git: 749c249dc3b6 - main - subr_pctrie: use ilog2(x) instead of fls(x)-1

[Doug Moore]

The branch main has been updated by dougm:

URL: 
https://cgit.FreeBSD.org/src/commit/?id=749c249dc3b656fea69149f8f7ad3e1a7f935986

commit 749c249dc3b656fea69149f8f7ad3e1a7f935986
Author:     Doug Moore <do...@freebsd.org>
AuthorDate: 2024-06-03 18:31:19 +0000
Commit:     Doug Moore <do...@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);
        }