Re: -Os versus -O2

[Willy Tarreau <w@1wt.eu>]

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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