Reapply "Initial implementation of on-stack replacement (OSR)."

runtime/vm/assembler_x64.h

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #ifndef VM_ASSEMBLER_X64_H_
 #define VM_ASSEMBLER_X64_H_
 
 #ifndef VM_ASSEMBLER_H_
 #error Do not include assembler_x64.h directly; use assembler.h instead.
 #endif
@@ skipping 717 matching lines @@
   //   locals space  <=== RSP
   //   .....
   // This code sets this up with the sequence:
   //   pushq rbp
   //   movq rbp, rsp
   //   call L
   //   L: <code to adjust saved pc if there is any intrinsification code>
   //   .....
   void EnterDartFrame(intptr_t frame_size);
 
+  // Set up a Dart frame for a function compiled for on-stack replacement.
+  // The frame layout is a normal Dart frame, but the frame is partially set
+  // up on entry (it is the frame of the unoptimized code).
+  void EnterOsrFrame(intptr_t extra_size);
+
   // Set up a stub frame so that the stack traversal code can easily identify
   // a stub frame.
   // The stub frame layout is as follows:
   //   ....
   //   ret PC
   //   saved RBP
   //   pc (used to derive the RawInstruction Object of the stub)
   //   .....
   // This code sets this up with the sequence:
   //   pushq rbp
@@ skipping 170 matching lines @@
 }
 
 
 inline void Assembler::EmitOperandSizeOverride() {
   EmitUint8(0x66);
 }
 
 }  // namespace dart
 
 #endif  // VM_ASSEMBLER_X64_H_