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1829 lines
73 KiB
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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
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"http://www.w3.org/TR/html4/strict.dtd">
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<html>
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<head>
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<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
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<title>Writing an LLVM Pass</title>
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<link rel="stylesheet" href="llvm.css" type="text/css">
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</head>
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<body>
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<div class="doc_title">
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Writing an LLVM Pass
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</div>
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<ol>
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<li><a href="#introduction">Introduction - What is a pass?</a></li>
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<li><a href="#quickstart">Quick Start - Writing hello world</a>
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<ul>
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<li><a href="#makefile">Setting up the build environment</a></li>
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<li><a href="#basiccode">Basic code required</a></li>
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<li><a href="#running">Running a pass with <tt>opt</tt></a></li>
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</ul></li>
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<li><a href="#passtype">Pass classes and requirements</a>
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<ul>
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<li><a href="#ImmutablePass">The <tt>ImmutablePass</tt> class</a></li>
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<li><a href="#ModulePass">The <tt>ModulePass</tt> class</a>
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<ul>
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<li><a href="#runOnModule">The <tt>runOnModule</tt> method</a></li>
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</ul></li>
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<li><a href="#CallGraphSCCPass">The <tt>CallGraphSCCPass</tt> class</a>
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<ul>
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<li><a href="#doInitialization_scc">The <tt>doInitialization(CallGraph
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&)</tt> method</a></li>
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<li><a href="#runOnSCC">The <tt>runOnSCC</tt> method</a></li>
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<li><a href="#doFinalization_scc">The <tt>doFinalization(CallGraph
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&)</tt> method</a></li>
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</ul></li>
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<li><a href="#FunctionPass">The <tt>FunctionPass</tt> class</a>
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<ul>
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<li><a href="#doInitialization_mod">The <tt>doInitialization(Module
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&)</tt> method</a></li>
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<li><a href="#runOnFunction">The <tt>runOnFunction</tt> method</a></li>
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<li><a href="#doFinalization_mod">The <tt>doFinalization(Module
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&)</tt> method</a></li>
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</ul></li>
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<li><a href="#LoopPass">The <tt>LoopPass</tt> class</a>
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<ul>
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<li><a href="#doInitialization_loop">The <tt>doInitialization(Loop *,
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LPPassManager &)</tt> method</a></li>
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<li><a href="#runOnLoop">The <tt>runOnLoop</tt> method</a></li>
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<li><a href="#doFinalization_loop">The <tt>doFinalization()
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</tt> method</a></li>
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</ul></li>
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<li><a href="#BasicBlockPass">The <tt>BasicBlockPass</tt> class</a>
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<ul>
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<li><a href="#doInitialization_fn">The <tt>doInitialization(Function
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&)</tt> method</a></li>
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<li><a href="#runOnBasicBlock">The <tt>runOnBasicBlock</tt>
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method</a></li>
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<li><a href="#doFinalization_fn">The <tt>doFinalization(Function
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&)</tt> method</a></li>
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</ul></li>
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<li><a href="#MachineFunctionPass">The <tt>MachineFunctionPass</tt>
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class</a>
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<ul>
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<li><a href="#runOnMachineFunction">The
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<tt>runOnMachineFunction(MachineFunction &)</tt> method</a></li>
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</ul></li>
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</ul>
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<li><a href="#registration">Pass Registration</a>
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<ul>
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<li><a href="#print">The <tt>print</tt> method</a></li>
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</ul></li>
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<li><a href="#interaction">Specifying interactions between passes</a>
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<ul>
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<li><a href="#getAnalysisUsage">The <tt>getAnalysisUsage</tt>
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method</a></li>
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<li><a href="#AU::addRequired">The <tt>AnalysisUsage::addRequired<></tt> and <tt>AnalysisUsage::addRequiredTransitive<></tt> methods</a></li>
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<li><a href="#AU::addPreserved">The <tt>AnalysisUsage::addPreserved<></tt> method</a></li>
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<li><a href="#AU::examples">Example implementations of <tt>getAnalysisUsage</tt></a></li>
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<li><a href="#getAnalysis">The <tt>getAnalysis<></tt> and
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<tt>getAnalysisIfAvailable<></tt> methods</a></li>
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</ul></li>
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<li><a href="#analysisgroup">Implementing Analysis Groups</a>
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<ul>
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<li><a href="#agconcepts">Analysis Group Concepts</a></li>
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<li><a href="#registerag">Using <tt>RegisterAnalysisGroup</tt></a></li>
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</ul></li>
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<li><a href="#passStatistics">Pass Statistics</a>
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<li><a href="#passmanager">What PassManager does</a>
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<ul>
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<li><a href="#releaseMemory">The <tt>releaseMemory</tt> method</a></li>
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</ul></li>
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<li><a href="#registering">Registering dynamically loaded passes</a>
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<ul>
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<li><a href="#registering_existing">Using existing registries</a></li>
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<li><a href="#registering_new">Creating new registries</a></li>
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</ul></li>
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<li><a href="#debughints">Using GDB with dynamically loaded passes</a>
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<ul>
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<li><a href="#breakpoint">Setting a breakpoint in your pass</a></li>
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<li><a href="#debugmisc">Miscellaneous Problems</a></li>
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</ul></li>
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<li><a href="#future">Future extensions planned</a>
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<ul>
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<li><a href="#SMP">Multithreaded LLVM</a></li>
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</ul></li>
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</ol>
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<div class="doc_author">
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<p>Written by <a href="mailto:sabre@nondot.org">Chris Lattner</a> and
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<a href="mailto:jlaskey@mac.com">Jim Laskey</a></p>
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</div>
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<!-- *********************************************************************** -->
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<div class="doc_section">
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<a name="introduction">Introduction - What is a pass?</a>
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</div>
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<!-- *********************************************************************** -->
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<div class="doc_text">
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<p>The LLVM Pass Framework is an important part of the LLVM system, because LLVM
