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

runtime/vm/constants_arm.h

 // 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.
 
 #ifndef VM_CONSTANTS_ARM_H_
 #define VM_CONSTANTS_ARM_H_
 
 #include "platform/globals.h"
 #include "platform/assert.h"
 
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 // Register aliases.
 const Register TMP = IP;  // Used as scratch register by assembler.
 const Register TMP2 = kNoRegister;  // There is no second assembler temporary.
 const Register CTX = R9;  // Location of current context at method entry.
 const Register PP = R10;  // Caches object pool pointer in generated code.
 const Register SPREG = SP;  // Stack pointer register.
 const Register FPREG = FP;  // Frame pointer register.
 const Register LRREG = LR;  // Link register.
 const Register ICREG = R5;  // IC data register.
 const Register ARGS_DESC_REG = R4;
+const Register THR = R8;  // Caches current thread in generated code.
 
 // R15 encodes APSR in the vmrs instruction.
 const Register APSR = R15;
 
 // 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.
@@ skipping 454 matching lines @@
   static Instr* At(uword pc) { return reinterpret_cast<Instr*>(pc); }
 
  private:
   DISALLOW_ALLOCATION();
   DISALLOW_IMPLICIT_CONSTRUCTORS(Instr);
 };
 
 }  // namespace dart
 
 #endif  // VM_CONSTANTS_ARM_H_