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

[Philip Reames via llvm-commits]

Make sense!

Philip

On 1/9/23 10:59, Sanjay Patel wrote:
> Thanks! That’s how I had drafted it initially, but we canonicalize 
> that to the select form already. So I just made this go directly to 
> the end result.
>
> On Mon, Jan 9, 2023 at 1:35 PM Philip Reames 
> <listmail at philipreames.com> wrote:
>
>     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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