[llvm] 89a8a0c - [InstCombine] Inherit exact flags on extended shifts in trunc (lshr (sext A), C) --> (ashr A, C)

[Roman Lebedev via llvm-commits]

Thanks

On Tue, Sep 29, 2020 at 5:32 PM Simon Pilgrim via llvm-commits
<llvm-commits at lists.llvm.org> wrote:
>
>
> Author: Simon Pilgrim
> Date: 2020-09-29T15:32:09+01:00
> New Revision: 89a8a0c910422b9d363120769e2eebda03394b0f
>
> URL: https://github.com/llvm/llvm-project/commit/89a8a0c910422b9d363120769e2eebda03394b0f
> DIFF: https://github.com/llvm/llvm-project/commit/89a8a0c910422b9d363120769e2eebda03394b0f.diff
>
> LOG: [InstCombine] Inherit exact flags on extended shifts in trunc (lshr (sext A), C) --> (ashr A, C)
>
> This was missed in D88475
>
> Added:
>
>
> Modified:
>     llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
>     llvm/test/Transforms/InstCombine/cast.ll
>
> Removed:
>
>
>
> ################################################################################
> diff  --git a/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp b/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
> index ca55c8f5a887..fb885790d448 100644
> --- a/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
> +++ b/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
> @@ -831,6 +831,8 @@ Instruction *InstCombinerImpl::visitTrunc(TruncInst &Trunc) {
>    if (match(Src, m_LShr(m_SExt(m_Value(A)), m_Constant(C)))) {
>      unsigned AWidth = A->getType()->getScalarSizeInBits();
>      unsigned MaxShiftAmt = SrcWidth - std::max(DestWidth, AWidth);
> +    auto *OldSh = cast<Instruction>(Src);
> +    bool IsExact = OldSh->isExact();
>
>      // If the shift is small enough, all zero bits created by the shift are
>      // removed by the trunc.
> @@ -843,7 +845,8 @@ Instruction *InstCombinerImpl::visitTrunc(TruncInst &Trunc) {
>          Constant *MaxAmt = ConstantInt::get(SrcTy, DestWidth - 1, false);
>          Constant *ShAmt = ConstantExpr::getUMin(C, MaxAmt);
>          ShAmt = ConstantExpr::getTrunc(ShAmt, A->getType());
> -        return BinaryOperator::CreateAShr(A, ShAmt);
> +        return IsExact ? BinaryOperator::CreateExactAShr(A, ShAmt)
> +                       : BinaryOperator::CreateAShr(A, ShAmt);
>        }
>        // The types are mismatched, so create a cast after shifting:
>        // trunc (lshr (sext A), C) --> sext/trunc (ashr A, C)
> @@ -851,7 +854,7 @@ Instruction *InstCombinerImpl::visitTrunc(TruncInst &Trunc) {
>          Constant *MaxAmt = ConstantInt::get(SrcTy, AWidth - 1, false);
>          Constant *ShAmt = ConstantExpr::getUMin(C, MaxAmt);
>          ShAmt = ConstantExpr::getTrunc(ShAmt, A->getType());
> -        Value *Shift = Builder.CreateAShr(A, ShAmt);
> +        Value *Shift = Builder.CreateAShr(A, ShAmt, "", IsExact);
>          return CastInst::CreateIntegerCast(Shift, DestTy, true);
>        }
>      }
>
> diff  --git a/llvm/test/Transforms/InstCombine/cast.ll b/llvm/test/Transforms/InstCombine/cast.ll
> index db6b550f5faa..97439606973c 100644
> --- a/llvm/test/Transforms/InstCombine/cast.ll
> +++ b/llvm/test/Transforms/InstCombine/cast.ll
> @@ -1548,7 +1548,7 @@ define i8 @trunc_lshr_sext(i8 %A) {
>
>  define i8 @trunc_lshr_sext_exact(i8 %A) {
>  ; ALL-LABEL: @trunc_lshr_sext_exact(
> -; ALL-NEXT:    [[D:%.*]] = ashr i8 [[A:%.*]], 6
> +; ALL-NEXT:    [[D:%.*]] = ashr exact i8 [[A:%.*]], 6
>  ; ALL-NEXT:    ret i8 [[D]]
>  ;
>    %B = sext i8 %A to i32
> @@ -1718,7 +1718,7 @@ define i8 @trunc_lshr_sext_wide_input(i16 %A) {
>
>  define i8 @trunc_lshr_sext_wide_input_exact(i16 %A) {
>  ; ALL-LABEL: @trunc_lshr_sext_wide_input_exact(
> -; ALL-NEXT:    [[TMP1:%.*]] = ashr i16 [[A:%.*]], 9
> +; ALL-NEXT:    [[TMP1:%.*]] = ashr exact i16 [[A:%.*]], 9
>  ; ALL-NEXT:    [[D:%.*]] = trunc i16 [[TMP1]] to i8
>  ; ALL-NEXT:    ret i8 [[D]]
>  ;
>
>
>
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