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

Segher Boessenkool segher@kernel.crashing.org
Mon Oct 31 15:35:00 GMT 2016


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.

> >        /* 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



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