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

["Mattias Rönnblom" <hofors@lysator.liu.se>]

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.

__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.

> 
> 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
>