Claude Helmstetter
2010-Jul-01 10:48 UTC
[LLVMdev] Fail to unroll loop on simple examples.
Hi all, I have tried to use the loop-unroll pass on a large example, but without success. So I have tried on smaller examples, and I did not get more success. My simplest example is attached. It comes from this C code (using llvm-gcc, but similar result with clang): int main (int argc, char *argv[]) { int i; for (i = 0; i!=5; ++i) {} return 0; } I use this command (version 2.8svn, from July 1st) ~/Build/llvm-trunk/Release/bin/opt -mem2reg -indvars -loop-unroll -stats loop.ll | llvm-dis Whereas ScalarEvolution can compute the loop count, LoopInfo fails because the conditional branch is not in the expected block. If I provide the loop count with -unroll-count=5, then the function UnrollLoop fails form the same reason. The pass "-loopsimplify" has no effect. If I add the pass "-loop-rotate" before "-indvars", then the loop is successfully unrolled. However, I think this pass may increase the size of the code for all loops, including the loops that cannot be unrolled. Is this a bug? Or is there any other pass that can help UnrollLoop? -- Claude Helmstetter +86 186 0895 4884 / +33 6 32 99 06 00 http://formes.asia/people/Claude.Helmstetter/ -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20100701/ca0cf370/attachment.html> -------------- next part -------------- A non-text attachment was scrubbed... Name: loop.ll Type: application/octet-stream Size: 920 bytes Desc: not available URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20100701/ca0cf370/attachment.obj>
[I am a newcomer to llvm, so don't believe me.] Hi Claude, I added -debug to the command line in an attempt to study your example, and got out this: Loop Unroll: F[main] Loop %bb1 Loop Size = 4 Can't unroll; loop not terminated by a conditional branch. The source code for that is on line 116 (or so) in lib/Transforms/Utils/LoopUnroll.cpp Based on a quick look at the source code, this looks like a bug in LLVM to me, e.g. perhaps the test is applied on a wrong block? But more probably I am just missing something. --Pekka Nikander On 2010-07 -01, at 13:48 , Claude Helmstetter wrote:> Hi all, > > I have tried to use the loop-unroll pass on a large example, but without success. So I have tried on smaller examples, and I did not get more success. > > My simplest example is attached. It comes from this C code (using llvm-gcc, but similar result with clang): > > int main (int argc, char *argv[]) { > int i; > for (i = 0; i!=5; ++i) {} > return 0; > } > > I use this command (version 2.8svn, from July 1st) > ~/Build/llvm-trunk/Release/bin/opt -mem2reg -indvars -loop-unroll -stats loop.ll | llvm-dis > > Whereas ScalarEvolution can compute the loop count, LoopInfo fails because the conditional branch is not in the expected block. If I provide the loop count with -unroll-count=5, then the function UnrollLoop fails form the same reason. The pass "-loopsimplify" has no effect. > > If I add the pass "-loop-rotate" before "-indvars", then the loop is successfully unrolled. However, I think this pass may increase the size of the code for all loops, including the loops that cannot be unrolled. > > Is this a bug? Or is there any other pass that can help UnrollLoop? > > > -- > Claude Helmstetter > +86 186 0895 4884 / +33 6 32 99 06 00 > http://formes.asia/people/Claude.Helmstetter/ > > <loop.ll>_______________________________________________ > LLVM Developers mailing list > LLVMdev at cs.uiuc.edu http://llvm.cs.uiuc.edu > http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
On Thu, Jul 1, 2010 at 12:33 PM, Pekka Nikander <pekka.nikander at nomadiclab.com> wrote:> [I am a newcomer to llvm, so don't believe me.] > > Hi Claude, > > I added -debug to the command line in an attempt to study your example, and got out this: > > Loop Unroll: F[main] Loop %bb1 > Loop Size = 4 > Can't unroll; loop not terminated by a conditional branch. > > The source code for that is on line 116 (or so) in lib/Transforms/Utils/LoopUnroll.cpp > > Based on a quick look at the source code, this looks like a bug in LLVM to me, e.g. perhaps the test is applied on a wrong block? But more probably I am just missing something.Missed optimization... the loop unroller needs work to work on more than extremely simple loops. -Eli
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