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

src/compiler/ia32/instruction-selector-ia32.cc

Index: src/compiler/ia32/instruction-selector-ia32.cc
diff --git a/src/compiler/ia32/instruction-selector-ia32.cc b/src/compiler/ia32/instruction-selector-ia32.cc
index ce8cb0f6306410a2c5999553418d0388b011ad80..4a314ae46740abacc57d9dd6c88214b6751de30a 100644
--- a/src/compiler/ia32/instruction-selector-ia32.cc
+++ b/src/compiler/ia32/instruction-selector-ia32.cc
@@ -37,6 +37,10 @@ class IA32OperandGenerator FINAL : public OperandGenerator {
         return false;
     }
   }
+
+  bool CanBeBetterLeftOperand(Node* node) const {
+    return !selector()->IsLive(node);
+  }
 };
 
 
@@ -166,20 +170,24 @@ static void VisitBinop(InstructionSelector* selector, Node* node,
                        InstructionCode opcode, FlagsContinuation* cont) {
   IA32OperandGenerator 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()) {
@@ -296,16 +304,16 @@ void InstructionSelector::VisitInt32Sub(Node* node) {
 
 void InstructionSelector::VisitInt32Mul(Node* node) {
   IA32OperandGenerator g(this);
-  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)) {
     Emit(kIA32Imul, g.DefineAsRegister(node), g.Use(left),
          g.UseImmediate(right));
-  } else if (g.CanBeImmediate(left)) {
-    Emit(kIA32Imul, g.DefineAsRegister(node), g.Use(right),
-         g.UseImmediate(left));
   } else {
-    // TODO(turbofan): select better left operand.
+    if (g.CanBeBetterLeftOperand(right)) {
+      std::swap(left, right);
+    }
     Emit(kIA32Imul, g.DefineSameAsFirst(node), g.UseRegister(left),
          g.Use(right));
   }