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Re: [PATCH 06/11] Improve generic memrchr
This implementation has a lot of issues, checking on both x86_64 and i386
(by manually removing the assembly implementation to use the new default
one) I am seeing a lot of issues with string/testers and others.
Unfortunately the test-memrchr itself does not trigger most of the issues.
On 17/12/2016 04:57, Richard Henderson wrote:
> * string/memrchr.c: Use haszero.h, whichzero.h.
> ---
> string/memrchr.c | 180 ++++++++++---------------------------------------------
> 1 file changed, 30 insertions(+), 150 deletions(-)
>
> diff --git a/string/memrchr.c b/string/memrchr.c
> index b9b0c9e..d45b261 100644
> --- a/string/memrchr.c
> +++ b/string/memrchr.c
> @@ -21,180 +21,60 @@
> License along with the GNU C Library; if not, see
> <http://www.gnu.org/licenses/>. */
>
> -#include <stdlib.h>
> -
> -#ifdef HAVE_CONFIG_H
> -# include <config.h>
> -#endif
> -
> -#undef __ptr_t
> -#define __ptr_t void *
> -
> -#if defined _LIBC
> -# include <string.h>
> -# include <memcopy.h>
> -#endif
> -
> -#if defined HAVE_LIMITS_H || defined _LIBC
> -# include <limits.h>
> -#endif
> -
> -#define LONG_MAX_32_BITS 2147483647
> -
> -#ifndef LONG_MAX
> -# define LONG_MAX LONG_MAX_32_BITS
> -#endif
> -
> -#include <sys/types.h>
> +#include <string.h>
> +#include <stdint.h>
> +#include <haszero.h>
> +#include <whichzero.h>
>
> #undef __memrchr
> #undef memrchr
>
> -#ifndef weak_alias
> -# define __memrchr memrchr
> -#endif
> -
> /* Search no more than N bytes of S for C. */
> -__ptr_t
> -#ifndef MEMRCHR
> -__memrchr
> -#else
> -MEMRCHR
> -#endif
We can not get rid of this indirections without change the architectures
that uses the default implementation on ifunc (i386 for instance).
> - (const __ptr_t s, int c_in, size_t n)
> +void *
> +__memrchr (const void *s, int c_in, size_t n)
> {
> - const unsigned char *char_ptr;
> + const unsigned char *char_ptr = (const unsigned char *) s + n;
> const unsigned long int *longword_ptr;
> - unsigned long int longword, magic_bits, charmask;
> + unsigned long int longword, repeated_c;
> + uintptr_t i, align;
> unsigned char c;
>
> c = (unsigned char) c_in;
>
> /* Handle the last 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 + n;
> - n > 0 && ((unsigned long int) char_ptr
> - & (sizeof (longword) - 1)) != 0;
> - --n)
> + align = (uintptr_t)char_ptr % sizeof(longword);
> + for (i = 0; i < align; ++i)
> if (*--char_ptr == c)
> - return (__ptr_t) char_ptr;
> + return (void *) char_ptr;
We need to take care of 'n' while interacting over the string to align
it. It might the case where 's' is unaligned and 'n' is less than
the aligned value.
>
> - /* All these elucidatory comments refer to 4-byte longwords,
> - but the theory applies equally well to 8-byte longwords. */
> + /* Set up a longword, each of whose bytes is C. */
> + repeated_c = (-1ul / 0xff) * c;
>
> longword_ptr = (const unsigned long int *) char_ptr;
> + n -= align;
> + if (__glibc_unlikely(n == 0))
> + return NULL;
>
> - /* 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 LONG_MAX > LONG_MAX_32_BITS
> - charmask |= charmask << 32;
> -#endif
> -
> - /* 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. */
> - while (n >= sizeof (longword))
> + /* Loop while we have more than one longword remaining. */
> + while (n > sizeof (longword))
> {
> - /* 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, not 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 ^ charmask;
> -
> - /* 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)
> - {
> - /* Which of the bytes was C? If none of them were, it was
> - a misfire; continue the search. */
> -
> - const unsigned char *cp = (const unsigned char *) longword_ptr;
> -
> -#if LONG_MAX > 2147483647
> - if (cp[7] == c)
> - return (__ptr_t) &cp[7];
> - if (cp[6] == c)
> - return (__ptr_t) &cp[6];
> - if (cp[5] == c)
> - return (__ptr_t) &cp[5];
> - if (cp[4] == c)
> - return (__ptr_t) &cp[4];
> -#endif
> - if (cp[3] == c)
> - return (__ptr_t) &cp[3];
> - if (cp[2] == c)
> - return (__ptr_t) &cp[2];
> - if (cp[1] == c)
> - return (__ptr_t) &cp[1];
> - if (cp[0] == c)
> - return (__ptr_t) cp;
> - }
> -
> + longword = *--longword_ptr ^ repeated_c;
> + if (haszero (longword))
> + goto found;
> n -= sizeof (longword);
> }
>
> - char_ptr = (const unsigned char *) longword_ptr;
> -
> - while (n-- > 0)
> + /* Since our pointer is aligned, we can always read the last longword. */
> + longword = *--longword_ptr ^ repeated_c;
> + if (haszero (longword))
> {
> - if (*--char_ptr == c)
> - return (__ptr_t) char_ptr;
> + found:
> + i = whichzero (longword);
> + if (sizeof (longword) - 1 - i < n)
> + return (char *) longword_ptr + i;
We need to check longword in reverse word since we are checking for last
occurrence.
Below it is a workable implementation (on x86_64, I haven't checked on
a BE machine since I do not have access to one). It passes all the string
tests:
--
#ifndef weak_alias
# define __memrchr memrchr
#endif
static inline unsigned int
whichzero_reverse (unsigned long int x)
{
if (sizeof (x) == 4)
return whichzero (__builtin_bswap32 (x));
else
return whichzero (__builtin_bswap64 (x));
}
/* Search no more than N bytes of S for C. */
void *
#ifndef MEMRCHR
__memrchr
#else
MEMRCHR
#endif
(const void *s, int c_in, size_t n)
{
const unsigned char *char_ptr = (const unsigned char *) s + n;
const unsigned long int *longword_ptr;
unsigned long int longword, repeated_c;
uintptr_t i;
unsigned char c;
c = (unsigned char) c_in;
/* Align s to size_t reading one character at time. */
for (;
((uintptr_t)char_ptr & (sizeof(longword) - 1)) && (n != 0);
--n)
if (*--char_ptr == c_in)
return (void *) char_ptr;
/* Set up a longword, each of whose bytes is C. */
repeated_c = (-1ul / 0xff) * c;
/* Loop while we have more than one longword remaining. */
for (longword_ptr = (const void*)char_ptr;
n > sizeof(size_t);
n -= sizeof (size_t))
{
longword = *--longword_ptr ^ repeated_c;
if (haszero (longword))
goto found;
}
longword = *--longword_ptr ^ repeated_c;
if (haszero (longword))
{
found:
i = whichzero_reverse (longword);
if (i < n)
return (char*) (longword_ptr) + (sizeof (longword) - 1 - i);
}
return NULL;
}
#ifndef MEMRCHR
# ifdef weak_alias
weak_alias (__memrchr, memrchr)
# endif
#endif