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From: Adhemerval Zanella <azanella@sourceware.org> To: glibc-cvs@sourceware.org Subject: [glibc] string: Improve generic strlen Date: Mon, 6 Feb 2023 20:13:20 +0000 (GMT) [thread overview] Message-ID: <20230206201320.E780C3858C54@sourceware.org> (raw) https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=350d8d13661a863e6b189f02d876fa265fe71302 commit 350d8d13661a863e6b189f02d876fa265fe71302 Author: Adhemerval Zanella <adhemerval.zanella@linaro.org> Date: Tue Jan 10 18:00:54 2023 -0300 string: Improve generic strlen New algorithm read the first aligned address and mask off the unwanted bytes (this strategy is similar to arch-specific implementations used on powerpc, sparc, and sh). The loop now read word-aligned address and check using the has_zero macro. Checked on x86_64-linux-gnu, i686-linux-gnu, powerpc-linux-gnu, and powercp64-linux-gnu by removing the arch-specific assembly implementation and disabling multi-arch (it covers both LE and BE for 64 and 32 bits). Co-authored-by: Richard Henderson <richard.henderson@linaro.org> Reviewed-by: Noah Goldstein <goldstein.w.n@gmail.com> Diff: --- string/strlen.c | 92 ++++++++++++------------------------------------- sysdeps/s390/strlen-c.c | 10 +++--- 2 files changed, 28 insertions(+), 74 deletions(-) diff --git a/string/strlen.c b/string/strlen.c index ee1aae0fff..5a4424f9a5 100644 --- a/string/strlen.c +++ b/string/strlen.c @@ -15,86 +15,38 @@ License along with the GNU C Library; if not, see <https://www.gnu.org/licenses/>. */ +#include <libc-pointer-arith.h> +#include <string-fzb.h> +#include <string-fzc.h> +#include <string-fzi.h> +#include <string-shift.h> #include <string.h> -#include <stdlib.h> -#undef strlen - -#ifndef STRLEN -# define STRLEN strlen +#ifdef STRLEN +# define __strlen STRLEN #endif /* Return the length of the null-terminated string STR. Scan for the null terminator quickly by testing four bytes at a time. */ size_t -STRLEN (const char *str) +__strlen (const char *str) { - const char *char_ptr; - const unsigned long int *longword_ptr; - unsigned long int longword, himagic, lomagic; - - /* Handle the first few characters by reading one character at a time. - Do this until CHAR_PTR is aligned on a longword boundary. */ - for (char_ptr = str; ((unsigned long int) char_ptr - & (sizeof (longword) - 1)) != 0; - ++char_ptr) - if (*char_ptr == '\0') - return char_ptr - str; - - /* All these elucidatory comments refer to 4-byte longwords, - but the theory applies equally well to 8-byte longwords. */ - - longword_ptr = (unsigned long int *) char_ptr; + /* Align pointer to sizeof op_t. */ + const uintptr_t s_int = (uintptr_t) str; + const op_t *word_ptr = (const op_t*) PTR_ALIGN_DOWN (str, sizeof (op_t)); - /* Computing (longword - lomagic) sets the high bit of any corresponding - byte that is either zero or greater than 0x80. The latter case can be - filtered out by computing (~longword & himagic). The final result - will always be non-zero if one of the bytes of longword is zero. */ - himagic = 0x80808080L; - lomagic = 0x01010101L; - if (sizeof (longword) > 4) - { - /* 64-bit version of the magic. */ - /* Do the shift in two steps to avoid a warning if long has 32 bits. */ - himagic = ((himagic << 16) << 16) | himagic; - lomagic = ((lomagic << 16) << 16) | lomagic; - } - if (sizeof (longword) > 8) - abort (); + op_t word = *word_ptr; + find_t mask = shift_find (find_zero_all (word), s_int); + if (mask != 0) + return index_first (mask); - /* Instead of the traditional loop which tests each character, - we will test a longword at a time. The tricky part is testing - if *any of the four* bytes in the longword in question are zero. */ - for (;;) - { - longword = *longword_ptr++; + do + word = *++word_ptr; + while (! has_zero (word)); - if (((longword - lomagic) & ~longword & himagic) != 0) - { - /* Which of the bytes was the zero? */ - - const char *cp = (const char *) (longword_ptr - 1); - - if (cp[0] == 0) - return cp - str; - if (cp[1] == 0) - return cp - str + 1; - if (cp[2] == 0) - return cp - str + 2; - if (cp[3] == 0) - return cp - str + 3; - if (sizeof (longword) > 4) - { - if (cp[4] == 0) - return cp - str + 4; - if (cp[5] == 0) - return cp - str + 5; - if (cp[6] == 0) - return cp - str + 6; - if (cp[7] == 0) - return cp - str + 7; - } - } - } + return ((const char *) word_ptr) + index_first_zero (word) - str; } +#ifndef STRLEN +weak_alias (__strlen, strlen) libc_hidden_builtin_def (strlen) +#endif diff --git a/sysdeps/s390/strlen-c.c b/sysdeps/s390/strlen-c.c index b829ef2452..0a33a6f8e5 100644 --- a/sysdeps/s390/strlen-c.c +++ b/sysdeps/s390/strlen-c.c @@ -21,12 +21,14 @@ #if HAVE_STRLEN_C # if HAVE_STRLEN_IFUNC # define STRLEN STRLEN_C +# endif + +# include <string/strlen.c> + +# if HAVE_STRLEN_IFUNC # if defined SHARED && IS_IN (libc) -# undef libc_hidden_builtin_def -# define libc_hidden_builtin_def(name) \ - __hidden_ver1 (__strlen_c, __GI_strlen, __strlen_c); +__hidden_ver1 (__strlen_c, __GI_strlen, __strlen_c); # endif # endif -# include <string/strlen.c> #endif
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