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Optimize bitcount32 by replacing 6 logical operations with 2. The key

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observation here is that it doesn't matter what garbage accumulates in
bits which we're going to end up masking away anyway, as long as the
garbage doesn't overflow into bits which we care about.

This improved version may not be the fastest possible on all systems,
but it's certainly going to be better than what was here before.
This commit is contained in:
cperciva 2007-02-14 05:21:22 +00:00
parent 5b027888f7
commit 93537b1cea
1 changed files with 3 additions and 3 deletions
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6
sys/sys/systm.h
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@ -391,9 +391,9 @@ bitcount32(uint32_t x)
x = (x & 0x55555555) + ((x & 0xaaaaaaaa) >> 1);
x = (x & 0x33333333) + ((x & 0xcccccccc) >> 2);
x = (x & 0x0f0f0f0f) + ((x & 0xf0f0f0f0) >> 4);
x = (x & 0x00ff00ff) + ((x & 0xff00ff00) >> 8);
x = (x & 0x0000ffff) + ((x & 0xffff0000) >> 16);
x = (x + (x >> 4)) & 0x0f0f0f0f;
x = (x + (x >> 8));
x = (x + (x >> 16)) & 0x000000ff;
return (x);
}