></li>
<li><tt>unsigned getOpcode()</tt>
<p>Returns the opcode for the <tt>Instruction</tt>.</p></li>
<li><tt><a href="#Instruction">Instruction</a> *clone() const</tt>
<p>Returns another instance of the specified instruction, identical
in all ways to the original except that the instruction has no parent
(ie it's not embedded into a <a href="#BasicBlock"><tt>BasicBlock</tt></a>),
and it has no name</p></li>
</ul>
</div>
<!-- ======================================================================= -->
<div class="doc_subsection">
<a name="Constant">The <tt>Constant</tt> class and subclasses</a>
</div>
<div class="doc_text">
<p>Constant represents a base class for different types of constants. It
is subclassed by ConstantInt, ConstantArray, etc. for representing
the various types of Constants. <a href="#GlobalValue">GlobalValue</a> is also
a subclass, which represents the address of a global variable or function.
</p>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">Important Subclasses of Constant </div>
<div class="doc_text">
<ul>
<li>ConstantInt : This subclass of Constant represents an integer constant of
any width.
<ul>
<li><tt>const APInt&amp; getValue() const</tt>: Returns the underlying
value of this constant, an APInt value.</li>
<li><tt>int64_t getSExtValue() const</tt>: Converts the underlying APInt
value to an int64_t via sign extension. If the value (not the bit width)
of the APInt is too large to fit in an int64_t, an assertion will result.
For this reason, use of this method is discouraged.</li>
<li><tt>uint64_t getZExtValue() const</tt>: Converts the underlying APInt
value to a uint64_t via zero extension. IF the value (not the bit width)
of the APInt is too large to fit in a uint64_t, an assertion will result.
For this reason, use of this method is discouraged.</li>
<li><tt>static ConstantInt* get(const APInt&amp; Val)</tt>: Returns the
ConstantInt object that represents the value provided by <tt>Val</tt>.
The type is implied as the IntegerType that corresponds to the bit width
of <tt>Val</tt>.</li>
<li><tt>static ConstantInt* get(const Type *Ty, uint64_t Val)</tt>:
Returns the ConstantInt object that represents the value provided by
<tt>Val</tt> for integer type <tt>Ty</tt>.</li>
</ul>
</li>
<li>ConstantFP : This class represents a floating point constant.
<ul>
<li><tt>double getValue() const</tt>: Returns the underlying value of
this constant. </li>
</ul>
</li>
<li>ConstantArray : This represents a constant array.
<ul>
<li><tt>const std::vector&lt;Use&gt; &amp;getValues() const</tt>: Returns
a vector of component constants that makeup this array. </li>
</ul>
</li>
<li>ConstantStruct : This represents a constant struct.
<ul>
<li><tt>const std::vector&lt;Use&gt; &amp;getValues() const</tt>: Returns
a vector of component constants that makeup this array. </li>
</ul>
</li>
<li>GlobalValue : This represents either a global variable or a function. In
either case, the value is a constant fixed address (after linking).
</li>
</ul>
</div>
<!-- ======================================================================= -->
<div class="doc_subsection">
<a name="GlobalValue">The <tt>GlobalValue</tt> class</a>
</div>
<div class="doc_text">
<p><tt>#include "<a
href="/doxygen/GlobalValue_8h-source.html">llvm/GlobalValue.h</a>"</tt><br>
doxygen info: <a href="/doxygen/classllvm_1_1GlobalValue.html">GlobalValue
Class</a><br>
Superclasses: <a href="#Constant"><tt>Constant</tt></a>,
<a href="#User"><tt>User</tt></a>, <a href="#Value"><tt>Value</tt></a></p>
<p>Global values (<a href="#GlobalVariable"><tt>GlobalVariable</tt></a>s or <a
href="#Function"><tt>Function</tt></a>s) are the only LLVM values that are
visible in the bodies of all <a href="#Function"><tt>Function</tt></a>s.
