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

[law at redhat dot com]

In message <20031201200143.GC20851@kam.mff.cuni.cz>, Jan Hubicka writes:
 >> > 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 l
 >et'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)
Reusing the SSA version #s is the least interesting part of SSA_NAME
management and there are certainly other ways to implement it.  The
only reason it was included at all was because it was trivial to
implement within the manager.

jeff