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

[Anton Erasmus]

On 18 Dec 2006 at 11:28, Shaun Jackman wrote:

> 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.
> 

Hi,

Not a macro, but the following seems to generate reasonable code.

typedef struct
{
  unsigned char h;
  unsigned char l;
}B2;


typedef union
{
  B2 x;
  unsigned short us;
}BU;



inline unsigned short byteswap(unsigned short xx)
{
  BU tmp;
  unsigned char tmp1;

  tmp.us=xx;
  tmp1=tmp.x.h;
  tmp.x.h=tmp.x.l;
  tmp.x.l=tmp1;
  return(tmp.us);
}

volatile unsigned short gX;

void test(void)
{
   gX=byteswap(gX);

}
                           

Regards
  Anton Erasmus

-- 
A J Erasmus