Btrfs: add branch prediction hints in the read page end IO function

This patch add some branch prediction hints into the end IO function
of the read page, it reduced the percentage of the branch misses from
5.5% to 4.9%.
Signed-off-by: NMiao Xie <miaox@cn.fujitsu.com>
Signed-off-by: NJosef Bacik <jbacik@fusionio.com>
Signed-off-by: NChris Mason <chris.mason@fusionio.com>

fs/btrfs/extent_io.c

@@ -2502,7 +2502,7 @@ static void end_bio_extent_readpage(struct bio *bio, int err)
 
 		spin_lock(&tree->lock);
 		state = find_first_extent_bit_state(tree, start, EXTENT_LOCKED);
-		if (state && state->start == start) {
+		if (likely(state && state->start == start)) {
 			/*
 			 * take a reference on the state, unlock will drop
 			 * the ref
@@ -2512,7 +2512,8 @@ static void end_bio_extent_readpage(struct bio *bio, int err)
 		spin_unlock(&tree->lock);
 
 		mirror = io_bio->mirror_num;
-		if (uptodate && tree->ops && tree->ops->readpage_end_io_hook) {
+		if (likely(uptodate && tree->ops &&
+			   tree->ops->readpage_end_io_hook)) {
 			ret = tree->ops->readpage_end_io_hook(page, start, end,
 							      state, mirror);
 			if (ret)
@@ -2521,12 +2522,15 @@ static void end_bio_extent_readpage(struct bio *bio, int err)
 				clean_io_failure(start, page);
 		}
 
-		if (!uptodate && tree->ops && tree->ops->readpage_io_failed_hook) {
+		if (likely(uptodate))
+			goto readpage_ok;
+
+		if (tree->ops && tree->ops->readpage_io_failed_hook) {
 			ret = tree->ops->readpage_io_failed_hook(page, mirror);
 			if (!ret && !err &&
 			    test_bit(BIO_UPTODATE, &bio->bi_flags))
 				uptodate = 1;
-		} else if (!uptodate) {
+		} else {
 			/*
 			 * The generic bio_readpage_error handles errors the
 			 * following way: If possible, new read requests are
@@ -2547,7 +2551,7 @@ static void end_bio_extent_readpage(struct bio *bio, int err)
 				continue;
 			}
 		}
-
+readpage_ok:
 		if (uptodate && tree->track_uptodate) {
 			set_extent_uptodate(tree, start, end, &cached,
 					    GFP_ATOMIC);