From: Noah Goldstein <goldstein.w.n@gmail.com>
To: Adhemerval Zanella <adhemerval.zanella@linaro.org>
Cc: libc-alpha@sourceware.org, Richard Henderson <rth@twiddle.net>
Subject: Re: [PATCH v5 07/17] string: Improve generic strchr
Date: Thu, 5 Jan 2023 15:19:10 -0800 [thread overview]
Message-ID: <CAFUsyfKrGiwTGgMu6Cyi938wd9tE4kjev1Vr6PuoK0pOLn5nSw@mail.gmail.com> (raw)
In-Reply-To: <CAFUsyfK_+YYwpTZ9d=1i93PyVs2cAFY1Mh5inkHyZe9xcdmpng@mail.gmail.com>
On Thu, Jan 5, 2023 at 3:09 PM Noah Goldstein <goldstein.w.n@gmail.com> wrote:
>
> On Mon, Sep 19, 2022 at 1:01 PM Adhemerval Zanella via Libc-alpha
> <libc-alpha@sourceware.org> wrote:
> >
> > 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} and string-extbyte function.
> >
> > 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).
> >
> > Co-authored-by: Richard Henderson <rth@twiddle.net>
> > ---
> > string/strchr.c | 172 +++++++---------------------------------
> > sysdeps/s390/strchr-c.c | 11 +--
> > 2 files changed, 34 insertions(+), 149 deletions(-)
> >
> > diff --git a/string/strchr.c b/string/strchr.c
> > index bfd0c4e4bc..6bbee7f79d 100644
> > --- a/string/strchr.c
> > +++ b/string/strchr.c
> > @@ -22,164 +22,48 @@
> >
> > #include <string.h>
> > #include <stdlib.h>
> > +#include <stdint.h>
> > +#include <string-fza.h>
> > +#include <string-fzb.h>
> > +#include <string-fzi.h>
> > +#include <string-extbyte.h>
> > +#include <string-maskoff.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.
> > + /* Set up a word, each of whose bytes is C. */
> > + unsigned char c = (unsigned char) c_in;
> > + 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.
> > + /* Align the input address to op_t. */
> > + uintptr_t s_int = (uintptr_t) s;
> > + const op_t *word_ptr = word_containing (s);
> >
> > - 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!
> > + /* Read the first aligned word, but force bytes before the string to
> > + match neither zero nor goal (we make sure the high bit of each byte
> > + is 1, and the low 7 bits are all the opposite of the goal byte). */
> > + op_t bmask = create_mask (s_int);
> > + op_t word = (*word_ptr | bmask) ^ (repeated_c & highbit_mask (bmask));
> >
> > - So it ignores everything except 128's, when they're aligned
> > - properly.
> > + while (! has_zero_eq (word, repeated_c))
> > + word = *++word_ptr;
> >
> > - 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;
> > - }
> > - }
> > - }
> > + op_t found = index_first_zero_eq (word, repeated_c);
> >
> > + if (extractbyte (word, found) == c)
> > + return (char *) (word_ptr) + found;
> > return NULL;
> > }
> > -
> > -#ifdef weak_alias
> > -# undef index
> > +#ifndef STRCHR
> > weak_alias (strchr, index)
> > -#endif
> > libc_hidden_builtin_def (strchr)
> > +#endif
> > diff --git a/sysdeps/s390/strchr-c.c b/sysdeps/s390/strchr-c.c
> > index 4ac3a62fba..a5a1781b1c 100644
> > --- a/sysdeps/s390/strchr-c.c
> > +++ b/sysdeps/s390/strchr-c.c
> > @@ -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
> > --
> > 2.34.1
> >
>
> Can this just be implemented as:
>
> char * r = strchrnul(p, c);
> return *r ? r : NULL;
Thats wrong, should be: `return (*r == c) ? r : NULL;`
>
> then only have strchrnul impl to worry about?
