[tree-ssa] Avoid nondeterminism in tree-ssanames

[Geoff Keating]

> 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.

> 2. Long term SSA_NAME management needs to go away.  
> 
>    2a. First we need to be able to GC between the SSA passes.
> 
>    2b. We need to be able to tell the GC system that a particular object
>        is no longer being used and that the GC system should re-use that
>        memory if it is convenient to do so.

A well-implemented freelist would help with both these goals.  It will
also be more efficient than a general-purpose allocator, and safer
than free() or equivalent.

-- 
- Geoffrey Keating <geoffk@geoffk.org>