[llvm] r360366 - [CodeGen] Add comment about FSUB <-> FNEG xforms

[Cameron McInally via llvm-commits]

Author: mcinally
Date: Thu May  9 12:28:52 2019
New Revision: 360366

URL: http://llvm.org/viewvc/llvm-project?rev=360366&view=rev
Log:
[CodeGen] Add comment about FSUB <-> FNEG xforms

Differential Revision: https://reviews.llvm.org/D61741

Modified:
    llvm/trunk/lib/CodeGen/SelectionDAG/DAGCombiner.cpp

Modified: llvm/trunk/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/CodeGen/SelectionDAG/DAGCombiner.cpp?rev=360366&r1=360365&r2=360366&view=diff
==============================================================================
--- llvm/trunk/lib/CodeGen/SelectionDAG/DAGCombiner.cpp (original)
+++ llvm/trunk/lib/CodeGen/SelectionDAG/DAGCombiner.cpp Thu May  9 12:28:52 2019
@@ -11629,6 +11629,10 @@ SDValue DAGCombiner::visitFSUB(SDNode *N
   }
 
   // (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())) {