On Fri, Mar 11, 2016 at 4:01 AM, Jeff Law <l...@redhat.com> wrote:
>
> As discussed in the BZ, we have multiple problems with how we sort the
> coalesce list during out-of-ssa coalescing.
>
> First, the sort is not stable. If the cost of two coalesce pairs is the
> same, we break the tie by looking at the underlying SSA_NAME_VERSION of the
> first, then the second elements in the coalesce pairs.
>
> As a result, changes in SSA_NAME_VERSIONs in the IL can result in different
> coalescing during out-of-ssa. That in turn can cause changes in what
> objects are coalesced, which in turn causes random performance changes.
>
> This patch addresses that problem by recording an index for each coalescing
> pair discovered and using that index as the final tiebreaker rather than
> looking at SSA_NAME_VERSIONs. That brings stability to the coalescing
> process and avoids a lot of unnecessary differences in the code we generate
> when SSA_NAME_VERSIONs change.
>
> The second problem is our costing heuristic only looks at edge frequencies &
> flags. It's actually a pretty good heuristic and captures the main goal of
> coalescing -- reducing the most commonly executed copies. However, in the
> case where the edge frequencies/flags result in the same cost we can do
> better.
>
> When we coalesce two SSA_NAMEs, we have to build the union of the conflicts
> of each of the SSA_NAMEs -- which means the resulting union object is less
> likely to be able to participate in further coalescing.
>
> So given two coalescing pairs with the same primary cost, preferring the
> coalescing pair with the smaller resulting conflict set gives us a better
> chance that the resulting object will be able to participate in further
> coalescing.
>
> That heuristic broadly mirrors one aspect of how iterated conservative
> coalescing works. The other interesting heuristic (that I did not
> implement) was to favor coalescing of the pair which had a higher degree of
> common conflicts between the two nodes -- which broadly falls into the same
> category as what we're doing with this patch. The key being that the
> conflict sets are an important thing to consider when coalescing.
>
> Using the conflict sizes as a tie-breaker eliminates the regression in 64058
> and AFAICT also eliminates the regression in 68654 (the latter doesn't
> include a testcase or as in-depth analysis as 64058, but my testing
> indicates this patch should generate the desired code for both cases).
>
> The patch has (of course) bootstrapped and regression tested on
> x86_64-linux-gnu.
>
> I'd be curious for thoughts on how to build a testcase for this. I could
> emit the conflict sizes along with the coalescing cost in the dumps, but
> that won't positively verify that we've done the preferred set of
> coalescings.
>
> I might be able to look at the .expand dumps and perhaps look for copies on
> edges. However, unless the only copies are the ones that were causing the
> regression, I suspect such a test would end up being rather fragile.
>
> Other thoughts on how to get this under regression testing? And of course,
> thoughts on the patch itself?
Can you please split out the 'index' introduction as a separate patch
and apply that?
I think it is quite obviously a good idea and might make regression
hunting easier
later if needed.
For the other part I noticed a few things
1) having a bitmap_count_ior_bits () would be an improvement
2) you might end up with redundant work(?) as you are iterating
over SSA name coalesce candidates but look at partition conflicts
3) having this extra heuristic might be best guarded by
flag_expensive_optimizations
as it is a quite expensive "tie breaker" - maybe even improve things
by first sorting
after cost and then only doing the tie breaking when necessary, re-sorting the
sub-sequence with same original cost. It may also be enough to only perform
this for "important" candidates, say within the first 100 of the function or so
or with cost > X.
And finally - if we really think that looking at the conflict size
increase is the way to go
it would maybe be better to use a fibheap updating keys in attempt_coalesce
when we merge the conflicts. That would also mean to work on a list (fibheap)
of coalesces of partitions rather than SSA names.
I think the patch is reasonable enough for GCC 6 if we can bring compile-time
cost down a bit (it can be quadratic in the number of SSA names if we have
a lot of coalesce candidates and nearly full conflict bitmaps - of course that's
not a case we handle very well right now but still). I would have hoped the
index part of the patch fixed the regression (by luck)...
As far as a testcase goes we want to scan the dumps for the actual coalesces
being done. Might be a bit fragile though...
Thanks,
Richard.
> Thanks,
> Jeff
>
> diff --git a/gcc/ChangeLog b/gcc/ChangeLog
> index cc91e84..f28baa2 100644
> --- a/gcc/ChangeLog
> +++ b/gcc/ChangeLog
> @@ -1,3 +1,18 @@
> +2016-03-10 Jeff Law <l...@redhat.com>
> +
> + PR tree-optimization/64058
> + * tree-ssa-coalesce.c (struct coalesce_pair): Add new fields
> + CONFLICT_COUNT and INDEX.
> + (num_coalesce_pairs): Move up earlier in the file.
> + (find_coalesce_pair): Initialize the INDEX field for each pair
> + discovered.
> + (add_conflict_counts): New function to initialize the CONFLICT_COUNT
> + field for each conflict pair.
> + (coalesce_ssa_name): Call it.
> + (compare_pairs): No longer sort on the elements of each pair.
> + Instead break ties with the conflict size and finally the index
> + of the coalesce pair.
