Make some constants' meaning clear for X64

src/x64/assembler-x64.h

Index: src/x64/assembler-x64.h
diff --git a/src/x64/assembler-x64.h b/src/x64/assembler-x64.h
index 07afc129dc82c76b76a5eadb2c6bd3425eb0289e..4e36b6e4bc4062ead3023e978af5e2115b7c99db 100644
--- a/src/x64/assembler-x64.h
+++ b/src/x64/assembler-x64.h
@@ -579,29 +579,36 @@ class Assembler : public AssemblerBase {
   // Distance between the address of the code target in the call instruction
   // and the return address pushed on the stack.
   static const int kCallTargetAddressOffset = 4;  // Use 32-bit displacement.
-  // Distance between the start of the JS return sequence and where the
-  // 32-bit displacement of a near call would be, relative to the pushed
-  // return address.  TODO: Use return sequence length instead.
-  // Should equal Debug::kX64JSReturnSequenceLength - kCallTargetAddressOffset;
-  static const int kPatchReturnSequenceAddressOffset = 13 - 4;
-  // Distance between start of patched debug break slot and where the
-  // 32-bit displacement of a near call would be, relative to the pushed
-  // return address.  TODO: Use return sequence length instead.
-  // Should equal Debug::kX64JSReturnSequenceLength - kCallTargetAddressOffset;
-  static const int kPatchDebugBreakSlotAddressOffset = 13 - 4;
-  // TODO(X64): Rename this, removing the "Real", after changing the above.
-  static const int kRealPatchReturnSequenceAddressOffset = 2;
-
-  // Some x64 JS code is padded with int3 to make it large
-  // enough to hold an instruction when the debugger patches it.
-  static const int kJumpInstructionLength = 13;
-  static const int kCallInstructionLength = 13;
-  static const int kJSReturnSequenceLength = 13;
+  // The length of call(kScratchRegister).
+  static const int kCallScratchRegisterInstructionLength = 3;
+  // The length of call(Immediate32).
   static const int kShortCallInstructionLength = 5;
-  static const int kPatchDebugBreakSlotReturnOffset = 4;
-
-  // The debug break slot must be able to contain a call instruction.
-  static const int kDebugBreakSlotLength = kCallInstructionLength;
+  // The length of movq(kScratchRegister, address).
+  static const int kMoveAddressIntoScratchRegisterInstructionLength =
+      2 + kPointerSize;
+  // The length of movq(kScratchRegister, address) and call(kScratchRegister).
+  static const int kCallSequenceLength =
+      kMoveAddressIntoScratchRegisterInstructionLength +
+      kCallScratchRegisterInstructionLength;
+
+  // The js return and debug break slot must be able to contain an indirect
+  // call sequence, some x64 JS code is padded with int3 to make it large
+  // enough to hold an instruction when the debugger patches it.
+  static const int kJSReturnSequenceLength = kCallSequenceLength;
+  static const int kDebugBreakSlotLength = kCallSequenceLength;
+  static const int kPatchDebugBreakSlotReturnOffset = kCallTargetAddressOffset;
+  // Distance between the start of the JS return sequence and where the
+  // 32-bit displacement of a short call would be. The short call is from
+  // SetDebugBreakAtIC from debug-x64.cc.
+  static const int kPatchReturnSequenceAddressOffset =
+      kJSReturnSequenceLength - kPatchDebugBreakSlotReturnOffset;
+  // Distance between the start of the JS return sequence and where the
+  // 32-bit displacement of a short call would be. The short call is from
+  // SetDebugBreakAtIC from debug-x64.cc.
+  static const int kPatchDebugBreakSlotAddressOffset =
+      kDebugBreakSlotLength - kPatchDebugBreakSlotReturnOffset;
+  static const int kRealPatchReturnSequenceAddressOffset =
+      kMoveAddressIntoScratchRegisterInstructionLength - kPointerSize;
 
   // One byte opcode for test eax,0xXXXXXXXX.
   static const byte kTestEaxByte = 0xA9;