[PATCH] RFC: use C11 alignas instead of GCC attribute aligned
Tyler Retzlaff
roretzla at linux.microsoft.com
Fri Jan 26 00:31:43 CET 2024
On Thu, Jan 25, 2024 at 11:53:04PM +0100, 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.
>
> 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
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