[PATCH] D148599: ValueTrackng: Handle sign bit for fptrunc

[Jay Foad via Phabricator via llvm-commits]

foad added a comment.

I feel like there are undocumented assumptions here.

Does fptrunc +/- 0.0 preserve sign?

Is Known.SignBit supposed to be accurate even for Nans? That seems unfortunate, since for a lot of operations you won't be able to propagate it unless you can prove something isn't Nan.



================
Comment at: llvm/lib/Analysis/ValueTracking.cpp:4083
+  // Sign should be preserved
+  if (KnownSrc.cannotBeOrderedLessThanZero())
+    Known.knownNot(NegExcept0);
----------------
Writing this as `KnownSrc.isKnownNot(NegExcept0)` would make it clearer that the source and result classes are the same here.

Also why can't this logic be mirrored for PosExcept0?


CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D148599/new/

https://reviews.llvm.org/D148599