"http://www.w3.org/TR/html4/strict.dtd">
<html>
<head>
+ <meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<title>Writing an LLVM Pass</title>
<link rel="stylesheet" href="llvm.css" type="text/css">
</head>
<ul>
<li><a href="#makefile">Setting up the build environment</a></li>
<li><a href="#basiccode">Basic code required</a></li>
- <li><a href="#running">Running a pass with <tt>opt</tt>
- or <tt>analyze</tt></a></li>
+ <li><a href="#running">Running a pass with <tt>opt</tt></a></li>
</ul></li>
<li><a href="#passtype">Pass classes and requirements</a>
<ul>
<li><a href="#ImmutablePass">The <tt>ImmutablePass</tt> class</a></li>
- <li><a href="#Pass">The <tt>Pass</tt> class</a>
+ <li><a href="#ModulePass">The <tt>ModulePass</tt> class</a>
<ul>
- <li><a href="#run">The <tt>run</tt> method</a></li>
+ <li><a href="#runOnModule">The <tt>runOnModule</tt> method</a></li>
+ </ul></li>
+ <li><a href="#CallGraphSCCPass">The <tt>CallGraphSCCPass</tt> class</a>
+ <ul>
+ <li><a href="#doInitialization_scc">The <tt>doInitialization(CallGraph
+ &)</tt> method</a></li>
+ <li><a href="#runOnSCC">The <tt>runOnSCC</tt> method</a></li>
+ <li><a href="#doFinalization_scc">The <tt>doFinalization(CallGraph
+ &)</tt> method</a></li>
</ul></li>
<li><a href="#FunctionPass">The <tt>FunctionPass</tt> class</a>
<ul>
<li><a href="#doFinalization_mod">The <tt>doFinalization(Module
&)</tt> method</a></li>
</ul></li>
+ <li><a href="#LoopPass">The <tt>LoopPass</tt> class</a>
+ <ul>
+ <li><a href="#doInitialization_loop">The <tt>doInitialization(Loop *,
+ LPPassManager &)</tt> method</a></li>
+ <li><a href="#runOnLoop">The <tt>runOnLoop</tt> method</a></li>
+ <li><a href="#doFinalization_loop">The <tt>doFinalization()
+ </tt> method</a></li>
+ </ul></li>
<li><a href="#BasicBlockPass">The <tt>BasicBlockPass</tt> class</a>
<ul>
<li><a href="#doInitialization_fn">The <tt>doInitialization(Function
<ul>
<li><a href="#getAnalysisUsage">The <tt>getAnalysisUsage</tt>
method</a></li>
- <li><a href="#getAnalysis">The <tt>getAnalysis</tt> method</a></li>
+ <li><a href="#AU::addRequired">The <tt>AnalysisUsage::addRequired<></tt> and <tt>AnalysisUsage::addRequiredTransitive<></tt> methods</a></li>
+ <li><a href="#AU::addPreserved">The <tt>AnalysisUsage::addPreserved<></tt> method</a></li>
+ <li><a href="#AU::examples">Example implementations of <tt>getAnalysisUsage</tt></a></li>
+ <li><a href="#getAnalysis">The <tt>getAnalysis<></tt> and
+<tt>getAnalysisIfAvailable<></tt> methods</a></li>
</ul></li>
<li><a href="#analysisgroup">Implementing Analysis Groups</a>
<ul>
<li><a href="#agconcepts">Analysis Group Concepts</a></li>
<li><a href="#registerag">Using <tt>RegisterAnalysisGroup</tt></a></li>
</ul></li>
+ <li><a href="#passStatistics">Pass Statistics</a>
<li><a href="#passmanager">What PassManager does</a>
<ul>
<li><a href="#releaseMemory">The <tt>releaseMemory</tt> method</a></li>
</ul></li>
+ <li><a href="#registering">Registering dynamically loaded passes</a>
+ <ul>
+ <li><a href="#registering_existing">Using existing registries</a></li>
+ <li><a href="#registering_new">Creating new registries</a></li>
+ </ul></li>
<li><a href="#debughints">Using GDB with dynamically loaded passes</a>
<ul>
<li><a href="#breakpoint">Setting a breakpoint in your pass</a></li>
<li><a href="#future">Future extensions planned</a>
<ul>
<li><a href="#SMP">Multithreaded LLVM</a></li>
- <li><a href="#ModuleSource">A new <tt>ModuleSource</tt> interface</a></li>
- <li><a href="#PassFunctionPass"><tt>Pass</tt>es requiring
- <tt>FunctionPass</tt>es</a></li>
</ul></li>
</ol>
-<div class="doc_text">
- <p><b>Written by <a href="mailto:sabre@nondot.org">Chris Lattner</a></b><p>
+<div class="doc_author">
+ <p>Written by <a href="mailto:sabre@nondot.org">Chris Lattner</a> and
+ <a href="mailto:jlaskey@mac.com">Jim Laskey</a></p>
</div>
<!-- *********************************************************************** -->
<div class="doc_text">
<p>The LLVM Pass Framework is an important part of the LLVM system, because LLVM
-passes are where the interesting parts of the compiler exist. Passes perform
-the transformations and optimizations that make up the compiler, they build
-the analysis results that are used by these transformations, and they are, above
-all, a structuring technique for compiler code.</p>
+passes are where most of the interesting parts of the compiler exist. Passes
+perform the transformations and optimizations that make up the compiler, they
+build the analysis results that are used by these transformations, and they are,
+above all, a structuring technique for compiler code.</p>
<p>All LLVM passes are subclasses of the <tt><a
-href="http://llvm.cs.uiuc.edu/doxygen/classllvm_1_1Pass.html">Pass</a></tt>
+href="http://llvm.org/doxygen/classllvm_1_1Pass.html">Pass</a></tt>
class, which implement functionality by overriding virtual methods inherited
-from <tt>Pass</tt>. Depending on how your pass works, you may be able to
-inherit from the <tt><a
-href="http://llvm.cs.uiuc.edu/doxygen/structllvm_1_1FunctionPass.html">FunctionPass</a></tt>
-or <tt><a
-href="http://llvm.cs.uiuc.edu/doxygen/structllvm_1_1BasicBlockPass.html">BasicBlockPass</a></tt>,
-which gives the system more information about what your pass does, and how it
-can be combined with other passes. One of the main features of the LLVM Pass
-Framework is that it schedules passes to run in an efficient way based on the
-constraints that your pass has.</p>
+from <tt>Pass</tt>. Depending on how your pass works, you should inherit from
+the <tt><a href="#ModulePass">ModulePass</a></tt>, <tt><a
+href="#CallGraphSCCPass">CallGraphSCCPass</a></tt>, <tt><a
+href="#FunctionPass">FunctionPass</a></tt>, or <tt><a
+href="#LoopPass">LoopPass</a></tt>, or <tt><a
+href="#BasicBlockPass">BasicBlockPass</a></tt> classes, which gives the system
+more information about what your pass does, and how it can be combined with
+other passes. One of the main features of the LLVM Pass Framework is that it
+schedules passes to run in an efficient way based on the constraints that your
+pass meets (which are indicated by which class they derive from).</p>
<p>We start by showing you how to construct a pass, everything from setting up
the code, to compiling, loading, and executing it. After the basics are down,
<p>Here we describe how to write the "hello world" of passes. The "Hello" pass
is designed to simply print out the name of non-external functions that exist in
-the program being compiled. It does not modify the program at all, just
+the program being compiled. It does not modify the program at all, it just
inspects it. The source code and files for this pass are available in the LLVM
source tree in the <tt>lib/Transforms/Hello</tt> directory.</p>
<div class="doc_text">
-<p>First thing you need to do is create a new directory somewhere in the LLVM
-source base. For this example, we'll assume that you made
-"<tt>lib/Transforms/Hello</tt>". The first thing you must do is set up a build
-script (Makefile) that will compile the source code for the new pass. To do
-this, copy this into "<tt>Makefile</tt>":</p>
-
-<hr>
+ <p>First, you need to create a new directory somewhere in the LLVM source
+ base. For this example, we'll assume that you made
+ <tt>lib/Transforms/Hello</tt>. Next, you must set up a build script
+ (Makefile) that will compile the source code for the new pass. To do this,
+ copy the following into <tt>Makefile</tt>:</p>
+ <hr/>
-<pre>
+<div class="doc_code"><pre>
# Makefile for hello pass
-# Path to top level of LLVM heirarchy
+# Path to top level of LLVM hierarchy
LEVEL = ../../..
# Name of the library to build
-LIBRARYNAME = hello
+LIBRARYNAME = Hello
-# Build a dynamically loadable shared object
-SHARED_LIBRARY = 1
+# Make the shared library become a loadable module so the tools can
+# dlopen/dlsym on the resulting library.
+LOADABLE_MODULE = 1
+
+# Tell the build system which LLVM libraries your pass needs. You'll probably
+# need at least LLVMSystem.a, LLVMSupport.a, LLVMCore.a but possibly several
+# others too.
