[PATCH v5 08/17] string: Improve generic strchrnul
Adhemerval Zanella Netto
adhemerval.zanella@linaro.org
Tue Jan 10 18:16:10 GMT 2023
On 10/01/23 14:17, Noah Goldstein wrote:
> On Tue, Jan 10, 2023 at 6:18 AM Adhemerval Zanella Netto
> <adhemerval.zanella@linaro.org> wrote:
>>
>>
>>
>> On 09/01/23 20:33, Richard Henderson wrote:
>>> On 1/9/23 12:35, Adhemerval Zanella Netto 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);
>>>
>>> Oh, two fine points:
>>>
>>> (1) big-endian would want shifting left,
>>> (2) alpha would want shifting by bits not bytes,
>>> because the cmpbge insn produces an 8-bit mask.
>>>
>>> so you'd need to hide this shift in the headers like create_mask().
>>
>> Alright, the following works:
>>
>>
>> static __always_inline op_t
>> check_mask (op_t word, uintptr_t s_int)
>> {
>> if (__BYTE_ORDER == __LITTLE_ENDIAN)
>> return word >> (CHAR_BIT * (s_int % sizeof (s_int)));
>> else
>> return word << (CHAR_BIT * (s_int % sizeof (s_int)));
>> }
>
> Imo put this in with "[PATCH v5 03/17] Add string-maskoff.h generic header"
> think may also be needed for memchr.
Yeap, this is what I have done.
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