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

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"
 
 namespace dart {
 
 // We support both VFPv3-D16 and VFPv3-D32 profiles, but currently only one at
 // a time.
 #if defined(__ARM_ARCH_7A__)
 #define VFPv3_D32
 #elif defined(TARGET_ARCH_ARM) && !defined(HOST_ARCH_ARM)
 // If we're running in the simulator, use all 32.
 #define VFPv3_D32
 #else
 #define VFPv3_D16
 #endif
 #if defined(VFPv3_D16) == defined(VFPv3_D32)
 #error "Exactly one of VFPv3_D16 or VFPv3_D32 can be defined at a time."
 #endif
 
 
 enum Register {
-  kNoRegister = -1,
-  kFirstFreeCpuRegister = 0,
   R0  =  0,
   R1  =  1,
   R2  =  2,
   R3  =  3,
   R4  =  4,
   R5  =  5,
   R6  =  6,
   R7  =  7,
-  R8  =  8,
-  R9  =  9,
-  R10 = 10,
-  R11 = 11,
-  R12 = 12,
-  R13 = 13,
-  R14 = 14,
-  kLastFreeCpuRegister = 14,
-  R15 = 15,
+  R8  =  8,  // THR
+  R9  =  9,  // PP
+  R10 = 10,  // CTX
+  R11 = 11,  // FP
+  R12 = 12,  // IP aka TMP
+  R13 = 13,  // SP
+  R14 = 14,  // LR
+  R15 = 15,  // PC
+  kNumberOfCpuRegisters = 16,
+  kNoRegister = -1,  // Signals an illegal register.
+
+  // Aliases.
   FP  = R11,
   IP  = R12,
   SP  = R13,
   LR  = R14,
   PC  = R15,
-  kNumberOfCpuRegisters = 16,
 };
 
 
 // Values for single-precision floating point registers.
 enum SRegister {
   kNoSRegister = -1,
   S0  =  0,
   S1  =  1,
   S2  =  2,
   S3  =  3,
@@ skipping 198 matching lines @@
 const RegList kAbiArgumentCpuRegs =
     (1 << R0) | (1 << R1) | (1 << R2) | (1 << R3);
 const RegList kAbiPreservedCpuRegs =
     (1 << R4) | (1 << R5) | (1 << R6) | (1 << R7) |
     (1 << R8) | (1 << R9) | (1 << R10);
 const int kAbiPreservedCpuRegCount = 7;
 const QRegister kAbiFirstPreservedFpuReg = Q4;
 const QRegister kAbiLastPreservedFpuReg = Q7;
 const int kAbiPreservedFpuRegCount = 4;
 
+const RegList kReservedCpuRegisters =
+     (1 << SPREG) |
+     (1 << FPREG) |
+     (1 << TMP)   |
+     (1 << PP)    |
+     (1 << THR)   |
+     (1 << PC);
 // CPU registers available to Dart allocator.
 const RegList kDartAvailableCpuRegs =
-    (1 << R0) | (1 << R1) | (1 << R2) | (1 << R3) |
-    (1 << R4) | (1 << R5) | (1 << R6) | (1 << R7) |
-    (1 << R8) | (1 << R14);
-
+    kAllCpuRegistersList & ~kReservedCpuRegisters;
 // Registers available to Dart that are not preserved by runtime calls.
 const RegList kDartVolatileCpuRegs =
     kDartAvailableCpuRegs & ~kAbiPreservedCpuRegs;
 const int kDartVolatileCpuRegCount = 5;
 const QRegister kDartFirstVolatileFpuReg = Q0;
 const QRegister kDartLastVolatileFpuReg = Q3;
 const int kDartVolatileFpuRegCount = 4;
 
 
 // Values for the condition field as defined in section A3.2.
@@ skipping 419 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_