diff --git a/lib/libc/string/strlen.c b/lib/libc/string/strlen.c index 0bdc81d7bb9a..a862ffc245ca 100644 --- a/lib/libc/string/strlen.c +++ b/lib/libc/string/strlen.c @@ -1,105 +1,131 @@ /*- * SPDX-License-Identifier: BSD-2-Clause-FreeBSD * * Copyright (c) 2009, 2010 Xin LI * * Redistribution and use in source and binary forms, with or without * modification, are permitted provided that the following conditions * are met: * 1. Redistributions of source code must retain the above copyright * notice, this list of conditions and the following disclaimer. * 2. Redistributions in binary form must reproduce the above copyright * notice, this list of conditions and the following disclaimer in the * documentation and/or other materials provided with the distribution. * * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE * ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF * SUCH DAMAGE. */ #include __FBSDID("$FreeBSD$"); #include #include #include /* * Portable strlen() for 32-bit and 64-bit systems. * + * Rationale: it is generally much more efficient to do word length + * operations and avoid branches on modern computer systems, as + * compared to byte-length operations with a lot of branches. + * * The expression: * * ((x - 0x01....01) & ~x & 0x80....80) * * would evaluate to a non-zero value iff any of the bytes in the * original word is zero. * + * On multi-issue processors, we can divide the above expression into: + * a) (x - 0x01....01) + * b) (~x & 0x80....80) + * c) a & b + * + * Where, a) and b) can be partially computed in parallel. + * * The algorithm above is found on "Hacker's Delight" by * Henry S. Warren, Jr. - * - * Note: this leaves performance on the table and each architecture - * would be best served with a tailor made routine instead. */ +/* Magic numbers for the algorithm */ #if LONG_BIT == 32 static const unsigned long mask01 = 0x01010101; static const unsigned long mask80 = 0x80808080; #elif LONG_BIT == 64 static const unsigned long mask01 = 0x0101010101010101; static const unsigned long mask80 = 0x8080808080808080; #else #error Unsupported word size #endif #define LONGPTR_MASK (sizeof(long) - 1) -#if BYTE_ORDER == LITTLE_ENDIAN -#define FINDZERO __builtin_ctzl -#else -#define FINDZERO __builtin_clzl -#endif +/* + * Helper macro to return string length if we caught the zero + * byte. + */ +#define testbyte(x) \ + do { \ + if (p[x] == '\0') \ + return (p - str + x); \ + } while (0) size_t strlen(const char *str) { + const char *p; const unsigned long *lp; - unsigned long mask; long va, vb; - long val; - lp = (unsigned long *) (uintptr_t) str; - if ((uintptr_t)lp & LONGPTR_MASK) { - lp = (__typeof(lp)) ((uintptr_t)lp & ~LONGPTR_MASK); -#if BYTE_ORDER == LITTLE_ENDIAN - mask = ~(~0UL << (((uintptr_t)str & LONGPTR_MASK) << 3)); -#else - mask = ~(~0UL >> (((uintptr_t)str & LONGPTR_MASK) << 3)); -#endif - val = *lp | mask; - va = (val - mask01); - vb = ((~val) & mask80); - if (va & vb) { - return ((const char *)lp - str + (FINDZERO(va & vb) >> 3)); - } - lp++; - } + /* + * Before trying the hard (unaligned byte-by-byte access) way + * to figure out whether there is a nul character, try to see + * if there is a nul character is within this accessible word + * first. + * + * p and (p & ~LONGPTR_MASK) must be equally accessible since + * they always fall in the same memory page, as long as page + * boundaries is integral multiple of word size. + */ + lp = (const unsigned long *)((uintptr_t)str & ~LONGPTR_MASK); + va = (*lp - mask01); + vb = ((~*lp) & mask80); + lp++; + if (va & vb) + /* Check if we have \0 in the first part */ + for (p = str; p < (const char *)lp; p++) + if (*p == '\0') + return (p - str); + /* Scan the rest of the string using word sized operation */ for (; ; lp++) { va = (*lp - mask01); vb = ((~*lp) & mask80); if (va & vb) { - return ((const char *)lp - str + (FINDZERO(va & vb) >> 3)); + p = (const char *)(lp); + testbyte(0); + testbyte(1); + testbyte(2); + testbyte(3); +#if (LONG_BIT >= 64) + testbyte(4); + testbyte(5); + testbyte(6); + testbyte(7); +#endif } } - __builtin_unreachable(); + /* NOTREACHED */ return (0); } diff --git a/sys/libkern/strlen.c b/sys/libkern/strlen.c index 8fa5f3927ea9..a8c7964f69a3 100644 --- a/sys/libkern/strlen.c +++ b/sys/libkern/strlen.c @@ -1,101 +1,130 @@ /*- * SPDX-License-Identifier: BSD-2-Clause-FreeBSD * * Copyright (c) 2009, 2010 Xin LI * * Redistribution and use in source and binary forms, with or without * modification, are permitted provided that the following conditions * are met: * 1. Redistributions of source code must retain the above copyright * notice, this list of conditions and the following disclaimer. * 2. Redistributions in binary form must reproduce the above copyright * notice, this list of conditions and the following disclaimer in the * documentation and/or other materials provided with the distribution. * * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE * ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF * SUCH DAMAGE. */ #include __FBSDID("$FreeBSD$"); #include #include /* * Portable strlen() for 32-bit and 64-bit systems. * + * Rationale: it is generally much more efficient to do word length + * operations and avoid branches on modern computer systems, as + * compared to byte-length operations with a lot of branches. + * * The expression: * * ((x - 0x01....01) & ~x & 0x80....80) * * would evaluate to a non-zero value iff any of the bytes in the * original word is zero. * + * On multi-issue processors, we can divide the above expression into: + * a) (x - 0x01....01) + * b) (~x & 0x80....80) + * c) a & b + * + * Where, a) and b) can be partially computed in parallel. + * * The algorithm above is found on "Hacker's Delight" by * Henry S. Warren, Jr. */ +/* Magic numbers for the algorithm */ #if LONG_BIT == 32 static const unsigned long mask01 = 0x01010101; static const unsigned long mask80 = 0x80808080; #elif LONG_BIT == 64 static const unsigned long mask01 = 0x0101010101010101; static const unsigned long mask80 = 0x8080808080808080; #else #error Unsupported word size #endif #define LONGPTR_MASK (sizeof(long) - 1) -#if BYTE_ORDER == LITTLE_ENDIAN -#define FINDZERO __builtin_ctzl -#else -#define FINDZERO __builtin_clzl -#endif +/* + * Helper macro to return string length if we caught the zero + * byte. + */ +#define testbyte(x) \ + do { \ + if (p[x] == '\0') \ + return (p - str + x); \ + } while (0) size_t (strlen)(const char *str) { + const char *p; const unsigned long *lp; - unsigned long mask; long va, vb; - long val; - lp = (unsigned long *) (uintptr_t) str; - if ((uintptr_t)lp & LONGPTR_MASK) { - lp = (__typeof(lp)) ((uintptr_t)lp & ~LONGPTR_MASK); -#if BYTE_ORDER == LITTLE_ENDIAN - mask = ~(~0UL << (((uintptr_t)str & LONGPTR_MASK) << 3)); -#else - mask = ~(~0UL >> (((uintptr_t)str & LONGPTR_MASK) << 3)); -#endif - val = *lp | mask; - va = (val - mask01); - vb = ((~val) & mask80); - if (va & vb) { - return ((const char *)lp - str + (FINDZERO(va & vb) >> 3)); - } - lp++; - } + /* + * Before trying the hard (unaligned byte-by-byte access) way + * to figure out whether there is a nul character, try to see + * if there is a nul character is within this accessible word + * first. + * + * p and (p & ~LONGPTR_MASK) must be equally accessible since + * they always fall in the same memory page, as long as page + * boundaries is integral multiple of word size. + */ + lp = (const unsigned long *)((uintptr_t)str & ~LONGPTR_MASK); + va = (*lp - mask01); + vb = ((~*lp) & mask80); + lp++; + if (va & vb) + /* Check if we have \0 in the first part */ + for (p = str; p < (const char *)lp; p++) + if (*p == '\0') + return (p - str); + /* Scan the rest of the string using word sized operation */ for (; ; lp++) { va = (*lp - mask01); vb = ((~*lp) & mask80); if (va & vb) { - return ((const char *)lp - str + (FINDZERO(va & vb) >> 3)); + p = (const char *)(lp); + testbyte(0); + testbyte(1); + testbyte(2); + testbyte(3); +#if (LONG_BIT >= 64) + testbyte(4); + testbyte(5); + testbyte(6); + testbyte(7); +#endif } } - __builtin_unreachable(); + /* NOTREACHED */ return (0); }