[PATCH] bb-reorder: Improve compgotos pass (PR71785)

[Richard Biener]

On Mon, Oct 31, 2016 at 4:35 PM, Segher Boessenkool
<segher@kernel.crashing.org> wrote:
> On Mon, Oct 31, 2016 at 04:09:48PM +0100, Steven Bosscher wrote:
>> On Sun, Oct 30, 2016 at 8:10 PM, Segher Boessenkool wrote:
>> > This patch solves this problem by simply running the duplicate_computed_gotos
>> > pass again, as long as it does any work.  The patch looks much bigger than
>> > it is, because I factored out two routines to simplify the control flow.
>>
>> It's made the patch a bit difficult to read. Condensing it a bit...
>
> Well, it would have a goto crossing a loop, or two gotos crossing each
> other, otherwise.  I should have done it as two patches I guess (first
> factor, then change).
>
>> > +  for (;;)
>> >      {
>> > +      if (n_basic_blocks_for_fn (fun) <= NUM_FIXED_BLOCKS + 1)
>> > +       return 0;
>>
>> This test should not be needed in the loop. This pass can never
>> collapse the function to a single basic block.
>
> Yeah maybe, but that relies on quite a few assumptions.  This is strictly
> an optimisation anyway, will move it outside the loop.
>
>> > +      basic_block bb;
>> > +      FOR_EACH_BB_FN (bb, fun)
>> > +       {
>> > +         /* Build the reorder chain for the original order of blocks.  */
>> > +         if (bb->next_bb != EXIT_BLOCK_PTR_FOR_FN (fun))
>> > +           bb->aux = bb->next_bb;
>> > +       }
>> >
>> > +      duplicate_computed_gotos_find_candidates (fun, candidates, max_size);
>> >
>> > +      bool changed = false;
>> > +      if (!bitmap_empty_p (candidates))
>> > +       changed = duplicate_computed_gotos_do_duplicate (fun, candidates);
>> >
>> > +      if (changed)
>> > +       fixup_partitions ();
>> > +
>> > +      cfg_layout_finalize ();
>>
>> I don't think you have to go into/out-of cfglayout mode for each iteration.
>
> Yeah probably.  I was too lazy :-)  It needs the cleanup_cfg every
> iteration though, I was afraid that interacts.

Ick.  Why would it need a cfg-cleanup every iteration?  I fear this is quadratic
complexity in the number of edges to the compgoto block (and the size of the
function).  Can't the unfactoring perform the "cleanup" we rely on here?

>> >        /* Merge the duplicated blocks into predecessors, when possible.  */
>> > +      if (changed)
>> > +       cleanup_cfg (0);
>> > +      else
>> > +       break;
>> >      }
>>
>> Maybe a gcc_assert that the loop doesn't iterate more often than num_edges?
>
> Good plan (num blocks even).
>
> Thanks,
>
>
> Segher