#include <stdio.h>
#include <stdlib.h>

#ifndef MY_S_IS_LOCAL
struct my_s
{
	int a[128];
	int b[128];
	int c[128];
	int d[128];
} my_s;
#endif /* MY_S_IS_LOCAL */

int main(int ac, char *av[])
{
#ifdef MY_S_IS_LOCAL
	struct my_s
	{
		int a[128];
		int b[128];
		int c[128];
		int d[128];
	} my_s;
#endif /* MY_S_IS_LOCAL */

	/* silence warnings about unused variables*/
	(void)ac;
	(void)av;
	(void)my_s;

	for(int i=0; i< 256; i++)
	{
		my_s.b[i]=0xdeadbeef;
	}

	/* prevent optimiser from eliminating |my_s| */	
	(void)&my_s;

	return EXIT_SUCCESS;
}

##############################################

# warnings are generated for global variables...
$ g++ -g -gdwarf-2 -O2 -Wall -Wextra -Waggressive-loop-optimizations x.cpp 
x.cpp: In function ‘int main(int, char**)’:
x.cpp:33:23: warning: iteration 128u invokes undefined behavior [-Waggressive-loop-optimizations]
   my_s.b[i]=0xdeadbeef;
                       ^
x.cpp:31:2: note: containing loop
  for(int i=0; i< 256; i++)
  ^

# ... but NO warnings are generated for local variables
$ g++ -g -gdwarf-2 -DMY_S_IS_LOCAL -O2 -Wall -Wextra -Waggressive-loop-optimizations x.cpp 
$

# EOF.