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passes are where most of the interesting parts of the compiler exist. Passes
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perform the transformations and optimizations that make up the compiler, they
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build the analysis results that are used by these transformations, and they are,
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above all, a structuring technique for compiler code.</p>
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<p>All LLVM passes are subclasses of the <tt><a
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href="http://llvm.org/doxygen/classllvm_1_1Pass.html">Pass</a></tt>
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class, which implement functionality by overriding virtual methods inherited
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from <tt>Pass</tt>. Depending on how your pass works, you should inherit from
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the <tt><a href="#ModulePass">ModulePass</a></tt>, <tt><a
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href="#CallGraphSCCPass">CallGraphSCCPass</a></tt>, <tt><a
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href="#FunctionPass">FunctionPass</a></tt>, or <tt><a
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href="#LoopPass">LoopPass</a></tt>, or <tt><a
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href="#BasicBlockPass">BasicBlockPass</a></tt> classes, which gives the system
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more information about what your pass does, and how it can be combined with
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other passes. One of the main features of the LLVM Pass Framework is that it
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schedules passes to run in an efficient way based on the constraints that your
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pass meets (which are indicated by which class they derive from).</p>
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<p>We start by showing you how to construct a pass, everything from setting up
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the code, to compiling, loading, and executing it. After the basics are down,
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more advanced features are discussed.</p>
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</div>
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<!-- *********************************************************************** -->
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<div class="doc_section">
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<a name="quickstart">Quick Start - Writing hello world</a>
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</div>
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<!-- *********************************************************************** -->
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<div class="doc_text">
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<p>Here we describe how to write the "hello world" of passes. The "Hello" pass
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is designed to simply print out the name of non-external functions that exist in
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the program being compiled. It does not modify the program at all, it just
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inspects it. The source code and files for this pass are available in the LLVM
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source tree in the <tt>lib/Transforms/Hello</tt> directory.</p>
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</div>
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<!-- ======================================================================= -->
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<div class="doc_subsection">
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<a name="makefile">Setting up the build environment</a>
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</div>
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<div class="doc_text">
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<p>First, you need to create a new directory somewhere in the LLVM source
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base. For this example, we'll assume that you made
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<tt>lib/Transforms/Hello</tt>. Next, you must set up a build script
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(Makefile) that will compile the source code for the new pass. To do this,
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copy the following into <tt>Makefile</tt>:</p>
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<hr/>
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<div class="doc_code"><pre>
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# Makefile for hello pass
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# Path to top level of LLVM heirarchy
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LEVEL = ../../..
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# Name of the library to build
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LIBRARYNAME = Hello
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# Make the shared library become a loadable module so the tools can
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# dlopen/dlsym on the resulting library.
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LOADABLE_MODULE = 1
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# Tell the build system which LLVM libraries your pass needs. You'll probably
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# need at least LLVMSystem.a, LLVMSupport.a, LLVMCore.a but possibly several
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# others too.
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LLVMLIBS = LLVMCore.a LLVMSupport.a LLVMSystem.a
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# Include the makefile implementation stuff
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include $(LEVEL)/Makefile.common
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</pre></div>
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<p>This makefile specifies that all of the <tt>.cpp</tt> files in the current
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directory are to be compiled and linked together into a
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<tt>Debug/lib/Hello.so</tt> shared object that can be dynamically loaded by
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the <tt>opt</tt> or <tt>bugpoint</tt> tools via their <tt>-load</tt> options.
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If your operating system uses a suffix other than .so (such as windows or
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Mac OS/X), the appropriate extension will be used.</p>
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<p>Now that we have the build scripts set up, we just need to write the code for
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the pass itself.</p>
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</div>
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<!-- ======================================================================= -->
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<div class="doc_subsection">
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<a name="basiccode">Basic code required</a>
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</div>
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<div class="doc_text">
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<p>Now that we have a way to compile our new pass, we just have to write it.
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Start out with:</p>
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<div class="doc_code"><pre>
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<b>#include</b> "<a href="http://llvm.org/doxygen/Pass_8h-source.html">llvm/Pass.h</a>"
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<b>#include</b> "<a href="http://llvm.org/doxygen/Function_8h-source.html">llvm/Function.h</a>"
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</pre></div>
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<p>Which are needed because we are writing a <tt><a
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href="http://llvm.org/doxygen/classllvm_1_1Pass.html">Pass</a></tt>, and
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we are operating on <tt><a
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href="http://llvm.org/doxygen/classllvm_1_1Function.html">Function</a></tt>'s.</p>
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<p>Next we have:</p>
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<div class="doc_code"><pre>
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<b>using namespace llvm;</b>
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</pre></div>
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<p>... which is required because the functions from the include files
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live in the llvm namespace.
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</p>
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<p>Next we have:</p>
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<div class="doc_code"><pre>
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<b>namespace</b> {
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</pre></div>
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<p>... which starts out an anonymous namespace. Anonymous namespaces are to C++
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what the "<tt>static</tt>" keyword is to C (at global scope). It makes the
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things declared inside of the anonymous namespace only visible to the current
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file. If you're not familiar with them, consult a decent C++ book for more
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information.</p>
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<p>Next, we declare our pass itself:</p>
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<div class="doc_code"><pre>
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<b>struct</b> Hello : <b>public</b> <a href="#FunctionPass">FunctionPass</a> {
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</pre></div><p>
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<p>This declares a "<tt>Hello</tt>" class that is a subclass of <tt><a
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href="http://llvm.org/doxygen/classllvm_1_1FunctionPass.html">FunctionPass</a></tt>.
