[PATCH] fixing SSE detection in solid

[Thiago Macieira]

On Friday 08 February 2008 17:20:08 Friedrich W. H. Kossebau wrote:
> Am Freitag, 8. Februar 2008, um 15:10 Uhr, schrieb Thiago Macieira:
> > On Friday 08 February 2008 07:27:52 Tobias Koenig wrote:
> > > Hmm isn't msb the bit that represents 2^0? You see I'm not good at
> > > binary level ;)
> >
> > MSB = Most significant bit.
>
> As in: the bit which makes the greatest change in the value if swapped.
>
> > 2^0 is always LSB (Least significant bit).
>
> As in: the bit which makes the smallest change.
>
> > Little-endian == LSB-first or LSB left or left-to-right
> > Big-endian == MSB-first or MSB left or right-to-left (like our normal
> > numbers)
>
> You mixed up bytes and bits ;)

No, I didn't. The only thing is that you only get access to the bits by the 
byteful. So you never know how the memory arranges them. :-)

Try an architecture that supports both endiannesses and then try sharing 
memory between two different programs. At least on the IA-64, you'll see all 
bits inverted, not just the bytes (like ntohl/htonl).

This is specially important with bitfield structures. Here's an example, 
extracted from /usr/include/arpa/nameser_compat.h:

typedef struct {
        unsigned        id :16;         /* query identification number */
#if BYTE_ORDER == BIG_ENDIAN
                        /* fields in third byte */
        unsigned        qr: 1;          /* response flag */
        unsigned        opcode: 4;      /* purpose of message */
        unsigned        aa: 1;          /* authoritive answer */
        unsigned        tc: 1;          /* truncated message */
        unsigned        rd: 1;          /* recursion desired */
                        /* fields in fourth byte */
        unsigned        ra: 1;          /* recursion available */
        unsigned        unused :1;      /* unused bits (MBZ as of 4.9.3a3) */
        unsigned        ad: 1;          /* authentic data from named */
        unsigned        cd: 1;          /* checking disabled by resolver */
        unsigned        rcode :4;       /* response code */
#endif
#if BYTE_ORDER == LITTLE_ENDIAN || BYTE_ORDER == PDP_ENDIAN
                        /* fields in third byte */
        unsigned        rd :1;          /* recursion desired */
        unsigned        tc :1;          /* truncated message */
        unsigned        aa :1;          /* authoritive answer */
        unsigned        opcode :4;      /* purpose of message */
        unsigned        qr :1;          /* response flag */
                        /* fields in fourth byte */
        unsigned        rcode :4;       /* response code */
        unsigned        cd: 1;          /* checking disabled by resolver */
        unsigned        ad: 1;          /* authentic data from named */
        unsigned        unused :1;      /* unused bits (MBZ as of 4.9.3a3) */
        unsigned        ra :1;          /* recursion available */
#endif

The "aa" field is the third bit in the little-endian order. It is accessed as 
follows in Q3Dns:
    bool aa = (answer[2] & 4) != 0;

Same for the "rd" field:
   p[2] = 1; // recursion desired, rest is 0
 
> Endianess is about bytes, not bits (at least with the machines we care
> about AFAIK). Cmp. http://en.wikipedia.org/wiki/Endianness
>
> Inside a byte the MSB is the first and the LSB is the last bit.

The example above disproves that. The LSB is "rd", which is last in big endian 
architectures.

-- 
  Thiago Macieira  -  thiago (AT) macieira.info - thiago (AT) kde.org
    PGP/GPG: 0x6EF45358; fingerprint:
    E067 918B B660 DBD1 105C  966C 33F5 F005 6EF4 5358
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