[PATCH v5 07/17] string: Improve generic strchr
Adhemerval Zanella Netto
adhemerval.zanella@linaro.org
Mon Jan 9 19:39:36 GMT 2023
On 05/01/23 20:19, Noah Goldstein wrote:
> 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?
Yes, although I think strchr is a more used symbol than strchrnul. However,
we can optimize it later by adding a __strchrnul_inline and expand it
on both strchr and strchrnul. I will change to use strchrnul as you suggested.
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