From 4358c5217dc7a16d17c931a291489ec1d3f69902 Mon Sep 17 00:00:00 2001
From: Amaury Sechet <deadalnix@gmail.com>
Date: Mon, 31 Jul 2017 16:56:25 +0000
Subject: [PATCH] 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

llvm-svn: 309578
---
 llvm/lib/Target/X86/X86ISelLowering.cpp | 20 ++++++++++++++++----
 1 file changed, 16 insertions(+), 4 deletions(-)

diff --git a/llvm/lib/Target/X86/X86ISelLowering.cpp b/llvm/lib/Target/X86/X86ISelLowering.cpp
index da9adcb50fba..5448c28c6da8 100644
--- a/llvm/lib/Target/X86/X86ISelLowering.cpp
+++ b/llvm/lib/Target/X86/X86ISelLowering.cpp
@@ -34517,6 +34517,7 @@ static SDValue combineFMA(SDNode *N, SelectionDAG &DAG,
 
   // 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, SelectionDAG &DAG,
   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, SelectionDAG &DAG,
     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,