> From: Thomas Monjalon [mailto:tho...@monjalon.net] > Sent: Monday, 14 August 2023 15.46 > > mercredi 9 août 2023, Morten Brørup: > > > From: Tyler Retzlaff [mailto:roret...@linux.microsoft.com] > > > Sent: Tuesday, 8 August 2023 22.50 > > > > > > On Tue, Aug 08, 2023 at 10:22:09PM +0200, Morten Brørup wrote: > > > > > From: Tyler Retzlaff [mailto:roret...@linux.microsoft.com] > > > > > Sent: Tuesday, 8 August 2023 21.20 > > > > > > > > > > On Tue, Aug 08, 2023 at 07:23:41PM +0100, Bruce Richardson > wrote: > > > > > > On Tue, Aug 08, 2023 at 10:53:03AM -0700, Tyler Retzlaff > wrote: > > > > > > > Hi folks, > > > > > > > > > > > > > > Moving this discussion to the dev mailing list for broader > > > comment. > > > > > > > > > > > > > > Unfortunately, we've hit a roadblock with integrating C11 > > > atomics > > > > > > > for DPDK. The main issue is that GNU C++ prior to - > std=c++23 > > > > > explicitly > > > > > > > cannot be integrated with C11 stdatomic.h. Basically, you > can't > > > > > include > > > > > > > the header and you can't use `_Atomic' type specifier to > declare > > > > > atomic > > > > > > > types. This is not a problem with LLVM or MSVC as they both > > > allow > > > > > > > integration with C11 stdatomic.h, but going forward with C11 > > > atomics > > > > > > > would break using DPDK in C++ programs when building with > GNU > > > g++. > > > > > > > > > > > > > > Essentially you cannot compile the following with g++. > > > > > > > > > > > > > > #include <stdatomic.h> > > > > > > > > > > > > > > int main(int argc, char *argv[]) { return 0; } > > > > > > > > > > > > > > In file included from atomic.cpp:1: > > > > > > > /usr/lib/gcc/x86_64-pc-cygwin/11/include/stdatomic.h:40:9: > > > error: > > > > > > > ‘_Atomic’ does not name a type > > > > > > > 40 | typedef _Atomic _Bool atomic_bool; > > > > > > > > > > > > > > ... more errors of same ... > > > > > > > > > > > > > > It's also acknowledged as something known and won't fix by > GNU > > > g++ > > > > > > > maintainers. > > > > > > > > > > > > > > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=60932 > > > > > > > > > > > > > > Given the timeframe I would like to propose the minimally > > > invasive, > > > > > > > lowest risk solution as follows. > > > > > > > > > > > > > > 1. Adopt stdatomic.h for all Windows targets, leave all > > > Linux/BSD > > > > > targets > > > > > > > using GCC builtin C++11 memory model atomics. > > > > > > > 2. Introduce a macro that allows _Atomic type specifier to > be > > > > > applied to > > > > > > > function parameter, structure field types and variable > > > > > declarations. > > > > > > > > > > > > > > * The macro would expand empty for Linux/BSD targets. > > > > > > > * The macro would expand to C11 _Atomic keyword for > Windows > > > > > targets. > > > > > > > > > > > > > > 3. Introduce basic macro that allows __atomic_xxx for > > > normalized > > > > > use > > > > > > > internal to DPDK. > > > > > > > > > > > > > > * The macro would not be defined for Linux/BSD targets. > > > > > > > * The macro would expand __atomic_xxx to corresponding > > > > > stdatomic.h > > > > > > > atomic_xxx operations for Windows targets. > > > > > > > > > > > > > > > Regarding naming of these macros (suggested in 2. and 3.), they > should > > > probably bear the rte_ prefix instead of overlapping existing names, > so > > > applications can also use them directly. > > > > > > > > E.g.: > > > > #define rte_atomic for _Atomic or nothing, > > > > #define rte_atomic_fetch_add() for atomic_fetch_add() or > > > __atomic_fetch_add(), and > > > > #define RTE_MEMORY_ORDER_SEQ_CST for memory_order_seq_cst or > > > __ATOMIC_SEQ_CST. > > > > > > > > Maybe that is what you meant already. I'm not sure of the scope > and > > > details of your suggestion here. > > > > > > I'm shy to do anything in the rte_ namespace because I don't want to > > > formalize it as an API. > > > > > > I was envisioning the following. > > > > > > Internally DPDK code just uses __atomic_fetch_add directly, the > macros > > > are provided for Windows targets to expand to __atomic_fetch_add. > > > > > > Externally DPDK applications that don't care about being portable > may > > > use __atomic_fetch_add (BSD/Linux) or atomic_fetch_add (Windows) > > > directly. > > > > > > Externally DPDK applications that care to be portable may do what is > > > done Internally and <<use>> the __atomic_fetch_add directly. By > > > including say rte_stdatomic.h indirectly (Windows) gets the macros > > > expanded to atomic_fetch_add and for BSD/Linux it's a noop include. > > > > > > Basically I'm placing a little ugly into Windows built code and in > trade > > > we don't end up with a bunch of rte_ APIs that were strongly > objected to > > > previously. > > > > > > It's a compromise. > > > > OK, we probably need to offer a public header file to wrap the > atomics, using either names prefixed with rte_ or names similar to the > gcc builtin atomics. > > > > I guess the objections were based on the assumption that we were > switching to C11 atomics with DPDK 23.11, so the rte_ prefixed atomic > APIs would be very short lived (DPDK 23.07 to 23.11 only). But with this > new information about GNU C++ incompatibility, that seems not to be the > case, so the naming discussion can be reopened. > > > > If we don't introduce such a wrapper header, all portable code needs > to surround the use of atomics with #ifdef USE_STDATOMIC_H. > > > > BTW: Can the compilers that understand both builtin atomics and C11 > stdatomics.h handle code with #define __atomic_fetch_add > atomic_fetch_add and #define __ATOMIC_SEQ_CST memory_order_seq_cst? If > not, then we need to use rte_ prefixed atomics. > > > > And what about C++ atomics... Do we want (or need?) a third variant > using C++ atomics, e.g. "atomic<int> x;" instead of "_Atomic int x;"? (I > hope not!) For reference, the "atomic_int" type is "_Atomic int" in C, > but "std::atomic<int>" in C++. > > > > C++23 provides the C11 compatibility macro "_Atomic(T)", which means > "_Atomic T" in C and "std::atomic<T>" in C++. Perhaps we can somewhat > rely on this, and update our coding standards to require using e.g. > "_Atomic(int)" for atomic types, and disallow using "_Atomic int". > > You mean the syntax _Atomic(T) is working well in both C and C++?
This syntax is API compatible across C11 and C++23, so it would work with (C11 and C++23) applications building DPDK from scratch. But it is only "recommended" ABI compatible for compilers [1], so DPDK in distros cannot rely on. [1]: https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2020/p0943r6.html It would be future-proofing for the benefit of C++23 based applications... I was mainly mentioning it for completeness, now that we are switching to a new standard for atomics. Realistically, considering that 1. such a coding standard (requiring "_Atomic(T)" instead of "_Atomic T") would only be relevant for a 2023 standard, and 2. that we are now upgrading to a standard from 2011, we would probably have to wait for a very distant future (12 years?) before C++ applications can reap the benefits of such a coding standard.
