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

runtime/vm/flow_graph_allocator.cc

 // 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.
 
 #include "vm/flow_graph_allocator.h"
 
 #include "vm/bit_vector.h"
 #include "vm/intermediate_language.h"
 #include "vm/il_printer.h"
 #include "vm/flow_graph.h"
@@ skipping 83 matching lines @@
     intrinsic_mode_(intrinsic_mode) {
   for (intptr_t i = 0; i < vreg_count_; i++) {
     live_ranges_.Add(NULL);
   }
   for (intptr_t i = 0; i < vreg_count_; i++) {
     value_representations_.Add(kNoRepresentation);
   }
 
   // All registers are marked as "not blocked" (array initialized to false).
   // Mark the unavailable ones as "blocked" (true).
-  for (intptr_t i = 0; i < kFirstFreeCpuRegister; i++) {
-    blocked_cpu_registers_[i] = true;
+  for (intptr_t i = 0; i < kNumberOfCpuRegisters; i++) {
+    if ((kDartAvailableCpuRegs & (1 << i)) == 0) {
+      blocked_cpu_registers_[i] = true;
+    }
   }
-  for (intptr_t i = kLastFreeCpuRegister + 1; i < kNumberOfCpuRegisters; i++) {
-    blocked_cpu_registers_[i] = true;
-  }
-  if (TMP != kNoRegister) {
-    blocked_cpu_registers_[TMP] = true;
-  }
-  if (TMP2 != kNoRegister) {
-    blocked_cpu_registers_[TMP2] = true;
-  }
-  if (PP != kNoRegister) {
-    blocked_cpu_registers_[PP] = true;
-  }
-  blocked_cpu_registers_[SPREG] = true;
-  blocked_cpu_registers_[FPREG] = true;
-  blocked_cpu_registers_[THR] = true;
 
   // FpuTMP is used as scratch by optimized code and parallel move resolver.
   blocked_fpu_registers_[FpuTMP] = true;
 
   // Block additional registers needed preserved when generating intrinsics.
   // TODO(fschneider): Handle saving and restoring these registers when
   // generating intrinsic code.
   if (intrinsic_mode) {
     blocked_cpu_registers_[ARGS_DESC_REG] = true;
   }
@@ skipping 2887 matching lines @@
     THR_Print("-- [after ssa allocator] ir [%s] -------------\n",
               function.ToFullyQualifiedCString());
     FlowGraphPrinter printer(flow_graph_, true);
     printer.PrintBlocks();
     THR_Print("----------------------------------------------\n");
   }
 }
 
 
 }  // namespace dart