Re: [avr-gcc-list] Avr-gcc Produces Incorrect Code with -Os

[Mark Litwack]

On Friday 16 May 2008 08:05:41 am Paulo Marques wrote:
> ....
>
> Note! This is not even the worst case for this optimization. The worst
> case I've seen is something like:
>
> int my_flag;
>
> ....
> cli();
> my_flag = 0x1234;
> sei();
> ....
>
> Contrary to what you might expect, that my_flag access isn't being
> protected by the cli/sei pair, and might be reordered by the compiler
> (and "might" here means "it has happened", not some theoretical scenario).
>
> Currently, the official way of doing something like that is using the
> macros provided in <util/atomic.h>.

Wow.  I think the cli()/sei() construct is used fairly
universally, and it's also in the avr-libc docs.

What gives ATOMIC_BLOCK() it's special immunity?  The memory
clobber?

-mark