</head>
<body>
-<div class="doc_title">
+<h1>
Writing an LLVM Pass
-</div>
+</h1>
<ol>
<li><a href="#introduction">Introduction - What is a pass?</a></li>
</div>
<!-- *********************************************************************** -->
-<div class="doc_section">
+<h2>
<a name="introduction">Introduction - What is a pass?</a>
-</div>
+</h2>
<!-- *********************************************************************** -->
-<div class="doc_text">
+<div>
<p>The LLVM Pass Framework is an important part of the LLVM system, because LLVM
passes are where most of the interesting parts of the compiler exist. Passes
</div>
<!-- *********************************************************************** -->
-<div class="doc_section">
+<h2>
<a name="quickstart">Quick Start - Writing hello world</a>
-</div>
+</h2>
<!-- *********************************************************************** -->
-<div class="doc_text">
+<div>
<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
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>
-
<!-- ======================================================================= -->
-<div class="doc_subsection">
+<h3>
<a name="makefile">Setting up the build environment</a>
-</div>
+</h3>
-<div class="doc_text">
+<div>
<p>First, configure and build LLVM. This needs to be done directly inside the
LLVM source tree rather than in a separate objects directory.
<tt>lib/Transforms/Hello</tt>. Finally, 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/>
+ <hr>
<div class="doc_code"><pre>
# Makefile for hello pass
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>If you are used CMake to build LLVM, see
+<a href="CMake.html#passdev">Developing an LLVM pass with CMake</a>.</p>
+
<p>Now that we have the build scripts set up, we just need to write the code for
the pass itself.</p>
</div>
<!-- ======================================================================= -->
-<div class="doc_subsection">
+<h3>
<a name="basiccode">Basic code required</a>
-</div>
+</h3>
-<div class="doc_text">
+<div>
<p>Now that we have a way to compile our new pass, we just have to write it.
Start out with:</p>
-<div class="doc_code"><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>
+</pre>
+</div>
<p>Which are needed because we are writing a <tt><a
href="http://llvm.org/doxygen/classllvm_1_1Pass.html">Pass</a></tt>,
and we will be doing some printing.</p>
<p>Next we have:</p>
-<div class="doc_code"><pre>
+
+<div class="doc_code">
+<pre>
<b>using namespace llvm;</b>
-</pre></div>
+</pre>
+</div>
+
<p>... which is required because the functions from the include files
-live in the llvm namespace.
-</p>
+live in the llvm namespace.</p>
<p>Next we have:</p>
-<div class="doc_code"><pre>
+<div class="doc_code">
+<pre>
<b>namespace</b> {
-</pre></div>
+</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
-things declared inside of the anonymous namespace only visible to the current
+things declared inside of the anonymous namespace visible only to the current
file. If you're not familiar with them, consult a decent C++ book for more
information.</p>
<p>Next, we declare our pass itself:</p>
-<div class="doc_code"><pre>
+<div class="doc_code">
+<pre>
<b>struct</b> Hello : <b>public</b> <a href="#FunctionPass">FunctionPass</a> {
-</pre></div><p>
+</pre>
+</div>
<p>This declares a "<tt>Hello</tt>" class that is a subclass of <tt><a
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
+href="#FunctionPass"><tt>FunctionPass</tt></a>'s operate on a function at a
time.</p>
-<div class="doc_code"><pre>
- static char ID;
- Hello() : FunctionPass(ID) {}
-</pre></div><p>
+<div class="doc_code">
+<pre>
+ static char ID;
+ Hello() : FunctionPass(ID) {}
+</pre>
+</div>
-<p> This declares pass identifier used by LLVM to identify pass. This allows LLVM to
-avoid using expensive C++ runtime information.</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>
+<div class="doc_code">
+<pre>
<b>virtual bool</b> <a href="#runOnFunction">runOnFunction</a>(Function &F) {
- errs() << "<i>Hello: </i>" << F.getName() << "\n";
+ errs() << "<i>Hello: </i>";
+ errs().write_escaped(F.getName()) << "\n";
<b>return false</b>;
}
}; <i>// end of struct Hello</i>
-</pre></div>
+} <i>// end of anonymous namespace</i>
+</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>
-<div class="doc_code"><pre>
- char Hello::ID = 0;
-</pre></div>
+<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
+<p>We initialize pass ID here. LLVM uses ID's address to identify a pass, so
initialization value is not important.</p>
-<div class="doc_code"><pre>
- static 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></div>
+<div class="doc_code">
+<pre>
+static RegisterPass<Hello> X("<i>hello</i>", "<i>Hello World Pass</i>",
+ false /* Only looks at CFG */,
+ false /* Analysis Pass */);
+</pre>
+</div>
-<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 arguments describe its behavior.
