[llvm] Update foldFMulReassoc to respect absent fast-math flags (PR #88589)

[Andy Kaylor via llvm-commits]

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@@ -631,31 +631,38 @@ Instruction *InstCombinerImpl::foldFMulReassoc(BinaryOperator &I) {
   Value *Op1 = I.getOperand(1);
   Value *X, *Y;
   Constant *C;
+  BinaryOperator *Op0BinOp;
 
   // Reassociate constant RHS with another constant to form constant
   // expression.
-  if (match(Op1, m_Constant(C)) && C->isFiniteNonZeroFP()) {
+  if (match(Op1, m_Constant(C)) && C->isFiniteNonZeroFP() &&
+      match(Op0, m_AllowReassoc(m_BinOp(Op0BinOp)))) {
+    // Everything in this scope folds I with Op0, intersecting their FMF.
+    FastMathFlags FMF = I.getFastMathFlags() & Op0BinOp->getFastMathFlags();
+    IRBuilder<>::FastMathFlagGuard FMFGuard(Builder);
+    Builder.setFastMathFlags(FMF);
     Constant *C1;
     if (match(Op0, m_OneUse(m_FDiv(m_Constant(C1), m_Value(X))))) {
       // (C1 / X) * C --> (C * C1) / X
       Constant *CC1 =
           ConstantFoldBinaryOpOperands(Instruction::FMul, C, C1, DL);
       if (CC1 && CC1->isNormalFP())
-        return BinaryOperator::CreateFDivFMF(CC1, X, &I);
+        return BinaryOperator::CreateFDivFMF(CC1, X, FMF);
     }
     if (match(Op0, m_FDiv(m_Value(X), m_Constant(C1)))) {
+      // FIXME: This seems like it should also be checking for arcp
----------------
andykaylor wrote:

My thinking is that division is not associative, and you can only apply the associative property if you replace division with multiplication by the reciprocal. I agree that we're not consistent about this, but I don't see how skipping the arcp check can be justified.

FWIW, gcc thinks this requires reciprocal math: https://godbolt.org/z/T7xTTshbf

https://github.com/llvm/llvm-project/pull/88589