From: Eric Blake Date: Wed, 23 Apr 2008 21:03:40 +0000 (-0600) Subject: Improve memchr2 performance. X-Git-Url: https://pintos-os.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=092621651ba401797a2e671eb24207c49e5e9b64;p=pspp Improve memchr2 performance. * lib/memchr2.c (memchr2): Further optimize parallel detection of NUL bytes. * modules/memchr2 (Depends-on): Use intprops.h. Signed-off-by: Eric Blake --- diff --git a/ChangeLog b/ChangeLog index 4b87fda684..769f0f3e14 100644 --- a/ChangeLog +++ b/ChangeLog @@ -1,3 +1,10 @@ +2008-04-23 Eric Blake + + Improve memchr2 performance. + * lib/memchr2.c (memchr2): Further optimize parallel detection of + NUL bytes. + * modules/memchr2 (Depends-on): Use intprops.h. + 2008-04-23 Simon Josefsson * lib/sys_socket.in.h (setsockopt): Be more type safe by declaring diff --git a/lib/memchr2.c b/lib/memchr2.c index 38533434d4..81de6138a9 100644 --- a/lib/memchr2.c +++ b/lib/memchr2.c @@ -29,19 +29,28 @@ along with this program. If not, see . */ #include #include +#include "intprops.h" + /* Return the first address of either C1 or C2 (treated as unsigned char) that occurs within N bytes of the memory region S. If neither byte appears, return NULL. */ void * memchr2 (void const *s, int c1_in, int c2_in, size_t n) { + /* On 32-bit hardware, choosing longword to be a 32-bit unsigned + long instead of a 64-bit uintmax_t tends to give better + performance. On 64-bit hardware, unsigned long is generally 64 + bits already. Change this typedef to experiment with + performance. */ + typedef unsigned long longword; + const unsigned char *char_ptr; - const uintmax_t *longword_ptr; - uintmax_t longword1; - uintmax_t longword2; - uintmax_t magic_bits; - uintmax_t charmask1; - uintmax_t charmask2; + const longword *longword_ptr; + longword longword1; + longword longword2; + longword magic_bits; + longword charmask1; + longword charmask2; unsigned char c1; unsigned char c2; int i; @@ -63,31 +72,17 @@ memchr2 (void const *s, int c1_in, int c2_in, size_t n) /* All these elucidatory comments refer to 4-byte longwords, but the theory applies equally well to any size longwords. */ - longword_ptr = (const uintmax_t *) 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. */ - - /* Set MAGIC_BITS to be this pattern of 1 and 0 bits. - Set CHARMASK to be a longword, each of whose bytes is C. */ - - magic_bits = 0xfefefefe; + longword_ptr = (const longword *) char_ptr; + magic_bits = 0x01010101; charmask1 = c1 | (c1 << 8); charmask2 = c2 | (c2 << 8); charmask1 |= charmask1 << 16; charmask2 |= charmask2 << 16; - if (0xffffffffU < UINTMAX_MAX) + if (0xffffffffU < TYPE_MAXIMUM (longword)) { - magic_bits |= magic_bits << 32; - charmask1 |= charmask1 << 32; - charmask2 |= charmask2 << 32; + magic_bits |= magic_bits << 31 << 1; + charmask1 |= charmask1 << 31 << 1; + charmask2 |= charmask2 << 31 << 1; if (8 < sizeof longword1) for (i = 64; i < sizeof longword1 * 8; i *= 2) { @@ -96,89 +91,28 @@ memchr2 (void const *s, int c1_in, int c2_in, size_t n) charmask2 |= charmask2 << i; } } - magic_bits = (UINTMAX_MAX >> 1) & (magic_bits | 1); /* 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. */ + if *any of the four* bytes in the longword in question are zero. + + We first use an xor to convert target bytes into a NUL byte, + since the test for a zero byte is more efficient. For all byte + values except 0x00 and 0x80, subtracting 1 from the byte will + leave the most significant bit unchanged. So detecting 0 is + simply a matter of subtracting from all bytes in parallel, and + checking for a most significant bit that changed to 1. */ + while (n >= sizeof longword1) { - /* 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. - - 3) But wait! Aren't we looking for C, not zero? - Good point. So what we do is XOR LONGWORD with a longword, - each of whose bytes is C. This turns each byte that is C - into a zero. */ - longword1 = *longword_ptr ^ charmask1; - longword2 = *longword_ptr++ ^ charmask2; - - /* Add MAGIC_BITS to LONGWORD. */ - if ((((longword1 + magic_bits) - - /* Set those bits that were unchanged by the addition. */ - ^ ~longword1) - - /* 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) != 0 - || (((longword2 + magic_bits) ^ ~longword2) & ~magic_bits) != 0) - { - /* Which of the bytes was C? If none of them were, it was - a misfire; continue the search. */ - - const unsigned char *cp = (const unsigned char *) (longword_ptr - 1); - - if (cp[0] == c1 || cp[0] == c2) - return (void *) cp; - if (cp[1] == c1 || cp[1] == c2) - return (void *) &cp[1]; - if (cp[2] == c1 || cp[2] == c2) - return (void *) &cp[2]; - if (cp[3] == c1 || cp[3] == c2) - return (void *) &cp[3]; - if (4 < sizeof longword1 && (cp[4] == c1 || cp[4] == c2)) - return (void *) &cp[4]; - if (5 < sizeof longword1 && (cp[5] == c1 || cp[5] == c2)) - return (void *) &cp[5]; - if (6 < sizeof longword1 && (cp[6] == c1 || cp[6] == c2)) - return (void *) &cp[6]; - if (7 < sizeof longword1 && (cp[7] == c1 || cp[7] == c2)) - return (void *) &cp[7]; - if (8 < sizeof longword1) - for (i = 8; i < sizeof longword1; i++) - if (cp[i] == c1 || cp[i] == c2) - return (void *) &cp[i]; - } + longword2 = *longword_ptr ^ charmask2; + if (((((longword1 - magic_bits) & ~longword1) + | ((longword2 - magic_bits) & ~longword2)) + & (magic_bits << 7)) != 0) + break; + longword_ptr++; n -= sizeof longword1; } @@ -191,5 +125,5 @@ memchr2 (void const *s, int c1_in, int c2_in, size_t n) ++char_ptr; } - return 0; + return NULL; } diff --git a/modules/memchr2 b/modules/memchr2 index 7e834157a2..da5671de11 100644 --- a/modules/memchr2 +++ b/modules/memchr2 @@ -6,6 +6,7 @@ lib/memchr2.h lib/memchr2.c Depends-on: +intprops stdint memchr