[libcxx-commits] [PATCH] D46443: [libc++] Add missing cstdalign header

Marek Kurdej via Phabricator via libcxx-commits libcxx-commits at lists.llvm.org
Wed Mar 10 00:35:01 PST 2021

curdeius requested changes to this revision.
curdeius added inline comments.
This revision now requires changes to proceed.

Comment at: libcxx/include/cstdalign:24
+#include <__config>
+#include <stdalign.h>
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.

Comment at: libcxx/test/std/language.support/cstdalign/cstdalign.pass.cpp:16-18
+#ifdef alignas
+#error alignas should not be a macro in C++
Please do the same for `alignof`.

Comment at: libcxx/test/std/language.support/cstdalign/cstdalign.pass.cpp:27
+#ifndef __alignof_is_defined
+#error __alignof_is_defined not defined
hubert.reinterpretcast wrote:
> sbc100 wrote:
> > hubert.reinterpretcast wrote:
> > > sbc100 wrote:
> > > > ldionne wrote:
> > > > > I'm not seeing `__alignof_is_defined` anywhere in the spec?
> > > > Removed
> > > Seems like a defect in the old standard. The prose doesn't match the synopsis. `__alignof_is_defined` is a macro in C11's `stdalign.h` (and so is `alignof`). That the C++ committee did not intend for an `alignof` macro can probably be assumed. I suspect the lack of an `__alignof_is_defined` macro was also unintended.
> > So should I add back the check for `__alignof_is_defined`?
> I think so (in addition to also checking that `alignof` is not defined as a macro). I think the patch needs to be confirmed again either way by the libc++ approvers though.
Please check `__alignof_is_defined` as well, but guard it with `#ifdef _LIBCPP_VERSION` to be conforming.
I agree that it seems to be a defect in the old standard.

  rG LLVM Github Monorepo



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