On 2024-01-26 11:18, Morten Brørup wrote:
From: Mattias Rönnblom [mailto:hof...@lysator.liu.se]
Sent: Friday, 26 January 2024 11.05
On 2024-01-25 23:53, Morten Brørup wrote:
From: Tyler Retzlaff [mailto:roret...@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.
OK, in that case, what is the relevance of question 1 above?
__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-
b31ec10c0...@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
