block: optimise bvec_iter_advance()

bvec_iter_advance() is quite popular, but compilers fail to do proper
alias analysis and optimise it good enough. The assembly is checked
for gcc 9.2, x86-64.

- remove @iter->bi_size from min(...), as it's always less than @bytes.
Modify at the beginning and forget about it.

- the compiler isn't able to collapse memory dependencies and remove
writes in the loop. Help it by explicitely using local vars.
Signed-off-by: NArvind Sankar <nivedita@alum.mit.edu>
Signed-off-by: NPavel Begunkov <asml.silence@gmail.com>
Signed-off-by: NJens Axboe <axboe@kernel.dk>

include/linux/bvec.h

@@ -87,26 +87,24 @@ struct bvec_iter_all {
 static inline bool bvec_iter_advance(const struct bio_vec *bv,
 		struct bvec_iter *iter, unsigned bytes)
 {
+	unsigned int idx = iter->bi_idx;
+
 	if (WARN_ONCE(bytes > iter->bi_size,
 		     "Attempted to advance past end of bvec iter\n")) {
 		iter->bi_size = 0;
 		return false;
 	}
 
-	while (bytes) {
-		const struct bio_vec *cur = bv + iter->bi_idx;
-		unsigned len = min3(bytes, iter->bi_size,
-				    cur->bv_len - iter->bi_bvec_done);
-
-		bytes -= len;
-		iter->bi_size -= len;
-		iter->bi_bvec_done += len;
+	iter->bi_size -= bytes;
+	bytes += iter->bi_bvec_done;
 
-		if (iter->bi_bvec_done == cur->bv_len) {
-			iter->bi_bvec_done = 0;
-			iter->bi_idx++;
-		}
+	while (bytes && bytes >= bv[idx].bv_len) {
+		bytes -= bv[idx].bv_len;
+		idx++;
 	}
+
+	iter->bi_idx = idx;
+	iter->bi_bvec_done = bytes;
 	return true;
 }