[turbofan] Improve typing for string::length property.

src/compiler/typer.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/base/flags.h"
 #include "src/bootstrapper.h"
 #include "src/compiler/graph-reducer.h"
 #include "src/compiler/js-operator.h"
 #include "src/compiler/node.h"
 #include "src/compiler/node-properties.h"
@@ skipping 1500 matching lines @@
     case Runtime::kInlineIsFunction:
     case Runtime::kInlineIsRegExp:
       return Bounds(Type::None(zone()), Type::Boolean(zone()));
     case Runtime::kInlineDoubleLo:
     case Runtime::kInlineDoubleHi:
       return Bounds(Type::None(zone()), Type::Signed32());
     case Runtime::kInlineConstructDouble:
     case Runtime::kInlineMathFloor:
     case Runtime::kInlineMathSqrt:
       return Bounds(Type::None(zone()), Type::Number());
+    case Runtime::kInlineStringGetLength:
+      // The string::length property is always an unsigned smi.
+      return Bounds(Type::None(), Type::Intersect(Type::UnsignedSmall(),
+                                                  Type::TaggedSigned()));
     default:
       break;
   }
   return Bounds::Unbounded(zone());
 }
 
 
 Bounds Typer::Visitor::TypeJSDebugger(Node* node) {
   UNREACHABLE();
   return Bounds();
@@ skipping 692 matching lines @@
       TYPED_ARRAYS(TYPED_ARRAY_CASE)
 #undef TYPED_ARRAY_CASE
     }
   }
   return Type::Constant(value, zone());
 }
 
 }  // namespace compiler
 }  // namespace internal
 }  // namespace v8