Re: C++: EGCS perf. vs GCC

[Marc Lehmann <pcg at goof dot com>]

On Tue, Dec 08, 1998 at 10:35:16AM +0000, Alex Maranda wrote:
> -f<stuff> pack (you can see it when compiling with gcc -v -Q -O<optlevel>)
> and is my understanding that a greater <optlevel> is meant for speed. BTW,
> did you know there are undocumented levels -O4...-O8?

in PGCC, yes, in eGCS/GCC, no. BTW, -O3 already includes many optimizations
more than -O§ in EGCS.

On Tue, Dec 08, 1998 at 11:02:26AM +0000, Alex Maranda wrote:
> and =2. I will format the results in an intuitive way and post them
> tomorrow. It's enough to say now that pgcc went to warp by 3% vs. the best

If that are 3% in -O2 I think its quite respectable (if its with -O6 its
not). The only difefrences between egcs -O2 and pgcc -O2 are:

- no scheduling in pgcc
- a few small changes in the heuristics "to taste" in pgcc

It could be interesting to time these

-funroll-loops
-funroll-all-loops
-funroll-loops -fschedule-insns
-funroll-all-loops -fschedule-insns

with egcs, and, additionally with pgcc, "-O6 -frisc", "-O6 -fno-risc" (and
unrolling/no unrolling/scheduling)

You could also try -mk6 (or -mamdk6, dependent on version), I sometimes saw
interesting differences. Ah, and -mstack-align-double might change results
even more.

If you have success with any of these we have just another case where
optimizations slow down one program and speed up another. Its getting tricky.

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