[PATCH 10/17] string: Improve generic memchr

[Noah Goldstein]

On Mon, Sep 19, 2022 at 12:17 PM Adhemerval Zanella Netto
<adhemerval.zanella@linaro.org> wrote:
>
>
>
> On 03/09/22 00:47, Noah Goldstein wrote:
>
> >>
> >> -  longword_ptr = (const longword *) char_ptr;
> >> +  /* Compute the address of the word containing the last byte. */
> >> +  const op_t *lword = word_containing (lbyte);
> >>
> >> -  /* All these elucidatory comments refer to 4-byte longwords,
> >> -     but the theory applies equally well to any size longwords.  */
> >> +  /* Read the first word, but munge it so that bytes before the array
> >> +     will not match goal.  */
> >> +  const op_t * word_ptr = word_containing (s);
> >> +  op_t word = (*word_ptr | before_mask) ^ (repeated_c & before_mask);
> >
> > Why do you xor with repeated_c & before_mask here?
> >
> > Doesn't the has_eq(word, repeated_c) do that?
>
> For the case of c_in being 0xff, since for this case or with before_mask
> will make has_eq to return early.  The test-memchr does not trigger it,
> but test-memccpy does fail without the XOR.

I see. Since a match in the first several bytes is fairly common
maybe it would be better to special case the first iteration and just do

has_eq(word, repeated_c) >> (CHAR_BIT * (addr % sizeof(addr)).
The result can just be added to `s` if there is a match.