X64: Let debugger patch JSReturn with a debug break.

src/x64/debug-x64.cc

 // Copyright 2009 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 20 matching lines @@
 #include "codegen-inl.h"
 #include "debug.h"
 
 
 namespace v8 {
 namespace internal {
 
 #ifdef ENABLE_DEBUGGER_SUPPORT
 
 bool Debug::IsDebugBreakAtReturn(v8::internal::RelocInfo* rinfo) {
-  UNIMPLEMENTED();
-  return false;
+  ASSERT(RelocInfo::IsJSReturn(rinfo->rmode()));
+  // 11th byte of patch is 0x49, 11th byte of JS return is 0xCC (int3).
+  ASSERT(*(rinfo->pc() + 10) == 0x49 || *(rinfo->pc() + 10) == 0xCC);
+  return (*(rinfo->pc() + 10) == 0x49);
 }
 
 void Debug::GenerateCallICDebugBreak(MacroAssembler* masm) {
   masm->int3();  // UNIMPLEMENTED
 }
 
 void Debug::GenerateConstructCallDebugBreak(MacroAssembler* masm) {
   masm->int3();  // UNIMPLEMENTED
 }
 
@@ skipping 21 matching lines @@
   masm->int3();  // UNIMPLEMENTED
 }
 
 void Debug::GenerateStubNoRegistersDebugBreak(MacroAssembler* masm) {
   masm->int3();  // UNIMPLEMENTED
 }
 
 #endif  // ENABLE_DEBUGGER_SUPPORT
 
 } }  // namespace v8::internal