From: "Morten Brørup" <mb@smartsharesystems.com>
To: "Mattias Rönnblom" <hofors@lysator.liu.se>,
"Tyler Retzlaff" <roretzla@linux.microsoft.com>,
dev@dpdk.org
Cc: "Mattias Rönnblom" <mattias.ronnblom@ericsson.com>,
"Anatoly Burakov" <anatoly.burakov@intel.com>,
"Bruce Richardson" <bruce.richardson@intel.com>,
"David Christensen" <drc@linux.vnet.ibm.com>,
"Harry van Haaren" <harry.van.haaren@intel.com>,
"Konstantin Ananyev" <konstantin.v.ananyev@yandex.ru>,
"Min Zhou" <zhoumin@loongson.cn>,
"Ruifeng Wang" <ruifeng.wang@arm.com>,
"Stanislaw Kardach" <kda@semihalf.com>,
thomas@monjalon.net
Subject: RE: [PATCH] RFC: use C11 alignas instead of GCC attribute aligned
Date: Fri, 26 Jan 2024 11:18:50 +0100 [thread overview]
Message-ID: <98CBD80474FA8B44BF855DF32C47DC35E9F1A7@smartserver.smartshare.dk> (raw)
In-Reply-To: <91ba1ece-10dd-4698-acd9-6b51cfc63cd9@lysator.liu.se>
> From: Mattias Rönnblom [mailto:hofors@lysator.liu.se]
> Sent: Friday, 26 January 2024 11.05
>
> On 2024-01-25 23:53, Morten Brørup wrote:
> >> From: Tyler Retzlaff [mailto:roretzla@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@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
> >
next prev parent reply other threads:[~2024-01-26 10:18 UTC|newest]
Thread overview: 29+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-11-15 17:39 Tyler Retzlaff
2023-11-15 17:39 ` [PATCH] eal: " Tyler Retzlaff
2023-11-15 18:13 ` Bruce Richardson
2023-11-15 18:27 ` Tyler Retzlaff
2023-11-15 20:08 ` Morten Brørup
2023-11-15 21:03 ` Tyler Retzlaff
2023-11-15 22:43 ` Stanisław Kardach
2023-11-16 10:12 ` Mattias Rönnblom
2024-01-25 18:37 ` [PATCH] RFC: " Tyler Retzlaff
2024-01-25 22:53 ` Morten Brørup
2024-01-25 23:31 ` Tyler Retzlaff
2024-01-26 10:05 ` Mattias Rönnblom
2024-01-26 10:18 ` Morten Brørup [this message]
2024-01-27 19:15 ` Mattias Rönnblom
2024-01-28 8:57 ` Morten Brørup
2024-01-28 10:00 ` Mattias Rönnblom
2024-01-29 19:43 ` Tyler Retzlaff
2024-01-30 8:08 ` Mattias Rönnblom
2024-01-30 17:39 ` Tyler Retzlaff
2024-01-30 17:59 ` Bruce Richardson
2024-01-30 18:01 ` Bruce Richardson
2024-01-30 18:04 ` Tyler Retzlaff
2024-01-30 18:18 ` Mattias Rönnblom
2024-01-31 16:04 ` Mattias Rönnblom
2024-01-30 8:09 ` Morten Brørup
2024-01-30 9:28 ` Mattias Rönnblom
2024-01-30 10:17 ` Morten Brørup
2024-01-30 13:00 ` Morten Brørup
2024-01-30 17:54 ` Tyler Retzlaff
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