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Date: Mon, 3 Jun 2024 18:32:08 GMT
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To: src-committers@FreeBSD.org, dev-commits-src-all@FreeBSD.org,
dev-commits-src-main@FreeBSD.org
From: Doug Moore <dougm@FreeBSD.org>
Subject: git: 749c249dc3b6 - main - subr_pctrie: use ilog2(x) instead
of fls(x)-1
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X-Git-Committer: dougm
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X-Git-Commit: 749c249dc3b656fea69149f8f7ad3e1a7f935986
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The branch main has been updated by dougm:
URL: https://cgit.FreeBSD.org/src/commit/?id=749c249dc3b656fea69149f8f7ad3e1a7f935986
commit 749c249dc3b656fea69149f8f7ad3e1a7f935986
Author: Doug Moore <dougm@FreeBSD.org>
AuthorDate: 2024-06-03 18:31:19 +0000
Commit: Doug Moore <dougm@FreeBSD.org>
CommitDate: 2024-06-03 18:31:19 +0000
subr_pctrie: use ilog2(x) instead of fls(x)-1
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
---
sys/kern/subr_pctrie.c | 6 +++---
1 file changed, 3 insertions(+), 3 deletions(-)
diff --git a/sys/kern/subr_pctrie.c b/sys/kern/subr_pctrie.c
index 85df0a9bf9e4..76f4ee17a8ca 100644
--- a/sys/kern/subr_pctrie.c
+++ b/sys/kern/subr_pctrie.c
@@ -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);
}