+LLVMLIBS = LLVMCore.a LLVMSupport.a LLVMSystem.a
# Include the makefile implementation stuff
include $(LEVEL)/Makefile.common
-</pre>
+</pre></div>
<p>This makefile specifies that all of the <tt>.cpp</tt> files in the current
directory are to be compiled and linked together into a
-<tt>lib/Debug/libhello.so</tt> shared object that can be dynamically loaded by
-the <tt>opt</tt> or <tt>analyze</tt> tools.</p>
+<tt>Debug/lib/Hello.so</tt> shared object that can be dynamically loaded by
+the <tt>opt</tt> or <tt>bugpoint</tt> tools via their <tt>-load</tt> options.
+If your operating system uses a suffix other than .so (such as windows or
+Mac OS/X), the appropriate extension will be used.</p>
<p>Now that we have the build scripts set up, we just need to write the code for
the pass itself.</p>
<p>Now that we have a way to compile our new pass, we just have to write it.
Start out with:</p>
-<pre>
-<b>#include</b> "<a href="http://llvm.cs.uiuc.edu/doxygen/Pass_8h-source.html">llvm/Pass.h</a>"
-<b>#include</b> "<a href="http://llvm.cs.uiuc.edu/doxygen/Function_8h-source.html">llvm/Function.h</a>"
-</pre>
+<div class="doc_code"><pre>
+<b>#include</b> "<a href="http://llvm.org/doxygen/Pass_8h-source.html">llvm/Pass.h</a>"
+<b>#include</b> "<a href="http://llvm.org/doxygen/Function_8h-source.html">llvm/Function.h</a>"
+<b>#include</b> "<a href="http://llvm.org/doxygen/raw__ostream_8h.html">llvm/Support/raw_ostream.h</a>"
+</pre></div>
<p>Which are needed because we are writing a <tt><a
-href="http://llvm.cs.uiuc.edu/doxygen/classllvm_1_1Pass.html">Pass</a></tt>, and
+href="http://llvm.org/doxygen/classllvm_1_1Pass.html">Pass</a></tt>,
we are operating on <tt><a
-href="http://llvm.cs.uiuc.edu/doxygen/classllvm_1_1Function.html">Function</a></tt>'s.</p>
+href="http://llvm.org/doxygen/classllvm_1_1Function.html">Function</a></tt>'s,
+and we will be doing some printing.</p>
+
+<p>Next we have:</p>
+<div class="doc_code"><pre>
+<b>using namespace llvm;</b>
+</pre></div>
+<p>... which is required because the functions from the include files
+live in the llvm namespace.
+</p>
<p>Next we have:</p>
-<pre>
+<div class="doc_code"><pre>
<b>namespace</b> {
-</pre>
+</pre></div>
<p>... which starts out an anonymous namespace. Anonymous namespaces are to C++
what the "<tt>static</tt>" keyword is to C (at global scope). It makes the
<p>Next, we declare our pass itself:</p>
-<pre>
+<div class="doc_code"><pre>
<b>struct</b> Hello : <b>public</b> <a href="#FunctionPass">FunctionPass</a> {
-</pre><p>
+</pre></div><p>
<p>This declares a "<tt>Hello</tt>" class that is a subclass of <tt><a
-href="http://llvm.cs.uiuc.edu/doxygen/structllvm_1_1FunctionPass.html">FunctionPass</a></tt>.
+href="http://llvm.org/doxygen/classllvm_1_1FunctionPass.html">FunctionPass</a></tt>.
The different builtin pass subclasses are described in detail <a
href="#passtype">later</a>, but for now, know that <a
href="#FunctionPass"><tt>FunctionPass</tt></a>'s operate a function at a
time.</p>
-<pre>
+<div class="doc_code"><pre>
+ static char ID;
+ Hello() : FunctionPass(&ID) {}
+</pre></div><p>
+
+<p> This declares pass identifier used by LLVM to identify pass. This allows LLVM to
+avoid using expensive C++ runtime information.</p>
+
+<div class="doc_code"><pre>
<b>virtual bool</b> <a href="#runOnFunction">runOnFunction</a>(Function &F) {
- std::cerr << "<i>Hello: </i>" << F.getName() << "\n";
+ errs() << "<i>Hello: </i>" << F.getName() << "\n";
<b>return false</b>;
}
}; <i>// end of struct Hello</i>
-</pre>
+</pre></div>
<p>We declare a "<a href="#runOnFunction"><tt>runOnFunction</tt></a>" method,
which overloads an abstract virtual method inherited from <a
to do our thing, so we just print out our message with the name of each
function.</p>
-<pre>
- RegisterOpt<Hello> X("<i>hello</i>", "<i>Hello World Pass</i>");
+<div class="doc_code"><pre>
+ char Hello::ID = 0;
+</pre></div>
+
+<p> We initialize pass ID here. LLVM uses ID's address to identify pass so
+initialization value is not important.</p>
+
+<div class="doc_code"><pre>
+ RegisterPass<Hello> X("<i>hello</i>", "<i>Hello World Pass</i>",
+ false /* Only looks at CFG */,
+ false /* Analysis Pass */);
} <i>// end of anonymous namespace</i>
-</pre>
+</pre></div>
-<p>Lastly, we register our class <tt>Hello</tt>, giving it a command line
-argument "<tt>hello</tt>", and a name "<tt>Hello World Pass</tt>". There are
-several different ways of <a href="#registration">registering your pass</a>,
-depending on what it is to be used for. For "optimizations" we use the
-<tt>RegisterOpt</tt> template.</p>
+<p>Lastly, we <a href="#registration">register our class</a> <tt>Hello</tt>,
+giving it a command line
+argument "<tt>hello</tt>", and a name "<tt>Hello World Pass</tt>".
+Last two RegisterPass arguments are optional. Their default value is false.
+If a pass walks CFG without modifying it then third argument is set to true.
+If a pass is an analysis pass, for example dominator tree pass, then true
+is supplied as fourth argument. </p>
<p>As a whole, the <tt>.cpp</tt> file looks like:</p>
-<pre>
-<b>#include</b> "<a href="http://llvm.cs.uiuc.edu/doxygen/Pass_8h-source.html">llvm/Pass.h</a>"
-<b>#include</b> "<a href="http://llvm.cs.uiuc.edu/doxygen/Function_8h-source.html">llvm/Function.h</a>"
+<div class="doc_code"><pre>
+<b>#include</b> "<a href="http://llvm.org/doxygen/Pass_8h-source.html">llvm/Pass.h</a>"
+<b>#include</b> "<a href="http://llvm.org/doxygen/Function_8h-source.html">llvm/Function.h</a>"
+<b>#include</b> "<a href="http://llvm.org/doxygen/raw__ostream_8h.html">llvm/Support/raw_ostream.h</a>"
+
+<b>using namespace llvm;</b>
<b>namespace</b> {
<b>struct Hello</b> : <b>public</b> <a href="#FunctionPass">FunctionPass</a> {
+
+ static char ID;
+ Hello() : FunctionPass(&ID) {}
+
<b>virtual bool</b> <a href="#runOnFunction">runOnFunction</a>(Function &F) {
- std::cerr << "<i>Hello: </i>" << F.getName() << "\n";
+ errs() << "<i>Hello: </i>" << F.getName() << "\n";
<b>return false</b>;
}
};
- RegisterOpt<Hello> X("<i>hello</i>", "<i>Hello World Pass</i>");
+ char Hello::ID = 0;
+ RegisterPass<Hello> X("<i>hello</i>", "<i>Hello World Pass</i>");
}
-</pre>
+</pre></div>
<p>Now that it's all together, compile the file with a simple "<tt>gmake</tt>"
command in the local directory and you should get a new
-"<tt>lib/Debug/libhello.so</tt> file. Note that everything in this file is
+"<tt>Debug/lib/Hello.so</tt> file. Note that everything in this file is
contained in an anonymous namespace: this reflects the fact that passes are self
contained units that do not need external interfaces (although they can have
them) to be useful.</p>
<!-- ======================================================================= -->
<div class="doc_subsection">
- <a name="running">Running a pass with <tt>opt</tt> or <tt>analyze</tt></a>
+ <a name="running">Running a pass with <tt>opt</tt></a>
</div>
<div class="doc_text">
-<p>Now that you have a brand new shiny <tt>.so</tt> file, we can use the
+<p>Now that you have a brand new shiny shared object file, we can use the
<tt>opt</tt> command to run an LLVM program through your pass. Because you
-registered your pass with the <tt>RegisterOpt</tt> template, you will be able to
+registered your pass with the <tt>RegisterPass</tt> template, you will be able to
use the <tt>opt</tt> tool to access it, once loaded.</p>
<p>To test it, follow the example at the end of the <a
href="GettingStarted.html">Getting Started Guide</a> to compile "Hello World" to
-LLVM. We can now run the bytecode file (<tt>hello.bc</tt>) for the program
-through our transformation like this (or course, any bytecode file will
+LLVM. We can now run the bitcode file (<tt>hello.bc</tt>) for the program
+through our transformation like this (or course, any bitcode file will
work):</p>
-<pre>
-$ opt -load ../../../lib/Debug/libhello.so -hello < hello.bc > /dev/null
+<div class="doc_code"><pre>
+$ opt -load ../../../Debug/lib/Hello.so -hello < hello.bc > /dev/null
Hello: __main
Hello: puts
Hello: main
-</pre>
+</pre></div>
<p>The '<tt>-load</tt>' option specifies that '<tt>opt</tt>' should load your
pass as a shared object, which makes '<tt>-hello</tt>' a valid command line
<p>To see what happened to the other string you registered, try running
<tt>opt</tt> with the <tt>--help</tt> option:</p>
-<pre>
-$ opt -load ../../../lib/Debug/libhello.so --help
+<div class="doc_code"><pre>
+$ opt -load ../../../Debug/lib/Hello.so --help
OVERVIEW: llvm .bc -> .bc modular optimizer
-USAGE: opt [options] <input bytecode>
+USAGE: opt [options] <input bitcode>
OPTIONS:
Optimizations available:
-inline - Function Integration/Inlining
-instcombine - Combine redundant instructions
...
