This is the mail archive of the
libc-alpha@sourceware.org
mailing list for the glibc project.
Re: [PATCH] Improve generic strcspn performance
- From: Adhemerval Zanella <adhemerval dot zanella at linaro dot org>
- To: libc-alpha at sourceware dot org
- Date: Fri, 8 Jan 2016 18:05:03 -0200
- Subject: Re: [PATCH] Improve generic strcspn performance
- Authentication-results: sourceware.org; auth=none
- References: <AM3PR08MB0088F82AB469E058650B493283F60 at AM3PR08MB0088 dot eurprd08 dot prod dot outlook dot com>
Some comments below:
On 08-01-2016 16:40, Wilco Dijkstra wrote:
> Improve strcspn performance using a much faster algorithm. It is kept simple
> so it works well on most targets. It is generally at least 10 times faster
> than the existing implementation on bench-strcspn on a few AArch64
> implementations, and for some tests 100 times as fast (repeatedly calling
> strchr on a small string is extremely slow...). In fact the string/bits/string2.h
> inlines make no longer sense, as GCC already uses strlen if reject is an empty
> string, strchrnul is 5 times as fast as __strcspn_c1, while __strcspn_c2 and
> __strcspn_c3 are slower than the strcspn main loop for large strings (though reject
> length 2-4 could be special cased in the future to gain even more performance).
>
> Built and tested on AArch64. OK for GLIBC 2.23?
>
> ChangeLog:
> 2016-01-08 Wilco Dijkstra <wdijkstr@arm.com>
>
> * string/strcspn.c (strcspn): Rewrite function.
> * string/bits/string2.h (strcspn): Use __builtin_strcspn.
> (__strcspn_c1) Remove inline function.
> (__strcspn_c2) Likewise.
> (__strcspn_c3) Likewise.
>
> diff --git a/string/bits/string2.h b/string/bits/string2.h
> index e18c67530ec78338c9591015f3d95c785de54726..d0926f1e7898a13852081f45d54bff5145751695 100644
> --- a/string/bits/string2.h
> +++ b/string/bits/string2.h
> @@ -199,62 +199,10 @@ extern void *__rawmemchr (const void *__s, int __c);
>
> /* Return the length of the initial segment of S which
> consists entirely of characters not in REJECT. */
> -#if !defined _HAVE_STRING_ARCH_strcspn
> -# ifndef _HAVE_STRING_ARCH_strcspn
> -# if __GNUC_PREREQ (3, 2)
> -# define strcspn(s, reject) \
> - __extension__ \
> - ({ char __r0, __r1, __r2; \
> - (__builtin_constant_p (reject) && __string2_1bptr_p (reject) \
> - ? ((__builtin_constant_p (s) && __string2_1bptr_p (s)) \
> - ? __builtin_strcspn (s, reject) \
> - : ((__r0 = ((const char *) (reject))[0], __r0 == '\0') \
> - ? strlen (s) \
> - : ((__r1 = ((const char *) (reject))[1], __r1 == '\0') \
> - ? __strcspn_c1 (s, __r0) \
> - : ((__r2 = ((const char *) (reject))[2], __r2 == '\0') \
> - ? __strcspn_c2 (s, __r0, __r1) \
> - : (((const char *) (reject))[3] == '\0' \
> - ? __strcspn_c3 (s, __r0, __r1, __r2) \
> - : __builtin_strcspn (s, reject)))))) \
> - : __builtin_strcspn (s, reject)); })
> -# endif
> +#ifndef _HAVE_STRING_ARCH_strcspn
> +# if __GNUC_PREREQ (3, 2)
> +# define strcspn(s, reject) __builtin_strcspn (s, reject)
> # endif
> -
> -__STRING_INLINE size_t __strcspn_c1 (const char *__s, int __reject);
> -__STRING_INLINE size_t
> -__strcspn_c1 (const char *__s, int __reject)
> -{
> - size_t __result = 0;
> - while (__s[__result] != '\0' && __s[__result] != __reject)
> - ++__result;
> - return __result;
> -}
> -
> -__STRING_INLINE size_t __strcspn_c2 (const char *__s, int __reject1,
> - int __reject2);
> -__STRING_INLINE size_t
> -__strcspn_c2 (const char *__s, int __reject1, int __reject2)
> -{
> - size_t __result = 0;
> - while (__s[__result] != '\0' && __s[__result] != __reject1
> - && __s[__result] != __reject2)
> - ++__result;
> - return __result;
> -}
> -
> -__STRING_INLINE size_t __strcspn_c3 (const char *__s, int __reject1,
> - int __reject2, int __reject3);
> -__STRING_INLINE size_t
> -__strcspn_c3 (const char *__s, int __reject1, int __reject2,
> - int __reject3)
> -{
> - size_t __result = 0;
> - while (__s[__result] != '\0' && __s[__result] != __reject1
> - && __s[__result] != __reject2 && __s[__result] != __reject3)
> - ++__result;
> - return __result;
> -}
> #endif
>
>
> diff --git a/string/strcspn.c b/string/strcspn.c
> index 2694d2ab0e807d0712d6b103dbdfd75ef164ebf1..cdb2df315c394889fe04b69980c63ea4ddbdb286 100644
> --- a/string/strcspn.c
> +++ b/string/strcspn.c
> @@ -26,16 +26,48 @@
> /* Return the length of the maximum initial segment of S
> which contains no characters from REJECT. */
> size_t
> -STRCSPN (const char *s, const char *reject)
> +STRCSPN (const char *str, const char *reject)
> {
> - size_t count = 0;
> + unsigned char table[256];
> + unsigned char *p, *s;
> + unsigned int c0, c1, c2, c3;
> + size_t count;
>
> - while (*s != '\0')
> - if (strchr (reject, *s++) == NULL)
> - ++count;
> - else
> - return count;
> + if (reject[0] == '\0')
> + return strlen (str);
> + if (reject[1] == '\0')
> + return __strchrnul (str, reject [0]) - str;
I am not sure how often strcspn is used with empty or one char argument to
validate this optimization in specific since it adds more branch cases for
more general inputs.
>
> - return count;
> + /* Use multiple small memsets to enable inlining on most targets. */
> + p = memset (table, 0, 64);
> + memset (p + 64, 0, 64);
> + memset (p + 128, 0, 64);
> + memset (p + 192, 0, 64);
It is unfortunate we need to use this to force inline instead to let the
compiler handle it directly (and also simplifying the code by using
c99 initializers). I noted x86_64 does no inline, although aarch64 and
powerpc64le calls memset. How bad is avoiding this explicit calls now
and work on compiler side to detect this aligned memset?
> +
> + s = (unsigned char*) reject;
> + do
> + p[c0 = *s++] = 1;
> + while (c0);
> +
> + s = (unsigned char*) str;
> + if (p[s[0]]) return 0;
> + if (p[s[1]]) return 1;
> + if (p[s[2]]) return 2;
> + if (p[s[3]]) return 3;
> +
> + s = (unsigned char *) ((size_t)s & ~3);
> +
> + do
> + {
> + s += 4;
> + c0 = p[s[0]];
> + c1 = p[s[1]];
> + c2 = p[s[2]];
> + c3 = p[s[3]];
> + }
> + while ((c0 | c1 | c2 | c3) == 0);
> +
> + count = s - (unsigned char *) str;
> + return (c0 | c1) != 0 ? count - c0 + 1 : count - c2 + 3;
> }
> libc_hidden_builtin_def (strcspn)
>