-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>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>". The last two arguments describe its behavior: if a pass walks CFG
+without modifying it then the third argument is set to <tt>true</tt>; if a pass
+is an analysis pass, for example dominator tree pass, then <tt>true</tt> is
+supplied as the fourth argument.</p>
<p>As a whole, the <tt>.cpp</tt> file looks like:</p>
-<div class="doc_code"><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>"
Hello() : FunctionPass(ID) {}
<b>virtual bool</b> <a href="#runOnFunction">runOnFunction</a>(Function &F) {
- errs() << "<i>Hello: </i>" << F.getName() << "\n";
+ errs() << "<i>Hello: </i>";
+ errs().write_escaped(F.getName()) << '\n';
<b>return false</b>;
}
+
};
-
- char Hello::ID = 0;
- static RegisterPass<Hello> X("hello", "Hello World Pass", false, false);
}
-
-</pre></div>
+
+char Hello::ID = 0;
+static RegisterPass<Hello> X("hello", "Hello World Pass", false, false);
+</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 file
"<tt>Debug+Asserts/lib/Hello.so</tt>" under the top level directory of the LLVM
source tree (not in the local directory). 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>
+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>
<!-- ======================================================================= -->
-<div class="doc_subsection">
+<h3>
<a name="running">Running a pass with <tt>opt</tt></a>
-</div>
+</h3>
-<div class="doc_text">
+<div>
<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
</div>
+</div>
+
<!-- *********************************************************************** -->
-<div class="doc_section">
+<h2>
<a name="passtype">Pass classes and requirements</a>
-</div>
+</h2>
<!-- *********************************************************************** -->
-<div class="doc_text">
+<div>
<p>One of the first things that you should do when designing a new pass is to
decide what class you should subclass for your pass. The <a
optimize how passes are run, so that the resultant compiler isn't unnecessarily
slow.</p>
-</div>
-
<!-- ======================================================================= -->
-<div class="doc_subsection">
+<h3>
<a name="ImmutablePass">The <tt>ImmutablePass</tt> class</a>
-</div>
+</h3>
-<div class="doc_text">
+<div>
<p>The most plain and boring type of pass is the "<tt><a
href="http://llvm.org/doxygen/classllvm_1_1ImmutablePass.html">ImmutablePass</a></tt>"
</div>
<!-- ======================================================================= -->
-<div class="doc_subsection">
+<h3>
<a name="ModulePass">The <tt>ModulePass</tt> class</a>
-</div>
+</h3>
-<div class="doc_text">
+<div>
<p>The "<tt><a
href="http://llvm.org/doxygen/classllvm_1_1ModulePass.html">ModulePass</a></tt>"
<tt>ModulePass</tt> and overload the <tt>runOnModule</tt> method with the
following signature:</p>
-</div>
-
<!-- _______________________________________________________________________ -->
-<div class="doc_subsubsection">
+<h4>
<a name="runOnModule">The <tt>runOnModule</tt> method</a>
-</div>
+</h4>
-<div class="doc_text">
+<div>
<div class="doc_code"><pre>
<b>virtual bool</b> runOnModule(Module &M) = 0;
</div>
+</div>
+
<!-- ======================================================================= -->
-<div class="doc_subsection">
+<h3>
<a name="CallGraphSCCPass">The <tt>CallGraphSCCPass</tt> class</a>
-</div>
+</h3>
-<div class="doc_text">
+<div>
<p>The "<tt><a
href="http://llvm.org/doxygen/classllvm_1_1CallGraphSCCPass.html">CallGraphSCCPass</a></tt>"
