[InstCombine] Preserve FMF for powi simplifications.

[spatel]

LGTM

llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp

887

It would be another patch, but if you're looking at powi code, it would be nice to update this with something like:

match(II->getArgOperand, m_APInt(ExponentC))

That would allow us to also match vector powi calls in addition to the current scalar calls.

[vdmitrie]

llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp

887

Thank you Sanjay for review. Regarding your suggestion, according to LangRef the exponent argument is always scalar for powi. So that would probably be just a cosmetic change as existing code handles vector types for "base" argument.

llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp

Instruction *InstCombinerImpl::visitCallInst(CallInst &CI) {
   case Intrinsic::masked_scatter:
     return simplifyMaskedScatter(*II);
   case Intrinsic::launder_invariant_group:
   case Intrinsic::strip_invariant_group:
     if (auto *SkippedBarrier = simplifyInvariantGroupIntrinsic(*II, *this))
       return replaceInstUsesWith(*II, SkippedBarrier);
     break;
   case Intrinsic::powi:
     if (ConstantInt *Power = dyn_cast<ConstantInt>(II->getArgOperand(1))) {
       // 0 and 1 are handled in instsimplify
-
       // powi(x, -1) -> 1/x
       if (Power->isMinusOne())
-        return BinaryOperator::CreateFDiv(ConstantFP::get(CI.getType(), 1.0),
-                                          II->getArgOperand(0));
+        return BinaryOperator::CreateFDivFMF(ConstantFP::get(CI.getType(), 1.0),
+                                             II->getArgOperand(0), II);
       // powi(x, 2) -> x*x
       if (Power->equalsInt(2))
-        return BinaryOperator::CreateFMul(II->getArgOperand(0),
-                                          II->getArgOperand(0));
+        return BinaryOperator::CreateFMulFMF(II->getArgOperand(0),
+                                             II->getArgOperand(0), II);
     }
     break;
 
   case Intrinsic::cttz:
   case Intrinsic::ctlz:
     if (auto *I = foldCttzCtlz(*II, *this))
       return I;
     break;

llvm/test/Transforms/InstCombine/intrinsics.ll

 define void @powi(double %V, double *%P) {
   %A = tail call fast double @llvm.powi.f64(double %V, i32 -1) nounwind
   store volatile double %A, double* %P
 
   %D = tail call fast double @llvm.powi.f64(double %V, i32 2) nounwind
   store volatile double %D, double* %P
   ret void
 ; CHECK-LABEL: @powi(
-; CHECK: %A = fdiv double 1.0{{.*}}, %V
+; CHECK: %A = fdiv fast double 1.0{{.*}}, %V
 ; CHECK: store volatile double %A,
-; CHECK: %D = fmul double %V, %V
+; CHECK: %D = fmul fast double %V, %V
 ; CHECK: store volatile double %D
 }
 
 define i32 @cttz(i32 %a) {
 ; CHECK-LABEL: @cttz(
 ; CHECK-NEXT:    ret i32 3
 ;
   %or = or i32 %a, 8