[PATCH] RFC: use C11 alignas instead of GCC attribute aligned

[Morten Brørup]

> From: Mattias Rönnblom [mailto:hofors at lysator.liu.se]
> Sent: Friday, 26 January 2024 11.05
> 
> On 2024-01-25 23:53, Morten Brørup wrote:
> >> From: Tyler Retzlaff [mailto:roretzla at linux.microsoft.com]
> >> Sent: Thursday, 25 January 2024 19.37
> >>
> >> ping.
> >>
> >> Please review this thread if you have time, the main point of
> >> discussion
> >> I would like to receive consensus on the following questions.
> >>
> >> 1. Should we continue to expand common alignments behind an
> __rte_macro
> >>
> >>    i.e. what do we prefer to appear in code
> >>
> >>    alignas(RTE_CACHE_LINE_MIN_SIZE)
> >>
> >>    -- or --
> >>
> >>    __rte_cache_aligned
> >>
> >> One of the benefits of dropping the macro is it provides a clear
> visual
> >> indicator that it is not placed in the same location or get applied
> >> to types as is done with __attribute__((__aligned__(n))).
> >
> > We don't want our own proprietary variant of something that already
> exists in the C standard. Now that we have moved to C11, the __rte
> alignment macros should be considered obsolete.
> 
> Making so something cache-line aligned is not in C11.

We are talking about the __rte_aligned() macro, not the cache alignment macro.

> 
> __rte_cache_aligned is shorter, provides a tiny bit of abstraction, and
> is already an established DPDK standard. So just keep the macro. If it
> would change, I would argue for it to be changed to rte_cache_aligned
> (i.e., just moving it out of __ namespace, and maybe making it
> all-uppercase).
> 
> Non-trivial C programs wrap things all the time, standard or not. It's
> not something to be overly concerned about, imo.

Using the cache alignment macro was obviously a bad example for discussing the __rte_aligned() macro.

FYI, Tyler later agreed to introducing the RTE_CACHE_ALIGNAS you had proposed in an earlier correspondence.

> 
> >
> > Note: I don't mind convenience macros for common use cases, so we
> could also introduce the macro suggested by Mattias [1]:
> >
> > #define RTE_CACHE_ALIGNAS alignas(RTE_CACHE_LINE_SIZE)
> >
> > [1]: https://inbox.dpdk.org/dev/dc3f3131-38e6-4219-861e-
> b31ec10c08bb at lysator.liu.se/
> >
> >>
> >> 2. where should we place alignas(n) or __rte_macro (if we use a
> macro)
> >>
> >> Should it be on the same line as the variable or field or on the
> >> preceeding line?
> >>
> >>    /* same line example struct */
> >>    struct T {
> >>        /* alignas(64) applies to field0 *not* struct T type
> declaration
> >> */
> >>        alignas(64) void *field0;
> >>        void *field1;
> >>
> >>        ... other fields ...
> >>
> >>        alignas(64) uint64_t field5;
> >>        uint32_t field6;
> >>
> >>        ... more fields ...
> >>
> >>    };
> >>
> >>    /* same line example array */
> >>    alignas(64) static const uint32_t array[4] = { ... };
> >>
> >>    -- or --
> >>
> >>    /* preceeding line example struct */
> >>    struct T {
> >>        /* alignas(64) applies to field0 *not* struct T type
> declaration
> >> */
> >>        alignas(64)
> >>        void *field0;
> >>        void *field1;
> >>
> >>        ... other fields ...
> >>
> >>        alignas(64)
> >>        uint64_t field5;
> >>        uint32_t field6;
> >>
> >>        ... more fields ...
> >>
> >>    };
> >>
> >>    /* preceeding line example array */
> >>    alignas(64)
> >>    static const uint32_t array[4] = { ... };
> >>
> >
> > Searching the net for what other projects do, I came across this
> required placement [2]:
> >
> > uint64_t alignas(64) field5;
> >
> > [2]:
> https://lore.kernel.org/buildroot/20230730000851.6faa3391@windsurf/T/
> >
> > So let's follow the standard's intention and put them on the same
> line.
> > On an case-by-case basis, we can wrap lines if it improves
> readability, like we do with function headers that have a lot of
> attributes.
> >
> >>
> >> I'll submit patches for lib/* once the discussion is concluded.
> >>
> >> thanks folks
> >