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The different builtin pass subclasses are described in detail <a
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href="#passtype">later</a>, but for now, know that <a
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href="#FunctionPass"><tt>FunctionPass</tt></a>'s operate a function at a
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time.</p>
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<div class="doc_code"><pre>
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static char ID;
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Hello() : FunctionPass(&ID) {}
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</pre></div><p>
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<p> This declares pass identifier used by LLVM to identify pass. This allows LLVM to
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avoid using expensive C++ runtime information.</p>
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<div class="doc_code"><pre>
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<b>virtual bool</b> <a href="#runOnFunction">runOnFunction</a>(Function &F) {
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llvm::cerr << "<i>Hello: </i>" << F.getName() << "\n";
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<b>return false</b>;
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}
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}; <i>// end of struct Hello</i>
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</pre></div>
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<p>We declare a "<a href="#runOnFunction"><tt>runOnFunction</tt></a>" method,
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which overloads an abstract virtual method inherited from <a
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href="#FunctionPass"><tt>FunctionPass</tt></a>. This is where we are supposed
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to do our thing, so we just print out our message with the name of each
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function.</p>
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<div class="doc_code"><pre>
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char Hello::ID = 0;
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</pre></div>
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<p> We initialize pass ID here. LLVM uses ID's address to identify pass so
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initialization value is not important.</p>
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<div class="doc_code"><pre>
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RegisterPass<Hello> X("<i>hello</i>", "<i>Hello World Pass</i>",
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false /* Only looks at CFG */,
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false /* Analysis Pass */);
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} <i>// end of anonymous namespace</i>
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</pre></div>
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<p>Lastly, we <a href="#registration">register our class</a> <tt>Hello</tt>,
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giving it a command line
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argument "<tt>hello</tt>", and a name "<tt>Hello World Pass</tt>".
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Last two RegisterPass arguments are optional. Their default value is false.
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If a pass walks CFG without modifying it then third argument is set to true.
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If a pass is an analysis pass, for example dominator tree pass, then true
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is supplied as fourth argument. </p>
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<p>As a whole, the <tt>.cpp</tt> file looks like:</p>
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<div class="doc_code"><pre>
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<b>#include</b> "<a href="http://llvm.org/doxygen/Pass_8h-source.html">llvm/Pass.h</a>"
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<b>#include</b> "<a href="http://llvm.org/doxygen/Function_8h-source.html">llvm/Function.h</a>"
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<b>using namespace llvm;</b>
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<b>namespace</b> {
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<b>struct Hello</b> : <b>public</b> <a href="#FunctionPass">FunctionPass</a> {
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static char ID;
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Hello() : FunctionPass(&ID) {}
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<b>virtual bool</b> <a href="#runOnFunction">runOnFunction</a>(Function &F) {
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llvm::cerr << "<i>Hello: </i>" << F.getName() << "\n";
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<b>return false</b>;
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}
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};
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char Hello::ID = 0;
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RegisterPass<Hello> X("<i>hello</i>", "<i>Hello World Pass</i>");
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}
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</pre></div>
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<p>Now that it's all together, compile the file with a simple "<tt>gmake</tt>"
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command in the local directory and you should get a new
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"<tt>Debug/lib/Hello.so</tt> file. Note that everything in this file is
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contained in an anonymous namespace: this reflects the fact that passes are self
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contained units that do not need external interfaces (although they can have
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them) to be useful.</p>
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</div>
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<!-- ======================================================================= -->
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<div class="doc_subsection">
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<a name="running">Running a pass with <tt>opt</tt></a>
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</div>
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<div class="doc_text">
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<p>Now that you have a brand new shiny shared object file, we can use the
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<tt>opt</tt> command to run an LLVM program through your pass. Because you
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registered your pass with the <tt>RegisterPass</tt> template, you will be able to
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use the <tt>opt</tt> tool to access it, once loaded.</p>
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<p>To test it, follow the example at the end of the <a
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href="GettingStarted.html">Getting Started Guide</a> to compile "Hello World" to
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LLVM. We can now run the bitcode file (<tt>hello.bc</tt>) for the program
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through our transformation like this (or course, any bitcode file will
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work):</p>
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<div class="doc_code"><pre>
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$ opt -load ../../../Debug/lib/Hello.so -hello < hello.bc > /dev/null
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Hello: __main
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Hello: puts
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Hello: main
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</pre></div>
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<p>The '<tt>-load</tt>' option specifies that '<tt>opt</tt>' should load your
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pass as a shared object, which makes '<tt>-hello</tt>' a valid command line
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argument (which is one reason you need to <a href="#registration">register your
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pass</a>). Because the hello pass does not modify the program in any
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interesting way, we just throw away the result of <tt>opt</tt> (sending it to
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<tt>/dev/null</tt>).</p>
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<p>To see what happened to the other string you registered, try running
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<tt>opt</tt> with the <tt>--help</tt> option:</p>
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<div class="doc_code"><pre>
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$ opt -load ../../../Debug/lib/Hello.so --help
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OVERVIEW: llvm .bc -> .bc modular optimizer
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USAGE: opt [options] <input bitcode>
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OPTIONS:
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Optimizations available:
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...
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-funcresolve - Resolve Functions
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-gcse - Global Common Subexpression Elimination
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-globaldce - Dead Global Elimination
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<b>-hello - Hello World Pass</b>
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-indvars - Canonicalize Induction Variables
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-inline - Function Integration/Inlining
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-instcombine - Combine redundant instructions
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...
