diff options
| -rw-r--r-- | string/strlen.c | 16 |
1 files 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); |