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Initial checkin of the "Writting an LLVM Pass" document git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@3267 91177308-0d34-0410-b5e6-96231b3b80d8
2002-08-08 20:11:18 +00:00
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html><head><title>Writing an LLVM Pass</title></head>
<!--
I. General Structure of an LLVM Program
I.1 "What is a 'Value'?": Value & User class
I.2 Type & Derived Types
I.3 GlobalVariable, Function
I.4 BasicBlock
I.5 Instruction & Subclasses
1.6 Argument
1.7 Constants
III. Useful things to know about the LLVM source base:
III.1 Useful links that introduce the STL
III.2 isa<>, cast<>, dyn_cast<>
III.3 Makefiles, useful options
III.4 How to use opt & analyze to debug stuff
III.5 How to write a regression test
III.6 DEBUG() and Statistics (-debug & -stats)
III.7 The -time-passes option
III.8 ... more as needed ...
I think that writing Section #1 would be very helpful and that's the most
stable portion of the sourcebase. #3 can be started on, but will probably
just grow as time goes on. I'd like to do Section #2 once I finish some
changes up that effect it.
-->
<body bgcolor=white>
<table width="100%" bgcolor="#330077" border=0 cellpadding=4 cellspacing=0>
<tr><td>&nbsp; <font size=+3 color="#EEEEFF" face="Georgia,Palatino,Times,Roman"><b>Writing an LLVM Pass</b></font></td>
</tr></table>
<ol>
<li><a href="#introduction">Introduction - What is a pass?</a>
<li><a href="#quickstart">Quick Start - Writing hello world</a>
<ul>
<li><a href="#makefile">Setting up the build environment</a>
<li><a href="#basiccode">Basic code required</a>
<li><a href="#running">Running a pass with <tt>opt</tt>
or <tt>analyze</tt></a>
</ul>
<li><a href="#passtype">Pass classes and requirements</a>
<ul>
<li><a href="#Pass">The <tt>Pass</tt> class</a>
<ul>
<li><a href="#run">The <tt>run</tt> method</a>
</ul>
<li><a href="#FunctionPass">The <tt>FunctionPass</tt> class</a>
<ul>
<li><a href="#doInitialization">The <tt>doInitialization</tt> method</a>
<li><a href="#runOnFunction">The <tt>runOnFunction</tt> method</a>
<li><a href="#doFinalization">The <tt>doFinalization</tt> method</a>
</ul>
<li><a href="#BasicBlockPass">The <tt>BasicBlockPass</tt> class</a>
<ul>
<li><a href="#runOnBasicBlock">The <tt>runOnBasicBlock</tt> method</a>
</ul>
</ul>
<li><a href="#registration">Pass Registration</a>
<ul>
<li><a href="#print">The <tt>print</tt> method</a>
</ul>
<li><a href="#interaction">Specifying interactions between passes</a>
<ul>
<li><a href="#getAnalysisUsage">The <tt>getAnalysisUsage</tt> method</a>
<li><a href="#getAnalysis">The <tt>getAnalysis</tt> method</a>
</ul>
<li><a href="#passmanager">What PassManager does</a>
<ul>
<li><a href="#releaseMemory">The <tt>releaseMemory</tt> method</a>
</ul>
<li><a href="#future">Future extensions planned</a>
<ul>
<li><a href="#SMP">Multithreaded LLVM</a>
<li><a href="#ModuleSource">A new <tt>ModuleSource</tt> interface</a>
<li><a href="#PassFunctionPass"><tt>Pass</tt>'s requiring <tt>FunctionPass</tt>'s</a>
</ul>
</ol><p>
<!-- *********************************************************************** -->
<table width="100%" bgcolor="#330077" border=0 cellpadding=4 cellspacing=0>
<tr><td align=center><font color="#EEEEFF" size=+2 face="Georgia,Palatino"><b>
<a name="introduction">Introduction - What is a pass?
