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

[Nadav Rotem]

On Mar 27, 2014, at 11:06 AM, Eric Christopher <echristo at gmail.com> wrote:

> On Thu, Mar 27, 2014 at 11:03 AM, Nadav Rotem <nrotem at apple.com> wrote:
>> 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.
> 
> How many? I haven't looked at it in detail, but the codegen out of the
> compiler seems to be cleaned up pretty well, post legalization. Do you
> have an idea of how few we could do it in?
> 

We already get  "fptosi <8 x float> %x to <8 x i32>” right, and it generates vcvttps2dq.  To get from <8 x i32> to <8 x i8>  I think that vpacksi32 (v packed i32) can do the trick. 

If that does not work then we can probably shuffle twice and blend. 


> -eric
> 
>> 
>> 
>> 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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