io_uring: optimise submission left counting

Considering all inlining io_submit_sqe() is huge and usually ends up
calling some other functions.

We decrement @left in io_submit_sqes() just before calling
io_submit_sqe() and use it later after the call. Considering how huge
io_submit_sqe() is, there is not much hope @left will be treated
gracefully by compilers.

Decrement it after the call, not only it's easier on register spilling
and probably saves stack write/read, but also at least for x64 uses
CPU flags set by the dec instead of doing (read/write and tests).
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/807f9a276b54ee8ff4e42e2b78721484f1c71743.1649771823.git.asml.silence@gmail.comSigned-off-by: Jens Axboe <axboe@kernel.dk>

fs/io_uring.c

@@ -7877,17 +7877,17 @@ static int io_submit_sqes(struct io_ring_ctx *ctx, unsigned int nr)
 			io_req_add_to_cache(req, ctx);
 			break;
 		}
-		/* will complete beyond this point, count as submitted */
-		left--;
-		if (io_submit_sqe(ctx, req, sqe)) {
-			/*
-			 * Continue submitting even for sqe failure if the
-			 * ring was setup with IORING_SETUP_SUBMIT_ALL
-			 */
-			if (!(ctx->flags & IORING_SETUP_SUBMIT_ALL))
-				break;
+
+		/*
+		 * Continue submitting even for sqe failure if the
+		 * ring was setup with IORING_SETUP_SUBMIT_ALL
+		 */
+		if (unlikely(io_submit_sqe(ctx, req, sqe)) &&
+		    !(ctx->flags & IORING_SETUP_SUBMIT_ALL)) {
+			left--;
+			break;
 		}
-	} while (left);
+	} while (--left);
 
 	if (unlikely(left)) {
 		ret -= left;