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ifset: Use a slightly better portable ctz().

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The behavior is identical but the codegen is improved.
This commit is contained in:
Nicholas J. Kain 2022-09-24 14:12:02 -04:00
parent 95c70ba2a7
commit 1c1ea86604
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1 changed files with 6 additions and 7 deletions
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ifset.c
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@ -29,15 +29,14 @@
static uint32_t ifset_nl_seq = 1; static uint32_t ifset_nl_seq = 1;
// 32-bit position values are relatively prime to 37, so the residue mod37 // Portable ctz() that gives a defined result for x == 0.
// gives a unique mapping for each value. Gives correct result for v=0. static int trailz(uint32_t x)
static int trailz(uint32_t v)
{ {
static const int bpm37[] = { static const int ctzt[32] = {
32, 0, 1, 26, 2, 23, 27, 0, 3, 16, 24, 30, 28, 11, 0, 13, 4, 7, 17, 0, 1, 28, 2, 29, 14, 24, 3, 30, 22, 20, 15, 25, 17, 4, 8,
0, 25, 22, 31, 15, 29, 10, 12, 6, 0, 21, 14, 9, 5, 20, 8, 19, 18 31, 27, 13, 23, 21, 19, 16, 7, 26, 12, 18, 6, 11, 5, 10,
}; };
return bpm37[(-v & v) % 37]; return x ? ctzt[((x & -x) * 0x077cb531u) >> 27] : 32;
} }
// sn must be in network order // sn must be in network order