Checkpoint for GC *after* saving object reference.

It turns out, surprisingly enough, it's much easier to see the object we
are trying to trace if we put it where we can see it first.

machine/instructions/r_ret.hpp

@@ -7,7 +7,7 @@ namespace rubinius {
         call_frame->scope->flush_to_heap(state);
       }
 
-      Object* obj = call_frame->return_value = RVAL(r0);
+      Object* obj = RVAL(r0);
 
       if(obj->nil_p()) {
         if(NIL_TAG_ID(obj) == 0) {
@@ -16,6 +16,8 @@ namespace rubinius {
         }
       }
 
+      call_frame->return_value = obj;
+
       return reinterpret_cast<intptr_t>(obj);
     }
   }

machine/instructions/ret.hpp

@@ -7,7 +7,7 @@ namespace rubinius {
         call_frame->scope->flush_to_heap(state);
       }
 
-      Object* obj = call_frame->return_value = stack_top();
+      Object* obj = stack_top();
 
       if(obj->nil_p()) {
         if(NIL_TAG_ID(obj) == 0) {
@@ -16,6 +16,8 @@ namespace rubinius {
         }
       }
 
+      call_frame->return_value = obj;
+
       return reinterpret_cast<intptr_t>(obj);
     }
   }

machine/interpreter/r_ret.cpp

@@ -3,9 +3,11 @@
 namespace rubinius {
   namespace interpreter {
     intptr_t r_ret(STATE, CallFrame* call_frame, intptr_t const opcodes[]) {
+      intptr_t value = instructions::r_ret(state, call_frame, argument(0));
+
       state->vm()->checkpoint(state);
 
-      return instructions::r_ret(state, call_frame, argument(0));
+      return value;
     }
   }
 }

machine/interpreter/ret.cpp

@@ -3,9 +3,11 @@
 namespace rubinius {
   namespace interpreter {
     intptr_t ret(STATE, CallFrame* call_frame, intptr_t const opcodes[]) {
+      intptr_t value = instructions::ret(state, call_frame);
+
       state->vm()->checkpoint(state);
 
-      return instructions::ret(state, call_frame);
+      return value;
     }
   }
 }