Re: -Os versus -O2

On Mon, Jun 25, 2007 at 09:08:23AM +0200, Segher Boessenkool wrote:
In my experience, -Os produced faster code on gcc-2.95 than -O2 or -O3.

On what CPU? The effect of different optimisations varies
hugely between different CPUs (and architectures).

x86

It was not only because of cache considerations, but because gcc used
different tricks to avoid poor optimizations, and at the end, the CPU
ended executing the alternative code faster.

-Os is "as fast as you can without bloating the code size",
so that is the expected result for CPUs that don't need
special hand-holding around certain performance pitfalls.

With gcc-3.3, -Os show roughly the same performance as -O2 for me on
various programs. However, with gcc-3.4, I noticed a slow down with
-Os. And with gcc-4, using -Os optimizes only for size, even if the
output code is slow as hell. I've had programs whose speed dropped
by 70% using -Os on gcc-4.

Well you better report those! <http://gcc.gnu.org/bugzilla>

No, -Os is for size only :

-Os Optimize for size. -Os enables all -O2 optimizations
that do not typically increase code size. It also
performs further optimizations designed to reduce code
size.

So it is expected that speed can be reduced using -Os. I won't report
a thing which is already documented !

But in some situtations, it's desirable to have the smallest possible
kernel whatever its performance. This goes for installation CDs for
instance.

There are much better ways to achieve that.

Optimizing is not a matter of choosing *one* way, but cumulating
everything you have. For instance, on a smart boot loader, I have
a kernel which is about 300 kB, or 700 kB with the initramfs. Among
the tricks I used :
- -Os
- -march=i386
- align everything to 0
- replace gzip with p7zip

Even if each of them reduces overall size by 5%, the net result is
0.95^4 = 0.81 = 19% gain, for the same set of features. This is
something to consider.

Regards,
Willy

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