[tree-ssa] Avoid nondeterminism in tree-ssanames

[Andrew MacLeod]

On Mon, 2003-12-01 at 16:10, Jan Hubicka wrote:
> > In message <20031201200143.GC20851@kam.mff.cuni.cz>, Jan Hubicka writes:
> >  >> > From: law@redhat.com

> >  >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.
> It makes 11% performance difference on Gerald's testcase on my setup,
> just FYI.
> 
Is it specific optimizations that are suspect to it? ie, can you tell
which optimizations are responsible for taking the performance hit when
we dont reuse SSA_NAMES?

Presuming that is isn't SSA->normal, which I would expect to be flat
with and without SSA_NAME resuse, the same routines could be used if its
one place.  SSA->normal compresses the SSA_NAME list into a partition
list which is dense, and then allocated memory and manipulates the dense
list.  

If there is a specific optimization that is impacted by sparser
SSA_NAMEs (ie, allocating large vectors over SSA_NAMES indexes), the
partitioning data structure could be used to compress it to just
elements that matter. If it makes sense. If the problem is spread across
a lot of the optimizations then something else might be better.

I dont particularly like re-using SSA_NAMEs versions. Expecially in
large programs Im debugging, I tend to grep for VAR_v in a.c.* listing
files to see where uses appear and dissappear. I guess its not a huge
thing to filter out, but I can see it being very confusing sometimes,
especially if the new use is at all similar to what the old use was.

RTL never recycles register numbers does it? I dont recall ever doing it
on any other compiler I used either. Thats not to say there isn't some
benefits from it :-)

Andrew