Re: MPS: hash tables / obarrays

[Gerd Möllmann]

Helmut Eller <eller.helmut@gmail.com> writes:

>> I'd say a reference we didn't scan :-(.
>>
>> The is_mps with a Lisp_Object doesn't work with symbols, BTW, because
>> the Lisp_Object doesn't contain a pointer, but an offset from lispsym.
>
> The function that tries to print is comp--final and the argument is the
> expr.  When I call safe_debug_print on that it goes on for many pages
> and ends with this:
>
>    #2314# 635 #2194# 636 #2181# 637 #2022# 638 #1949# 639 #1891# 640 #1640#)) 
> #s(comp-data-container nil #s(hash-table test 
>   igc.c:343: Emacs fatal error: assertion failed: h->obj_type != IGC_OBJ_FWD
>   
>   Breakpoint 1, terminate_due_to_signal (sig=6, backtrace_limit=2147483647)
>       at emacs.c:443
>   443       signal (sig, SIG_DFL);
>   The program being debugged stopped while in a function called from GDB.
>   Evaluation of the expression containing the function
>   (safe_debug_print) will be abandoned.
>   When the function is done executing, GDB will silently stop.
>   (gdb)
>
> So it could have something to do with hash tables after all.

Could be, of course.

Maybe you could try the following: In igc_check_fwd, where we find the
header has IGC_OBJ_FWD,

  void
  igc_check_fwd (void *client)
  {
    if (is_mps (client))
      {
        struct igc_header *h = client_to_base (client);
        igc_assert (h->obj_type != IGC_OBJ_FWD);
      }
  }

  struct igc_fwd
  {
    struct igc_header header;
    mps_addr_t new_base_addr;
  };

When h->obj_type == IGC_OBJ_FWD, then the header is actually an igc_fwd,
and the new_base_addr points to where the object was forwarded to.
dflt_fwd does that. Some phantasy debug commands woould look like

(lldb) p *(struct igc_fwd *) h
=> ...new_base_addr = 0x1234

(lldb) p *(struct igc_header *) 0x1234
(lldb) p base_to_client ((void *) 0x1234)

We could then perhaps, not always, see what the real object looks like.
For example, if it is a symbol, its name etc. Maybe that gives us a
clue.