diff --git a/acinclude.m4 b/acinclude.m4
index 41ef6d1a4c..ba157c0aeb 100644
--- a/acinclude.m4
+++ b/acinclude.m4
@@ -200,7 +200,7 @@ AC_ARG_ENABLE([optimize-kernel],
         [disable compilation of the kernel module with optimization (defaults
          based on platform)])],
     , 
-    [enable_optimize_kernel="yes"])
+    [enable_optimize_kernel=""])
 AC_ARG_ENABLE([debug-lwp],
     [AS_HELP_STRING([--enable-debug-lwp],
         [enable compilation of the LWP code with debugging information
diff --git a/src/cf/osconf.m4 b/src/cf/osconf.m4
index a42d33b249..93a5279dee 100644
--- a/src/cf/osconf.m4
+++ b/src/cf/osconf.m4
@@ -725,6 +725,20 @@ case $AFS_SYSNAME in
 		;;
 esac
 
+#
+# Defaults for --enable-optimize-kernel
+#
+if test "x$enable_optimize_kernel" = "x" ; then
+  AS_CASE([$AFS_SYSNAME],
+    [sunx86_510|sunx86_511],
+      dnl Somewhere around Solaris Studio 12.*, the compiler started adding SSE
+      dnl instructions to optimized code, without any ability to turn it off.
+      dnl So just default to not optimizing kernel code for the relevant
+      dnl platforms, until we get a better autoconf test for this.
+      [enable_optimize_kernel=no],
+    [enable_optimize_kernel=yes])
+fi
+
 #
 # Special build targets
 #