> +
> 2016-03-10 Ulrich Weigand <ulrich.weig...@de.ibm.com>
>
> PR target/70168
> diff --git a/gcc/tree-ssa-coalesce.c b/gcc/tree-ssa-coalesce.c
> index 6624e7e..b8a2e0d 100644
> --- a/gcc/tree-ssa-coalesce.c
> +++ b/gcc/tree-ssa-coalesce.c
> @@ -50,6 +50,19 @@ struct coalesce_pair
> int first_element;
> int second_element;
> int cost;
> +
> + /* A count of the number of unique partitions this pair would conflict
> + with if coalescing was successful. This is the secondary sort key,
> + given two pairs with equal costs, we will prefer the pair with a
> smaller
> + conflict set.
> +
> + Note this is not updated and propagated as pairs are coalesced. */
> + int conflict_count;
> +
> + /* The order in which coalescing pairs are discovered is recorded in this
> + field, which is used as the final tie breaker when sorting coalesce
> + pairs. */
> + int index;
> };
>
> /* Coalesce pair hashtable helpers. */
> @@ -254,6 +267,13 @@ delete_coalesce_list (coalesce_list *cl)
> free (cl);
> }
>
> +/* Return the number of unique coalesce pairs in CL. */
> +
> +static inline int
> +num_coalesce_pairs (coalesce_list *cl)
> +{
> + return cl->list->elements ();
> +}
>
> /* Find a matching coalesce pair object in CL for the pair P1 and P2. If
> one isn't found, return NULL if CREATE is false, otherwise create a new
> @@ -290,6 +310,7 @@ find_coalesce_pair (coalesce_list *cl, int p1, int p2,
> bool create)
> pair->first_element = p.first_element;
> pair->second_element = p.second_element;
> pair->cost = 0;
> + pair->index = num_coalesce_pairs (cl);
> *slot = pair;
> }
>
> @@ -343,29 +364,21 @@ compare_pairs (const void *p1, const void *p2)
> int result;
>
> result = (* pp1)->cost - (* pp2)->cost;
> - /* Since qsort does not guarantee stability we use the elements
> - as a secondary key. This provides us with independence from
> - the host's implementation of the sorting algorithm. */
> + /* We use the size of the resulting conflict set as the secondary sort
> key.
> + Given two equal costing coalesce pairs, we want to prefer the pair
> that
> + has the smaller conflict set. */
> if (result == 0)
> {
> - result = (* pp2)->first_element - (* pp1)->first_element;
> + result = (*pp2)->conflict_count - (*pp1)->conflict_count;
> + /* And if everything else is equal, then sort based on which
> + coalesce pair was found first. */
> if (result == 0)
> - result = (* pp2)->second_element - (* pp1)->second_element;
> + result = (*pp2)->index - (*pp1)->index;
> }
>
> return result;
> }
>
> -
> -/* Return the number of unique coalesce pairs in CL. */
> -
> -static inline int
> -num_coalesce_pairs (coalesce_list *cl)
> -{
> - return cl->list->elements ();
> -}
> -
> -
> /* Iterate over CL using ITER, returning values in PAIR. */
>
> #define FOR_EACH_PARTITION_PAIR(PAIR, ITER, CL) \
> @@ -578,6 +591,42 @@ ssa_conflicts_merge (ssa_conflicts *ptr, unsigned x,
> unsigned y)
> }
> }
>
> +/* Compute and record how many unique conflicts would exist for the
> + representative partition for each coalesce pair in CL.
> +
> + CONFLICTS is the conflict graph and MAP is the current partition view.
> */
> +
> +static void
> +add_conflict_counts (ssa_conflicts *conflicts, var_map map, coalesce_list
> *cl)
> +{
> + coalesce_pair *p;
> + coalesce_iterator_type ppi;
> + bitmap tmp = BITMAP_ALLOC (NULL);
> +
> + FOR_EACH_PARTITION_PAIR (p, ppi, cl)
> + {
> + int p1 = var_to_partition (map, ssa_name (p->first_element));
> + int p2 = var_to_partition (map, ssa_name (p->second_element));
> +
> + /* 4 cases. If both P1 and P2 have conflicts, then build their
> + union and count the members. Else handle the degenerate cases
> + in the obvious ways. */
> + if (conflicts->conflicts[p1] && conflicts->conflicts[p2])
> + {
> + bitmap_ior (tmp, conflicts->conflicts[p1],
> conflicts->conflicts[p2]);
> + p->conflict_count = bitmap_count_bits (tmp);
> + bitmap_clear (tmp);
> + }
> + else if (conflicts->conflicts[p1])
> + p->conflict_count = bitmap_count_bits (conflicts->conflicts[p1]);
> + else if (conflicts->conflicts[p2])
> + p->conflict_count = bitmap_count_bits (conflicts->conflicts[p2]);
> + else
> + p->conflict_count = 0;
> + }
> +
> + BITMAP_FREE (tmp);
> +}
>
> /* Dump a conflicts graph. */
>
> @@ -1802,6 +1851,7 @@ coalesce_ssa_name (void)
> if (dump_file && (dump_flags & TDF_DETAILS))
> ssa_conflicts_dump (dump_file, graph);
>
> + add_conflict_counts (graph, map, cl);
> sort_coalesce_list (cl);
>
> if (dump_file && (dump_flags & TDF_DETAILS))
>
