[llvm] r309578 - Do not recombine FMA when that is not needed.

[Amaury Sechet via llvm-commits]

Author: deadalnix
Date: Mon Jul 31 09:56:25 2017
New Revision: 309578

URL: http://llvm.org/viewvc/llvm-project?rev=309578&view=rev
Log:
Do not recombine FMA when that is not needed.

Summary: As per title. This creates useless recombines.

Reviewers: jyknight, nemanjai, mkuper, spatel, RKSimon, zvi, bkramer

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D33848

Modified:
    llvm/trunk/lib/Target/X86/X86ISelLowering.cpp

Modified: llvm/trunk/lib/Target/X86/X86ISelLowering.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Target/X86/X86ISelLowering.cpp?rev=309578&r1=309577&r2=309578&view=diff
==============================================================================
--- llvm/trunk/lib/Target/X86/X86ISelLowering.cpp (original)
+++ llvm/trunk/lib/Target/X86/X86ISelLowering.cpp Mon Jul 31 09:56:25 2017
@@ -34517,6 +34517,7 @@ static SDValue combineFMA(SDNode *N, Sel
 
   // Negative multiplication when NegA xor NegB
   bool NegMul = (NegA != NegB);
+  bool HasNeg = NegA || NegB || NegC;
 
   unsigned NewOpcode;
   if (!NegMul)
@@ -34524,6 +34525,14 @@ static SDValue combineFMA(SDNode *N, Sel
   else
     NewOpcode = (!NegC) ? X86ISD::FNMADD : X86ISD::FNMSUB;
 
+  // For FMA and FAMDD, we risk reconstructing the node we started with.
+  // In order to avoid this, we check for negation or opcode change. If
+  // one of the two happened, then it is a new node and we return it.
+  if (N->getOpcode() == X86ISD::FMADD || N->getOpcode() == ISD::FMA) {
+    if (HasNeg || NewOpcode != N->getOpcode())
+      return DAG.getNode(NewOpcode, dl, VT, A, B, C);
+    return SDValue();
+  }
 
   if (N->getOpcode() == X86ISD::FMADD_RND) {
     switch (NewOpcode) {
@@ -34547,12 +34556,15 @@ static SDValue combineFMA(SDNode *N, Sel
     case X86ISD::FNMSUB: NewOpcode = X86ISD::FNMSUBS3_RND; break;
     }
   } else {
-    assert((N->getOpcode() == X86ISD::FMADD || N->getOpcode() == ISD::FMA) &&
-           "Unexpected opcode!");
-    return DAG.getNode(NewOpcode, dl, VT, A, B, C);
+    llvm_unreachable("Unexpected opcode!");
   }
 
-  return DAG.getNode(NewOpcode, dl, VT, A, B, C, N->getOperand(3));
+  // Only return the node is the opcode was changed or one of the
+  // operand was negated. If not, we'll just recreate the same node.
+  if (HasNeg || NewOpcode != N->getOpcode())
+    return DAG.getNode(NewOpcode, dl, VT, A, B, C, N->getOperand(3));
+
+  return SDValue();
 }
 
 static SDValue combineZext(SDNode *N, SelectionDAG &DAG,