diff mbox series

[2/2] lib/string: Improve str_has_prefix() performance

Message ID 20250407-imp_str_perf-v1-2-ed95d52964a4@quicinc.com (mailing list archive)
State Rejected
Headers show
Series lib/string: Improve performance for both strstarts() and str_has_prefix() | expand

Commit Message

Zijun Hu April 7, 2025, 1:15 p.m. UTC 
From: Zijun Hu <quic_zijuhu@quicinc.com>

str_has_prefix() is frequently invoked to return length of the prefix
string if a string has another string as prefix, but its performance
is degraded by the strlen() loop contained.

Improve its performance by eliminating the strlen() loop.

Link: https://lore.kernel.org/all/20250113234643.GA3631169-robh@kernel.org
Signed-off-by: Zijun Hu <quic_zijuhu@quicinc.com>
Cc: Rob Herring (Arm) <robh@kernel.org>
---
 include/linux/string.h | 21 +--------------------
 lib/string.c           | 33 +++++++++++++++++++++++++++++++++
 2 files changed, 34 insertions(+), 20 deletions(-)

Comments

Andy Shevchenko April 7, 2025, 1:52 p.m. UTC | #1 
On Mon, Apr 07, 2025 at 09:15:05PM +0800, Zijun Hu wrote:
> From: Zijun Hu <quic_zijuhu@quicinc.com>
> 
> str_has_prefix() is frequently invoked to return length of the prefix
> string if a string has another string as prefix, but its performance
> is degraded by the strlen() loop contained.
> 
> Improve its performance by eliminating the strlen() loop.

NAK.
diff mbox series

Patch

diff --git a/include/linux/string.h b/include/linux/string.h
index e5f7defa277572719e1dbfdd264f3de6ef8544f1..394b76666ece0924c50264aaca39784d5630a2fe 100644
--- a/include/linux/string.h
+++ b/include/linux/string.h
@@ -533,25 +533,6 @@  void memcpy_and_pad(void *dest, size_t dest_len, const void *src, size_t count,
 	       sizeof(*(obj)) - offsetof(typeof(*(obj)), member));	\
 })
 
-/**
- * str_has_prefix - Test if a string has a given prefix
- * @str: The string to test
- * @prefix: The string to see if @str starts with
- *
- * A common way to test a prefix of a string is to do:
- *  strncmp(str, prefix, sizeof(prefix) - 1)
- *
- * But this can lead to bugs due to typos, or if prefix is a pointer
- * and not a constant. Instead use str_has_prefix().
- *
- * Returns:
- * * strlen(@prefix) if @str starts with @prefix
- * * 0 if @str does not start with @prefix
- */
-static __always_inline size_t str_has_prefix(const char *str, const char *prefix)
-{
-	size_t len = strlen(prefix);
-	return strncmp(str, prefix, len) == 0 ? len : 0;
-}
+size_t str_has_prefix(const char *str, const char *prefix);
 
 #endif /* _LINUX_STRING_H_ */
diff --git a/lib/string.c b/lib/string.c
index ea52c8509328358e436766b1186a82419d45089d..17f1a070b190debc3eaeff6d1ae45b55bae69b29 100644
--- a/lib/string.c
+++ b/lib/string.c
@@ -332,6 +332,39 @@  int strncmp(const char *cs, const char *ct, size_t count)
 EXPORT_SYMBOL(strncmp);
 #endif
 
+/**
+ * str_has_prefix - Test if a string has a given prefix
+ * @str: The string to test
+ * @prefix: The string to see if @str starts with
+ *
+ * A common way to test a prefix of a string is to do:
+ *  strncmp(str, prefix, sizeof(prefix) - 1)
+ *
+ * But this can lead to bugs due to typos, or if prefix is a pointer
+ * and not a constant. Instead use str_has_prefix().
+ *
+ * Returns:
+ * * strlen(@prefix) if @str starts with @prefix
+ * * 0 if @str does not start with @prefix
+ */
+size_t str_has_prefix(const char *str, const char *prefix)
+{
+	const char *p = prefix;
+	unsigned char c1, c2;
+
+	do {
+		c1 = *str++;
+		c2 = *p++;
+
+		if (c1 != c2)
+			return c2 == '\0' ? p - 1 - prefix : 0;
+
+	} while (c2 != '\0');
+
+	return p - 1 - prefix;
+}
+EXPORT_SYMBOL(str_has_prefix);
+
 #ifndef __HAVE_ARCH_STRCHR
 /**
  * strchr - Find the first occurrence of a character in a string