+/*-
+ * Copyright (c) 2009 Xin LI <delphij@FreeBSD.org>
+ * All rights reserved.
+ *
+ * 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 <sys/cdefs.h>
+__FBSDID("$FreeBSD: src/lib/libc/string/strlen.c,v 1.7 2009/01/26 07:31:28 delphij Exp $");
+
+#include <limits.h>
+#include <sys/types.h>
+#include <platform/string.h>
+
+#if !_PLATFORM_OPTIMIZED_STRLEN
+
+/*
+ * 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. However, we can further reduce ~1/3 of
+ * time if we consider that strlen() usually operate on 7-bit ASCII
+ * by employing the following expression, which allows false positive
+ * when high bit of 1 and use the tail case to catch these case:
+ *
+ * ((x - 0x01....01) & 0x80....80)
+ *
+ * This is more than 5.2 times as fast as the raw implementation on
+ * Intel T7300 under long mode for strings longer than word length.
+ */
+
+/* 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)
+
+/*
+ * 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
+_platform_strlen(const char *str)
+{
+ const char *p;
+ const unsigned long *lp;
+
+ /* Skip the first few bytes until we have an aligned p */
+ for (p = str; (uintptr_t)p & LONGPTR_MASK; p++)
+ if (*p == '\0')
+ return (p - str);
+
+ /* Scan the rest of the string using word sized operation */
+ for (lp = (const unsigned long *)p; ; lp++)
+ if ((*lp - mask01) & mask80) {
+ 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
+ }
+
+ /* NOTREACHED */
+ return (0);
+}
+
+#if VARIANT_STATIC
+size_t
+strlen(const char *str)
+{
+ return _platform_strlen(str);
+}
+#endif
+
+#endif
+
projects / apple / libplatform.git / commitdiff
