[turbofan] Add constant-folding for Float64RoundDown.

R=jarin@chromium.org

Review-Url: https://codereview.chromium.org/2616613002
Cr-Commit-Position: refs/heads/master@{#42058}

src/compiler/machine-operator-reducer.cc

@@ -664,6 +664,8 @@ Reduction MachineOperatorReducer::Reduce(Node* node) {
     case IrOpcode::kFloat64LessThan:
     case IrOpcode::kFloat64LessThanOrEqual:
       return ReduceFloat64Compare(node);
+    case IrOpcode::kFloat64RoundDown:
+      return ReduceFloat64RoundDown(node);
     default:
       break;
   }
@@ -1391,6 +1393,14 @@ Reduction MachineOperatorReducer::ReduceFloat64Compare(Node* node) {
   return NoChange();
 }
 
+Reduction MachineOperatorReducer::ReduceFloat64RoundDown(Node* node) {
+  DCHECK_EQ(IrOpcode::kFloat64RoundDown, node->opcode());
+  Float64Matcher m(node->InputAt(0));
+  if (m.HasValue()) {
+    return ReplaceFloat64(Floor(m.Value()));
+  }
+  return NoChange();
+}
 
 CommonOperatorBuilder* MachineOperatorReducer::common() const {
   return jsgraph()->common();

src/compiler/machine-operator-reducer.h

@@ -96,6 +96,7 @@ class V8_EXPORT_PRIVATE MachineOperatorReducer final
   Reduction ReduceFloat64InsertLowWord32(Node* node);
   Reduction ReduceFloat64InsertHighWord32(Node* node);
   Reduction ReduceFloat64Compare(Node* node);
+  Reduction ReduceFloat64RoundDown(Node* node);
 
   Graph* graph() const;
   JSGraph* jsgraph() const { return jsgraph_; }

test/unittests/compiler/machine-operator-reducer-unittest.cc

@@ -2085,8 +2085,19 @@ TEST_F(MachineOperatorReducerTest, Float64LessThanOrEqualWithFloat32Constant) {
 
 
 // -----------------------------------------------------------------------------
-// Store
+// Float64RoundDown
 
+TEST_F(MachineOperatorReducerTest, Float64RoundDownWithConstant) {
+  TRACED_FOREACH(double, x, kFloat64Values) {
+    Reduction r = Reduce(graph()->NewNode(
+        machine()->Float64RoundDown().placeholder(), Float64Constant(x)));
+    ASSERT_TRUE(r.Changed());
+    EXPECT_THAT(r.replacement(), IsFloat64Constant(Floor(x)));
+  }
+}
+
+// -----------------------------------------------------------------------------
+// Store
 
 TEST_F(MachineOperatorReducerTest, StoreRepWord8WithWord32And) {
   const StoreRepresentation rep(MachineRepresentation::kWord8, kNoWriteBarrier);