kristof.beyls added a comment. In D74918#1898191 <https://reviews.llvm.org/D74918#1898191>, @__simt__ wrote:
> In D74918#1897636 <https://reviews.llvm.org/D74918#1897636>, @kristof.beyls > wrote: > > > If these values are part of the C++ target platform ABI, it seems to me the > > values for std::hardware_{constructive,destructive}_interference_size > > should be set by whoever has the authority to decide C++ platform ABI for > > specific platforms. > > Assuming my thought in the previous sentence is correct; discussions on > > which values to chose for > > std::hardware_{constructive,destructive}_interference_size should happen in > > whichever forums decide C++ platform ABI for the various platforms? (Maybe > > for some platforms that forum might be clang-related fora like > > reviews.llvm.org, but probably not for all platforms). > > With my (probably limited) understanding of the requirements, it seems > > like deriving std::hardware_{constructive,destructive}_interference_size > > from actual cache line size on a specific micro-architecture doesn't seem > > to be the right approach? > > > They will be in the library ABI, meaning the libc++ ABI. > > It's valid for libstdc++ and libc++ to have different values there. If we > wish, we could try for an alignment (no pun intended) on these values, but > even then that's just between these two libraries. > > Which is good and encouraging, because I don't know what forum we would have > to go to. I see. So IIUC, this is library C++ ABI, to be defined by the C++ library. Since no 2 C++ libraries can co-exist in a single application, there is no need for different C++ libraries to agree? I think there is still an issue then with getting the values of std::hardware_{constructive,destructive}_interference_size in the library implementation derived from compiler builtins. There are quite a few systems where clang supports targeting a different C++ library than libc++, e.g. libstdc++ on linux or (IIUC) the MSVC C++ library on Windows. If these C++ libraries implement std::hardware_{constructive,destructive}_interference_size based on a value returned by a compiler builtin, and the different compilers that are used with these libraries return different values for such a builtin, then the library C++ ABI here will be dependent on which compiler used? Doesn't this indicate that either: - std::hardware_{constructive,destructive}_interference_size should not be implemented using a compiler builtin, or - all compilers must return the same value for the builtin; for all targets they support. Overall, that makes me doubt that using a compiler builtin to implement std::hardware_{constructive,destructive}_interference_size is the right direction. If the functionality in this patch does not need to support implementing std::hardware_{constructive,destructive}_interference_size, then the design constraints for this patch change? Repository: rG LLVM Github Monorepo CHANGES SINCE LAST ACTION https://reviews.llvm.org/D74918/new/ https://reviews.llvm.org/D74918 _______________________________________________ cfe-commits mailing list cfe-commits@lists.llvm.org https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-commits
