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pure string functions
- To: libc-alpha at sourceware dot cygnus dot com
- Subject: pure string functions
- From: Andreas Jaeger <aj at suse dot de>
- Date: 11 May 2000 10:32:54 +0200
- Cc: Jan Hubicka <jh at suse dot cz>
Hi,
Jan has developed a page for gcc to optimize pure functions. He also
developed a patch to add the pure attribute to a number of string
functions in glibc.
The gcc manual documents pure functions as:
======================================================================
Many functions have no effects except the return value and their
return value depends only on the parameters and/or global variables.
Such a function can be subject
to common subexpression elimination and loop optimization just as an
arithmetic operator would be. These functions should be declared
with the attribute @code{pure}. For example,
@smallexample
int square (int) __attribute__ ((pure));
@end smallexample
@noindent
says that the hypothetical function @code{square} is safe to call
fewer times than the program says.
Some of common examples of pure functions are @code{strlen} or @code{memcmp}.
Interesting non-pure functions are functions with infinite loops or those
depending on volatile memory or other system resource, that may change between
two consetuctive calls (such as @code{feof} in multithreding environment).
======================================================================
I've tested his glibc patch thoroughly on Alpha and Intel and it a
does make a difference in glibc.
Uli, can I commit this patch?
After this patch is in, others might want to go through gcc and add
pure to a number of other functions.
Andreas
2000-05-11 Jan Hubicka <jh@suse.cz>
* misc/sys/cdefs.h: Add support for pure attribute.
* include/string.h: Add __attribute_pure__ to various functions.
* string/string.h: Likewise.
============================================================
Index: include/string.h
--- include/string.h 1999/12/19 18:32:05 1.9
+++ include/string.h 2000/05/11 08:23:44
@@ -5,23 +5,29 @@
extern void *__memccpy (void *__dest, __const void *__src,
int __c, size_t __n);
-extern size_t __strnlen (__const char *__string, size_t __maxlen);
+extern size_t __strnlen (__const char *__string, size_t __maxlen)
+ __attribute_pure__;
extern char *__strsep (char **__stringp, __const char *__delim);
-extern int __strverscmp (__const char *__s1, __const char *__s2);
+extern int __strverscmp (__const char *__s1, __const char *__s2)
+ __attribute_pure__;
extern int __strncasecmp (__const char *__s1, __const char *__s2,
- size_t __n);
+ size_t __n)
+ __attribute_pure__;
extern char *__strndup (__const char *__string, size_t __n)
__attribute_malloc__;
-extern void *__rawmemchr (__const void *__s, int __c);
+extern void *__rawmemchr (__const void *__s, int __c)
+ __attribute_pure__;
-extern char *__strchrnul (__const char *__s, int __c);
+extern char *__strchrnul (__const char *__s, int __c)
+ __attribute_pure__;
-extern void *__memrchr (__const void *__s, int __c, size_t __n);
+extern void *__memrchr (__const void *__s, int __c, size_t __n)
+ __attribute_pure__;
/* Now the real definitions. We do this here since some of the functions
above are defined as macros in the headers. */
============================================================
Index: string/string.h
--- string/string.h 2000/02/24 08:17:11 1.73
+++ string/string.h 2000/05/11 08:23:44
@@ -55,18 +55,20 @@
/* Compare N bytes of S1 and S2. */
extern int memcmp (__const void *__s1, __const void *__s2, size_t __n)
- __THROW;
+ __THROW __attribute_pure__;
/* Search N bytes of S for C. */
-extern void *memchr (__const void *__s, int __c, size_t __n) __THROW;
+extern void *memchr (__const void *__s, int __c, size_t __n)
+ __THROW __attribute_pure__;
#ifdef __USE_GNU
/* Search in S for C. This is similar to `memchr' but there is no
length limit. */
-extern void *rawmemchr (__const void *__s, int __c) __THROW;
+extern void *rawmemchr (__const void *__s, int __c) __THROW __attribute_pure__;
