Re: [PATCH] shrink-wrap: New spread_components

[Kyrill Tkachov <kyrylo.tkachov@foss.arm.com>]

On 09/11/16 21:46, Segher Boessenkool wrote:
> This patch changes spread_components to use a simpler algorithm that
> puts prologue components as early as possible, and epilogue components
> as late as possible.  This allows better scheduling, and also saves a
> bit of code size.  The blocks that run with some specific component
> enabled after this patch is a strict superset of those that had it
> before the patch.
>
> It does this by finding for every component the basic blocks where that
> component is not needed on some path from the entry block (it reuses
> head_components to store this), and similarly the blocks where the
> component is not needed on some path to the exit block (or the exit can
> not be reached from that block) (stored in tail_components).  Blocks
> that then are in neither of those two sets get the component active.
>
> Tested on powerpc64-linux {-m32,-m64}.  Is this okay for trunk?

This also passess bootstrap and regtest on aarch64-none-linux-gnu with
the hooks implemented [1]. It doesn't fix the gobmk performance regression that
I reported in that patch, but I'm working on improving that patch in other
ways so there's still benchmarking and analysis to do.

Thanks,
Kyrill

[1] https://gcc.gnu.org/ml/gcc-patches/2016-11/msg00945.html


>
> Segher
>
>
> 2016-11-09  Segher Boessenkool  <segher@kernel.crashing.org>
>
> 	* shrink-wrap.c (init_separate_shrink_wrap): Do not clear
> 	head_components and tail_components.
> 	(spread_components): New algorithm.
> 	(emit_common_tails_for_components): Clear head_components and
> 	tail_components.
> 	(insert_prologue_epilogue_for_components): Write extra output to the
> 	dump file for sibcalls and abnormal exits.
>
> ---
>   gcc/shrink-wrap.c | 181 +++++++++++++++++++++++++++++++++++++++++++-----------
>   1 file changed, 146 insertions(+), 35 deletions(-)
>
> diff --git a/gcc/shrink-wrap.c b/gcc/shrink-wrap.c
> index 4395d8a..e480d4d 100644
> --- a/gcc/shrink-wrap.c
> +++ b/gcc/shrink-wrap.c
> @@ -1131,8 +1131,6 @@ init_separate_shrink_wrap (sbitmap components)
>         SW (bb)->head_components = sbitmap_alloc (SBITMAP_SIZE (components));
>         SW (bb)->tail_components = sbitmap_alloc (SBITMAP_SIZE (components));
>         bitmap_clear (SW (bb)->has_components);
> -      bitmap_clear (SW (bb)->head_components);
> -      bitmap_clear (SW (bb)->tail_components);
>       }
>   }
>   
> @@ -1253,48 +1251,151 @@ place_prologue_for_one_component (unsigned int which, basic_block head)
>       }
>   }
>   
> -/* Mark HAS_COMPONENTS for every block dominated by at least one block with
> -   HAS_COMPONENTS set for the respective components, starting at HEAD.  */
> +/* Set HAS_COMPONENTS in every block to the maximum it can be set to without
> +   setting it on any path from entry to exit where it was not already set
> +   somewhere (or, for blocks that have no path to the exit, consider only
> +   paths from the entry to the block itself).  */
>   static void
> -spread_components (basic_block head)
> +spread_components (sbitmap components)
>   {
> -  basic_block bb = head;
> -  bool first_visit = true;
> -  /* This keeps a tally of all components active.  */
> -  sbitmap components = SW (head)->has_components;
