DAG: Stop trying to fold FP -(x-y) -> y-x in getNode with nsz

This was increasing the number of instructions when fsub was legalized
on AMDGPU with no signed zeros enabled. This fold should be guarded by
hasOneUse, and I don't think getNode should be doing that. The same
fold is already done as a regular combine through isNegatibleForFree.

This does require duplicating, even though isNegatibleForFree does
this combine already (and properly checks hasOneUse) to avoid one PPC
regression. In the regression, the outer fneg has nsz but the fsub
operand does not. isNegatibleForFree only sees the operand, and
doesn't see it's used from a nsz context. A nsz parameter needs to be
added and threaded through isNegatibleForFree to avoid this.

2 files changed, 10 insertions, 5 deletions

diff --git a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
index 02ae11d8a00..1415b1e37d1 100644
--- a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -13350,6 +13350,16 @@ SDValue DAGCombiner::visitFNEG(SDNode *N) {
   if (TLI.isNegatibleForFree(N0, DAG, LegalOperations, ForCodeSize))
     return TLI.getNegatedExpression(N0, DAG, LegalOperations, ForCodeSize);
 
+  // -(X-Y) -> (Y-X) is unsafe because when X==Y, -0.0 != +0.0 FIXME: This is
+  // duplicated in isNegatibleForFree, but isNegatibleForFree doesn't know it
+  // was called from a context with a nsz flag if the input fsub does not.
+  if (N0.getOpcode() == ISD::FSUB &&
+      (DAG.getTarget().Options.NoSignedZerosFPMath ||
+       N->getFlags().hasNoSignedZeros()) && N0.hasOneUse()) {
+    return DAG.getNode(ISD::FSUB, SDLoc(N), VT, N0.getOperand(1),
+                       N0.getOperand(0), N->getFlags());
+  }
+
   // Transform fneg(bitconvert(x)) -> bitconvert(x ^ sign) to avoid loading
   // constant pool values.
   if (!TLI.isFNegFree(VT) &&
diff --git a/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp b/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
index a808b399596..6dcf271a81c 100644
--- a/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
@@ -4688,11 +4688,6 @@ SDValue SelectionDAG::getNode(unsigned Opcode, const SDLoc &DL, EVT VT,
     if (OpOpcode == ISD::UNDEF)
       return getUNDEF(VT);
 
-    // -(X-Y) -> (Y-X) is unsafe because when X==Y, -0.0 != +0.0
-    if ((getTarget().Options.NoSignedZerosFPMath || Flags.hasNoSignedZeros()) &&
-        OpOpcode == ISD::FSUB)
-      return getNode(ISD::FSUB, DL, VT, Operand.getOperand(1),
-                     Operand.getOperand(0), Flags);
     if (OpOpcode == ISD::FNEG)  // --X -> X
       return Operand.getOperand(0);
     break;