[x64] Make r12 allocatable.

src/x64/regexp-macro-assembler-x64.cc

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "src/v8.h"
 
 #if V8_TARGET_ARCH_X64
 
 #include "src/cpu-profiler.h"
 #include "src/log.h"
@@ skipping 25 matching lines @@
  *         RegExp registers.
  * - rsp : Points to tip of C stack.
  * - rcx : Points to tip of backtrack stack.  The backtrack stack contains
  *         only 32-bit values.  Most are offsets from some base (e.g., character
  *         positions from end of string or code location from Code* pointer).
  * - r8  : Code object pointer.  Used to convert between absolute and
  *         code-object-relative addresses.
  *
  * The registers rax, rbx, r9 and r11 are free to use for computations.
  * If changed to use r12+, they should be saved as callee-save registers.
- * The macro assembler special registers r12 and r13 (kSmiConstantRegister,
- * kRootRegister) aren't special during execution of RegExp code (they don't
- * hold the values assumed when creating JS code), so no Smi or Root related
- * macro operations can be used.
+ * The macro assembler special register r13 (kRootRegister) isn't special
+ * during execution of RegExp code (it doesn't hold the value assumed when
+ * creating JS code), so Root related macro operations can be used.
  *
  * Each call to a C++ method should retain these registers.
  *
  * The stack will have the following content, in some order, indexable from the
  * frame pointer (see, e.g., kStackHighEnd):
  *    - Isolate* isolate     (address of the current isolate)
  *    - direct_call          (if 1, direct call from JavaScript code, if 0 call
  *                            through the runtime system)
  *    - stack_area_base      (high end of the memory area to use as
  *                            backtracking stack)
@@ skipping 1374 matching lines @@
   }
 }
 
 #undef __
 
 #endif  // V8_INTERPRETED_REGEXP
 
 }}  // namespace v8::internal
 
 #endif  // V8_TARGET_ARCH_X64