Re: [PATCH] middle-end/114604 - ranger allocates bitmap without initialized obstack

[Aldy Hernandez <aldyh@redhat.com>]

On Mon, Apr 8, 2024 at 6:29 PM Richard Biener <rguenther@suse.de> wrote:
>
>
>
> > Am 08.04.2024 um 18:09 schrieb Aldy Hernandez <aldyh@redhat.com>:
> >
> > On Mon, Apr 8, 2024 at 5:54 PM Jakub Jelinek <jakub@redhat.com> wrote:
> >>
> >> On Mon, Apr 08, 2024 at 05:40:23PM +0200, Aldy Hernandez wrote:
> >>>>        PR middle-end/114604
> >>>>        * gimple-range.cc (enable_ranger): Initialize the global
> >>>>        bitmap obstack.
> >>>>        (disable_ranger): Release it.
> >>>> ---
> >>>> gcc/gimple-range.cc | 4 ++++
> >>>> 1 file changed, 4 insertions(+)
> >>>>
> >>>> diff --git a/gcc/gimple-range.cc b/gcc/gimple-range.cc
> >>>> index c16b776c1e3..4d3b1ce8588 100644
> >>>> --- a/gcc/gimple-range.cc
> >>>> +++ b/gcc/gimple-range.cc
> >>>> @@ -689,6 +689,8 @@ enable_ranger (struct function *fun, bool use_imm_uses)
> >>>> {
> >>>>   gimple_ranger *r;
> >>>>
> >>>> +  bitmap_obstack_initialize (NULL);
> >>>> +
> >>>>   gcc_checking_assert (!fun->x_range_query);
> >>>>   r = new gimple_ranger (use_imm_uses);
> >>>>   fun->x_range_query = r;
> >>>> @@ -705,6 +707,8 @@ disable_ranger (struct function *fun)
> >>>>   gcc_checking_assert (fun->x_range_query);
> >>>>   delete fun->x_range_query;
> >>>>   fun->x_range_query = NULL;
> >>>> +
> >>>> +  bitmap_obstack_release (NULL);
> >>>
> >>> Are you not allowed to initialize/use obstacks unless
> >>> bitmap_obstack_initialize(NULL) is called?
> >>
> >> You can use it with some other obstack, just not the default one.
> >>
> >>> If so, wouldn't it be
> >>> better to lazily initialize it downstream (bitmap_alloc, or whomever
> >>> needs it initialized)?
> >>
> >> No, you still need to decide where is the safe point to release it.
> >> Unlike the non-default bitmap_obstack_initialize/bitmap_obstack_release,
> >> the default one can nest (has associated nesting counter).  So, the above
> >> patch just says that ranger starts using the default obstack in
> >> enable_ranger and stops using it in disable_ranger and anything ranger
> >> associated in the obstack can be freed at that point.
> >
> > I thought ranger never used the default one:
> >
> > $ grep bitmap_obstack_initialize *value* *range*
> > value-relation.cc:  bitmap_obstack_initialize (&m_bitmaps);
> > value-relation.cc:  bitmap_obstack_initialize (&m_bitmaps);
> > gimple-range-cache.cc:  bitmap_obstack_initialize (&m_bitmaps);
> > gimple-range-gori.cc:  bitmap_obstack_initialize (&m_bitmaps);
> > gimple-range-infer.cc:  bitmap_obstack_initialize (&m_bitmaps);
> > gimple-range-phi.cc:  bitmap_obstack_initialize (&m_bitmaps);
> >
> > or even:
> >
> > $ grep obstack.*NULL *value* *range*
> > value-range-storage.cc:    obstack_free (&m_obstack, NULL);
> > value-relation.cc:  obstack_free (&m_chain_obstack, NULL);
> > value-relation.cc:  obstack_free (&m_chain_obstack, NULL);
> > gimple-range-infer.cc:  obstack_free (&m_list_obstack, NULL);
> > value-range-storage.cc:    obstack_free (&m_obstack, NULL);
> >
> > I'm obviously missing something here.
>
> Look for BITMAP_ALLOC (NULL) in the backtrace in the PR

Ahh!  Thanks.

A few default obstack uses snuck in while I wasn't looking.

$ grep BITMAP_ALLOC.*NULL *range*
gimple-range-cache.cc:  m_propfail = BITMAP_ALLOC (NULL);
gimple-range-cache.h:  inline ssa_lazy_cache () { active_p =
BITMAP_ALLOC (NULL); }
gimple-range.cc:  m_pop_list = BITMAP_ALLOC (NULL);

I wonder if it would be cleaner to just change these to use named obstacks.

Andrew, is there a reason we were using the default obstack for these?
 For reference, they are  class update_list used in the ranger cache,
ssa_lazy_cache, and dom_ranger.

Aldy