Use a reserved stack local variable to store the Code object so that it can be looked up easily whe…

vm/assembler_macros_x64.h

Index: vm/assembler_macros_x64.h
===================================================================
--- vm/assembler_macros_x64.h	(revision 7669)
+++ vm/assembler_macros_x64.h	(working copy)
@@ -33,6 +33,49 @@
                           Register class_reg,
                           Label* failure,
                           Register instance_reg);
+
+  // Set up a dart frame on entry with a frame pointer and PC information to
+  // enable easy access to the RawInstruction object of code corresponding
+  // to this frame.
+  // The dart frame layout is as follows:
+  //   ....
+  //   ret PC
+  //   saved RBP     <=== RBP
+  //   pc (used to derive the RawInstruction Object of the dart code)
+  //   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>
+  //   .....
+  static void EnterDartFrame(Assembler* assembler, intptr_t frame_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
+  //   movq rbp, rsp
+  //   pushq immediate(0)
+  //   .....
+  static void EnterStubFrame(Assembler* assembler);
+
+  // Instruction pattern from entrypoint is used in dart frame prologs
+  // to set up the frame and save a PC which can be used to figure out the
+  // RawInstruction object corresponding to the code running in the frame.
+  // entrypoint:
+  //   pushq rbp          (size is 1 byte)
+  //   movq rbp, rsp      (size is 3 bytes)
+  //   call L             (size is 5 bytes)
+  //   L:
+  static const intptr_t kOffsetOfSavedPCfromEntrypoint = 9;
 };
 
 }  // namespace dart.