| Message ID | 20200919001021.21690-1-mateusznosek0@gmail.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | fs/open.c: micro-optimization by avoiding branch on common path | expand |
On Sat, Sep 19, 2020 at 02:10:21AM +0200, mateusznosek0@gmail.com wrote: > From: Mateusz Nosek <mateusznosek0@gmail.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. Does it measurably show in any profiles? > Signed-off-by: Mateusz Nosek <mateusznosek0@gmail.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; > + } If anything, turn the second if into return S_ISDIR(...) ? -EISDIR : -EINVAL;
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)