Re: [PATCH 4/6] Port ipa-cp to use cgraph_edge summary.

["Martin Liška" <mliska@suse.cz>]

On 07/10/2015 04:18 PM, Martin Jambor wrote:
> Hi,
> 
> I know the patch has been approved by Jeff, but please do not commit
> it before considering the following:
> 
> On Thu, Jul 09, 2015 at 11:13:53AM +0200, Martin Liska wrote:
>> gcc/ChangeLog:
>>
>> 2015-07-03  Martin Liska  <mliska@suse.cz>
>>
>> 	* ipa-cp.c (struct edge_clone_summary): New structure.
>> 	(class edge_clone_summary_t): Likewise.
>> 	(edge_clone_summary_t::initialize): New method.
>> 	(edge_clone_summary_t::duplicate): Likewise.
>> 	(get_next_cgraph_edge_clone): Remove.
>> 	(get_info_about_necessary_edges): Refactor using the new
>> 	data structure.
>> 	(gather_edges_for_value): Likewise.
>> 	(perhaps_add_new_callers): Likewise.
>> 	(ipcp_driver): Allocate and deallocate newly added
>> 	instance.
>> ---
>>  gcc/ipa-cp.c | 198 ++++++++++++++++++++++++++++++++++-------------------------
>>  1 file changed, 113 insertions(+), 85 deletions(-)
>>
>> diff --git a/gcc/ipa-cp.c b/gcc/ipa-cp.c
>> index 16b9cde..8a50b63 100644
>> --- a/gcc/ipa-cp.c
>> +++ b/gcc/ipa-cp.c
>> @@ -2888,54 +2888,79 @@ ipcp_discover_new_direct_edges (struct cgraph_node *node,
>>      inline_update_overall_summary (node);
>>  }
>>  
>> -/* Vector of pointers which for linked lists of clones of an original crgaph
>> -   edge. */
>> +/* Edge clone summary.  */
>>  
>> -static vec<cgraph_edge *> next_edge_clone;
>> -static vec<cgraph_edge *> prev_edge_clone;
>> -
>> -static inline void
>> -grow_edge_clone_vectors (void)
>> +struct edge_clone_summary
> 
> I's got constructors and destructors so it should be a class, reaally.
> 
>>  {
>> -  if (next_edge_clone.length ()
>> -      <=  (unsigned) symtab->edges_max_uid)
>> -    next_edge_clone.safe_grow_cleared (symtab->edges_max_uid + 1);
>> -  if (prev_edge_clone.length ()
>> -      <=  (unsigned) symtab->edges_max_uid)
>> -    prev_edge_clone.safe_grow_cleared (symtab->edges_max_uid + 1);
>> -}
>> +  /* Default constructor.  */
>> +  edge_clone_summary (): edge_set (NULL), edge (NULL) {}
>>  
>> -/* Edge duplication hook to grow the appropriate linked list in
>> -   next_edge_clone. */
>> +  /* Default destructor.  */
>> +  ~edge_clone_summary ()
>> +  {
>> +    gcc_assert (edge_set != NULL);
>>  
>> -static void
>> -ipcp_edge_duplication_hook (struct cgraph_edge *src, struct cgraph_edge *dst,
>> -			    void *)
>> +    if (edge != NULL)
>> +      {
>> +	gcc_checking_assert (edge_set->contains (edge));
>> +	edge_set->remove (edge);
>> +      }
>> +
>> +    /* Release memory for an empty set.  */
>> +    if (edge_set->elements () == 0)
>> +      delete edge_set;
>> +  }
>> +
>> +  hash_set <cgraph_edge *> *edge_set;
>> +  cgraph_edge *edge;
> 
> If the hash set is supposed to replace the linked list of edge clones,
> then a removal mechanism seems to be missing.  The whole point of
> prev_edge_clone vector was to allow removal of edges from the linked
> list, because as speculative edges are thrown away, clones can be too
> and then we must remove the pointer from the list, or hash set.
> 
> Have you tried -O3 LTOing Firefox with these changes?
> 
> But I must say that I'm not convinced that converting the linked list
> into a hash_set is a good idea at all.  Apart from the self-removal
> operation, the lists are always traversed linearly and in full, so
> except for using a C++-style iterator, I really do not see any point.
> 
> Moreover, you seem to create a hash table for each and every edge,
> even when it has no clones, just to be able to enter the edge itself
> into it, and so not skip it when you iterate over all clones.  That
> really seems like unjustifiable overhead.  And the deletion in
> duplication hook is also very unappealing.  So the bottom line is that
> while I like turning the two vectors into a summary, I do not like the
> hash set at all.  If absolutely think it is a good idea, please make
> that change in a separate patch so that we can better argue about its
> merits.
> 
> On the other hand, since the summaries are hash-based themselves, it
> would be great if they had a predicate to find out whether there is
> any summary for a given edge at all and have get_next_cgraph_edge_clone
> return false if there was none.  That would actually save memory.
> 
> Thanks,
> 
> Martin
> 

After a discussion with Martin, we made a decision to preserve current implementation
and not to port the IPA CP to he newly introduced summary.

Martin