diff mbox series

[RFC] fs: micro-optimization remove branches by adjusting flag values

Message ID 20200914174338.9808-1-mateusznosek0@gmail.com (mailing list archive)
State New, archived
Headers show
Series [RFC] fs: micro-optimization remove branches by adjusting flag values | expand

Commit Message

Mateusz Nosek Sept. 14, 2020, 5:43 p.m. UTC 
From: Mateusz Nosek <mateusznosek0@gmail.com>

When flags A and B have equal values than the following code

if(flags1 & A)
	flags2 |= B;

is equivalent to

flags2 |= (flags1 & A);

The latter code should generate less instructions and be faster as one
branch is omitted in it.

Introduced patch changes the value of 'LOOKUP_EMPTY' and makes it equal
to the value of 'AT_EMPTY_PATH'. Thanks to that, few branches can be
changed in a way showed above which improves both performance and the
size of the code.

Signed-off-by: Mateusz Nosek <mateusznosek0@gmail.com>
---
 fs/exec.c             | 14 ++++++++++----
 fs/fhandle.c          |  4 ++--
 fs/namespace.c        |  4 ++--
 fs/open.c             |  8 ++++----
 fs/stat.c             |  4 ++--
 fs/utimes.c           |  6 +++---
 include/linux/namei.h |  4 ++--
 7 files changed, 25 insertions(+), 19 deletions(-)

Comments

Matthew Wilcox Sept. 14, 2020, 6:06 p.m. UTC | #1 
On Mon, Sep 14, 2020 at 07:43:38PM +0200, mateusznosek0@gmail.com wrote:
> From: Mateusz Nosek <mateusznosek0@gmail.com>
> 
> When flags A and B have equal values than the following code
> 
> if(flags1 & A)
> 	flags2 |= B;
> 
> is equivalent to
> 
> flags2 |= (flags1 & A);
> 
> The latter code should generate less instructions and be faster as one
> branch is omitted in it.

[citation needed]

$ cat test.c 
int a(int x)
{
	int y = 0;

	if (x & 1)
		y |= 1;
	if (x & 2)
		y |= 2;

	return y;
}

$ objdump -d test.o
0000000000000000 <a>:
   0:   89 f8                   mov    %edi,%eax
   2:   83 e0 03                and    $0x3,%eax
   5:   c3                      retq   

Please stop submitting uglifying patches without checking they actually
improve anything.  GCC is smarter than you think it is.
Al Viro Sept. 15, 2020, 12:31 a.m. UTC | #2 
On Mon, Sep 14, 2020 at 07:43:38PM +0200, mateusznosek0@gmail.com wrote:
> From: Mateusz Nosek <mateusznosek0@gmail.com>
> 
> When flags A and B have equal values than the following code
> 
> if(flags1 & A)
> 	flags2 |= B;
> 
> is equivalent to
> 
> flags2 |= (flags1 & A);
> 
> The latter code should generate less instructions and be faster as one
> branch is omitted in it.
> 
> Introduced patch changes the value of 'LOOKUP_EMPTY' and makes it equal
> to the value of 'AT_EMPTY_PATH'. Thanks to that, few branches can be
> changed in a way showed above which improves both performance and the
> size of the code.

No.  AT_EMPTY_PATH is a part of userland ABI; to tie LOOKUP_EMPTY to it
means that we can't ever modify the sucker.  Worse, it restricts any
possible reshuffling of the LOOKUP_... bits in the future.

So unless you can show an effect on the real-world profiles, there are
fairly strong reasons to avoid that headache.
Al Viro Sept. 15, 2020, 12:38 a.m. UTC | #3 
On Mon, Sep 14, 2020 at 07:06:29PM +0100, Matthew Wilcox wrote:
> $ objdump -d test.o
> 0000000000000000 <a>:
>    0:   89 f8                   mov    %edi,%eax
>    2:   83 e0 03                and    $0x3,%eax
>    5:   c3                      retq   
> 
> Please stop submitting uglifying patches without checking they actually
> improve anything.  GCC is smarter than you think it is.

