[llvm] [InstCombine] Resolve TODO: nnan nsz X / -0.0 -> copysign(inf, X) (PR #79766)

[Matt Arsenault via llvm-commits]

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@@ -1600,7 +1600,17 @@ Instruction *InstCombinerImpl::foldFDivConstantDivisor(BinaryOperator &I) {
   // nnan X / +0.0 -> copysign(inf, X)
   if (I.hasNoNaNs() && match(I.getOperand(1), m_Zero())) {
     IRBuilder<> B(&I);
-    // TODO: nnan nsz X / -0.0 -> copysign(inf, X)
+    CallInst *CopySign = B.CreateIntrinsic(
+        Intrinsic::copysign, {C->getType()},
+        {ConstantFP::getInfinity(I.getType()), I.getOperand(0)}, &I);
+    CopySign->takeName(&I);
+    return replaceInstUsesWith(I, CopySign);
+  } else if (I.hasNoNaNs() && I.hasNoSignedZeros() &&
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arsenm wrote:

In a follow up patch, if you wanted to be more aggressive you could also check for known-non-nan sources 

https://github.com/llvm/llvm-project/pull/79766