[llvm] 0eedc9e - [InstCombine] bitrev (zext i1 X) --> select X, SMinC, 0

[Philip Reames via llvm-commits]

I think you could replace the bitreverse with a left-shift by bitwidth-1 
if you wanted.

Not sure if that's better or worse than the select representation.

Philip

On 1/9/23 09:31, Sanjay Patel via llvm-commits wrote:
> Author: Sanjay Patel
> Date: 2023-01-09T12:27:37-05:00
> New Revision: 0eedc9e56712938ce4c01e14ba755dbdb2a0df18
>
> URL: https://github.com/llvm/llvm-project/commit/0eedc9e56712938ce4c01e14ba755dbdb2a0df18
> DIFF: https://github.com/llvm/llvm-project/commit/0eedc9e56712938ce4c01e14ba755dbdb2a0df18.diff
>
> LOG: [InstCombine] bitrev (zext i1 X) --> select X, SMinC, 0
>
> https://alive2.llvm.org/ce/z/ZXCtgi
>
> This breaks the infinite combine loop for issue #59897,
> but we may still need more changes to avoid those loops.
>
> Added:
>      
>
> Modified:
>      llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
>      llvm/test/Transforms/InstCombine/bitreverse.ll
>
> Removed:
>      
>
>
> ################################################################################
> diff  --git a/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp b/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
> index 59486312448e..65248eae6acb 100644
> --- a/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
> +++ b/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
> @@ -1441,6 +1441,18 @@ Instruction *InstCombinerImpl::visitCallInst(CallInst &CI) {
>   
>       break;
>     }
> +  case Intrinsic::bitreverse: {
> +    // bitrev (zext i1 X to ?) --> X ? SignBitC : 0
> +    Value *X;
> +    if (match(II->getArgOperand(0), m_ZExt(m_Value(X))) &&
> +        X->getType()->isIntOrIntVectorTy(1)) {
> +      Type *Ty = II->getType();
> +      APInt SignBit = APInt::getSignMask(Ty->getScalarSizeInBits());
> +      return SelectInst::Create(X, ConstantInt::get(Ty, SignBit),
> +                                ConstantInt::getNullValue(Ty));
> +    }
> +    break;
> +  }
>     case Intrinsic::bswap: {
>       Value *IIOperand = II->getArgOperand(0);
>   
>
> diff  --git a/llvm/test/Transforms/InstCombine/bitreverse.ll b/llvm/test/Transforms/InstCombine/bitreverse.ll
> index 232d69da9a02..ab41debc65c7 100644
> --- a/llvm/test/Transforms/InstCombine/bitreverse.ll
> +++ b/llvm/test/Transforms/InstCombine/bitreverse.ll
> @@ -323,7 +323,7 @@ define i32 @rev_i1(i1 %x) {
>   ; CHECK-LABEL: @rev_i1(
>   ; CHECK-NEXT:    [[Z:%.*]] = zext i1 [[X:%.*]] to i32
>   ; CHECK-NEXT:    call void @use_i32(i32 [[Z]])
> -; CHECK-NEXT:    [[R:%.*]] = call i32 @llvm.bitreverse.i32(i32 [[Z]])
> +; CHECK-NEXT:    [[R:%.*]] = select i1 [[X]], i32 -2147483648, i32 0
>   ; CHECK-NEXT:    ret i32 [[R]]
>   ;
>     %z = zext i1 %x to i32
> @@ -334,8 +334,7 @@ define i32 @rev_i1(i1 %x) {
>   
>   define <2 x i8> @rev_v2i1(<2 x i1> %x) {
>   ; CHECK-LABEL: @rev_v2i1(
> -; CHECK-NEXT:    [[Z:%.*]] = zext <2 x i1> [[X:%.*]] to <2 x i8>
> -; CHECK-NEXT:    [[R:%.*]] = call <2 x i8> @llvm.bitreverse.v2i8(<2 x i8> [[Z]])
> +; CHECK-NEXT:    [[R:%.*]] = select <2 x i1> [[X:%.*]], <2 x i8> <i8 -128, i8 -128>, <2 x i8> zeroinitializer
>   ; CHECK-NEXT:    ret <2 x i8> [[R]]
>   ;
>     %z = zext <2 x i1> %x to <2 x i8>
> @@ -353,3 +352,19 @@ define i32 @rev_i2(i2 %x) {
>     %r = call i32 @llvm.bitreverse.i32(i32 %z)
>     ret i32 %r
>   }
> +
> +; This used to infinite loop.
> +
> +define i64 @PR59897(i1 %X1_2) {
> +; CHECK-LABEL: @PR59897(
> +; CHECK-NEXT:    [[NOT_X1_2:%.*]] = xor i1 [[X1_2:%.*]], true
> +; CHECK-NEXT:    [[X0_3X2X5X0:%.*]] = zext i1 [[NOT_X1_2]] to i64
> +; CHECK-NEXT:    ret i64 [[X0_3X2X5X0]]
> +;
> +  %X1_3 = zext i1 %X1_2 to i32
> +  %X8_3x2x2x0 = call i32 @llvm.bitreverse.i32(i32 %X1_3)
> +  %X8_4x2x3x0 = xor i32 %X8_3x2x2x0, -1
> +  %X0_3x2x4x0 = lshr i32 %X8_4x2x3x0, 31
> +  %X0_3x2x5x0 = zext i32 %X0_3x2x4x0 to i64
> +  ret i64 %X0_3x2x5x0
> +}
>
>
>          
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