His main point isn't that - it's reshuffling LOOKUP_... bits to make that
kind of optimisation possible.  However, doing that sets us up for PITA
down the road (e.g. reshuffling LOOKUP_... bits becomes forbidden, etc.)
and I'd rather not go there unless we have a real-world evidence that it
does buy us anything.
diff mbox series

Patch

diff --git a/fs/exec.c b/fs/exec.c
index a91003e28eaa..39e1ada1ee6c 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -904,8 +904,8 @@  static struct file *do_open_execat(int fd, struct filename *name, int flags)
 		return ERR_PTR(-EINVAL);
 	if (flags & AT_SYMLINK_NOFOLLOW)
 		open_exec_flags.lookup_flags &= ~LOOKUP_FOLLOW;
-	if (flags & AT_EMPTY_PATH)
-		open_exec_flags.lookup_flags |= LOOKUP_EMPTY;
+	BUILD_BUG_ON(AT_EMPTY_PATH != LOOKUP_EMPTY);
+	open_exec_flags.lookup_flags |= (flags & AT_EMPTY_PATH);
 
 	file = do_filp_open(fd, name, &open_exec_flags);
 	if (IS_ERR(file))
@@ -2176,7 +2176,10 @@  SYSCALL_DEFINE5(execveat,
 		const char __user *const __user *, envp,
 		int, flags)
 {
-	int lookup_flags = (flags & AT_EMPTY_PATH) ? LOOKUP_EMPTY : 0;
+	int lookup_flags;
+
+	BUILD_BUG_ON(AT_EMPTY_PATH != LOOKUP_EMPTY);
+	lookup_flags = (flags & AT_EMPTY_PATH);
 
 	return do_execveat(fd,
 			   getname_flags(filename, lookup_flags, NULL),
@@ -2197,7 +2200,10 @@  COMPAT_SYSCALL_DEFINE5(execveat, int, fd,
 		       const compat_uptr_t __user *, envp,
 		       int,  flags)
 {
-	int lookup_flags = (flags & AT_EMPTY_PATH) ? LOOKUP_EMPTY : 0;
+	int lookup_flags;
+
+	BUILD_BUG_ON(AT_EMPTY_PATH != LOOKUP_EMPTY);
+	lookup_flags = (flags & AT_EMPTY_PATH);
 
 	return compat_do_execveat(fd,
 				  getname_flags(filename, lookup_flags, NULL),
diff --git a/fs/fhandle.c b/fs/fhandle.c
index 01263ffbc4c0..579bf462bf89 100644
--- a/fs/fhandle.c
+++ b/fs/fhandle.c
@@ -102,8 +102,8 @@  SYSCALL_DEFINE5(name_to_handle_at, int, dfd, const char __user *, name,
 		return -EINVAL;
 
 	lookup_flags = (flag & AT_SYMLINK_FOLLOW) ? LOOKUP_FOLLOW : 0;
-	if (flag & AT_EMPTY_PATH)
-		lookup_flags |= LOOKUP_EMPTY;
+	BUILD_BUG_ON(AT_EMPTY_PATH != LOOKUP_EMPTY);
+	lookup_flags |= (flag & AT_EMPTY_PATH);
 	err = user_path_at(dfd, name, lookup_flags, &path);
 	if (!err) {
 		err = do_sys_name_to_handle(&path, handle, mnt_id);
diff --git a/fs/namespace.c b/fs/namespace.c
index 098f981dce54..319f42d11236 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -2456,8 +2456,8 @@  SYSCALL_DEFINE3(open_tree, int, dfd, const char __user *, filename, unsigned, fl
 		lookup_flags &= ~LOOKUP_AUTOMOUNT;
 	if (flags & AT_SYMLINK_NOFOLLOW)
 		lookup_flags &= ~LOOKUP_FOLLOW;
-	if (flags & AT_EMPTY_PATH)
-		lookup_flags |= LOOKUP_EMPTY;
+	BUILD_BUG_ON(AT_EMPTY_PATH != LOOKUP_EMPTY);
+	lookup_flags |= (flags & AT_EMPTY_PATH);
 
 	if (detached && !may_mount())
 		return -EPERM;
diff --git a/fs/open.c b/fs/open.c
index 9af548fb841b..8b6fe1e89811 100644
--- a/fs/open.c
+++ b/fs/open.c
@@ -410,8 +410,8 @@  static long do_faccessat(int dfd, const char __user *filename, int mode, int fla
 