next prev parent reply other threads:[~2023-01-05 23:19 UTC|newest]
Thread overview: 55+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-09-19 19:59 [PATCH v5 00/17] Improve generic string routines Adhemerval Zanella
2022-09-19 19:59 ` [PATCH v5 01/17] Parameterize op_t from memcopy.h Adhemerval Zanella
2022-09-19 19:59 ` [PATCH v5 02/17] Parameterize OP_T_THRES " Adhemerval Zanella
2022-09-20 10:49 ` Carlos O'Donell
2022-09-19 19:59 ` [PATCH v5 03/17] Add string-maskoff.h generic header Adhemerval Zanella
2022-09-20 11:43 ` Carlos O'Donell
2022-09-22 17:31 ` Adhemerval Zanella Netto
2023-01-05 22:49 ` Noah Goldstein
2023-01-05 23:26 ` Alejandro Colomar
2023-01-09 18:19 ` Adhemerval Zanella Netto
2023-01-09 18:02 ` Adhemerval Zanella Netto
2022-09-19 19:59 ` [PATCH v5 04/17] Add string vectorized find and detection functions Adhemerval Zanella
2023-01-05 22:53 ` Noah Goldstein
2023-01-09 18:51 ` Adhemerval Zanella Netto
2023-01-05 23:04 ` Noah Goldstein
2023-01-09 19:34 ` Adhemerval Zanella Netto
2022-09-19 19:59 ` [PATCH v5 05/17] string: Improve generic strlen Adhemerval Zanella
2022-09-19 19:59 ` [PATCH v5 06/17] string: Improve generic strnlen Adhemerval Zanella
2022-09-19 19:59 ` [PATCH v5 07/17] string: Improve generic strchr Adhemerval Zanella
2023-01-05 23:09 ` Noah Goldstein
2023-01-05 23:19 ` Noah Goldstein [this message]
2023-01-09 19:39 ` Adhemerval Zanella Netto
2022-09-19 19:59 ` [PATCH v5 08/17] string: Improve generic strchrnul Adhemerval Zanella
2023-01-05 23:17 ` Noah Goldstein
2023-01-09 20:35 ` Adhemerval Zanella Netto
2023-01-09 20:49 ` Richard Henderson
2023-01-09 20:59 ` Noah Goldstein
2023-01-09 21:01 ` Noah Goldstein
2023-01-09 23:33 ` Richard Henderson
2023-01-10 14:18 ` Adhemerval Zanella Netto
2023-01-10 16:24 ` Richard Henderson
2023-01-10 17:16 ` Noah Goldstein
2023-01-10 18:19 ` Adhemerval Zanella Netto
2023-01-10 17:17 ` Noah Goldstein
2023-01-10 18:16 ` Adhemerval Zanella Netto
2022-09-19 19:59 ` [PATCH v5 09/17] string: Improve generic strcmp Adhemerval Zanella
2022-09-19 19:59 ` [PATCH v5 10/17] string: Improve generic memchr Adhemerval Zanella
2023-01-05 23:47 ` Noah Goldstein
2023-01-09 20:50 ` Adhemerval Zanella Netto
2023-01-05 23:49 ` Noah Goldstein
2023-01-09 20:51 ` Adhemerval Zanella Netto
2023-01-09 21:26 ` Noah Goldstein
2023-01-10 14:33 ` Adhemerval Zanella Netto
2022-09-19 19:59 ` [PATCH v5 11/17] string: Improve generic memrchr Adhemerval Zanella
2023-01-05 23:51 ` Noah Goldstein
2022-09-19 19:59 ` [PATCH v5 12/17] hppa: Add memcopy.h Adhemerval Zanella
2022-09-19 19:59 ` [PATCH v5 13/17] hppa: Add string-fzb.h and string-fzi.h Adhemerval Zanella
2022-09-19 19:59 ` [PATCH v5 14/17] alpha: " Adhemerval Zanella
2022-09-19 19:59 ` [PATCH v5 15/17] arm: Add string-fza.h Adhemerval Zanella
2022-09-19 19:59 ` [PATCH v5 16/17] powerpc: " Adhemerval Zanella
2022-09-19 19:59 ` [PATCH v5 17/17] sh: Add string-fzb.h Adhemerval Zanella
2022-12-05 17:07 ` [PATCH v5 00/17] Improve generic string routines Xi Ruoyao
2023-01-05 21:56 ` Adhemerval Zanella Netto
2023-01-05 23:52 ` Noah Goldstein
2023-01-06 13:43 ` Adhemerval Zanella Netto
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