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

[Tyler Retzlaff <roretzla@linux.microsoft.com>]

On Tue, Jan 30, 2024 at 09:08:21AM +0100, Mattias Rönnblom wrote:
> On 2024-01-29 20:43, Tyler Retzlaff wrote:
> >On Sun, Jan 28, 2024 at 11:00:31AM +0100, Mattias Rönnblom wrote:
> >>On 2024-01-28 09:57, Morten Brørup wrote:
> >>>>From: Mattias Rönnblom [mailto:hofors@lysator.liu.se]
> >>>>Sent: Saturday, 27 January 2024 20.15
> >>>>
> >>>>On 2024-01-26 11:18, Morten Brørup wrote:
> >>>>>>From: Mattias Rönnblom [mailto:hofors@lysator.liu.se]
> >>>>>>Sent: Friday, 26 January 2024 11.05
> >>>>>>
> >>>>>>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.
> >>>>>
> >>>>>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?
> >>>
> >>>With this in mind, try re-reading Tyler's clarifications in this tread.
> >>>
> >>>Briefly: alignas() can be attached to variables and structure fields, but not to types (like __rte_aligned()), so to align a structure:
> >>>
> >>>struct foo {
> >>>	int alignas(64) bar; /* alignas(64) must be here */
> >>>	int             baz;
> >>>}; /* __rte_aligned(64) was here, but alignas(64) cannot be here. */
> >>>
> >>>So the question is: Do we want to eliminate the __rte_aligned() macro - which relies on compiler attributes - and migrate to using the C11 standard alignas()?
> >>>
> >>>I think yes; after updating to C11, the workaround for pre-C11 not offering alignment is obsolete, and its removal should be on the roadmap.
> >>>
> >>
> >>OK, thanks for the explanation. Interesting limitation in the standard.
> >>
> >>If the construct the standard is offering is less effective (in this
> >>case, less readable) and the non-standard-based option is possible
> >>to implement on all compilers (i.e., on MSVC too), then we should
> >>keep the custom option. Especially if it's already there, but also
> >>in cases where it isn't.
> >>
> >>In fact, one could argue *everything* related to alignment should go
> >>through something rte_, __rte_ or RTE_-prefixed. So, "int
> >>RTE_ALIGNAS(64) bar;". Maybe that would be silly, but it would be
> >>consistent with RTE_CACHE_ALIGNAS.
> >>
> >>I would worry more about allowing DPDK developers writing clean and
> >>readable code, than very slightly lowering the bar for the fraction
> >>of newcomers experienced with the latest and greatest from the C
> >>standard, and *not* familiar with age-old GCC extensions.
> >
> >I’d just like to summarize where my understanding is at after reviewing
> >this discussion and my downstream branch. But I also want to make it
> >clear that we probably need to use both standard C and non-standard
> >attribute/declspec for object and struct/union type alignment
> >respectively.
> >
> >I've assumed we prefer avoiding per-compiler conditional expansion when
> >possible through the use of standard C mechanisms. But there are
> >instances when alignas is awkward.
> >
> >So I think the following is consistent with what Mattias is advocating
> >sans any discussions related to actual naming of macros.
> >
> >We should have 2 macros, upon which others may be built to expand to
> >well-known values for e.g. cache line size.
> >
> >RTE_ALIGNAS(n) object;
> >
> >* This macro is used to align C objects i.e. variable, array, struct/union
> >   fields etc.
> >* Trivially expands to alignas(n) for all toolchains.
> >* Placed in a location that both C and C++ translation units accept that
> >   is on the same line preceeding the object type.
> >   example:
> >   // RTE_ALIGNAS(n) object;
> >   RTE_ALIGNAS(16) char somearray[16];
> >
> >RTE_ALIGN_TYPE(n)
> >
> >* This macro is used to align struct/union types.
> >* Conditionally expands to __declspec(align(n)) (msvc) and
> >   __attribute__((__aligned__(n))) (for all other toolchains)
> >* Placed in a location that for all gcc,clang,msvc and both C and C++
> >   translation units accept.
> >   example:
> >   // {struct,union} RTE_ALIGN_TYPE(n) tag { ... };
> >   struct RTE_ALIGN_TYPE(64) sometype { ... };
> >
> 
> Sorry if I've missed some discussion on the list, but the current
> pattern of putting __rte_aligned(X) at the end doesn't work with
> MSVC, or why are we doing this? C11 purism doesn't seem like much of
> a driving force.

__rte_aligned(X) at the end doesn't work with MSVC __declspec(align(n))

> 
> If one defined a macro as __declspec(align(X)) on MSVC and
> __attribute__(__aligned__(X)) on other compilers, could it do the
> work of both the above RTE_ALIGNAS() and RTE_ALIGN_TYPE()?
> 
> <a> struct <b> { int a; } <c>;

yes for struct/union. but only when placed at location you mark as <b>
when compiling both C and C++ for all toolchains.

maybe, for objects but ideally, we prefer alignas for consistent semantics
defined by standard rather than accomodating potential implementation
differences when conditionally expanding __aligned vs __declspec. as you
have noted __declspec has limitations/variations when compared to
__attribute__((__aligned__(n))).

> 
> You would have to mandate the placement of such a __rte_aligned
> plug-in replacement being at <b> rather than (the more intuitive?)
> <a>, since clang doesn't like __attribute__s before the struct/union
> keyword, correct?

for struct/union there is a single placement accepted by all toolchains
for both C and C++ and it is <b>.

> 
> What about other <rte_common.h> __attribute__ wrappers like
> __rte_packed; would they also need to change placement to make DPDK
> work with MSVC?

packing is a different problem that needs a separate RFC and discussion
of it's own.

> 
> >I'm not picky about what the names actualy are if you have better
> >suggestions i'm happy to adopt them.
> >
> >Thoughts? Comments?
> >
> >Appreciate the discussion this has been helpful.
> >
> >ty
> >