Because they are visible at global scope, they are also subject to linking with
other globals defined in different translation units. To control the linking
process, <tt>GlobalValue</tt>s know their linkage rules. Specifically,
<tt>GlobalValue</tt>s know whether they have internal or external linkage, as
defined by the <tt>LinkageTypes</tt> enumeration.</p>
<p>If a <tt>GlobalValue</tt> has internal linkage (equivalent to being
<tt>static</tt> in C), it is not visible to code outside the current translation
unit, and does not participate in linking. If it has external linkage, it is
visible to external code, and does participate in linking. In addition to
linkage information, <tt>GlobalValue</tt>s keep track of which <a
href="#Module"><tt>Module</tt></a> they are currently part of.</p>
<p>Because <tt>GlobalValue</tt>s are memory objects, they are always referred to
by their <b>address</b>. As such, the <a href="#Type"><tt>Type</tt></a> of a
global is always a pointer to its contents. It is important to remember this
when using the <tt>GetElementPtrInst</tt> instruction because this pointer must
be dereferenced first. For example, if you have a <tt>GlobalVariable</tt> (a
subclass of <tt>GlobalValue)</tt> that is an array of 24 ints, type <tt>[24 x
i32]</tt>, then the <tt>GlobalVariable</tt> is a pointer to that array. Although
the address of the first element of this array and the value of the
<tt>GlobalVariable</tt> are the same, they have different types. The
<tt>GlobalVariable</tt>'s type is <tt>[24 x i32]</tt>. The first element's type
is <tt>i32.</tt> Because of this, accessing a global value requires you to
dereference the pointer with <tt>GetElementPtrInst</tt> first, then its elements
can be accessed. This is explained in the <a href="LangRef.html#globalvars">LLVM
Language Reference Manual</a>.</p>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
<a name="m_GlobalValue">Important Public Members of the <tt>GlobalValue</tt>
class</a>
</div>
<div class="doc_text">
<ul>
<li><tt>bool hasInternalLinkage() const</tt><br>
<tt>bool hasExternalLinkage() const</tt><br>
<tt>void setInternalLinkage(bool HasInternalLinkage)</tt>
<p> These methods manipulate the linkage characteristics of the <tt>GlobalValue</tt>.</p>
<p> </p>
</li>
<li><tt><a href="#Module">Module</a> *getParent()</tt>
<p> This returns the <a href="#Module"><tt>Module</tt></a> that the
GlobalValue is currently embedded into.</p></li>
</ul>
</div>
<!-- ======================================================================= -->
<div class="doc_subsection">
<a name="Function">The <tt>Function</tt> class</a>
</div>
<div class="doc_text">
<p><tt>#include "<a
href="/doxygen/Function_8h-source.html">llvm/Function.h</a>"</tt><br> doxygen
info: <a href="/doxygen/classllvm_1_1Function.html">Function Class</a><br>
Superclasses: <a href="#GlobalValue"><tt>GlobalValue</tt></a>,
<a href="#Constant"><tt>Constant</tt></a>,
<a href="#User"><tt>User</tt></a>,
<a href="#Value"><tt>Value</tt></a></p>
<p>The <tt>Function</tt> class represents a single procedure in LLVM. It is
actually one of the more complex classes in the LLVM heirarchy because it must
keep track of a large amount of data. The <tt>Function</tt> class keeps track
of a list of <a href="#BasicBlock"><tt>BasicBlock</tt></a>s, a list of formal
<a href="#Argument"><tt>Argument</tt></a>s, and a
<a href="#SymbolTable"><tt>SymbolTable</tt></a>.</p>
<p>The list of <a href="#BasicBlock"><tt>BasicBlock</tt></a>s is the most
commonly used part of <tt>Function</tt> objects. The list imposes an implicit
ordering of the blocks in the function, which indicate how the code will be
layed out by the backend. Additionally, the first <a
href="#BasicBlock"><tt>BasicBlock</tt></a> is the implicit entry node for the
<tt>Function</tt>. It is not legal in LLVM to explicitly branch to this initial
block. There are no implicit exit nodes, and in fact there may be multiple exit
nodes from a single <tt>Function</tt>. If the <a
href="#BasicBlock"><tt>BasicBlock</tt></a> list is empty, this indicates that
the <tt>Function</tt> is actually a function declaration: the actual body of the