-</pre>
+</pre></div>
<p>The pass name get added as the information string for your pass, giving some
documentation to users of <tt>opt</tt>. Now that you have a working pass, you
the execution time of your pass along with the other passes you queue up. For
example:</p>
-<pre>
-$ opt -load ../../../lib/Debug/libhello.so -hello -time-passes < hello.bc > /dev/null
+<div class="doc_code"><pre>
+$ opt -load ../../../Debug/lib/Hello.so -hello -time-passes < hello.bc > /dev/null
Hello: __main
Hello: puts
Hello: main
Total Execution Time: 0.02 seconds (0.0479059 wall clock)
---User Time--- --System Time-- --User+System-- ---Wall Time--- --- Pass Name ---
- 0.0100 (100.0%) 0.0000 ( 0.0%) 0.0100 ( 50.0%) 0.0402 ( 84.0%) Bytecode Writer
+ 0.0100 (100.0%) 0.0000 ( 0.0%) 0.0100 ( 50.0%) 0.0402 ( 84.0%) Bitcode Writer
0.0000 ( 0.0%) 0.0100 (100.0%) 0.0100 ( 50.0%) 0.0031 ( 6.4%) Dominator Set Construction
0.0000 ( 0.0%) 0.0000 ( 0.0%) 0.0000 ( 0.0%) 0.0013 ( 2.7%) Module Verifier
<b> 0.0000 ( 0.0%) 0.0000 ( 0.0%) 0.0000 ( 0.0%) 0.0033 ( 6.9%) Hello World Pass</b>
0.0100 (100.0%) 0.0100 (100.0%) 0.0200 (100.0%) 0.0479 (100.0%) TOTAL
-</pre>
+</pre></div>
<p>As you can see, our implementation above is pretty fast :). The additional
passes listed are automatically inserted by the '<tt>opt</tt>' tool to verify
<div class="doc_text">
<p>The most plain and boring type of pass is the "<tt><a
-href="http://llvm.cs.uiuc.edu/doxygen/structllvm_1_1ImmutablePass.html">ImmutablePass</a></tt>"
+href="http://llvm.org/doxygen/classllvm_1_1ImmutablePass.html">ImmutablePass</a></tt>"
class. This pass type is used for passes that do not have to be run, do not
change state, and never need to be updated. This is not a normal type of
transformation or analysis, but can provide information about the current
<!-- ======================================================================= -->
<div class="doc_subsection">
- <a name="Pass">The <tt>Pass</tt> class</a>
+ <a name="ModulePass">The <tt>ModulePass</tt> class</a>
</div>
<div class="doc_text">
<p>The "<tt><a
-href="http://llvm.cs.uiuc.edu/doxygen/classllvm_1_1Pass.html">Pass</a></tt>"
+href="http://llvm.org/doxygen/classllvm_1_1ModulePass.html">ModulePass</a></tt>"
class is the most general of all superclasses that you can use. Deriving from
-<tt>Pass</tt> indicates that your pass uses the entire program as a unit,
+<tt>ModulePass</tt> indicates that your pass uses the entire program as a unit,
refering to function bodies in no predictable order, or adding and removing
-functions. Because nothing is known about the behavior of direct <tt>Pass</tt>
+functions. Because nothing is known about the behavior of <tt>ModulePass</tt>
subclasses, no optimization can be done for their execution.</p>
-<p>To write a correct <tt>Pass</tt> subclass, derive from <tt>Pass</tt> and
-overload the <tt>run</tt> method with the following signature:</p>
+<p>A module pass can use function level passes (e.g. dominators) using
+the getAnalysis interface
+<tt>getAnalysis<DominatorTree>(llvm::Function *)</tt> to provide the
+function to retrieve analysis result for, if the function pass does not require
+any module or immutable passes. Note that this can only be done for functions for which the
+analysis ran, e.g. in the case of dominators you should only ask for the
+DominatorTree for function definitions, not declarations.</p>
+
+<p>To write a correct <tt>ModulePass</tt> subclass, derive from
+<tt>ModulePass</tt> and overload the <tt>runOnModule</tt> method with the
+following signature:</p>
+
+</div>
+
+<!-- _______________________________________________________________________ -->
+<div class="doc_subsubsection">
+ <a name="runOnModule">The <tt>runOnModule</tt> method</a>
+</div>
+
+<div class="doc_text">
+
+<div class="doc_code"><pre>
+ <b>virtual bool</b> runOnModule(Module &M) = 0;
+</pre></div>
+
+<p>The <tt>runOnModule</tt> method performs the interesting work of the pass.
+It should return true if the module was modified by the transformation and
+false otherwise.</p>
+
+</div>
+
+<!-- ======================================================================= -->
+<div class="doc_subsection">
+ <a name="CallGraphSCCPass">The <tt>CallGraphSCCPass</tt> class</a>
+</div>
+
+<div class="doc_text">
+
+<p>The "<tt><a
+href="http://llvm.org/doxygen/classllvm_1_1CallGraphSCCPass.html">CallGraphSCCPass</a></tt>"
+is used by passes that need to traverse the program bottom-up on the call graph
+(callees before callers). Deriving from CallGraphSCCPass provides some
+mechanics for building and traversing the CallGraph, but also allows the system
+to optimize execution of CallGraphSCCPass's. If your pass meets the
+requirements outlined below, and doesn't meet the requirements of a <tt><a
+href="#FunctionPass">FunctionPass</a></tt> or <tt><a
+href="#BasicBlockPass">BasicBlockPass</a></tt>, you should derive from
+<tt>CallGraphSCCPass</tt>.</p>
+
+<p><b>TODO</b>: explain briefly what SCC, Tarjan's algo, and B-U mean.</p>
+
+<p>To be explicit, <tt>CallGraphSCCPass</tt> subclasses are:</p>
+
+<ol>
+
+<li>... <em>not allowed</em> to modify any <tt>Function</tt>s that are not in
+the current SCC.</li>
+
+<li>... <em>not allowed</em> to inspect any Function's other than those in the
+current SCC and the direct callees of the SCC.</li>
+
+<li>... <em>required</em> to preserve the current CallGraph object, updating it
+to reflect any changes made to the program.</li>
+
+<li>... <em>not allowed</em> to add or remove SCC's from the current Module,
+though they may change the contents of an SCC.</li>
+
+<li>... <em>allowed</em> to add or remove global variables from the current
+Module.</li>
+
+<li>... <em>allowed</em> to maintain state across invocations of
+ <a href="#runOnSCC"><tt>runOnSCC</tt></a> (including global data).</li>
+</ol>
+
+<p>Implementing a <tt>CallGraphSCCPass</tt> is slightly tricky in some cases
+because it has to handle SCCs with more than one node in it. All of the virtual
+methods described below should return true if they modified the program, or
+false if they didn't.</p>
+
+</div>
+
+<!-- _______________________________________________________________________ -->
+<div class="doc_subsubsection">
+ <a name="doInitialization_scc">The <tt>doInitialization(CallGraph &)</tt>
+ method</a>
+</div>
+
+<div class="doc_text">
+
+<div class="doc_code"><pre>
+ <b>virtual bool</b> doInitialization(CallGraph &CG);
+</pre></div>
+
+<p>The <tt>doIninitialize</tt> method is allowed to do most of the things that
+<tt>CallGraphSCCPass</tt>'s are not allowed to do. They can add and remove
+functions, get pointers to functions, etc. The <tt>doInitialization</tt> method
+is designed to do simple initialization type of stuff that does not depend on
+the SCCs being processed. The <tt>doInitialization</tt> method call is not
+scheduled to overlap with any other pass executions (thus it should be very
+fast).</p>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
- <a name="run">The <tt>run</tt> method</a>
+ <a name="runOnSCC">The <tt>runOnSCC</tt> method</a>
</div>
<div class="doc_text">
-<pre>
- <b>virtual bool</b> run(Module &M) = 0;
-</pre>
+<div class="doc_code"><pre>
+ <b>virtual bool</b> runOnSCC(const std::vector<CallGraphNode *> &SCCM) = 0;
+</pre></div>
-<p>The <tt>run</tt> method performs the interesting work of the pass, and should
-return true if the module was modified by the transformation, false
+<p>The <tt>runOnSCC</tt> method performs the interesting work of the pass, and
+should return true if the module was modified by the transformation, false
otherwise.</p>
</div>
+<!-- _______________________________________________________________________ -->
+<div class="doc_subsubsection">
+ <a name="doFinalization_scc">The <tt>doFinalization(CallGraph
+ &)</tt> method</a>
+</div>
+
+<div class="doc_text">
+
+<div class="doc_code"><pre>
+ <b>virtual bool</b> doFinalization(CallGraph &CG);
+</pre></div>
+
+<p>The <tt>doFinalization</tt> method is an infrequently used method that is
+called when the pass framework has finished calling <a
+href="#runOnFunction"><tt>runOnFunction</tt></a> for every function in the
+program being compiled.</p>
+
+</div>
+
<!-- ======================================================================= -->
<div class="doc_subsection">
<a name="FunctionPass">The <tt>FunctionPass</tt> class</a>
<div class="doc_text">
-<p>In contrast to
direct <tt>Pass</tt> subclasses, direct <tt><a
-href="http://llvm.cs.uiuc.edu/doxygen/classllvm_1_1Pass.html">FunctionPass</a></tt>
+<p>In contrast to
<tt>ModulePass</tt> subclasses, <tt><a
+href="http://llvm.org/doxygen/classllvm_1_1Pass.html">FunctionPass</a></tt>
subclasses do have a predictable, local behavior that can be expected by the
system. All <tt>FunctionPass</tt> execute on each function in the program
independent of all of the other functions in the program.
