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

[Paulo Marques]

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.

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.

--
Paulo Marques
Software Development Department - Grupo PIE, S.A.
Phone: +351 252 290600, Fax: +351 252 290601
Web: www.grupopie.com

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