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Re: [PATCH] Sanitize block partitioning under -freorder-blocks-and-partition


> Great! Is this the LTO merging you were talking about in an earlier
> message, or the gcov runtime fix (that would presumably not be
> lto-specific)?

It is the LTO path - we need to merge profiles there anyway for his code unification
work.

> > I have patch to track this.  Moreover vforks seems to produce repeated
> > merging of results.
> 
> Aha, does this explain the gimp issue as well?

Not really - we still need to debug why we hit cold section so many times with
partitioning.  I sitll think easier approach will be to lock the cold section and
then start probably with testsuite (i.e. write script to compile the small testcases
with FDO + partitioning and see what crash by hitting cold section).

> >
> > Is it really necessary to run this from internal loop of the cfgcleanup?
> 
> The reason I added it here is that just below there is a call to
> verify_flow_info, and that will fail with the new verification.

Hmm, OK, I suppose we run the cleanup after partitioning just once or twice, right?
We can track this incrementally - I am not sure if we put it from the internal iteration
loop we would get anything substantial either.
Removing unreachable blocks twice is however ugly.

> +/* Ensure that all hot bbs are included in a hot path through the
> +   procedure. This is done by calling this function twice, once
> +   with WALK_UP true (to look for paths from the entry to hot bbs) and
> +   once with WALK_UP false (to look for paths from hot bbs to the exit).
> +   Returns the updated value of COLD_BB_COUNT and adds newly-hot bbs
> +   to BBS_IN_HOT_PARTITION.  */
> +
> +static unsigned int
> +sanitize_hot_paths (bool walk_up, unsigned int cold_bb_count,
> +                    vec<basic_block> *bbs_in_hot_partition)
> +{
> +  /* Callers check this.  */
> +  gcc_checking_assert (cold_bb_count);
> +
> +  /* Keep examining hot bbs while we still have some left to check
> +     and there are remaining cold bbs.  */
> +  vec<basic_block> hot_bbs_to_check = bbs_in_hot_partition->copy ();
> +  while (! hot_bbs_to_check.is_empty ()
> +         && cold_bb_count)
> +    {
> +      basic_block bb = hot_bbs_to_check.pop ();
> +      vec<edge, va_gc> *edges = walk_up ? bb->preds : bb->succs;
> +      edge e;
> +      edge_iterator ei;
> +      int highest_probability = 0;
> +      int highest_freq = 0;
> +      gcov_type highest_count = 0;
> +      bool found = false;
> +
> +      /* Walk the preds/succs and check if there is at least one already
> +         marked hot. Keep track of the most frequent pred/succ so that we
> +         can mark it hot if we don't find one.  */
> +      FOR_EACH_EDGE (e, ei, edges)
> +        {
> +          basic_block reach_bb = walk_up ? e->src : e->dest;
> +
> +          if (e->flags & EDGE_DFS_BACK)
> +            continue;
> +
> +          if (BB_PARTITION (reach_bb) != BB_COLD_PARTITION)
> +          {
> +            found = true;
> +            break;
> +          }
> +          /* The following loop will look for the hottest edge via
> +             the edge count, if it is non-zero, then fallback to the edge
> +             frequency and finally the edge probability.  */
> +          if (e->count > highest_count)
> +            highest_count = e->count;
> +          int edge_freq = EDGE_FREQUENCY (e);
> +          if (edge_freq > highest_freq)
> +            highest_freq = edge_freq;
> +          if (e->probability > highest_probability)
> +            highest_probability = e->probability;
> +        }
> +
> +      /* If bb is reached by (or reaches, in the case of !WALK_UP) another hot
> +         block (or unpartitioned, e.g. the entry block) then it is ok. If not,
> +         then the most frequent pred (or succ) needs to be adjusted.  In the
> +         case where multiple preds/succs have the same frequency (e.g. a
> +         50-50 branch), then both will be adjusted.  */
> +      if (found)
> +        continue;
> +
> +      FOR_EACH_EDGE (e, ei, edges)
> +        {
> +          if (e->flags & EDGE_DFS_BACK)
> +            continue;
> +          /* Select the hottest edge using the edge count, if it is non-zero,
> +             then fallback to the edge frequency and finally the edge
> +             probability.  */
> +          if (highest_count)
> +            {
> +              if (e->count < highest_count)
> +                continue;
> +            }
> +          else if (highest_freq)

The frequency condition needs to be done only when you walk predecestors - when
you walk down the edge probabilities are just fine.

The patch seems OK to me now.  I will make our FDO tester to use partitioning so we get
this benchmarked a bit.

Honza


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