</div>
<!-- _______________________________________________________________________ -->
-<div class="subsubsection">
+<div class="doc_subsubsection">
<a name="doInitialization_mod">The <tt>doInitialization(Module &)</tt>
method</a>
</div>
<div class="doc_text">
-<pre>
+<div class="doc_code"><pre>
<b>virtual bool</b> doInitialization(Module &M);
-</pre>
+</pre></div>
<p>The <tt>doIninitialize</tt> method is allowed to do most of the things that
<tt>FunctionPass</tt>'s are not allowed to do. They can add and remove
fast).</p>
<p>A good example of how this method should be used is the <a
-href="http://llvm.cs.uiuc.edu/doxygen/LowerAllocations_8cpp-source.html">LowerAllocations</a>
+href="http://llvm.org/doxygen/LowerAllocations_8cpp-source.html">LowerAllocations</a>
pass. This pass converts <tt>malloc</tt> and <tt>free</tt> instructions into
platform dependent <tt>malloc()</tt> and <tt>free()</tt> function calls. It
uses the <tt>doInitialization</tt> method to get a reference to the malloc and
</div>
<!-- _______________________________________________________________________ -->
-<div class="subsubsection">
+<div class="doc_subsubsection">
<a name="runOnFunction">The <tt>runOnFunction</tt> method</a>
</div>
<div class="doc_text">
-<pre>
+<div class="doc_code"><pre>
<b>virtual bool</b> runOnFunction(Function &F) = 0;
-</pre><p>
+</pre></div><p>
<p>The <tt>runOnFunction</tt> method must be implemented by your subclass to do
the transformation or analysis work of your pass. As usual, a true value should
</div>
<!-- _______________________________________________________________________ -->
-<div class="subsubsection">
+<div class="doc_subsubsection">
<a name="doFinalization_mod">The <tt>doFinalization(Module
&)</tt> method</a>
</div>
<div class="doc_text">
-<pre>
+<div class="doc_code"><pre>
<b>virtual bool</b> doFinalization(Module &M);
-</pre>
+</pre></div>
<p>The <tt>doFinalization</tt> method is an infrequently used method that is
called when the pass framework has finished calling <a
</div>
+<!-- ======================================================================= -->
+<div class="doc_subsection">
+ <a name="LoopPass">The <tt>LoopPass</tt> class </a>
+</div>
+
+<div class="doc_text">
+
+<p> All <tt>LoopPass</tt> execute on each loop in the function independent of
+all of the other loops in the function. <tt>LoopPass</tt> processes loops in
+loop nest order such that outer most loop is processed last. </p>
+
+<p> <tt>LoopPass</tt> subclasses are allowed to update loop nest using
+<tt>LPPassManager</tt> interface. Implementing a loop pass is usually
+straightforward. <tt>Looppass</tt>'s may overload three virtual methods to
+do their work. All these methods should return true if they modified the
+program, or false if they didn't. </p>
+</div>
+
+<!-- _______________________________________________________________________ -->
+<div class="doc_subsubsection">
+ <a name="doInitialization_loop">The <tt>doInitialization(Loop *,
+ LPPassManager &)</tt>
+ method</a>
+</div>
+
+<div class="doc_text">
+
+<div class="doc_code"><pre>
+ <b>virtual bool</b> doInitialization(Loop *, LPPassManager &LPM);
+</pre></div>
+
+<p>The <tt>doInitialization</tt> method is designed to do simple initialization
+type of stuff that does not depend on the functions being processed. The
+<tt>doInitialization</tt> method call is not scheduled to overlap with any
+other pass executions (thus it should be very fast). LPPassManager
+interface should be used to access Function or Module level analysis
+information.</p>
+
+</div>
+
+
+<!-- _______________________________________________________________________ -->
+<div class="doc_subsubsection">
+ <a name="runOnLoop">The <tt>runOnLoop</tt> method</a>
+</div>
+
+<div class="doc_text">
+
+<div class="doc_code"><pre>
+ <b>virtual bool</b> runOnLoop(Loop *, LPPassManager &LPM) = 0;
+</pre></div><p>
+
+<p>The <tt>runOnLoop</tt> method must be implemented by your subclass to do
+the transformation or analysis work of your pass. As usual, a true value should
+be returned if the function is modified. <tt>LPPassManager</tt> interface
+should be used to update loop nest.</p>
+
+</div>
+
+<!-- _______________________________________________________________________ -->
+<div class="doc_subsubsection">
+ <a name="doFinalization_loop">The <tt>doFinalization()</tt> method</a>
+</div>
+
+<div class="doc_text">
+
+<div class="doc_code"><pre>
+ <b>virtual bool</b> doFinalization();
+</pre></div>
+
+<p>The <tt>doFinalization</tt> method is an infrequently used method that is
+called when the pass framework has finished calling <a
+href="#runOnLoop"><tt>runOnLoop</tt></a> for every loop in the
+program being compiled. </p>
+
+</div>
+
+
+
<!-- ======================================================================= -->
<div class="doc_subsection">
<a name="BasicBlockPass">The <tt>BasicBlockPass</tt> class</a>
<li>Modify or inspect any basic blocks outside of the current one</li>
<li>Maintain state across invocations of
<a href="#runOnBasicBlock"><tt>runOnBasicBlock</tt></a></li>
-<li>Modify the constrol flow graph (by altering terminator instructions)</li>
-<li>Any of the things verboten for
+<li>Modify the control flow graph (by altering terminator instructions)</li>
+<li>Any of the things forbidden for
<a href="#FunctionPass"><tt>FunctionPass</tt></a>es.</li>
</ol>
</div>
<!-- _______________________________________________________________________ -->
-<div class="subsubsection">
+<div class="doc_subsubsection">
<a name="doInitialization_fn">The <tt>doInitialization(Function
&)</tt> method</a>
</div>
<div class="doc_text">
-<pre>
+<div class="doc_code"><pre>
<b>virtual bool</b> doInitialization(Function &F);
-</pre>
+</pre></div>
<p>The <tt>doIninitialize</tt> method is allowed to do most of the things that
<tt>BasicBlockPass</tt>'s are not allowed to do, but that
<tt>FunctionPass</tt>'s can. The <tt>doInitialization</tt> method is designed
-to do simple initialization type of stuff that does not depend on the
+to do simple initialization that does not depend on the
BasicBlocks being processed. The <tt>doInitialization</tt> method call is not
scheduled to overlap with any other pass executions (thus it should be very
fast).</p>
</div>
<!-- _______________________________________________________________________ -->
-<div class="subsubsection">
+<div class="doc_subsubsection">
<a name="runOnBasicBlock">The <tt>runOnBasicBlock</tt> method</a>
</div>
<div class="doc_text">
-<pre>
+<div class="doc_code"><pre>
<b>virtual bool</b> runOnBasicBlock(BasicBlock &BB) = 0;
-</pre>
+</pre></div>
<p>Override this function to do the work of the <tt>BasicBlockPass</tt>. This
function is not allowed to inspect or modify basic blocks other than the
</div>
<!-- _______________________________________________________________________ -->
-<div class="subsubsection">
+<div class="doc_subsubsection">
<a name="doFinalization_fn">The <tt>doFinalization(Function &)</tt>
method</a>
</div>
<div class="doc_text">
-<pre>
+<div class="doc_code"><pre>
<b>virtual bool</b> doFinalization(Function &F);
-</pre>
+</pre></div>
<p>The <tt>doFinalization</tt> method is an infrequently used method that is
called when the pass framework has finished calling <a
<div class="doc_text">
-<p>A <tt>MachineFunctionPass</tt> executes on the machine-dependent
-representation of each LLVM function in the program,
-independent of all of the other functions in the program.
