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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>How to submit an LLVM bug report</title>
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<link rel="stylesheet" href="_static/llvm.css" type="text/css">
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</head>
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<body>
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<h1>
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How to submit an LLVM bug report
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</h1>
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<table class="layout" style="width: 90%" >
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<tr class="layout">
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<td class="left">
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<ol>
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<li><a href="#introduction">Introduction - Got bugs?</a></li>
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<li><a href="#crashers">Crashing Bugs</a>
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<ul>
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<li><a href="#front-end">Front-end bugs</a>
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<li><a href="#ct_optimizer">Compile-time optimization bugs</a>
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<li><a href="#ct_codegen">Code generator bugs</a>
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</ul></li>
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<li><a href="#miscompilations">Miscompilations</a></li>
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<li><a href="#codegen">Incorrect code generation (JIT and LLC)</a></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="http://misha.brukman.net">Misha Brukman</a></p>
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</div>
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</td>
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</tr>
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</table>
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<!-- *********************************************************************** -->
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<h2>
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<a name="introduction">Introduction - Got bugs?</a>
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</h2>
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<!-- *********************************************************************** -->
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<div>
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<p>If you're working with LLVM and run into a bug, we definitely want to know
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about it. This document describes what you can do to increase the odds of
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getting it fixed quickly.</p>
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<p>Basically you have to do two things at a minimum. First, decide whether the
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bug <a href="#crashers">crashes the compiler</a> (or an LLVM pass), or if the
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compiler is <a href="#miscompilations">miscompiling</a> the program (i.e., the
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compiler successfully produces an executable, but it doesn't run right). Based
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on
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what type of bug it is, follow the instructions in the linked section to narrow
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down the bug so that the person who fixes it will be able to find the problem
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more easily.</p>
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<p>Once you have a reduced test-case, go to <a
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href="http://llvm.org/bugs/enter_bug.cgi">the LLVM Bug Tracking
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System</a> and fill out the form with the necessary details (note that you don't
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need to pick a category, just use the "new-bugs" category if you're not sure).
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The bug description should contain the following
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information:</p>
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<ul>
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<li>All information necessary to reproduce the problem.</li>
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<li>The reduced test-case that triggers the bug.</li>
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<li>The location where you obtained LLVM (if not from our Subversion
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repository).</li>
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</ul>
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<p>Thanks for helping us make LLVM better!</p>
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</div>
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<!-- *********************************************************************** -->
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<h2>
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<a name="crashers">Crashing Bugs</a>
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</h2>
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<!-- *********************************************************************** -->
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<div>
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<p>More often than not, bugs in the compiler cause it to crash—often due
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to an assertion failure of some sort. The most important
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piece of the puzzle is to figure out if it is crashing in the GCC front-end
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or if it is one of the LLVM libraries (e.g. the optimizer or code generator)
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that has problems.</p>
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<p>To figure out which component is crashing (the front-end,
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optimizer or code generator), run the
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<tt><b>llvm-gcc</b></tt> command line as you were when the crash occurred, but
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with the following extra command line options:</p>
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<ul>
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<li><tt><b>-O0 -emit-llvm</b></tt>: If <tt>llvm-gcc</tt> still crashes when
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passed these options (which disable the optimizer and code generator), then
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the crash is in the front-end. Jump ahead to the section on <a
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href="#front-end">front-end bugs</a>.</li>
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<li><tt><b>-emit-llvm</b></tt>: If <tt>llvm-gcc</tt> crashes with this option
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(which disables the code generator), you found an optimizer bug. Jump ahead
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to <a href="#ct_optimizer"> compile-time optimization bugs</a>.</li>
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<li>Otherwise, you have a code generator crash. Jump ahead to <a
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href="#ct_codegen">code generator bugs</a>.</li>
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</ul>
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<!-- ======================================================================= -->
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<h3>
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<a name="front-end">Front-end bugs</a>
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</h3>
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<div>
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<p>If the problem is in the front-end, you should re-run the same
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<tt>llvm-gcc</tt> command that resulted in the crash, but add the
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<tt>-save-temps</tt> option. The compiler will crash again, but it will leave
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behind a <tt><i>foo</i>.i</tt> file (containing preprocessed C source code) and
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possibly <tt><i>foo</i>.s</tt> for each
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compiled <tt><i>foo</i>.c</tt> file. Send us the <tt><i>foo</i>.i</tt> file,
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along with the options you passed to llvm-gcc, and a brief description of the
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error it caused.</p>
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<p>The <a href="http://delta.tigris.org/">delta</a> tool helps to reduce the
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preprocessed file down to the smallest amount of code that still replicates the
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problem. You're encouraged to use delta to reduce the code to make the
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developers' lives easier. <a
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href="http://gcc.gnu.org/wiki/A_guide_to_testcase_reduction">This website</a>
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has instructions on the best way to use delta.</p>
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</div>
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<!-- ======================================================================= -->
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<h3>
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<a name="ct_optimizer">Compile-time optimization bugs</a>
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</h3>
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<div>
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<p>If you find that a bug crashes in the optimizer, compile your test-case to a
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<tt>.bc</tt> file by passing "<tt><b>-emit-llvm -O0 -c -o foo.bc</b></tt>".
