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    SubjectRe: [Cbe-oss-dev] [RFC, PATCH 4/4] Add support to OProfile for profiling Cell BE SPUs -- update
    On Wednesday 31 January 2007 00:31, Carl Love wrote:
    > Unfortunately, the only way we know how to
    > figure out what the LFSR value that corresponds to the number in the
    > sequence that is N before the last value (0xFFFFFF) is to calculate the
    > previous value N times.  It is like trying to ask what is the pseudo
    > random number that is N before this pseudo random number?

    Well, you can at least implement the lfsr both ways, and choose the one
    that is faster to get at, like

    u32 get_lfsr(u32 v)
    int i;
    u32 r = 0xffffff;
    if (v < 0x7fffff) {
    for (i = 0; i < v; i++)
    r = lfsr_forwards(r);
    } else {
    for (i = 0; i < (0x1000000 - v); i++)
    r = lfsr_backwards(r);
    return r;

    Also, if the value doesn't have to be really exact, you could have
    a small lookup table with precomputed values, like:

    u32 get_lfsr(u32 v)
    static const lookup[256] = {
    0xab3492, 0x3e3f34, 0xc47610c, ... /* insert actual values */

    return lookup[v >> 16];

    Arnd <><
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      Last update: 2007-01-31 07:09    [W:0.021 / U:5.168 seconds]
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