RE: [PATCH v4] eventdev: ensure 16-byte alignment for events

["Morten Brørup" <mb@smartsharesystems.com>]

> From: Stephen Hemminger [mailto:stephen@networkplumber.org]
> Sent: Monday, 13 November 2023 00.32
> 
> On Sun, 12 Nov 2023 09:30:24 +0100
> Morten Brørup <mb@smartsharesystems.com> wrote:
> 
> > > > +static_assert(sizeof(struct rte_event) == 16, "Event structure
> size
> > > is not 16-bytes in size");
> > > > +
> > > >  static struct rte_eventdev
> rte_event_devices[RTE_EVENT_MAX_DEVS];
> > >
> > > Please don't reinvent RTE_BUILD_BUG_ON().
> > > Instead fix that to be a static_assert()
> >
> > I would say the opposite:
> > With our upgrade to the C11 standard, let's get rid of the
> RTE_BUILD_BUG_ON() workaround for the lack of static_assert() in older
> C standards.
> >
> > Unfortunately, the static_assert(expression) variant without the
> "message" parameter, which would make our RTE_BUILD_BUG_ON() macro
> completely obsolete, requires C23. And I don't see how we can make this
> variant available with C11. So we probably have to wait until DPDK
> requires C23.
> >
> > Until then, let's gradually phase out the DPDK-specific
> RTE_BUILD_BUG_ON() in favor of standard C's static_assert(), and live
> with the inconvenience of having to provide a message parameter for it.
> >
> > Please also note that static_assert() can be used outside code
> blocks, which makes it handy for use in header files.
> 
> If you look at my RFC, the message is just as good as the one in this
> code.
> It ends up being stringified version of the expression. Which is more
> exact than the wording used in some other places.

I agree that the output of your RFC is better than that of static_assert(expr, msg).
I'm arguing that RTE_BUILD_BUG_ON() would be considered reinventing the wheel if introduced today, because the C11 standard already offers something similar. So we should prefer that over RTE_BUILD_BUG_ON(), not the other way around.

It's a difference in opinion. Both RTE_BUILD_BUG_ON() and static_assert() are viable; you seem to prefer the first, I prefer the latter.

Both occur in existing DPDK code, and Bruce happened to chose static_assert() for this patch.

The overall question is a choice between three options:
1. prefer using RTE_BUILD_BUG_ON() (using your RFC implementation),
2. phase out RTE_BUILD_BUG_ON() in favor of the C11 standard's static_assert(), or
3. continue using both?

I'm not strongly opposed to either of the three, as long as the community agrees.