Re: [PATCH v2] Make loops_list support an optional loop_p root

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

On Fri, Jul 30, 2021 at 7:20 AM Kewen.Lin <linkw@linux.ibm.com> wrote:
>
> on 2021/7/29 下午4:01, Richard Biener wrote:
> > On Fri, Jul 23, 2021 at 10:41 AM Kewen.Lin <linkw@linux.ibm.com> wrote:
> >>
> >> on 2021/7/22 下午8:56, Richard Biener wrote:
> >>> On Tue, Jul 20, 2021 at 4:37
> >>> PM Kewen.Lin <linkw@linux.ibm.com> wrote:
> >>>>
> >>>> Hi,
> >>>>
> >>>> This v2 has addressed some review comments/suggestions:
> >>>>
> >>>>   - Use "!=" instead of "<" in function operator!= (const Iter &rhs)
> >>>>   - Add new CTOR loops_list (struct loops *loops, unsigned flags)
> >>>>     to support loop hierarchy tree rather than just a function,
> >>>>     and adjust to use loops* accordingly.
> >>>
> >>> I actually meant struct loop *, not struct loops * ;)  At the point
> >>> we pondered to make loop invariant motion work on single
> >>> loop nests we gave up not only but also because it iterates
> >>> over the loop nest but all the iterators only ever can process
> >>> all loops, not say, all loops inside a specific 'loop' (and
> >>> including that 'loop' if LI_INCLUDE_ROOT).  So the
> >>> CTOR would take the 'root' of the loop tree as argument.
> >>>
> >>> I see that doesn't trivially fit how loops_list works, at least
> >>> not for LI_ONLY_INNERMOST.  But I guess FROM_INNERMOST
> >>> could be adjusted to do ONLY_INNERMOST as well?
> >>>
> >>
> >>
> >> Thanks for the clarification!  I just realized that the previous
> >> version with struct loops* is problematic, all traversal is
> >> still bounded with outer_loop == NULL.  I think what you expect
> >> is to respect the given loop_p root boundary.  Since we just
> >> record the loops' nums, I think we still need the function* fn?
> >
> > Would it simplify things if we recorded the actual loop *?
> >
>
> I'm afraid it's unsafe to record the loop*.  I had the same
> question why the loop iterator uses index rather than loop* when
> I read this at the first time.  I guess the design of processing
> loops allows its user to update or even delete the folllowing
> loops to be visited.  For example, when the user does some tricks
> on one loop, then it duplicates the loop and its children to
> somewhere and then removes the loop and its children, when
> iterating onto its children later, the "index" way will check its
> validity by get_loop at that point, but the "loop *" way will
> have some recorded pointers to become dangling, can't do the
> validity check on itself, seems to need a side linear search to
> ensure the validity.
>
> > There's still the to_visit reserve which needs a bound on
> > the number of loops for efficiency reasons.
> >
>
> Yes, I still keep the fn in the updated version.
>
> >> So I add one optional argument loop_p root and update the
> >> visiting codes accordingly.  Before this change, the previous
> >> visiting uses the outer_loop == NULL as the termination condition,
> >> it perfectly includes the root itself, but with this given root,
> >> we have to use it as the termination condition to avoid to iterate
> >> onto its possible existing next.
> >>
> >> For LI_ONLY_INNERMOST, I was thinking whether we can use the
> >> code like:
> >>
> >>     struct loops *fn_loops = loops_for_fn (fn)->larray;
> >>     for (i = 0; vec_safe_iterate (fn_loops, i, &aloop); i++)
> >>         if (aloop != NULL
> >>             && aloop->inner == NULL
> >>             && flow_loop_nested_p (tree_root, aloop))
> >>              this->to_visit.quick_push (aloop->num);
> >>
> >> it has the stable bound, but if the given root only has several
> >> child loops, it can be much worse if there are many loops in fn.
> >> It seems impossible to predict the given root loop hierarchy size,
> >> maybe we can still use the original linear searching for the case
> >> loops_for_fn (fn) == root?  But since this visiting seems not so
> >> performance critical, I chose to share the code originally used
> >> for FROM_INNERMOST, hope it can have better readability and
> >> maintainability.
> >
> > I was indeed looking for something that has execution/storage
> > bound on the subtree we're interested in.  If we pull the CTOR
> > out-of-line we can probably keep the linear search for
> > LI_ONLY_INNERMOST when looking at the whole loop tree.
> >
>
> OK, I've moved the suggested single loop tree walker out-of-line
> to cfgloop.c, and brought the linear search back for
> LI_ONLY_INNERMOST when looking at the whole loop tree.
>
> > It just seemed to me that we can eventually re-use a
> > single loop tree walker for all orders, just adjusting the
> > places we push.
> >
>
> Wow, good point!  Indeed, I have further unified all orders
> handlings into a single function walk_loop_tree.
>
> >>
> >> Bootstrapped and regtested on powerpc64le-linux-gnu P9,
> >> x86_64-redhat-linux and aarch64-linux-gnu, also
> >> bootstrapped on ppc64le P9 with bootstrap-O3 config.
> >>
> >> Does the attached patch meet what you expect?
> >
> > So yeah, it's probably close to what is sensible.  Not sure
> > whether optimizing the loops for the !only_push_innermost_p
> > case is important - if we manage to produce a single
> > walker with conditionals based on 'flags' then IPA-CP should
> > produce optimal clones as well I guess.
> >
>
> Thanks for the comments, the updated v2 is attached.
> Comparing with v1, it does:
>
>   - Unify one single loop tree walker for all orders.
>   - Move walk_loop_tree out-of-line to cfgloop.c.
>   - Keep the linear search for LI_ONLY_INNERMOST with
>     tree_root of fn loops.
>   - Use class loop * instead of loop_p.
>
> Bootstrapped & regtested on powerpc64le-linux-gnu Power9
> (with/without the hunk for LI_ONLY_INNERMOST linear search,
> it can have the coverage to exercise LI_ONLY_INNERMOST
> in walk_loop_tree when "without").
>
> Is it ok for trunk?

Looks good to me.  I think that the 'mn' was an optimization
for the linear walk and it's cheaper to pointer test against
the actual 'root' loop (no need to dereference).  Thus

+  if (flags & LI_ONLY_INNERMOST && tree_root == loops->tree_root)
     {
-      for (i = 0; vec_safe_iterate (loops_for_fn (fn)->larray, i, &aloop); i++)
+      class loop *aloop;
+      unsigned int i;
+      for (i = 0; vec_safe_iterate (loops->larray, i, &aloop); i++)
        if (aloop != NULL
            && aloop->inner == NULL
-           && aloop->num >= mn)
+           && aloop->num != mn)
          this->to_visit.quick_push (aloop->num);

could elide the aloop->num != mn check and start iterating from 1,
since loops->tree_root->num == 0

and the walk_loop_tree could simply do

  class loop *exclude = flags & LI_INCLUDE_ROOT ? NULL : root;

and pointer test aloop against exclude.  That avoids the idea that
'mn' is a vehicle to exclude one random loop from the iteration.

Richard.

> BR,
> Kewen
> -----
> gcc/ChangeLog:
>
>         * cfgloop.h (loops_list::loops_list): Add one optional argument root
>         and adjust accordingly, update loop tree walking and factor out
>         to ...
>         * cfgloop.c (loops_list::walk_loop_tree): ...this.  New function.