function hasn't been linked in yet.</p>
<p>In addition to a list of <a href="#BasicBlock"><tt>BasicBlock</tt></a>s, the
<tt>Function</tt> class also keeps track of the list of formal <a
href="#Argument"><tt>Argument</tt></a>s that the function receives. This
container manages the lifetime of the <a href="#Argument"><tt>Argument</tt></a>
nodes, just like the <a href="#BasicBlock"><tt>BasicBlock</tt></a> list does for
the <a href="#BasicBlock"><tt>BasicBlock</tt></a>s.</p>
<p>The <a href="#SymbolTable"><tt>SymbolTable</tt></a> is a very rarely used
LLVM feature that is only used when you have to look up a value by name. Aside
from that, the <a href="#SymbolTable"><tt>SymbolTable</tt></a> is used
internally to make sure that there are not conflicts between the names of <a
href="#Instruction"><tt>Instruction</tt></a>s, <a
href="#BasicBlock"><tt>BasicBlock</tt></a>s, or <a
href="#Argument"><tt>Argument</tt></a>s in the function body.</p>
<p>Note that <tt>Function</tt> is a <a href="#GlobalValue">GlobalValue</a>
and therefore also a <a href="#Constant">Constant</a>. The value of the function
is its address (after linking) which is guaranteed to be constant.</p>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
<a name="m_Function">Important Public Members of the <tt>Function</tt>
class</a>
</div>
<div class="doc_text">
<ul>
<li><tt>Function(const </tt><tt><a href="#FunctionType">FunctionType</a>
*Ty, LinkageTypes Linkage, const std::string &amp;N = "", Module* Parent = 0)</tt>
<p>Constructor used when you need to create new <tt>Function</tt>s to add
the the program. The constructor must specify the type of the function to
create and what type of linkage the function should have. The <a
href="#FunctionType"><tt>FunctionType</tt></a> argument
specifies the formal arguments and return value for the function. The same
<a href="#FunctionType"><tt>FunctionType</tt></a> value can be used to
create multiple functions. The <tt>Parent</tt> argument specifies the Module
in which the function is defined. If this argument is provided, the function
will automatically be inserted into that module's list of
functions.</p></li>
<li><tt>bool isExternal()</tt>
<p>Return whether or not the <tt>Function</tt> has a body defined. If the
function is "external", it does not have a body, and thus must be resolved
by linking with a function defined in a different translation unit.</p></li>
<li><tt>Function::iterator</tt> - Typedef for basic block list iterator<br>
<tt>Function::const_iterator</tt> - Typedef for const_iterator.<br>
<tt>begin()</tt>, <tt>end()</tt>
<tt>size()</tt>, <tt>empty()</tt>
<p>These are forwarding methods that make it easy to access the contents of
a <tt>Function</tt> object's <a href="#BasicBlock"><tt>BasicBlock</tt></a>
list.</p></li>
<li><tt>Function::BasicBlockListType &amp;getBasicBlockList()</tt>
<p>Returns the list of <a href="#BasicBlock"><tt>BasicBlock</tt></a>s. This
is necessary to use when you need to update the list or perform a complex
action that doesn't have a forwarding method.</p></li>
<li><tt>Function::arg_iterator</tt> - Typedef for the argument list
iterator<br>
<tt>Function::const_arg_iterator</tt> - Typedef for const_iterator.<br>
<tt>arg_begin()</tt>, <tt>arg_end()</tt>
<tt>arg_size()</tt>, <tt>arg_empty()</tt>
<p>These are forwarding methods that make it easy to access the contents of
a <tt>Function</tt> object's <a href="#Argument"><tt>Argument</tt></a>
list.</p></li>
<li><tt>Function::ArgumentListType &amp;getArgumentList()</tt>
<p>Returns the list of <a href="#Argument"><tt>Argument</tt></a>s. This is
necessary to use when you need to update the list or perform a complex
action that doesn't have a forwarding method.</p></li>
<li><tt><a href="#BasicBlock">BasicBlock</a> &amp;getEntryBlock()</tt>
<p>Returns the entry <a href="#BasicBlock"><tt>BasicBlock</tt></a> for the
function. Because the entry block for the function is always the first
block, this returns the first block of the <tt>Function</tt>.</p></li>
<li><tt><a href="#Type">Type</a> *getReturnType()</tt><br>
