[avr-gcc-list] AVR byte swap optimization

[Shaun Jackman]

The following macro expands to some rather frightful code on the AVR:

#define BSWAP_16(x) \
    ((((x) >> 8) & 0xff) | (((x) & 0xff) << 8))

uint16_t bswap_16(uint16_t x)
{
  0:    9c 01           movw    r18, r24
  2:    89 2f           mov     r24, r25
  4:    99 27           eor     r25, r25
  6:    32 2f           mov     r19, r18
  8:    22 27           eor     r18, r18
        return BSWAP_16(x);
}
  a:    82 2b           or      r24, r18
  c:    93 2b           or      r25, r19
  e:    08 95           ret

Ideally, this macro would expand to three mov instructions and a ret.
Is there anything I can do to help GCC along here? I'm using GCC 4.1.0
with -O2.

I won't bother to show bswap_32 here, which produces a real disaster!
Think 47 instructions, for what should be 6.

Cheers,
Shaun

$ avr-gcc --version |head -1
avr-gcc (GCC) 4.1.0