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* Remove notes at the top of the file
* Add information about how to debug a dynamically loaded pass. git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@3583 91177308-0d34-0410-b5e6-96231b3b80d8
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Chris Lattner
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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
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<html><head><title>Writing an LLVM Pass</title></head>
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<!--
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I. General Structure of an LLVM Program
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I.1 "What is a 'Value'?": Value & User class
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I.2 Type & Derived Types
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I.3 GlobalVariable, Function
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I.4 BasicBlock
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I.5 Instruction & Subclasses
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1.6 Argument
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1.7 Constants
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III. Useful things to know about the LLVM source base:
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III.1 Useful links that introduce the STL
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III.2 isa<>, cast<>, dyn_cast<>
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III.3 Makefiles, useful options
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III.4 How to use opt & analyze to debug stuff
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III.5 How to write a regression test
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III.6 DEBUG() and Statistics (-debug & -stats)
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III.7 The -time-passes option
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III.8 ... more as needed ...
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I think that writing Section #1 would be very helpful and that's the most
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stable portion of the sourcebase. #3 can be started on, but will probably
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just grow as time goes on.
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-->
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<body bgcolor=white>
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<table width="100%" bgcolor="#330077" border=0 cellpadding=4 cellspacing=0>
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@ -80,6 +52,11 @@ just grow as time goes on.
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<ul>
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<li><a href="#releaseMemory">The <tt>releaseMemory</tt> method</a>
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</ul>
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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
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<li><a href="#debugmisc">Miscellaneous Problems
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</ul>
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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>
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@ -1025,6 +1002,92 @@ internal state. This method is called after the <tt>run*</tt> method for the
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class, before the next call of <tt>run*</tt> in your pass.<p>
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<!-- *********************************************************************** -->
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</ul><table width="100%" bgcolor="#330077" border=0 cellpadding=4 cellspacing=0>
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<tr><td align=center><font color="#EEEEFF" size=+2 face="Georgia,Palatino"><b>
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<a name="debughints">Using GDB with dynamically loaded passes
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</b></font></td></tr></table><ul>
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<!-- *********************************************************************** -->
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Unfortunately, using GDB with dynamically loaded passes is not as easy as it
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should be. First of all, you can't set a breakpoint in a shared object that has
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not been loaded yet, and second of all there are problems with inlined functions
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in shared objects. Here are some suggestions to debugging your pass with
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GDB.<p>
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For sake of discussion, I'm going to assume that you are debugging a
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transformation invoked by <tt>opt</tt>, although nothing described here depends
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on that.<p>
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<!-- _______________________________________________________________________ -->
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</ul><h4><a name="breakpoint"><hr size=0>Setting a breakpoint in your pass</h4><ul>
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First thing you do is start <tt>gdb</tt> on the <tt>opt</tt> process:<p>
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<pre>
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$ <b>gdb opt</b>
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GNU gdb 5.0
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Copyright 2000 Free Software Foundation, Inc.
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GDB is free software, covered by the GNU General Public License, and you are
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welcome to change it and/or distribute copies of it under certain conditions.
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Type "show copying" to see the conditions.
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There is absolutely no warranty for GDB. Type "show warranty" for details.
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This GDB was configured as "sparc-sun-solaris2.6"...
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(gdb)
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</pre><p>
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Note that <tt>opt</tt> has a lot of debugging information in it, so it takes
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time to load. Be patient. Since we cannot set a breakpoint in our pass yet
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(the shared object isn't loaded until runtime), we must execute the process, and
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have it stop before it invokes our pass, but after it has loaded the shared
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object. The most foolproof way of doing this is to set a breakpoint in
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<tt>PassManager::run</tt> and then run the process with the arguments you
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want:<p>
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<pre>
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(gdb) <b>break PassManager::run</b>
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Breakpoint 1 at 0x2413bc: file Pass.cpp, line 70.
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(gdb) <b>run test.bc -load /shared/lattner/cvs/llvm/lib/Debug/[libname].so -[passoption]</b>
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Starting program: /shared/lattner/cvs/llvm/tools/Debug/opt test.bc
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-load /shared/lattner/cvs/llvm/lib/Debug/[libname].so -[passoption]
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Breakpoint 1, PassManager::run (this=0xffbef174, M=@0x70b298) at Pass.cpp:70
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70 bool PassManager::run(Module &M) { return PM->run(M); }
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(gdb)
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</pre></p>
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Once the <tt>opt</tt> stops in the <tt>PassManager::run</tt> method you are now
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free to set breakpoints in your pass so that you can trace through execution or
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do other standard debugging stuff.<p>
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<!-- _______________________________________________________________________ -->
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</ul><h4><a name="debugmisc"><hr size=0>Miscellaneous Problems</h4><ul>
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Once you have the basics down, there are a couple of problems that GDB has, some
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with solutions, some without.<p>
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<ul>
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<li>Inline functions have bogus stack information. In general, GDB does a
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pretty good job getting stack traces and stepping through inline functions.
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When a pass is dynamically loaded however, it somehow completely loses this
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capability. The only solution I know of is to de-inline a function (move it
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from the body of a class to a .cpp file).<p>
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<li>Restarting the program breaks breakpoints. After following the information
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above, you have succeeded in getting some breakpoints planted in your pass. Nex
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thing you know, you restart the program (i.e., you type '<tt>run</tt>' again),
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and you start getting errors about breakpoints being unsettable. The only way I
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have found to "fix" this problem is to <tt>delete</tt> the breakpoints that are
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already set in your pass, run the program, and re-set the breakpoints once
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execution stops in <tt>PassManager::run</tt>.<p>
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</ul>
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Hopefully these tips will help with common case debugging situations. If you'd
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like to contribute some tips of your own, just contact <a
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href="mailto:sabre@nondot.org">Chris</a>.<p>
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<!-- *********************************************************************** -->
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</ul><table width="100%" bgcolor="#330077" border=0 cellpadding=4 cellspacing=0>
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<tr><td align=center><font color="#EEEEFF" size=+2 face="Georgia,Palatino"><b>
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@ -1098,9 +1161,9 @@ href="#Pass"><tt>Pass</tt></a>, only the other way around.<p>
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<!-- *********************************************************************** -->
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<hr><font size-1>
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<address><a href="mailto:sabre@nondot.org">Christopher Lattner</a></address>
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<address><a href="mailto:sabre@nondot.org">Chris Lattner</a></address>
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<!-- Created: Tue Aug 6 15:00:33 CDT 2002 -->
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<!-- hhmts start -->
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Last modified: Thu Aug 22 14:19:43 CDT 2002
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Last modified: Thu Sep 5 15:06:01 CDT 2002
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<!-- hhmts end -->
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</font></body></html>
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