Re: [PATCH 2/2] lib/string.c: Optimize memchr()

[Yu-Jen Chang]

Andrey Semashev <andrey.semashev@xxxxxxxxx> 於 2022年7月11日 週一 凌晨4:01寫道：
>
> On 7/10/22 17:28, Yu-Jen Chang wrote:
> > The original version of memchr() is implemented with the byte-wise
> > comparing technique, which does not fully use 64-bits or 32-bits
> > registers in CPU. We use word-wide comparing so that 8 characters
> > can be compared at the same time on CPU. This code is base on
> > David Laight's implementation.
> >
> > We create two files to measure the performance. The first file
> > contains on average 10 characters ahead the target character.
> > The second file contains at least 1000 characters ahead the
> > target character. Our implementation of “memchr()” is slightly
> > better in the first test and nearly 4x faster than the orginal
> > implementation in the second test.
> >
> > Signed-off-by: Yu-Jen Chang <arthurchang09@xxxxxxxxx>
> > Signed-off-by: Ching-Chun (Jim) Huang <jserv@xxxxxxxxxxxxxxxx>
> > ---
> >  lib/string.c | 28 +++++++++++++++++++++-------
> >  1 file changed, 21 insertions(+), 7 deletions(-)
> >
> > diff --git a/lib/string.c b/lib/string.c
> > index 80469e6c3..8ca965431 100644
> > --- a/lib/string.c
> > +++ b/lib/string.c
> > @@ -905,21 +905,35 @@ EXPORT_SYMBOL(strnstr);
> >  #ifndef __HAVE_ARCH_MEMCHR
> >  /**
> >   * memchr - Find a character in an area of memory.
> > - * @s: The memory area
> > + * @p: The memory area
> >   * @c: The byte to search for
> > - * @n: The size of the area.
> > + * @length: The size of the area.
> >   *
> >   * returns the address of the first occurrence of @c, or %NULL
> >   * if @c is not found
> >   */
> > -void *memchr(const void *s, int c, size_t n)
> > +void *memchr(const void *p, int c, unsigned long length)
> >  {
> > -     const unsigned char *p = s;
> > -     while (n-- != 0) {
> > -             if ((unsigned char)c == *p++) {
> > -                     return (void *)(p - 1);
> > +     u64 mask, val;
> > +     const void *end = p + length;
> > +
> > +     c &= 0xff;
> > +     if (p <= end - 8) {
> > +             mask = c;
> > +             MEMCHR_MASK_GEN(mask);
> > +
> > +             for (; p <= end - 8; p += 8) {
> > +                     val = *(u64 *)p ^ mask;
>
> What if p is not aligned to 8 (or 4 on 32-bit targets) bytes? Not all
> targets support (efficient) unaligned loads, do they?

I think it works if p is not aligned to 8 or 4 bytes.

Let's say the string is 10 bytes. The for loop here will search the first
8 bytes. If the target character is in the last 2 bytes, the second for
loop will find it. It also work like this on 32-bit machine.

>
> > +                     if ((val + 0xfefefefefefefeffu) &
> > +                         (~val & 0x8080808080808080u))
> > +                             break;
> >               }
> >       }
> > +
> > +     for (; p < end; p++)
> > +             if (*(unsigned char *)p == c)
> > +                     return (void *)p;
> > +
> >       return NULL;
> >  }
> >  EXPORT_SYMBOL(memchr);
>