Disable optimizations on one function (was: 'pragma optimize' ...)

[Jeffrey Walton]

>> Many folks try and cast ptr to volatile, but that's an abuse because
>> GCC considers volatile something for memory mapped hardware. Volatile
>> should not be used in an attempt to tame the optimizer.
>
> GCC does not consider volatile to be something for memory mapped
> hardware.

OK, this appears to be creating a moving definition (or the definition
has changed since I took note of it). I took the last definition from
Ian Lance Taylor. See http://www.airs.com/blog/archives/154 and, for
example, https://gcc.gnu.org/ml/gcc-help/2012-03/msg00257.html.

> ...
> If you don't mind being nonportable, a memoryclobber should do it:
>
>     memset(ptr, 0x00, n);
>     asm volatile("": : : "memory");
>
> or even:
>
>     memset(ptr, 0x00, n);
>     __atomic_thread_fence(__ATOMIC_RELAXED);

Perfect, thanks.

Jeff