On Wed, Feb 09, 2011 at 08:42:05AM +0000, Jonathan Wakely wrote:
> On 9 February 2011 08:34, Sebastian Pop wrote:
> >
> > For example x264 defines CFLAGS="-O4 -ffast-math $CFLAGS", and so
> > building this benchmark with CFLAGS="-O2" would have no effect.
>
> Why not?
>
> Ignoring the fact -O3 is the highest level for GCC, the manual says:
> "If you use multiple -O options, with or without level numbers, the
> last such option is the one that is effective."
> http://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html
>
> And CFLAGS="-fno-fast-math -O2" would cancel the effects of -ffast-math too.
Last time I've checked e.g. their byte-benchmark uses
OPTON = -O
...
$(CC) -o $(PROGDIR)/arithoh ${CFLAGS} ${OPTON} -Darithoh
$(SRCDIR)/arith.c
and similarly for most of the other files, so it really doesn't matter what -O
level you provide in CFLAGS there, they are just comparing -O performance.
Jakub
