Jakob Stoklund Olesen
2012-Apr-17 22:22 UTC
[LLVMdev] InstCombine adds bit masks, confuses self, others
On Apr 16, 2012, at 3:30 PM, Chandler Carruth <chandlerc at google.com> wrote:> Does sinking these into the DAGCombine layer help? How much does it break?I tried disabling just the InstCombine transforms that hide shl instructions behind bitmasks. Even though DAGCombine has the same transforms, it causes some pretty bad regressions: External/SPEC/CINT95/147_vortex/147_vortex 0.294 0.322 +9.6% +40mB MultiSource/Benchmarks/Olden/tsp/tsp 0.680 0.748 +9.9% +41mB SingleSource/Benchmarks/Shootout-C++/except 0.116 0.128 +10.8% +45mB SingleSource/Benchmarks/Shootout/strcat 0.102 0.113 +11.1% +46mB SingleSource/Benchmarks/Shootout-C++/hash 0.455 0.507 +11.4% +47mB External/Povray/povray 2.015 2.246 +11.5% +47mB External/SPEC/CINT2000/255_vortex/255_vortex 1.814 2.044 +12.7% +52mB SingleSource/Benchmarks/Shootout-C++/heapsort 1.871 2.132 +13.9% +57mB SingleSource/Benchmarks/Shootout-C++/ary3 1.087 1.264 +16.3% +65mB MultiSource/Benchmarks/SciMark2-C/scimark2 27.491 23.596 -14.2% -66mB MultiSource/Benchmarks/Olden/bisort/bisort 0.360 0.428 +19.0% +75mB MultiSource/Benchmarks/Olden/bh/bh 1.074 1.287 +19.9% +79mB (Running on Sandy Bridge, x86-64) I'll try to figure out why. /jakob -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20120417/c7c5a56a/attachment.html>
Chandler Carruth
2012-Apr-17 22:29 UTC
[LLVMdev] InstCombine adds bit masks, confuses self, others
On Wed, Apr 18, 2012 at 12:22 AM, Jakob Stoklund Olesen <stoklund at 2pi.dk>wrote:> I tried disabling just the InstCombine transforms that hide shl > instructions behind bitmasks. Even though DAGCombine has the same > transforms, it causes some pretty bad regressions: >I wonder about your idea of still combining most of the shifts, but leaving the last bits uncombined... Certainly the combining effects seem particularly important to capture because we'll only look through a finite number of shifts to determine demanded bits, etc. I would expect that, even with the fix, demanded bits might need adjusting to cope with the pattern. But this is all theorizing. =] The actually regressions will likely tell more. -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20120418/6d713c36/attachment.html>
Jakob Stoklund Olesen
2012-Apr-17 23:15 UTC
[LLVMdev] InstCombine adds bit masks, confuses self, others
On Apr 17, 2012, at 3:29 PM, Chandler Carruth wrote:> On Wed, Apr 18, 2012 at 12:22 AM, Jakob Stoklund Olesen <stoklund at 2pi.dk> wrote: > I tried disabling just the InstCombine transforms that hide shl instructions behind bitmasks. Even though DAGCombine has the same transforms, it causes some pretty bad regressions: > > I wonder about your idea of still combining most of the shifts, but leaving the last bits uncombined... Certainly the combining effects seem particularly important to capture because we'll only look through a finite number of shifts to determine demanded bits, etc.Yes, the idea is to only preserve shl instructions because of their arithmetic/logical duality. I'll keep folding ashr/lshr. These transformations all still run: X << C1 >> C2 --> X << (C1-C2) & Mask X << C1 >> C2 --> X >> (C2-C1) & Mask X << C1 << C2 --> X << (C1+C2) X >> C1 >> C2 --> X >> (C1+C2) I also allow folding of exact right shifts: X >>exact C1 <<nuw C2 --> X <<nuw (C2-C1) X >>exact C1 << C2 --> X >>exact (C1-C2) Since a bit mask isn't needed, scalar evolution should be able to reason about this. Long chains of shifts should still be collapsed this way.> But this is all theorizing. =] The actually regressions will likely tell more.Yeah, all of the regressions I posted actually had identical assembly. I'll try measuring again while holding my breath. /jakob -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20120417/b1d03c47/attachment.html>
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