[v2,04/16] Improve generic strchr
Commit Message
[BZ #5806]
* string/strchr.c: Use string-fzb.h, string-fzi.h, string-extbyte.h.
---
string/strchr.c | 164 ++++++++++----------------------------------------------
1 file changed, 29 insertions(+), 135 deletions(-)
@@ -22,164 +22,58 @@
#include <string.h>
#include <stdlib.h>
+#include <stdint.h>
+#include <string-fzb.h>
+#include <string-fzi.h>
+#include <string-extbyte.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;
+ const op_t *word_ptr;
+ op_t word, repeated_c, found;
+ uintptr_t i, align;
unsigned char c;
c = (unsigned char) c_in;
+ char_ptr = (const unsigned char *) s;
/* 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)
+ Do this until CHAR_PTR is aligned on a word boundary. */
+ align = -(uintptr_t)char_ptr % sizeof(word);
+ for (i = 0; i < align; ++i, ++char_ptr)
if (*char_ptr == c)
- return (void *) char_ptr;
+ return (char *) 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 ();
+ /* Set up a word, each of whose bytes is C. */
+ repeated_c = ((op_t)-1 / 0xff) * c;
- /* 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 (;;)
+ word_ptr = (const op_t *) char_ptr;
+ do
{
- /* 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;
- }
- }
+ word = *word_ptr++;
}
+ while (!has_zero_eq (word, repeated_c));
- return NULL;
+ found = index_first_zero_eq (word, repeated_c);
+ if (extractbyte (word, found) == c)
+ return (char *) (word_ptr - 1) + found;
+ else
+ return NULL;
}
-#ifdef weak_alias
-# undef index
+#ifndef STRCHR
weak_alias (strchr, index)
-#endif
libc_hidden_builtin_def (strchr)
+#endif