[avr-gcc-list] Re: AVR byte swap optimization

[Shaun Jackman]

On 11/26/06, Denis Vlasenko <address@hidden> wrote:
> On Saturday 18 November 2006 00:30, Shaun Jackman wrote:
> > The following macro expands to some rather frightful code on the AVR:
> >
> > #define BSWAP_16(x) \
> >      ((((x) >> 8) & 0xff) | (((x) & 0xff) << 8))
>
> Sometimes gcc is generating better code if you cast
> values instead of masking. Try:
>
>       ( (uint8_t)((x) >> 8) | ((uint8_t)(x)) << 8 )

Your suggestion seemed like a good one and gave me some hope.
Unfortunately, it produces identical results to my BSWAP_16 macro. I
also tried the following:

        uint8_t a = x >> 8, b = x;
        return b << 8 | a;

Different register allocations this time, but identical instructions.

Cheers,
Shaun