re: __atomic_test_and_set() and mips o32 - help wanted

On Sat, 22 Nov 2025, matthew green wrote:

> >  Hmm, so where does the original issue reported with this thread come 
> > from?  It seems like something that should have been long sorted.
> 
> it comes from GDB calling C++'s "test_and_set()" which in GCC 14's
> libstdc++ is implemented as direct calls to __atomic_test_and_set().
> 
>    libgdb.a(minsyms.o): in function `minimal_symbol_reader::install()':
>    minsyms.c:(.text+0x4224): undefined reference to `__atomic_test_and_set'

 Ah, I see.  I think I got confused by your original description.

 So I investigated it a little and this is a compiler intrinsic, for which
to be available the backend needs to implement one of four alternative RTL 
machine instructions, in this order of preference: `atomic_test_and_set',
`atomic_exchange', `atomic_compare_and_swap' or `sync_lock_test_and_set'.

 The MIPS backend implements `atomic_exchange' with an LL/SC loop or, for 
the XLP CPU (that was an interesting one!), with the SWAP instruction.  
It also implements `atomic_compare_and_swap' and `sync_lock_test_and_set', 
with LL/SC.  There's no fallback to a libcall implemented AFAICT, hence 
the link failure, and the API would have to be provided by libatomic.  I 
can only see code for an ARM target there.

 A couple more atomic functions might be good having provided then beyond 
`__atomic_test_and_set' for better code generation in the absence of LL/SC 
(be it real or emulated).

  Maciej