[Bug target/104338] RISC-V: Subword atomics result in library calls

["palmer at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104338

--- Comment #5 from palmer at gcc dot gnu.org ---
(In reply to rvalue from comment #4)
> In short term, maybe we can change the spec to link against libatomic by
> default (implemented in
> https://github.com/riscv-collab/riscv-gcc/commit/
> 2c4857d0981501b7c50bbf228de9e287611f8ae5). It will solve a lot of build
> errors if we revert the value of `LIB_SPEC` instead of only link against
> libatomic when `-pthread` is present.
> 
> Detailed talk about this:
> https://github.com/riscv-collab/riscv-gcc/issues/337

We talked through some options like that and decided it was too risky for
GCC-12.  We already found one ABI break related to this (see 84568), and want
to make sure we give distros adequate advance notice before something that we
know to break ABIs.

That said, it's really not a GCC ABI break, it's a per-package configure issue.
 We can fix the libstdcxx fallout, which is the only bit we know about right
now (though it's not like we've scrubbed builds for this).  If the folks
building distros think it's better to risk the ABI breaks rather than chase
around the build failures, then I'm fine rushing something in to GCC-12.

I see Andreas is already here, I'm having some trouble adding anyone else
though (I can never quite figure out Bugzilla...).