linux-kernel - [PATCH] fs/open.c: micro-optimization by avoiding branch on common path

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Date:   Sat, 19 Sep 2020 02:10:21 +0200
From:   mateusznosek0@...il.com
To:     linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Cc:     Mateusz Nosek <mateusznosek0@...il.com>, viro@...iv.linux.org.uk
Subject: [PATCH] fs/open.c: micro-optimization by avoiding branch on common path

From: Mateusz Nosek <mateusznosek0@...il.com>

If file is a directory it is surely not regular. Therefore, if 'S_ISREG'
check returns false one can be sure that vfs_truncate must returns with
error. Introduced patch refactors code to avoid one branch in 'likely'
control flow path. Moreover, it marks the proper check with 'unlikely'
macro to improve both branch prediction and readability. Changes were
tested with gcc 8.3.0 on x86 architecture and it is confirmed that
slightly better assembly is generated.

Signed-off-by: Mateusz Nosek <mateusznosek0@...il.com>
---
 fs/open.c | 10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/fs/open.c b/fs/open.c
index 9af548fb841b..69658ea27530 100644
--- a/fs/open.c
+++ b/fs/open.c
@@ -74,10 +74,12 @@ long vfs_truncate(const struct path *path, loff_t length)
 	inode = path->dentry->d_inode;

 	/* For directories it's -EISDIR, for other non-regulars - -EINVAL */
-	if (S_ISDIR(inode->i_mode))
-		return -EISDIR;
-	if (!S_ISREG(inode->i_mode))
-		return -EINVAL;
+	if (unlikely(!S_ISREG(inode->i_mode))) {
+		if (S_ISDIR(inode->i_mode))
+			return -EISDIR;
+		else
+			return -EINVAL;
+	}

 	error = mnt_want_write(path->mnt);
 	if (error)
-- 
2.20.1