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

[Nadav Rotem]

This looks like a codegen problem and not an ISA problem.  Can we convert  v8f32->v8i32 efficiently? Yes.  Can we convert v8i32->v8i8 efficiently? Yes.  Can we do it in less than 7 instructions? Probably. 
 

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.
> 
> -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
> > >> +}
> > >>
> > >>
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