[PATCH] D139310: InstCombine: Fold fneg (copysign x, y) -> copysign x, (fneg y)

[Matt Arsenault via Phabricator via llvm-commits]

arsenm closed this revision.
arsenm added a comment.

93ea500a3d15bb7d4ddfb1f682c97002e2969304



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Comment at: llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp:2443-2446
+    // The source copysign has an additional value input, so we can't propagate
+    // flags the copysign doesn't also have.
+    FastMathFlags FMF = I.getFastMathFlags();
+    FMF &= cast<FPMathOperator>(OneUse)->getFastMathFlags();
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jcranmer-intel wrote:
> It took me some thinking through all the cases, but I'm now convinced that it's required that both the `fneg` and `copysign` *need* to get the intersection of fast-math flags.
I eventually just wrote out all the cases and let alive2 tell me it was wrong


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