</b></font></td></tr></table><ul>
<!-- *********************************************************************** -->
The LLVM Pass Framework is an important part of the LLVM system, because LLVM
passes are where the interesting parts of the compiler exist. Passes perform
the transformations and optimizations that make up the compiler, they build
the analysis results that are used by these transformations, and they are, above
all, a structuring technique for compiler code.<p>
All LLVM passes are subclasses of the <tt><a
href="http://llvm.cs.uiuc.edu/doxygen/classPass.html">Pass</a></tt> class, which
implement functionality by overriding virtual methods inherited from
<tt>Pass</tt>. Depending on how your pass works, you may be able to inherit
from the <tt><a
href="http://llvm.cs.uiuc.edu/doxygen/structFunctionPass.html">FunctionPass</a></tt>
or <tt><a
href="http://llvm.cs.uiuc.edu/doxygen/structBasicBlockPass.html">BasicBlockPass</a></tt>,
which gives the system more information about what your pass does, and how it
can be combined with other passes. One of the main features of the LLVM Pass
Framework is that it schedules passes to run in an efficient way based on the
constraints that your pass has.<p>
We start by showing you how to construct a pass, everything from setting up the
code, to compiling, loading, and executing it. After the basics are down, more
advanced features are discussed.<p>
<!-- *********************************************************************** -->
</ul><table width="100%" bgcolor="#330077" border=0 cellpadding=4 cellspacing=0>
<tr><td align=center><font color="#EEEEFF" size=+2 face="Georgia,Palatino"><b>
<a name="quickstart">Quick Start - Writing hello world
</b></font></td></tr></table><ul>
<!-- *********************************************************************** -->
Here we describe how to write the "hello world" of passes. The "Hello" pass is
designed to simply print out the name of non-external functions that exist in
the program being compiled. It does not modify the program at all, just
inspects it. The source code and files for this pass are available in the LLVM
source tree in the <tt>lib/Transforms/Hello</tt> directory.<p>
<!-- ======================================================================= -->
</ul><table width="100%" bgcolor="#441188" border=0 cellpadding=4 cellspacing=0>
<tr><td>&nbsp;</td><td width="100%">&nbsp;
<font color="#EEEEFF" face="Georgia,Palatino"><b>
<a name="makefile">Setting up the build environment
</b></font></td></tr></table><ul>
First thing you need to do is create a new directory somewhere in the LLVM
source base. For this example, we'll assume that you made
"<tt>lib/Transforms/Hello</tt>". The first thing you must do is set up a build
script (Makefile) that will compile the source code for the new pass. To do
this, copy this into "<tt>Makefile</tt>":<p>
</ul><hr><ul><pre>
# Makefile for hello pass
LEVEL = ../../.. # Path to top level of LLVM heirarchy
LIBRARYNAME = hello # Name of the library to build
SHARED_LIBRARY = 1 # Build a dynamically loadable shared object
include $(LEVEL)/Makefile.common # Include the makefile implementation stuff
</pre></ul><hr><ul><p>
This makefile specifies that all of the <tt>.cpp</tt> files in the current
directory are to be compiled and linked together into a
<tt>lib/Debug/libhello.so</tt> shared object that can be dynamically loaded by
the <tt>opt</tt> or <tt>analyze</tt> tools.<p>
Now that we have the build scripts set up, we just need to write the code for
the pass itself.<p>
<!-- ======================================================================= -->
</ul><table width="100%" bgcolor="#441188" border=0 cellpadding=4 cellspacing=0>
<tr><td>&nbsp;</td><td width="100%">&nbsp;
<font color="#EEEEFF" face="Georgia,Palatino"><b>
<a name="basiccode">Basic code required
</b></font></td></tr></table><ul>
Now that we have a way to compile our new pass, we just have to write it. Start
out with:<p>
<pre>
<b>#include</b> "<a href="http://llvm.cs.uiuc.edu/doxygen/Pass_8h-source.html">llvm/Pass.h</a>"
<b>#include</b> "<a href="http://llvm.cs.uiuc.edu/doxygen/Function_8h-source.html">llvm/Function.h</a>"
</pre>
Which are needed because we are writing a <tt><a
href="http://llvm.cs.uiuc.edu/doxygen/classPass.html">Pass</a></tt>, and we are
operating on <tt><a
href="http://llvm.cs.uiuc.edu/doxygen/classFunction.html">Function</a></tt>'s.<p>
Next we have:<p>
<pre>
<b>namespace</b> {
</pre><p>
... which starts out an anonymous namespace. Anonymous namespaces are to C++
what the "<tt>static</tt>" keyword is to C (at global scope). It makes the
things declared inside of the anonymous namespace only visible to the current
file. If you're not familiar with them, consult a decent C++ book for more
information.<p>
Next, we declare our pass itself:<p>
<pre>
<b>struct</b> Hello : <b>public</b> <a href="#FunctionPass">FunctionPass</a> {
</pre><p>
This declares a "<tt>Hello</tt>" class that is a subclass of <tt><a
href="http://llvm.cs.uiuc.edu/doxygen/structFunctionPass.html">FunctionPass</a></tt>.
