Port `strlen` in gcc, which enhance performance over 10 times Please refer to these following articles 1. [Determine if a word has a byte less than n] (https://graphics.stanford.edu/~seander/bithacks.html#HasLessInWord) 2. [Determine if a word has a zero byte] (https://graphics.stanford.edu/~seander/bithacks.html#ZeroInWord)
Signed-off-by: Hsin-Yu.Chen <[email protected]> --- lib/string.c | 77 +++++++++++++++++++++++++++++++++++++++++++++++++--- 1 file changed, 73 insertions(+), 4 deletions(-) diff --git a/lib/string.c b/lib/string.c index 6891d15ce991..31e8642422af 100644 --- a/lib/string.c +++ b/lib/string.c @@ -398,11 +398,80 @@ EXPORT_SYMBOL(strnchr); #ifndef __HAVE_ARCH_STRLEN size_t strlen(const char *s) { - const char *sc; + const char *char_ptr; + const unsigned long *longword_ptr; + unsigned long longword, himagic, lomagic; - for (sc = s; *sc != '\0'; ++sc) - /* nothing */; - return sc - s; + /* Handle the first few characters by reading one character at a time. + * Do this until CHAR_PTR is aligned on a longword boundary. + */ + for (char_ptr = s; ((unsigned long) char_ptr + & (sizeof(longword) - 1)) != 0; + ++char_ptr) + if (*char_ptr == '\0') + return char_ptr - s; + + /* All these elucidatory comments refer to 4-byte longwords, + * but the theory applies equally well to 8-byte longwords. + */ + longword_ptr = (unsigned long *) 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. + */ + himagic = 0x80808080L; + lomagic = 0x01010101L; + + if (sizeof(longword) > 4) { + /* 64-bit version of the magic. */ + /* Do the shift in two steps to avoid a warning if long has 32 bits. + */ + himagic = ((himagic << 16) << 16) | himagic; + lomagic = ((lomagic << 16) << 16) | lomagic; + } + + if (sizeof(longword) > 8) + abort(); + + /* Instead of the traditional loop which tests each character, + * we will test a longword at a time. The tricky part is testing + * if *any of the four* bytes in the longword in question are zero. + */ + for (;;) { + longword = *longword_ptr++; + 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. + */ + const char *cp = (const char *) (longword_ptr - 1); + + if (cp[0] == 0) + return cp - s; + else if (cp[1] == 0) + return cp - s + 1; + else if (cp[2] == 0) + return cp - s + 2; + else if (cp[3] == 0) + return cp - s + 3; + if (sizeof(longword) > 4) { + if (cp[4] == 0) + return cp - s + 4; + else if (cp[5] == 0) + return cp - s + 5; + else if (cp[6] == 0) + return cp - s + 6; + else if (cp[7] == 0) + return cp - s + 7; + } + } + } } EXPORT_SYMBOL(strlen); #endif -- 2.38.1
