Bug#727621: armv5 and ATOMIC_INT_LOCK_FREE

[Yvan Roux <yvan.roux@linaro.org>]

On 3 December 2013 01:19, Nicolas Pitre <nicolas.pitre@linaro.org> wrote:
> On Mon, 2 Dec 2013, Riku Voipio wrote:
>
>> Hi,
>>
>> According the debian bug report [1], it is not possible to use std::future
>> on armv5 targetting toolchains. This is because libstdc++ will only enable
>> std::future if ATOMIC_INT_LOCK_FREE > 1.  There is no LDREX for armv5 and
>> older, so this definition is set to ATOMIC_INT_LOCK_FREE when compiling for
>> ARMv4t or ARMv5.
>>
>> My impression is that you should be able to use the kernel helpers for
>> atomic operations in lockfree (?) manner, so the ATOMIC_INT_LOCK_FREE
>> definition is probably incorrect on older arm archs then?
>
> You're absolutely right.
>
> Please see Documentation/arm/kernel_user_helpers.txt in the kernel
> source tree for details.  There is even an example implementation for
> atomic_add() there.
>
> Also I remember gcc being added some native atomic operation support at
> some point and they were implemented in terms of the kernel helpers when
> the architecture level wasn't high enough to benefit from load/store
> exclusive instructions.
>
>
> Nicolas


Nicolas is right, GCC has now a set of bult-in functions which try to
match the Cxx11 memory model.  The description is in the manual here:

http://gcc.gnu.org/onlinedocs/gcc-4.8.2/gcc/_005f_005fatomic-Builtins.html#_005f_005fatomic-Builtins

Yvan