[PATCH 1/1] libsepol: make ebitmap_cardinality() of linear complexity

[Nicolas Iooss <nicolas.iooss@m4x.org>]

As ebitmap_get_bit() complexity is linear in the size of the bitmap, the
complexity of ebitmap_cardinality() is quadratic. This can be optimized
by browsing the nodes of the bitmap directly in ebitmap_cardinality().

While at it, use built-in function __builtin_popcountll() to count the
ones in the 64-bit value n->map for each bitmap node. This seems better
suited than "count++". This seems to work on gcc and clang on x86,
x86_64, ARM and ARM64 but if it causes compatibility issues with some
compilers or architectures (or with older versions of gcc or clang),
the use of __builtin_popcountll() can be replaced by a C implementation
of a popcount algorithm.

Signed-off-by: Nicolas Iooss <nicolas.iooss@m4x.org>
---
 libsepol/src/ebitmap.c | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/libsepol/src/ebitmap.c b/libsepol/src/ebitmap.c
index d23444ce5064..a5108b7184c5 100644
--- a/libsepol/src/ebitmap.c
+++ b/libsepol/src/ebitmap.c
@@ -128,14 +128,15 @@ int ebitmap_andnot(ebitmap_t *dst, ebitmap_t *e1, ebitmap_t *e2, unsigned int ma
 
 unsigned int ebitmap_cardinality(ebitmap_t *e1)
 {
-	unsigned int i, count = 0;
+	unsigned int count = 0;
+	ebitmap_node_t *n;
 
 	if (e1->cardinality || e1->highbit == 0)
 		return e1->cardinality;
 
-	for (i=ebitmap_startbit(e1); i < ebitmap_length(e1); i++)
-		if (ebitmap_get_bit(e1, i))
-			count++;
+	for (n = e1->node; n; n = n->next) {
+		count += __builtin_popcountll(n->map);
+	}
 	e1->cardinality = count;
 	return count;
 }
-- 
2.25.0