libstdc++ DT_NEEDED on libatomic after PR81358

[Joseph Myers]

On Fri, 10 Oct 2025, Jonathan Wakely wrote:

> On Fri, 10 Oct 2025 at 21:11, Joseph Myers wrote:
> 
> > On Fri, 10 Oct 2025, Matthew Malcomson wrote:
> >
> > > Questions for others:
> > > 1) We'd like to double-check that this sounds like a problem before
> > trying to
> > > change it.
> > > 2) Does this trigger thoughts of anything else to look out for?
> > > 3) Would the best solution be to add some `--as-needed` push/pop state
> > > arguments into the link line?
> >
> > Yes, I'd expect libstdc++ to be linked (DT_NEEDED) with libatomic only if
> > it actually has undefined references that libatomic resolves.
> >
> >
> We try quite hard to avoid libstdc++ having any undefined references to
> libatomic functions. The atomics inside the library are defined in terms of
> the config/cpu/**/atomicity.h functions, which only operate on int and
> which are defined using inline assembly (and spinlocks when needed). That's
> partly because those functions predate libatomic, but partly because it
> makes libstdc++ self-sufficient.
> 
> So until now, users only needed to link to libatomic if their own code used
> atomics on types that require libatomic calls, but libstdc++.so doesn't do
> that.

My build-many-glibcs.py bot is showing a testsuite build failure for 
sparcv9-linux-gnu, where static libstdc++ has an undefined reference to 
__atomic_fetch_add_4, resulting in failure to link 
nptl/tst-cancel24-static.  Either it's a bug that these undefined 
references have appeared, or else the glibc build system needs to know 
about linking such static tests with -latomic as well as -lstdc++ (glibc 
tests necessarily use -nostdlib, so GCC knowing about linking with 
-latomic as needed by default doesn't help them).  (The shared libstdc++ 
for the same target has DT_NEEDED for libatomic, but it's a static test 
that fails to link, static libraries not having any such way to 
communicate their dependencies on other libraries.)

-- 
Joseph S. Myers
josmyers@redhat.com