[llvm] r239701 - [InstSimplify] Add self-fdiv identities for -ffinite-math-only.

[Sean Silva]

On Sun, Jun 14, 2015 at 11:53 AM, Benjamin Kramer <benny.kra at googlemail.com>
wrote:

> Author: d0k
> Date: Sun Jun 14 13:53:58 2015
> New Revision: 239701
>
> URL: http://llvm.org/viewvc/llvm-project?rev=239701&view=rev
> Log:
> [InstSimplify] Add self-fdiv identities for -ffinite-math-only.
>
> When NaNs and Infs are ignored we can fold
>  X /  X -> 1.0
> -X /  X -> -1.0
>  X / -X -> -1.0
>
> Modified:
>     llvm/trunk/lib/Analysis/InstructionSimplify.cpp
>     llvm/trunk/test/Transforms/InstSimplify/fast-math.ll
>
> Modified: llvm/trunk/lib/Analysis/InstructionSimplify.cpp
> URL:
> http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Analysis/InstructionSimplify.cpp?rev=239701&r1=239700&r2=239701&view=diff
>
> ==============================================================================
> --- llvm/trunk/lib/Analysis/InstructionSimplify.cpp (original)
> +++ llvm/trunk/lib/Analysis/InstructionSimplify.cpp Sun Jun 14 13:53:58
> 2015
> @@ -1126,6 +1126,21 @@ static Value *SimplifyFDivInst(Value *Op
>    if (FMF.noNaNs() && FMF.noSignedZeros() && match(Op0, m_AnyZero()))
>      return Op0;
>
> +  if (FMF.noNaNs() && FMF.noInfs()) {
> +    // X / X -> 1.0 iff NaNs and infinities are ignored.
>

I'm not sure "iff NaNs and infinities are ignored" (in particular the
"iff") is the best way to put it. The situation I think is more like "is
legal when NaNs and infinities are ignored". Merely ignoring NaNs and
infinities does not imply that X/X is guaranteed to be 1.0.

Also, I don't think you need the noInfs assumption. The cases where
X/X->1.0 will fail have X/X->NaN.

-- Sean Silva


> +    if (Op0 == Op1)
> +      return ConstantFP::get(Op0->getType(), 1.0);
> +
> +    // -X /  X -> -1.0 and
> +    //  X / -X -> -1.0 iff NaNs and infinities are ignored.
> +    // We can ignore signed zeros because +-0.0/+-0.0 is NaN and ignored.
> +    if ((BinaryOperator::isFNeg(Op0, /*IgnoreZeroSign=*/true) &&
> +         BinaryOperator::getFNegArgument(Op0) == Op1) ||
> +        (BinaryOperator::isFNeg(Op1, /*IgnoreZeroSign=*/true) &&
> +         BinaryOperator::getFNegArgument(Op1) == Op0))
> +      return ConstantFP::get(Op0->getType(), -1.0);
> +  }
> +
>    return nullptr;
>  }
>
>
> Modified: llvm/trunk/test/Transforms/InstSimplify/fast-math.ll
> URL:
> http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/InstSimplify/fast-math.ll?rev=239701&r1=239700&r2=239701&view=diff
>
> ==============================================================================
> --- llvm/trunk/test/Transforms/InstSimplify/fast-math.ll (original)
> +++ llvm/trunk/test/Transforms/InstSimplify/fast-math.ll Sun Jun 14
> 13:53:58 2015
> @@ -114,3 +114,80 @@ define double @fdiv_zero_by_x(double %X)
>    ret double %r
>  ; CHECK: ret double 0
>  }
> +
> +define float @fdiv_self(float %f)  {
> +  %div = fdiv nnan ninf float %f, %f
> +  ret float %div
> +; CHECK-LABEL: fdiv_self
> +; CHECK: ret float 1.000000e+00
> +}
> +
> +define float @fdiv_self_invalid1(float %f)  {
> +  %div = fdiv ninf float %f, %f
> +  ret float %div
> +; CHECK-LABEL: fdiv_self_invalid1
> +; CHECK: %div = fdiv ninf float %f, %f
> +; CHECK-NEXT: ret float %div
> +}
> +
> +define float @fdiv_self_invalid2(float %f)  {
> +  %div = fdiv nnan float %f, %f
> +  ret float %div
> +; CHECK-LABEL: fdiv_self_invalid2
> +; CHECK: %div = fdiv nnan float %f, %f
> +; CHECK-NEXT: ret float %div
> +}
> +
> +define float @fdiv_self_invalid3(float %f)  {
> +  %div = fdiv float %f, %f
> +  ret float %div
> +; CHECK-LABEL: fdiv_self_invalid3
> +; CHECK: %div = fdiv float %f, %f
> +; CHECK-NEXT: ret float %div
> +}
> +
> +define float @fdiv_neg(float %f) {
> +  %neg = fsub fast float -0.000000e+00, %f
> +  %div = fdiv nnan ninf float %neg, %f
> +  ret float %div
> +; CHECK-LABEL: fdiv_neg
> +; CHECK: ret float -1.000000e+00
> +}
> +
> +define float @fdiv_neg_invalid1(float %f) {
> +  %neg = fsub fast float -0.000000e+00, %f
> +  %div = fdiv ninf float %neg, %f
> +  ret float %div
> +; CHECK-LABEL: fdiv_neg_invalid1
> +; CHECK: %neg = fsub fast float -0.000000e+00, %f
> +; CHECK-NEXT: %div = fdiv ninf float %neg, %f
> +; CHECK-NEXT: ret float %div
> +}
> +
> +define float @fdiv_neg_invalid2(float %f) {
> +  %neg = fsub fast float -0.000000e+00, %f
> +  %div = fdiv nnan float %neg, %f
> +  ret float %div
> +; CHECK-LABEL: fdiv_neg_invalid2
> +; CHECK: %neg = fsub fast float -0.000000e+00, %f
> +; CHECK-NEXT: %div = fdiv nnan float %neg, %f
> +; CHECK-NEXT: ret float %div
> +}
> +
> +define float @fdiv_neg_invalid3(float %f) {
> +  %neg = fsub fast float -0.000000e+00, %f
> +  %div = fdiv float %neg, %f
> +  ret float %div
> +; CHECK-LABEL: fdiv_neg_invalid3
> +; CHECK: %neg = fsub fast float -0.000000e+00, %f
> +; CHECK-NEXT: %div = fdiv float %neg, %f
> +; CHECK-NEXT: ret float %div
> +}
> +
> +define float @fdiv_neg_swapped(float %f) {
> +  %neg = fsub float 0.000000e+00, %f
> +  %div = fdiv nnan ninf float %f, %neg
> +  ret float %div
> +; CHECK-LABEL: fdiv_neg_swapped
> +; CHECK: ret float -1.000000e+00
> +}
>
>
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