Add machine-specific, optimized implementations of bcmp and memcmp.

PR: 73111
Submitted by: Ville-Pertti Keinonen <will@iki.fi> (taken from NetBSD)
MFC after: 3 weeks

lib/libc/amd64/string/Makefile.inc

@@ -1,3 +1,3 @@
 # $FreeBSD$
 
-MDSRCS+= bcopy.S bzero.S memcpy.S memmove.S memset.S
+MDSRCS+= bcmp.S bcopy.S bzero.S memcmp.S memcpy.S memmove.S memset.S

lib/libc/amd64/string/bcmp.S

@@ -0,0 +1,25 @@
+#include <machine/asm.h>
+__FBSDID("$FreeBSD$");
+
+#if 0
+	RCSID("$NetBSD: bcmp.S,v 1.1 2001/06/19 00:25:04 fvdl Exp $")
+#endif
+
+ENTRY(bcmp)
+	xorl	%eax,%eax		/* clear return value */
+	cld				/* set compare direction forward */
+
+	movq	%rdx,%rcx		/* compare by words */
+	shrq	$3,%rcx
+	repe
+	cmpsq
+	jne	L1
+
+	movq	%rdx,%rcx		/* compare remainder by bytes */
+	andq	$7,%rcx
+	repe
+	cmpsb
+	je	L2
+
+L1:	incl	%eax
+L2:	ret

lib/libc/amd64/string/memcmp.S

@@ -0,0 +1,41 @@
+/*
+ * Written by J.T. Conklin <jtc@NetBSD.org>.
+ * Public domain.
+ * Adapted for NetBSD/x86_64 by Frank van der Linden <fvdl@wasabisystems.com>
+ */
+
+#include <machine/asm.h>
+__FBSDID("$FreeBSD$");
+
+#if 0
+	RCSID("$NetBSD: memcmp.S,v 1.2 2003/07/26 19:24:39 salo Exp $")
+#endif
+
+ENTRY(memcmp)
+	cld				/* set compare direction forward */
+	movq	%rdx,%rcx		/* compare by longs */
+	shrq	$3,%rcx
+	repe
+	cmpsq
+	jne	L5			/* do we match so far? */
+
+	movq	%rdx,%rcx		/* compare remainder by bytes */
+	andq	$7,%rcx
+	repe
+	cmpsb
+	jne	L6			/* do we match? */
+
+	xorl	%eax,%eax		/* we match, return zero	*/
+	ret
+
+L5:	movl	$8,%ecx			/* We know that one of the next	*/
+	subq	%rcx,%rdi		/* eight pairs of bytes do not	*/
+	subq	%rcx,%rsi		/* match.			*/
+	repe
+	cmpsb
+L6:	xorl	%eax,%eax		/* Perform unsigned comparison	*/
+	movb	-1(%rdi),%al
+	xorl	%edx,%edx
+	movb	-1(%rsi),%dl
+	subl    %edx,%eax
+	ret