Re: [tree-ssa] Avoid nondeterminism in tree-ssanames

[Jan Hubicka <jh at suse dot cz>]

> > From: law@redhat.com
> > Date: Mon, 01 Dec 2003 10:28:13 -0700
> 
> > 1. Re-using SSA_NAMES across different functions is a bad idea.  Please let's
> >    not do that.  The whole point of the SSA_NAME cache is to avoid silly 
> >    GC allocations within the SSA path for a single function.
> 
> So, maybe the best thing to do is to reintroduce a freelist, but do it
> right?
> 
> A 'deletable' freelist is supposed to look like:
> 
> if (freelist_head)
> {
>   <type> tmp;
>   tmp = freelist_head;
>   freelist_head = tmp->next;
>   memset (tmp, 0, sizeof (*tmp));
>   return tmp;
> }
> else
>   return ggc_alloc_cleared (sizeof (<type>));
> 
> That is, the new object should be treated as uninitialised data.

I did that, but it did increase memory usages.
The idea of re-using freed SSA_NAMES seems to be important factor.
So I think that even with ggc_free, we will want some scheme to re-use
SSA_NAMEs in this particular case.  (or compact them in between passes
that would however make dumps dificult to read)

Honza