> +  basic_block entry_block = ENTRY_BLOCK_PTR_FOR_FN (cfun);
> +  basic_block exit_block = EXIT_BLOCK_PTR_FOR_FN (cfun);
>   
> -  for (;;)
> +  /* A stack of all blocks left to consider, and a bitmap of all blocks
> +     on that stack.  */
> +  vec<basic_block> todo;
> +  todo.create (n_basic_blocks_for_fn (cfun));
> +  bitmap seen = BITMAP_ALLOC (NULL);
> +
> +  sbitmap old = sbitmap_alloc (SBITMAP_SIZE (components));
> +
> +  /* Find for every block the components that are *not* needed on some path
> +     from the entry to that block.  Do this with a flood fill from the entry
> +     block.  Every block can be visited at most as often as the number of
> +     components (plus one), and usually much less often.  */
> +
> +  if (dump_file)
> +    fprintf (dump_file, "Spreading down...\n");
> +
> +  basic_block bb;
> +  FOR_ALL_BB_FN (bb, cfun)
> +    bitmap_clear (SW (bb)->head_components);
> +
> +  bitmap_copy (SW (entry_block)->head_components, components);
> +
> +  edge e;
> +  edge_iterator ei;
> +
> +  todo.quick_push (single_succ (entry_block));
> +  bitmap_set_bit (seen, single_succ (entry_block)->index);
> +  while (!todo.is_empty ())
>       {
> -      if (first_visit)
> -	{
> -	  bitmap_ior (SW (bb)->has_components, SW (bb)->has_components,
> -		      components);
> +      bb = todo.pop ();
>   
> -	  if (first_dom_son (CDI_DOMINATORS, bb))
> -	    {
> -	      components = SW (bb)->has_components;
> -	      bb = first_dom_son (CDI_DOMINATORS, bb);
> -	      continue;
> -	    }
> -	}
> +      bitmap_copy (old, SW (bb)->head_components);
>   
> -      components = SW (bb)->has_components;
> +      FOR_EACH_EDGE (e, ei, bb->preds)
> +	bitmap_ior (SW (bb)->head_components, SW (bb)->head_components,
> +		    SW (e->src)->head_components);
>   
> -      if (next_dom_son (CDI_DOMINATORS, bb))
> +      bitmap_and_compl (SW (bb)->head_components, SW (bb)->head_components,
> +			SW (bb)->has_components);
> +
> +      if (!bitmap_equal_p (old, SW (bb)->head_components))
> +	FOR_EACH_EDGE (e, ei, bb->succs)
> +	  if (bitmap_set_bit (seen, e->dest->index))
> +	    todo.quick_push (e->dest);
> +
> +      bitmap_clear_bit (seen, bb->index);
> +    }
> +
> +  /* Find for every block the components that are *not* needed on some reverse
> +     path from the exit to that block.  */
> +
> +  if (dump_file)
> +    fprintf (dump_file, "Spreading up...\n");
> +
> +  /* First, mark all blocks not reachable from the exit block as not needing
> +     any component on any path to the exit.  Mark everything, and then clear
> +     again by a flood fill.  */
> +
> +  FOR_ALL_BB_FN (bb, cfun)
> +    bitmap_copy (SW (bb)->tail_components, components);
> +
> +  FOR_EACH_EDGE (e, ei, exit_block->preds)
> +    {
> +      todo.quick_push (e->src);
> +      bitmap_set_bit (seen, e->src->index);
> +    }
> +
> +  while (!todo.is_empty ())
> +    {
> +      bb = todo.pop ();
> +
> +      if (!bitmap_empty_p (SW (bb)->tail_components))
> +	FOR_EACH_EDGE (e, ei, bb->preds)
> +	  if (bitmap_set_bit (seen, e->src->index))
> +	    todo.quick_push (e->src);
> +
> +      bitmap_clear (SW (bb)->tail_components);
> +
> +      bitmap_clear_bit (seen, bb->index);
> +    }
> +
> +  /* And then, flood fill backwards to find for every block the components
> +     not needed on some path to the exit.  */
> +
> +  bitmap_copy (SW (exit_block)->tail_components, components);
