[PATCH] D68364: Implement C++20's P0784 (More constexpr containers)

[Richard Smith - zygoloid via Phabricator via cfe-commits]

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:
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  https://reviews.llvm.org/D68364/new/

https://reviews.llvm.org/D68364