Re: [Qemu-devel] [PATCH 0/1] Change default CFLAGS to -O3

[Peter Maydell]

On 9 January 2017 at 10:01, Jin Guojie <address@hidden> wrote:
> * in configure, the default optimization level is changed to O3.
> * Tested successfully on following machines:
>
>   | Host machine | Host OS  | qemu-system | Debian ISO  |
>   |-----------------------------------------------------|
>   |   x86 64     | Fedora25 |    i386     |    i386     |
>   | mips 64 le   | Fedora25 |    i386     |    i386     |
>
> Signed-off-by: Jin Guojie <address@hidden>
>
> Jin Guojie (1):
>   Change default CFLAGS to -O3
>
>  configure | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)

...so what are the measured performance improvements from
this change? As I understand it -O3 is where the "trade off
code size for speed" optimisations kick in, so we need
to benchmark to be sure that the increased icache usage
doesn't outweigh any speed gains. -O3 is not guaranteed
to be strictly better than -O2 (which is why for instance
Debian and Gentoo recommend -O2 as their default).

thanks
-- PMM