rsmith added inline comments.
================ Comment at: libcxx/include/version:254 // # define __cpp_lib_concepts 201806L +# define __cpp_lib_constexpr_dynamic_alloc 201907L // # define __cpp_lib_constexpr_misc 201811L ---------------- ldionne wrote: > rsmith wrote: > > ldionne wrote: > > > rsmith wrote: > > > > Should this be conditioned on compiler support being available? > > > So.. I've decided not to do that in this patch so far. > > > > > > The support for constexpr allocation was checked into Clang about a year > > > ago, right? I actually expect this to be a slightly contentious point, > > > but I'd like to assume that we're using a reasonably recent Clang. I > > > don't see a strong point for being able to use new libc++ headers with an > > > old Clang anyway, since vendors usually release the two together. IOW, > > > supporting this would add complexity for virtually no benefit. I do agree > > > it's a slightly more aggressive stance than we've had so far, but this > > > sort of reasonable assumption makes it so much easier to write stuff for > > > libc++. > > OK, just a few thoughts then I'm going to bow out of this; this seems like > > a policy decision for the libc++ maintainers to make. > > > > In favor of dropping support for new libc++ + old clang: we generally don't > > permit version skew between different components of LLVM. It seems > > reasonable to expect all wanted parts of a particular LLVM release to be > > built together. > > > > Against dropping support for new libc++ + old clang: we do support > > installing more than one version of LLVM (and in particular more than one > > version of Clang) on the same system, but because libc++ defaults to being > > installed in `/usr/include/c++/v1`, we don't seem to encourage installing > > more than one version of libc++, so -- even assuming we only support the > > *newest* version of libc++ going into `/usr/include/c++/v1` -- new versions > > of libc++ need to work with old versions of Clang. > > > > I think (largely by accident) Clang will prefer a libc++ installed into > > `/usr/lib/clang/$VER/include` over one from `/usr/include/c++/v1`. If we > > switched to installing libc++ there, I don't see any technical barrier to > > version-locking them, though I'm not sure what story that leaves for use of > > libc++ with GCC and other compilers. It seems worth noting that this is > > exactly what libstdc++ does in order to need to support only one version of > > GCC. > > OK, just a few thoughts then I'm going to bow out of this; this seems like > > a policy decision for the libc++ maintainers to make. > > Hear hear! > > > > I think (largely by accident) Clang will prefer a libc++ installed into > > `/usr/lib/clang/$VER/include` over one from `/usr/include/c++/v1`. If we > > switched to installing libc++ there, I don't see any technical barrier to > > version-locking them, though I'm not sure what story that leaves for use of > > libc++ with GCC and other compilers. It seems worth noting that this is > > exactly what libstdc++ does in order to need to support only one version of > > GCC. > > I think it would be great to do that. Honestly, this is a huge simplification > and makes implementing new features much easier. Also, I think it's > reasonable to support not-trunk compilers, like 6 months old or something > like that. But not several years back. > > One last question: do you know what controls where the libc++ headers are > installed as part of the LLVM distribution? Install paths are set in `libcxx/include/CMakeLists.txt` for the headers and in `libcxx/src/CMakeLists.txt` for the libraries (search for `install(`) based on cmake variables LIBCXX_INSTALL_HEADER_PREFIX, LIBCXX_INSTALL_PREFIX, and LIBCXX_INSTALL_LIBRARY_DIR. It would probably make sense to put the libc++ headers into somewhere like `/usr/lib/clang/$VER/include/c++` instead of directly in `/usr/lib/clang/$VER/include`; that'll need some changes to the clang driver to make sure we look there. But of course that's doable if the two are version-locked :) Repository: rG LLVM Github Monorepo CHANGES SINCE LAST ACTION https://reviews.llvm.org/D68364/new/ https://reviews.llvm.org/D68364 _______________________________________________ cfe-commits mailing list cfe-commits@lists.llvm.org https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-commits
