Re: [Mspgcc-users] Atomic bit-twiddling instructions

[David B. <da...@we...>]

On 23/08/2012 22:52, William Swanson wrote:
> On Thu, Aug 23, 2012 at 12:49 PM, William Swanson <swa...@gm...> wrote:
>>> Another option is a single inline assembly call for the "|=" if the above
>>> code split does not help.
>>
>> I've had to do this in a few places, like when I need to atomically
>> update a hardware flag.
>
> I just realized something horrible. Code like this is not safe:
>
>    P1IFG &= ~P1_PWR_SW;
>
> There is no assurance that the "&=" operator will work atomically.
> Even if it does compile to the correct "bis.b" instruction, what
> happens if another IO pin toggles mid-instruction? Could the
> transition be lost? Hopefully the chip designers accounted for this,
> but it's still a scary thought.
>

There is no guarantee that the compiler will produce a single "bis" 
instruction for such code (though it normally will) - as I said before, 
"volatile" gives you very few guarantees according to the standards.