[turbofan] Add constant-folding for Float64RoundDown.

src/compiler/machine-operator-reducer.cc

 // Copyright 2014 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "src/compiler/machine-operator-reducer.h"
 
 #include "src/base/bits.h"
 #include "src/base/division-by-constant.h"
 #include "src/base/ieee754.h"
 #include "src/codegen.h"
@@ skipping 646 matching lines @@
     case IrOpcode::kFloat64InsertHighWord32:
       return ReduceFloat64InsertHighWord32(node);
     case IrOpcode::kStore:
     case IrOpcode::kUnalignedStore:
     case IrOpcode::kCheckedStore:
       return ReduceStore(node);
     case IrOpcode::kFloat64Equal:
     case IrOpcode::kFloat64LessThan:
     case IrOpcode::kFloat64LessThanOrEqual:
       return ReduceFloat64Compare(node);
+    case IrOpcode::kFloat64RoundDown:
+      return ReduceFloat64RoundDown(node);
     default:
       break;
   }
   return NoChange();
 }
 
 Reduction MachineOperatorReducer::ReduceInt32Add(Node* node) {
   DCHECK_EQ(IrOpcode::kInt32Add, node->opcode());
   Int32BinopMatcher m(node);
   if (m.right().Is(0)) return Replace(m.left().node());  // x + 0 => x
@@ skipping 707 matching lines @@
                : m.left().InputAt(0));
     node->ReplaceInput(
         1, m.right().HasValue()
                ? Float32Constant(static_cast<float>(m.right().Value()))
                : m.right().InputAt(0));
     return Changed(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();
 }
 
 
 MachineOperatorBuilder* MachineOperatorReducer::machine() const {
   return jsgraph()->machine();
 }
 
 
 Graph* MachineOperatorReducer::graph() const { return jsgraph()->graph(); }
 
 }  // namespace compiler
 }  // namespace internal
 }  // namespace v8