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..0d7d058db09e58e83fc0bc51f5ad48a87101311d 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).

[danno, 2013/08/02 14:01:20]

I am not sure that it is help to include "Scratch" in the name here an below.
Perhaps a little more generic is better, e.g.

kCallRegisterShortInstructionSize = 3;
kCallImmediateShortInstructionSize = 5;
kMoveAddressIntoRegisterShortInstructionSize

etc...

[haitao.feng, 2013/08/02 15:10:56]

If the register number is less than 8, then the call instruction size is 2. For
move instruction, it is true for 64-bit system for REX.W, but for x32, if the
register number is less than 8, we do not need the REX prefix.

kShortCallInstruction is better as it is short indeed. It only could access
[-2G, 2G] relative to the current pc address.

+  static const int kCallScratchRegisterInstructionSize = 3;
+  // The length of call(Immediate32).
   static const int kShortCallInstructionLength = 5;

[danno, 2013/08/02 14:01:20]

Unify to either "...Length" or "...Size". I think "Length" is better.

[haitao.feng, 2013/08/02 15:10:56]

Done.

-  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 kMoveAddressIntoScratchRegisterInstructionSize =
+      2 + kPointerSize;
+  // The length of movq(kScratchRegister, address) and call(kScratchRegister).
+  static const int kCallSequenceLength =
+      kMoveAddressIntoScratchRegisterInstructionSize +
+      kCallScratchRegisterInstructionSize;
+
+  // 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;

[haitao.feng, 2013/08/02 04:41:17]

It seems that we could use short call and RUNTIME_ENTRY to patch the JS return
and debug break slot as the patched site and target are all in the code range
for X64.  I leave it to another CL.

+  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 =
+      kMoveAddressIntoScratchRegisterInstructionSize - kPointerSize;
 
   // One byte opcode for test eax,0xXXXXXXXX.
   static const byte kTestEaxByte = 0xA9;