Can different optimization levels lead to functionally different code?

Question

I am curious about the liberties that a compiler has when optimizing. Let\'s limit this question to GCC and C/C++ (any version, any flavour of standard):

Is it possi

Answer 1

Two different C programs:

foo6.c

void p2(void);

int main() {
    p2();
    return 0;
}

bar6.c

#include 

char main;

void p2() {
    printf("0x%x\n", main);
}

When both modules are compiled into one excecutable with optimization levels one and zero, they print out two different values. 0x48 for -O1 and 0x55 for -O0

Screenshot of terminal

Here is an example of it working in my environment