[PATCH] Link static C++ tests against libatomic.a if needed

[Adhemerval Zanella Netto]

On 05/05/26 09:33, H.J. Lu wrote:
> On Tue, May 5, 2026 at 8:27 PM Adhemerval Zanella Netto
> <adhemerval.zanella@linaro.org> wrote:
>>
>>
>>
>> On 04/05/26 22:50, H.J. Lu wrote:
>>> On Tue, May 5, 2026 at 9:42 AM Sam James <sam@gentoo.org> wrote:
>>>>
>>>> "H.J. Lu" <hjl.tools@gmail.com> writes:
>>>>
>>>>> On Mon, May 4, 2026 at 10:33 AM H.J. Lu <hjl.tools@gmail.com> wrote:
>>>>>>
>>>>>> Define cxx-static-link-libatomic to -latomic if DT_NEEDED entries in
>>>>>> libstdc++.so include libatomic.so.  Update link-extra-libs-static to
>>>>>> include $(cxx-static-link-libatomic) if -lstdc++ is used.
>>>>>>
>>>>>> Tested with sparcv8-linux-gnu-leon3 and sparcv9-linux-gnu using
>>>>>> uild-many-glibcs.py.  This fixes BZ #34121.
>>>>>>
>>>>>
>>>>> FWIW, sparc g++ does pass -latomic_asneeded to ld.  But glibc tests
>>>>> are built with -nostdlib -nostartfiles using gcc, not g++.
>>>>
>>>> https://inbox.sourceware.org/gcc-patches/CACb0b4m+ex47Tpyp1qvW9+0p4onyEvsfXeXE_4zZrteMO5q5EQ@mail.gmail.com/
>>>>
>>>> I don't remember what happened with that, there were a few issues with
>>>> the libatomic autolinking so I lost track.
>>>
>>> One way to fix the gcc is to create libstdc++.a as linker script, similar to
>>>
>>> [hjl@gnu-tgl-3 x86-glibc]$ cat /usr/lib64/libm.a
>>> /* GNU ld script
>>> */
>>> OUTPUT_FORMAT(elf64-x86-64)
>>> GROUP ( /usr/lib64/libm-2.43.a /usr/lib64/libmvec.a )
>>> [hjl@gnu-tgl-3 x86-glibc]$
>>>
>>> to include libatomic.a in libstdc++.a if needed.  But I don't know if it works
>>> everywhere.
>>>
>>
>> It seems gcc-16 has another issue, it also fails on building glibc after a bootstrap
>> with build-many-glibcs.py with:
>>
>> [..-m32 -mlong-double-128 -mcpu=v9 -o /home/azanella/Projects/glibc/build/sparcv9-linux-gnu-gcc16/support/links-dso-program -pie  -Wl,-O1 -nostdlib -nostartfiles [...]
>> [...]/sparc64-linux-gnu/sparc64-glibc-linux-gnu/bin/ld: [...]/sparc64-linux-gnu/bin/../lib/gcc/sparc64-glibc-linux-gnu/16.1.1/../../../../sparc64-glibc-linux-gnu/lib/../lib/libstdc++.so: undefined reference to `__atomic_fetch_add_4@LIBATOMIC_1.0'
>> [...]//sparc64-glibc-lisparc64-linux-gnunux-gnu/bin/ld: [...]/sparc64-linux-gnu/bin/../lib/gcc/sparc64-glibc-linux-gnu/16.1.1/../../../../sparc64-glibc-linux-gnu/lib/../lib/libstdc++.so: undefined reference to `__atomic_fetch_sub_4@LIBATOMIC_1.0'
>>
>> And dumping the linker invocation, it shows another issue:
>>
>> ld: warning: libatomic.so.1, needed by [...]/sparc64-linux-gnu/bin/../lib/gcc/sparc64-glibc-linux-gnu/16.1.1/../../../../sparc64-glibc-linux-gnu/lib/../lib/libstdc++.so, not found (try using -rpath or -rpath-link)
> 
> Which version of binutils are you using?  Binutils master branch works
> for me for sparc64-glibc-linux-gnu.

It is a pretty recent one:

$ ./bin/sparc64-glibc-linux-gnu-ld -v
GNU ld (GNU Binutils) 2.46.0.20260504

> 
>> It seems now that libstdc++.so is dynamic linker against libatomic.so:
>>
>> $ readelf -d ./sparc64-glibc-linux-gnu/lib/libstdc++.so | grep NEEDED
>>  0x00000001 (NEEDED)                     Shared library: [libm.so.6]
>>  0x00000001 (NEEDED)                     Shared library: [libatomic.so.1]
>>  0x00000001 (NEEDED)                     Shared library: [libc.so.6]
>>  0x00000001 (NEEDED)                     Shared library: [ld-linux.so.2]
>>  0x00000001 (NEEDED)                     Shared library: [libgcc_s.so.1]
>>
>> Where previously (gcc-15) it seems to be statically linked:
>>
>> $ readelf -d ./sparc64-glibc-linux-gnu/lib/libstdc++.so | grep NEEDED
>>  0x00000001 (NEEDED)                     Shared library: [libm.so.6]
>>  0x00000001 (NEEDED)                     Shared library: [libc.so.6]
>>  0x00000001 (NEEDED)                     Shared library: [ld-linux.so.2]
>>  0x00000001 (NEEDED)                     Shared library: [libgcc_s.so.1]
>>
> 
>