<tt><a href="#FunctionType">FunctionType</a> *getFunctionType()</tt>
<p>This traverses the <a href="#Type"><tt>Type</tt></a> of the
<tt>Function</tt> and returns the return type of the function, or the <a
href="#FunctionType"><tt>FunctionType</tt></a> of the actual
function.</p></li>
<li><tt><a href="#SymbolTable">SymbolTable</a> *getSymbolTable()</tt>
<p> Return a pointer to the <a href="#SymbolTable"><tt>SymbolTable</tt></a>
for this <tt>Function</tt>.</p></li>
</ul>
</div>
<!-- ======================================================================= -->
<div class="doc_subsection">
<a name="GlobalVariable">The <tt>GlobalVariable</tt> class</a>
</div>
<div class="doc_text">
<p><tt>#include "<a
href="/doxygen/GlobalVariable_8h-source.html">llvm/GlobalVariable.h</a>"</tt>
<br>
doxygen info: <a href="/doxygen/classllvm_1_1GlobalVariable.html">GlobalVariable
Class</a><br>
Superclasses: <a href="#GlobalValue"><tt>GlobalValue</tt></a>,
<a href="#Constant"><tt>Constant</tt></a>,
<a href="#User"><tt>User</tt></a>,
<a href="#Value"><tt>Value</tt></a></p>
<p>Global variables are represented with the (suprise suprise)
<tt>GlobalVariable</tt> class. Like functions, <tt>GlobalVariable</tt>s are also
subclasses of <a href="#GlobalValue"><tt>GlobalValue</tt></a>, and as such are
always referenced by their address (global values must live in memory, so their
"name" refers to their constant address). See
<a href="#GlobalValue"><tt>GlobalValue</tt></a> for more on this. Global
variables may have an initial value (which must be a
<a href="#Constant"><tt>Constant</tt></a>), and if they have an initializer,
they may be marked as "constant" themselves (indicating that their contents
never change at runtime).</p>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
<a name="m_GlobalVariable">Important Public Members of the
<tt>GlobalVariable</tt> class</a>
</div>
<div class="doc_text">
<ul>
<li><tt>GlobalVariable(const </tt><tt><a href="#Type">Type</a> *Ty, bool
isConstant, LinkageTypes&amp; Linkage, <a href="#Constant">Constant</a>
*Initializer = 0, const std::string &amp;Name = "", Module* Parent = 0)</tt>
<p>Create a new global variable of the specified type. If
<tt>isConstant</tt> is true then the global variable will be marked as
unchanging for the program. The Linkage parameter specifies the type of
linkage (internal, external, weak, linkonce, appending) for the variable. If
the linkage is InternalLinkage, WeakLinkage, or LinkOnceLinkage,&nbsp; then
the resultant global variable will have internal linkage. AppendingLinkage
concatenates together all instances (in different translation units) of the
variable into a single variable but is only applicable to arrays. &nbsp;See
the <a href="LangRef.html#modulestructure">LLVM Language Reference</a> for
further details on linkage types. Optionally an initializer, a name, and the
module to put the variable into may be specified for the global variable as
well.</p></li>
<li><tt>bool isConstant() const</tt>
<p>Returns true if this is a global variable that is known not to
be modified at runtime.</p></li>
<li><tt>bool hasInitializer()</tt>
<p>Returns true if this <tt>GlobalVariable</tt> has an intializer.</p></li>
<li><tt><a href="#Constant">Constant</a> *getInitializer()</tt>
<p>Returns the intial value for a <tt>GlobalVariable</tt>. It is not legal
to call this method if there is no initializer.</p></li>
</ul>
</div>
<!-- ======================================================================= -->
<div class="doc_subsection">
<a name="BasicBlock">The <tt>BasicBlock</tt> class</a>
</div>
<div class="doc_text">
<p><tt>#include "<a
href="/doxygen/BasicBlock_8h-source.html">llvm/BasicBlock.h</a>"</tt><br>
doxygen info: <a href="/doxygen/structllvm_1_1BasicBlock.html">BasicBlock
Class</a><br>
Superclass: <a href="#Value"><tt>Value</tt></a></p>
<p>This class represents a single entry multiple exit section of the code,
commonly known as a basic block by the compiler community. The
<tt>BasicBlock</tt> class maintains a list of <a
href="#Instruction"><tt>Instruction</tt></a>s, which form the body of the block.
