[Bug rtl-optimization/50065] -Os, -O2, -O3 optimization breaks LD/ST ordering on 32-bit SPARC

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

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50065

--- Comment #6 from Eric Botcazou <ebotcazou at gcc dot gnu.org> 2011-08-14 12:42:47 UTC ---
> The code is equivalent to
> 
> volatile unsigned char lock;
> int remap_barrier;
> 
> while (atomic_test_and_set(lock)) {
>    while (lock) {
>      ;
>    }
> }
> remap_barrier++;
> lock = 0;
> 
> Eric: could you let me know you you think the code inside function  
> spinlock_lock(&remap_lock) is a NOP?

I don't, you simply misquoted, I wrote "the end of the code".  The first part
of the spinlock implementation is correct, in particular you have the required
memory barrier in spinlock_is_locked.  The second part is not correct, as you
don't have the memory barrier in spinlock_unlock.

> Also, the arch_write_lock/unlock in the SPARC port of Linux uses a very 
> similar implementation.

No, it precisely doesn't, it has the memory barrier in spinlock_unlock.