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</pre></div>
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<p>The pass name get added as the information string for your pass, giving some
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documentation to users of <tt>opt</tt>. Now that you have a working pass, you
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would go ahead and make it do the cool transformations you want. Once you get
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it all working and tested, it may become useful to find out how fast your pass
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is. The <a href="#passManager"><tt>PassManager</tt></a> provides a nice command
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line option (<tt>--time-passes</tt>) that allows you to get information about
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the execution time of your pass along with the other passes you queue up. For
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example:</p>
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<div class="doc_code"><pre>
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$ opt -load ../../../Debug/lib/Hello.so -hello -time-passes < hello.bc > /dev/null
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Hello: __main
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Hello: puts
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Hello: main
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===============================================================================
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... Pass execution timing report ...
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===============================================================================
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Total Execution Time: 0.02 seconds (0.0479059 wall clock)
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---User Time--- --System Time-- --User+System-- ---Wall Time--- --- Pass Name ---
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0.0100 (100.0%) 0.0000 ( 0.0%) 0.0100 ( 50.0%) 0.0402 ( 84.0%) Bitcode Writer
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0.0000 ( 0.0%) 0.0100 (100.0%) 0.0100 ( 50.0%) 0.0031 ( 6.4%) Dominator Set Construction
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0.0000 ( 0.0%) 0.0000 ( 0.0%) 0.0000 ( 0.0%) 0.0013 ( 2.7%) Module Verifier
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<b> 0.0000 ( 0.0%) 0.0000 ( 0.0%) 0.0000 ( 0.0%) 0.0033 ( 6.9%) Hello World Pass</b>
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0.0100 (100.0%) 0.0100 (100.0%) 0.0200 (100.0%) 0.0479 (100.0%) TOTAL
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</pre></div>
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<p>As you can see, our implementation above is pretty fast :). The additional
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passes listed are automatically inserted by the '<tt>opt</tt>' tool to verify
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that the LLVM emitted by your pass is still valid and well formed LLVM, which
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hasn't been broken somehow.</p>
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|
|
<p>Now that you have seen the basics of the mechanics behind passes, we can talk
|
|
about some more details of how they work and how to use them.</p>
|
|
|
|
</div>
|
|
|
|
<!-- *********************************************************************** -->
|
|
<div class="doc_section">
|
|
<a name="passtype">Pass classes and requirements</a>
|
|
</div>
|
|
<!-- *********************************************************************** -->
|
|
|
|
<div class="doc_text">
|
|
|
|
<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
|
|
href="#basiccode">Hello World</a> example uses the <tt><a
|
|
href="#FunctionPass">FunctionPass</a></tt> class for its implementation, but we
|
|
did not discuss why or when this should occur. Here we talk about the classes
|
|
available, from the most general to the most specific.</p>
|
|
|
|
<p>When choosing a superclass for your Pass, you should choose the <b>most
|
|
specific</b> class possible, while still being able to meet the requirements
|
|
listed. This gives the LLVM Pass Infrastructure information necessary to
|
|
optimize how passes are run, so that the resultant compiler isn't unneccesarily
|
|
slow.</p>
|
|
|
|
</div>
|
|
|
|
<!-- ======================================================================= -->
|
|
<div class="doc_subsection">
|
|
<a name="ImmutablePass">The <tt>ImmutablePass</tt> class</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
|
|
<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>"
|
|
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
|
|
compiler configuration.</p>
|
|
|
|
<p>Although this pass class is very infrequently used, it is important for
|
|
providing information about the current target machine being compiled for, and
|
|
other static information that can affect the various transformations.</p>
|
|
|
|
<p><tt>ImmutablePass</tt>es never invalidate other transformations, are never
|
|
invalidated, and are never "run".</p>
|
|
|
|
</div>
|
|
|
|
<!-- ======================================================================= -->
|
|
<div class="doc_subsection">
|
|
<a name="ModulePass">The <tt>ModulePass</tt> class</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>The "<tt><a
|
|
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>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 <tt>ModulePass</tt>
|
|
subclasses, no optimization can be done for their execution. A module pass
|
|
can use function level passes (e.g. dominators) using getAnalysis interface
|
|
<tt> getAnalysis<DominatorTree>(Function)</tt>, if the function pass
|
|
does not require any module passes. </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="runOnSCC">The <tt>runOnSCC</tt> method</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
|
|
<div class="doc_code"><pre>
|
|
<b>virtual bool</b> runOnSCC(const std::vector<CallGraphNode *> &SCCM) = 0;
|
|
</pre></div>
|
|
|
|
<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>
|
|
|
|
<div class="doc_text">
|
|
|
|
<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.
