[CodeGen] Add comment about FSUB <-> FNEG xforms
Differential Revision: https://reviews.llvm.org/D61741
llvm-svn: 360366
diff --git a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
index 4c9e34b..59f7af0 100644
--- a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -11629,6 +11629,10 @@
}
// (fsub -0.0, N1) -> -N1
+ // NOTE: It is safe to transform an FSUB(-0.0,X) into an FNEG(X), since the
+ // FSUB does not specify the sign bit of a NaN. Also note that for
+ // the same reason, the inverse transform is not safe, unless fast math
+ // flags are in play.
if (N0CFP && N0CFP->isZero()) {
if (N0CFP->isNegative() ||
(Options.NoSignedZerosFPMath || Flags.hasNoSignedZeros())) {
