swap bytes in a ushort/ulong

[Jurgen Eidt 0]

What is the fasted way to swap bytes in a ushort/ulong? For example: 0x1234 -> 0x3412 0x12345678 -> 0x78563412

unsigned short short_exch(const unsigned short a)
{
    // 0x1234 -> 0x3412
    return (a << 8) + (a >> 8);
};


unsigned long long_exch(unsigned long a)
{
    // 0x12345678 -> 0x78563412

    unsigned long t = a&0xff;
    a >>= 8;
    t <<= 8;
    t += a&0xff;

    a >>= 8;
    t <<= 8;
    t += a&0xff;

    a >>= 8;
    t <<= 8;

    return t + a;
};

Jürgen Eidt http://cpicture.de/en

[Rick York]

Try a union : typedef union { ULONG itemlong; UCHAR itemchar[4]; } UnionLong; then you can swap the bytes through a temporary variable. a two cent stamp short of going postal.

[Jurgen Eidt 0]

Didn't thought of a union :) But even swapping with the xor trick I'm not sure how the compiler optimization would be. I guess looking at the asm code would make the final judgement. Thanks. Jürgen Eidt http://cpicture.de/en

[Gary R Wheeler]

unsigned long Swap(unsigned long value)
{
return ((value & 0xFF000000) >> 24) |
(value & 0x00FF0000) >> 8) |
(value & 0x0000FF00) << 8) |
(value & 0x000000FF) << 24);
}

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Software Zen: delete this;

[Jurgen Eidt 0]

Thanks Gary, that a good way to do it! Jürgen Eidt http://cpicture.de/en

[Gary R Wheeler]

You're welcome. If you make the function inline, the compiler will compile it in place without the function call overhead (I forgot to do that).

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Software Zen: delete this;

[Jurgen Eidt 0]

Of course inline! Actually you can remove the masking for the 24 bit shifts. For the inner two bytes I don't see an improvement. Thanks Gary for the follow up :) Jürgen Eidt http://cpicture.de/en