[llvm-commits] [PATCH][fast-math, Instcombine] X * (cond ? 1.0 : 0.0) => cond ? X : 0.0

[Shuxin Yang]

Hi, Jakub:

    Yes, you are right. My code will miss the case when both operands 
are Select.
Please feel free to replace my code with yours (Or, do I need to revert 
my code first).

     Please note that I only "READ" your code, I didn't "REVIEW" the code --
I'm not eligible for code review.

Thanks
Shuxin

On 12/14/12 11:08 AM, Jakub Staszak wrote:
> The only thing which concerns me is case where both of FMUL operands are SelectInst:
>
> define double @select4(i32 %cond, double %x, double %y) {
>    %tobool = icmp ne i32 %cond, 0
>    %tobool2 = icmp ne i32 %cond, 1
>    %v0 = select i1 %tobool2, double %x, double %y
>    %cond1 = select i1 %tobool, double 0.000000e+00, double 1.000000e+00
>    %mul = fmul nnan nsz double %cond1, %v0
>    %add = fadd double %mul, %y
>    ret double %add
> }
>
> Your code won't optimized because first "Match" will be successful, and then it will fail to match ConstantFP.
>
> I have an alternative, which might be a little bit more expensive, but handles the code above.
> (If we decide to drop "match" and use old-school dyn_cast<> it might be cheaper though.)
>
> +  // X * cond ? 1.0 : 0.0 => cond ? X : 0.0
> +  if (I.hasNoNaNs() && I.hasNoSignedZeros()) {
> +    Value *V0 = I.getOperand(0);
> +    Value *V1 = I.getOperand(1);
> +    Value *Cond;
> +
> +    Value *T  = 0;
> +    if (match(V0, m_Select(m_Value(Cond), m_Zero(), m_FPOne())))
> +      // (cond ? 0.0 : 1.0) * V1
> +      T = Builder->CreateSelect(Cond, cast<SelectInst>(V0)->getOperand(1), V1);
> +    else if (match(V0, m_Select(m_Value(Cond), m_FPOne(), m_Zero())))
> +      // (cond ? 1.0 : 0.0) * V1
> +      T = Builder->CreateSelect(Cond, cast<SelectInst>(V0)->getOperand(2), V1);
> +    else if (match(V1, m_Select(m_Value(Cond), m_Zero(), m_FPOne())))
> +      // V0 * (cond ? 0.0 : 1.0)
> +      T = Builder->CreateSelect(Cond, cast<SelectInst>(V1)->getOperand(1), V0);
> +    else if (match(V1, m_Select(m_Value(Cond), m_FPOne(), m_Zero())))
> +      // V0 * (cond ? 1.0 : 0.0)
> +      T = Builder->CreateSelect(Cond, cast<SelectInst>(V1)->getOperand(2), V0);
> +
> +    if (T)
> +      return ReplaceInstUsesWith(I, T);
> +  }
> +
>
>
> - Kuba
>
> On Dec 14, 2012, at 6:58 PM, Nadav Rotem <nrotem at apple.com> wrote:
>
>> LGTM.
>>
>>
>> On Dec 13, 2012, at 2:24 PM, Shuxin Yang <shuxin.llvm at gmail.com> wrote:
>>
>>> Hi,
>>>
>>> The attached patch is to implement rule:
>>>   X * (cond ? 1.0 : 0.0) => Cond ? X : 0.0
>>>
>>> The multiply must be flagged n-signed-zero and n-nan.
>>>
>>> Thanks
>>> Shuxin
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