On Thu, May 2, 2024 at 5:14 PM Hsin-Yu.Chen <[email protected]> wrote: > > Port `strlen` in gcc,
This is the Linux kernel project. What does this mean? Also note we refer to the function as strlen() (and no [back]quotes). > 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) Make these proper Link: tags Link: URL#1 [1] Link: URL#2 [2] ... > - const char *sc; > + const char *char_ptr; No need to change the var name for this. > + const unsigned long *longword_ptr; > + unsigned long longword, himagic, lomagic; Keep it in reversed xmas tree order. ... > + /* Handle the first few characters by reading one character at a time. > + * Do this until CHAR_PTR is aligned on a longword boundary. > + */ /* * This is the wrong comment style. You may * use this example. */ ... > + for (char_ptr = s; ((unsigned long) char_ptr > + & (sizeof(longword) - 1)) != 0; > + ++char_ptr) This is too verbose (too many unneeded symbols) and why pre-increment? What is special about it? ... > + /* All these elucidatory comments refer to 4-byte longwords, > + * but the theory applies equally well to 8-byte longwords. > + */ Use proper style. > + longword_ptr = (unsigned long *) char_ptr; No space after casting and why do you need it? ... > + /* 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. > + */ Use proper style. ... > + /* 64-bit version of the magic. */ > + /* Do the shift in two steps to avoid a warning if long has > 32 bits. > + */ Ditto. ... > + if (sizeof(longword) > 8) > + abort(); Huh?! ... > + /* 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. > + */ Proper style, please. ... > + 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 (... <= 4) continue; > + 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; A lot of redundant 'else':s. > + } > + } > + } -- With Best Regards, Andy Shevchenko