<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 <tt>Function</tt>s other than those
-in the current SCC and the direct callers and direct callees of the SCC.</li>
+<li>... <em>not allowed</em> to inspect or modify any <tt>Function</tt>s other
+than those in the current SCC and the direct callers and 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>
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>
+<h4>
+ <a name="doInitialization_scc">
+ The <tt>doInitialization(CallGraph &)</tt> method
+ </a>
+</h4>
-<div class="doc_text">
+<div>
<div class="doc_code"><pre>
<b>virtual bool</b> doInitialization(CallGraph &CG);
</div>
<!-- _______________________________________________________________________ -->
-<div class="doc_subsubsection">
+<h4>
<a name="runOnSCC">The <tt>runOnSCC</tt> method</a>
-</div>
+</h4>
-<div class="doc_text">
+<div>
<div class="doc_code"><pre>
<b>virtual bool</b> runOnSCC(CallGraphSCC &SCC) = 0;
</div>
<!-- _______________________________________________________________________ -->
-<div class="doc_subsubsection">
- <a name="doFinalization_scc">The <tt>doFinalization(CallGraph
- &)</tt> method</a>
-</div>
+<h4>
+ <a name="doFinalization_scc">
+ The <tt>doFinalization(CallGraph &)</tt> method
+ </a>
+</h4>
-<div class="doc_text">
+<div>
<div class="doc_code"><pre>
<b>virtual bool</b> doFinalization(CallGraph &CG);
</div>
+</div>
+
<!-- ======================================================================= -->
-<div class="doc_subsection">
+<h3>
<a name="FunctionPass">The <tt>FunctionPass</tt> class</a>
-</div>
+</h3>
-<div class="doc_text">
+<div>
<p>In contrast to <tt>ModulePass</tt> subclasses, <tt><a
href="http://llvm.org/doxygen/classllvm_1_1Pass.html">FunctionPass</a></tt>
may overload three virtual methods to do their work. All of 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_mod">The <tt>doInitialization(Module &)</tt>
- method</a>
-</div>
+<h4>
+ <a name="doInitialization_mod">
+ The <tt>doInitialization(Module &)</tt> method
+ </a>
+</h4>
-<div class="doc_text">
+<div>
<div class="doc_code"><pre>
<b>virtual bool</b> doInitialization(Module &M);
</div>
<!-- _______________________________________________________________________ -->
-<div class="doc_subsubsection">
+<h4>
<a name="runOnFunction">The <tt>runOnFunction</tt> method</a>
-</div>
+</h4>
-<div class="doc_text">
+<div>
<div class="doc_code"><pre>
<b>virtual bool</b> runOnFunction(Function &F) = 0;
</div>
<!-- _______________________________________________________________________ -->
-<div class="doc_subsubsection">
- <a name="doFinalization_mod">The <tt>doFinalization(Module
- &)</tt> method</a>
-</div>
+<h4>
+ <a name="doFinalization_mod">
+ The <tt>doFinalization(Module &)</tt> method
+ </a>
+</h4>
-<div class="doc_text">
+<div>
<div class="doc_code"><pre>
<b>virtual bool</b> doFinalization(Module &M);
</div>
+</div>
+
<!-- ======================================================================= -->
-<div class="doc_subsection">
+<h3>
<a name="LoopPass">The <tt>LoopPass</tt> class </a>
-</div>
+</h3>
-<div class="doc_text">
+<div>
<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
<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
+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>
+<h4>
+ <a name="doInitialization_loop">
+ The <tt>doInitialization(Loop *,LPPassManager &)</tt> method
+ </a>
+</h4>
-<div class="doc_text">
+<div>
<div class="doc_code"><pre>
<b>virtual bool</b> doInitialization(Loop *, LPPassManager &LPM);
<!-- _______________________________________________________________________ -->
