[PATCH] D148230: ValueTracking: fadd +0 cannot return -0

[Jay Foad via Phabricator via llvm-commits]

foad added inline comments.


================
Comment at: llvm/lib/Analysis/ValueTracking.cpp:4369
+    if (KnownRHS.KnownFPClasses == fcPosZero)
+      Known.knownNot(fcNegZero);
+
----------------
First, this is wrong for FSub, since -0 - +0 = -0

Second, you can generalize `KnownRHS.KnownFPClasses == fcPosZero` to `KnownRHS.isKnownNot(fcNegZero)` (or `fcPosZero` for FSub).

Third you could do a similar check on the LHS. For both FAdd and FSub, if the LHS is not -0 then the result can't be -0.


CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D148230/new/

https://reviews.llvm.org/D148230