Add comment about FSUB(-0.0,X) <--> FNEG(X) transforms

[spatel]

LGTM - thanks!

llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp

11632–11634

Please spell out "transform" - "xform" may not be obvious even for native English readers.

llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp

SDValue DAGCombiner::visitFSUB(SDNode *N) {
 
   if (N0 == N1) {
     // (fsub x, x) -> 0.0
     if (Options.UnsafeFPMath || Flags.hasNoNaNs())
       return DAG.getConstantFP(0.0f, DL, VT);
   }
 
   // (fsub -0.0, N1) -> -N1
+  // NOTE: It is safe to xform 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 xform is not safe, unless fast math
+  //       flags are in play.
   if (N0CFP && N0CFP->isZero()) {
     if (N0CFP->isNegative() ||
         (Options.NoSignedZerosFPMath || Flags.hasNoSignedZeros())) {
       if (isNegatibleForFree(N1, LegalOperations, TLI, &Options, ForCodeSize))
         return GetNegatedExpression(N1, DAG, LegalOperations, ForCodeSize);
       if (!LegalOperations || TLI.isOperationLegal(ISD::FNEG, VT))
         return DAG.getNode(ISD::FNEG, DL, VT, N1, Flags);
     }