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

[Jakub Staszak]

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
>> <mpy_select.patch>_______________________________________________
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