jwakely added a comment. In D87974#2438723 <https://reviews.llvm.org/D87974#2438723>, @zoecarver wrote:
> In D87974#2438682 <https://reviews.llvm.org/D87974#2438682>, @BillyONeal > wrote: > >>> Are they actually the same, with the same handling of corner cases like >>> unions and tail padding? >>> There's more to this than just the name, and if they aren't the same, it >>> seems better to have two names. >> >> They are both implementing the same C++ feature, with the same desired >> semantics of zeroing out any bits in the object representation that aren't >> in the value representation. If they differ, one or the other would have a >> bug. Do they support non-trivially copyable types? That isn't required for the atomic compare exchange feature, but is relevant for a feature exposed to users. What about extensions like zero-sized arrays or C99 flexible array members? > I agree, they either need to be identical (including corner cases) or there > needs to be two of them (i.e., GCC ships both `__builtin_zero_non_value_bits` > and `__builtin_clear_padding` and the first is the same as MSVC, Clang, and > NVCC). GCC doesn't need to support both. It only works with libstdc++ so it only needs to support the one used by libstdc++ (although there is a patch to add `-stdlib=libc++` to GCC). If libstdc++ uses `__has_builtin` to check what the compiler supports then Clang doesn't even need to support GCC's built-in, because libstdc++ wouldn't use it if not supported (and could use `__builtin_zero_non_value_bits` instead when supported). The Intel compiler would need to support both though. >>> Is there a specification for __builtin_zero_non_value_bits available >>> somewhere? >> >> I don't know if there is a formal spec for it beyond the actual C++ standard. > > I think P0528 is the relevant paper but other than that, no, there's not a > spec. I think that's going to be the most time sensitive part of implementing > this: coming up with the spec and making sure all the tests pass on all the > implementations. GCC has publicly available documentation describing its built-in, and publicly available tests for it. That's the kind of spec I'm looking for. Repository: rG LLVM Github Monorepo CHANGES SINCE LAST ACTION https://reviews.llvm.org/D87974/new/ https://reviews.llvm.org/D87974 _______________________________________________ cfe-commits mailing list cfe-commits@lists.llvm.org https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-commits