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Then run:</p>
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<div class="doc_code">
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<p><tt><b>opt</b> -std-compile-opts -debug-pass=Arguments foo.bc
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-disable-output</tt></p>
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</div>
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<p>This command should do two things: it should print out a list of passes, and
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then it should crash in the same way as llvm-gcc. If it doesn't crash, please
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follow the instructions for a <a href="#front-end">front-end bug</a>.</p>
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<p>If this does crash, then you should be able to debug this with the following
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bugpoint command:</p>
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<div class="doc_code">
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<p><tt><b>bugpoint</b> foo.bc <list of passes printed by
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<b>opt</b>></tt></p>
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</div>
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<p>Please run this, then file a bug with the instructions and reduced .bc files
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that bugpoint emits. If something goes wrong with bugpoint, please submit the
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"foo.bc" file and the list of passes printed by <b>opt</b>.</p>
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</div>
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<!-- ======================================================================= -->
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<h3>
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<a name="ct_codegen">Code generator bugs</a>
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</h3>
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<div>
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<p>If you find a bug that crashes llvm-gcc in the code generator, compile your
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source file to a .bc file by passing "<tt><b>-emit-llvm -c -o foo.bc</b></tt>"
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to llvm-gcc (in addition to the options you already pass). Once your have
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foo.bc, one of the following commands should fail:</p>
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<ol>
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<li><tt><b>llc</b> foo.bc</tt></li>
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<li><tt><b>llc</b> foo.bc -relocation-model=pic</tt></li>
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<li><tt><b>llc</b> foo.bc -relocation-model=static</tt></li>
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</ol>
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<p>If none of these crash, please follow the instructions for a
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<a href="#front-end">front-end bug</a>. If one of these do crash, you should
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be able to reduce this with one of the following bugpoint command lines (use
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the one corresponding to the command above that failed):</p>
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<ol>
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<li><tt><b>bugpoint</b> -run-llc foo.bc</tt></li>
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<li><tt><b>bugpoint</b> -run-llc foo.bc --tool-args
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-relocation-model=pic</tt></li>
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<li><tt><b>bugpoint</b> -run-llc foo.bc --tool-args
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-relocation-model=static</tt></li>
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</ol>
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<p>Please run this, then file a bug with the instructions and reduced .bc file
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that bugpoint emits. If something goes wrong with bugpoint, please submit the
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"foo.bc" file and the option that llc crashes with.</p>
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</div>
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</div>
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<!-- *********************************************************************** -->
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<h2>
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<a name="miscompilations">Miscompilations</a>
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</h2>
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<!-- *********************************************************************** -->
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<div>
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<p>If llvm-gcc successfully produces an executable, but that executable doesn't
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run right, this is either a bug in the code or a bug in the
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compiler. The first thing to check is to make sure it is not using undefined
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behavior (e.g. reading a variable before it is defined). In particular, check
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to see if the program <a href="http://valgrind.org/">valgrind</a>s clean,
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passes purify, or some other memory checker tool. Many of the "LLVM bugs" that
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we have chased down ended up being bugs in the program being compiled, not
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LLVM.</p>
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<p>Once you determine that the program itself is not buggy, you should choose
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which code generator you wish to compile the program with (e.g. LLC or the JIT)
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and optionally a series of LLVM passes to run. For example:</p>
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<div class="doc_code">
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<p><tt>
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<b>bugpoint</b> -run-llc [... optzn passes ...] file-to-test.bc --args -- [program arguments]</tt></p>
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</div>
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<p><tt>bugpoint</tt> will try to narrow down your list of passes to the one pass
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that causes an error, and simplify the bitcode file as much as it can to assist
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you. It will print a message letting you know how to reproduce the resulting
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error.</p>