The different builting pass subclasses are described in detail <a
href="#passtype">later</a>, but for now, know that <a href="#FunctionPass"><tt>FunctionPass</tt></a>'s
operate a function at a time.<p>
<pre>
<b>virtual bool</b> <a href="#runOnFunction">runOnFunction</a>(Function &F) {
std::cerr &lt;&lt; "<i>Hello: </i>" &lt;&lt; F.getName() &lt;&lt; "\n";
<b>return false</b>;
}
}; <i>// end of struct Hello</i>
</pre>
We declare a "<a href="#runOnFunction"><tt>runOnFunction</tt></a>" method, which
overloads an abstract virtual method inherited from <a
href="#FunctionPass"><tt>FunctionPass</tt></a>. This is where we are supposed
to do our thing, so we just print out our message with the name of each
function.<p>
<pre>
RegisterOpt&lt;Hello&gt; X("<i>hello</i>", "<i>Hello World Pass</i>");
} <i>// end of anonymous namespace</i>
</pre><p>
Lastly, we register our class <tt>Hello</tt>, giving it a command line argument
"<tt>hello</tt>", and a name "<tt>Hello World Pass</tt>". There are several
different ways of <a href="#registration">registering your pass</a>, depending
on what it is to be used for. For "optimizations" we use the
<tt>RegisterOpt</tt> template.<p>
As a whole, the <tt>.cpp</tt> file looks like:<p>
<pre>
<b>#include</b> "<a href="http://llvm.cs.uiuc.edu/doxygen/Pass_8h-source.html">llvm/Pass.h</a>"
<b>#include</b> "<a href="http://llvm.cs.uiuc.edu/doxygen/Function_8h-source.html">llvm/Function.h</a>"
<b>namespace</b> {
<b>struct Hello</b> : <b>public</b> <a href="#FunctionPass">FunctionPass</a> {
<b>virtual bool</b> <a href="#runOnFunction">runOnFunction</a>(Function &F) {
std::cerr &lt;&lt; "<i>Hello: </i>" &lt;&lt; F.getName() &lt;&lt; "\n";
<b>return false</b>;
}
};
RegisterOpt&lt;Hello&gt; X("<i>hello</i>", "<i>Hello World Pass</i>");
}
</pre><p>
Now that it's all together, compile the file with a simple "<tt>gmake</tt>"
command in the local directory and you should get a new
"<tt>lib/Debug/libhello.so</tt> file. Note that everything in this file is
contained in an anonymous namespace: this reflects the fact that passes are self
contained units that do not need external interfaces (although they can have
them) to be useful.<p>
<!-- ======================================================================= -->
</ul><table width="100%" bgcolor="#441188" border=0 cellpadding=4 cellspacing=0>
<tr><td>&nbsp;</td><td width="100%">&nbsp;
<font color="#EEEEFF" face="Georgia,Palatino"><b>
<a name="running">Running a pass with <tt>opt</tt> or <tt>analyze</tt>
</b></font></td></tr></table><ul>
Now that you have a brand new shiny <tt>.so</tt> file, we can use the
<tt>opt</tt> command to run an LLVM program through your pass. Because you
registered your pass with the <tt>RegisterOpt</tt> template, you will be able to
use the <tt>opt</tt> tool to access it, once loaded.<p>
To test it, follow the example at the end of the <a
href="GettingStarted.html">Getting Started Guide</a> to compile "Hello World" to
LLVM. We can now run the bytecode file (<tt>hello.bc</tt>) for the program
through our transformation like this (or course, any bytecode file will
work):<p>
<pre>
$ opt -load ../../../lib/Debug/libhello.so -hello &lt; hello.bc &gt; /dev/null
Hello: __main
Hello: puts
Hello: main
</pre><p>
The '<tt>-load</tt>' option specifies that '<tt>opt</tt>' should load your pass
as a shared object, which makes '<tt>-hello</tt>' a valid command line argument
(which is one reason you need to <a href="#registration">register your
pass</a>). Because the hello pass does not modify the program in any
interesting way, we just throw away the result of <tt>opt</tt> (sending it to
<tt>/dev/null</tt>).<p>
To see what happened to the other string you registered, try running
<tt>opt</tt> with the <tt>--help</tt> option:<p>
<pre>
$ opt -load ../../../lib/Debug/libhello.so --help
OVERVIEW: llvm .bc -&gt; .bc modular optimizer
USAGE: opt [options] &lt;input bytecode&gt;
OPTIONS:
Optimizations available:
...
