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

[Jonathan Wakely <jwakely@redhat.com>]

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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? 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.

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commit eea73ea3bdd44a8f7d8c0f54b15bfba9058f6ce8
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Thu Aug 31 18:31:32 2023

    libstdc++: Avoid useless dependency on read_symlink from tzdb
    
    chrono::tzdb::current_zone uses filesystem::read_symlink, which creates
    a dependency on the fs_ops.o object in libstdc++.a, which then creates
    dependencies on several OS functions if --gc-sections isn't used.
    
    In the cases where that causes linker failures, we probably don't have
    readlink anyway, so the filesystem::read_symlink call will always fail.
    Repeat the preprocessor conditions for filesystem::read_symlink in the
    body of chrono::tzdb::current_zone so that we don't create the
    dependency on fs_ops.o if it's not even going to be able to read the
    symlink.
    
    libstdc++-v3/ChangeLog:
    
            * src/c++20/tzdb.cc (tzdb::current_zone): Check configure macros
            for POSIX readlink before using filesystem::read_symlink.

diff --git a/libstdc++-v3/src/c++20/tzdb.cc b/libstdc++-v3/src/c++20/tzdb.cc
index 0fcbf6a4824..24044bb60f8 100644
--- a/libstdc++-v3/src/c++20/tzdb.cc
+++ b/libstdc++-v3/src/c++20/tzdb.cc
@@ -1635,6 +1635,9 @@ namespace std::chrono
     // TODO cache this function's result?
 
 #ifndef _AIX
+    // Repeat the preprocessor condition used by filesystem::read_symlink,
+    // to avoid a dependency on src/c++17/tzdb.o if it won't work anyway.
+#if defined(_GLIBCXX_HAVE_READLINK) && defined(_GLIBCXX_HAVE_SYS_STAT_H)
     error_code ec;
     // This should be a symlink to e.g. /usr/share/zoneinfo/Europe/London
     auto path = filesystem::read_symlink("/etc/localtime", ec);
@@ -1653,6 +1656,7 @@ namespace std::chrono
 	      return tz;
 	  }
       }
+#endif
     // Otherwise, look for a file naming the time zone.
     string_view files[] {
       "/etc/timezone",    // Debian derivates