[v9,06/22] string: Improve generic strchr
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Commit Message
New algorithm now calls strchrnul.
Checked on x86_64-linux-gnu, i686-linux-gnu, powerpc-linux-gnu,
and powerpc64-linux-gnu by removing the arch-specific assembly
implementation and disabling multi-arch (it covers both LE and BE
for 64 and 32 bits).
---
string/strchr.c | 159 ++--------------------------------------
sysdeps/s390/strchr-c.c | 11 +--
2 files changed, 14 insertions(+), 156 deletions(-)
Comments
On 1/17/23 09:59, Adhemerval Zanella wrote:
> New algorithm now calls strchrnul.
>
> Checked on x86_64-linux-gnu, i686-linux-gnu, powerpc-linux-gnu,
> and powerpc64-linux-gnu by removing the arch-specific assembly
> implementation and disabling multi-arch (it covers both LE and BE
> for 64 and 32 bits).
> ---
> string/strchr.c | 159 ++--------------------------------------
> sysdeps/s390/strchr-c.c | 11 +--
> 2 files changed, 14 insertions(+), 156 deletions(-)
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
r~
@@ -21,165 +21,22 @@
<https://www.gnu.org/licenses/>. */
#include <string.h>
-#include <stdlib.h>
#undef strchr
+#undef index
-#ifndef STRCHR
-# define STRCHR strchr
+#ifdef STRCHR
+# define strchr STRCHR
#endif
/* Find the first occurrence of C in S. */
char *
-STRCHR (const char *s, int c_in)
+strchr (const char *s, int c_in)
{
- const unsigned char *char_ptr;
- const unsigned long int *longword_ptr;
- unsigned long int longword, magic_bits, charmask;
- unsigned char c;
-
- c = (unsigned char) c_in;
-
- /* 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 = (const unsigned char *) s;
- ((unsigned long int) char_ptr & (sizeof (longword) - 1)) != 0;
- ++char_ptr)
- if (*char_ptr == c)
- return (void *) char_ptr;
- else if (*char_ptr == '\0')
- return NULL;
-
- /* 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;
-
- /* Bits 31, 24, 16, and 8 of this number are zero. Call these bits
- the "holes." Note that there is a hole just to the left of
- each byte, with an extra at the end:
-
- bits: 01111110 11111110 11111110 11111111
- bytes: AAAAAAAA BBBBBBBB CCCCCCCC DDDDDDDD
-
- The 1-bits make sure that carries propagate to the next 0-bit.
- The 0-bits provide holes for carries to fall into. */
- magic_bits = -1;
- magic_bits = magic_bits / 0xff * 0xfe << 1 >> 1 | 1;
-
- /* Set up a longword, each of whose bytes is C. */
- charmask = c | (c << 8);
- charmask |= charmask << 16;
- if (sizeof (longword) > 4)
- /* Do the shift in two steps to avoid a warning if long has 32 bits. */
- charmask |= (charmask << 16) << 16;
- 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 (;;)
- {
- /* We tentatively exit the loop if adding MAGIC_BITS to
- LONGWORD fails to change any of the hole bits of LONGWORD.
-
- 1) Is this safe? Will it catch all the zero bytes?
- Suppose there is a byte with all zeros. Any carry bits
- propagating from its left will fall into the hole at its
- least significant bit and stop. Since there will be no
- carry from its most significant bit, the LSB of the
- byte to the left will be unchanged, and the zero will be
- detected.
-
- 2) Is this worthwhile? Will it ignore everything except
- zero bytes? Suppose every byte of LONGWORD has a bit set
- somewhere. There will be a carry into bit 8. If bit 8
- is set, this will carry into bit 16. If bit 8 is clear,
- one of bits 9-15 must be set, so there will be a carry
- into bit 16. Similarly, there will be a carry into bit
- 24. If one of bits 24-30 is set, there will be a carry
- into bit 31, so all of the hole bits will be changed.
-
- The one misfire occurs when bits 24-30 are clear and bit
- 31 is set; in this case, the hole at bit 31 is not
- changed. If we had access to the processor carry flag,
- we could close this loophole by putting the fourth hole
- at bit 32!
-
- So it ignores everything except 128's, when they're aligned
- properly.
-
- 3) But wait! Aren't we looking for C as well as zero?
- Good point. So what we do is XOR LONGWORD with a longword,
- each of whose bytes is C. This turns each byte that is C
- into a zero. */
-
- longword = *longword_ptr++;
-
- /* Add MAGIC_BITS to LONGWORD. */
- if ((((longword + magic_bits)
-
- /* Set those bits that were unchanged by the addition. */
- ^ ~longword)
-
- /* Look at only the hole bits. If any of the hole bits
- are unchanged, most likely one of the bytes was a
- zero. */
- & ~magic_bits) != 0
-
- /* That caught zeroes. Now test for C. */
- || ((((longword ^ charmask) + magic_bits) ^ ~(longword ^ charmask))
- & ~magic_bits) != 0)
- {
- /* Which of the bytes was C or zero?
- If none of them were, it was a misfire; continue the search. */
-
- const unsigned char *cp = (const unsigned char *) (longword_ptr - 1);
-
- if (*cp == c)
- return (char *) cp;
- else if (*cp == '\0')
- return NULL;
- if (*++cp == c)
- return (char *) cp;
- else if (*cp == '\0')
- return NULL;
- if (*++cp == c)
- return (char *) cp;
- else if (*cp == '\0')
- return NULL;
- if (*++cp == c)
- return (char *) cp;
- else if (*cp == '\0')
- return NULL;
- if (sizeof (longword) > 4)
- {
- if (*++cp == c)
- return (char *) cp;
- else if (*cp == '\0')
- return NULL;
- if (*++cp == c)
- return (char *) cp;
- else if (*cp == '\0')
- return NULL;
- if (*++cp == c)
- return (char *) cp;
- else if (*cp == '\0')
- return NULL;
- if (*++cp == c)
- return (char *) cp;
- else if (*cp == '\0')
- return NULL;
- }
- }
- }
-
- return NULL;
+ char *r = __strchrnul (s, c_in);
+ return (*(unsigned char *)r == (unsigned char)c_in) ? r : NULL;
}
-
-#ifdef weak_alias
-# undef index
+#ifndef STRCHR
weak_alias (strchr, index)
-#endif
libc_hidden_builtin_def (strchr)
+#endif
@@ -21,13 +21,14 @@
#if HAVE_STRCHR_C
# if HAVE_STRCHR_IFUNC
# define STRCHR STRCHR_C
-# undef weak_alias
+# endif
+
+# include <string/strchr.c>
+
+# if HAVE_STRCHR_IFUNC
# if defined SHARED && IS_IN (libc)
-# undef libc_hidden_builtin_def
-# define libc_hidden_builtin_def(name) \
- __hidden_ver1 (__strchr_c, __GI_strchr, __strchr_c);
+__hidden_ver1 (__strchr_c, __GI_strchr, __strchr_c);
# endif
# endif
-# include <string/strchr.c>
#endif