[PATCH] D46443: [libc++] Add missing cstdalign header

[Hubert Tong via Phabricator via cfe-commits]

hubert.reinterpretcast added inline comments.


================
Comment at: libcxx/include/cstdalign:24
+#include <__config>
+#include <stdalign.h>
+
----------------
sbc100 wrote:
> hubert.reinterpretcast wrote:
> > curdeius wrote:
> > > hubert.reinterpretcast wrote:
> > > > curdeius wrote:
> > > > > curdeius wrote:
> > > > > > hubert.reinterpretcast wrote:
> > > > > > > sbc100 wrote:
> > > > > > > > hubert.reinterpretcast wrote:
> > > > > > > > > This seems to be assuming that the underlying C library's `stdalign.h` is C++ friendly. A C11 `stdalign.h` //does// define `alignof` and `alignas` as macros.
> > > > > > > > Should I just remove this `#include` then?
> > > > > > > The idea would be to //add// a `stdalign.h` alongside this header that doesn't `#include_next` the underlying C library's `stdalign.h`.
> > > > > > I'm not sure if that should be the solution. At least gcc's libstdc++ assumes that `stdalign.h` is C++-compatbile (cf. https://github.com/gcc-mirror/gcc/blob/16e2427f50c208dfe07d07f18009969502c25dc8/libstdc%2B%2B-v3/include/c_global/cstdalign).
> > > > > > 
> > > > > > Clang provides a compatible header: https://github.com/llvm/llvm-project/commit/8acb4044d83ecc9df81b1c9f327d5bd4325e1756.
> > > > > > Gcc too of course: https://github.com/gcc-mirror/gcc/blob/16e2427f50c208dfe07d07f18009969502c25dc8/gcc/ginclude/stdalign.h.
> > > > > > 
> > > > > > MSVC's STL on the other hand, doesn't include `<stdalign.h>` (https://github.com/microsoft/STL/blob/main/stl/inc/cstdalign).
> > > > > > 
> > > > > > @hubert.reinterpretcast, are you aware of an environment which has non-friendly `stdalign.h`?
> > > > > FYI, musl is also C++ friendly: https://git.musl-libc.org/cgit/musl/tree/include/stdalign.h.
> > > > >>! Quote:
> > > > > @hubert.reinterpretcast, are you aware of an environment which has non-friendly stdalign.h?
> > > > 
> > > > The one GCC provides disagrees with the interpretation I gave of which macros should be present. The one that Clang provides //does// match my interpretation. It seems the GCC one is non-friendly (albeit a different form of non-friendly than the one I opened with).
> > > Oh, you mean that `__alignas_is_defined` and `__alignof_is_defined` won't be defined in this case, right?
> > > In this case, I guess we won't avoid having `stdalign.h` as you had suggested.
> > > And indeed the test fails with gcc:
> > > ```
> > > bin/llvm-lit -vv ../../libcxx/test/std/language.support/cstdalign/ --param=std=c++17 --param=cxx_under_test=`which g++`
> > > ...
> > > libcxx/test/std/language.support/cstdalign/cstdalign.pass.cpp:21:2: error: #error __alignas_is_defined not defined
> > >    21 | #error __alignas_is_defined not defined
> > >       |  ^~~~~
> > > ```
> > > 
> > > That's unfortunately a configuration which is not tested in the CI.
> > Yes, it looks like adding a `stdalign.h` for libc++ is needed to reliably get `__alignas_is_defined` and `__alignof_is_defined`.
> Alternatively, could we just defined them if they are not already and skip including `stdalign.h` here?
`stdalign.h` is a header defined in C++17 like `stdbool.h` is one. libc++ has a `stdbool.h`, so I think having a `stdalign.h` makes sense (since we know that `stdalign.h` in some environments does not do what we want).


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