Re: [Algorithms] Byte Swapping

[Charles B. <cb...@cb...>]

Hmm... I'm not sure what's wrong with it, it's some C order of operations
problem
that I'm not familiar with.  If you expand it out into simple steps :

y = x = (x>>16)|(x<<16);
y ^= (x = ((x^(x>>8)) & 0x00FF00FF));
y ^= (x<<8);

then it works just fine.  If you have four bytes ABCD, the first line
turns them into CDAB.  The second line gives you (0D0B^0C0A).  The
last line is thus (D0B0^C0A0).  Put them all together :

y = CDAB^0D0B^0C0A^D0B0^C0A0 = DCBA

Anyway, this is all just a curiosity because I don't think that's the
most efficient way to do it :^)

At 12:36 PM 7/18/2000 +1000, you wrote:
>It looks like the way this is meant to work is that it swaps the top and
>bottom 16 bits then proceeds to do the old 3 xor byte swapping  trick
>(x^=y^=x) for each of the two 16 bit segments but I think the code is a
>little bit broken. I can't see how that second bit works.
>
>
>-----Original Message-----
>From: brian sharon <br...@hu...>
>To: gda...@li...
><gda...@li...>
>Date: Tuesday, 18 July 2000 12:02
>Subject: Re: [Algorithms] Byte Swapping
>
>
>>
>>> From: Charles Bloom <cb...@cb...>
>>>
>>> Jumping in late : I'm sure you don't need it now, but
>>> here's a crazy ANSI C byte-swapper :
>>>
>>> let x = a 32 bit ulong , ABCD
>>> then y = DCBA
>>>
>>> y = (x = x>>16 | x<<16) ^ (x = (x^x>>8) & 0x00FF00FF) ^ (x<<8);
>>>
>>
>>wow, I'll say that's crazy...but unfortunately non-functional.  see the
>>results of this program:
>>
>>#include <stdio.h>
>>
>>long swap1(long x)
>>{
>>    return (x = x>>16 | x<<16) ^ (x = (x^x>>8) & 0x00FF00FF) ^ (x<<8);
>>}
>>
>>long swap2(long x)
>>{
>>    return         ((x >> 24) & 0x000000FF)
>>            |    ((x >> 8 ) & 0x0000FF00)
>>            |    ((x << 8 ) & 0x00FF0000)
>>            |    ((x << 24) & 0xFF000000)
>>            ;
>>}
>>
>>int main(int argc, char **argv)
>>{
>>    long test = 0x01020304;
>>
>>    printf("swap1: 0x%08X 0x%08X 0x%08X \n", test, swap1(test),
>>swap1(swap1(test)));
>>
>>    printf("swap2: 0x%08X 0x%08X 0x%08X \n", test, swap2(test),
>>swap2(swap2(test)));
>>}
>>
>>swap1: 0x01020304 0x07000300 0x03000700
>>swap2: 0x01020304 0x04030201 0x01020304
>>
>>
>>
>>anyway, in case anyone hasn't gotten their fill of this stuff, here is our
>>comparatively lame byteswapping code that uses those fast swappers on
>>PowerPC:
>>
>>#ifndef _MBSWAP_H_
>>#define _MBSWAP_H_
>>
>>// swapping macros/inlines
>>#include <Runtime/MBBase.h>
>>
>>inline MBint16 MBSwap(const MBint16 & swap)
>>{
>>#if defined(MB_MAC) && defined(MB_POWERPC)
>>    return __lhbrx(const_cast<MBint16 *>(&swap), 0);
>>#else
>>    return         ((swap >> 8 ) & 0x00FF)
>>            |    ((swap << 8 ) & 0xFF00)
>>            ;
>>#endif
>>}
>>
>>inline MBuint16 MBSwap(const MBuint16 & swap)
>>{
>>#if defined(MB_MAC) && defined(MB_POWERPC)
>>    return __lhbrx(const_cast<MBuint16 *>(&swap), 0);
>>#else
>>    return         ((swap >> 8 ) & 0x00FF)
>>            |    ((swap << 8 ) & 0xFF00)
>>            ;
>>#endif
>>}
>>
>>inline MBint32 MBSwap(const MBint32 & swap)
>>{
>>#if defined(MB_MAC) && defined(MB_POWERPC)
>>    return __lwbrx(const_cast<MBint32 *>(&swap), 0);
>>#else
>>    return         ((swap >> 24) & 0x000000FF)
>>            |    ((swap >> 8 ) & 0x0000FF00)
>>            |    ((swap << 8 ) & 0x00FF0000)
>>            |    ((swap << 24) & 0xFF000000)
>>            ;
>>#endif
>>}
>>
>>inline MBuint32 MBSwap(const MBuint32 & swap)
>>{
>>#if defined(MB_MAC) && defined(MB_POWERPC)
>>    return __lwbrx(const_cast<MBuint32 *>(&swap), 0);
>>#else
>>    return         ((swap >> 24) & 0x000000FF)
>>            |    ((swap >> 8 ) & 0x0000FF00)
>>            |    ((swap << 8 ) & 0x00FF0000)
>>            |    ((swap << 24) & 0xFF000000)
>>            ;
>>#endif
>>}
>>
>>inline MBfloat MBSwap(const MBfloat & swap)
>>{
>>    MBuint32 temp = *(const MBuint32*)&swap;
>>    temp = MBSwap(temp);
>>    return *(const MBfloat *)&temp;
>>}
>>
>>#ifdef MB_MSB
>>
>>    #define MBSwapMSBToNative( s ) s
>>    #define MBSwapNativeToMSB( s ) s
>>
>>    #define MBSwapLSBToNative MBSwap
>>    #define MBSwapNativeToLSB MBSwap
>>
>>#elif defined(MB_LSB)
>>
>>    #define MBSwapLSBToNative( s ) s
>>    #define MBSwapNativeToLSB( s ) s
>>
>>    #define MBSwapMSBToNative MBSwap
>>    #define MBSwapNativeToMSB MBSwap
>>
>>#endif
>>
>>#endif // header guard
>>
>>
>>
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>>
>
>
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--------------------------------------
Charles Bloom           www.cbloom.com