[v8,05/17] string: Improve generic strlen
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Commit Message
From: Adhemerval Zanella Netto <adhemerval.zanella@linaro.org>
New algorithm have the following key differences:
- Reads first word unaligned and use string-maskoff functions to
remove unwanted data. This strategy follow arch-specific
optimization used on powerpc, sparc, and SH.
- Use of has_zero and index_first_zero parametrized functions.
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>
Message-Id: <20230111204558.2402155-6-adhemerval.zanella@linaro.org>
---
string/strlen.c | 87 +++++++++--------------------------------
sysdeps/s390/strlen-c.c | 12 +++---
2 files changed, 25 insertions(+), 74 deletions(-)
@@ -17,84 +17,33 @@
#include <string.h>
#include <stdlib.h>
+#include <stdint.h>
+#include <string-fzb.h>
+#include <string-fzc.h>
+#include <string-maskoff.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;
+ /* Align pointer to sizeof op_t. */
+ const uintptr_t s_int = (uintptr_t) str;
+ const op_t *word_ptr = word_containing (str);
- /* 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;
+ /* Read and MASK the first word. */
+ op_t word = *word_ptr | create_mask (s_int);
- /* All these elucidatory comments refer to 4-byte longwords,
- but the theory applies equally well to 8-byte longwords. */
+ while (! has_zero (word))
+ word = *++word_ptr;
- longword_ptr = (unsigned long int *) char_ptr;
-
- /* 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 ();
-
- /* 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++;
-
- 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
@@ -21,12 +21,14 @@
#if HAVE_STRLEN_C
# if HAVE_STRLEN_IFUNC
# define STRLEN STRLEN_C
-# 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);
-# endif
# endif
# include <string/strlen.c>
+
+# if HAVE_STRLEN_IFUNC
+# if defined SHARED && IS_IN (libc)
+__hidden_ver1 (__strlen_c, __GI_strlen, __strlen_c);
+# endif
+# endif
+
#endif