[PATCH] string: Replace outdated comments in strlen().
Noah Goldstein
goldstein.w.n@gmail.com
Sun Mar 20 04:41:08 GMT 2022
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.
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