> +
> +  FOR_EACH_EDGE (e, ei, exit_block->preds)
> +    {
> +      todo.quick_push (e->src);
> +      bitmap_set_bit (seen, e->src->index);
> +    }
> +
> +  while (!todo.is_empty ())
> +    {
> +      bb = todo.pop ();
> +
> +      bitmap_copy (old, SW (bb)->tail_components);
> +
> +      FOR_EACH_EDGE (e, ei, bb->succs)
> +	bitmap_ior (SW (bb)->tail_components, SW (bb)->tail_components,
> +		    SW (e->dest)->tail_components);
> +
> +      bitmap_and_compl (SW (bb)->tail_components, SW (bb)->tail_components,
> +			SW (bb)->has_components);
> +
> +      if (!bitmap_equal_p (old, SW (bb)->tail_components))
> +	FOR_EACH_EDGE (e, ei, bb->preds)
> +	  if (bitmap_set_bit (seen, e->src->index))
> +	    todo.quick_push (e->src);
> +
> +      bitmap_clear_bit (seen, bb->index);
> +    }
> +
> +  /* Finally, mark everything not not needed both forwards and backwards.  */
> +
> +  FOR_EACH_BB_FN (bb, cfun)
> +    {
> +      bitmap_and (SW (bb)->head_components, SW (bb)->head_components,
> +		  SW (bb)->tail_components);
> +      bitmap_and_compl (SW (bb)->has_components, components,
> +			SW (bb)->head_components);
> +    }
> +
> +  FOR_ALL_BB_FN (bb, cfun)
> +    {
> +      if (dump_file)
>   	{
> -	  bb = next_dom_son (CDI_DOMINATORS, bb);
> -	  basic_block parent = get_immediate_dominator (CDI_DOMINATORS, bb);
> -	  components = SW (parent)->has_components;
> -	  first_visit = true;
> -	}
> -      else
> -	{
> -	  if (bb == head)
> -	    return;
> -	  bb = get_immediate_dominator (CDI_DOMINATORS, bb);
> -	  first_visit = false;
> +	  fprintf (dump_file, "bb %d components:", bb->index);
> +	  dump_components ("has", SW (bb)->has_components);
> +	  fprintf (dump_file, "\n");
>   	}
>       }
> +
> +  sbitmap_free (old);
> +  BITMAP_FREE (seen);
>   }
>   
>   /* If we cannot handle placing some component's prologues or epilogues where
> @@ -1384,6 +1485,9 @@ emit_common_heads_for_components (sbitmap components)
>     sbitmap tmp = sbitmap_alloc (SBITMAP_SIZE (components));
>   
>     basic_block bb;
> +  FOR_ALL_BB_FN (bb, cfun)
> +    bitmap_clear (SW (bb)->head_components);
> +
>     FOR_EACH_BB_FN (bb, cfun)
>       {
>         /* Find which prologue resp. epilogue components are needed for all
> @@ -1470,6 +1574,9 @@ emit_common_tails_for_components (sbitmap components)
>     sbitmap tmp = sbitmap_alloc (SBITMAP_SIZE (components));
>   
>     basic_block bb;
> +  FOR_ALL_BB_FN (bb, cfun)
> +    bitmap_clear (SW (bb)->tail_components);
> +
>     FOR_EACH_BB_FN (bb, cfun)
>       {
>         /* Find which prologue resp. epilogue components are needed for all
> @@ -1608,6 +1715,10 @@ insert_prologue_epilogue_for_components (sbitmap components)
>   			   e->dest->index);
>   		  dump_components ("epi", epi);
>   		  dump_components ("pro", pro);
> +		  if (e->flags & EDGE_SIBCALL)
> +		    fprintf (dump_file, "  (SIBCALL)");
> +		  else if (e->flags & EDGE_ABNORMAL)
> +		    fprintf (dump_file, "  (ABNORMAL)");
>   		  fprintf (dump_file, "\n");
>   		}
>   
> @@ -1697,7 +1808,7 @@ try_shrink_wrapping_separate (basic_block first_bb)
>     EXECUTE_IF_SET_IN_BITMAP (components, 0, j, sbi)
>       place_prologue_for_one_component (j, first_bb);
>   
> -  spread_components (first_bb);
> +  spread_components (components);
>   
>     disqualify_problematic_components (components);
>