[llvm] [Reassociate] Resolve FIXME: No Lowering of FNeg to FMul unless it is safe (PR #85252)

[Yingwei Zheng via llvm-commits]

================
@@ -287,11 +287,16 @@ static Instruction *CreateNeg(Value *S1, const Twine &Name,
 static BinaryOperator *LowerNegateToMultiply(Instruction *Neg) {
   assert((isa<UnaryOperator>(Neg) || isa<BinaryOperator>(Neg)) &&
          "Expected a Negate!");
-  // FIXME: It's not safe to lower a unary FNeg into a FMul by -1.0.
   unsigned OpNo = isa<BinaryOperator>(Neg) ? 1 : 0;
   Type *Ty = Neg->getType();
-  Constant *NegOne = Ty->isIntOrIntVectorTy() ?
-    ConstantInt::getAllOnesValue(Ty) : ConstantFP::get(Ty, -1.0);
+  // Only lower to FMul if the operation is not a unary FNeg or Neg has the
+  // correct flags.
+  if (Ty->isFloatingPointTy() && isa<UnaryOperator>(Neg) &&
+      !hasFPAssociativeFlags(Neg))
----------------
dtcxzyw wrote:

```suggestion
  if (Ty->isFPOrFPVectorTy() && isa<UnaryOperator>(Neg) &&
      !Neg->hasNoNaNs())
```

Please add a scalar test and a vector test.


https://github.com/llvm/llvm-project/pull/85252