On 12/19/2016 01:14 PM, Szabolcs Nagy wrote:
> On 16/12/16 16:31, Stefan Liebler wrote:
>> Passing a volatile pointer to the atomic macros can lead to extra stores
>> and loads to stack if such a macro creates a temporary variable by using
>> __typeof (*(mem)). Thus the patch passes a int pointer to the atomic macros.
>
> the resolution of
> http://www.open-std.org/jtc1/sc22/wg14/www/docs/dr_423.htm
> says that the type of a cast expression is unqualified, so
>
>   __typeof ((__typeof (*(mem)))*(mem)) tmp = *(mem);
>
> would have the right type.. seems to work since gcc-5
>
> https://godbolt.org/g/eS0X5b
>
> (looks beautiful.)
>

Thanks for this.

I've adjusted the needed atomic-macros in include/atomic.h and use the 
typeof-construct "__typeof ((__typeof (*(mem)))*(mem)) tmp;".
Now we can pass the volatile int pointers to the atomic-macros.

See the updated patch.

Okay to commit?

Bye.
Stefan

ChangeLog:

	* include/atomic.h:
	(__atomic_val_bysize): Cast type to omit volatile qualifier.
	(atomic_exchange_acq): Likewise.
	(atomic_load_relaxed): Likewise.
	* nptl/pthread_spin_init.c (pthread_spin_init):
	Use atomic_store_relaxed.
	* nptl/pthread_spin_lock.c (pthread_spin_lock):
	Use C11-like atomic macros.
	* nptl/pthread_spin_trylock.c
	(pthread_spin_trylock): Likewise.  Use an explicit test if lock
	is free, if new SPIN_TRYLOCK_USE_CMPXCHG_INSTEAD_OF_XCHG macro
	is set to one.
	(SPIN_TRYLOCK_USE_CMPXCHG_INSTEAD_OF_XCHG): New define.
	* nptl/pthread_spin_unlock.c (pthread_spin_unlock):
	Use atomic_store_release.