/* Search N bytes of S for the final occurrence of C. */
-extern void *memrchr (__const void *__s, int __c, size_t __n) __THROW;
+extern void *memrchr (__const void *__s, int __c, size_t __n)
+ __THROW __attribute_pure__;
#endif
@@ -85,13 +87,15 @@
size_t __n) __THROW;
/* Compare S1 and S2. */
-extern int strcmp (__const char *__s1, __const char *__s2) __THROW;
+extern int strcmp (__const char *__s1, __const char *__s2)
+ __THROW __attribute_pure__;
/* Compare N characters of S1 and S2. */
extern int strncmp (__const char *__s1, __const char *__s2, size_t __n)
- __THROW;
+ __THROW __attribute_pure__;
/* Compare the collated forms of S1 and S2. */
-extern int strcoll (__const char *__s1, __const char *__s2) __THROW;
+extern int strcoll (__const char *__s1, __const char *__s2)
+ __THROW __attribute_pure__;
/* Put a transformation of SRC into no more than N bytes of DEST. */
extern size_t strxfrm (char *__restrict __dest,
__const char *__restrict __src, size_t __n) __THROW;
@@ -104,7 +108,7 @@
/* Compare the collated forms of S1 and S2 using rules from L. */
extern int __strcoll_l (__const char *__s1, __const char *__s2, __locale_t __l)
- __THROW;
+ __THROW __attribute_pure__;
/* Put a transformation of SRC into no more than N bytes of DEST. */
extern size_t __strxfrm_l (char *__dest, __const char *__src, size_t __n,
__locale_t __l) __THROW;
@@ -148,33 +152,37 @@
#endif
/* Find the first occurrence of C in S. */
-extern char *strchr (__const char *__s, int __c) __THROW;
+extern char *strchr (__const char *__s, int __c) __THROW __attribute_pure__;
/* Find the last occurrence of C in S. */
-extern char *strrchr (__const char *__s, int __c) __THROW;
+extern char *strrchr (__const char *__s, int __c) __THROW __attribute_pure__;
#ifdef __USE_GNU
/* This funciton is similar to `strchr'. But it returns a pointer to
the closing NUL byte in case C is not found in S. */
-extern char *strchrnul (__const char *__s, int __c) __THROW;
+extern char *strchrnul (__const char *__s, int __c) __THROW __attribute_pure__;
#endif
/* Return the length of the initial segment of S which
consists entirely of characters not in REJECT. */
-extern size_t strcspn (__const char *__s, __const char *__reject) __THROW;
+extern size_t strcspn (__const char *__s, __const char *__reject)
+ __THROW __attribute_pure__;
/* Return the length of the initial segment of S which
consists entirely of characters in ACCEPT. */
-extern size_t strspn (__const char *__s, __const char *__accept) __THROW;
+extern size_t strspn (__const char *__s, __const char *__accept)
+ __THROW __attribute_pure__;
/* Find the first occurrence in S of any character in ACCEPT. */
-extern char *strpbrk (__const char *__s, __const char *__accept) __THROW;
+extern char *strpbrk (__const char *__s, __const char *__accept)
+ __THROW __attribute_pure__;
/* Find the first occurrence of NEEDLE in HAYSTACK. */
-extern char *strstr (__const char *__haystack, __const char *__needle) __THROW;
+extern char *strstr (__const char *__haystack, __const char *__needle)
+ __THROW __attribute_pure__;
#ifdef __USE_GNU
/* Similar to `strstr' but this function ignores the case of both strings. */
extern char *__strcasestr (__const char *__haystack, __const char *__needle)
- __THROW;
+ __THROW __attribute_pure__;
extern char *strcasestr (__const char *__haystack, __const char *__needle)
- __THROW;
+ __THROW __attribute_pure__;
#endif
/* Divide S into tokens separated by characters in DELIM. */
@@ -196,7 +204,8 @@
NEEDLE is NEEDLELEN bytes long;
HAYSTACK is HAYSTACKLEN bytes long. */
extern void *memmem (__const void *__haystack, size_t __haystacklen,
- __const void *__needle, size_t __needlelen) __THROW;
+ __const void *__needle, size_t __needlelen)
+ __THROW __attribute_pure__;
/* Copy N bytes of SRC to DEST, return pointer to bytes after the
last written byte. */
@@ -208,12 +217,13 @@
/* Return the length of S. */
-extern size_t strlen (__const char *__s) __THROW;
+extern size_t strlen (__const char *__s) __THROW __attribute_pure__;
#ifdef __USE_GNU
/* Find the length of STRING, but scan at most MAXLEN characters.
