Re: [avr-gcc-list] Re: Optimisation of bit set/clear in uint32_t

[Dave Hylands]

Hi Alex,

On Wed, Apr 22, 2009 at 5:51 AM, Alex Wenger <address@hidden> wrote:
...snip...
>>> Try the following version of set_status:
>>>
>>> void set_status(uint32_t flag, uint8_t set) {
>>>      if (set) *(uint32_t* &status) |= flag;
>>>      else *(uint32_t* &status) &= ~flag;
>>>      asm("" : : : "memory");
>>> }
...snip...
> Why? The memory clobber does exactly what you need for using it in an
> interrupt and outside.

So what exactly does

     asm("" : : : "memory");

do?

Why is it significant for use inside an interrupt handler?

I'm guessing that it prevents re-ordering, but it isn't clear to me
why reordering would be bad in this situation.

-- 
Dave Hylands
Shuswap, BC, Canada
http://www.DaveHylands.com/