[Sablevm-developer] Re: SableVM

[Prof. E. M. G. <eti...@uq...>]

Hi Christian,

I am CC'ing the sablevm-developer ML, as my answer might also help others.

Christian ROY wrote:
>   I'm curious about something. I'm currently using marking to determine
> what elements of the stack are reference types. I've come accross a
> behavior I'd like to have clarified. Before a method is invoked the object
> refrerence is pushed onto the stack and so are the arguments. When the new
> method begins it creates it own stack frame and sets its local from
> the previous values of the old stack frame. So the object reference is
> placed in local[0] and the arguments in local[1] ... to local[n]. How do
> you distinguish which of these arguments are references and which are not?

There is a bitmap used to indicate which of the "parameter" locals are references.

Look at src/libsablevm/types.h:

...
struct _svmt_method_info_struct
{
...
   jint java_args_count;         /* int,ref,boolean,... = 1; double, long = 2 */
   _svmt_gc_map_node *parameters_gc_map;
...
This is not sufficient for tracing all locals, as you also have to consider
locals which are not parameters.  The reference-yet-not-parameter locals are all
grouped together, just after the parameter locals.  See:

...
struct _svmt_method_frame_info_struct
{
...
   /* number of non-parameter reference local variables */
   jint non_parameter_ref_locals_count;
...

Please look at src/libsablevm/gc_copying.c for an example of how to use them, e.g.

/* method formal parameters */
{
   jint i;
   jint count = parameters_gc_map->size;

   for (i = 0; i < count; i++)
     {
       if (_svmf_get_bit (parameters_gc_map->bits, i))
         {
           locals[i].reference =
             _svmf_copy_object (env, locals[i].reference,
                                pto_space_tail);
         }
     }
}

/* other ref locals */
{
   jint i;
   jint start = method->java_args_count;
   jint end = start + non_parameter_ref_locals_count;

   for (i = start; i < end; i++)
     {
       locals[i].reference =
         _svmf_copy_object (env, locals[i].reference,
                            pto_space_tail);
     }
}


I hope this helps.

Etienne

-- 
Etienne M. Gagnon, Ph.D.             http://www.info.uqam.ca/~egagnon/
SableVM:                                       http://www.sablevm.org/
SableCC:                                       http://www.sablecc.org/