-funcresolve - Resolve Functions
-gcse - Global Common Subexpression Elimination
-globaldce - Dead Global Elimination
<b>-hello - Hello World Pass</b>
-indvars - Cannonicalize Induction Variables
-inline - Function Integration/Inlining
-instcombine - Combine redundant instructions
...
</pre><p>
The pass name get added as the information string for your pass, giving some
documentation to users of <tt>opt</tt>. Now that you have a working pass, you
would go ahead and make it do the cool transformations you want. Once you get
it all working and tested, it may become useful to find out how fast your pass
is. The <a href="#passManager"><tt>PassManager</tt></a> provides a nice command
line option (<tt>--time-passes</tt>) that allows you to get information about
the execution time of your pass along with the other passes you queue up. For
example:<p>
<pre>
$ opt -load ../../../lib/Debug/libhello.so -hello -time-passes &lt; hello.bc &gt; /dev/null
Hello: __main
Hello: puts
Hello: main
===============================================================================
... Pass execution timing report ...
===============================================================================
Total Execution Time: 0.02 seconds (0.0479059 wall clock)
---User Time--- --System Time-- --User+System-- ---Wall Time--- --- Pass Name ---
0.0100 (100.0%) 0.0000 ( 0.0%) 0.0100 ( 50.0%) 0.0402 ( 84.0%) Bytecode Writer
0.0000 ( 0.0%) 0.0100 (100.0%) 0.0100 ( 50.0%) 0.0031 ( 6.4%) Dominator Set Construction
0.0000 ( 0.0%) 0.0000 ( 0.0%) 0.0000 ( 0.0%) 0.0013 ( 2.7%) Module Verifier
<b> 0.0000 ( 0.0%) 0.0000 ( 0.0%) 0.0000 ( 0.0%) 0.0033 ( 6.9%) Hello World Pass</b>
0.0100 (100.0%) 0.0100 (100.0%) 0.0200 (100.0%) 0.0479 (100.0%) TOTAL
</pre><p>
As you can see, our implementation above is pretty fast :). The additional
passes listed are automatically inserted by the '<tt>opt</tt>' tool to verify
that the LLVM emitted by your pass is still valid and well formed LLVM, which
hasn't been broken somehow.
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>
<!-- *********************************************************************** -->
</ul><table width="100%" bgcolor="#330077" border=0 cellpadding=4 cellspacing=0>
<tr><td align=center><font color="#EEEEFF" size=+2 face="Georgia,Palatino"><b>
<a name="passtype">Pass classes and requirements
</b></font></td></tr></table><ul>
<!-- *********************************************************************** -->
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>
When choosing a superclass for your Pass, you should choose the most specific
class possible, while still being able to meet the requirements listed. This
gives the LLVM Pass Infrastructure information neccesary to optimize how passes
are run, so that the resultant compiler isn't unneccesarily slow.<p>
<!-- ======================================================================= -->
</ul><table width="100%" bgcolor="#441188" border=0 cellpadding=4 cellspacing=0>
<tr><td>&nbsp;</td><td width="100%">&nbsp;
<font color="#EEEEFF" face="Georgia,Palatino"><b>
<a name="Pass">The <tt>Pass</tt> class
</b></font></td></tr></table><ul>
The "<tt><a href="http://llvm.cs.uiuc.edu/doxygen/classPass.html">Pass</a></tt>"
class is the most general of all superclasses that you can use. Deriving from
<tt>Pass</tt> indicates that your pass uses the entire program as a unit,
refering to function bodies in no predictable order, or adding and removing
functions. Because nothing is known about the behavior of direct <tt>Pass</tt>
subclasses, no optimization can be done for their execution.<p>
To write a correct <tt>Pass</tt> subclass, derive from <tt>Pass</tt> and
overload the <tt>run</tt> method with the following signature:<p>
<!-- _______________________________________________________________________ -->
</ul><h4><a name="run"><hr size=0>The <tt>run</tt> method</h4><ul>
<pre>
<b>virtual bool</b> run(Module &amp;M) = 0;
</pre><p>
The <tt>run</tt> method performs the interesting work of the pass, and should
return true if the module was modified by the transformation, false
otherwise.<p>
<!-- ======================================================================= -->
</ul><table width="100%" bgcolor="#441188" border=0 cellpadding=4 cellspacing=0>
<tr><td>&nbsp;</td><td width="100%">&nbsp;
<font color="#EEEEFF" face="Georgia,Palatino"><b>
<a name="FunctionPass">The <tt>FunctionPass</tt> class
</b></font></td></tr></table><ul>
In contrast to direct <tt>Pass</tt> subclasses, direct <tt><a
href="http://llvm.cs.uiuc.edu/doxygen/classPass.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
independant 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>
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>Add or remove Function's from the current Module.
