[Bug libstdc++/103626] _GLIBCXX_HOSTED should respect -ffreestanding

["redi at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103626

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
     Ever confirmed|0                           |1
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2022-09-16

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Jonathan Wakely from comment #0)
> Libstdc++ should not define _GLIBCXX_HOSTED to 1 when __STD_HOSTED__ == 0
> i.e. when -ffreestanding is used.

That should be as simple as:

> diff --git a/libstdc++-v3/acinclude.m4 b/libstdc++-v3/acinclude.m4
> index 9aa81e1ac13..1b404d7df22 100644
> --- a/libstdc++-v3/acinclude.m4
> +++ b/libstdc++-v3/acinclude.m4
> @@ -2992,7 +2992,7 @@ AC_DEFUN([GLIBCXX_ENABLE_HOSTED], [
>      fi
>    else
>      is_hosted=yes
> -    hosted_define=1
> +    hosted_define=__STDC_HOSTED__
>    fi
>    GLIBCXX_CONDITIONAL(GLIBCXX_HOSTED, test $is_hosted = yes)
>    AC_DEFINE_UNQUOTED(_GLIBCXX_HOSTED, $hosted_define,
> diff --git a/libstdc++-v3/include/Makefile.am b/libstdc++-v3/include/Makefile.am
> index 3eeb407a57f..3472e84f75c 100644
> --- a/libstdc++-v3/include/Makefile.am
> +++ b/libstdc++-v3/include/Makefile.am
> @@ -1326,7 +1326,7 @@ ${host_builddir}/c++config.h: ${CONFIG_HEADER} \
>       ${CONFIG_HEADER} > /dev/null 2>&1 \
>       && ldbl_alt128_compat='s,^#undef _GLIBCXX_LONG_DOUBLE_ALT128_COMPAT$$,#define _GLIBCXX_LONG_DOUBLE_ALT128_COMPAT 1,' ;\
>       verbose_assert='s,g,g,' ; \
> -     grep "^[         ]*#[    ]*define[       ][      ]*_GLIBCXX_HOSTED[      ][      ]*1[    ]*$$" \
> +     grep "^[         ]*#[    ]*define[       ][      ]*_GLIBCXX_HOSTED[      ][      ]*__STDC_HOSTED__[      ]*$$" \
>       ${CONFIG_HEADER} > /dev/null 2>&1 \
>       && grep "^[      ]*#[    ]*define[       ][      ]*_GLIBCXX_VERBOSE[     ][      ]*1[    ]*$$" \
>       ${CONFIG_HEADER} > /dev/null 2>&1 \


> The --disable-hosted-libstdcxx configure option will still allow
> installation of a library that can never be used as hosted, but it will also
> be possible to use an existing hosted implementation in freestanding mode.

This would mean that a full set of library headers would be available for
inclusion in programs compiled with -ffreestanding, so to prevent the
non-freestanding ones being included we should add to them:

#if !_GLIBCXX_HOSTED
# error "This header is not supported in freestanding mode"
#endif

Then we can teach the testsuite to recognise that error and skip a test as
UNSUPPORTED. That will allow the full testsuite to be run without changing
thousands of individual tests with something like "{
dg-require-effective-target hosted }". The tests that aren't valid for
freestanding will just be UNSUPPORTED when -ffreestanding is in the
runtestflags.

We might need to also strip -ffreestanding from the flags when building the
libtestc++.a support lib used by the testsuite, because that lib requires
hosted features.

This all seems doable though.