Ok, looks like ##resolve-referencing-objects is in place now!

> (define a "Hello world")
> (##resolve-referencing-objects a)
(#<absent> #<absent> #<absent>)
> (list a)
("Hello world")
> (##resolve-referencing-objects a)
(#<absent> #<absent> #<absent> ("Hello world"))
> (vector a)
#("Hello world")
> (##resolve-referencing-objects a)
(#<absent> #<absent> #<absent> #("Hello world") ("Hello world"))
> 0 1 2
0
> 1
> 2
> (##gc)
> (##resolve-referencing-objects a)
(#<absent> #<absent> #<absent>)


Implementation:

To _kernel.scm row 4388 and on are added:

;;;----------------------------------------------------------------------------

;;; Resolver of referencing objects

;;; Works for memory-allocated objects only, for details see the conditions for
;;; the validate_old_obj call in mem.c .

; For this procedure to work, up to the point where the explicitly induced GC is made,
; there must be no other GC.
;
; (##resolve-referencing-objects obj) => list of the objects that reference obj
(define (##resolve-referencing-objects obj)
  (##declare (not interrupts-enabled))

  ; Set the object to scan for
  (##c-code #<<end-of-code

// Note that this variable is declared even if the referencing objects resolver
// is not enabled.
resolve_objects_referencing = ___ARG1;

end-of-code
   obj)

  ; Trig garbage collection as to perform the scan
  (##gc)

  ; Reset the object to scan for setting
  (##c-code #<<end-of-code

  resolve_objects_referencing = -1;

end-of-code
   )

  ; Simplistic way of indicating to user whether this procedure is implemented.
  (and (##c-code #<<end-of-code

#ifdef ___DEBUG
___RESULT = ___TRU;
#else
___RESULT = ___FAL;
#endif

end-of-code
        )
       ; Download the resolved objects and return
       (let loop ((r '()))
         (let ((object (##c-code "___RESULT = get_next_resolved_referencing_object();")))
           (if object
               (loop (cons object r))
               r)))))

To mem.h row 69 and on are added:

extern ___SCMOBJ resolve_objects_referencing;
extern ___SCMOBJ get_next_resolved_referencing_object ___PVOID;


To mem.h are added starting at the following rows:

30:


/*
 * Defining the symbol ENABLE_REFERENCING_OBJECTS_RESOLVER will enable
 * the GC to resolve the references to a specified object, see
 * ##resolve-referencing-objects of _kernel.scm for more info.
 */
#define ENABLE_REFERENCING_OBJECTS_RESOLVER

423:

/* Object to look up all references to */
___WORD resolve_objects_referencing = -1;

typedef struct resolved_referencing_object_struct {
  ___SCMOBJ object;
  struct resolved_referencing_object_struct* next;
} resolved_referencing_object;

___HIDDEN resolved_referencing_object* resolved_referencing_objects = NULL;

void add_resolved_referencing_object(___SCMOBJ object) {
  resolved_referencing_object* r = (resolved_referencing_object*) malloc(sizeof(resolved_referencing_object));
  r->object = object;
  r->next = resolved_referencing_objects;
  resolved_referencing_objects = r;
}

// Get next object resolved as referencing resolve_objects_referencing .
// Used by ##resolve-referencing-objects
___SCMOBJ get_next_resolved_referencing_object ___PVOID
  {
    if (resolved_referencing_objects == NULL) {
      return ___FAL;
    } else {
      resolved_referencing_object* r = resolved_referencing_objects;
      ___SCMOBJ object = r->object;
      resolved_referencing_objects = r->next;
      free(r);
      return object;
    }
  }

2184 (replacing the previous ifdef-endif validate_old_obj block):

#if defined(ENABLE_CONSISTENCY_CHECKS) || defined(ENABLE_REFERENCING_OBJECTS_RESOLVER)

#ifdef ENABLE_REFERENCING_OBJECTS_RESOLVER
          if (obj == resolve_objects_referencing) {

            // This typecasting of container_body to ___SCMOBJ was inspired by the corresponding code in explain_problem .
            if (reference_location == IN_OBJECT) {
              // Reference from other Scheme object's body
              ___WORD container;
#if ___tPAIR != ___tSUBTYPED
              ___WORD head = container_body[-1];
              int subtype = ___HD_SUBTYPE(head);
              if (subtype == ___sPAIR)
                container = ___TAG(container_body-___BODY_OFS,___tPAIR);
              else
#endif
              container = ___TAG(container_body-___BODY_OFS,___tSUBTYPED);

