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

[Morten Brørup]

> 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]:

#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