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

[Benjamin Kramer]

On Tue, Jun 16, 2015 at 2:09 AM Sean Silva <chisophugis at gmail.com> wrote:

> 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.
>

Removed ninf check and rephrased comment in r239821. Thanks for the review
:)

- Ben


>
>
>> +    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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