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Re: [PATCH 02/11] Improve generic strchr
On 17/12/2016 04:57, Richard Henderson wrote:
> * string/strchr.c: Use haszero.h, whichzero.h, extractbyte.h.
Since you are changing strchrnul, why not just simplify strchr to just:
--
/* Find the first occurrence of C in S. */
char *
STRCHR (const char *s, int c_in)
{
char *r = __strchrnul (s, c_in);
return *(unsigned char *) r == (unsigned char) c_in ? r : NULL;
}
--
?
It will incur in a function call, but as least it has two main advantages:
- less icache pressure due potentially less code.
- an architecture that uses default strchr and strchrnull will require just
to provide an optimized strchrnul to get a boot on both symbols.
> ---
> string/strchr.c | 144 ++++++++------------------------------------------------
> 1 file changed, 19 insertions(+), 125 deletions(-)
>
> diff --git a/string/strchr.c b/string/strchr.c
> index 25c2fe4..a70d72a 100644
> --- a/string/strchr.c
> +++ b/string/strchr.c
> @@ -22,6 +22,10 @@
>
> #include <string.h>
> #include <stdlib.h>
> +#include <stdint.h>
> +#include <haszero.h>
> +#include <whichzero.h>
> +#include <extractbyte.h>
>
> #undef strchr
>
> @@ -35,147 +39,37 @@ 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 long int longword, 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)
> + align = -(uintptr_t)char_ptr % sizeof(longword);
> + 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 ();
> + repeated_c = (-1ul / 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 (;;)
> + longword_ptr = (unsigned long int *) 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;
> - }
> - }
> }
> + while (!haszero2(longword, longword ^ repeated_c));
>
> - return NULL;
> + found = whichzero2(longword, longword ^ repeated_c);
> + if (extractbyte(longword, found) == c)
> + return (char *) (longword_ptr - 1) + found;
> + else
> + return NULL;
> }
>
> #ifdef weak_alias
>