regression on Adobe-C++/loop_unroll

[Gerolf Hoflehner]

The key option is -flto. Just O3 does not show the regression.

Compiler version:
/Users/ghoflehner/dev/regress_builds/build/Debug+Asserts/bin/clang++ -v
clang version 3.5 (trunk 199970) (llvm/trunk 200067)
Target: x86_64-apple-darwin13.1.0
Thread model: posix

Options:
$CCROOT/bin/clang++ -I/Users/ghoflehner/dev/test_builds/test-2014-01-28_04-36-58/SingleSource/Benchmarks/Adobe-C++ -I/Users/ghoflehner/dev/llvm-test-suite/SingleSource/Benchmarks/Adobe-C++ -I/Users/ghoflehner/dev/llvm-test-suite/include -I../../../include -D_GNU_SOURCE -D__STDC_LIMIT_MACROS -DNDEBUG  -O3 -Wno-implicit-function-declaration -Wno-incompatible-pointer-types -flto -isysroot /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.9.sdk -mavx  -m64 -fomit-frame-pointer -mdynamic-no-pic -c /Users/ghoflehner/dev/llvm-test-suite/SingleSource/Benchmarks/Adobe-C++/loop_unroll.cpp -o loop_unroll.o
env DYLD_LIBRARY_PATH=$CCROOT/lib $CCROOT/bin/clang++ -o loop_unroll.simple loop_unroll.o -lm -lstdc++  -Wno-implicit-function-declaration -Wno-incompatible-pointer-types -isysroot /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.9.sdk -mavx -m64 -fomit-frame-pointer -mdynamic-no-pic


On Feb 7, 2014, at 1:33 AM, Chandler Carruth <chandlerc at gmail.com> wrote:

> I've diffed the IR at -O3 before and after r200067 and it is identical other than SSA value names that are minutely different.
> 
> I think i'll need before/after IR or an exact commandline to produce the exact same result you're seeing to make much progress here.
> 
> On Fri Feb 07 2014 at 1:16:35 AM, Chandler Carruth <chandlerc at gmail.com> wrote:
> So this doesn't reproduce for me at all when I just compile and loop_unroll.cpp from the test suite for x86 (sandybridge). Not really sure what all is required to observe this slowdown.
> 
> I've also checked and we have no interesting benchmark regressions on our internal benchmarks with that revision... Really weird.
> 
> Can you provide before/after bitcode? File a bug as well to track it if its this severe?
> 
> On Fri Feb 07 2014 at 12:32:34 AM, Chandler Carruth <chandlerc at gmail.com> wrote:
> It's really weird because the public performance bots don't show this regression:
> 
> http://llvm.org/perf/db_default/v4/nts/21277
> 
> It would be really nice to get the stuff that people have hard requirements on posted to the LNT dashboard...
> 
> It's also particularly weird because this shouldn't really have changed the way LICM works. Looking into it though...
> 
> On Thu Feb 06 2014 at 11:19:21 AM, Gerolf Hoflehner <ghoflehner at apple.com> wrote:
> Hi Chandler
> 
> your commit seems to cause a major regression (>20%)  on loop_unroll and other benchmarks as well. This is ( at least)  on x86 under -O3 -flto and seems to be due to the loss of LICM. Please take a look.
> 
> Cheers
> Gerolf
> 
> ------------------------------------------------------------------------
> r200067 | chandlerc | 2014-01-24 20:07:24 -0800 (Fri, 24 Jan 2014) | 44 lines
> 
> [LPM] Make LCSSA a utility with a FunctionPass that applies it to all
> the loops in a function, and teach LICM to work in the presance of
> LCSSA.
> 

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