[llvm] r325148 - [InstCombine] Don't fold select(C, Z, binop(select(C, X, Y), W)) -> select(C, Z, binop(Y, W)) if the binop is rem or div.

[Hans Wennborg via llvm-commits]

Merged to 6.0 in r325501.

On Wed, Feb 14, 2018 at 7:08 PM, Craig Topper via llvm-commits
<llvm-commits at lists.llvm.org> wrote:
> Author: ctopper
> Date: Wed Feb 14 10:08:33 2018
> New Revision: 325148
>
> URL: http://llvm.org/viewvc/llvm-project?rev=325148&view=rev
> Log:
> [InstCombine] Don't fold select(C, Z, binop(select(C, X, Y), W)) -> select(C, Z, binop(Y, W)) if the binop is rem or div.
>
> The select may have been preventing a division by zero or INT_MIN/-1 so removing it might not be safe.
>
> Fixes PR36362.
>
> Differential Revision: https://reviews.llvm.org/D43276
>
> Added:
>     llvm/trunk/test/Transforms/InstCombine/pr36362.ll
> Modified:
>     llvm/trunk/lib/Transforms/InstCombine/InstCombineSelect.cpp
>
> Modified: llvm/trunk/lib/Transforms/InstCombine/InstCombineSelect.cpp
> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/InstCombine/InstCombineSelect.cpp?rev=325148&r1=325147&r2=325148&view=diff
> ==============================================================================
> --- llvm/trunk/lib/Transforms/InstCombine/InstCombineSelect.cpp (original)
> +++ llvm/trunk/lib/Transforms/InstCombine/InstCombineSelect.cpp Wed Feb 14 10:08:33 2018
> @@ -1760,11 +1760,25 @@ Instruction *InstCombiner::visitSelectIn
>      }
>    }
>
> +  auto canMergeSelectThroughBinop = [](BinaryOperator *BO) {
> +    // The select might be preventing a division by 0.
> +    switch (BO->getOpcode()) {
> +    default:
> +      return true;
> +    case Instruction::SRem:
> +    case Instruction::URem:
> +    case Instruction::SDiv:
> +    case Instruction::UDiv:
> +      return false;
> +    }
> +  };
> +
>    // Try to simplify a binop sandwiched between 2 selects with the same
>    // condition.
>    // select(C, binop(select(C, X, Y), W), Z) -> select(C, binop(X, W), Z)
>    BinaryOperator *TrueBO;
> -  if (match(TrueVal, m_OneUse(m_BinOp(TrueBO)))) {
> +  if (match(TrueVal, m_OneUse(m_BinOp(TrueBO))) &&
> +      canMergeSelectThroughBinop(TrueBO)) {
>      if (auto *TrueBOSI = dyn_cast<SelectInst>(TrueBO->getOperand(0))) {
>        if (TrueBOSI->getCondition() == CondVal) {
>          TrueBO->setOperand(0, TrueBOSI->getTrueValue());
> @@ -1783,7 +1797,8 @@ Instruction *InstCombiner::visitSelectIn
>
>    // select(C, Z, binop(select(C, X, Y), W)) -> select(C, Z, binop(Y, W))
>    BinaryOperator *FalseBO;
> -  if (match(FalseVal, m_OneUse(m_BinOp(FalseBO)))) {
> +  if (match(FalseVal, m_OneUse(m_BinOp(FalseBO))) &&
> +      canMergeSelectThroughBinop(FalseBO)) {
>      if (auto *FalseBOSI = dyn_cast<SelectInst>(FalseBO->getOperand(0))) {
>        if (FalseBOSI->getCondition() == CondVal) {
>          FalseBO->setOperand(0, FalseBOSI->getFalseValue());
>
> Added: llvm/trunk/test/Transforms/InstCombine/pr36362.ll
> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/InstCombine/pr36362.ll?rev=325148&view=auto
> ==============================================================================
> --- llvm/trunk/test/Transforms/InstCombine/pr36362.ll (added)
> +++ llvm/trunk/test/Transforms/InstCombine/pr36362.ll Wed Feb 14 10:08:33 2018
> @@ -0,0 +1,17 @@
> +; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
> +;RUN: opt -instcombine -S %s | FileCheck %s
> +
> +; We shouldn't remove the select before the srem
> +define i32 @foo(i1 %a, i32 %b, i32 %c) {
> +; CHECK-LABEL: @foo(
> +; CHECK-NEXT:    [[SEL1:%.*]] = select i1 [[A:%.*]], i32 [[B:%.*]], i32 -1
> +; CHECK-NEXT:    [[REM:%.*]] = srem i32 [[C:%.*]], [[SEL1]]
> +; CHECK-NEXT:    [[SEL2:%.*]] = select i1 [[A]], i32 [[REM]], i32 0
> +; CHECK-NEXT:    ret i32 [[SEL2]]
> +;
> +  %sel1 = select i1 %a, i32 %b, i32 -1
> +  %rem = srem i32 %c, %sel1
> +  %sel2 = select i1 %a, i32 %rem, i32 0
> +  ret i32 %sel2
> +}
> +
>
>
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