> -----Original Message-----
> From: Paulo Marques [mailto:[EMAIL PROTECTED]
> Sent: Friday, May 16, 2008 12:07 PM
> To: Weddington, Eric
> Cc: Mark Litwack; [email protected]; Joerg Wunsch
> Subject: Re: [avr-gcc-list] Avr-gcc Produces Incorrect Code with -Os
>
> Weddington, Eric wrote:
> > [...]
> >> If ATOMIC_BLOCK() is *the* way to do it, perhaps the docs
> >> could be improved a little by emphasizing the use of
> >> ATOMIC_BLOCK() as a necessity rather than as a convenience.
> >
> > I'll go a step further: Is there any reason to NOT include
> the memory
> > barrier in the interrupt macros? Would that even work?
> >
> > Like this,
> >
> > # define sei() __asm__ __volatile__ ("sei" ::: "memory")
> > # define cli() __asm__ __volatile__ ("cli" ::: "memory")
>
> This has been discussed before (on the avr-libc list, December 2005,
> just search for '"cli" and "sei" should clobber memory'), and
> this exact
> solution was actually proposed by me.
Oh! Thanks for the reference! :-) That was a while ago...
> The downside is that a memory clobber like that drops all
> optimizations
> of global variables that the compiler might be temporarily storing in
> registers. So, it actually hurts performance for code
> unrelated to the
> critical section.
>
> Another thing we could do to help the common cases of reading
> / writing
> multi-byte vars (or even single byte non-volatile vars) was to write
> accessors like (totaly untested):
>
> static inline uint16_t atomic_read16(uint16_t *var)
> {
> uint16_t ret;
> uint8_t old_sreg = SREG;
> cli();
> ret = *((volatile uint16_t *)var);
> SREG = old_sreg;
> }
>
> static inline void atomic_write16(uint16_t *var, uint16_t value)
> {
> uint8_t old_sreg = SREG;
> cli();
> *((volatile uint16_t *)var) = value;
> SREG = old_sreg;
> }
>
> ....(repeat for other sizes)....
>
> I'd have to check this code better, because I always end up
> placing the
> volatile in the wrong place and casting the pointer to
> volatile instead
> of the variable ;)
>
> Anyway, the advantage of something like this over
> ATOMIC_BLOCK() is that
> it doesn't need any memory clobbers, since all operations are already
> volatile. So, this should produce slightly better code.
Interesting...
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