Re: [Qemu-devel] [PATCH 1/3] log: improve performance of qemu_log and qemu_log_mask if disabled

[Denis V. Lunev]

On 10/15/2015 08:23 PM, Alex Bennée wrote:
> Denis V. Lunev <address@hidden> writes:
>
> > The patch is intended to avoid to perform any operation including
> > calculation of log function arguments when the log is not enabled due to
> > various reasons.
> >
> > Functions qemu_log and qemu_log_mask are replaced with variadic macros.
> > Unfortunately the code is not C99 compatible and we can not use
> > portable __VA_ARGS__ way. There are a lot of warnings in the other
> > places with --std=c99. Thus the only way to achive the result is to use
> > args.. GCC extension.
> >
> > Format checking performed by compiler will not suffer by this patch. It
> > will be done inside in fprintf arguments checking.
> >
> > Signed-off-by: Denis V. Lunev <address@hidden>
> > Signed-off-by: Pavel Butsykin <address@hidden>
> > CC: Markus Armbruster <address@hidden>
> > CC: Luiz Capitulino <address@hidden>
> > CC: Eric Blake <address@hidden>
> > CC: Peter Maydell <address@hidden>
> > ---
> >   include/qemu/log.h | 17 ++++++++++++++---
> >   qemu-log.c         | 21 ---------------------
> >   2 files changed, 14 insertions(+), 24 deletions(-)
> >
> > diff --git a/include/qemu/log.h b/include/qemu/log.h
> > index f880e66..57b8c96 100644
> > --- a/include/qemu/log.h
> > +++ b/include/qemu/log.h
> > @@ -53,7 +53,13 @@ static inline bool qemu_loglevel_mask(int mask)
> >   
> >   /* main logging function
> >    */
> > -void GCC_FMT_ATTR(1, 2) qemu_log(const char *fmt, ...);
> > +#define qemu_log(args...)                   \
> > +    do {                                    \
> > +        if (!qemu_log_enabled()) {          \
> > +            break;                          \
> > +        }                                   \
> > +        fprintf(qemu_logfile, args);        \
> > +    } while (0)
> I've had one of Peter's patches in my logging improvements queue for a
> while although it uses a slightly different form which I prefer:
>
> -/* log only if a bit is set on the current loglevel mask
> +/* log only if a bit is set on the current loglevel mask:
> + * @mask: bit to check in the mask
> + * @fmt: printf-style format string
> + * @args: optional arguments for format string
>    */
> -void GCC_FMT_ATTR(2, 3) qemu_log_mask(int mask, const char *fmt, ...);
> -
> +#define qemu_log_mask(MASK, FMT, ...)                   \
> +    do {                                                \
> +        if (unlikely(qemu_loglevel_mask(MASK))) {       \
> +            qemu_log(FMT, ## __VA_ARGS__);              \
> +        }                                               \
>
> See the message:
>
> qemu-log: Avoid function call for disabled qemu_log_mask logging

as far as I can see that patch is one year old at least
and my version is slightly better, it does the same for
qemu_log.

The difference is that old patch has unlikely() hint and does not
contain break.

I can revert the condition and add the hint in a couple
of minutes if this will increase the chance to get
both things merged. We should have both functions
as macros.

Will you accept that?

Den