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

[Joshua Cranmer via llvm-commits]

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@@ -1701,6 +1701,31 @@ 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)
+  Value *Op0 = I.getOperand(0), *Op1 = I.getOperand(1);
+  auto *II = dyn_cast<IntrinsicInst>(Op1);
+  if (!II || II->getIntrinsicID() != Intrinsic::sqrt || !II->hasOneUse() ||
+      !I.hasAllowReassoc() || !I.hasAllowReciprocal())
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jcranmer-intel wrote:

Not quite, you'd need the `reassoc` on the first `fdiv` as well, and at that point, it's probably best to just require that all the operations have `reassoc` and `arcp`.

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