Re: [PATCH v3] eventdev: avoid non-burst shortcut for variable-size bursts

[Jerin Jacob <jerinjacobk@gmail.com>]

On Fri, May 12, 2023 at 7:26 PM Morten Brørup <mb@smartsharesystems.com> wrote:
>
> > From: Mattias Rönnblom [mailto:mattias.ronnblom@ericsson.com]
> > Sent: Friday, 12 May 2023 15.15
> >
> > On 2023-05-12 13:59, Jerin Jacob wrote:
> > > On Thu, May 11, 2023 at 2:00 PM Mattias Rönnblom
> > > <mattias.ronnblom@ericsson.com> wrote:
> > >>
> > >> Use non-burst event enqueue and dequeue calls from burst enqueue and
> > >> dequeue only when the burst size is compile-time constant (and equal
> > >> to one).
> > >>
> > >> Signed-off-by: Mattias Rönnblom <mattias.ronnblom@ericsson.com>
> > >>
> > >> ---
> > >>
> > >> v3: Actually include the change v2 claimed to contain.
> > >> v2: Wrap builtin call in __extension__, to avoid compiler warnings if
> > >>      application is compiled with -pedantic. (Morten Brørup)
> > >> ---
> > >>   lib/eventdev/rte_eventdev.h | 4 ++--
> > >>   1 file changed, 2 insertions(+), 2 deletions(-)
> > >>
> > >> diff --git a/lib/eventdev/rte_eventdev.h b/lib/eventdev/rte_eventdev.h
> > >> index a90e23ac8b..a471caeb6d 100644
> > >> --- a/lib/eventdev/rte_eventdev.h
> > >> +++ b/lib/eventdev/rte_eventdev.h
> > >> @@ -1944,7 +1944,7 @@ __rte_event_enqueue_burst(uint8_t dev_id, uint8_t
> > port_id,
> > >>           * Allow zero cost non burst mode routine invocation if
> > application
> > >>           * requests nb_events as const one
> > >>           */
> > >> -       if (nb_events == 1)
> > >> +       if (__extension__(__builtin_constant_p(nb_events)) && nb_events ==
> > 1)
> > >
> > > "Why" part is not clear from the commit message. Is this to avoid
> > > nb_events read if it is built-in const.
> >
> > The __builtin_constant_p() is introduced to avoid having the compiler
> > generate a conditional branch and two different code paths in case
> > nb_elem is a run-time variable.
> >
> > In particular, this matters if nb_elems is run-time variable and varies
> > between 1 and some larger value.
> >
> > I should have mention this in the commit message.
> >
> > A very slight performance improvement. It also makes the code better
> > match the comment, imo. Zero cost for const one enqueues, but no impact
> > non-compile-time-constant-length enqueues.
> >
> > Feel free to ignore.
> >
> > > If so, check should be following. Right?
> > >
> > > if (__extension__((__builtin_constant_p(nb_events)) && nb_events == 1)
> > > || nb_events  == 1)
>
> @Mattias: You missed the second part of this comparison, also catching nb_events == 1 with non-constant nb_events.
>
> @Jerin: Such a change has no effect, compared to the original code.


Yes. That was the reason for I was skipping __builtin_constant_p in
the initial version.