[PATCH review 5/7] dcache: Only read d_flags once in d_is_dir

Eric W. Biederman ebiederm at xmission.com
Sat Aug 15 18:38:49 UTC 2015

Cache the value of __d_entry_type in d_is_dir and test if it equal to

The generated assembly goes from:
	movl	(%rdi), %eax  # MEM[(volatile __u32 *)dentry_3(D)], tmp73
	andl	$7340032, %eax  #, tmp73
	cmpl	$2097152, %eax  #, tmp73
	je	.L1091	#,
	movl	(%rdi), %eax	# MEM[(volatile __u32 *)dentry_3(D)], tmp74
	andl	$7340032, %eax	#, tmp74
	cmpl	$3145728, %eax	#, tmp74
	je	.L1091	#,
	movl	(%rdi), %eax	# MEM[(volatile __u32 *)dentry_3(D)], tmp71
	andl	$6291456, %eax	#, tmp71
	cmpl	$2097152, %eax	#, tmp71
	jne	.L1091	  #,

Which with only one read of d_flags, one comparison and one jump is
dramatically better code.

As __d_entry_type is not written to allow the compiler to optimize
away anything that it does, when it is possible and reasonable to
optimize things away the optimization needs to be performend manually.

Signed-off-by: "Eric W. Biederman" <ebiederm at xmission.com>
 include/linux/dcache.h | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/include/linux/dcache.h b/include/linux/dcache.h
index 5b69856b45a2..82eb50aaf446 100644
--- a/include/linux/dcache.h
+++ b/include/linux/dcache.h
@@ -443,7 +443,8 @@ static inline bool d_is_autodir(const struct dentry *dentry)
 static inline bool d_is_dir(const struct dentry *dentry)
-	return d_can_lookup(dentry) || d_is_autodir(dentry);
+	unsigned type = __d_entry_type(dentry);
+	return (type == DCACHE_DIRECTORY_TYPE) || (type == DCACHE_AUTODIR_TYPE);
 static inline bool d_is_symlink(const struct dentry *dentry)

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