prevent silly overflow in memops benchmark, to make it more fair

tests/runner.py

@@ -3050,20 +3050,22 @@ else:
         #include<string.h>
         #include<stdlib.h>
         int main() {
-          int N = 4*1024*1024;
+          int N = 1024*1024;
+          int M = 600;
           int final = 0;
           char *buf = (char*)malloc(N);
-          for (int t = 0; t < 20; t++) {
+          for (int t = 0; t < M; t++) {
             for (int i = 0; i < N; i++)
-              buf[i] = (i*i + final)%256;
+              buf[i] = (i + final)%256;
             for (int i = 0; i < N; i++)
               final += buf[i] & 1;
+            final = final % 1000;
           }
           printf("final: %d.\\n", final);
           return 1;
         }      
       '''
-      self.do_benchmark(src, [], 'final: 41943040.')
+      self.do_benchmark(src, [], 'final: 800.')
 
     def test_fannkuch(self):
       src = open(path_from_root('tests', 'fannkuch.cpp'), 'r').read()