Re: [PATCH 5/6] make get_domminated_by_region return a auto_vec

[Richard Biener <richard.guenther@gmail.com>]

On Thu, Jun 17, 2021 at 4:43 PM Martin Sebor <msebor@gmail.com> wrote:
>
> On 6/17/21 12:03 AM, Richard Biener wrote:
> > On Wed, Jun 16, 2021 at 6:01 PM Martin Sebor <msebor@gmail.com> wrote:
> >>
> >> On 6/16/21 6:46 AM, Richard Sandiford via Gcc-patches wrote:
> >>> Richard Biener via Gcc-patches <gcc-patches@gcc.gnu.org> writes:
> >>>> On Tue, Jun 15, 2021 at 8:02 AM Trevor Saunders <tbsaunde@tbsaunde.org> wrote:
> >>>>>
> >>>>> This makes it clear the caller owns the vector, and ensures it is cleaned up.
> >>>>>
> >>>>> Signed-off-by: Trevor Saunders <tbsaunde@tbsaunde.org>
> >>>>>
> >>>>> bootstrapped and regtested on x86_64-linux-gnu, ok?
> >>>>
> >>>> OK.
> >>>>
> >>>> Btw, are "standard API" returns places we can use 'auto'?  That would avoid
> >>>> excessive indent for
> >>>>
> >>>> -  dom_bbs = get_dominated_by_region (CDI_DOMINATORS,
> >>>> -                                    bbs.address (),
> >>>> -                                    bbs.length ());
> >>>> +  auto_vec<basic_block> dom_bbs = get_dominated_by_region (CDI_DOMINATORS,
> >>>> +                                                          bbs.address (),
> >>>> +                                                          bbs.length ());
> >>>>
> >>>> and just uses
> >>>>
> >>>>     auto dom_bbs = get_dominated_by_region (...
> >>>>
> >>>> Not asking you to do this, just a question for the audience.
> >>>
> >>> Personally I think this would be surprising for something that doesn't
> >>> have copy semantics.  (Not that I'm trying to reopen that debate here :-)
> >>> FWIW, I agree not having copy semantics is probably the most practical
> >>> way forward for now.)
> >>
> >> But you did open the door for me to reiterate my strong disagreement
> >> with that.  The best C++ practice going back to the early 1990's is
> >> to make types safely copyable and assignable.  It is the default for
> >> all types, in both C++ and C, and so natural and expected.
> >>
> >> Preventing copying is appropriate in special and rare circumstances
> >> (e.g, a mutex may not be copyable, or a file or iostream object may
> >> not be because they represent a unique physical resource.)
> >>
> >> In the absence of such special circumstances preventing copying is
> >> unexpected, and in the case of an essential building block such as
> >> a container, makes the type difficult to use.
> >>
> >> The only argument for disabling copying that has been given is
> >> that it could be surprising(*).  But because all types are copyable
> >> by default the "surprise" is usually when one can't be.
> >>
> >> I think Richi's "surprising" has to do with the fact that it lets
> >> one inadvertently copy a large amount of data, thus leading to
> >> an inefficiency.  But by analogy, there are infinitely many ways
> >> to end up with inefficient code (e.g., deep recursion, or heap
> >> allocation in a loop), and they are not a reason to ban the coding
> >> constructs that might lead to it.
> >>
> >> IIUC, Jason's comment about surprising effects was about implicit
> >> conversion from auto_vec to vec.  I share that concern, and agree
> >> that it should be addressed by preventing the conversion (as Jason
> >> suggested).
> >
> > But fact is that how vec<> and auto_vec<> are used today in GCC
> > do not favor that.  In fact your proposed vec<> would be quite radically
> > different (and IMHO vec<> and auto_vec<> should be unified then to
> > form your proposed new container).  auto_vec<> at the moment simply
> > maintains ownership like a smart pointer - which is _also_ not copyable.
>
> Yes, as we discussed in the review below, vec is not a good model
> because (as you note again above) it's constrained by its legacy
> uses.  The best I think we can do for it is to make it safer to
> use.
> https://gcc.gnu.org/pipermail/gcc-patches/2021-June/571622.html

Which is what Trevors patches do by simply disallowing things
that do not work at the moment.

> (Smart pointers don't rule out copying.  A std::unique_ptr does
> and std::shared_ptr doesn't.  But vec and especially auto_vec
> are designed to be containers, not "unique pointers" so any
> relationship there is purely superficial and a distraction.)
>
> That auto_vec and vec share a name and an is-a relationship is
> incidental, an implementation detail leaked into the API.  A better
> name than vector is hard to come up with, but the public inheritance
> is a design flaw, a bug waiting to be introduced due to the conversion
> and the assumptions the base vec makes about POD-ness and shallow
> copying.  Hindsight is always 20/20 but past mistakes should not
> dictate the design of a general purpose vector-like container in
> GCC.

That auto_vec<> "decays" to vec<> was on purpose design.

By-value passing of vec<> is also on purpose to avoid an extra
pointer indirection on each access.