If no '\0' terminator is found in that many characters, return MAXLEN. */
-extern size_t strnlen (__const char *__string, size_t __maxlen) __THROW;
+extern size_t strnlen (__const char *__string, size_t __maxlen)
+ __THROW __attribute_pure__;
#endif
@@ -238,13 +248,14 @@
extern void bzero (void *__s, size_t __n) __THROW;
/* Compare N bytes of S1 and S2 (same as memcmp). */
-extern int bcmp (__const void *__s1, __const void *__s2, size_t __n) __THROW;
+extern int bcmp (__const void *__s1, __const void *__s2, size_t __n)
+ __THROW __attribute_pure__;
/* Find the first occurrence of C in S (same as strchr). */
-extern char *index (__const char *__s, int __c) __THROW;
+extern char *index (__const char *__s, int __c) __THROW __attribute_pure__;
/* Find the last occurrence of C in S (same as strrchr). */
-extern char *rindex (__const char *__s, int __c) __THROW;
+extern char *rindex (__const char *__s, int __c) __THROW __attribute_pure__;
/* Return the position of the first bit set in I, or 0 if none are set.
The least-significant bit is position 1, the most-significant 32. */
@@ -262,22 +273,25 @@
# endif
/* Compare S1 and S2, ignoring case. */
-extern int __strcasecmp (__const char *__s1, __const char *__s2) __THROW;
-extern int strcasecmp (__const char *__s1, __const char *__s2) __THROW;
+extern int __strcasecmp (__const char *__s1, __const char *__s2)
+ __THROW __attribute_pure__;
+extern int strcasecmp (__const char *__s1, __const char *__s2)
+ __THROW __attribute_pure__;
/* Compare no more than N chars of S1 and S2, ignoring case. */
extern int strncasecmp (__const char *__s1, __const char *__s2, size_t __n)
- __THROW;
+ __THROW __attribute_pure__;
#endif /* Use BSD. */
#ifdef __USE_GNU
/* Again versions of a few functions which use the given locale instead
of the global one. */
extern int __strcasecmp_l (__const char *__s1, __const char *__s2,
- __locale_t __loc) __THROW;
+ __locale_t __loc) __THROW __attribute_pure__;
extern int __strncasecmp_l (__const char *__s1, __const char *__s2,
- size_t __n, __locale_t __loc) __THROW;
+ size_t __n, __locale_t __loc)
+ __THROW __attribute_pure__;
#endif
#ifdef __USE_BSD
@@ -289,7 +303,8 @@
#ifdef __USE_GNU
/* Compare S1 and S2 as strings holding name & indices/version numbers. */
-extern int strverscmp (__const char *__s1, __const char *__s2) __THROW;
+extern int strverscmp (__const char *__s1, __const char *__s2)
+ __THROW __attribute_pure__;
/* Return a string describing the meaning of the signal number in SIG. */
extern char *strsignal (int __sig) __THROW;
============================================================
Index: misc/sys/cdefs.h
--- misc/sys/cdefs.h 2000/03/31 07:19:43 1.33
+++ misc/sys/cdefs.h 2000/05/11 08:23:44
@@ -123,6 +123,15 @@
# define __attribute_malloc__ /* Ignore */
#endif
+/* At some point during the gcc 2.96 development the `pure' attribute
+ for functions was introduced. We don't want to use it unconditionally
+ (although this would be possible) since it generates warnings. */
+#if __GNUC_PREREQ (2,96)
+# define __attribute_pure__ __attribute__ ((__pure__))
+#else
+# define __attribute_pure__ /* Ignore */
+#endif
+
/* It is possible to compile containing GCC extensions even if GCC is
run in pedantic mode if the uses are carefully marked using the
`__extension__' keyword. But this is not generally available before
--
Andreas Jaeger
SuSE Labs aj@suse.de
private aj@arthur.rhein-neckar.de