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

[Eric Weddington]

You could always equate the macro to some inline assembly like what is done
for a number of avr-libc macros.

Eric Weddington 

> -----Original Message-----
> From: 
> address@hidden 
> [mailto:address@hidden
> org] On Behalf Of Shaun Jackman
> Sent: Friday, November 17, 2006 4:31 PM
> To: address@hidden; address@hidden
> Subject: [avr-gcc-list] AVR byte swap optimization
> 
> 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
> 
> 
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