[llvm] r255124 - [InstCombine] fold bitcasts around an extractelement

[Mehdi Amini via llvm-commits]

Hi Sanjay,

I reverted in r255126, this broke a clang test: http://lab.llvm.org:8011/builders/llvm-clang-lld-x86_64-scei-ps4-ubuntu-fast/builds/4193/steps/test/logs/stdio

Best,

— 
Mehdi


> On Dec 9, 2015, at 8:17 AM, Sanjay Patel via llvm-commits <llvm-commits at lists.llvm.org> wrote:
> 
> Author: spatel
> Date: Wed Dec  9 10:17:20 2015
> New Revision: 255124
> 
> URL: http://llvm.org/viewvc/llvm-project?rev=255124&view=rev
> Log:
> [InstCombine] fold bitcasts around an extractelement
> 
> Example:
>  bitcast (extractelement (bitcast <2 x float> %X to <2 x i32>), 1) to float
>    --->
>  extractelement <2 x float> %X, i32 1
> 
> This is part of fixing PR25543:
> https://llvm.org/bugs/show_bug.cgi?id=25543
> 
> The next step will be to generalize this fold:
> trunc ( lshr ( bitcast X) ) -> extractelement (X)
> 
> Ie, I'm hoping to replace the existing transform of:
> bitcast ( trunc ( lshr ( bitcast X)))
> added by:
> http://reviews.llvm.org/rL112232
> 
> with 2 less specific transforms to catch the case in the bug report.
> 
> Differential Revision: http://reviews.llvm.org/D14879
> 
> 
> Modified:
>    llvm/trunk/lib/Transforms/InstCombine/InstCombineCasts.cpp
>    llvm/trunk/test/Transforms/InstCombine/bitcast.ll
> 
> Modified: llvm/trunk/lib/Transforms/InstCombine/InstCombineCasts.cpp
> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/InstCombine/InstCombineCasts.cpp?rev=255124&r1=255123&r2=255124&view=diff
> ==============================================================================
> --- llvm/trunk/lib/Transforms/InstCombine/InstCombineCasts.cpp (original)
> +++ llvm/trunk/lib/Transforms/InstCombine/InstCombineCasts.cpp Wed Dec  9 10:17:20 2015
> @@ -1715,6 +1715,40 @@ static Value *optimizeIntegerToVectorIns
>   return Result;
> }
> 
> +/// Given a bitcasted vector fed into an extract element instruction and then
> +/// bitcasted again, eliminate at least one bitcast by changing the vector type
> +/// of the extractelement instruction.
> +/// Example:
> +///   bitcast (extractelement (bitcast <2 x float> %X to <2 x i32>), 1) to float
> +///    --->
> +///   extractelement <2 x float> %X, i32 1
> +static Instruction *foldBitCastExtElt(BitCastInst &BitCast, InstCombiner &IC,
> +                                      const DataLayout &DL) {
> +  // TODO: Create and use a pattern matcher for ExtractElementInst.
> +  auto *ExtElt = dyn_cast<ExtractElementInst>(BitCast.getOperand(0));
> +  if (!ExtElt || !ExtElt->hasOneUse())
> +    return nullptr;
> +
> +  Value *InnerBitCast = nullptr;
> +  if (!match(ExtElt->getOperand(0), m_BitCast(m_Value(InnerBitCast))))
> +    return nullptr;
> +
> +  VectorType *VecType = cast<VectorType>(InnerBitCast->getType());
> +  Type *DestType = BitCast.getType();
> +
> +  // If the element type of the vector doesn't match the result type,
> +  // bitcast it to a vector type that we can extract from.
> +  if (VecType->getElementType() != DestType) {
> +    unsigned VecWidth = VecType->getPrimitiveSizeInBits();
> +    unsigned DestWidth = DestType->getPrimitiveSizeInBits();
> +    unsigned NumElts = VecWidth / DestWidth;
> +    VecType = VectorType::get(DestType, NumElts);
> +    InnerBitCast = IC.Builder->CreateBitCast(InnerBitCast, VecType, "bc");
> +  }
> +
> +  return ExtractElementInst::Create(InnerBitCast, ExtElt->getOperand(1));
> +}
> +
> static Instruction *foldVecTruncToExtElt(Value *VecInput, Type *DestTy,
>                                          unsigned ShiftAmt, InstCombiner &IC,
>                                          const DataLayout &DL) {
> @@ -1886,6 +1920,9 @@ Instruction *InstCombiner::visitBitCast(
>     }
>   }
> 
> +  if (Instruction *I = foldBitCastExtElt(CI, *this, DL))
> +    return I;
> +
>   if (SrcTy->isPointerTy())
>     return commonPointerCastTransforms(CI);
>   return commonCastTransforms(CI);
> 
> Modified: llvm/trunk/test/Transforms/InstCombine/bitcast.ll
> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/InstCombine/bitcast.ll?rev=255124&r1=255123&r2=255124&view=diff
> ==============================================================================
> --- llvm/trunk/test/Transforms/InstCombine/bitcast.ll (original)
> +++ llvm/trunk/test/Transforms/InstCombine/bitcast.ll Wed Dec  9 10:17:20 2015
> @@ -64,7 +64,7 @@ define float @test3(<2 x float> %A, <2 x
> ; CHECK-NEXT:  ret float %add
> }
> 
> -; TODO: Both bitcasts are unnecessary; change the extractelement.
> +; Both bitcasts are unnecessary; change the extractelement.
> 
> define float @bitcast_extelt1(<2 x float> %A) {
>   %bc1 = bitcast <2 x float> %A to <2 x i32>
> @@ -73,13 +73,11 @@ define float @bitcast_extelt1(<2 x float
>   ret float %bc2
> 
> ; CHECK-LABEL: @bitcast_extelt1(
> -; CHECK-NEXT:  %bc1 = bitcast <2 x float> %A to <2 x i32>
> -; CHECK-NEXT:  %ext = extractelement <2 x i32> %bc1, i32 0
> -; CHECK-NEXT:  %bc2 = bitcast i32 %ext to float
> +; CHECK-NEXT:  %bc2 = extractelement <2 x float> %A, i32 0
> ; CHECK-NEXT:  ret float %bc2
> }
> 
> -; TODO: Second bitcast can be folded into the first.
> +; Second bitcast can be folded into the first.
> 
> define i64 @bitcast_extelt2(<4 x float> %A) {
>   %bc1 = bitcast <4 x float> %A to <2 x double>
> @@ -88,9 +86,8 @@ define i64 @bitcast_extelt2(<4 x float>
>   ret i64 %bc2
> 
> ; CHECK-LABEL: @bitcast_extelt2(
> -; CHECK-NEXT:  %bc1 = bitcast <4 x float> %A to <2 x double>
> -; CHECK-NEXT:  %ext = extractelement <2 x double> %bc1, i32 1
> -; CHECK-NEXT:  %bc2 = bitcast double %ext to i64
> +; CHECK-NEXT:  %bc = bitcast <4 x float> %A to <2 x i64>
> +; CHECK-NEXT:  %bc2 = extractelement <2 x i64> %bc, i32 1
> ; CHECK-NEXT:  ret i64 %bc2
> }
> 
> 
> 
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