|
|
<tt>FunctionPass</tt>'s do not require that they are executed in a particular
|
|
order, and <tt>FunctionPass</tt>'s do not modify external functions.</p>
|
|
|
|
<p>To be explicit, <tt>FunctionPass</tt> subclasses are not allowed to:</p>
|
|
|
|
<ol>
|
|
<li>Modify a Function other than the one currently being processed.</li>
|
|
<li>Add or remove Function's from the current Module.</li>
|
|
<li>Add or remove global variables from the current Module.</li>
|
|
<li>Maintain state across invocations of
|
|
<a href="#runOnFunction"><tt>runOnFunction</tt></a> (including global data)</li>
|
|
</ol>
|
|
|
|
<p>Implementing a <tt>FunctionPass</tt> is usually straightforward (See the <a
|
|
href="#basiccode">Hello World</a> pass for example). <tt>FunctionPass</tt>'s
|
|
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>
|
|
|
|
<div class="doc_text">
|
|
|
|
<div class="doc_code"><pre>
|
|
<b>virtual bool</b> doInitialization(Module &M);
|
|
</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
|
|
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 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).</p>
|
|
|
|
<p>A good example of how this method should be used is the <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
|
|
free functions that it needs, adding prototypes to the module if necessary.</p>
|
|
|
|
</div>
|
|
|
|
<!-- _______________________________________________________________________ -->
|
|
<div class="doc_subsubsection">
|
|
<a name="runOnFunction">The <tt>runOnFunction</tt> method</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
|
|
<div class="doc_code"><pre>
|
|
<b>virtual bool</b> runOnFunction(Function &F) = 0;
|
|
</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
|
|
be returned if the function is modified.</p>
|
|
|
|
</div>
|
|
|
|
<!-- _______________________________________________________________________ -->
|
|
<div class="doc_subsubsection">
|
|
<a name="doFinalization_mod">The <tt>doFinalization(Module
|
|
&)</tt> method</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
|
|
<div class="doc_code"><pre>
|
|
<b>virtual bool</b> doFinalization(Module &M);
|
|
</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="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>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
|
|
<p><tt>BasicBlockPass</tt>'s are just like <a
|
|
href="#FunctionPass"><tt>FunctionPass</tt></a>'s, except that they must limit
|
|
their scope of inspection and modification to a single basic block at a time.
|
|
As such, they are <b>not</b> allowed to do any of the following:</p>
|
|
|
|
<ol>
|
|
<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 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>
|
|
|
|
<p><tt>BasicBlockPass</tt>es are useful for traditional local and "peephole"
|
|
optimizations. They may override the same <a
|
|
href="#doInitialization_mod"><tt>doInitialization(Module &)</tt></a> and <a
|
|
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>
|
|
|
|
<div class="doc_text">
|
|
|
|
<div class="doc_code"><pre>
|
|
<b>virtual bool</b> doInitialization(Function &F);
|
|
</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 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="doc_subsubsection">
|
|
<a name="runOnBasicBlock">The <tt>runOnBasicBlock</tt> method</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
|
|
<div class="doc_code"><pre>
|
|
<b>virtual bool</b> runOnBasicBlock(BasicBlock &BB) = 0;
|
|
</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
|
|
parameter, and are not allowed to modify the CFG. A true value must be returned
|
|
if the basic block is modified.</p>
|
|
|
|
</div>
|
|
|
|
<!-- _______________________________________________________________________ -->
|
|
<div class="doc_subsubsection">
|
|
<a name="doFinalization_fn">The <tt>doFinalization(Function &)</tt>
|
|
method</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
|
|
<div class="doc_code"><pre>
|
|
<b>virtual bool</b> doFinalization(Function &F);
|
|
</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="#runOnBasicBlock"><tt>runOnBasicBlock</tt></a> for every BasicBlock in the
|
|
program being compiled. This can be used to perform per-function
|
|
finalization.</p>
|
|
|
|
</div>
|
|
|
|
<!-- ======================================================================= -->
|
|
<div class="doc_subsection">
|
|
<a name="MachineFunctionPass">The <tt>MachineFunctionPass</tt> class</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
|
|
<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>
|
|
|
|
<ol>
|
|
<li>Modify any LLVM Instructions, BasicBlocks or Functions.</li>
|
|
<li>Modify a MachineFunction other than the one currently being processed.</li>
|
|
<li>Add or remove MachineFunctions from the current Module.</li>
|
|
<li>Add or remove global variables from the current Module.</li>
|
|
<li>Maintain state across invocations of <a
|
|
href="#runOnMachineFunction"><tt>runOnMachineFunction</tt></a> (including global
|
|
data)</li>
|
|
</ol>
|
|
|
|
</div>
|
|
|
|
<!-- _______________________________________________________________________ -->
|
|
<div class="doc_subsubsection">
|
|
<a name="runOnMachineFunction">The <tt>runOnMachineFunction(MachineFunction
|
|
&MF)</tt> method</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
|
|
<div class="doc_code"><pre>
|
|
<b>virtual bool</b> runOnMachineFunction(MachineFunction &MF) = 0;
|
|
</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
|
|
work of your <tt>MachineFunctionPass</tt>.</p>
|
|
|
|
<p>The <tt>runOnMachineFunction</tt> method is called on every
|
|
<tt>MachineFunction</tt> in a <tt>Module</tt>, so that the
|
|
<tt>MachineFunctionPass</tt> may perform optimizations on the machine-dependent
|
|
representation of the function. If you want to get at the LLVM <tt>Function</tt>
|
|
for the <tt>MachineFunction</tt> you're working on, use
|
|
<tt>MachineFunction</tt>'s <tt>getFunction()</tt> accessor method -- but
|
|
remember, you may not modify the LLVM <tt>Function</tt> or its contents from a
|
|
<tt>MachineFunctionPass</tt>.</p>
|
|
|
|
</div>
|
|
|
|
<!-- *********************************************************************** -->
|
|
<div class="doc_section">
|
|
<a name="registration">Pass registration</a>
|
|
</div>
|
|
<!-- *********************************************************************** -->
|
|
|
|
<div class="doc_text">
|
|
|
|
<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
|
|
what it does. Here we discuss how and why passes are registered.</p>
|
|
|
|
<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, 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 want your pass to be easily dumpable, you should
