[v7,08/17] string: Improve generic strchrnul
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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 function to
remove unwanted data. This strategy follow arch-specific
optimization used on aarch64 and powerpc.
- Use string-fz{b,i} functions.
Checked on x86_64-linux-gnu, i686-linux-gnu, powerpc64-linux-gnu,
and powerpc-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>
---
string/strchrnul.c | 153 +++---------------
.../power4/multiarch/strchrnul-ppc32.c | 4 -
sysdeps/s390/strchrnul-c.c | 2 -
3 files changed, 22 insertions(+), 137 deletions(-)
@@ -1,10 +1,5 @@
/* Copyright (C) 1991-2023 Free Software Foundation, Inc.
This file is part of the GNU C Library.
- Based on strlen implementation by Torbjorn Granlund (tege@sics.se),
- with help from Dan Sahlin (dan@sics.se) and
- bug fix and commentary by Jim Blandy (jimb@ai.mit.edu);
- adaptation to strchr suggested by Dick Karpinski (dick@cca.ucsf.edu),
- and implemented by Roland McGrath (roland@ai.mit.edu).
The GNU C Library is free software; you can redistribute it and/or
modify it under the terms of the GNU Lesser General Public
@@ -21,146 +16,42 @@
<https://www.gnu.org/licenses/>. */
#include <string.h>
-#include <memcopy.h>
#include <stdlib.h>
+#include <stdint.h>
+#include <string-fza.h>
+#include <string-fzb.h>
+#include <string-fzi.h>
+#include <string-maskoff.h>
#undef __strchrnul
#undef strchrnul
-#ifndef STRCHRNUL
-# define STRCHRNUL __strchrnul
+#ifdef STRCHRNUL
+# define __strchrnul STRCHRNUL
#endif
/* Find the first occurrence of C in S or the final NUL byte. */
char *
-STRCHRNUL (const char *s, int c_in)
+__strchrnul (const char *str, 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 || *char_ptr == '\0')
- return (void *) char_ptr;
-
- /* 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.
+ op_t repeated_c = repeat_bytes (c_in);
- 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.
+ uintptr_t s_int = (uintptr_t) str;
+ const op_t *word_ptr = word_containing (str);
- 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!
+ op_t word = *word_ptr;
- So it ignores everything except 128's, when they're aligned
- properly.
+ find_t mask = shift_find (find_zero_eq_all (word, repeated_c), s_int);
+ if (mask != 0)
+ return (char *) str + index_first (mask);
- 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. */
+ do
+ word = *++word_ptr;
+ while (! has_zero_eq (word, repeated_c));
- 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 || *cp == '\0')
- return (char *) cp;
- if (*++cp == c || *cp == '\0')
- return (char *) cp;
- if (*++cp == c || *cp == '\0')
- return (char *) cp;
- if (*++cp == c || *cp == '\0')
- return (char *) cp;
- if (sizeof (longword) > 4)
- {
- if (*++cp == c || *cp == '\0')
- return (char *) cp;
- if (*++cp == c || *cp == '\0')
- return (char *) cp;
- if (*++cp == c || *cp == '\0')
- return (char *) cp;
- if (*++cp == c || *cp == '\0')
- return (char *) cp;
- }
- }
- }
-
- /* This should never happen. */
- return NULL;
+ op_t found = index_first_zero_eq (word, repeated_c);
+ return (char *) word_ptr + found;
}
-
+#ifndef STRCHRNUL
weak_alias (__strchrnul, strchrnul)
+#endif
@@ -19,10 +19,6 @@
#include <string.h>
#define STRCHRNUL __strchrnul_ppc
-
-#undef weak_alias
-#define weak_alias(a,b )
-
extern __typeof (strchrnul) __strchrnul_ppc attribute_hidden;
#include <string/strchrnul.c>
@@ -22,8 +22,6 @@
# if HAVE_STRCHRNUL_IFUNC
# define STRCHRNUL STRCHRNUL_C
# define __strchrnul STRCHRNUL
-# undef weak_alias
-# define weak_alias(name, alias)
# endif
# include <string/strchrnul.c>