[turbofan] IA: support better left operand for commutative binops

src/compiler/x64/instruction-selector-x64.cc

Index: src/compiler/x64/instruction-selector-x64.cc
diff --git a/src/compiler/x64/instruction-selector-x64.cc b/src/compiler/x64/instruction-selector-x64.cc
index 96501e686b3b7eb0a73ee519c4893d7b14e61af4..c9c1aac780db853b1bb172ed6998d5bdce7241d2 100644
--- a/src/compiler/x64/instruction-selector-x64.cc
+++ b/src/compiler/x64/instruction-selector-x64.cc
@@ -52,6 +52,8 @@ class X64OperandGenerator FINAL : public OperandGenerator {
         return false;
     }
   }
+
+  bool CanBeBetterLeftOperand(Node* node) { return !selector()->IsUsed(node); }
 };
 
 
@@ -178,20 +180,24 @@ static void VisitBinop(InstructionSelector* selector, Node* node,
                        InstructionCode opcode, FlagsContinuation* cont) {
   X64OperandGenerator g(selector);
   Int32BinopMatcher m(node);
+  Node* left = m.left().node();
+  Node* right = m.right().node();
   InstructionOperand* inputs[4];
   size_t input_count = 0;
   InstructionOperand* outputs[2];
   size_t output_count = 0;
 
   // TODO(turbofan): match complex addressing modes.
-  // TODO(turbofan): if commutative, pick the non-live-in operand as the left as
-  // this might be the last use and therefore its register can be reused.
-  if (g.CanBeImmediate(m.right().node())) {
-    inputs[input_count++] = g.Use(m.left().node());
-    inputs[input_count++] = g.UseImmediate(m.right().node());
+  if (g.CanBeImmediate(right)) {
+    inputs[input_count++] = g.Use(left);
+    inputs[input_count++] = g.UseImmediate(right);
   } else {
-    inputs[input_count++] = g.UseRegister(m.left().node());
-    inputs[input_count++] = g.Use(m.right().node());
+    if (node->op()->HasProperty(Operator::kCommutative) &&
+        g.CanBeBetterLeftOperand(right)) {
+      std::swap(left, right);
+    }
+    inputs[input_count++] = g.UseRegister(left);
+    inputs[input_count++] = g.Use(right);
   }
 
   if (cont->IsBranch()) {
@@ -392,16 +398,16 @@ void InstructionSelector::VisitInt64Sub(Node* node) {
 static void VisitMul(InstructionSelector* selector, Node* node,
                      ArchOpcode opcode) {
   X64OperandGenerator g(selector);
-  Node* left = node->InputAt(0);
-  Node* right = node->InputAt(1);
+  Int32BinopMatcher m(node);
+  Node* left = m.left().node();
+  Node* right = m.right().node();
   if (g.CanBeImmediate(right)) {
     selector->Emit(opcode, g.DefineAsRegister(node), g.Use(left),
                    g.UseImmediate(right));
-  } else if (g.CanBeImmediate(left)) {
-    selector->Emit(opcode, g.DefineAsRegister(node), g.Use(right),
-                   g.UseImmediate(left));
   } else {
-    // TODO(turbofan): select better left operand.
+    if (g.CanBeBetterLeftOperand(right)) {
+      std::swap(left, right);
+    }
     selector->Emit(opcode, g.DefineSameAsFirst(node), g.UseRegister(left),
                    g.Use(right));
   }