Cache current thread in a reserved register and use it in LoadIsolate

runtime/vm/assembler_x64.cc

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #include "vm/globals.h"  // NOLINT
 #if defined(TARGET_ARCH_X64)
 
 #include "vm/assembler.h"
 #include "vm/cpu.h"
 #include "vm/heap.h"
@@ skipping 2792 matching lines @@
 
 void Assembler::LoadWordFromPoolOffset(Register dst, Register pp,
                                        int32_t offset) {
   // This sequence must be of fixed size. AddressBaseImm32
   // forces the address operand to use a fixed-size imm32 encoding.
   movq(dst, Address::AddressBaseImm32(pp, offset));
 }
 
 
 void Assembler::LoadIsolate(Register dst) {
-  movq(dst, Immediate(reinterpret_cast<uword>(Isolate::Current())));
+  movq(dst, Address(THR, Thread::isolate_offset()));
 }
 
 
 void Assembler::LoadObject(Register dst, const Object& object, Register pp) {
   if (CanLoadFromObjectPool(object)) {
     const int32_t offset =
         Array::element_offset(object_pool_.FindObject(object, kNotPatchable));
     LoadWordFromPoolOffset(dst, pp, offset - kHeapObjectTag);
   } else {
     ASSERT((Isolate::Current() == Dart::vm_isolate()) ||
@@ skipping 1072 matching lines @@
 
 
 const char* Assembler::FpuRegisterName(FpuRegister reg) {
   ASSERT((0 <= reg) && (reg < kNumberOfXmmRegisters));
   return xmm_reg_names[reg];
 }
 
 }  // namespace dart
 
 #endif  // defined TARGET_ARCH_X64