> I fully support fixing or at least mitigating the problems with
> the vec base class (unsafe copying, pass-by-value etc.).  As I
> mentioned, I already started working on this cleanup.  I also
> have no objection to introducing a non-copyable form of a vector
> template (I offered one in my patch), or even to making auto_vec
> non-copyable provided a copyable and assignable one is introduced
> at the same time, under some other name.

Why at the same time?  I'm still not convinced we need another
vector type here.  Yes, auto_vec<auto_vec<..> > would be convenient,
but then auto_vec<> doesn't bother to call the DTOR on its elements
either (it's actually vec<> again here).  So auto_vec<> is _not_
a fancier C++ vec<>, it's still just vec<> but with RAII for the container
itself.

> Having said that, and although I don't mind the cleanup being taken
> off my plate, I would have expected the courtesy of at least a heads
> up first.  I do find it disrespectful for someone else involved in
> the review of my work to at the same time submit a patch of their
> own that goes in the opposite direction, and for you to unilaterally
> approve it while the other review hasn't concluded yet.

Because the changes do not change anything as far as I understand.
They make more use of auto_vec<> ownership similar to when
I added the move ctor and adjusted a single loop API.  At the same
time it completes the move stuff and plugs some holes.

Richard.

> Martin
>
> >
> > Richard.
> >
> >> Martin
> >>
> >>>
> >>> Thanks,
> >>> Richard
> >>>
> >>>> Thanks,
> >>>> Richard.
> >>>>
> >>>>> gcc/ChangeLog:
> >>>>>
> >>>>>           * dominance.c (get_dominated_by_region): Return auto_vec<basic_block>.
> >>>>>           * dominance.h (get_dominated_by_region): Likewise.
> >>>>>           * tree-cfg.c (gimple_duplicate_sese_region): Adjust.
> >>>>>           (gimple_duplicate_sese_tail): Likewise.
> >>>>>           (move_sese_region_to_fn): Likewise.
> >>>>> ---
> >>>>>    gcc/dominance.c |  4 ++--
> >>>>>    gcc/dominance.h |  2 +-
> >>>>>    gcc/tree-cfg.c  | 18 +++++++-----------
> >>>>>    3 files changed, 10 insertions(+), 14 deletions(-)
> >>>>>
> >>>>> diff --git a/gcc/dominance.c b/gcc/dominance.c
> >>>>> index 0e464cb7282..4943102ff1d 100644
> >>>>> --- a/gcc/dominance.c
> >>>>> +++ b/gcc/dominance.c
> >>>>> @@ -906,13 +906,13 @@ get_dominated_by (enum cdi_direction dir, basic_block bb)
> >>>>>       direction DIR) by some block between N_REGION ones stored in REGION,
> >>>>>       except for blocks in the REGION itself.  */
> >>>>>
> >>>>> -vec<basic_block>
> >>>>> +auto_vec<basic_block>
> >>>>>    get_dominated_by_region (enum cdi_direction dir, basic_block *region,
> >>>>>                            unsigned n_region)
> >>>>>    {
> >>>>>      unsigned i;
> >>>>>      basic_block dom;
> >>>>> -  vec<basic_block> doms = vNULL;
> >>>>> +  auto_vec<basic_block> doms;
> >>>>>
> >>>>>      for (i = 0; i < n_region; i++)
> >>>>>        region[i]->flags |= BB_DUPLICATED;
> >>>>> diff --git a/gcc/dominance.h b/gcc/dominance.h
> >>>>> index 515a369aacf..c74ad297c6a 100644
> >>>>> --- a/gcc/dominance.h
> >>>>> +++ b/gcc/dominance.h
> >>>>> @@ -47,7 +47,7 @@ extern basic_block get_immediate_dominator (enum cdi_direction, basic_block);
> >>>>>    extern void set_immediate_dominator (enum cdi_direction, basic_block,
> >>>>>                                        basic_block);
> >>>>>    extern auto_vec<basic_block> get_dominated_by (enum cdi_direction, basic_block);
> >>>>> -extern vec<basic_block> get_dominated_by_region (enum cdi_direction,
> >>>>> +extern auto_vec<basic_block> get_dominated_by_region (enum cdi_direction,
> >>>>>                                                            basic_block *,
> >>>>>                                                            unsigned);
> >>>>>    extern vec<basic_block> get_dominated_to_depth (enum cdi_direction,
> >>>>> diff --git a/gcc/tree-cfg.c b/gcc/tree-cfg.c
> >>>>> index 6bdd1a561fd..c9403deed19 100644
> >>>>> --- a/gcc/tree-cfg.c
> >>>>> +++ b/gcc/tree-cfg.c
> >>>>> @@ -6495,7 +6495,6 @@ gimple_duplicate_sese_region (edge entry, edge exit,
> >>>>>      bool free_region_copy = false, copying_header = false;
> >>>>>      class loop *loop = entry->dest->loop_father;
