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lib: small update for strlen, strnlen, use less cpu instructions

PATCH In review Orestes Leal Rodriguez 2 participants 2 messages Jun 16, 2015

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Orestes Leal Rodriguez

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#0 lore raw

PATCH 1/1 lib: small update for strlen, strnlen, use less cpu instructions

Very small update to strlen and strnlen that now use less cpu instructions by using a counter to avoid the memory addresses substraction to find the length of the string.

Signed-off-by Orestes Leal Rodriguez <[email protected]>

---

lib/string.c +13 -8
index 992bf30..c873436 100644
@@ -17,6 +17,10 @@
  * * Sat Feb 09 2002, Jason Thomas <[email protected]>,
  *                    Matthew Hawkins <[email protected]>
  * -  Kissed strtok() goodbye
+ *
+ * * Tuesday June 16 2015, Orestes Leal Rodriguez <[email protected]>
+ * - strlen, strnlen: by using a single counter we use less cpu instructions
+ *   by avoiding substracting the memory addresses before return
  */
 
 #include <linux/types.h>

@@ -401,11 +405,11 @@ EXPORT_SYMBOL(strim);
  */
 size_t strlen(const char *s)
 {
-	const char *sc;
+	size_t sz = 0;
 
-	for (sc = s; *sc != '\0'; ++sc)
-		/* nothing */;
-	return sc - s;
+	for (; *s++ != '\0'; sz++)
+		/* empty */;
+	return sz;
 }
 EXPORT_SYMBOL(strlen);
 #endif

@@ -418,12 +422,13 @@ EXPORT_SYMBOL(strlen);
  */
 size_t strnlen(const char *s, size_t count)
 {
-	const char *sc;
+	size_t sz = 0;
 
-	for (sc = s; count-- && *sc != '\0'; ++sc)
-		/* nothing */;
-	return sc - s;
+	for (; count-- && *s++ != '\0'; sz++)
+		/* empty */;
+	return sz;
 }
+
 EXPORT_SYMBOL(strnlen);
 #endif
#1 lore raw
Quoted 3 lines · Orestes Leal Rodriguez

Very small update to strlen and strnlen that now use less cpu instructions by using a counter to avoid the memory addresses substraction to find the length of the string.

[]

lib/string.c EXPORT_SYMBOL(strlen); · line 429

Quoted 13 lines
lib/string.c +4 -4
@@ -418,12 +422,13 @@ EXPORT_SYMBOL(strlen);
   */
  size_t strnlen(const char *s, size_t count)
  {
-	const char *sc;
+	size_t sz = 0;
 
-	for (sc = s; count-- && *sc != '\0'; ++sc)
-		/* nothing */;
-	return sc - s;
+	for (; count-- && *s++ != '\0'; sz++)
+		/* empty */;
+	return sz;

That's one subtraction at end-of-string vs
a register increment for each non-zero byte.

smaller isn't worth slower.