CTX and TMP registers may not be allocated.

runtime/vm/locations.cc

 // Copyright (c) 2012, 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/locations.h"
 
 #include "vm/intermediate_language.h"
 
 namespace dart {
 
 
 static Register AllocateFreeRegister(
     EmbeddedArray<bool, kNumberOfCpuRegisters>* blocked_registers) {
   for (intptr_t regno = 0; regno < kNumberOfCpuRegisters; regno++) {
     if (!blocked_registers->At(regno)) {
       blocked_registers->SetAt(regno, true);
       return static_cast<Register>(regno);
     }
   }
   UNREACHABLE();
   return kNoRegister;
 }
 
 
 void LocationSummary::AllocateRegisters() {
   EmbeddedArray<bool, kNumberOfCpuRegisters> blocked_registers;
 
-  // Mark all registers free.
+  // Mark all available registers free.
   for (intptr_t i = 0; i < kNumberOfCpuRegisters; i++) {
     blocked_registers[i] = false;
   }
 
   // Mark all fixed registers as used.
   for (intptr_t i = 0; i < count(); i++) {
     Location loc = in(i);
     if (loc.kind() == Location::kRegister) {
       ASSERT(!blocked_registers[loc.reg()]);
       blocked_registers[loc.reg()] = true;
     }
   }
 
+  // Do not allocate known registers.
+  blocked_registers[CTX] = true;
+  if (TMP != kNoRegister) {
+    blocked_registers[TMP] = true;
+  }
+
   // Allocate all unallocated input locations.
   for (intptr_t i = 0; i < count(); i++) {
     Location loc = in(i);
     if (loc.kind() == Location::kUnallocated) {
       ASSERT(loc.policy() == Location::kRequiresRegister);
       set_in(i, Location::RegisterLocation(
           AllocateFreeRegister(&blocked_registers)));
     }
   }
 
   Location result_location = out();
   if (result_location.kind() == Location::kUnallocated) {
     switch (result_location.policy()) {
       case Location::kRequiresRegister:
         result_location = Location::RegisterLocation(
             AllocateFreeRegister(&blocked_registers));
         break;
       case Location::kSameAsFirstInput:
         result_location = in(0);
         break;
     }
     set_out(result_location);
   }
 }
 
 
 }  // namespace dart