X64: Let debugger patch JSReturn with a debug break.

src/x64/codegen-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 337 matching lines @@
   return_value->ToRegister(rax);
 
   // Add a label for checking the size of the code used for returning.
   Label check_exit_codesize;
   masm_->bind(&check_exit_codesize);
 
   // Leave the frame and return popping the arguments and the
   // receiver.
   frame_->Exit();
   masm_->ret((scope_->num_parameters() + 1) * kPointerSize);
+  // Add padding that will be overwritten by a debugger breakpoint.
+  // frame_->Exit() generates "movq rsp, rbp; pop rbp" length 5.
+  // "ret k" has length 2.
+  const int kPadding = kX64JSReturnSequenceLength - 5 - 2;
+  for (int i = 0; i < kPadding; ++i) {
+    masm_->int3();
+  }
   DeleteFrame();
 
-  // TODO(x64): introduce kX64JSReturnSequenceLength and enable assert.
-
   // Check that the size of the code used for returning matches what is
   // expected by the debugger.
-  // ASSERT_EQ(Debug::kIa32JSReturnSequenceLength,
-  //          masm_->SizeOfCodeGeneratedSince(&check_exit_codesize));
+  ASSERT_EQ(Debug::kX64JSReturnSequenceLength,
+            masm_->SizeOfCodeGeneratedSince(&check_exit_codesize));
 }
 
 
 #ifdef DEBUG
 bool CodeGenerator::HasValidEntryRegisters() {
   return (allocator()->count(rax) == (frame()->is_used(rax) ? 1 : 0))
       && (allocator()->count(rbx) == (frame()->is_used(rbx) ? 1 : 0))
       && (allocator()->count(rcx) == (frame()->is_used(rcx) ? 1 : 0))
       && (allocator()->count(rdx) == (frame()->is_used(rdx) ? 1 : 0))
       && (allocator()->count(rdi) == (frame()->is_used(rdi) ? 1 : 0))
@@ skipping 6560 matching lines @@
 int CompareStub::MinorKey() {
   // Encode the two parameters in a unique 16 bit value.
   ASSERT(static_cast<unsigned>(cc_) < (1 << 15));
   return (static_cast<unsigned>(cc_) << 1) | (strict_ ? 1 : 0);
 }
 
 
 #undef __
 
 } }  // namespace v8::internal