[llvm] [InstCombine] Optimise x / sqrt(y / z) with fast-math pattern. (PR #76737)

[Matt Arsenault via llvm-commits]

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@@ -1701,6 +1701,33 @@ static Instruction *foldFDivPowDivisor(BinaryOperator &I,
   return BinaryOperator::CreateFMulFMF(Op0, Pow, &I);
 }
 
+/// Convert div to mul if we have an sqrt divisor iff sqrt's operand is a fdiv
+/// instruction.
+static Instruction *foldFDivSqrtDivisor(BinaryOperator &I,
+                                        InstCombiner::BuilderTy &Builder) {
+  // X / sqrt(Y / Z) -->  X * sqrt(Z / Y)
+  if (!I.hasAllowReassoc() || !I.hasAllowReciprocal())
+    return nullptr;
+  Value *Op0 = I.getOperand(0), *Op1 = I.getOperand(1);
+  auto *II = dyn_cast<IntrinsicInst>(Op1);
+  if (!II || II->getIntrinsicID() != Intrinsic::sqrt || !II->hasOneUse() ||
+      !II->hasAllowReassoc() || !II->hasAllowReciprocal())
+    return nullptr;
+
+  Value *Y, *Z;
+  auto *DivOp = dyn_cast<Instruction>(II->getOperand(0));
+  if (!DivOp || !DivOp->hasOneUse() || !DivOp->hasAllowReassoc() ||
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arsenm wrote:

Yes

https://github.com/llvm/llvm-project/pull/76737