[llvm] r204880 - X86: Correct vectorization cost model for v8f32->v8i8.

[Quentin Colombet]

On Mar 27, 2014, at 9:40 AM, Eric Christopher <echristo at gmail.com> wrote:

> 
> On Mar 27, 2014 8:41 AM, "Jim Grosbach" <grosbach at apple.com> wrote:
> >
> >
> >
> > > On Mar 26, 2014, at 10:32 PM, Eric Christopher <echristo at gmail.com> wrote:
> > >
> > >> On Wed, Mar 26, 2014 at 5:04 PM, Jim Grosbach <grosbach at apple.com> wrote:
> > >> Author: grosbach
> > >> Date: Wed Mar 26 19:04:11 2014
> > >> New Revision: 204880
> > >>
> > >> URL: http://llvm.org/viewvc/llvm-project?rev=204880&view=rev
> > >> Log:
> > >> X86: Correct vectorization cost model for v8f32->v8i8.
> > >>
> > >> Fix the cost model to reflect the reality of our codegen.
> > >
> > > Reality of our codegen or reality of the processors?
> > >
> >
> > The latter, though the cost model should be accurately reflecting both.
> 
> It should, but in the latter a bug should be filed and a comment to that effect listed. "Realities of our CodeGen" really sounds like a deficiency we're papering over.
> 
That’s a good point and I’m tempted to think this is a deficiency of the current model. Indeed the same codegen could have different performance on different processors.
That said, currently the differences are small and likely in the noise. Moreover the cost model is already making approximations (e.g., moves, spill), and I am not sure adding complexity here would help the performances, at least for now.

Anyway, filing a bug may still be a good idea :).

What do you guys think?

Thanks,
-Quentin

> -eric
> 
> PS. Reading more of the patches I'll just CC Quentin on this response too.  :)
> 
> > > -eric
> > >
> > >>
> > >> rdar://16370633
> > >>
> > >> Added:
> > >>    llvm/trunk/test/Transforms/LoopVectorize/X86/fp_to_sint8-cost-model.ll
> > >> Modified:
> > >>    llvm/trunk/lib/Target/X86/X86TargetTransformInfo.cpp
> > >>
> > >> Modified: llvm/trunk/lib/Target/X86/X86TargetTransformInfo.cpp
> > >> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Target/X86/X86TargetTransformInfo.cpp?rev=204880&r1=204879&r2=204880&view=diff
> > >> ==============================================================================
> > >> --- llvm/trunk/lib/Target/X86/X86TargetTransformInfo.cpp (original)
> > >> +++ llvm/trunk/lib/Target/X86/X86TargetTransformInfo.cpp Wed Mar 26 19:04:11 2014
> > >> @@ -513,7 +513,7 @@ unsigned X86TTI::getCastInstrCost(unsign
> > >>     { ISD::UINT_TO_FP,  MVT::v4f64, MVT::v4i16, 2 },
> > >>     { ISD::UINT_TO_FP,  MVT::v4f64, MVT::v4i32, 6 },
> > >>
> > >> -    { ISD::FP_TO_SINT,  MVT::v8i8,  MVT::v8f32, 1 },
> > >> +    { ISD::FP_TO_SINT,  MVT::v8i8,  MVT::v8f32, 7 },
> > >>     { ISD::FP_TO_SINT,  MVT::v4i8,  MVT::v4f32, 1 },
> > >>   };
> > >>
> > >>
> > >> Added: llvm/trunk/test/Transforms/LoopVectorize/X86/fp_to_sint8-cost-model.ll
> > >> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/LoopVectorize/X86/fp_to_sint8-cost-model.ll?rev=204880&view=auto
> > >> ==============================================================================
> > >> --- llvm/trunk/test/Transforms/LoopVectorize/X86/fp_to_sint8-cost-model.ll (added)
> > >> +++ llvm/trunk/test/Transforms/LoopVectorize/X86/fp_to_sint8-cost-model.ll Wed Mar 26 19:04:11 2014
> > >> @@ -0,0 +1,24 @@
> > >> +; RUN: opt < %s  -loop-vectorize -mtriple=x86_64-apple-macosx10.8.0 -mcpu=corei7-avx -S -debug-only=loop-vectorize 2>&1 | FileCheck %s
> > >> +
> > >> +target datalayout = "e-p:64:64:64-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:64:64-f32:32:32-f64:64:64-v64:64:64-v128:128:128-a0:0:64-s0:64:64-f80:128:128-n8:16:32:64-S128"
> > >> +target triple = "x86_64-apple-macosx10.8.0"
> > >> +
> > >> +
> > >> +; CHECK: cost of 7 for VF 8 For instruction:   %conv = fptosi float %tmp to i8
> > >> +define void @float_to_sint8_cost(i8* noalias nocapture %a, float* noalias nocapture readonly %b) nounwind {
> > >> +entry:
> > >> +  br label %for.body
> > >> +for.body:
> > >> +  %indvars.iv = phi i64 [ 0, %entry ], [ %indvars.iv.next, %for.body ]
> > >> +  %arrayidx = getelementptr inbounds float* %b, i64 %indvars.iv
> > >> +  %tmp = load float* %arrayidx, align 4
> > >> +  %conv = fptosi float %tmp to i8
> > >> +  %arrayidx2 = getelementptr inbounds i8* %a, i64 %indvars.iv
> > >> +  store i8 %conv, i8* %arrayidx2, align 4
> > >> +  %indvars.iv.next = add nuw nsw i64 %indvars.iv, 1
> > >> +  %exitcond = icmp eq i64 %indvars.iv.next, 256
> > >> +  br i1 %exitcond, label %for.end, label %for.body
> > >> +
> > >> +for.end:
> > >> +  ret void
> > >> +}
> > >>
> > >>
> > >> _______________________________________________
> > >> llvm-commits mailing list
> > >> llvm-commits at cs.uiuc.edu
> > >> http://lists.cs.uiuc.edu/mailman/listinfo/llvm-commits

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