[llvm] [InstCombine] Fold more 'fcmp' 'select' instrs idioms into 'fabs' (PR #83381)

[Andy Kaylor via llvm-commits]

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@@ -2742,7 +2742,12 @@ static Instruction *foldSelectWithFCmpToFabs(SelectInst &SI,
     // Note: We require "nnan" for this fold because fcmp ignores the signbit
     //       of NAN, but IEEE-754 specifies the signbit of NAN values with
     //       fneg/fabs operations.
-    if (!SI.hasNoSignedZeros() || !SI.hasNoNaNs())
+    if (!SI.hasNoNaNs())
+      return nullptr;
+
+    bool FunctionHasNoSignedZeroes =
+        SI.getParent()->getParent()->hasFnAttribute("no-signed-zeros-fp-math");
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andykaylor wrote:

In principle, I agree with @arsenm about not introducing additional uses of the attribute. I said in #51601 that I couldn't see a way around it, but @nikic's suggestion about setting the flag somewhere based on the attribute might be a way around this. I can't see a reason that it wouldn't be legal for SROA to set the nsz flag when it eliminates the stores and loads in this example: https://godbolt.org/z/fahb8oM9o

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