Revert "Remove BITMAP_USE_TREE."

Some systems use a native 64 KiB page size, which means that the bitmap
for the smallest size class can be 8192 bits, not just 512 bits as when
the page size is 4 KiB.  Linear search in bitmap_{sfu,ffu}() is
unacceptably slow for such large bitmaps.

This reverts commit 7c00f04ff4.

src/bitmap.c

@@ -6,6 +6,82 @@
 
 /******************************************************************************/
 
+#ifdef BITMAP_USE_TREE
+
+void
+bitmap_info_init(bitmap_info_t *binfo, size_t nbits) {
+	unsigned i;
+	size_t group_count;
+
+	assert(nbits > 0);
+	assert(nbits <= (ZU(1) << LG_BITMAP_MAXBITS));
+
+	/*
+	 * Compute the number of groups necessary to store nbits bits, and
+	 * progressively work upward through the levels until reaching a level
+	 * that requires only one group.
+	 */
+	binfo->levels[0].group_offset = 0;
+	group_count = BITMAP_BITS2GROUPS(nbits);
+	for (i = 1; group_count > 1; i++) {
+		assert(i < BITMAP_MAX_LEVELS);
+		binfo->levels[i].group_offset = binfo->levels[i-1].group_offset
+		    + group_count;
+		group_count = BITMAP_BITS2GROUPS(group_count);
+	}
+	binfo->levels[i].group_offset = binfo->levels[i-1].group_offset
+	    + group_count;
+	assert(binfo->levels[i].group_offset <= BITMAP_GROUPS_MAX);
+	binfo->nlevels = i;
+	binfo->nbits = nbits;
+}
+
+static size_t
+bitmap_info_ngroups(const bitmap_info_t *binfo) {
+	return binfo->levels[binfo->nlevels].group_offset;
+}
+
+void
+bitmap_init(bitmap_t *bitmap, const bitmap_info_t *binfo, bool fill) {
+	size_t extra;
+	unsigned i;
+
+	/*
+	 * Bits are actually inverted with regard to the external bitmap
+	 * interface.
+	 */
+
+	if (fill) {
+		/* The "filled" bitmap starts out with all 0 bits. */
+		memset(bitmap, 0, bitmap_size(binfo));
+		return;
+	}
+
+	/*
+	 * The "empty" bitmap starts out with all 1 bits, except for trailing
+	 * unused bits (if any).  Note that each group uses bit 0 to correspond
+	 * to the first logical bit in the group, so extra bits are the most
+	 * significant bits of the last group.
+	 */
+	memset(bitmap, 0xffU, bitmap_size(binfo));
+	extra = (BITMAP_GROUP_NBITS - (binfo->nbits & BITMAP_GROUP_NBITS_MASK))
+	    & BITMAP_GROUP_NBITS_MASK;
+	if (extra != 0) {
+		bitmap[binfo->levels[1].group_offset - 1] >>= extra;
+	}
+	for (i = 1; i < binfo->nlevels; i++) {
+		size_t group_count = binfo->levels[i].group_offset -
+		    binfo->levels[i-1].group_offset;
+		extra = (BITMAP_GROUP_NBITS - (group_count &
+		    BITMAP_GROUP_NBITS_MASK)) & BITMAP_GROUP_NBITS_MASK;
+		if (extra != 0) {
+			bitmap[binfo->levels[i+1].group_offset - 1] >>= extra;
+		}
+	}
+}
+
+#else /* BITMAP_USE_TREE */
+
 void
 bitmap_info_init(bitmap_info_t *binfo, size_t nbits) {
 	assert(nbits > 0);
@@ -37,6 +113,8 @@ bitmap_init(bitmap_t *bitmap, const bitmap_info_t *binfo, bool fill) {
 	}
 }
 
+#endif /* BITMAP_USE_TREE */
+
 size_t
 bitmap_size(const bitmap_info_t *binfo) {
 	return (bitmap_info_ngroups(binfo) << LG_SIZEOF_BITMAP);