Re: RFA: Don't use obsavestring in dwarf2read

Elena Zannoni
2004-02-05T23:20:06Z

Daniel Jacobowitz writes:
> On Thu, Feb 05, 2004 at 03:34:30PM -0500, Elena Zannoni wrote:
> > > The obstacks themselves are probably a good idea. Once upon a time,
> > > Peter informed me, there was a plan to free the psymbol obstack when
> > > all symbols had been read in; but that doesn't seem like a useful
> > > optimization, and I can't think offhand of any use for separate symbol
> > > and type obstacks. I wouldn't object to having a per-objfile obstack
> > > instead, and un-seperating them.
> >
> > I think it would be worthwhile to see how much doing that would save us.
>
> Well, it wouldn't save anything by itself - there's immeasurable
> overhead to the obstacks. It would let us eliminate this sort of
> duplication, but they're pretty tricky to identify; it took me a couple
> of hours to convince myself about this set of 'em.
>

I meant in general, yes. Since the possible 'shorcuts' are difficult
to identify, and they are only for dwarf2 (that I've looked at) I am
bit worried about the cross pointers. I am thinking to kill the triad.

> > > > [BTW why are only few obstack properly initialized?]
> > >
> > > Which do you mean?
> > >
> >
> > I grepped for obstack_init, and only a few obstacks call that
> > function. Form the obstack doco, it seems that it needs to be
> > called. I wonder if the function was introduced later on in libiberty,
> > as an afterthought.
>
> It looks like obstack_specify_allocation and obstack_init fill the same
> role. The objfile's obstacks use the former.
>

Which is dumb:

# define obstack_init(h) \
_obstack_begin ((h), 0, 0, \
(void *(*) ()) obstack_chunk_alloc, (void (*) ()) obstack_chunk_free)

# define obstack_specify_allocation(h, size, alignment, chunkfun, freefun) \
_obstack_begin ((h), (size), (alignment), \
(void *(*) ()) (chunkfun), (void (*) ()) (freefun))

all the calls to obstack_specify_allocation use xmalloc and xfree, but
we also have:

/* Unless explicitly specified, GDB obstacks always use xmalloc() and
xfree(). */
#define obstack_chunk_alloc xmalloc
#define obstack_chunk_free xfree

There is only a call to obstack_specify_allocation that specifies size
and alignment.

I'd prefer them to use just one method. I'll clean up some.

> > Ah, ok, it's because of the nature of the program you were handling. I
> > was trying to imagine how the overhead of obstack themselves could be
> > that large. It seems to me that this is a good argument for an 'on
> > demand' symbol reading implementaion. But, yes the various dwarf2
> > sections are already in the psymbol_obstack. And we are duplicating
> > that again on the type_obstack. :-(
>
> Right. I'm not sure how much of this can be done on demand that isn't
> already; if I wasn't clear about this, the 100MB was a worst-case
> number (-readnow). Without -readnow it's much less.
>

ah. Yep, you didn't mention readnow.