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

runtime/vm/constants_mips.h

Index: runtime/vm/constants_mips.h
diff --git a/runtime/vm/constants_mips.h b/runtime/vm/constants_mips.h
index fed178f1ce07b59b678579fc7f4079a3268a2ff3..c4656d54b6423a13f8610c30a0a1ade0e6d71726 100644
--- a/runtime/vm/constants_mips.h
+++ b/runtime/vm/constants_mips.h
@@ -11,8 +11,7 @@ namespace dart {
 
 enum Register {
   R0  =  0,
-  R1  =  1,
-  kFirstFreeCpuRegister = 2,
+  R1  =  1,  // AT aka TMP
   R2  =  2,
   R3  =  3,
   R4  =  4,
@@ -30,23 +29,23 @@ enum Register {
   R16 = 16,
   R17 = 17,
   R18 = 18,
-  R19 = 19,
+  R19 = 19,  // THR
   R20 = 20,
   R21 = 21,
-  kLastFreeCpuRegister = 21,
-  R22 = 22,
-  R23 = 23,
+  R22 = 22,  // CTX
+  R23 = 23,  // PP
   R24 = 24,
   R25 = 25,
   R26 = 26,
   R27 = 27,
   R28 = 28,
-  R29 = 29,
-  R30 = 30,
-  R31 = 31,
+  R29 = 29,  // SP
+  R30 = 30,  // FP
+  R31 = 31,  // RA
   kNumberOfCpuRegisters = 32,
   IMM = 32,  // Positive value is easier to encode than kNoRegister in bitfield.
-  kNoRegister = -1,
+  kNoRegister = -1,  // Signals an illegal register.
+
 
   // Register aliases.
   ZR = R0,
@@ -188,6 +187,7 @@ const Register ICREG = S5;  // IC data register.
 const Register ARGS_DESC_REG = S4;
 const Register THR = S3;  // Caches current thread in generated code.
 
+
 // The code that generates a comparison can be far away from the code that
 // generates the branch that uses the result of that comparison. In this case,
 // CMPRES1 and CMPRES2 are used for the results of the comparison. We need two
@@ -207,7 +207,6 @@ const Register kStackTraceObjectReg = V1;
 typedef uint32_t RegList;
 const RegList kAllCpuRegistersList = 0xFFFFFFFF;
 
-
 const RegList kAbiArgumentCpuRegs =
     (1 << A0) | (1 << A1) | (1 << A2) | (1 << A3);
 const RegList kAbiPreservedCpuRegs =
@@ -221,13 +220,24 @@ const FRegister kAbiLastPreservedFpuReg =
     static_cast<FRegister>(kNumberOfFRegisters - 1);
 const int kAbiPreservedFpuRegCount = 12;
 
+const RegList kReservedCpuRegisters =
+    (1 << SPREG)   |
+    (1 << FPREG)   |
+    (1 << TMP)     |
+    (1 << PP)      |
+    (1 << THR)     |
+    (1 << CTX)     |
+    (1 << ZR)      |
+    (1 << CMPRES1) |
+    (1 << CMPRES2) |
+    (1 << K0)      |
+    (1 << K1)      |
+    (1 << GP)      |
+    (1 << RA);
 // CPU registers available to Dart allocator.
 const RegList kDartAvailableCpuRegs =
-    (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 << R16) | (1 << R17) |
-    (1 << R18) | (1 << R19) | (1 << R20) | (1 << R21);
+    kAllCpuRegistersList & ~kReservedCpuRegisters;
+// Registers available to Dart that are not preserved by runtime calls.
 const RegList kDartVolatileCpuRegs =
     kDartAvailableCpuRegs & ~kAbiPreservedCpuRegs;
 const int kDartVolatileCpuRegCount = 14;