Reducing compilation memory usage

[Alejandro Pulver <alepulver at FreeBSD dot org>]

Hello.

There is a Core War simulator (an interpreter for a minimalistic
machine, called MARS, with a language named redcode) called fmars
(http://www.v-lo.krakow.pl/~michal/fmars.html), which uses computed
gotos to achieve very good performance.

The original version only uses a subset of the available redcode
instructions (~250), instead of the full set (~8000). The author told
me GCC dumped core if trying to use all of them (I don't remember if he
was referring to 2.95 or 3.3).

I decided to try with GCC 4 (4.2.3 20071219 prerelease and 4.3.0
20080111 experimental), and it builds (both) without optimizations
(-O0), but runs twice as slower than the reduced fmars built with -O2.

Version 3.4.6 (20060305) runs out of memory (with and without
optimizations), as 4.x does even with -O1. I have 2GB of RAM (and
allowed processes to use it).

The strange thing is that when looking at options enabled at different
-O levels (trying to remove the ones that were using more memory),
ended up with a GCC 4.2.3 command-line that specified exactly all the
options enabled by -O2 (as listed in the manpage), and built. But ran
at the same speed as without them.

The source file is a function with the ~250/~8000 cases like this one
(they are generated by an intermediate program):

dat_f_immediate_direct:
      {if (!--w->nprocs)
      {
      *death_tab++ = w->id;
      if (--alive_cnt < 2)goto out; w->prev->next = w->next;
      w->next->prev = w->prev;
      cycles -= cycles / alive_cnt;
      }w = w->next;
      ip = *w->head;if (++w->head == queue_end)
            w->head = queue_start;if (!--cycles)
          goto out;
      goto *ip->i;}

So my question is, what optimizations options can I use in this file
without using too much memory (I've read the docs and tried with
various ones, but was annoyed about the fact that using equivalent -f*
options worked without effect but -O/-O2 didn't)?

If more information is needed, please ask. Also please CC me as I'm not
subscribed. I'm using FreeBSD 6.2 (with mentioned GCC 3.4.6 as base
compiler).

Thanks and Best Regards,
Ale

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