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subr_pctrie: use ilog2(x) instead of fls(x)-1

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In three instances where fls(x)-1 is used, the compiler does not know
that x is nonzero and so adds needless zero checks.  Using ilog(x)
instead saves, in each instance, about 4 instructions, including a
conditional, and 16 or so bytes, on an amd64 build.

Reviewed by:    alc
Differential Revision:  https://reviews.freebsd.org/D45330
This commit is contained in:
Doug Moore 2024-06-03 13:31:19 -05:00
parent 08f6f78f81
commit 749c249dc3
1 changed files with 3 additions and 3 deletions
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6
sys/kern/subr_pctrie.c
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@ -350,7 +350,7 @@ pctrie_insert_node(void *parentp, struct pctrie_node *parent, uint64_t *val)
"uint64 too wide");
_Static_assert(sizeof(uint64_t) * NBBY <=
(1 << (sizeof(parent->pn_clev) * NBBY)), "pn_clev too narrow");
parent->pn_clev = rounddown(flsll(index ^ newind) - 1, PCTRIE_WIDTH);
parent->pn_clev = rounddown(ilog2(index ^ newind), PCTRIE_WIDTH);
parent->pn_owner = PCTRIE_COUNT;
parent->pn_owner = index & -(parent->pn_owner << parent->pn_clev);
@ -546,14 +546,14 @@ pctrie_lookup_le(struct pctrie *ptree, uint64_t index)
KASSERT((pred->pn_popmap & ((1 << slot) - 1)) != 0,
("%s: no popmap siblings before slot %d in node %p",
__func__, slot, pred));
slot = fls(pred->pn_popmap & ((1 << slot) - 1)) - 1;
slot = ilog2(pred->pn_popmap & ((1 << slot) - 1));
pred = pctrie_node_load(&pred->pn_child[slot], NULL,
PCTRIE_LOCKED);
}
while (!pctrie_isleaf(pred)) {
KASSERT(pred->pn_popmap != 0,
("%s: no popmap children in node %p", __func__, pred));
slot = fls(pred->pn_popmap) - 1;
slot = ilog2(pred->pn_popmap);
pred = pctrie_node_load(&pred->pn_child[slot], NULL,
PCTRIE_LOCKED);
}