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

[Adhemerval Zanella Netto]

On 05/05/26 11:08, H.J. Lu wrote:
> On Tue, May 5, 2026 at 8:36 PM Adhemerval Zanella Netto
> <adhemerval.zanella@linaro.org> wrote:
>>
>>
>>
>> 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
> 
> For some reason, my ld works:
> 
> $ ./install/compilers/sparc64-linux-gnu/bin/sparc64-glibc-linux-gnu-ld -V
> GNU ld (GNU Binutils) 2.46.50.20260503
>   Supported emulations:
>    elf64_sparc
>    elf32_sparc
> 
> Please try the enclosed patch.  If it works for you, I will change it
> for both dynamic and static C++ executables.
> 

I still think the resulting libstdc++ being linked against libatomic.so is
gcc build issue due some missing flags (sparcv9 should have all required C11
atomic instruction that gcc should not be required to use libatomic).

And I think we will need to evaluate better if an ABI does require libatomic
for some reason.  One of the reason we remove sparcv8 and older was exactly 
because the ISA lacks proper atomic operations.