<li>Add or remove global variables from the current Module.
<li>Maintain state across invocations of
<a href="#runOnFunction"><tt>runOnFunction</tt></a> (including global data)
</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>
<!-- _______________________________________________________________________ -->
</ul><h4><a name="doInitialization"><hr size=0>The <tt>doInitialization</tt>
method</h4><ul>
<pre>
<b>virtual bool</b> doInitialization(Module &amp;M);
</pre><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>doInitialize</tt> method is
designed to do simple initialization type of stuff that does not depend on the
functions being processed. The <tt>doInitialization</tt> function call is not
scheduled to overlap with any other pass executions.<p>
A good example of how this method should be used is the <a
href="http://llvm.cs.uiuc.edu/doxygen/LowerAllocations_8cpp-source.html">LowerAllocations</a>
pass. This pass converts <tt>malloc</tt> and <tt>free</tt> instructions into
platform dependant <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 neccesary.<p>
<!-- _______________________________________________________________________ -->
</ul><h4><a name="runOnFunction"><hr size=0>The <tt>runOnFunction</tt> method</h4><ul>
<pre>
<b>virtual bool</b> runOnFunction(Function &amp;F) = 0;
</pre><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>
<!-- _______________________________________________________________________ -->
</ul><h4><a name="doFinalization"><hr size=0>The <tt>doFinalization</tt> method</h4><ul>
<pre>
<b>virtual bool</b> doFinalization(Module &amp;M);
</pre</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>
<!-- ======================================================================= -->
</ul><table width="100%" bgcolor="#441188" border=0 cellpadding=4 cellspacing=0>
<tr><td>&nbsp;</td><td width="100%">&nbsp;
<font color="#EEEEFF" face="Georgia,Palatino"><b>
<a name="BasicBlockPass">The <tt>BasicBlockPass</tt> class</a>
</b></font></td></tr></table><ul>
<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>Maintain state across invocations of
<a href="#runOnBasicBlock"><tt>runOnBasicBlock</tt></a>
<li>Modify the constrol flow graph (by altering terminator instructions)
<li>Any of the things verboten for
<a href="#FunctionPass"><tt>FunctionPass</tt></a>'s.
