| Original patch from: gentoo/src/patchsets/glibc/2.9/1020_all_glibc-2.9-strlen-hack.patch |
| |
| -= BEGIN original header =- |
| http://sourceware.org/bugzilla/show_bug.cgi?id=5807 |
| http://www.cl.cam.ac.uk/~am21/progtricks.html |
| |
| -= END original header =- |
| |
| diff -durN glibc-2_9.orig/string/strlen.c glibc-2_9/string/strlen.c |
| --- glibc-2_9.orig/string/strlen.c 2005-12-14 12:09:07.000000000 +0100 |
| +++ glibc-2_9/string/strlen.c 2009-02-02 22:00:51.000000000 +0100 |
| @@ -32,7 +32,7 @@ |
| { |
| const char *char_ptr; |
| const unsigned long int *longword_ptr; |
| - unsigned long int longword, magic_bits, himagic, lomagic; |
| + unsigned long int longword, himagic, lomagic; |
| |
| /* Handle the first few characters by reading one character at a time. |
| Do this until CHAR_PTR is aligned on a longword boundary. */ |
| @@ -42,28 +42,14 @@ |
| if (*char_ptr == '\0') |
| return char_ptr - str; |
| |
| - /* All these elucidatory comments refer to 4-byte longwords, |
| - but the theory applies equally well to 8-byte longwords. */ |
| - |
| 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. */ |
| - magic_bits = 0x7efefeffL; |
| 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. */ |
| - magic_bits = ((0x7efefefeL << 16) << 16) | 0xfefefeffL; |
| himagic = ((himagic << 16) << 16) | himagic; |
| lomagic = ((lomagic << 16) << 16) | lomagic; |
| } |
| @@ -75,56 +61,12 @@ |
| if *any of the four* bytes in the longword in question are zero. */ |
| for (;;) |
| { |
| - /* We tentatively exit the loop if adding MAGIC_BITS to |
| - LONGWORD fails to change any of the hole bits of LONGWORD. |
| - |
| - 1) Is this safe? Will it catch all the zero bytes? |
| - Suppose there is a byte with all zeros. Any carry bits |
| - propagating from its left will fall into the hole at its |
| - least significant bit and stop. Since there will be no |
| - carry from its most significant bit, the LSB of the |
| - byte to the left will be unchanged, and the zero will be |
| - detected. |
| - |
| - 2) Is this worthwhile? Will it ignore everything except |
| - zero bytes? Suppose every byte of LONGWORD has a bit set |
| - somewhere. There will be a carry into bit 8. If bit 8 |
| - is set, this will carry into bit 16. If bit 8 is clear, |
| - one of bits 9-15 must be set, so there will be a carry |
| - into bit 16. Similarly, there will be a carry into bit |
| - 24. If one of bits 24-30 is set, there will be a carry |
| - into bit 31, so all of the hole bits will be changed. |
| - |
| - The one misfire occurs when bits 24-30 are clear and bit |
| - 31 is set; in this case, the hole at bit 31 is not |
| - changed. If we had access to the processor carry flag, |
| - we could close this loophole by putting the fourth hole |
| - at bit 32! |
| - |
| - So it ignores everything except 128's, when they're aligned |
| - properly. */ |
| - |
| longword = *longword_ptr++; |
| |
| - if ( |
| -#if 0 |
| - /* Add MAGIC_BITS to LONGWORD. */ |
| - (((longword + magic_bits) |
| - |
| - /* Set those bits that were unchanged by the addition. */ |
| - ^ ~longword) |
| - |
| - /* Look at only the hole bits. If any of the hole bits |
| - are unchanged, most likely one of the bytes was a |
| - zero. */ |
| - & ~magic_bits) |
| -#else |
| - ((longword - lomagic) & himagic) |
| -#endif |
| - != 0) |
| + /* This hack taken from Alan Mycroft's HAKMEMC postings. |
| + See: http://www.cl.cam.ac.uk/~am21/progtricks.html */ |
| + 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); |
| |