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

runtime/vm/assembler_arm64.cc

 // Copyright (c) 2014, 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_ARM64)
 
 #include "vm/assembler.h"
 #include "vm/cpu.h"
 #include "vm/longjump.h"
@@ skipping 443 matching lines @@
                                        const ExternalLabel* label,
                                        Patchability patchable,
                                        Register pp) {
   const int32_t offset =
       Array::element_offset(object_pool_.FindExternalLabel(label, patchable));
   LoadWordFromPoolOffsetFixed(dst, pp, offset);
 }
 
 
 void Assembler::LoadIsolate(Register dst, Register pp) {
-  LoadImmediate(dst, reinterpret_cast<uword>(Isolate::Current()), pp);
+  ldr(dst, Address(THR, Thread::isolate_offset()));
 }
 
 
 void Assembler::LoadObject(Register dst, const Object& object, Register pp) {
   if (CanLoadObjectFromPool(object)) {
     const int32_t offset =
         Array::element_offset(object_pool_.FindObject(object, kNotPatchable));
     LoadWordFromPoolOffset(dst, pp, offset);
   } else {
     ASSERT((Isolate::Current() == Dart::vm_isolate()) ||
@@ skipping 987 matching lines @@
     add(base, array, Operand(index, LSL, shift));
   }
   const OperandSize size = Address::OperandSizeFor(cid);
   ASSERT(Address::CanHoldOffset(offset, Address::Offset, size));
   return Address(base, offset, Address::Offset, size);
 }
 
 }  // namespace dart
 
 #endif  // defined TARGET_ARCH_ARM64