|
|
implement the virtual <tt>print</tt> method:</p>
|
|
|
|
</div>
|
|
|
|
<!-- _______________________________________________________________________ -->
|
|
<div class="doc_subsubsection">
|
|
<a name="print">The <tt>print</tt> method</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
|
|
<div class="doc_code"><pre>
|
|
<b>virtual void</b> print(std::ostream &O, <b>const</b> Module *M) <b>const</b>;
|
|
</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. Use the <tt>opt -analyze</tt> argument to invoke this method.</p>
|
|
|
|
<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
|
|
debugger), so it should only be used to enhance debug output, it should not be
|
|
depended on.</p>
|
|
|
|
</div>
|
|
|
|
<!-- *********************************************************************** -->
|
|
<div class="doc_section">
|
|
<a name="interaction">Specifying interactions between passes</a>
|
|
</div>
|
|
<!-- *********************************************************************** -->
|
|
|
|
<div class="doc_text">
|
|
|
|
<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
|
|
the various passes. To track this, each pass can declare the set of passes that
|
|
are required to be executed before the current pass, and the passes which are
|
|
invalidated by the current pass.</p>
|
|
|
|
<p>Typically this functionality is used to require that analysis results are
|
|
computed before your pass is run. Running arbitrary transformation passes can
|
|
invalidate the computed analysis results, which is what the invalidation set
|
|
specifies. If a pass does not implement the <tt><a
|
|
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">
|
|
<a name="getAnalysisUsage">The <tt>getAnalysisUsage</tt> method</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
|
|
<div class="doc_code"><pre>
|
|
<b>virtual void</b> getAnalysisUsage(AnalysisUsage &Info) <b>const</b>;
|
|
</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.org/doxygen/classllvm_1_1AnalysisUsage.html">AnalysisUsage</a></tt>
|
|
object with information about which passes are required and not invalidated. To
|
|
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.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 class="doc_code"><pre>
|
|
<i>// This example modifies the program, but does not modify the CFG</i>
|
|
<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.org/doxygen/classllvm_1_1LoopInfo.html">LoopInfo</a>>();
|
|
}
|
|
</pre></div>
|
|
|
|
</div>
|
|
|
|
<!-- _______________________________________________________________________ -->
|
|
<div class="doc_subsubsection">
|
|
<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 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>
|
|
|
|
<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.
|
|
|
|
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_section">
|
|
<a name="analysisgroup">Implementing Analysis Groups</a>
|
|
</div>
|
|
<!-- *********************************************************************** -->
|
|
|
|
<div class="doc_text">
|
|
|
|
<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.
|
|
For many applications, this is great, for others, more flexibility is
|
|
required.</p>
|
|
|
|
<p>In particular, some analyses are defined such that there is a single simple
|
|
interface to the analysis results, but multiple ways of calculating them.
|
|
Consider alias analysis for example. The most trivial alias analysis returns
|
|
"may alias" for any alias query. The most sophisticated analysis a
|
|
flow-sensitive, context-sensitive interprocedural analysis that can take a
|
|
significant amount of time to execute (and obviously, there is a lot of room
|
|
between these two extremes for other implementations). To cleanly support
|
|
situations like this, the LLVM Pass Infrastructure supports the notion of
|
|
Analysis Groups.</p>
|
|
|
|
</div>
|
|
|
|
<!-- _______________________________________________________________________ -->
|
|
<div class="doc_subsubsection">
|
|
<a name="agconcepts">Analysis Group Concepts</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
|
|
<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
|
|
just like passes, but unlike passes, they need not derive from the <tt>Pass</tt>
|
|
class. An analysis group may have one or more implementations, one of which is
|
|
the "default" implementation.</p>
|
|
|
|
<p>Analysis groups are used by client passes just like other passes are: the
|
|
<tt>AnalysisUsage::addRequired()</tt> and <tt>Pass::getAnalysis()</tt> methods.
|
|
In order to resolve this requirement, the <a href="#passmanager">PassManager</a>
|
|
scans the available passes to see if any implementations of the analysis group
|
|
are available. If none is available, the default implementation is created for
|
|
the pass to use. All standard rules for <A href="#interaction">interaction
|
|
between passes</a> still apply.</p>
|
|
|
|
<p>Although <a href="#registration">Pass Registration</a> is optional for normal
|
|
passes, all analysis group implementations must be registered, and must use the
|
|
<A href="#registerag"><tt>RegisterAnalysisGroup</tt></a> template to join the
|
|
implementation pool. Also, a default implementation of the interface
|
|
<b>must</b> be registered with <A
|
|
href="#registerag"><tt>RegisterAnalysisGroup</tt></a>.</p>
|
|
|
|
<p>As a concrete example of an Analysis Group in action, consider the <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.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.org/doxygen/classllvm_1_1AliasAnalysis.html">AliasAnalysis</a></tt>
|
|
interface (for example the <tt><a
|
|
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>
|
|
|
|
<p>From the user's perspective, commands work just like normal. Issuing the
|
|
command '<tt>opt -gcse ...</tt>' will cause the <tt>basicaa</tt> class to be
|
|
instantiated and added to the pass sequence. Issuing the command '<tt>opt
|
|
-somefancyaa -gcse ...</tt>' will cause the <tt>gcse</tt> pass to use the
|
|
<tt>somefancyaa</tt> alias analysis (which doesn't actually exist, it's just a
|
|
hypothetical example) instead.</p>
|
|
|
|
</div>
|
|
|
|
<!-- _______________________________________________________________________ -->
|
|
<div class="doc_subsubsection">
|
|
<a name="registerag">Using <tt>RegisterAnalysisGroup</tt></a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>The <tt>RegisterAnalysisGroup</tt> template is used to register the analysis
|
|
group itself as well as add pass implementations to the analysis group. First,
|
|
an analysis should be registered, with a human readable name provided for it.
