Introducing memcchr(3).

It seems two of the file system drivers we have in the tree, namely ufs
and ext3, use a function called `skpc()'. The meaning of this function
does not seem to be documented in FreeBSD, but it turns out one needs to
be a VAX programmer to understand what it does.

SPKC is an instruction on the VAX that does the opposite of memchr(). It
searches for the non-equal character. Add a new function called
memcchr() to the tree that has the following advantages over skpc():

- It has a name that makes more sense than skpc(). Just like strcspn()
  matches the complement of strspn(), memcchr() is the complement of
  memchr().

- It is faster than skpc(). Similar to our strlen() in libc, it compares
  entire words, instead of single bytes. It seems that for this routine
  this yields a sixfold performance increase on amd64.

- It has a man page.

1 files changed, 115 insertions, 0 deletions

diff --git a/sys/libkern/memcchr.c b/sys/libkern/memcchr.c
new file mode 100644
index 0000000..eebb23a
--- /dev/null
+++ b/sys/libkern/memcchr.c
@@ -0,0 +1,115 @@
+/*-
+ * Copyright (c) 2012 Ed Schouten <ed@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("$FreeBSD$");
+
+#include <sys/libkern.h>
+#include <sys/limits.h>
+#include <sys/param.h>
+
+/*
+ * memcchr(): find first character in buffer not matching `c'.
+ *
+ * This function performs the complement of memchr().  To provide decent
+ * performance, this function compares data from the buffer one word at
+ * a time.
+ *
+ * This code is inspired by libc's strlen(), written by Xin Li.
+ */
+
+#if LONG_BIT != 32 && LONG_BIT != 64
+#error Unsupported word size
+#endif
+
+#define	LONGPTR_MASK (sizeof(long) - 1)
+
+#define	TESTBYTE				\
+	do {					\
+		if (*p != (unsigned char)c)	\
+			goto done;		\
+		p++;				\
+	} while (0)
+
+void *
+memcchr(const void *begin, int c, size_t n)
+{
+	const unsigned long *lp;
+	const unsigned char *p, *end;
+	unsigned long word;
+
+	/* Four or eight repetitions of `c'. */
+	word = (unsigned char)c;
+	word |= word << 8;
+	word |= word << 16;
+#if LONG_BIT >= 64
+	word |= word << 32;
+#endif
+
+	/* Don't perform memory I/O when passing a zero-length buffer. */
+	if (n == 0)
+		return (NULL);
+
+	/*
+	 * First determine whether there is a character unequal to `c'
+	 * in the first word.  As this word may contain bytes before
+	 * `begin', we may execute this loop spuriously.
+	 */
+	lp = (const unsigned long *)((uintptr_t)begin & ~LONGPTR_MASK);
+	end = (const unsigned char *)begin + n;
+	if (*lp++ != word)
+		for (p = begin; p < (const unsigned char *)lp;)
+			TESTBYTE;
+
+	/* Now compare the data one word at a time. */
+	for (; (const unsigned char *)lp < end; lp++) {
+		if (*lp != word) {
+			p = (const unsigned char *)lp;
+			TESTBYTE;
+			TESTBYTE;
+			TESTBYTE;
+#if LONG_BIT >= 64
+			TESTBYTE;
+			TESTBYTE;
+			TESTBYTE;
+			TESTBYTE;
+#endif
+			goto done;
+		}
+	}
+
+	return (NULL);
+
+done:
+	/*
+	 * If the end of the buffer is not word aligned, the previous
+	 * loops may obtain an address that's beyond the end of the
+	 * buffer.
+	 */
+	if (p < end)
+		return (__DECONST(void *, p));
+	return (NULL);
+}