-<div class="doc_subsubsection">
+<h4>
<a name="runOnLoop">The <tt>runOnLoop</tt> method</a>
-</div>
+</h4>
-<div class="doc_text">
+<div>
<div class="doc_code"><pre>
<b>virtual bool</b> runOnLoop(Loop *, LPPassManager &LPM) = 0;
</div>
<!-- _______________________________________________________________________ -->
-<div class="doc_subsubsection">
+<h4>
<a name="doFinalization_loop">The <tt>doFinalization()</tt> method</a>
-</div>
+</h4>
-<div class="doc_text">
+<div>
<div class="doc_code"><pre>
<b>virtual bool</b> doFinalization();
</div>
+</div>
+
<!-- ======================================================================= -->
-<div class="doc_subsection">
+<h3>
<a name="RegionPass">The <tt>RegionPass</tt> class </a>
-</div>
+</h3>
-<div class="doc_text">
+<div>
<p> <tt>RegionPass</tt> is similar to <a href="#LoopPass"><tt>LoopPass</tt></a>,
but executes on each single entry single exit region in the function.
<p> <tt>RegionPass</tt> subclasses are allowed to update the region tree by using
the <tt>RGPassManager</tt> interface. You may overload three virtual methods of
-<tt>RegionPass</tt> to implementing your own region pass is usually. All these
+<tt>RegionPass</tt> to implement your own region pass. All these
methods should return true if they modified the program, or false if they didn not.
</p>
-</div>
<!-- _______________________________________________________________________ -->
-<div class="doc_subsubsection">
- <a name="doInitialization_region">The <tt>doInitialization(Region *,
- RGPassManager &)</tt>
- method</a>
-</div>
+<h4>
+ <a name="doInitialization_region">
+ The <tt>doInitialization(Region *, RGPassManager &)</tt> method
+ </a>
+</h4>
-<div class="doc_text">
+<div>
<div class="doc_code"><pre>
<b>virtual bool</b> doInitialization(Region *, RGPassManager &RGM);
<!-- _______________________________________________________________________ -->
-<div class="doc_subsubsection">
+<h4>
<a name="runOnRegion">The <tt>runOnRegion</tt> method</a>
-</div>
+</h4>
-<div class="doc_text">
+<div>
<div class="doc_code"><pre>
<b>virtual bool</b> runOnRegion(Region *, RGPassManager &RGM) = 0;
</div>
<!-- _______________________________________________________________________ -->
-<div class="doc_subsubsection">
+<h4>
<a name="doFinalization_region">The <tt>doFinalization()</tt> method</a>
-</div>
+</h4>
-<div class="doc_text">
+<div>
<div class="doc_code"><pre>
<b>virtual bool</b> doFinalization();
</div>
-
+</div>
<!-- ======================================================================= -->
-<div class="doc_subsection">
+<h3>
<a name="BasicBlockPass">The <tt>BasicBlockPass</tt> class</a>
-</div>
+</h3>
-<div class="doc_text">
+<div>
<p><tt>BasicBlockPass</tt>'s are just like <a
href="#FunctionPass"><tt>FunctionPass</tt></a>'s, except that they must limit
href="#doFinalization_mod"><tt>doFinalization(Module &)</tt></a> methods that <a
href="#FunctionPass"><tt>FunctionPass</tt></a>'s have, but also have the following virtual methods that may also be implemented:</p>
-</div>
-
<!-- _______________________________________________________________________ -->
-<div class="doc_subsubsection">
- <a name="doInitialization_fn">The <tt>doInitialization(Function
- &)</tt> method</a>
-</div>
+<h4>
+ <a name="doInitialization_fn">
+ The <tt>doInitialization(Function &)</tt> method
+ </a>
+</h4>
-<div class="doc_text">
+<div>
<div class="doc_code"><pre>
<b>virtual bool</b> doInitialization(Function &F);
</div>
<!-- _______________________________________________________________________ -->
