Use a root array register for constant loads and stack guards on x64....

src/x64/register-allocator-x64-inl.h

 // 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 33 matching lines @@
 
 // The register allocator uses small integers to represent the
 // non-reserved assembler registers.
 int RegisterAllocator::ToNumber(Register reg) {
   ASSERT(reg.is_valid() && !IsReserved(reg));
   const int kNumbers[] = {
     0,   // rax
     2,   // rcx
     3,   // rdx
     1,   // rbx
-    -1,  // rsp
-    -1,  // rbp
-    -1,  // rsi
+    -1,  // rsp  Stack pointer.
+    -1,  // rbp  Frame pointer.
+    -1,  // rsi  Context.
     4,   // rdi
     5,   // r8
     6,   // r9
-    -1,  // r10
-    7,   // r11
-    11,  // r12
-    10,   // r13
-    8,   // r14
-    9   // r15
+    -1,  // r10  Scratch register.
+    9,   // r11
+    10,  // r12
+    -1,  // r13  Roots array.  This is callee saved.
+    7,   // r14
+    8    // r15
   };
   return kNumbers[reg.code()];
 }
 
 
 Register RegisterAllocator::ToRegister(int num) {
   ASSERT(num >= 0 && num < kNumRegisters);
   const Register kRegisters[] =
-      { rax, rbx, rcx, rdx, rdi, r8, r9, r11, r14, r15, r13, r12 };
+      { rax, rbx, rcx, rdx, rdi, r8, r9, r14, r15, r11, r12 };
   return kRegisters[num];
 }
 
 
 void RegisterAllocator::Initialize() {
   Reset();
   // The non-reserved rdi register is live on JS function entry.
   Use(rdi);  // JS function.
 }
 } }  // namespace v8::internal
 
 #endif  // V8_X64_REGISTER_ALLOCATOR_X64_INL_H_