[Bug tree-optimization/61515] [4.9/4.10 Regression] Extremely long compile time for generated code

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61515

--- Comment #13 from rguenther at suse dot de <rguenther at suse dot de> ---
On Tue, 17 Jun 2014, law at redhat dot com wrote:

> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61515
> 
> Jeffrey A. Law <law at redhat dot com> changed:
> 
>            What    |Removed                     |Added
> ----------------------------------------------------------------------------
>                  CC|                            |law at redhat dot com
>            Assignee|unassigned at gcc dot gnu.org      |law at redhat dot com
> 
> --- Comment #12 from Jeffrey A. Law <law at redhat dot com> ---
> Fundamentally we don't have a way to know what equivalences we need to
> invalidate.
> Invalidation is, umm, painful.  The question in my mind is why are we getting
> so many invalidations to start with.  That's the first thing to look at.

Well, it's easy to avoid the quadraticness - you can always create 
testcases that need a lot of invalidates.  But the current algorithm
really does not scale.

> Honestly though, I really wonder if handling backedges is worth the effort,
> even though it's important for one benchmark.

Not sure about that, but trivial improvements to the scalability are
possible here.  Walking all SSA names is O(number of stmts) - if you
do that O(number of stmts) time (as you do) that's clearly the bug.