              // ___printf("Found match to sought for object %lu : ",resolve_objects_referencing);
              // print_object(container,0,"",0);
              // ___printf("\n");

              add_resolved_referencing_object(container);
            } else {
              // Reference from global variable etc.
              add_resolved_referencing_object(___ABSENT);
            }

          }

#endif

#ifdef ENABLE_CONSISTENCY_CHECKS
          if (___DEBUG_SETTINGS_LEVEL(___setup_params.debug_settings) >= 1)
            validate_old_obj (obj);
#endif

#endif

and the #define ENABLE_CONSISTENCY_CHECKS :s that regard a block that define or set reference_location are replaced with #if defined(ENABLE_CONSISTENCY_CHECKS) || defined(ENABLE_REFERENCING_OBJECTS_RESOLVER) .


Compilation of a new ./gsc/gsc (g++):

cd lib
g++ -Wno-unused -Wno-write-strings -g -fno-trapping-math -fno-strict-aliasing -fwrapv -fPIC -fno-common -mieee-fp   -I"../include" -c -o "mem.o" -I. -DHAVE_CONFIG_H -D___GAMBCDIR="\"/usr/local/Gambit-C\"" -D___SYS_TYPE_CPU="\"i686\"" -D___SYS_TYPE_VENDOR="\"pc\"" -D___SYS_TYPE_OS="\"linux-gnu\"" -D___CONFIGURE_COMMAND="\"./configure '--enable-cplusplus' '--enable-single-host' '--enable-debug'"\" -D___OBJ_EXTENSION="\".o\"" -D___EXE_EXTENSION="\"\"" -D___BAT_EXTENSION="\"\"" -D___PRIMAL mem.c -D___LIBRARY
gsc -:~~bin=../bin,~~lib=../lib,~~include=../include -c  _kernel.scm
gsc -:~~bin=../bin,~~lib=../lib,~~include=../include -f -link -flat -o _gambc.c _kernel.c _system.c _num.c _std.c _eval.c _io.c _nonstd.c _thread.c _repl.c
g++ -Wno-unused -Wno-write-strings -g -fno-trapping-math -fno-strict-aliasing -fwrapv -fPIC -fno-common -mieee-fp   -I"../include" -c -o "_kernel.o" -I. -DHAVE_CONFIG_H -D___GAMBCDIR="\"/usr/local/Gambit-C\"" -D___SYS_TYPE_CPU="\"i686\"" -D___SYS_TYPE_VENDOR="\"pc\"" -D___SYS_TYPE_OS="\"linux-gnu\"" -D___CONFIGURE_COMMAND="\"./configure '--enable-cplusplus' '--enable-single-host' '--enable-debug'"\" -D___OBJ_EXTENSION="\".o\"" -D___EXE_EXTENSION="\"\"" -D___BAT_EXTENSION="\"\"" -D___PRIMAL _kernel.c -D___LIBRARY
cd ..
make


Space for improvement or things to note are:

 * ##resolve-referencing-objects is always compiled, but returns #f if Gambit is not compiled with --enable-debug .

   Possibly some neater or otherwise better degradation of the procedure could be devised for when Gambit is not compiled in debug mode?

 * When compiled with --debug, on the iteration over each memory-allocated object, two checks are always done, namely the two highlighted above.

   These could easily be united to one collective check, by making a global boolean variable sth like perform_individual_object_checks that's set to (resolve_objects_referencing != -1) || (___DEBUG_SETTINGS_LEVEL(___setup_params.debug_settings) >= 1) on ___garbage_collect .

   Those two checks would then be surrounded by if (perform_individual_object_checks) { .. } .

   This would save some clock cycles in the ordinary use case, but add ~5 lines of complexity to mem.c .

 * Note that object references that are from objects other than Gambit standard objects, are represented as an absent special object.

 * I did not implement this in validate_old_obj as ##resolve-referencing-objects is quite common to use
   and validate_old_obj in contrast looks like a very expensive, special-purpose routine.


Feedback?


Mikael


2013/6/18 Marc Feeley <feeley@iro.umontreal.ca>
Also, your code would be useful to others, so please do a pull request when you are done.

Marc

On Jun 18, 2013, at 12:00 PM, Mikael <mikael.rcv@gmail.com> wrote:

> Ah, yey, your code in explain_problem already provided an example of how to do this!
>
> Will write here soon how to do this lookup of referencing objects soon.
>