[PATCH] D77642: [InstCombine] exclude bitcast of ppc_fp128 in icmp signbit fold

[Eric Christopher via llvm-commits]

Could use a comment about what the transform is doing and why we're
disallowing the ppc float, otherwise LGTM.

On Tue, Apr 7, 2020, 5:23 AM Sanjay Patel via Phabricator <
reviews at reviews.llvm.org> wrote:

> spatel created this revision.
> spatel added reviewers: echristo, kbarton.
> Herald added subscribers: hiraditya, mcrosier.
>
> Based on the post-commit comments for rG0f56bbc <
> https://reviews.llvm.org/rG0f56bbc1a5b2ddc881d1c55c9024b9c473dac6f0>,
> there might be a problem with this transform:
>
>   (bitcast (fpext/fptrunc X)) to iX) < 0 --> (bitcast X to iY) < 0
>
> ...and the ppc_fp128 data type, so conservatively bypass if we are
> bitcasting a ppc_fp128.
>
>
> https://reviews.llvm.org/D77642
>
> Files:
>   llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
>   llvm/test/Transforms/InstCombine/icmp.ll
>
>
> Index: llvm/test/Transforms/InstCombine/icmp.ll
> ===================================================================
> --- llvm/test/Transforms/InstCombine/icmp.ll
> +++ llvm/test/Transforms/InstCombine/icmp.ll
> @@ -3700,8 +3700,9 @@
>
>  define i1 @signbit_bitcast_fpext_ppc_fp128(float %x) {
>  ; CHECK-LABEL: @signbit_bitcast_fpext_ppc_fp128(
> -; CHECK-NEXT:    [[TMP1:%.*]] = bitcast float [[X:%.*]] to i32
> -; CHECK-NEXT:    [[S4:%.*]] = icmp slt i32 [[TMP1]], 0
> +; CHECK-NEXT:    [[S2:%.*]] = fpext float [[X:%.*]] to ppc_fp128
> +; CHECK-NEXT:    [[S3:%.*]] = bitcast ppc_fp128 [[S2]] to i128
> +; CHECK-NEXT:    [[S4:%.*]] = icmp slt i128 [[S3]], 0
>  ; CHECK-NEXT:    ret i1 [[S4]]
>  ;
>    %s2 = fpext float %x to ppc_fp128
> Index: llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
> ===================================================================
> --- llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
> +++ llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
> @@ -2766,8 +2766,8 @@
>      // the FP cast because the FP cast does not change the sign-bit.
>      const APInt *C;
>      bool TrueIfSigned;
> -    if (match(Op1, m_APInt(C)) && Bitcast->hasOneUse() &&
> -        isSignBitCheck(Pred, *C, TrueIfSigned)) {
> +    if (!BCSrcOp->getType()->isPPC_FP128Ty() && match(Op1, m_APInt(C)) &&
> +        Bitcast->hasOneUse() && isSignBitCheck(Pred, *C, TrueIfSigned)) {
>        if (match(BCSrcOp, m_FPExt(m_Value(X))) ||
>            match(BCSrcOp, m_FPTrunc(m_Value(X)))) {
>          // (bitcast (fpext/fptrunc X)) to iX) < 0 --> (bitcast X to iY) <
> 0
>
>
>
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