</ol><p>
<tt>BasicBlockPass</tt>'s are useful for traditional local and "peephole"
optimizations. They may override the same <a
href="#doInitialization"><tt>doInitialization</tt></a> and <a
href="#doFinalization"><tt>doFinalization</tt></a> methods that <a
href="#FunctionPass"><tt>FunctionPass</tt></a>'s have, but also have a
<tt>runOnBasicBlock</tt> method:<p>
<!-- _______________________________________________________________________ -->
</ul><h4><a name="runOnBasicBlock"><hr size=0>The <tt>runOnBasicBlock</tt> method</h4><ul>
<pre>
<b>virtual bool</b> runOnBasicBlock(BasicBlock &amp;BB) = 0;
</pre><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>
<!-- *********************************************************************** -->
</ul><table width="100%" bgcolor="#330077" border=0 cellpadding=4 cellspacing=0>
<tr><td align=center><font color="#EEEEFF" size=+2 face="Georgia,Palatino"><b>
<a name="registration">Pass registration
</b></font></td></tr></table><ul>
<!-- *********************************************************************** -->
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>
Passes can be registered in several different ways. Depending on the general
classification of the pass, you should use one of the following templates to
register the pass:<p>
<ul>
<li><b><tt>RegisterOpt</tt></b> - This template should be used when you are
registering a pass that logically should be available for use in the
'<tt>opt</tt>' utility.<p>
<li><b><tt>RegisterAnalysis</tt></b> - This template should be used when you are
registering a pass that logically should be available for use in the
'<tt>analysis</tt>' utility.<p>
<li><b><tt>RegisterLLC</tt></b> - This template should be used when you are
registering a pass that logically should be available for use in the
'<tt>llc</tt>' utility.<p>
<li><b><tt>RegisterPass</tt></b> - This is the generic form of the
<tt>Register*</tt> templates that should be used if you want your pass listed by
multiple or no utilities. This template takes an extra third argument that
specifies which tools it should be listed in. See the <a
href="http://llvm.cs.uiuc.edu/doxygen/PassSupport_8h-source.html">PassSupport.h</a>
file for more information.<p>
</ul><p>
Regardless of how you register your pass, you must specify at least two
parameters. The first parameter is the name of the pass that is to be used on
the command line to specify that the pass should be added to a program (for
example <tt>opt</tt> or <tt>analyze</tt>). The second argument is the name of
the pass, which is to be used for the <tt>--help</tt> output of programs, as
well as for debug output generated by the <tt>--debug-pass</tt> option.<p>
If you pass is constructed by its default constructor, you only ever have to
pass these two arguments. If, on the other hand, you require other information
(like target specific information), you must pass an additional argument. This
argument is a pointer to a function used to create the pass. For an example of
how this works, look at the <a
href="http://llvm.cs.uiuc.edu/doxygen/LowerAllocations_8cpp-source.html">LowerAllocations.cpp</a>
file.<p>
If a pass is registered to be used by the <tt>analyze</tt> utility, you should
implement the virtual <tt>print</tt> method:<p>
<!-- _______________________________________________________________________ -->
</ul><h4><a name="print"><hr size=0>The <tt>print</tt> method</h4><ul>
<pre>
<b>virtual void</b> print(std::ostream &amp;O, <b>const</b> Module *M) <b>const</b>;
</pre><p>
The <tt>print</tt> method must be implemented by "analyses" in order to print a
human readable version of the analysis results. This is useful for debugging an
analysis itself, as well as for other people to figure out how an analysis
works. The <tt>analyze</tt> tool uses this method to generate its output.<p>
The <tt>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>
<!-- *********************************************************************** -->
</ul><table width="100%" bgcolor="#330077" border=0 cellpadding=4 cellspacing=0>
<tr><td align=center><font color="#EEEEFF" size=+2 face="Georgia,Palatino"><b>
<a name="interaction">Specifying interactions between passes
</b></font></td></tr></table><ul>
<!-- *********************************************************************** -->
One of the main responsibilities of the <tt>PassManager</tt> is the 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>
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>
<!-- _______________________________________________________________________ -->
</ul><h4><a name="getAnalysisUsage"><hr size=0>The <tt>getAnalysisUsage</tt> method</h4><ul>
<pre>
<b>virtual void</b> getAnalysisUsage(AnalysisUsage &amp;Info) <b>const</b>;
</pre><p>
By implementing the <tt>getAnalysisUsage</tt> method, the required and
invalidated sets may be specified for your transformation. The implementation
should fill in the <tt><a
href="http://llvm.cs.uiuc.edu/doxygen/classAnalysisUsage.html">AnalysisUsage</a></tt>
object with information about which passes are required and not invalidated. To do this, the following set methods are provided by the <tt><a
href="http://llvm.cs.uiuc.edu/doxygen/classAnalysisUsage.html">AnalysisUsage</a></tt> class:<p>
<pre>
<i>// addRequires - Add the specified pass to the required set for your pass.</i>
<b>template</b>&lt;<b>class</b> PassClass&gt;
AnalysisUsage &amp;AnalysisUsage::addRequired();
<i>// addPreserved - Add the specified pass to the set of analyses preserved by
// this pass</i>
<b>template</b>&lt;<b>class</b> PassClass&gt;
AnalysisUsage &amp;AnalysisUsage::addPreserved();
<i>// setPreservesAll - Call this if the pass does not modify its input at all</i>
<b>void</b> AnalysisUsage::setPreservesAll();
<i>// preservesCFG - This function should be called by the pass, iff they do not:
//
// 1. Add or remove basic blocks from the function
// 2. Modify terminator instructions in any way.