|
|
Unlike registration of passes, there is no command line argument to be specified
|
|
for the Analysis Group Interface itself, because it is "abstract":</p>
|
|
|
|
<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>
|
|
|
|
<div class="doc_code"><pre>
|
|
<b>namespace</b> {
|
|
//<i> Analysis Group implementations <b>must</b> be registered normally...</i>
|
|
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.org/doxygen/classllvm_1_1AliasAnalysis.html">AliasAnalysis</a>> C(B);
|
|
}
|
|
</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.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>
|
|
|
|
<div class="doc_code"><pre>
|
|
<b>namespace</b> {
|
|
//<i> Analysis Group implementations <b>must</b> be registered normally...</i>
|
|
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.org/doxygen/classllvm_1_1AliasAnalysis.html">AliasAnalysis</a>, <b>true</b>> E(D);
|
|
}
|
|
</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. 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>
|
|
<!-- *********************************************************************** -->
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>The <a
|
|
href="http://llvm.org/doxygen/PassManager_8h-source.html"><tt>PassManager</tt></a>
|
|
<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
|
|
passes.</p>
|
|
|
|
<p>The <tt>PassManager</tt> does two main things to try to reduce the execution
|
|
time of a series of passes:</p>
|
|
|
|
<ol>
|
|
<li><b>Share analysis results</b> - The PassManager attempts to avoid
|
|
recomputing analysis results as much as possible. This means keeping track of
|
|
which analyses are available already, which analyses get invalidated, and which
|
|
analyses are needed to be run for a pass. An important part of work is that the
|
|
<tt>PassManager</tt> tracks the exact lifetime of all analysis results, allowing
|
|
it to <a href="#releaseMemory">free memory</a> allocated to holding analysis
|
|
results as soon as they are no longer needed.</li>
|
|
|
|
<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
|
|
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,
|
|
etc... until the entire program has been run through the passes.
|
|
|
|
<p>This improves the cache behavior of the compiler, because it is only touching
|
|
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.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>
|
|
|
|
<p>The effectiveness of the <tt>PassManager</tt> is influenced directly by how
|
|
much information it has about the behaviors of the passes it is scheduling. For
|
|
example, the "preserved" set is intentionally conservative in the face of an
|
|
unimplemented <a href="#getAnalysisUsage"><tt>getAnalysisUsage</tt></a> method.
|
|
Not implementing when it should be implemented will have the effect of not
|
|
allowing any analysis results to live across the execution of your pass.</p>
|
|
|
|
<p>The <tt>PassManager</tt> class exposes a <tt>--debug-pass</tt> command line
|
|
options that is useful for debugging pass execution, seeing how things work, and
|
|
diagnosing when you should be preserving more analyses than you currently are
|
|
(To get information about all of the variants of the <tt>--debug-pass</tt>
|
|
option, just type '<tt>opt --help-hidden</tt>').</p>
|
|
|
|
<p>By using the <tt>--debug-pass=Structure</tt> option, for example, we can see
|
|
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>
|
|
|
|
<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
|
|
Immediate Dominators Construction
|
|
Global Common Subexpression Elimination
|
|
-- Immediate Dominators Construction
|
|
-- Global Common Subexpression Elimination
|
|
Natural Loop Construction
|
|
Loop Invariant Code Motion
|
|
-- Natural Loop Construction
|
|
-- Loop Invariant Code Motion
|
|
Module Verifier
|
|
-- Dominator Set Construction
|
|
-- Module Verifier
|
|
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
|
|
GCSE uses dominator and immediate dominator information to do its job. The LICM
|
|
pass uses natural loop information, which uses dominator sets, but not immediate
|
|
dominators. Because immediate dominators are no longer useful after the GCSE
|
|
pass, it is immediately destroyed. The dominator sets are then reused to
|
|
compute natural loop information, which is then used by the LICM pass.</p>
|
|
|
|
<p>After the LICM pass, the module verifier runs (which is automatically added
|
|
by the '<tt>opt</tt>' tool), which uses the dominator set to check that the
|
|
resultant LLVM code is well formed. After it finishes, the dominator set
|
|
information is destroyed, after being computed once, and shared by three
|
|
passes.</p>
|
|
|
|
<p>Lets see how this changes when we run the <a href="#basiccode">Hello
|
|
World</a> pass in between the two passes:</p>
|
|
|
|
<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
|
|
Immediate Dominators Construction
|
|
Global Common Subexpression Elimination
|
|
<b>-- Dominator Set Construction</b>
|
|
-- Immediate Dominators Construction
|
|
-- Global Common Subexpression Elimination
|
|
<b> Hello World Pass
|
|
-- Hello World Pass
|
|
Dominator Set Construction</b>
|
|
Natural Loop Construction
|
|
Loop Invariant Code Motion
|
|
-- Natural Loop Construction
|
|
-- Loop Invariant Code Motion
|
|
Module Verifier
|
|
-- Dominator Set Construction
|
|
-- Module Verifier
|
|
Bitcode Writer
|
|
--Bitcode Writer
|
|
Hello: __main
|
|
Hello: puts
|
|
Hello: main
|
|
</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>
|
|
|
|
<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></div>
|
|
|
|
<p>Now when we run our pass, we get this output:</p>
|
|
|
|
<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
|
|
Dominator Set Construction
|
|
Immediate Dominators Construction
|
|
Global Common Subexpression Elimination
|
|
-- Immediate Dominators Construction
|
|
-- Global Common Subexpression Elimination
|
|
Hello World Pass
|
|
-- Hello World Pass
|
|
Natural Loop Construction
|
|
Loop Invariant Code Motion
|
|
-- Loop Invariant Code Motion
|
|
-- Natural Loop Construction
|
|
Module Verifier
|
|
-- Dominator Set Construction
|
|
-- Module Verifier
|
|
Bitcode Writer
