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

runtime/vm/constants_arm64.h

Index: runtime/vm/constants_arm64.h
diff --git a/runtime/vm/constants_arm64.h b/runtime/vm/constants_arm64.h
index ab76fd16b9c433fb055da7f951df57882f71f34b..b14ce5533515bfc00bd3419083aa1cc39d1d1215 100644
--- a/runtime/vm/constants_arm64.h
+++ b/runtime/vm/constants_arm64.h
@@ -10,7 +10,6 @@
 namespace dart {
 
 enum Register {
-  kFirstFreeCpuRegister = 0,
   R0  =  0,
   R1  =  1,
   R2  =  2,
@@ -29,16 +28,15 @@ enum Register {
   R15 = 15,
   R16 = 16,  // IP0 aka TMP
   R17 = 17,  // IP1 aka TMP2
-  R18 = 18,  // SP in Dart code.
-  R19 = 19,
-  R20 = 20,
+  R18 = 18,  // "platform register" on iOS.
+  R19 = 19,  // SP in Dart code.
+  R20 = 20,  // THR
   R21 = 21,
   R22 = 22,
   R23 = 23,
   R24 = 24,
   R25 = 25,
   R26 = 26,
-  kLastFreeCpuRegister = 26,
   R27 = 27,  // PP
   R28 = 28,  // CTX
   R29 = 29,  // FP
@@ -55,11 +53,12 @@ enum Register {
   // Aliases.
   IP0 = R16,
   IP1 = R17,
-  SP = R18,
+  SP = R19,
   FP = R29,
   LR = R30,
 };
 
+
 enum VRegister {
   V0  =  0,
   V1  =  1,
@@ -113,12 +112,13 @@ const Register CTX = R28;  // Location of current context at method entry.
 const Register PP = R27;  // Caches object pool pointer in generated code.
 const Register CODE_REG = R24;
 const Register FPREG = FP;  // Frame pointer register.
-const Register SPREG = R18;  // Stack pointer register.
+const Register SPREG = R19;  // 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;
@@ -143,25 +143,31 @@ const RegList kAbiArgumentCpuRegs =
     (1 << R0) | (1 << R1) | (1 << R2) | (1 << R3) |
     (1 << R4) | (1 << R5) | (1 << R6) | (1 << R7);
 const RegList kAbiPreservedCpuRegs =
-    (1 << R19) | (1 << R20) | (1 << R21) | (1 << R22) |
-    (1 << R23) | (1 << R24) | (1 << R25) | (1 << R26) |
-    (1 << R27) | (1 << R28);
-const Register kAbiFirstPreservedCpuReg = R19;
+    (1 << R20) | (1 << R21) | (1 << R22) | (1 << R23) |
+    (1 << R24) | (1 << R25) | (1 << R26) | (1 << R27) |
+    (1 << R28);
+const Register kAbiFirstPreservedCpuReg = R20;
 const Register kAbiLastPreservedCpuReg = R28;
-const int kAbiPreservedCpuRegCount = 10;
+const int kAbiPreservedCpuRegCount = 9;
 const VRegister kAbiFirstPreservedFpuReg = V8;
 const VRegister kAbiLastPreservedFpuReg = V15;
 const int kAbiPreservedFpuRegCount = 8;
 
+const intptr_t kReservedCpuRegisters =
+    (1 << SPREG) |  // Dart SP
+    (1 << FPREG) |
+    (1 << TMP)   |
+    (1 << TMP2)  |
+    (1 << PP)    |
+    (1 << THR)   |
+    (1 << LR)    |
+    (1 << R31)   |  // C++ SP
+    (1 << CTX)   |
+    (1 << R18);     // iOS platform register.
+                    // TODO(rmacnak): Only reserve on Mac & iOS.
 // 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 << R9)  | (1 << R10) | (1 << R11) |
-    (1 << R12) | (1 << R13) | (1 << R14) | (1 << R15) |
-    (1 << R19) | (1 << R20) | (1 << R21) | (1 << R22) |
-    (1 << R23) | (1 << R24) | (1 << R25) | (1 << R26);
-
+    kAllCpuRegistersList & ~kReservedCpuRegisters;
 // Registers available to Dart that are not preserved by runtime calls.
 const RegList kDartVolatileCpuRegs =
     kDartAvailableCpuRegs & ~kAbiPreservedCpuRegs;