-<div class="doc_subsubsection">
+<h4>
<a name="runOnBasicBlock">The <tt>runOnBasicBlock</tt> method</a>
-</div>
+</h4>
-<div class="doc_text">
+<div>
<div class="doc_code"><pre>
<b>virtual bool</b> runOnBasicBlock(BasicBlock &BB) = 0;
</div>
<!-- _______________________________________________________________________ -->
-<div class="doc_subsubsection">
- <a name="doFinalization_fn">The <tt>doFinalization(Function &)</tt>
- method</a>
-</div>
+<h4>
+ <a name="doFinalization_fn">
+ The <tt>doFinalization(Function &)</tt> method
+ </a>
+</h4>
-<div class="doc_text">
+<div>
<div class="doc_code"><pre>
<b>virtual bool</b> doFinalization(Function &F);
</div>
+</div>
+
<!-- ======================================================================= -->
-<div class="doc_subsection">
+<h3>
<a name="MachineFunctionPass">The <tt>MachineFunctionPass</tt> class</a>
-</div>
+</h3>
-<div class="doc_text">
+<div>
<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
data)</li>
</ol>
-</div>
-
<!-- _______________________________________________________________________ -->
-<div class="doc_subsubsection">
- <a name="runOnMachineFunction">The <tt>runOnMachineFunction(MachineFunction
- &MF)</tt> method</a>
-</div>
+<h4>
+ <a name="runOnMachineFunction">
+ The <tt>runOnMachineFunction(MachineFunction &MF)</tt> method
+ </a>
+</h4>
-<div class="doc_text">
+<div>
<div class="doc_code"><pre>
<b>virtual bool</b> runOnMachineFunction(MachineFunction &MF) = 0;
</div>
+</div>
+
+</div>
+
<!-- *********************************************************************** -->
-<div class="doc_section">
+<h2>
<a name="registration">Pass registration</a>
-</div>
+</h2>
<!-- *********************************************************************** -->
-<div class="doc_text">
+<div>
<p>In the <a href="#basiccode">Hello World</a> example pass we illustrated how
pass registration works, and discussed some of the reasons that it is used and
<p>If you want your pass to be easily dumpable, you should
implement the virtual <tt>print</tt> method:</p>
-</div>
-
<!-- _______________________________________________________________________ -->
-<div class="doc_subsubsection">
+<h4>
<a name="print">The <tt>print</tt> method</a>
-</div>
+</h4>
-<div class="doc_text">
+<div>
<div class="doc_code"><pre>
<b>virtual void</b> print(std::ostream &O, <b>const</b> Module *M) <b>const</b>;
</div>
+</div>
+
<!-- *********************************************************************** -->
-<div class="doc_section">
+<h2>
<a name="interaction">Specifying interactions between passes</a>
-</div>
+</h2>
<!-- *********************************************************************** -->
-<div class="doc_text">
+<div>
<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>
href="#getAnalysisUsage">getAnalysisUsage</a></tt> method, it defaults to not
having any prerequisite passes, and invalidating <b>all</b> other passes.</p>
-</div>
-
<!-- _______________________________________________________________________ -->
-<div class="doc_subsubsection">
+<h4>
<a name="getAnalysisUsage">The <tt>getAnalysisUsage</tt> method</a>
-</div>
+</h4>
-<div class="doc_text">
+<div>
<div class="doc_code"><pre>
<b>virtual void</b> getAnalysisUsage(AnalysisUsage &Info) <b>const</b>;
</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">
+<h4>
+ <a name="AU::addRequired">
+ The <tt>AnalysisUsage::addRequired<></tt>
+ and <tt>AnalysisUsage::addRequiredTransitive<></tt> methods
+ </a>
+</h4>
+
+<div>
<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.
