diff --git a/configure.ac b/configure.ac
index daa87d0d..0a5c5dd7 100644
--- a/configure.ac
+++ b/configure.ac
@@ -298,28 +298,28 @@ AC_ARG_ENABLE(assembler, AC_HELP_STRING([--disable-assembler],
 		[], [enable_assembler=yes])
 if test "x$enable_assembler" = xyes; then
 	case $host_cpu in
-		i?86)
-			# Darwin has different ABI than GNU+Linux and Solaris,
-			# and the x86 assembler code doesn't assemble.
-			case $host_os in
-				darwin*)
-					enable_assembler=no
-					;;
-				*)
-					enable_assembler=x86
-					;;
-			esac
-			;;
-		*)
-			enable_assembler=no
-			;;
+		i?86)   enable_assembler=x86 ;;
+		x86_64) enable_assembler=x86_64 ;;
+		*)      enable_assembler=no ;;
+	esac
+	# Darwin has different ABI than GNU+Linux and Solaris,
+	# and the assembler code doesn't assemble.
+	case $host_os in
+		darwin*) enable_assembler=no ;;
+		*)       ;;
 	esac
 fi
 case $enable_assembler in
-	x86|no)
+	x86)
 		AC_DEFINE([HAVE_ASM_X86], 1,
 			[Define to 1 if using x86 assembler optimizations.])
 		;;
+	x86_64)
+		AC_DEFINE([HAVE_ASM_X86_64], 1,
+			[Define to 1 if using x86_64 assembler optimizations.])
+		;;
+	no)
+		;;
 	*)
 		AC_MSG_RESULT([])
 		AC_MSG_ERROR([--enable-assembler accepts only \`yes', \`no', or \`x86'.])