gcc -g -O0 -o thread-ex ex1.c -Wall -Werror -lpthread
   0x000000000040071a <+84>:	mov    0x200934(%rip),%eax # 0x601054 <global_val>
   0x0000000000400720 <+90>:	cmp    $0x1,%eax
   0x0000000000400723 <+93>:	jne    0x40071a <main+84>


gcc -g -O1 -o thread-ex ex1.c -Wall -Werror -lpthread
   0x000000000040075c <+76>:	mov    0x2008f2(%rip),%eax  # 0x601054 <global_val>
   0x0000000000400762 <+82>:	cmp    $0x1,%eax
   0x0000000000400765 <+85>:	jne    0x400762 <main+82>