Rewrite fabs.S to use pure SSE2 operations. I got the clues how to do

this from looking at code generated by gcc.

lib/libc/amd64/gen/fabs.S

@@ -1,18 +1,43 @@
+/*-
+ * Copyright (c) 2004 Peter Wemm
+ * 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 <machine/asm.h>
-#if defined(LIBC_SCCS)
-	RCSID("$NetBSD: fabs.S,v 1.4 1997/07/16 14:37:16 christos Exp $")
-#endif
 __FBSDID("$FreeBSD$");
 
 /*
- * Ok, this sucks. Is there really no way to push an xmm register onto
- * the FP stack directly?
+ * Return floating point absolute value of a double.
  */
 
+	.text
 ENTRY(fabs)
-	movsd	%xmm0, -8(%rsp)
-	fldl	-8(%rsp)
-	fabs
-	fstpl	-8(%rsp)
-	movsd	-8(%rsp),%xmm0
+	movsd	%xmm0, %xmm1
+	movsd	signbit(%rip), %xmm0
+	andnpd  %xmm1, %xmm0
 	ret
+
+	.data
+signbit:
+	.quad	0x8000000000000000