Re: [PATCH] libstdc++: Use GLIBCXX_CHECK_LINKER_FEATURES for cross-builds (PR111238)

[Jonathan Wakely <jwakely@redhat.com>]

On Thu, 31 Aug 2023 at 18:42, Jonathan Wakely <jwakely@redhat.com> wrote:
>
> On Thu, 31 Aug 2023 at 16:26, Christophe Lyon
> <christophe.lyon@linaro.org> wrote:
> >
> > As discussed in PR104167 (comments #8 and below), and PR111238, using
> > -Wl,-gc-sections in the libstdc++ testsuite for arm-eabi
> > (cross-toolchain) avoids link failures for a few tests:
> >
> > 27_io/filesystem/path/108636.cc
>
> I think this one probably just needs { dg-require-filesystem-ts "" }
> because there's no point testing that we can link to the
> std::filesystem definitions if some of those definitions are unusable
> on the target.
>
> // { dg-additional-options "-Wl,--gc-sections" { target gc_sections } }
>
> For the rest of them, does the attached patch help?

I've tested the patch for an arm-eabi cross compiler, and it fixes the
linker errors.

It doesn't change the fact that almost any use of the std::filesystem
APIs will hit the linker errors and so require users to link with
--gc-sections (or provide stubs for the missing functions) but that's
for users to deal with (if anybody using newlib targets is even making
use of those std::filesystem APIs anyway). With the patch to tzdb.cc
we don't need to change how libstdc++ is tested for the arm-eabi cross
target.

> If arm-eabi
> doesn't define _GLIBCXX_HAVE_READLINK then there's no point even
> trying to call filesystem::read_symlink. We can avoid a useless
> dependency on it by reusing the same preprocessor condition that
> filesystem::read_symlink uses.
>
> > std/time/clock/gps/1.cc
> > std/time/clock/gps/io.cc
> > std/time/clock/tai/1.cc
> > std/time/clock/tai/io.cc
> > std/time/clock/utc/1.cc
> > std/time/clock/utc/io.cc
> > std/time/clock/utc/leap_second_info.cc
> > std/time/exceptions.cc
> > std/time/format.cc
> > std/time/time_zone/get_info_local.cc
> > std/time/time_zone/get_info_sys.cc
> > std/time/tzdb/1.cc
> > std/time/tzdb/leap_seconds.cc
> > std/time/tzdb_list/1.cc
> > std/time/zoned_time/1.cc
> > std/time/zoned_time/custom.cc
> > std/time/zoned_time/io.cc
> > std/time/zoned_traits.cc
> >
> > This patch achieves this by calling GLIBCXX_CHECK_LINKER_FEATURES in
> > cross-build cases, like we already do for native builds. We keep not
> > doing so in Canadian-cross builds.
> >
> > However, this would hide the fact that libstdc++ somehow forces the
> > user to use -Wl,-gc-sections to avoid undefined references to chdir,
> > mkdir, chmod, pathconf, ... so maybe it's better to keep the status
> > quo and not apply this patch?
>
> I'm undecided about this for now, but let's wait for HP's cris-elf
> testing anyway.