Re: [PATCH] string: Replace outdated comments in strlen().

[Noah Goldstein <goldstein.w.n@gmail.com>]

On Sat, Mar 19, 2022 at 8:05 PM Ricardo Bittencourt via Libc-alpha
<libc-alpha@sourceware.org> wrote:
>
> Copyright The GNU Toolchain Authors.
>
> The comments on strlen() don't match what the actual code does. They
> describe an older algorithm which is no longer in use. This change
> replace the old comments with new ones describing the algorithm used.
>
> I am a first time contributor, and I believe there is no need for
> copyright assignment, since the file changed is not in the shared
> source files list.
>
> This patch only changes comments, but for safety I have run the tests in
> my x64 ubuntu machine, with the following results:
>
> Summary of test results:
>    5051 PASS
>      80 UNSUPPORTED
>      16 XFAIL
>       6 XPASS
>
> Signed-off-by: Ricardo Bittencourt <bluepenguin@gmail.com>
> ---
>  string/strlen.c | 16 +++++-----------
>  1 file changed, 5 insertions(+), 11 deletions(-)
>
> diff --git a/string/strlen.c b/string/strlen.c
> index 0b8aefb812..54f3fb8167 100644
> --- a/string/strlen.c
> +++ b/string/strlen.c
> @@ -46,15 +46,10 @@ STRLEN (const char *str)
>
>    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.  */
> +  /* Computing (longword - lomagic) sets the high bit of any corresponding
> +     byte that is either zero or greater than 0x80.  The latter case can be
> +     filtered out by computing (~longword & himagic).  The final result
> +     will always be non-zero if one of the bytes of longword is zero.  */
>    himagic = 0x80808080L;
>    lomagic = 0x01010101L;
>    if (sizeof (longword) > 4)
> @@ -76,8 +71,7 @@ STRLEN (const char *str)
>
>        if (((longword - lomagic) & ~longword & himagic) != 0)
>         {
> -         /* Which of the bytes was the zero?  If none of them were, it was
> -            a misfire; continue the search.  */
> +         /* Which of the bytes was the zero?  */
>
>           const char *cp = (const char *) (longword_ptr - 1);
>
> --
> 2.25.1
>

LGTM. But wait to commit until monday so others can give feedback
if they want to.