Checked on x86_64-linux-gnu.

Reviewed-by: Sam James <sam@gentoo.org>

Although binutils has supported --no-undefined-version for a long timei
(319416359200 back in 2002), --undefined-version was only added more
recently (27fb6a1a7fcd on 2.40).

Reviewed-by: Sam James <sam@gentoo.org>

The check was initially used to define HAVE_BUILTIN_REDIRECTION, which
enables or not libc_hidden_builtin_proto support. It was later removed
with 3ce1f2959437e952b9db4eaeed2407424f11a4d1, making the feature
mandatory. The configure check was kept as a transition knob.

Current minimum gcc/linker always supports this, as well as clang with
some extra care. Also, missing hidden_proto/hidden_def support is
already flagged in the check-localplt test.

Reviewed-by: H.J. Lu <hjl.tools@gmail.com>

The 53807741fb44edb8e7c094cb5e7d4ff4e92a6ec1 added a configure check
for 64-bit atomic operations that were not previously enabled on some
32-bit ABIs.

However, the NPTL semaphore code casts a sem_t to a new_sem and issues
a 64-bit atomic operation for __HAVE_64B_ATOMICS.  Since sem_t has
32-bit alignment on 32-bit architectures, this prevents the use of
64-bit atomics even if the ABI supports them.

Assume 64-bit atomic support from __WORDSIZE, which maps to how glibc
defines it before the broken change.  Also rename __HAVE_64B_ATOMICS
to USE_64B_ATOMICS to define better the flag meaning.

Checked on x86_64-linux-gnu and i686-linux-gnu.

Reviewed-by: Wilco Dijkstra  <Wilco.Dijkstra@arm.com>

clang might generate an abort call when cleanup functions (set by
__attribute__ ((cleanup)) calls functions not marked as nothrow.

The hurd already provides abort for the loader at
sysdeps/mach/hurd/dl-sysdep.c, and adding it rtld-stubbed-symbols
triggers duplicate symbols.

Reviewed-by: H.J. Lu <hjl.tools@gmail.com>

Some symbols that might be auto-generated by the compiler are redefined
to internal alias (for instance mempcpy to __mempcpy).  However, if fortify
is enabled, the fortify wrapper is define before the alias re-defined and
clang warns attribute declaration must precede definition.

Use an asm alias if compiler does not support it, instead of an
attribute.

Reviewed-by: H.J. Lu <hjl.tools@gmail.com>

Now that atomic builtins are used by default, we can rely on the
compiler to define when to use 64-bit atomic operations.

It allows the use of 64-bit atomic operations on some 32-bit ABIs where
they were not previously enabled due to missing pre-processor handling:
hppa, mips64n32, s390, and sparcv9.

Co-authored-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>
Reviewed-by: Uros Bizjak <ubizjak@gmail.com>
Reviewed-by: Wilco Dijkstra  <Wilco.Dijkstra@arm.com>

Reviewed-by: Collin Funk <collin.funk1@gmail.com>

clang does not support the flag.

Reviewed-by: Sam James <sam@gentoo.org>

The configure check always fail with clang:

conftest.c:5:58: error: expected string literal as argument of 'alias' attribute
    5 | extern __typeof (__foo) foo __attribute__ ((weak, alias (__foo)));
      |                                                          ^
conftest.c:6:58: error: expected string literal as argument of 'alias' attribute
    6 | extern __typeof (__foo) bar __attribute__ ((weak, alias (foo)));
      |                                                          ^

Reviewed-by: Sam James <sam@gentoo.org>