Add faster, but unsafe version of LoadInternalField.

src/fast-accessor-assembler.h

 // Copyright 2015 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.
 
 #ifndef V8_FAST_ACCESSOR_ASSEMBLER_H_
 #define V8_FAST_ACCESSOR_ASSEMBLER_H_
 
 #include <stdint.h>
 #include <memory>
 #include <vector>
@@ skipping 36 matching lines @@
   typedef v8::experimental::FastAccessorBuilder::LabelId LabelId;
   typedef v8::FunctionCallback FunctionCallback;
 
   explicit FastAccessorAssembler(Isolate* isolate);
   ~FastAccessorAssembler();
 
   // Builder / assembler functions:
   ValueId IntegerConstant(int int_constant);
   ValueId GetReceiver();
   ValueId LoadInternalField(ValueId value_id, int field_no);
+
+  // Unsafe but fast version, does NOT perform any sanity checks.

[vogelheim, 2016/07/27 09:50:56]

nitpick: Your description is correct, but unnecessarily dramatic. :) How about
this:

Loads internal field and assumes the object is indeed a valid API object with
the proper internal fields present.

The intended use is to call this on an object whose structure has already been
checked previously, e.g. the accessor's receiver, which is map-checked before
the fast accessor is called on it. Using this on an arbitrary object will result
in unsafe memory accesses.

[Alfonso, 2016/07/28 14:39:13]

Acknowledged. I lost objectivity.

+  // This is plain unsafe, basically gives read access to memory.
+  // Use ONLY if can be verified 100% that all access paths have proper
+  // checks; and the checks are indeed redundant. Security have clear priority
+  // over performance, there's no trade-off to play here. Use the safe method
+  // otherwise, it's quite fast.
+  ValueId LoadInternalFieldUnsafe(ValueId value_id, int field_no);
+
   ValueId LoadValue(ValueId value_id, int offset);
   ValueId LoadObject(ValueId value_id, int offset);
 
   // Builder / assembler functions for control flow.
   void ReturnValue(ValueId value_id);
   void CheckFlagSetOrReturnNull(ValueId value_id, int mask);
   void CheckNotZeroOrReturnNull(ValueId value_id);
   LabelId MakeLabel();
   void SetLabel(LabelId label_id);
   void CheckNotZeroOrJump(ValueId value_id, LabelId label_id);
@@ skipping 28 matching lines @@
   // strict as this class will be exposed at the API.)
   enum { kBuilding, kBuilt, kError } state_;
 
   DISALLOW_COPY_AND_ASSIGN(FastAccessorAssembler);
 };
 
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_FAST_ACCESSOR_ASSEMBLER_H_