diff --git a/tests/lib_knuth_morris_pratt/lib_knuth_morris_pratt.fz b/tests/lib_knuth_morris_pratt/lib_knuth_morris_pratt.fz
index 554fe96a4f..f93f8cf01a 100644
--- a/tests/lib_knuth_morris_pratt/lib_knuth_morris_pratt.fz
+++ b/tests/lib_knuth_morris_pratt/lib_knuth_morris_pratt.fz
@@ -56,17 +56,19 @@ lib_knuth_morris_pratt =>
     test_kmp(n i32) =>
       ("a"*2*n).utf8.find ("a"*n+"b").utf8
 
+    say (test_kmp 1000)
 
     len := 250
 
+    if (envir.args.index_of "-benchmark")??
 
-    # for len=250 and interpreter takes ~70s on my machine
-    brute_force := time.stopwatch (()-> _:= test_brute_force len)
-    # for len=250 and interpreter takes ~1s on my machine
-    kmp := time.stopwatch (()-> _:= test_kmp len)
+      # for len=250 and interpreter takes ~70s on my machine
+      brute_force := time.stopwatch (()-> _:= test_brute_force len)
+      # for len=250 and interpreter takes ~1s on my machine
+      kmp := time.stopwatch (()-> _:= test_kmp len)
 
-    # check that kmp is at least four times faster
-    check (kmp*4 < brute_force)
+      # check that kmp is at least four times faster
+      check (kmp*4 < brute_force)
 
 
   functionality
diff --git a/tests/lib_knuth_morris_pratt/lib_knuth_morris_pratt.fz.expected_out b/tests/lib_knuth_morris_pratt/lib_knuth_morris_pratt.fz.expected_out
index 33a0755763..29d6290f6e 100644
--- a/tests/lib_knuth_morris_pratt/lib_knuth_morris_pratt.fz.expected_out
+++ b/tests/lib_knuth_morris_pratt/lib_knuth_morris_pratt.fz.expected_out
@@ -7,3 +7,4 @@
 0
 --nil--
 0
+--nil--