On Mon, Mar 23, 2009 at 09:56:45PM +0000, Mark Alan wrote:

> mike wrote:
> >What does this do?
> >
> ># ensure that there is only one -O in CFLAGS and that it is -O2
>
> When we use --with-cc-opt= to specify any alternative -On C compiler
> optimization, there seems to be a bug (or is it a designed feature?) in
> nginx's ./configure that leads to the following line in the created
> objs/Makefile file:
>
> CFLAGS = -O -pipe -O -W -Wall -Wpointer-arith -Wno-unused-parameter
> -Wno-unused-function -Wunused-variable -Wunused-value -Werror -g -O2
>
>
> If I am not wrong, that line should only have one instance of -O:
>
> CFLAGS = -pipe -W -Wall -Wpointer-arith -Wno-unused-parameter
> -Wno-unused-function -Wunused-variable -Wunused-value -Werror -g -O2
>
>
> Hence the usage of the command sed -i '/CFLAGS/s/ \-O //g' objs/Makefile
> to clean the other instances of -O.
>
> From man cc:
> " -O2 Optimize even more.
> (...)
> As compared to -O, this option increases both compilation time and the
> performance of the generated code."

As I understand gcc uses the lastest -O option, therefore -O -O2 is the same
as -O2. Also you may disable optimizations using -O -O0. As to -O2 itself,
I believe you would not see any significant changes in nginx with -O2
if compared with just -O. However, -O0 vs -O change is dramatically.


--
Igor Sysoev
http://sysoev.ru/en/