[Bug target/78357] nios2 uses non-standard atomic functions

[cltang at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78357

--- Comment #3 from Chung-Lin Tang <cltang at gcc dot gnu.org> ---
(In reply to Sebastian Huber from comment #2)
> (In reply to Chung-Lin Tang from comment #1)
> > Sebastian, I'm not sure what your problem is.  The atomics in nios2 are
> > implemented by __sync_* functions placed in libgcc. The built-in function
> > expansion inside GCC is aware of this, and __atomic_* functions get mapped
> > to those. Is there anything affecting your use?
> 
> I think this mapping of __atomic_* functions to __sync_* functions is
> wrong/outdated.  Is this a speciality of the Nios II support?  I am not
> aware of a second target support in GCC which does this.  The __sync_*
> builtins are a legacy API:
> 
> https://gcc.gnu.org/onlinedocs/gcc/_005f_005fsync-Builtins.
> html#g_t_005f_005fsync-Builtins

Nios II is not the only target which implements __sync_*, see the libgcc source
code for details.

In the current GCC code expansion paths, the __atomic_* functions are meant to
expand to hardware instruction sequences.  In cases where we need to generate
library calls for atomics, GCC only generates __sync_* calls.

> > 
> > libatomic is usually supported by those targets with more "rich" atomic
> > instructions, and nios2 in its current form is obviously not one of them.
> 
> The libatomic is for architectures which lack atomic instructions.

To clarify/correct my above statement, we do build libatomic like other
targets, but the basic __atomic_* functions used inside it is also generated as
__sync_* calls.

libatomic does NOT directly "implement" the basic __atomic_* operations, that's
supposed to be done inside the compiler.

Can you more specifically describe what you're trying to do? Or is this just a
general query?