|
|
--Bitcode Writer
|
|
Hello: __main
|
|
Hello: puts
|
|
Hello: main
|
|
</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>
|
|
|
|
<!-- _______________________________________________________________________ -->
|
|
<div class="doc_subsubsection">
|
|
<a name="releaseMemory">The <tt>releaseMemory</tt> method</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
|
|
<div class="doc_code"><pre>
|
|
<b>virtual void</b> releaseMemory();
|
|
</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
|
|
object itself is effectively the entire duration of the compilation process, we
|
|
need some way to free analysis results when they are no longer useful. The
|
|
<tt>releaseMemory</tt> virtual method is the way to do this.</p>
|
|
|
|
<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
|
|
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>
|
|
</div>
|
|
<!-- *********************************************************************** -->
|
|
|
|
<div class="doc_text">
|
|
|
|
<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
|
|
not been loaded yet, and second of all there are problems with inlined functions
|
|
in shared objects. Here are some suggestions to debugging your pass with
|
|
GDB.</p>
|
|
|
|
<p>For sake of discussion, I'm going to assume that you are debugging a
|
|
transformation invoked by <tt>opt</tt>, although nothing described here depends
|
|
on that.</p>
|
|
|
|
</div>
|
|
|
|
<!-- _______________________________________________________________________ -->
|
|
<div class="doc_subsubsection">
|
|
<a name="breakpoint">Setting a breakpoint in your pass</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>First thing you do is start <tt>gdb</tt> on the <tt>opt</tt> process:</p>
|
|
|
|
<div class="doc_code"><pre>
|
|
$ <b>gdb opt</b>
|
|
GNU gdb 5.0
|
|
Copyright 2000 Free Software Foundation, Inc.
|
|
GDB is free software, covered by the GNU General Public License, and you are
|
|
welcome to change it and/or distribute copies of it under certain conditions.
|
|
Type "show copying" to see the conditions.
|
|
There is absolutely no warranty for GDB. Type "show warranty" for details.
|
|
This GDB was configured as "sparc-sun-solaris2.6"...
|
|
(gdb)
|
|
</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
|
|
(the shared object isn't loaded until runtime), we must execute the process, and
|
|
have it stop before it invokes our pass, but after it has loaded the shared
|
|
object. The most foolproof way of doing this is to set a breakpoint in
|
|
<tt>PassManager::run</tt> and then run the process with the arguments you
|
|
want:</p>
|
|
|
|
<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/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></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
|
|
or do other standard debugging stuff.</p>
|
|
|
|
</div>
|
|
|
|
<!-- _______________________________________________________________________ -->
|
|
<div class="doc_subsubsection">
|
|
<a name="debugmisc">Miscellaneous Problems</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>Once you have the basics down, there are a couple of problems that GDB has,
|
|
some with solutions, some without.</p>
|
|
|
|
<ul>
|
|
<li>Inline functions have bogus stack information. In general, GDB does a
|
|
pretty good job getting stack traces and stepping through inline functions.
|
|
When a pass is dynamically loaded however, it somehow completely loses this
|
|
capability. The only solution I know of is to de-inline a function (move it
|
|
from the body of a class to a .cpp file).</li>
|
|
|
|
<li>Restarting the program breaks breakpoints. After following the information
|
|
above, you have succeeded in getting some breakpoints planted in your pass. Nex
|
|
thing you know, you restart the program (i.e., you type '<tt>run</tt>' again),
|
|
and you start getting errors about breakpoints being unsettable. The only way I
|
|
have found to "fix" this problem is to <tt>delete</tt> the breakpoints that are
|
|
already set in your pass, run the program, and re-set the breakpoints once
|
|
execution stops in <tt>PassManager::run</tt>.</li>
|
|
|
|
</ul>
|
|
|
|
<p>Hopefully these tips will help with common case debugging situations. If
|
|
you'd like to contribute some tips of your own, just contact <a
|
|
href="mailto:sabre@nondot.org">Chris</a>.</p>
|
|
|
|
</div>
|
|
|
|
<!-- *********************************************************************** -->
|
|
<div class="doc_section">
|
|
<a name="future">Future extensions planned</a>
|
|
</div>
|
|
<!-- *********************************************************************** -->
|
|
|
|
<div class="doc_text">
|
|
|
|
<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">
|
|
<a name="SMP">Multithreaded LLVM</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
|
|
<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
|
|
the semantics defined for passes above (specifically they cannot maintain state
|
|
across invocations of their <tt>run*</tt> methods), a nice clean way to
|
|
implement a multithreaded compiler would be for the <tt>PassManager</tt> class
|
|
to create multiple instances of each pass object, and allow the separate
|
|
instances to be hacking on different parts of the program at the same time.</p>
|
|
|
|
<p>This implementation would prevent each of the passes from having to implement
|
|
multithreaded constructs, requiring only the LLVM core to have locking in a few
|
|
places (for global resources). Although this is a simple extension, we simply
|
|
haven't had time (or multiprocessor machines, thus a reason) to implement this.
|
|
Despite that, we have kept the LLVM passes SMP ready, and you should too.</p>
|
|
|
|
</div>
|
|
|
|
<!-- *********************************************************************** -->
|
|
<hr>
|
|
<address>
|
|
<a href="http://jigsaw.w3.org/css-validator/check/referer"><img
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<a href="mailto:sabre@nondot.org">Chris Lattner</a><br>
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<a href="http://llvm.org">The LLVM Compiler Infrastructure</a><br>
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