> >>>>>      edge exit_copy;
> >>>>> -  vec<basic_block> doms = vNULL;
> >>>>>      edge redirected;
> >>>>>      profile_count total_count = profile_count::uninitialized ();
> >>>>>      profile_count entry_count = profile_count::uninitialized ();
> >>>>> @@ -6549,9 +6548,9 @@ gimple_duplicate_sese_region (edge entry, edge exit,
> >>>>>
> >>>>>      /* Record blocks outside the region that are dominated by something
> >>>>>         inside.  */
> >>>>> +  auto_vec<basic_block> doms;
> >>>>>      if (update_dominance)
> >>>>>        {
> >>>>> -      doms.create (0);
> >>>>>          doms = get_dominated_by_region (CDI_DOMINATORS, region, n_region);
> >>>>>        }
> >>>>>
> >>>>> @@ -6596,7 +6595,6 @@ gimple_duplicate_sese_region (edge entry, edge exit,
> >>>>>          set_immediate_dominator (CDI_DOMINATORS, entry->dest, entry->src);
> >>>>>          doms.safe_push (get_bb_original (entry->dest));
> >>>>>          iterate_fix_dominators (CDI_DOMINATORS, doms, false);
> >>>>> -      doms.release ();
> >>>>>        }
> >>>>>
> >>>>>      /* Add the other PHI node arguments.  */
> >>>>> @@ -6662,7 +6660,6 @@ gimple_duplicate_sese_tail (edge entry, edge exit,
> >>>>>      class loop *loop = exit->dest->loop_father;
> >>>>>      class loop *orig_loop = entry->dest->loop_father;
> >>>>>      basic_block switch_bb, entry_bb, nentry_bb;
> >>>>> -  vec<basic_block> doms;
> >>>>>      profile_count total_count = profile_count::uninitialized (),
> >>>>>                   exit_count = profile_count::uninitialized ();
> >>>>>      edge exits[2], nexits[2], e;
> >>>>> @@ -6705,7 +6702,8 @@ gimple_duplicate_sese_tail (edge entry, edge exit,
> >>>>>
> >>>>>      /* Record blocks outside the region that are dominated by something
> >>>>>         inside.  */
> >>>>> -  doms = get_dominated_by_region (CDI_DOMINATORS, region, n_region);
> >>>>> +  auto_vec<basic_block> doms = get_dominated_by_region (CDI_DOMINATORS, region,
> >>>>> +                                                       n_region);
> >>>>>
> >>>>>      total_count = exit->src->count;
> >>>>>      exit_count = exit->count ();
> >>>>> @@ -6785,7 +6783,6 @@ gimple_duplicate_sese_tail (edge entry, edge exit,
> >>>>>      /* Anything that is outside of the region, but was dominated by something
> >>>>>         inside needs to update dominance info.  */
> >>>>>      iterate_fix_dominators (CDI_DOMINATORS, doms, false);
> >>>>> -  doms.release ();
> >>>>>      /* Update the SSA web.  */
> >>>>>      update_ssa (TODO_update_ssa);
> >>>>>
> >>>>> @@ -7567,7 +7564,7 @@ basic_block
> >>>>>    move_sese_region_to_fn (struct function *dest_cfun, basic_block entry_bb,
> >>>>>                           basic_block exit_bb, tree orig_block)
> >>>>>    {
> >>>>> -  vec<basic_block> bbs, dom_bbs;
> >>>>> +  vec<basic_block> bbs;
> >>>>>      basic_block dom_entry = get_immediate_dominator (CDI_DOMINATORS, entry_bb);
> >>>>>      basic_block after, bb, *entry_pred, *exit_succ, abb;
> >>>>>      struct function *saved_cfun = cfun;
> >>>>> @@ -7599,9 +7596,9 @@ move_sese_region_to_fn (struct function *dest_cfun, basic_block entry_bb,
> >>>>>
> >>>>>      /* The blocks that used to be dominated by something in BBS will now be
> >>>>>         dominated by the new block.  */
> >>>>> -  dom_bbs = get_dominated_by_region (CDI_DOMINATORS,
> >>>>> -                                    bbs.address (),
> >>>>> -                                    bbs.length ());
> >>>>> +  auto_vec<basic_block> dom_bbs = get_dominated_by_region (CDI_DOMINATORS,
> >>>>> +                                                          bbs.address (),
> >>>>> +                                                          bbs.length ());
> >>>>>
> >>>>>      /* Detach ENTRY_BB and EXIT_BB from CFUN->CFG.  We need to remember
> >>>>>         the predecessor edges to ENTRY_BB and the successor edges to
> >>>>> @@ -7937,7 +7934,6 @@ move_sese_region_to_fn (struct function *dest_cfun, basic_block entry_bb,
> >>>>>      set_immediate_dominator (CDI_DOMINATORS, bb, dom_entry);
> >>>>>      FOR_EACH_VEC_ELT (dom_bbs, i, abb)
> >>>>>        set_immediate_dominator (CDI_DOMINATORS, abb, bb);
> >>>>> -  dom_bbs.release ();
> >>>>>
> >>>>>      if (exit_bb)
> >>>>>        {
> >>>>> --
> >>>>> 2.20.1
> >>>>>
> >>
>