[turbofan] Jump threading awareness of frame elision.

src/compiler/jump-threading.h

 // 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.
 
 #ifndef V8_COMPILER_JUMP_THREADING_H_
 #define V8_COMPILER_JUMP_THREADING_H_
 
 #include "src/compiler/instruction.h"
 
 namespace v8 {
 namespace internal {
 namespace compiler {
 
 // Forwards jumps to empty basic blocks that end with a second jump to the
 // destination of the second jump, transitively.
 class JumpThreading {
  public:
   // Compute the forwarding map of basic blocks to their ultimate destination.
   // Returns {true} if there is at least one block that is forwarded.
   static bool ComputeForwarding(Zone* local_zone, ZoneVector<RpoNumber>& result,
-                                InstructionSequence* code);
+                                InstructionSequence* code, bool frame_at_start);

[Jarin, 2016/01/28 05:26:58]

Please explain what the {frame_at_start} parameter means in the comment above.

 
   // Rewrite the instructions to forward jumps and branches.
   // May also negate some branches.
   static void ApplyForwarding(ZoneVector<RpoNumber>& forwarding,
                               InstructionSequence* code);
 };
 
 }  // namespace compiler
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_COMPILER_JUMP_THREADING_H