-A <tt>MachineFunctionPass</tt> is also a <tt>FunctionPass</tt>, so all
+<p>A <tt>MachineFunctionPass</tt> is a part of the LLVM code generator that
+executes on the machine-dependent representation of each LLVM function in the
+program. A <tt>MachineFunctionPass</tt> is also a <tt>FunctionPass</tt>, so all
the restrictions that apply to a <tt>FunctionPass</tt> also apply to it.
-<tt>MachineFunctionPass</tt>es also have additional restrictions. In
-particular, <tt>MachineFunctionPass</tt>es are not allowed to do any of
-the following:</p>
+<tt>MachineFunctionPass</tt>es also have additional restrictions. In particular,
+<tt>MachineFunctionPass</tt>es are not allowed to do any of the following:</p>
<ol>
<li>Modify any LLVM Instructions, BasicBlocks or Functions.</li>
</div>
<!-- _______________________________________________________________________ -->
-<div class="subsubsection">
+<div class="doc_subsubsection">
<a name="runOnMachineFunction">The <tt>runOnMachineFunction(MachineFunction
&MF)</tt> method</a>
</div>
<div class="doc_text">
-<pre>
+<div class="doc_code"><pre>
<b>virtual bool</b> runOnMachineFunction(MachineFunction &MF) = 0;
-</pre>
+</pre></div>
<p><tt>runOnMachineFunction</tt> can be considered the main entry point of a
<tt>MachineFunctionPass</tt>; that is, you should override this method to do the
pass registration works, and discussed some of the reasons that it is used and
what it does. Here we discuss how and why passes are registered.</p>
-<p>Passes can be registered in several different ways. Depending on the general
-classification of the pass, you should use one of the following templates to
-register the pass:</p>
-
-<ul>
-<li><b><tt>RegisterOpt</tt></b> - This template should be used when you are
-registering a pass that logically should be available for use in the
-'<tt>opt</tt>' utility.</li>
-
-<li><b><tt>RegisterAnalysis</tt></b> - This template should be used when you are
-registering a pass that logically should be available for use in the
-'<tt>analysis</tt>' utility.</li>
-
-<li><b><tt>RegisterLLC</tt></b> - This template should be used when you are
-registering a pass that logically should be available for use in the
-'<tt>llc</tt>' utility.</li>
-
-<li><b><tt>RegisterPass</tt></b> - This is the generic form of the
-<tt>Register*</tt> templates that should be used if you want your pass listed by
-multiple or no utilities. This template takes an extra third argument that
-specifies which tools it should be listed in. See the <a
-href="http://llvm.cs.uiuc.edu/doxygen/PassSupport_8h-source.html">PassSupport.h</a>
-file for more information.</li>
-
-</ul>
-
-<p>Regardless of how you register your pass, you must specify at least two
+<p>As we saw above, passes are registered with the <b><tt>RegisterPass</tt></b>
+template, which requires you to pass at least two
parameters. The first parameter is the name of the pass that is to be used on
the command line to specify that the pass should be added to a program (for
-example <tt>opt</tt> or <tt>analyze</tt>). The second argument is the name of
-the pass, which is to be used for the <tt>--help</tt> output of programs, as
+example, with <tt>opt</tt> or <tt>bugpoint</tt>). The second argument is the
+name of the pass, which is to be used for the <tt>--help</tt> output of
+programs, as
well as for debug output generated by the <tt>--debug-pass</tt> option.</p>
-<p>If you pass is constructed by its default constructor, you only ever have to
-pass these two arguments. If, on the other hand, you require other information
-(like target specific information), you must pass an additional argument. This
-argument is a pointer to a function used to create the pass. For an example of
-how this works, look at the <a
-href="http://llvm.cs.uiuc.edu/doxygen/LowerAllocations_8cpp-source.html">LowerAllocations.cpp</a>
-file.</p>
-
-<p>If a pass is registered to be used by the <tt>analyze</tt> utility, you
-should implement the virtual <tt>print</tt> method:</p>
+<p>If you want your pass to be easily dumpable, you should
+implement the virtual <tt>print</tt> method:</p>
</div>
<div class="doc_text">
-<pre>
+<div class="doc_code"><pre>
<b>virtual void</b> print(std::ostream &O, <b>const</b> Module *M) <b>const</b>;
-</pre>
+</pre></div>
<p>The <tt>print</tt> method must be implemented by "analyses" in order to print
a human readable version of the analysis results. This is useful for debugging
an analysis itself, as well as for other people to figure out how an analysis
-works. The <tt>analyze</tt> tool uses this method to generate its output.</p>
+works. Use the <tt>opt -analyze</tt> argument to invoke this method.</p>
-<p>The <tt>ostream</tt> parameter specifies the stream to write the results on,
+<p>The <tt>llvm::OStream</tt> parameter specifies the stream to write the results on,
and the <tt>Module</tt> parameter gives a pointer to the top level module of the
program that has been analyzed. Note however that this pointer may be null in
certain circumstances (such as calling the <tt>Pass::dump()</tt> from a
<div class="doc_text">
-<p>One of the main responsibilities of the <tt>PassManager</tt> is the make sure
+<p>One of the main responsibilities of the <tt>PassManager</tt> is to make sure
that passes interact with each other correctly. Because <tt>PassManager</tt>
tries to <a href="#passmanager">optimize the execution of passes</a> it must
know how the passes interact with each other and what dependencies exist between
<div class="doc_text">
-<pre>
+<div class="doc_code"><pre>
<b>virtual void</b> getAnalysisUsage(AnalysisUsage &Info) <b>const</b>;
-</pre>
+</pre></div>
<p>By implementing the <tt>getAnalysisUsage</tt> method, the required and
invalidated sets may be specified for your transformation. The implementation
should fill in the <tt><a
-href="http://llvm.cs.uiuc.edu/doxygen/classllvm_1_1AnalysisUsage.html">AnalysisUsage</a></tt>
+href="http://llvm.org/doxygen/classllvm_1_1AnalysisUsage.html">AnalysisUsage</a></tt>
object with information about which passes are required and not invalidated. To
-do this, the following set methods are provided by the <tt><a
-href="http://llvm.cs.uiuc.edu/doxygen/classllvm_1_1AnalysisUsage.html">AnalysisUsage</a></tt>
-class:</p>
-
-<pre>
- <i>// addRequires - Add the specified pass to the required set for your pass.</i>
- <b>template</b><<b>class</b> PassClass>
- AnalysisUsage &AnalysisUsage::addRequired();
-
- <i>// addPreserved - Add the specified pass to the set of analyses preserved by
- // this pass</i>
- <b>template</b><<b>class</b> PassClass>
- AnalysisUsage &AnalysisUsage::addPreserved();
-
- <i>// setPreservesAll - Call this if the pass does not modify its input at all</i>
- <b>void</b> AnalysisUsage::setPreservesAll();
-
- <i>// setPreservesCFG - This function should be called by the pass, iff they do not:
- //
- // 1. Add or remove basic blocks from the function
- // 2. Modify terminator instructions in any way.
- //
- // This is automatically implied for <a href="#BasicBlockPass">BasicBlockPass</a>'s
- //</i>
- <b>void</b> AnalysisUsage::setPreservesCFG();
-</pre>
-
-<p>Some examples of how to use these methods are:</p>
-
-<pre>
+do this, a pass may call any of the following methods on the AnalysisUsage
+object:</p>
+</div>
+
+<!-- _______________________________________________________________________ -->
+<div class="doc_subsubsection">
+ <a name="AU::addRequired">The <tt>AnalysisUsage::addRequired<></tt> and <tt>AnalysisUsage::addRequiredTransitive<></tt> methods</a>
+</div>
+
+<div class="doc_text">
+<p>
+If your pass requires a previous pass to be executed (an analysis for example),
+it can use one of these methods to arrange for it to be run before your pass.
+LLVM has many different types of analyses and passes that can be required,
+spanning the range from <tt>DominatorSet</tt> to <tt>BreakCriticalEdges</tt>.
+Requiring <tt>BreakCriticalEdges</tt>, for example, guarantees that there will
+be no critical edges in the CFG when your pass has been run.
+</p>
+
+<p>
+Some analyses chain to other analyses to do their job. For example, an <a
+href="AliasAnalysis.html">AliasAnalysis</a> implementation is required to <a
+href="AliasAnalysis.html#chaining">chain</a> to other alias analysis passes. In
+cases where analyses chain, the <tt>addRequiredTransitive</tt> method should be
+used instead of the <tt>addRequired</tt> method. This informs the PassManager
+that the transitively required pass should be alive as long as the requiring
+pass is.
+</p>
+</div>
+
+<!-- _______________________________________________________________________ -->
+<div class="doc_subsubsection">
+ <a name="AU::addPreserved">The <tt>AnalysisUsage::addPreserved<></tt> method</a>
+</div>
+
+<div class="doc_text">
+<p>
+One of the jobs of the PassManager is to optimize how and when analyses are run.
+In particular, it attempts to avoid recomputing data unless it needs to. For
+this reason, passes are allowed to declare that they preserve (i.e., they don't
+invalidate) an existing analysis if it's available. For example, a simple
+constant folding pass would not modify the CFG, so it can't possibly affect the
+results of dominator analysis. By default, all passes are assumed to invalidate
+all others.