</div>
<!-- _______________________________________________________________________ -->
-<div class="doc_subsubsection">
- <a name="AU::addPreserved">The <tt>AnalysisUsage::addPreserved<></tt> method</a>
-</div>
+<h4>
+ <a name="AU::addPreserved">
+ The <tt>AnalysisUsage::addPreserved<></tt> method
+ </a>
+</h4>
-<div class="doc_text">
+<div>
<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
</div>
<!-- _______________________________________________________________________ -->
-<div class="doc_subsubsection">
- <a name="AU::examples">Example implementations of <tt>getAnalysisUsage</tt></a>
-</div>
+<h4>
+ <a name="AU::examples">
+ Example implementations of <tt>getAnalysisUsage</tt>
+ </a>
+</h4>
-<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.org/doxygen/classllvm_1_1PostDominanceFrontier.html">PostDominanceFrontier</a>::getAnalysisUsage(AnalysisUsage &AU) <b>const</b> {
- AU.setPreservesAll();
- AU.addRequired<<a href="http://llvm.org/doxygen/classllvm_1_1PostDominatorTree.html">PostDominatorTree</a>>();
- }
-</pre></div>
-
-<p>and:</p>
+<div>
<div class="doc_code"><pre>
<i>// This example modifies the program, but does not modify the CFG</i>
</div>
<!-- _______________________________________________________________________ -->
-<div class="doc_subsubsection">
- <a name="getAnalysis">The <tt>getAnalysis<></tt> and
-<tt>getAnalysisIfAvailable<></tt> methods</a>
-</div>
+<h4>
+ <a name="getAnalysis">
+ The <tt>getAnalysis<></tt> and
+ <tt>getAnalysisIfAvailable<></tt> methods
+ </a>
+</h4>
-<div class="doc_text">
+<div>
<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
</div>
+</div>
+
<!-- *********************************************************************** -->
-<div class="doc_section">
+<h2>
<a name="analysisgroup">Implementing Analysis Groups</a>
-</div>
+</h2>
<!-- *********************************************************************** -->
-<div class="doc_text">
+<div>
<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
situations like this, the LLVM Pass Infrastructure supports the notion of
Analysis Groups.</p>
-</div>
-
<!-- _______________________________________________________________________ -->
-<div class="doc_subsubsection">
+<h4>
<a name="agconcepts">Analysis Group Concepts</a>
-</div>
+</h4>
-<div class="doc_text">
+<div>
<p>An Analysis Group is a single simple interface that may be implemented by
multiple different passes. Analysis Groups can be given human readable names
</div>
<!-- _______________________________________________________________________ -->
-<div class="doc_subsubsection">
+<h4>
<a name="registerag">Using <tt>RegisterAnalysisGroup</tt></a>
-</div>
+</h4>
-<div class="doc_text">
+<div>
<p>The <tt>RegisterAnalysisGroup</tt> template is used to register the analysis
group itself, while the <tt>INITIALIZE_AG_PASS</tt> is used to add pass
</div>
+</div>
+
<!-- *********************************************************************** -->
-<div class="doc_section">
+<h2>
<a name="passStatistics">Pass Statistics</a>
-</div>
+</h2>
<!-- *********************************************************************** -->
-<div class="doc_text">
+<div>
<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
<!-- *********************************************************************** -->
-<div class="doc_section">
+<h2>
<a name="passmanager">What PassManager does</a>
-</div>
+</h2>
<!-- *********************************************************************** -->
-<div class="doc_text">
+<div>
<p>The <a
href="http://llvm.org/doxygen/PassManager_8h-source.html"><tt>PassManager</tt></a>
<li><b>Pipeline the execution of passes on the program</b> - The
<tt>PassManager</tt> attempts to get better cache and memory usage behavior out
of a series of passes by pipelining the passes together. This means that, given
-a series of consequtive <a href="#FunctionPass"><tt>FunctionPass</tt></a>'s, it
+a series of consecutive <a href="#FunctionPass"><tt>FunctionPass</tt></a>'s, it
will execute all of the <a href="#FunctionPass"><tt>FunctionPass</tt></a>'s on
