[llvm] [SROA] Propagate no-signed-zeros(nsz) fast-math flag on the phi node using function attribute (PR #83381)

[Matt Arsenault via llvm-commits]

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@@ -1113,6 +1113,14 @@ void PromoteMem2Reg::RenamePass(BasicBlock *BB, BasicBlock *Pred,
         for (unsigned i = 0; i != NumEdges; ++i)
           APN->addIncoming(IncomingVals[AllocaNo], Pred);
 
+        if (APN->isComplete() &&
+            APN->getFunction()
+                ->getFnAttribute("no-signed-zeros-fp-math")
+                .getValueAsBool() &&
+            isa<FPMathOperator>(APN)) {
+          APN->setHasNoSignedZeros(true);
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arsenm wrote:

I'd probably reorder these checks; FPOp = dyn_cast<FPMathOperator>(); if (FPop && fnAttrIsNSZ) { FPOp->setHasNoSignedZeros() } 

This also needs a comment explaining why this needs to be done 

https://github.com/llvm/llvm-project/pull/83381