$ uname -a
NetBSD chieftec 8.99.8 NetBSD 8.99.8 (GENERIC) #1: Thu Dec  7 02:50:00 CET 2017  root@chieftec:/public/netbsd-root/sys/arch/amd64/compile/GENERIC amd64
$ cat umr2.c                                                                                                                 
#include <stdio.h>
#include <stdlib.h>

int main(int argc, char** argv) {
  int *a = malloc(sizeof(int) * 10);
  printf("a=%p\n", a);
  a[5] = 0;
  if (a[argc])
    printf("xx\n");
  return 0;
}
$ ./bin/clang -fsanitize=memory umr2.c -g -O0 
$ ./a.out                                     
a=0x703000000000
==19741==WARNING: MemorySanitizer: use-of-uninitialized-value
    #0 0x44c95a in main /public/llvm-build/umr2.c:8:7
    #1 0x40a5da in ___start (/public/llvm-build/./a.out+0x40a5da)

SUMMARY: MemorySanitizer: use-of-uninitialized-value /public/llvm-build/umr2.c:8:7 in main
Exiting

-------------------------------------------------------------------------------

$ uname -a
Linux a.b 4.9.0-3-amd64 #1 SMP Debian 4.9.30-2+deb9u2 (2017-06-26) x86_64 GNU/Linux
$ cat umr2.c 
#include <stdio.h>
#include <stdlib.h>

int main(int argc, char** argv) {
  int *a = malloc(sizeof(int) * 10);
  printf("a=%p\n", a);
  a[5] = 0;
  if (a[argc])
    printf("xx\n");
  return 0;
}
$ clang -fsanitize=memory umr2.c -g -O0
$ ./a.out 
a=0x70600000efd0
==9444==WARNING: MemorySanitizer: use-of-uninitialized-value
    #0 0x48925b in main /home/kamil/umr2.c:8:7
    #1 0x7f58e07e22b0 in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x202b0)
    #2 0x41beb9 in _start (/home/kamil/a.out+0x41beb9)

SUMMARY: MemorySanitizer: use-of-uninitialized-value /home/kamil/umr2.c:8:7 in main
Exiting