+</p>
+
+<p>
+The <tt>AnalysisUsage</tt> class provides several methods which are useful in
+certain circumstances that are related to <tt>addPreserved</tt>. In particular,
+the <tt>setPreservesAll</tt> method can be called to indicate that the pass does
+not modify the LLVM program at all (which is true for analyses), and the
+<tt>setPreservesCFG</tt> method can be used by transformations that change
+instructions in the program but do not modify the CFG or terminator instructions
+(note that this property is implicitly set for <a
+href="#BasicBlockPass">BasicBlockPass</a>'s).
+</p>
+
+<p>
+<tt>addPreserved</tt> is particularly useful for transformations like
+<tt>BreakCriticalEdges</tt>. This pass knows how to update a small set of loop
+and dominator related analyses if they exist, so it can preserve them, despite
+the fact that it hacks on the CFG.
+</p>
+</div>
+
+<!-- _______________________________________________________________________ -->
+<div class="doc_subsubsection">
+ <a name="AU::examples">Example implementations of <tt>getAnalysisUsage</tt></a>
+</div>
+
+<div class="doc_text">
+
+<div class="doc_code"><pre>
<i>// This is an example implementation from an analysis, which does not modify
// the program at all, yet has a prerequisite.</i>
- <b>void</b> <a href="http://llvm.cs.uiuc.edu/doxygen/structllvm_1_1PostDominanceFrontier.html">PostDominanceFrontier</a>::getAnalysisUsage(AnalysisUsage &AU) <b>const</b> {
+ <b>void</b> <a href="http://llvm.org/doxygen/classllvm_1_1PostDominanceFrontier.html">PostDominanceFrontier</a>::getAnalysisUsage(AnalysisUsage &AU) <b>const</b> {
AU.setPreservesAll();
- AU.addRequired<<a href="http://llvm.cs.uiuc.edu/doxygen/structllvm_1_1PostDominatorTree.html">PostDominatorTree</a>>();
+ AU.addRequired<<a href="http://llvm.org/doxygen/classllvm_1_1PostDominatorTree.html">PostDominatorTree</a>>();
}
-</pre>
+</pre></div>
<p>and:</p>
-<pre>
+<div class="doc_code"><pre>
<i>// This example modifies the program, but does not modify the CFG</i>
- <b>void</b> <a href="http://llvm.cs.uiuc.edu/doxygen/structLICM.html">LICM</a>::getAnalysisUsage(AnalysisUsage &AU) <b>const</b> {
+ <b>void</b> <a href="http://llvm.org/doxygen/structLICM.html">LICM</a>::getAnalysisUsage(AnalysisUsage &AU) <b>const</b> {
AU.setPreservesCFG();
- AU.addRequired<<a href="http://llvm.cs.uiuc.edu/doxygen/classllvm_1_1LoopInfo.html">LoopInfo</a>>();
+ AU.addRequired<<a href="http://llvm.org/doxygen/classllvm_1_1LoopInfo.html">LoopInfo</a>>();
}
-</pre>
+</pre></div>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
- <a name="getAnalysis">The <tt>getAnalysis<></tt> method</a>
+ <a name="getAnalysis">The <tt>getAnalysis<></tt> and
+<tt>getAnalysisIfAvailable<></tt> methods</a>
</div>
<div class="doc_text">
-<p>The <tt>Pass::getAnalysis<></tt> method is inherited by your class,
-providing you with access to the passes that you declared that you required with
-the <a href="#getAnalysisUsage"><tt>getAnalysisUsage</tt></a> method. It takes
-a single template argument that specifies which pass class you want, and returns
-a reference to that pass.</p>
+<p>The <tt>Pass::getAnalysis<></tt> method is automatically inherited by
+your class, providing you with access to the passes that you declared that you
+required with the <a href="#getAnalysisUsage"><tt>getAnalysisUsage</tt></a>
+method. It takes a single template argument that specifies which pass class you
+want, and returns a reference to that pass. For example:</p>
-<pre>
- <b>template</b><<b>typename</b> PassClass>
- AnalysisType &getAnalysis();
-</pre>
+<div class="doc_code"><pre>
+ bool LICM::runOnFunction(Function &F) {
+ LoopInfo &LI = getAnalysis<LoopInfo>();
+ ...
+ }
+</pre></div>
<p>This method call returns a reference to the pass desired. You may get a
runtime assertion failure if you attempt to get an analysis that you did not
declare as required in your <a
href="#getAnalysisUsage"><tt>getAnalysisUsage</tt></a> implementation. This
method can be called by your <tt>run*</tt> method implementation, or by any
-other local method invoked by your <tt>run*</tt> method.</p>
+other local method invoked by your <tt>run*</tt> method.
+
+A module level pass can use function level analysis info using this interface.
+For example:</p>
+
+<div class="doc_code"><pre>
+ bool ModuleLevelPass::runOnModule(Module &M) {
+ ...
+ DominatorTree &DT = getAnalysis<DominatorTree>(Func);
+ ...
+ }
+</pre></div>
+
+<p>In above example, runOnFunction for DominatorTree is called by pass manager
+before returning a reference to the desired pass.</p>
+
+<p>
+If your pass is capable of updating analyses if they exist (e.g.,
+<tt>BreakCriticalEdges</tt>, as described above), you can use the
+<tt>getAnalysisIfAvailable</tt> method, which returns a pointer to the analysis
+if it is active. For example:</p>
+
+<div class="doc_code"><pre>
+ ...
+ if (DominatorSet *DS = getAnalysisIfAvailable<DominatorSet>()) {
+ <i>// A DominatorSet is active. This code will update it.</i>
+ }
+ ...
+</pre></div>
</div>
<div class="doc_text">
-<p>Now that we understand the basics of how passes are defined, how the are
+<p>Now that we understand the basics of how passes are defined, how they are
used, and how they are required from other passes, it's time to get a little bit
fancier. All of the pass relationships that we have seen so far are very
simple: one pass depends on one other specific pass to be run before it can run.
</div>
<!-- _______________________________________________________________________ -->
-<div class="subsubsection">
+<div class="doc_subsubsection">
<a name="agconcepts">Analysis Group Concepts</a>
</div>
href="#registerag"><tt>RegisterAnalysisGroup</tt></a>.</p>
<p>As a concrete example of an Analysis Group in action, consider the <a
-href="http://llvm.cs.uiuc.edu/doxygen/classllvm_1_1AliasAnalysis.html">AliasAnalysis</a>
+href="http://llvm.org/doxygen/classllvm_1_1AliasAnalysis.html">AliasAnalysis</a>
analysis group. The default implementation of the alias analysis interface (the
<tt><a
-href="http://llvm.cs.uiuc.edu/doxygen/structBasicAliasAnalysis.html">basicaa</a></tt>
+href="http://llvm.org/doxygen/structBasicAliasAnalysis.html">basicaa</a></tt>
pass) just does a few simple checks that don't require significant analysis to
compute (such as: two different globals can never alias each other, etc).