//
// This is automatically implied for <a href="#BasicBlockPass">BasicBlockPass</a>'s
//</i>
<b>void</b> AnalysisUsage::preservesCFG();
</pre><p>
Some examples of how to use these methods are:<p>
<pre>
<i>// This is an example implementation from an analysis, which does not modify
// the program at all, yet has a prerequisite.</i>
<b>void</b> <a href="http://llvm.cs.uiuc.edu/doxygen/structPostDominanceFrontier.html">PostDominanceFrontier</a>::getAnalysisUsage(AnalysisUsage &amp;AU) <b>const</b> {
AU.setPreservesAll();
AU.addRequired&lt;<a href="http://llvm.cs.uiuc.edu/doxygen/structPostDominatorTree.html">PostDominatorTree</a>&gt;();
}
</pre><p>
and:<p>
<pre>
<i>// This example modifies the program, but does not modify the CFG</i>
<b>void</b> <a href="http://llvm.cs.uiuc.edu/doxygen/structLICM.html">LICM</a>::getAnalysisUsage(AnalysisUsage &amp;AU) <b>const</b> {
AU.preservesCFG();
AU.addRequired&lt;<a href="http://llvm.cs.uiuc.edu/doxygen/classLoopInfo.html">LoopInfo</a>&gt;();
}
</pre><p>
<!-- _______________________________________________________________________ -->
</ul><h4><a name="getAnalysis"><hr size=0>The <tt>getAnalysis&lt;&gt;</tt> method</h4><ul>
The <tt>Pass::getAnalysis&lt;&gt;</tt> method is inherited by your class,
providing you with access to the passes that you declared that you required with
the <a href="#getAnalysisUsage"><tt>getAnalysisUsage</tt></a> method. It takes
a single template argument that specifies which pass class you want, and returns
a reference to that pass.<p>
<pre>
<b>template</b>&lt;<b>typename</b> PassClass&gt;
AnalysisType &amp;getAnalysis();
</pre><p>
This method call returns a reference to the pass desired. You may get a runtime
assertion failure if you attempt to get an analysis that you did not declare as
required in your <a href="#getAnalysisUsage"><tt>getAnalysisUsage</tt></a>
implementation. This method can be called by your <tt>run*</tt> method
implementation, or by any other local method invoked by your <tt>run*</tt>
method.<p>
<!-- *********************************************************************** -->
</ul><table width="100%" bgcolor="#330077" border=0 cellpadding=4 cellspacing=0>
<tr><td align=center><font color="#EEEEFF" size=+2 face="Georgia,Palatino"><b>
<a name="passmanager">What PassManager does
</b></font></td></tr></table><ul>
<!-- *********************************************************************** -->
The <a
href="http://llvm.cs.uiuc.edu/doxygen/PassManager_8h-source.html"><tt>PassManager</tt></a>
<a href="http://llvm.cs.uiuc.edu/doxygen/classPassManager.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>
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.<p>
<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>'s 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.cs.uiuc.edu/doxygen/structDominatorSet.html"><tt>DominatorSet</tt></a>
needs to be calculated at a time. This also makes it possible some <a
href="#SMP">interesting enhancements</a> in the future.<p>
</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>
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>
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>
<pre>
$ opt -load ../../../lib/Debug/libhello.so -gcse -licm --debug-pass=Structure &lt; hello.bc &gt; /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
Bytecode Writer
--Bytecode Writer
</pre><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>
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>
Lets see how this changes when we run the <a href="#basiccode">Hello World</a>
pass in between the two passes:<p>
<pre>
$ opt -load ../../../lib/Debug/libhello.so -gcse -hello -licm --debug-pass=Structure &lt; hello.bc &gt; /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
Bytecode Writer
--Bytecode Writer
Hello: __main
Hello: puts
Hello: main
</pre><p>
Here we see that the <a href="#basiccode">Hello World</a> pass has killed the
Dominator Set pass, even though it doesn't modify the code at all! To fix this,
we need to add the following <a
href="#getAnalysisUsage"><tt>getAnalysisUsage</tt></a> method to our pass:<p>
<pre>
<i>// We don't modify the program, so we preserve all analyses</i>
<b>virtual void</b> getAnalysisUsage(AnalysisUsage &amp;AU) <b>const</b> {
AU.setPreservesAll();
