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

[Larry Barello]

Although a bit wordy, the following produces good code under a lot of
different circumstances.  It allows the compiler to shove things around &
optimize.


static inline uint16_t ByteSwap(uint16_t data)
{
        union byteswap
        {
                uint16_t word;
                struct
                {
                        uint8_t low;
                        uint8_t high;
                };
        } foo;
        foo.word = data;

        uint8_t t = foo.high;
        foo.high = foo.low;
        foo.low = t;
        return foo.word;
}


| -----Original Message-----
| From: address@hidden [mailto:avr-gcc-
| address@hidden On Behalf Of Shaun Jackman
| Sent: Friday, November 17, 2006 3:31 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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