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Re: [PATCH] Use strlen when searching for a nul char


ping

________________________________________
From: Wilco Dijkstra
Sent: 25 February 2016 13:04
To: 'GNU C Library'
Cc: nd
Subject: [PATCH] Use strlen when searching for a nul char

Remove the strchr (s, '\0') to rawmemchr optimization as using rawmemchr is
a bad idea - I have a patch to add strchr (s, '\0') -> strlen to GCC7.
Like strchr (s, '\0'), rawmemchr (s, '\0') appears a common idiom for finding
the end of a string, however it is not the most efficient way of doing so.
Strlen is a simpler operation which is significantly faster on larger inputs
(eg. on x86 strlen is 50% faster than rawmemchr on strings of 1KB).


ChangeLog:
2016-02-25  Wilco Dijkstra  <wdijkstr@arm.com>

        * string/bits/string2.h (strchr): Remove define.
        (__rawmemchr): Add new define.
        (rawmemchr): Likewise.

--

diff --git a/string/bits/string2.h b/string/bits/string2.h
index bebd158c5ff0f7bd7d9e4a4c3e120cd45b6e2143..f34fedb170352eaca0ed784ca6e76d7bbbfaefc2 100644
--- a/string/bits/string2.h
+++ b/string/bits/string2.h
@@ -62,14 +62,15 @@
 #endif


-#ifndef _HAVE_STRING_ARCH_strchr
+#ifndef _HAVE_STRING_ARCH_rawmemchr
 extern void *__rawmemchr (const void *__s, int __c);
-# if __GNUC_PREREQ (3, 2)
-#  define strchr(s, c) \
-  (__extension__ (__builtin_constant_p (c) && !__builtin_constant_p (s)              \
-                 && (c) == '\0'                                              \
-                 ? (char *) __rawmemchr (s, c)                               \
-                 : __builtin_strchr (s, c)))
+# define __rawmemchr(s, c) \
+    (__extension__ ({ char *__s = (char *)(s);         \
+      __builtin_constant_p (c) && (c) == '\0'          \
+      ? (void *)(__s + strlen (__s))                   \
+      : __rawmemchr (__s, (c));}))
+# ifdef __USE_GNU
+#  define rawmemchr(s,c) __rawmemchr ((s), (c))
 # endif
 #endif


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