From: Tyler Retzlaff <roretzla@linux.microsoft.com>
To: "Morten Brørup" <mb@smartsharesystems.com>
Cc: dev@dpdk.org, "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: Thu, 25 Jan 2024 15:31:43 -0800 [thread overview]
Message-ID: <20240125233143.GA1920@linuxonhyperv3.guj3yctzbm1etfxqx2vob5hsef.xx.internal.cloudapp.net> (raw)
In-Reply-To: <98CBD80474FA8B44BF855DF32C47DC35E9F1A3@smartserver.smartshare.dk>
On Thu, Jan 25, 2024 at 11:53:04PM +0100, 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.
>
> Note: I don't mind convenience macros for common use cases, so we could also introduce the macro suggested by Mattias [1]:
ack
>
> #define RTE_CACHE_ALIGNAS alignas(RTE_CACHE_LINE_SIZE)
>
> [1]: https://inbox.dpdk.org/dev/dc3f3131-38e6-4219-861e-b31ec10c08bb@lysator.liu.se/
i'm good with this, it satisfies that it is a different name than the
original and therefore achieves the same intent. i'll spin the patch
series with this macro.
>
> >
> > 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.
just fyi.
the linked code is c++ and standard c++ has both semantic and syntactic
differences from standard c. notably standard c is moving away
from the notion that you can alignas types and instead you align
variables/fields/members.
further restricting placement is the need to choose an intersecting
placement that works when consumed in either a c or c++ translation
unit. so the options i present above are that intersection.
ty
>
>
> >
> > I'll submit patches for lib/* once the discussion is concluded.
> >
> > thanks folks
next prev parent reply other threads:[~2024-01-25 23:31 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 [this message]
2024-01-26 10:05 ` Mattias Rönnblom
2024-01-26 10:18 ` Morten Brørup
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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