the first function, then all of the <a
href="#FunctionPass"><tt>FunctionPass</tt></a>es on the second function,
<p>Which shows that we don't accidentally invalidate dominator information
anymore, and therefore do not have to compute it twice.</p>
-</div>
-
<!-- _______________________________________________________________________ -->
-<div class="doc_subsubsection">
+<h4>
<a name="releaseMemory">The <tt>releaseMemory</tt> method</a>
-</div>
+</h4>
-<div class="doc_text">
+<div>
<div class="doc_code"><pre>
<b>virtual void</b> releaseMemory();
</div>
+</div>
+
<!-- *********************************************************************** -->
-<div class="doc_section">
+<h2>
<a name="registering">Registering dynamically loaded passes</a>
-</div>
+</h2>
<!-- *********************************************************************** -->
-<div class="doc_text">
+<div>
<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
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">
+<h3>
<a name="registering_existing">Using existing registries</a>
-</div>
+</h3>
-<div class="doc_text">
+<div>
<p>There are predefined registries to track instruction scheduling
(<tt>RegisterScheduler</tt>) and register allocation (<tt>RegisterRegAlloc</tt>)
<!-- _______________________________________________________________________ -->
-<div class="doc_subsection">
+<h3>
<a name="registering_new">Creating new registries</a>
-</div>
+</h3>
-<div class="doc_text">
+<div>
<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
</div>
+</div>
+
<!-- *********************************************************************** -->
-<div class="doc_section">
+<h2>
<a name="debughints">Using GDB with dynamically loaded passes</a>
-</div>
+</h2>
<!-- *********************************************************************** -->
-<div class="doc_text">
+<div>
<p>Unfortunately, using GDB with dynamically loaded passes is not as easy as it
should be. First of all, you can't set a breakpoint in a shared object that has
transformation invoked by <tt>opt</tt>, although nothing described here depends
on that.</p>
-</div>
-
<!-- _______________________________________________________________________ -->
-<div class="doc_subsubsection">
+<h4>
<a name="breakpoint">Setting a breakpoint in your pass</a>
-</div>
+</h4>
-<div class="doc_text">
+<div>
<p>First thing you do is start <tt>gdb</tt> on the <tt>opt</tt> process:</p>
</div>
<!-- _______________________________________________________________________ -->
-<div class="doc_subsubsection">
+<h4>
<a name="debugmisc">Miscellaneous Problems</a>
-</div>
+</h4>
-<div class="doc_text">
+<div>
<p>Once you have the basics down, there are a couple of problems that GDB has,
some with solutions, some without.</p>
</div>
+</div>
+
<!-- *********************************************************************** -->
-<div class="doc_section">
+<h2>
<a name="future">Future extensions planned</a>
-</div>
+</h2>
<!-- *********************************************************************** -->
-<div class="doc_text">
+<div>
<p>Although the LLVM Pass Infrastructure is very capable as it stands, and does
some nifty stuff, there are things we'd like to add in the future. Here is
where we are going:</p>
-</div>
-
<!-- _______________________________________________________________________ -->
-<div class="doc_subsubsection">
+<h4>
<a name="SMP">Multithreaded LLVM</a>
-</div>
+</h4>
-<div class="doc_text">
+<div>
<p>Multiple CPU machines are becoming more common and compilation can never be
fast enough: obviously we should allow for a multithreaded compiler. Because of
</div>
+</div>
+
<!-- *********************************************************************** -->
<hr>
<address>
src="http://www.w3.org/Icons/valid-html401-blue" alt="Valid HTML 4.01"></a>
<a href="mailto:sabre@nondot.org">Chris Lattner</a><br>
- <a href="http://llvm.org">The LLVM Compiler Infrastructure</a><br>
+ <a href="http://llvm.org/">The LLVM Compiler Infrastructure</a><br>
Last modified: $Date$
</address>