}
</pre><p>
Now when we run our pass, we get this output:<p>
<pre>
$ opt -load ../../../lib/Debug/libhello.so -gcse -hello -licm --debug-pass=Structure < hello.bc > /dev/null
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
Bytecode Writer
--Bytecode Writer
Hello: __main
Hello: puts
Hello: main
</pre><p>
Which shows that we don't accidentally invalidate dominator information
anymore, and therefore do not have to compute it twice.<p>
<!-- _______________________________________________________________________ -->
</ul><h4><a name="releaseMemory"><hr size=0>The <tt>releaseMemory</tt> method</h4><ul>
<pre>
<b>virtual void</b> releaseMemory();
</pre><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>
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>
<!-- *********************************************************************** -->
</ul><table width="100%" bgcolor="#330077" border=0 cellpadding=4 cellspacing=0>
<tr><td align=center><font color="#EEEEFF" size=+2 face="Georgia,Palatino"><b>
<a name="future">Future extensions planned
</b></font></td></tr></table><ul>
<!-- *********************************************************************** -->
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>
<!-- _______________________________________________________________________ -->
</ul><h4><a name="SMP"><hr size=0>Multithreaded LLVM</h4><ul>
Multiple CPU machines are becoming more command 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 seperate
instances to be hacking on different parts of the program at the same time.<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>
<!-- _______________________________________________________________________ -->
</ul><h4><a name="ModuleSource"><hr size=0>A new <tt>ModuleSource</tt> interface</h4><ul>
Currently, the <tt>PassManager</tt>'s <tt>run</tt> method takes a <tt><a
href="http://llvm.cs.uiuc.edu/doxygen/classModule.html">Module</a></tt> as
input, and runs all of the passes on this module. The problem with this
approach is that none of the <tt>PassManager</tt> features can be used for
timing and debugging the actual <b>loading</b> of the module from disk or
standard input.<p>
To solve this problem, eventually the <tt>PassManger</tt> class will accept a
<tt>ModuleSource</tt> object instead of a Module itself. When complete, this
will also allow for streaming of functions out of the bytecode representation,
allowing us to avoid holding the entire program in memory at once if we only are
dealing with <a href="#FunctionPass">FunctionPass</a>'s.<p>
As part of a different issue, eventually the bytecode loader will be extended to
allow on-demand loading of functions from the bytecode representation, in order
to better support the runtime reoptimizer. The bytecode format is already
capable of this, the loader just needs to be reworked a bit.<p>
<!-- _______________________________________________________________________ -->
</ul><h4><a name="PassFunctionPass"><hr size=0><tt>Pass</tt>'s requiring <tt>FunctionPass</tt>'s</h4><ul>
Currently it is illegal for a <a href="#Pass"><tt>Pass</tt></a> to require a <a
href="#FunctionPass"><tt>FunctionPass</tt></a>. This is because there is only
one instance of the <a href="#FunctionPass"><tt>FunctionPass</tt></a> object
ever created, thus nowhere to store information for all of the functions in the
program at the same time. Although this has come up a couple of times before,
this has always been worked around by factoring one big complicated pass into a
global and an interprocedural part, both of which are distinct. In the future,
it would be nice to have this though.<p>
Note that it is no problem for a <a
href="#FunctionPass"><tt>FunctionPass</tt></a> to require the results of a <a
href="#Pass"><tt>Pass</tt></a>, only the other way around.<p>
<!-- *********************************************************************** -->
</ul>
<!-- *********************************************************************** -->
<hr><font size-1>
<address><a href="mailto:sabre@nondot.org">Christopher Lattner</a></address>
<!-- Created: Tue Aug 6 15:00:33 CDT 2002 -->
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Last modified: Thu Aug 8 15:07:23 CDT 2002
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