From: Noah Goldstein <goldstein.w.n@gmail.com>
To: Adhemerval Zanella Netto <adhemerval.zanella@linaro.org>
Cc: libc-alpha@sourceware.org, Richard Henderson <rth@twiddle.net>
Subject: Re: [PATCH v5 08/17] string: Improve generic strchrnul
Date: Mon, 9 Jan 2023 13:01:09 -0800 [thread overview]
Message-ID: <CAFUsyfJ+1Y7EPhbbBiT22m55KS=0=9nNeVmF-8TQQ1wpapzAvw@mail.gmail.com> (raw)
In-Reply-To: <CAFUsyfLhK3FEPwCjWutZ+q0q4q-ZsS357q-ePwJ6d0wBKfAMng@mail.gmail.com>
On Mon, Jan 9, 2023 at 12:59 PM Noah Goldstein <goldstein.w.n@gmail.com> wrote:
>
> On Mon, Jan 9, 2023 at 12:35 PM Adhemerval Zanella Netto
> <adhemerval.zanella@linaro.org> wrote:
> >
> >
> >
> > On 05/01/23 20:17, Noah Goldstein wrote:
> > > On Mon, Sep 19, 2022 at 1:04 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} 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 <rth@twiddle.net>
> > >> ---
> > >> string/strchrnul.c | 156 +++---------------
> > >> .../power4/multiarch/strchrnul-ppc32.c | 4 -
> > >> sysdeps/s390/strchrnul-c.c | 2 -
> > >> 3 files changed, 24 insertions(+), 138 deletions(-)
> > >>
> > >> diff --git a/string/strchrnul.c b/string/strchrnul.c
> > >> index 0cc1fc6bb0..67defa3dab 100644
> > >> --- a/string/strchrnul.c
> > >> +++ b/string/strchrnul.c
> > >> @@ -1,10 +1,5 @@
> > >> /* Copyright (C) 1991-2022 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 +16,43 @@
> > >> <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.
> > >> + /* Set up a word, each of whose bytes is C. */
> > >> + 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) 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!
> > >> + /* 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));
> > >
> > > Think much clearer (and probably better codegen) is:
> > > find_zero_eq_low/all(word, repeated) >> (s_int * CHAR_BIT)
> >
> > It does not seem to work, at least not replacing the two lines with:
> >
> > op_t word = find_zero_eq_all/low (*word_ptr, repeated_c) >> (s_int * CHAR_BIT);
> >
> > The loader itself can not loader anything (which means strchr is failing
> > somewhere).
>
> The following works for me (only checking test-strchrnul, so maybe its missing
> a case), also as I have it here its only little endian. (Would need an
> ifdef here
> or API for shifting out out-of-bounds bits for cross platform).
> ```
> op_t word = *word_ptr;
> op_t mask = find_zero_eq_low(word, repeated_c) >> (CHAR_BIT * (s_int
> % sizeof(uintptr_t)));
> if(mask) {
> return (char *) str + index_first_(mask);
> }
>
word = *++word_ptr;
> while (! has_zero_eq (word, repeated_c))
> word = *++word_ptr;
>
> op_t found = index_first_zero_eq (word, repeated_c);
> return (char *) (word_ptr) + found;
> ```
next prev parent reply other threads:[~2023-01-09 21:01 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
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 [this message]
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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