Matching the language definition, the last element of this list of instructions
is always a terminator instruction (a subclass of the <a
href="#TerminatorInst"><tt>TerminatorInst</tt></a> class).</p>
<p>In addition to tracking the list of instructions that make up the block, the
<tt>BasicBlock</tt> class also keeps track of the <a
href="#Function"><tt>Function</tt></a> that it is embedded into.</p>
<p>Note that <tt>BasicBlock</tt>s themselves are <a
href="#Value"><tt>Value</tt></a>s, because they are referenced by instructions
like branches and can go in the switch tables. <tt>BasicBlock</tt>s have type
<tt>label</tt>.</p>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
<a name="m_BasicBlock">Important Public Members of the <tt>BasicBlock</tt>
class</a>
</div>
<div class="doc_text">
<ul>
<li><tt>BasicBlock(const std::string &amp;Name = "", </tt><tt><a
href="#Function">Function</a> *Parent = 0)</tt>
<p>The <tt>BasicBlock</tt> constructor is used to create new basic blocks for
insertion into a function. The constructor optionally takes a name for the new
block, and a <a href="#Function"><tt>Function</tt></a> to insert it into. If
the <tt>Parent</tt> parameter is specified, the new <tt>BasicBlock</tt> is
automatically inserted at the end of the specified <a
href="#Function"><tt>Function</tt></a>, if not specified, the BasicBlock must be
manually inserted into the <a href="#Function"><tt>Function</tt></a>.</p></li>
<li><tt>BasicBlock::iterator</tt> - Typedef for instruction list iterator<br>
<tt>BasicBlock::const_iterator</tt> - Typedef for const_iterator.<br>
<tt>begin()</tt>, <tt>end()</tt>, <tt>front()</tt>, <tt>back()</tt>,
<tt>size()</tt>, <tt>empty()</tt>
STL-style functions for accessing the instruction list.
<p>These methods and typedefs are forwarding functions that have the same
semantics as the standard library methods of the same names. These methods
expose the underlying instruction list of a basic block in a way that is easy to
manipulate. To get the full complement of container operations (including
operations to update the list), you must use the <tt>getInstList()</tt>
method.</p></li>
<li><tt>BasicBlock::InstListType &amp;getInstList()</tt>
<p>This method is used to get access to the underlying container that actually
holds the Instructions. This method must be used when there isn't a forwarding
function in the <tt>BasicBlock</tt> class for the operation that you would like
to perform. Because there are no forwarding functions for "updating"
operations, you need to use this if you want to update the contents of a
<tt>BasicBlock</tt>.</p></li>
<li><tt><a href="#Function">Function</a> *getParent()</tt>
<p> Returns a pointer to <a href="#Function"><tt>Function</tt></a> the block is
embedded into, or a null pointer if it is homeless.</p></li>
<li><tt><a href="#TerminatorInst">TerminatorInst</a> *getTerminator()</tt>
<p> Returns a pointer to the terminator instruction that appears at the end of
the <tt>BasicBlock</tt>. If there is no terminator instruction, or if the last
instruction in the block is not a terminator, then a null pointer is
returned.</p></li>
</ul>
</div>
<!-- ======================================================================= -->
<div class="doc_subsection">
<a name="Argument">The <tt>Argument</tt> class</a>
</div>
<div class="doc_text">
<p>This subclass of Value defines the interface for incoming formal
arguments to a function. A Function maintains a list of its formal
arguments. An argument has a pointer to the parent Function.</p>
</div>
<!-- *********************************************************************** -->
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