[m-dev.] Re: About to abandon .par grades... (fwd)

[Julien Fischer]

---------- Forwarded message ----------
Date: Tue, 19 Oct 2004 17:00:08 +1000
From: Ralph Becket <rafe at cs.mu.OZ.AU>
To: Fergus Henderson <fjh007 at galois.com>
Cc: Ralph Becket <rafe at cs.mu.OZ.AU>, Julien Fischer <juliensf at cs.mu.OZ.AU>,
     Zoltan Somogyi <zs at cs.mu.OZ.AU>
Subject: Re: About to abandon .par grades...

Thanks for the rapid response, Fergus.  I chatted briefly with Peter
Stuckey and he's keen that Mercury retain the .par grades, so maybe we
shouldn't be too hasty to abandon them.  Zoltan, would you be satisfied
if we (a) adopt one of Fergus' recommendations for the fix and (b) make
it clear in the NEWS file et cetera that, for various reasons, we may be
unable to fix problems to do with parallelism?

I have to drop this one for now and head off to sunny Bali.  See you all
in two weeks!

-- Ralph

Fergus Henderson, Monday, 18 October 2004:
> On 19-Oct-2004, Ralph Becket <rafe at cs.mu.OZ.AU> wrote:
> > Fergus Henderson, Monday, 18 October 2004:
> > >
> > > Well, I don't understand the fix either.
> > > Your diagnosis that without it "the Mercury engine is not initialized"
> > > doesn't make much sense to me, since there are calls to
> > > "MR_init_thread(MR_use_now)" immediately after the call to
> > > MR_init_memory() and the code to initialize the trail.
>
> Ignore that, I misread the #ifdefs.
>
> > > Where does it actually crash?
> >
> > The code crashes in mercury_wrapper.c:mercury_runtime_init() when it
> > executes the macro MR_save_registers().
>
> Ah, I see now.
>
> The code that is causing trouble is in mercury_wrapper.c from line 508
>
> 	#if defined(MR_HIGHLEVEL_CODE) && defined(MR_CONSERVATIVE_GC)
> 		MR_init_memory();
> 	  #ifdef MR_USE_TRAIL
> 		/* initialize the trail */
> 		MR_trail_zone = MR_create_zone("trail", 0,
> 			MR_trail_size, MR_next_offset(),
> 			MR_trail_zone_size, MR_default_handler);
> 		MR_trail_ptr = (MR_TrailEntry *) MR_trail_zone->min;
> 		MR_ticket_counter = 1;
> 		MR_ticket_high_water = 1;
> 	  #endif
> 	#else
> 		/* start up the Mercury engine */
> 		...
> 	#endif
>
> This code was part of my long-term goal of eventually eliminating the need
> for the low-level back-end.  This code is deliberately trying to avoid
> starting up the Mercury engine in the case when both MR_HIGHLEVEL_CODE
> and MR_CONSERVATIVE are defined.  The idea is that if we're using the
> conservative collector and we're generating ANSI C, then there's no need
> for the code to have any dependencies on all of that complicated code
> which manages the Mercury engine.
>
> Unfortunately this appears to not be the case any more, because
> various bits and pieces of code have crept in that depend on the
> Mercury engine stuff and are which are not inside appropriate #ifdefs.
>
> > In order to see what's going on
> > there we've tried replacing the contents of mercury_wrapper.c with
> > mercury_wrapper.i, but that seems to make every version of gcc we have
> > either crash or generate code that doesn't work!
> >
> > The part of the .i file showing the expansion of MR_save_registers is
> >
> > do {
> > 	((void) 0);
> > 	(((((MercuryEngine *) ((MercuryEngine *)
> > 	pthread_getspecific((MR_engine_base_key))))->
> > 		MR_eng_fake_reg))[((32 + 6) + 3) ]) = (MR_Word) (MR_sol_hp_var);
> > 	(((((MercuryEngine *) ((MercuryEngine *)
> > 	pthread_getspecific((MR_engine_base_key))))->
> > 		MR_eng_fake_reg))[((32 + 6) + 4) ]) = (MR_Word) (MR_min_hp_rec_var);
> > 	(((((MercuryEngine *) ((MercuryEngine *)
> > 	pthread_getspecific((MR_engine_base_key))))->
> > 		MR_eng_fake_reg))[((32 + 6) + 5) ]) = (MR_Word) (MR_min_sol_hp_rec_var);
> > 	(((((MercuryEngine *) ((MercuryEngine *)
> > 	pthread_getspecific((MR_engine_base_key))))->
> > 		MR_eng_fake_reg))[((32 + 6) + 6) ]) = (MR_Word) (MR_global_hp_var);
> >
> > 	((void) 0);
> > 	((void) 0);
> > } while (0);
>
> Hmm, perhaps all of the stuff which accesses MR_*hp* should be
> made conditional on #ifndef MR_CONSERVATIVE_GC.  Also the call to
> MR_save_registers() in mercury_init() should be conditional on whether
> we're using a Mercury engine.
>
> So, to summarize: instead of the "voodoo" fix, I suggest either
> 	- change the "#if defined(MR_HIGHLEVEL_CODE) && defined(MR_CONSERVATIVE_GC)"
> 	  at line to "#if 0" and add a comment explaining the intent of
> 	  this code and why it doesn't work anymore
> or
> 	- ensure that any uses of the Mercury engine's virtual machine registers
> 	  are conditionalized with #ifdefs so that they do not happen
> 	  in the case when there is no Mercury engine.
>
> --
> Fergus J. Henderson                 |  "I have always known that the pursuit
> Galois Connections, Inc.            |  of excellence is a lethal habit"
> Phone: +1 503 626 6616              |     -- the last words of T. S. Garp.
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