[libvirt] [PATCH 2/4] fixup? util: Optimize virBitmapUnion()

[Andrea Bolognani]

The original implementation is extremely straightforward but
not too efficient, because it uses the public API instead of
poking the innards directly. This second implementation does
the latter, and as a consequence can afford to make @b const,
which is nice even though most existing virBitmap APIs use
non-const pointers even for read-only bitmaps.

That said, I'm not entirely sure I got it quite right, so a
review from someone who's well versed in the virBitmap
internals (*cough* Peter *cough*) would be appreciated, and
if my implementation passes muster or can be amended through
review comments I'll gladly squash this commit into the
previous one.

Signed-off-by: Andrea Bolognani <abologna at redhat.com>
---
 src/util/virbitmap.c | 18 ++++++++++++------
 src/util/virbitmap.h |  2 +-
 2 files changed, 13 insertions(+), 7 deletions(-)

diff --git a/src/util/virbitmap.c b/src/util/virbitmap.c
index 1f0db563ab..f2127c053d 100644
--- a/src/util/virbitmap.c
+++ b/src/util/virbitmap.c
@@ -1271,17 +1271,23 @@ virBitmapIntersect(virBitmapPtr a,
  */
 int
 virBitmapUnion(virBitmapPtr a,
-               virBitmapPtr b)
+               const virBitmap *b)
 {
     size_t i;
+    size_t max;
 
-    for (i = 0; i < b->nbits; i++) {
-        if (virBitmapIsBitSet(b, i)) {
-            if (virBitmapSetBitExpand(a, i) < 0)
-                return -1;
-        }
+    if (a->nbits < b->nbits &&
+        virBitmapExpand(a, b->nbits) < 0) {
+        return -1;
     }
 
+    max = a->map_len;
+    if (max > b->map_len)
+        max = b->map_len;
+
+    for (i = 0; i < max; i++)
+        a->map[i] |= b->map[i];
+
     return 0;
 }
 
diff --git a/src/util/virbitmap.h b/src/util/virbitmap.h
index 49d9910fa7..8696214da8 100644
--- a/src/util/virbitmap.h
+++ b/src/util/virbitmap.h
@@ -150,7 +150,7 @@ void virBitmapIntersect(virBitmapPtr a, virBitmapPtr b)
     ATTRIBUTE_NONNULL(1) ATTRIBUTE_NONNULL(2);
 
 int virBitmapUnion(virBitmapPtr a,
-                   virBitmapPtr b)
+                   const virBitmap *b)
     ATTRIBUTE_NONNULL(1) ATTRIBUTE_NONNULL(2);
 
 void virBitmapSubtract(virBitmapPtr a, virBitmapPtr b)
-- 
2.21.0