[turbofan] Better narrow the derived type for the right shift operation.

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/bootstrapper.h"
 #include "src/compiler/graph-inl.h"
 #include "src/compiler/graph-reducer.h"
 #include "src/compiler/js-operator.h"
 #include "src/compiler/node.h"
 #include "src/compiler/node-properties-inl.h"
@@ skipping 898 matching lines @@
 
 Type* Typer::Visitor::JSShiftRightTyper(Type* lhs, Type* rhs, Typer* t) {
   lhs = NumberToInt32(ToNumber(lhs, t), t);
   rhs = NumberToUint32(ToNumber(rhs, t), t);
   double min = kMinInt;
   double max = kMaxInt;
   if (lhs->Min() >= 0) {
     // Right-shifting a non-negative value cannot make it negative, nor larger.
     min = std::max(min, 0.0);
     max = std::min(max, lhs->Max());
+    if (rhs->Min() > 0 && rhs->Max() <= 31)

[Benedikt Meurer, 2015/01/26 07:23:47]

Nit: please add { and }.

+      max = static_cast<int>(max) >> static_cast<int>(rhs->Min());
   }
   if (lhs->Max() < 0) {
     // Right-shifting a negative value cannot make it non-negative, nor smaller.
     min = std::max(min, lhs->Min());
     max = std::min(max, -1.0);
+    if (rhs->Min() > 0 && rhs->Max() <= 31)

[Benedikt Meurer, 2015/01/26 07:23:47]

Nit: please add { and }.

+      min = static_cast<int>(min) >> static_cast<int>(rhs->Min());
   }
   if (rhs->Min() > 0 && rhs->Max() <= 31) {
     // Right-shifting by a positive value yields a small integer value.
     double shift_min = kMinInt >> static_cast<int>(rhs->Min());
     double shift_max = kMaxInt >> static_cast<int>(rhs->Min());
     min = std::max(min, shift_min);
     max = std::min(max, shift_max);
   }
   // TODO(jarin) Ideally, the following micro-optimization should be performed
   // by the type constructor.
@@ skipping 1183 matching lines @@
       TYPED_ARRAYS(TYPED_ARRAY_CASE)
 #undef TYPED_ARRAY_CASE
     }
   }
   return Type::Constant(value, zone());
 }
 
 }  // namespace compiler
 }  // namespace internal
 }  // namespace v8