[PATCH] dcache: faster dentry_cmp()

[Alexey Dobriyan <adobriyan@gmail.com>]

1) consistently use "unsigned int" for dentry name length,
2) reuse subtraction result for return value, exact value doesn't matter
   because function is only used in boolean context,
3) use *p++ idiom for even better code.

All of this results in performance speedup of "git diff"
which is way out of statistical error (0.4% vs 0.15% of 3 sigma):

$ PAGER= perf stat -r 256 git-diff

 Performance counter stats for 'git-diff' (256 runs):

        115.033582 task-clock                #    0.993 CPUs utilized            ( +-  0.06% )
                 0 context-switches          #    0.000 M/sec                    ( +- 17.95% )
                 0 CPU-migrations            #    0.000 M/sec                    ( +- 19.47% )
             2,321 page-faults               #    0.020 M/sec                    ( +-  0.00% )
       384,540,991 cycles                    #    3.343 GHz                      ( +-  0.05% )
       121,833,562 stalled-cycles-frontend   #   31.68% frontend cycles idle     ( +-  0.16% )
        51,731,784 stalled-cycles-backend    #   13.45% backend  cycles idle     ( +-  0.37% )
       586,327,441 instructions              #    1.52  insns per cycle
                                             #    0.21  stalled cycles per insn  ( +-  0.00% )
       155,449,246 branches                  # 1351.338 M/sec                    ( +-  0.00% )
           542,511 branch-misses             #    0.35% of all branches          ( +-  0.07% )

       0.115856505 seconds time elapsed                                          ( +-  0.06% )

----------------------------------
after

 Performance counter stats for 'git-diff' (256 runs):

        114.486145 task-clock                #    0.993 CPUs utilized            ( +-  0.05% )
                 0 context-switches          #    0.000 M/sec                    ( +- 15.46% )
                 0 CPU-migrations            #    0.000 M/sec                    ( +- 20.06% )
             2,282 page-faults               #    0.020 M/sec                    ( +-  0.00% )
       382,725,382 cycles                    #    3.343 GHz                      ( +-  0.05% )
       119,808,563 stalled-cycles-frontend   #   31.30% frontend cycles idle     ( +-  0.15% )
        51,780,030 stalled-cycles-backend    #   13.53% backend  cycles idle     ( +-  0.33% )
       585,114,727 instructions              #    1.53  insns per cycle
                                             #    0.20  stalled cycles per insn  ( +-  0.00% )
       155,146,262 branches                  # 1355.153 M/sec                    ( +-  0.00% )
           526,739 branch-misses             #    0.34% of all branches          ( +-  0.12% )

       0.115315823 seconds time elapsed                                          ( +-  0.05% )

Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
---

 fs/dcache.c            |    6 +++---
 include/linux/dcache.h |   14 +++++++-------
 2 files changed, 10 insertions(+), 10 deletions(-)

--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -1368,7 +1368,7 @@ static struct dentry *__d_instantiate_unique(struct dentry *entry,
 					     struct inode *inode)
 {
 	struct dentry *alias;
-	int len = entry->d_name.len;
+	unsigned int len = entry->d_name.len;
 	const char *name = entry->d_name.name;
 	unsigned int hash = entry->d_name.hash;
 
@@ -1750,7 +1750,7 @@ struct dentry *__d_lookup_rcu(struct dentry *parent, struct qstr *name,
 	hlist_bl_for_each_entry_rcu(dentry, node, b, d_hash) {
 		struct inode *i;
 		const char *tname;
-		int tlen;
+		unsigned int tlen;
 
 		if (dentry->d_name.hash != hash)
 			continue;
@@ -1869,7 +1869,7 @@ struct dentry *__d_lookup(struct dentry *parent, struct qstr *name)
 	
 	hlist_bl_for_each_entry_rcu(dentry, node, b, d_hash) {
 		const char *tname;
-		int tlen;
+		unsigned int tlen;
 
 		if (dentry->d_name.hash != hash)
 			continue;
--- a/include/linux/dcache.h
+++ b/include/linux/dcache.h
@@ -51,18 +51,18 @@ extern struct dentry_stat_t dentry_stat;
  * Compare 2 name strings, return 0 if they match, otherwise non-zero.
  * The strings are both count bytes long, and count is non-zero.
  */
-static inline int dentry_cmp(const unsigned char *cs, size_t scount,
-				const unsigned char *ct, size_t tcount)
+static inline int dentry_cmp(const unsigned char *cs, unsigned int scount,
+			     const unsigned char *ct, unsigned int tcount)
 {
 	int ret;
-	if (scount != tcount)
-		return 1;
+
+	ret = scount - tcount;
+	if (ret)
+		return ret;
 	do {
-		ret = (*cs != *ct);
+		ret = *cs++ - *ct++;
 		if (ret)
 			break;
-		cs++;
-		ct++;
 		tcount--;
 	} while (tcount);
 	return ret;