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

runtime/vm/constants_arm64.h

 // 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.
 
 #ifndef VM_CONSTANTS_ARM64_H_
 #define VM_CONSTANTS_ARM64_H_
 
 #include "platform/assert.h"
 
 namespace dart {
@@ skipping 99 matching lines @@
 const Register TMP = R16;  // Used as scratch register by assembler.
 const Register TMP2 = R17;
 const Register CTX = R28;  // Location of current context at method entry.
 const Register PP = R27;  // Caches object pool pointer in generated code.
 const Register kNoPP = kNoRegister;
 const Register FPREG = FP;  // Frame pointer register.
 const Register SPREG = R18;  // Stack pointer register.
 const Register LRREG = LR;  // Link register.
 const Register ICREG = R5;  // IC data register.
 const Register ARGS_DESC_REG = R4;  // Arguments descriptor register.
+const Register THR = R20;  // Caches current thread in generated code.
 
 // Exception object is passed in this register to the catch handlers when an
 // exception is thrown.
 const Register kExceptionObjectReg = R0;
 
 // Stack trace object is passed in this register to the catch handlers when
 // an exception is thrown.
 const Register kStackTraceObjectReg = R1;
 
 // Masks, sizes, etc.
@@ skipping 937 matching lines @@
   static Instr* At(uword pc) { return reinterpret_cast<Instr*>(pc); }
 
  private:
   DISALLOW_ALLOCATION();
   DISALLOW_IMPLICIT_CONSTRUCTORS(Instr);
 };
 
 }  // namespace dart
 
 #endif  // VM_CONSTANTS_ARM64_H_