[heap] Remove store buffer top from roots

src/x64/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.
 
 #if V8_TARGET_ARCH_X64
 
 #include "src/base/bits.h"
 #include "src/base/division-by-constant.h"
 #include "src/bootstrapper.h"
 #include "src/codegen.h"
@@ skipping 197 matching lines @@
                                          Register scratch,
                                          SaveFPRegsMode save_fp,
                                          RememberedSetFinalAction and_then) {
   if (emit_debug_code()) {
     Label ok;
     JumpIfNotInNewSpace(object, scratch, &ok, Label::kNear);
     int3();
     bind(&ok);
   }
   // Load store buffer top.
-  LoadRoot(scratch, Heap::kStoreBufferTopRootIndex);
+  ExternalReference store_buffer =
+      ExternalReference::store_buffer_top(isolate());
+  movp(scratch, ExternalOperand(store_buffer));
   // Store pointer to buffer.
   movp(Operand(scratch, 0), addr);
   // Increment buffer top.
   addp(scratch, Immediate(kPointerSize));
   // Write back new top of buffer.
-  StoreRoot(scratch, Heap::kStoreBufferTopRootIndex);
+  movp(ExternalOperand(store_buffer), scratch);
   // Call stub on end of buffer.
   Label done;
   // Check for end of buffer.
   testp(scratch, Immediate(StoreBuffer::kStoreBufferOverflowBit));
   if (and_then == kReturnAtEnd) {
     Label buffer_overflowed;
     j(not_equal, &buffer_overflowed, Label::kNear);
     ret(0);
     bind(&buffer_overflowed);
   } else {
@@ skipping 5446 matching lines @@
   movl(rax, dividend);
   shrl(rax, Immediate(31));
   addl(rdx, rax);
 }
 
 
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_TARGET_ARCH_X64