[Devel] [PATCH 1/3] blk-cbt: Refactor cbt_find_next_extent for readability

[Nikolay Borisov]

Replace all shift rights/shift lefts by divide and multiply operations.
I've checked that this doesn't generate worse assembly and that shifts
are indeed continued to be used. PAGE_SHIFT + 3 is actually
BITS_PER_PAGE and is a constant integer expression so the compiler is
perfectly capable of figuring that out and generatin optimal assembly.

Replace an if/else construct with just an if which handles the error
condition. This makes the code somewhat easier to follow in a linear
fashion.

Remove a superfluous continue and make off2 variable local the sole
context it's being used.

Signed-off-by: Nikolay Borisov <nikolay.borisov at virtuozzo.com>
---
 block/blk-cbt.c | 21 +++++++++++----------
 1 file changed, 11 insertions(+), 10 deletions(-)

diff --git a/block/blk-cbt.c b/block/blk-cbt.c
index 967533a2a7a3..8baeb2804a61 100644
--- a/block/blk-cbt.c
+++ b/block/blk-cbt.c
@@ -758,14 +758,14 @@ static void cbt_flush_cache(struct cbt_info *cbt)
 
 static void cbt_find_next_extent(struct cbt_info *cbt, blkcnt_t block, struct cbt_extent *ex)
 {
-	unsigned long off, off2, idx;
+	unsigned long off, idx;
 	struct page *page;
 	bool found = 0;
 
 	ex->start = cbt->block_max;
 	ex->len = 0;
 
-	idx = block >> (PAGE_SHIFT + 3);
+	idx = block / BITS_PER_PAGE;
 	while (block < cbt->block_max) {
 		off = block & (BITS_PER_PAGE -1);
 		page = CBT_PAGE(cbt, idx);
@@ -778,17 +778,19 @@ static void cbt_find_next_extent(struct cbt_info *cbt, blkcnt_t block, struct cb
 		/* Find extent start */
 		if (!found) {
 			ex->start = find_next_bit(page_address(page), BITS_PER_PAGE, off);
-			if (ex->start != BITS_PER_PAGE) {
-				off = ex->start;
-				ex->start += idx << (PAGE_SHIFT + 3);
-				found = 1;
-			} else {
+			/* No set bits in current page, skip to next one */
+			if (ex->start == BITS_PER_PAGE) {
 				unlock_page(page);
 				goto next;
 			}
+
+			off = ex->start;
+			ex->start += idx / BITS_PER_PAGE;
+			found = 1;
 		}
 		if (found) {
-			off2 = find_next_zero_bit(page_address(page), BITS_PER_PAGE, off);
+			unsigned long off2 = find_next_zero_bit(page_address(page),
+								BITS_PER_PAGE, off);
 			ex->len += off2 - off;
 			if (off2 != BITS_PER_PAGE) {
 				unlock_page(page);
@@ -798,8 +800,7 @@ static void cbt_find_next_extent(struct cbt_info *cbt, blkcnt_t block, struct cb
 		unlock_page(page);
 	next:
 		idx++;
-		block = idx << (PAGE_SHIFT + 3);
-		continue;
+		block = idx * BITS_PER_PAGE;
 	}
 }
 
-- 
2.34.1