[MSAN] Fix memcmp_test on MIPS

[slthakur]

Summary

As per the test the 4th element of both arrays are not initialized and hence will contain garbage values. Memcmp returns the difference between the garbage values of the 4th element which will be different on every run of the test. And since the return value of memcmp is returned from main, we are getting random exit code every time.

[eugenis]

... which can happen to be zero, right? 1 out of 256.

Please do if (!res) printf("equals") to force the check, then return 0.

[slthakur]

... which can happen to be zero, right? 1 out of 256.
Please do if (!res) printf("equals") to force the check, then return 0.

In cases when value of res is not zero, "equals" will not be printed and hence test will fail.
Since the value of res in unpredictable, could we just do return 0 or ignore the exit code?

[eugenis]

Yes, that's what I meant. A check for the return value of memcmp so that it is not optimized out (though it probably won't anyway at -O0), and then always return 0.

[slthakur]

Addressed review comment.

[eugenis]

LGTM

[slthakur]

Committed in revision 261739

test/msan/memcmp_test.cc

 // RUN: %clangxx_msan -O0 -g %s -o %t
 // RUN: not %run %t 2>&1 | FileCheck %s
 // RUN: MSAN_OPTIONS=intercept_memcmp=0 %run %t
 
 #include <string.h>
+#include <stdio.h>
 int main(int argc, char **argv) {
   char a1[4];
   char a2[4];
   for (int i = 0; i < argc * 3; i++)
     a2[i] = a1[i] = i;
   int res = memcmp(a1, a2, 4);
-  return res;
+  if (!res)
+    printf("equals");
+  return 0;
   // CHECK: Uninitialized bytes in __interceptor_memcmp at offset 3
   // CHECK: MemorySanitizer: use-of-uninitialized-value
 }