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</div>
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<!-- *********************************************************************** -->
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<h2>
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<a name="codegen">Incorrect code generation</a>
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</h2>
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<!-- *********************************************************************** -->
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<div>
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<p>Similarly to debugging incorrect compilation by mis-behaving passes, you can
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debug incorrect code generation by either LLC or the JIT, using
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<tt>bugpoint</tt>. The process <tt>bugpoint</tt> follows in this case is to try
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to narrow the code down to a function that is miscompiled by one or the other
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method, but since for correctness, the entire program must be run,
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<tt>bugpoint</tt> will compile the code it deems to not be affected with the C
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Backend, and then link in the shared object it generates.</p>
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<p>To debug the JIT:</p>
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<div class="doc_code">
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<pre>
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bugpoint -run-jit -output=[correct output file] [bitcode file] \
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--tool-args -- [arguments to pass to lli] \
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--args -- [program arguments]
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</pre>
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</div>
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<p>Similarly, to debug the LLC, one would run:</p>
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<div class="doc_code">
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<pre>
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bugpoint -run-llc -output=[correct output file] [bitcode file] \
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--tool-args -- [arguments to pass to llc] \
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--args -- [program arguments]
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</pre>
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</div>
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<p><b>Special note:</b> if you are debugging MultiSource or SPEC tests that
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already exist in the <tt>llvm/test</tt> hierarchy, there is an easier way to
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debug the JIT, LLC, and CBE, using the pre-written Makefile targets, which
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will pass the program options specified in the Makefiles:</p>
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<div class="doc_code">
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<p><tt>
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cd llvm/test/../../program<br>
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make bugpoint-jit
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</tt></p>
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</div>
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<p>At the end of a successful <tt>bugpoint</tt> run, you will be presented
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with two bitcode files: a <em>safe</em> file which can be compiled with the C
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backend and the <em>test</em> file which either LLC or the JIT
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mis-codegenerates, and thus causes the error.</p>
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<p>To reproduce the error that <tt>bugpoint</tt> found, it is sufficient to do
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the following:</p>
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<ol>
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<li><p>Regenerate the shared object from the safe bitcode file:</p>
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<div class="doc_code">
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<p><tt>
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<b>llc</b> -march=c safe.bc -o safe.c<br>
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<b>gcc</b> -shared safe.c -o safe.so
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</tt></p>
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</div></li>
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<li><p>If debugging LLC, compile test bitcode native and link with the shared
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object:</p>
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<div class="doc_code">
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<p><tt>
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<b>llc</b> test.bc -o test.s<br>
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<b>gcc</b> test.s safe.so -o test.llc<br>
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./test.llc [program options]
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</tt></p>
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</div></li>
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<li><p>If debugging the JIT, load the shared object and supply the test
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bitcode:</p>
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<div class="doc_code">
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<p><tt><b>lli</b> -load=safe.so test.bc [program options]</tt></p>
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</div></li>
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</ol>
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</div>
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<!-- *********************************************************************** -->
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<hr>
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<address>
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<a href="http://jigsaw.w3.org/css-validator/check/referer"><img
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src="http://jigsaw.w3.org/css-validator/images/vcss-blue" alt="Valid CSS"></a>
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src="http://www.w3.org/Icons/valid-html401-blue" alt="Valid HTML 4.01"></a>
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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>
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<br>
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Last modified: $Date$
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</address>
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</body>
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</html>
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