Introduce PushInt64AsTwoSmis and PopInt64AsTwoSmis macro instructions for X64

src/x64/macro-assembler-x64.h

Index: src/x64/macro-assembler-x64.h
diff --git a/src/x64/macro-assembler-x64.h b/src/x64/macro-assembler-x64.h
index 61abc206e108ec2f85e0ff83393eb778c58dde79..ae3e4fd7e49d1e1fc3ed8644578784fe8e62af7d 100644
--- a/src/x64/macro-assembler-x64.h
+++ b/src/x64/macro-assembler-x64.h
@@ -720,6 +720,14 @@ class MacroAssembler: public Assembler {
   }
 
   void Push(Smi* smi);
+
+  // Push a 64-bit integer as two "general" Smis on stack so that the garbage

[danno, 2013/08/19 22:00:40]

I find the comments a bit confusing. I think the following is probably clearer:

Save away a 64-bit integer on the stack as two 32-bits integers masquerading as
smis so that the garbage collector skips visiting them.

[haitao.feng, 2013/08/20 13:20:15]

Done.

+  // collector could iterate stack safely. The Smis are general, i.e., all the
+  // lower kSmiShift bits are zero.
+  void PushInt64AsTwoSmis(Register src, Register scratch = kScratchRegister);
+  // Reconstruct a 64-bit integer from two "general" Smis from stack.

[danno, 2013/08/19 22:00:40]

// Reconstruct a 64-bit integer from two 32-bits integers masquerading as smis
on the top of stack.

[haitao.feng, 2013/08/20 13:20:15]

Done.

+  void PopInt64AsTwoSmis(Register dst, Register scratch = kScratchRegister);
+
   void Test(const Operand& dst, Smi* source);