Re: [avr-chat] Missed Optimisation ?

[bob]

On 01/03/11 15:33, Colin O'Flynn wrote:
> Hi Bob,
> 
> One consideration is that reading or writing might have side-effects on a
> memory-mapped peripheral.
> 
> So if you were accessing a peripheral with a pointer to a volatile uint32_t,
> and issue the |= command, you might actually be expecting the CPU to perform
> the memory read & write. The simple act of reading the memory could clear a
> flag for example.
> 
> My personal thought process with the volatile keyword is I assume the
> compiler does *all* memory accesses, even the unnecessary ones.
> 
> Regards,
> 
>   -Colin
Hi Colin,
by this logic the missing 'OR' instructions could also affect external
hardware as they are read-modify-write, but these are optimised away.

Cheers,

Bob