Don't touch x18 on ARM64 - it is globally reserved on iOS.

runtime/vm/constants_x64.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_X64_H_
 #define VM_CONSTANTS_X64_H_
 
 namespace dart {
 
 enum Register {
-  kFirstFreeCpuRegister = 0,
   RAX = 0,
   RCX = 1,
   RDX = 2,
   RBX = 3,
   RSP = 4,
   RBP = 5,
   RSI = 6,
   RDI = 7,
   R8  = 8,
   R9  = 9,
   R10 = 10,
   R11 = 11,
   R12 = 12,
   R13 = 13,
   R14 = 14,
   R15 = 15,
-  kLastFreeCpuRegister = 15,
   kNumberOfCpuRegisters = 16,
-  kNoRegister = -1  // Signals an illegal register.
+  kNoRegister = -1,  // Signals an illegal register.
 };
 
 
 enum ByteRegister {
   AL = 0,
   CL = 1,
   DL = 2,
   BL = 3,
   AH = 4,
   CH = 5,
@@ skipping 48 matching lines @@
 const Register CTX = R12;  // Location of current context at method entry.
 // Caches object pool pointer in generated code.
 const Register PP = R15;
 const Register SPREG = RSP;  // Stack pointer register.
 const Register FPREG = RBP;  // Frame pointer register.
 const Register ICREG = RBX;  // IC data register.
 const Register ARGS_DESC_REG = R10;  // Arguments descriptor register.
 const Register CODE_REG = R12;
 const Register THR = R14;  // 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 = RAX;
 
 // Stack trace object is passed in this register to the catch handlers when
 // an exception is thrown.
 const Register kStackTraceObjectReg = RDX;
 
 
+typedef uint32_t RegList;
+const RegList kAllCpuRegistersList = 0xFFFF;
+
+const RegList kReservedCpuRegisters =
+    (1 << SPREG) |
+    (1 << FPREG) |
+    (1 << TMP)   |
+    (1 << PP)    |
+    (1 << THR);
+// CPU registers available to Dart allocator.
+const RegList kDartAvailableCpuRegs =
+    kAllCpuRegistersList & ~kReservedCpuRegisters;
+
+
 enum ScaleFactor {
   TIMES_1 = 0,
   TIMES_2 = 1,
   TIMES_4 = 2,
   TIMES_8 = 3,
   TIMES_16 = 4,
   TIMES_HALF_WORD_SIZE = kWordSizeLog2 - 1
 };
 
 
@@ skipping 105 matching lines @@
 };
 
 
 // The largest multibyte nop we will emit.  This could go up to 15 if it
 // becomes important to us.
 const int MAX_NOP_SIZE = 8;
 
 }  // namespace dart
 
 #endif  // VM_CONSTANTS_X64_H_