Passes that use the <tt><a
-href="http://llvm.cs.uiuc.edu/doxygen/classllvm_1_1AliasAnalysis.html">AliasAnalysis</a></tt>
+href="http://llvm.org/doxygen/classllvm_1_1AliasAnalysis.html">AliasAnalysis</a></tt>
interface (for example the <tt><a
-href="http://llvm.cs.uiuc.edu/doxygen/classGCSE.html">gcse</a></tt> pass), do
+href="http://llvm.org/doxygen/structGCSE.html">gcse</a></tt> pass), do
not care which implementation of alias analysis is actually provided, they just
use the designated interface.</p>
</div>
<!-- _______________________________________________________________________ -->
-<div class="subsubsection">
+<div class="doc_subsubsection">
<a name="registerag">Using <tt>RegisterAnalysisGroup</tt></a>
</div>
Unlike registration of passes, there is no command line argument to be specified
for the Analysis Group Interface itself, because it is "abstract":</p>
-<pre>
- <b>static</b> RegisterAnalysisGroup<<a href="http://llvm.cs.uiuc.edu/doxygen/classllvm_1_1AliasAnalysis.html">AliasAnalysis</a>> A("<i>Alias Analysis</i>");
-</pre>
+<div class="doc_code"><pre>
+ <b>static</b> RegisterAnalysisGroup<<a href="http://llvm.org/doxygen/classllvm_1_1AliasAnalysis.html">AliasAnalysis</a>> A("<i>Alias Analysis</i>");
+</pre></div>
<p>Once the analysis is registered, passes can declare that they are valid
implementations of the interface by using the following code:</p>
-<pre>
+<div class="doc_code"><pre>
<b>namespace</b> {
//<i> Analysis Group implementations <b>must</b> be registered normally...</i>
- RegisterOpt<FancyAA>
+ RegisterPass<FancyAA>
B("<i>somefancyaa</i>", "<i>A more complex alias analysis implementation</i>");
//<i> Declare that we implement the AliasAnalysis interface</i>
- RegisterAnalysisGroup<<a href="http://llvm.cs.uiuc.edu/doxygen/classllvm_1_1AliasAnalysis.html">AliasAnalysis</a>, FancyAA> C;
+ RegisterAnalysisGroup<<a href="http://llvm.org/doxygen/classllvm_1_1AliasAnalysis.html">AliasAnalysis</a>> C(B);
}
-</pre>
+</pre></div>
<p>This just shows a class <tt>FancyAA</tt> that is registered normally, then
uses the <tt>RegisterAnalysisGroup</tt> template to "join" the <tt><a
-href="http://llvm.cs.uiuc.edu/doxygen/classllvm_1_1AliasAnalysis.html">AliasAnalysis</a></tt>
+href="http://llvm.org/doxygen/classllvm_1_1AliasAnalysis.html">AliasAnalysis</a></tt>
analysis group. Every implementation of an analysis group should join using
this template. A single pass may join multiple different analysis groups with
no problem.</p>
-<pre>
+<div class="doc_code"><pre>
<b>namespace</b> {
//<i> Analysis Group implementations <b>must</b> be registered normally...</i>
- RegisterOpt<<a href="http://llvm.cs.uiuc.edu/doxygen/structBasicAliasAnalysis.html">BasicAliasAnalysis</a>>
+ RegisterPass<<a href="http://llvm.org/doxygen/structBasicAliasAnalysis.html">BasicAliasAnalysis</a>>
D("<i>basicaa</i>", "<i>Basic Alias Analysis (default AA impl)</i>");
//<i> Declare that we implement the AliasAnalysis interface</i>
- RegisterAnalysisGroup<<a href="http://llvm.cs.uiuc.edu/doxygen/classllvm_1_1AliasAnalysis.html">AliasAnalysis</a>, <a href="http://llvm.cs.uiuc.edu/doxygen/structBasicAliasAnalysis.html">BasicAliasAnalysis</a>, <b>true</b>> E;
+ RegisterAnalysisGroup<<a href="http://llvm.org/doxygen/classllvm_1_1AliasAnalysis.html">AliasAnalysis</a>, <b>true</b>> E(D);
}
-</pre>
+</pre></div>
<p>Here we show how the default implementation is specified (using the extra
argument to the <tt>RegisterAnalysisGroup</tt> template). There must be exactly
one default implementation available at all times for an Analysis Group to be
-used. Here we declare that the <tt><a
-href="http://llvm.cs.uiuc.edu/doxygen/structBasicAliasAnalysis.html">BasicAliasAnalysis</a></tt>
+used. Only default implementation can derive from <tt>ImmutablePass</tt>.
+Here we declare that the
+ <tt><a href="http://llvm.org/doxygen/structBasicAliasAnalysis.html">BasicAliasAnalysis</a></tt>
pass is the default implementation for the interface.</p>
</div>
+<!-- *********************************************************************** -->
+<div class="doc_section">
+ <a name="passStatistics">Pass Statistics</a>
+</div>
+<!-- *********************************************************************** -->
+
+<div class="doc_text">
+<p>The <a
+href="http://llvm.org/doxygen/Statistic_8h-source.html"><tt>Statistic</tt></a>
+class is designed to be an easy way to expose various success
+metrics from passes. These statistics are printed at the end of a
+run, when the -stats command line option is enabled on the command
+line. See the <a href="http://llvm.org/docs/ProgrammersManual.html#Statistic">Statistics section</a> in the Programmer's Manual for details.
+
+</div>
+
+
<!-- *********************************************************************** -->
<div class="doc_section">
<a name="passmanager">What PassManager does</a>
<div class="doc_text">
<p>The <a
-href="http://llvm.cs.uiuc.edu/doxygen/PassManager_8h-source.html"><tt>PassManager</tt></a>
+href="http://llvm.org/doxygen/PassManager_8h-source.html"><tt>PassManager</tt></a>
<a
-href="http://llvm.cs.uiuc.edu/doxygen/classllvm_1_1PassManager.html">class</a>
+href="http://llvm.org/doxygen/classllvm_1_1PassManager.html">class</a>
takes a list of passes, ensures their <a href="#interaction">prerequisites</a>
are set up correctly, and then schedules passes to run efficiently. All of the
LLVM tools that run passes use the <tt>PassManager</tt> for execution of these
the LLVM program representation for a single function at a time, instead of
traversing the entire program. It reduces the memory consumption of compiler,
because, for example, only one <a
-href="http://llvm.cs.uiuc.edu/doxygen/structllvm_1_1DominatorSet.html"><tt>DominatorSet</tt></a>
-needs to be calculated at a time. This also makes it possible some <a
+href="http://llvm.org/doxygen/classllvm_1_1DominatorSet.html"><tt>DominatorSet</tt></a>
+needs to be calculated at a time. This also makes it possible to implement
+some <a
href="#SMP">interesting enhancements</a> in the future.</p></li>
</ol>
how our <a href="#basiccode">Hello World</a> pass interacts with other passes.
Lets try it out with the <tt>gcse</tt> and <tt>licm</tt> passes:</p>
-<pre>
-$ opt -load ../../../lib/Debug/libhello.so -gcse -licm --debug-pass=Structure < hello.bc > /dev/null
+<div class="doc_code"><pre>
+$ opt -load ../../../Debug/lib/Hello.so -gcse -licm --debug-pass=Structure < hello.bc > /dev/null
Module Pass Manager
Function Pass Manager
Dominator Set Construction
Module Verifier
-- Dominator Set Construction
-- Module Verifier
- Bytecode Writer
---Bytecode Writer
-</pre>
+ Bitcode Writer
+--Bitcode Writer
+</pre></div>
<p>This output shows us when passes are constructed and when the analysis
results are known to be dead (prefixed with '<tt>--</tt>'). Here we see that
<p>Lets see how this changes when we run the <a href="#basiccode">Hello
World</a> pass in between the two passes:</p>
-<pre>
-$ opt -load ../../../lib/Debug/libhello.so -gcse -hello -licm --debug-pass=Structure < hello.bc > /dev/null
+<div class="doc_code"><pre>
+$ opt -load ../../../Debug/lib/Hello.so -gcse -hello -licm --debug-pass=Structure < hello.bc > /dev/null
Module Pass Manager
Function Pass Manager
Dominator Set Construction
Module Verifier
-- Dominator Set Construction
-- Module Verifier
- Bytecode Writer
---Bytecode Writer
+ Bitcode Writer
+--Bitcode Writer
Hello: __main
Hello: puts
Hello: main
-</pre>
+</pre></div>
<p>Here we see that the <a href="#basiccode">Hello World</a> pass has killed the
Dominator Set pass, even though it doesn't modify the code at all! To fix this,
we need to add the following <a
href="#getAnalysisUsage"><tt>getAnalysisUsage</tt></a> method to our pass:</p>
-<pre>
+<div class="doc_code"><pre>
<i>// We don't modify the program, so we preserve all analyses</i>
<b>virtual void</b> getAnalysisUsage(AnalysisUsage &AU) <b>const</b> {
AU.setPreservesAll();
}
-</pre>
+</pre></div>
<p>Now when we run our pass, we get this output:</p>
-<pre>
-$ opt -load ../../../lib/Debug/libhello.so -gcse -hello -licm --debug-pass=Structure < hello.bc > /dev/null
+<div class="doc_code"><pre>
+$ opt -load ../../../Debug/lib/Hello.so -gcse -hello -licm --debug-pass=Structure < hello.bc > /dev/null
Pass Arguments: -gcse -hello -licm
Module Pass Manager
Function Pass Manager
Module Verifier
-- Dominator Set Construction
-- Module Verifier
- Bytecode Writer
---Bytecode Writer
+ Bitcode Writer
+--Bitcode Writer
Hello: __main
Hello: puts
Hello: main
-</pre>
+</pre></div>
<p>Which shows that we don't accidentally invalidate dominator information
anymore, and therefore do not have to compute it twice.</p>
<div class="doc_text">
-<pre>
+<div class="doc_code"><pre>
<b>virtual void</b> releaseMemory();
-</pre>
+</pre></div>
<p>The <tt>PassManager</tt> automatically determines when to compute analysis
results, and how long to keep them around for. Because the lifetime of the pass
<p>If you are writing an analysis or any other pass that retains a significant
amount of state (for use by another pass which "requires" your pass and uses the
<a href="#getAnalysis">getAnalysis</a> method) you should implement
-<tt>releaseMEmory</tt> to, well, release the memory allocated to maintain this
+<tt>releaseMemory</tt> to, well, release the memory allocated to maintain this
internal state. This method is called after the <tt>run*</tt> method for the
class, before the next call of <tt>run*</tt> in your pass.</p>
</div>
+<!-- *********************************************************************** -->
+<div class="doc_section">
+ <a name="registering">Registering dynamically loaded passes</a>
+</div>
+<!-- *********************************************************************** -->
+
+<div class="doc_text">
+
+<p><i>Size matters</i> when constructing production quality tools using llvm,
+both for the purposes of distribution, and for regulating the resident code size
+when running on the target system. Therefore, it becomes desirable to
+selectively use some passes, while omitting others and maintain the flexibility
+to change configurations later on. You want to be able to do all this, and,
+provide feedback to the user. This is where pass registration comes into
+play.</p>
+
+<p>The fundamental mechanisms for pass registration are the
+<tt>MachinePassRegistry</tt> class and subclasses of
+<tt>MachinePassRegistryNode</tt>.</p>
+
+<p>An instance of <tt>MachinePassRegistry</tt> is used to maintain a list of
+<tt>MachinePassRegistryNode</tt> objects. This instance maintains the list and
+communicates additions and deletions to the command line interface.</p>
+
+<p>An instance of <tt>MachinePassRegistryNode</tt> subclass is used to maintain
+information provided about a particular pass. This information includes the
+command line name, the command help string and the address of the function used
+to create an instance of the pass. A global static constructor of one of these
+instances <i>registers</i> with a corresponding <tt>MachinePassRegistry</tt>,
+the static destructor <i>unregisters</i>. Thus a pass that is statically linked
+in the tool will be registered at start up. A dynamically loaded pass will
+register on load and unregister at unload.</p>
+
+</div>
+
+<!-- _______________________________________________________________________ -->
+<div class="doc_subsection">
+ <a name="registering_existing">Using existing registries</a>
+</div>
+
+<div class="doc_text">
+
+<p>There are predefined registries to track instruction scheduling
+(<tt>RegisterScheduler</tt>) and register allocation (<tt>RegisterRegAlloc</tt>)
+machine passes. Here we will describe how to <i>register</i> a register
+allocator machine pass.</p>
+
+<p>Implement your register allocator machine pass. In your register allocator
+.cpp file add the following include;</p>
+
+<div class="doc_code"><pre>
+ #include "llvm/CodeGen/RegAllocRegistry.h"
+</pre></div>
+
+<p>Also in your register allocator .cpp file, define a creator function in the
+form; </p>
+
+<div class="doc_code"><pre>
+ FunctionPass *createMyRegisterAllocator() {
+ return new MyRegisterAllocator();
+ }
+</pre></div>
+
+<p>Note that the signature of this function should match the type of
+<tt>RegisterRegAlloc::FunctionPassCtor</tt>. In the same file add the
+"installing" declaration, in the form;</p>
+
+<div class="doc_code"><pre>
+ static RegisterRegAlloc myRegAlloc("myregalloc",
+ " my register allocator help string",
+ createMyRegisterAllocator);
+</pre></div>
+
+<p>Note the two spaces prior to the help string produces a tidy result on the
+--help query.</p>
+
+<div class="doc_code"><pre>
+$ llc --help
+ ...
