Added sections about debugging mis-compilations and incorrect code generation.
llvm-svn: 8584
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Misha Brukman
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@ -20,8 +20,10 @@
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<li><a href="#passes">Bugs in LLVM passes</a>
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</ul>
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<li><a href="#miscompilations">Miscompilations</a>
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<li><a href="#codegen">Incorrect code generation (JIT and LLC)</a>
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<p><b>Written by <a href="mailto:sabre@nondot.org">Chris Lattner</a></b><p>
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<p><b>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></b><p>
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</ol><p></font>
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</td><td valign=top align=right>
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<img src="Debugging.gif" width=444 height=314>
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@ -138,7 +140,7 @@ being linked together (the "<tt><b>llvm-gcc</b> -v</tt>" output should include
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the full list of objects linked). Then run:<p>
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<pre>
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<b>as</b> < /dev/null > null.bc
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<b>llvm-as</b> < /dev/null > null.bc
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<b>gccld</b> -debug-pass=Arguments null.bc
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</pre><p>
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@ -180,13 +182,80 @@ to reproduce the problem to the llvmbugs mailing list.<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="miscompilations">Miscompilations
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<a name="miscompilations">Miscompilations</a>
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</b></font></td></tr></table><ul>
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<!-- *********************************************************************** -->
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Fortunately we haven't had too many miscompilations. Because of this, this
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section is a TODO. Basically, use bugpoint to track down the problem.<p>
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A miscompilation occurs when a pass does not correctly transform a program, thus
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producing errors that are only noticed during execution. This is different from
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producing invalid LLVM code (i.e., code not in SSA form, using values before
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defining them, etc.) which the verifier will check for after a pass finishes its
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run.<p>
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To debug a miscompilation, you should choose which program you wish to run the
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output through, e.g. C backend, the JIT, or LLC, and a selection of passes, one
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of which may be causing the error, and run, for example:
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<pre>
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<b>bugpoint</b> -run-cbe -mode=compile [... optimization passes ...] file-to-test.bc
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</pre>
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<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 bytecode 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.
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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="codegen">Incorrect code generation</a>
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</b></font></td></tr></table><ul>
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<!-- *********************************************************************** -->
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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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To debug the JIT:
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<pre>
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<b>bugpoint</b> -run-jit -mode=codegen -output=[correct output file] [bytecodefile]
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</pre>
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Similarly, to debug the LLC, one would run:
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<pre>
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<b>bugpoint</b> -run-llc -mode=codegen -output=[correct output file] [bytecodefile]
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</pre>
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At the end of a successful <tt>bugpoint</tt> run, you will be presented
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with two bytecode 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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To reproduce the error that <tt>bugpoint</tt> found, it is sufficient to do the
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following:
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<ol>
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<li>Regenerate the shared object from the safe bytecode file:<br>
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<pre>
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<b>llvm-dis</b> -c safe.bc -o safe.c<br>
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<b>gcc</b> -shared safe.c -o safe.so
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</pre></li>
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<li>If debugging LLC, compile test bytecode native and link with the shared object:<br>
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<pre>
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<b>llc</b> test.bc -o test.s -f<br>
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gcc test.s safe.so -o test.llc<br>
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./test.llc [program options]
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</pre></li>
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<p>
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If debugging the JIT, load the shared object and supply the test bytecode:<br>
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<pre>
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<b>lli</b> -load=safe.so test.bc [program options]
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</pre></li>
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</ol>
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<!-- *********************************************************************** -->
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</ul>
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@ -196,6 +265,6 @@ section is a TODO. Basically, use bugpoint to track down the problem.<p>
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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: Fri May 23 09:48:53 CDT 2003
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Last modified: Wed Sep 17 13:49:15 CDT 2003
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<!-- hhmts end -->
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</font></body></html>
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