diff --git a/acinclude.m4 b/acinclude.m4
index 433542d05a..92cad6f293 100644
--- a/acinclude.m4
+++ b/acinclude.m4
@@ -199,7 +199,7 @@ 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 ab64a04920..04ed6a708f 100644
--- a/src/cf/osconf.m4
+++ b/src/cf/osconf.m4
@@ -652,6 +652,20 @@ esac
 
 MT_CFLAGS="${MT_CFLAGS} -DAFS_PTHREAD_ENV"
 
+#
+# 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
 #