[Bug c/35608] imit-structnest.c fails due to O(n^2) memory usage in record_component_alias

["rguenther at suse dot de" <gcc-bugzilla at gcc dot gnu dot org>]

------- Comment #9 from rguenther at suse dot de  2009-11-03 12:21 -------
Subject: Re:  imit-structnest.c fails due to O(n^2) memory usage
 in record_component_alias

On Tue, 3 Nov 2009, bonzini at gnu dot org wrote:

> ------- Comment #8 from bonzini at gnu dot org  2009-11-03 12:18 -------
> Memory allocation is O(n^2) in the nesting of the structures: when changing
> LIM4 from 6 to 7 invocations of the lower-level, memory goes from 583976 KB to
> 798116 KB (which is 1.366 times more, almost exactly a factor of (7/6)^2).
> 
> The culprit is record_component_aliases which creates a subset relationship
> from each structure to each containing structure.  Maybe we could reuse the
> et-forest here since these subset relationships form a forest (do all of them
> have this property? would we pessimize a lot by enforcing that even when it is
> conservative?)

Yes, they all form a forest.  It shouldn't pessimize anything to enforce
this.

> I suppose the bug has always been there since alias.c was introduced, it's just
> that previously an unused struct did not call record_component_aliases.

Right.

Richard.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=35608