 	if (flags & AT_SYMLINK_NOFOLLOW)
 		lookup_flags &= ~LOOKUP_FOLLOW;
-	if (flags & AT_EMPTY_PATH)
-		lookup_flags |= LOOKUP_EMPTY;
+	BUILD_BUG_ON(AT_EMPTY_PATH != LOOKUP_EMPTY);
+	lookup_flags |= (flags & AT_EMPTY_PATH);
 
 	if (!(flags & AT_EACCESS)) {
 		old_cred = access_override_creds();
@@ -692,8 +692,8 @@  int do_fchownat(int dfd, const char __user *filename, uid_t user, gid_t group,
 		goto out;
 
 	lookup_flags = (flag & AT_SYMLINK_NOFOLLOW) ? 0 : LOOKUP_FOLLOW;
-	if (flag & AT_EMPTY_PATH)
-		lookup_flags |= LOOKUP_EMPTY;
+	BUILD_BUG_ON(AT_EMPTY_PATH != LOOKUP_EMPTY);
+	lookup_flags |= (flag & AT_EMPTY_PATH);
 retry:
 	error = user_path_at(dfd, filename, lookup_flags, &path);
 	if (error)
diff --git a/fs/stat.c b/fs/stat.c
index 44f8ad346db4..a9feb7a7e9ec 100644
--- a/fs/stat.c
+++ b/fs/stat.c
@@ -168,8 +168,8 @@  static inline unsigned vfs_stat_set_lookup_flags(unsigned *lookup_flags,
 		*lookup_flags &= ~LOOKUP_FOLLOW;
 	if (flags & AT_NO_AUTOMOUNT)
 		*lookup_flags &= ~LOOKUP_AUTOMOUNT;
-	if (flags & AT_EMPTY_PATH)
-		*lookup_flags |= LOOKUP_EMPTY;
+	BUILD_BUG_ON(AT_EMPTY_PATH != LOOKUP_EMPTY);
+	*lookup_flags |= (flags & AT_EMPTY_PATH);
 
 	return 0;
 }
diff --git a/fs/utimes.c b/fs/utimes.c
index fd3cc4226224..95a48dbda7e1 100644
--- a/fs/utimes.c
+++ b/fs/utimes.c
@@ -79,15 +79,15 @@  static int do_utimes_path(int dfd, const char __user *filename,
 		struct timespec64 *times, int flags)
 {
 	struct path path;
-	int lookup_flags = 0, error;
+	int lookup_flags, error;
 
 	if (flags & ~(AT_SYMLINK_NOFOLLOW | AT_EMPTY_PATH))
 		return -EINVAL;
 
+	BUILD_BUG_ON(AT_EMPTY_PATH != LOOKUP_EMPTY);
+	lookup_flags = (flags & AT_EMPTY_PATH);
 	if (!(flags & AT_SYMLINK_NOFOLLOW))
 		lookup_flags |= LOOKUP_FOLLOW;
-	if (flags & AT_EMPTY_PATH)
-		lookup_flags |= LOOKUP_EMPTY;
 
 retry:
 	error = user_path_at(dfd, filename, lookup_flags, &path);
diff --git a/include/linux/namei.h b/include/linux/namei.h
index a4bb992623c4..52f8015717c0 100644
--- a/include/linux/namei.h
+++ b/include/linux/namei.h
@@ -21,7 +21,7 @@  enum {LAST_NORM, LAST_ROOT, LAST_DOT, LAST_DOTDOT};
 #define LOOKUP_FOLLOW		0x0001	/* follow links at the end */
 #define LOOKUP_DIRECTORY	0x0002	/* require a directory */
 #define LOOKUP_AUTOMOUNT	0x0004  /* force terminal automount */
-#define LOOKUP_EMPTY		0x4000	/* accept empty path [user_... only] */
+#define LOOKUP_EMPTY		0x1000	/* accept empty path [user_... only], AT_EMPTY_PATH set */
 #define LOOKUP_DOWN		0x8000	/* follow mounts in the starting point */
 #define LOOKUP_MOUNTPOINT	0x0080	/* follow mounts in the end */
 
@@ -36,7 +36,7 @@  enum {LAST_NORM, LAST_ROOT, LAST_DOT, LAST_DOTDOT};
 
 /* internal use only */
 #define LOOKUP_PARENT		0x0010
-#define LOOKUP_JUMPED		0x1000
+#define LOOKUP_JUMPED		0x4000
 #define LOOKUP_ROOT		0x2000
 #define LOOKUP_ROOT_GRABBED	0x0008