+ -regalloc - Register allocator to use: (default = linearscan)
+ =linearscan - linear scan register allocator
+ =local - local register allocator
+ =simple - simple register allocator
+ =myregalloc - my register allocator help string
+ ...
+</pre></div>
+
+<p>And that's it. The user is now free to use <tt>-regalloc=myregalloc</tt> as
+an option. Registering instruction schedulers is similar except use the
+<tt>RegisterScheduler</tt> class. Note that the
+<tt>RegisterScheduler::FunctionPassCtor</tt> is significantly different from
+<tt>RegisterRegAlloc::FunctionPassCtor</tt>.</p>
+
+<p>To force the load/linking of your register allocator into the llc/lli tools,
+add your creator function's global declaration to "Passes.h" and add a "pseudo"
+call line to <tt>llvm/Codegen/LinkAllCodegenComponents.h</tt>.</p>
+
+</div>
+
+
+<!-- _______________________________________________________________________ -->
+<div class="doc_subsection">
+ <a name="registering_new">Creating new registries</a>
+</div>
+
+<div class="doc_text">
+
+<p>The easiest way to get started is to clone one of the existing registries; we
+recommend <tt>llvm/CodeGen/RegAllocRegistry.h</tt>. The key things to modify
+are the class name and the <tt>FunctionPassCtor</tt> type.</p>
+
+<p>Then you need to declare the registry. Example: if your pass registry is
+<tt>RegisterMyPasses</tt> then define;</p>
+
+<div class="doc_code"><pre>
+MachinePassRegistry RegisterMyPasses::Registry;
+</pre></div>
+
+<p>And finally, declare the command line option for your passes. Example:</p>
+
+<div class="doc_code"><pre>
+ cl::opt<RegisterMyPasses::FunctionPassCtor, false,
+ RegisterPassParser<RegisterMyPasses> >
+ MyPassOpt("mypass",
+ cl::init(&createDefaultMyPass),
+ cl::desc("my pass option help"));
+</pre></div>
+
+<p>Here the command option is "mypass", with createDefaultMyPass as the default
+creator.</p>
+
+</div>
+
<!-- *********************************************************************** -->
<div class="doc_section">
<a name="debughints">Using GDB with dynamically loaded passes</a>
<p>First thing you do is start <tt>gdb</tt> on the <tt>opt</tt> process:</p>
-<pre>
+<div class="doc_code"><pre>
$ <b>gdb opt</b>
GNU gdb 5.0
Copyright 2000 Free Software Foundation, Inc.
There is absolutely no warranty for GDB. Type "show warranty" for details.
This GDB was configured as "sparc-sun-solaris2.6"...
(gdb)
-</pre>
+</pre></div>
<p>Note that <tt>opt</tt> has a lot of debugging information in it, so it takes
time to load. Be patient. Since we cannot set a breakpoint in our pass yet
<tt>PassManager::run</tt> and then run the process with the arguments you
want:</p>
-<pre>
-(gdb) <b>break PassManager::run</b>
+<div class="doc_code"><pre>
+(gdb) <b>break llvm::PassManager::run</b>
Breakpoint 1 at 0x2413bc: file Pass.cpp, line 70.
-(gdb) <b>run test.bc -load $(LLVMTOP)/llvm/lib/Debug/[libname].so -[passoption]</b>
-Starting program: opt test.bc -load $(LLVMTOP)/llvm/lib/Debug/[libname].so -[passoption]
+(gdb) <b>run test.bc -load $(LLVMTOP)/llvm/Debug/lib/[libname].so -[passoption]</b>
+Starting program: opt test.bc -load $(LLVMTOP)/llvm/Debug/lib/[libname].so -[passoption]
Breakpoint 1, PassManager::run (this=0xffbef174, M=@0x70b298) at Pass.cpp:70
70 bool PassManager::run(Module &M) { return PM->run(M); }
(gdb)
-</pre>
+</pre></div>
<p>Once the <tt>opt</tt> stops in the <tt>PassManager::run</tt> method you are
now free to set breakpoints in your pass so that you can trace through execution
</div>
<!-- _______________________________________________________________________ -->
-<div class="subsubsection">
+<div class="doc_subsubsection">
<a name="SMP">Multithreaded LLVM</a>
</div>
</div>
-<!-- _______________________________________________________________________ -->
-<div class="subsubsection">
- <a name="ModuleSource">A new <tt>ModuleSource</tt> interface</a>
-</div>
-
-<div class="doc_text">
-
-<p>Currently, the <tt>PassManager</tt>'s <tt>run</tt> method takes a <tt><a
-href="http://llvm.cs.uiuc.edu/doxygen/structllvm_1_1Module.html">Module</a></tt>
-as input, and runs all of the passes on this module. The problem with this
-approach is that none of the <tt>PassManager</tt> features can be used for
-timing and debugging the actual <b>loading</b> of the module from disk or
-standard input.</p>
-
-<p>To solve this problem, eventually the <tt>PassManger</tt> class will accept a
-<tt>ModuleSource</tt> object instead of a Module itself. When complete, this
-will also allow for streaming of functions out of the bytecode representation,
-allowing us to avoid holding the entire program in memory at once if we only are
-dealing with <a href="#FunctionPass">FunctionPass</a>es.</p>
-
-<p>As part of a different issue, eventually the bytecode loader will be extended
-to allow on-demand loading of functions from the bytecode representation, in
-order to better support the runtime reoptimizer. The bytecode format is already
-capable of this, the loader just needs to be reworked a bit.</p>
-
-</div>
-
-<!-- _______________________________________________________________________ -->
-<div class="subsubsection">
-<a name="PassFunctionPass"><tt>Pass</tt>'s requiring <tt>FunctionPass</tt>'s</a>
-</div>
-
-<div class="doc_text">
-
-<p>Currently it is illegal for a <a href="#Pass"><tt>Pass</tt></a> to require a
-<a href="#FunctionPass"><tt>FunctionPass</tt></a>. This is because there is
-only one instance of the <a href="#FunctionPass"><tt>FunctionPass</tt></a>
-object ever created, thus nowhere to store information for all of the functions
-in the program at the same time. Although this has come up a couple of times
-before, this has always been worked around by factoring one big complicated pass
-into a global and an interprocedural part, both of which are distinct. In the
-future, it would be nice to have this though.</p>
-
-<p>Note that it is no problem for a <a
-href="#FunctionPass"><tt>FunctionPass</tt></a> to require the results of a <a
-href="#Pass"><tt>Pass</tt></a>, only the other way around.</p>
-
-</div>
-
<!-- *********************************************************************** -->
<hr>
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<a href="mailto:sabre@nondot.org">Chris Lattner</a><br>
- <a href="http://llvm.cs.uiuc.edu">The LLVM Compiler Infrastructure</a><br>
+ <a href="http://llvm.org">The LLVM Compiler Infrastructure</a><br>
Last modified: $Date$
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