1 files changed, 45 insertions, 4 deletions

diff --git a/util/hbitmap.c b/util/hbitmap.c
index bcc0acdc6a..fd44c897ab 100644
--- a/util/hbitmap.c
+++ b/util/hbitmap.c
@@ -781,12 +781,33 @@ void hbitmap_truncate(HBitmap *hb, uint64_t size)
 
 bool hbitmap_can_merge(const HBitmap *a, const HBitmap *b)
 {
-    return (a->size == b->size) && (a->granularity == b->granularity);
+    return (a->orig_size == b->orig_size);
 }
 
 /**
- * Given HBitmaps A and B, let A := A (BITOR) B.
- * Bitmap B will not be modified.
+ * hbitmap_sparse_merge: performs dst = dst | src
+ * works with differing granularities.
+ * best used when src is sparsely populated.
+ */
+static void hbitmap_sparse_merge(HBitmap *dst, const HBitmap *src)
+{
+    uint64_t offset = 0;
+    uint64_t count = src->orig_size;
+
+    while (hbitmap_next_dirty_area(src, &offset, &count)) {
+        hbitmap_set(dst, offset, count);
+        offset += count;
+        if (offset >= src->orig_size) {
+            break;
+        }
+        count = src->orig_size - offset;
+    }
+}
+
+/**
+ * Given HBitmaps A and B, let R := A (BITOR) B.
+ * Bitmaps A and B will not be modified,
+ *     except when bitmap R is an alias of A or B.
  *
  * @return true if the merge was successful,
  *         false if it was not attempted.
@@ -801,7 +822,26 @@ bool hbitmap_merge(const HBitmap *a, const HBitmap *b, HBitmap *result)
     }
     assert(hbitmap_can_merge(b, result));
 
-    if (hbitmap_count(b) == 0) {
+    if ((!hbitmap_count(a) && result == b) ||
+        (!hbitmap_count(b) && result == a)) {
+        return true;
+    }
+
+    if (!hbitmap_count(a) && !hbitmap_count(b)) {
+        hbitmap_reset_all(result);
+        return true;
+    }
+
+    if (a->granularity != b->granularity) {
+        if ((a != result) && (b != result)) {
+            hbitmap_reset_all(result);
+        }
+        if (a != result) {
+            hbitmap_sparse_merge(result, a);
+        }
+        if (b != result) {
+            hbitmap_sparse_merge(result, b);
+        }
         return true;
     }
 
@@ -809,6 +849,7 @@ bool hbitmap_merge(const HBitmap *a, const HBitmap *b, HBitmap *result)
      * It may be possible to improve running times for sparsely populated maps
      * by using hbitmap_iter_next, but this is suboptimal for dense maps.
      */
+    assert(a->size == b->size);
     for (i = HBITMAP_LEVELS - 1; i >= 0; i--) {
         for (j = 0; j < a->sizes[i]; j++) {
             result->levels[i][j] = a->levels[i][j] | b->levels[i][j];