Possible missed optimization opportunity with const?

[Toshi Morita <tm314159 at yahoo dot com>]

I was involved in a discussion over the semantics of "const" in C, and the following code was posted: 

#include <stdio.h>
int foo = 0;
const int *pfoo = &foo;
void bar (void)
{
    foo +=3D;
}
int main(void)
{
   int a, b;
   a = *pfoo;
     bar();
     b = *pfoo;
   printf("a: %d, b: %d\n", a, b);
}
 

This code when compiled with gcc 4.8.2 using the optimization option -O3 produces: 

a: 0, b: 1 


So it appears even though pfoo is a const int *, the value *pfoo is read twice. 

Would it be valid for the code to print a:0, b: 0?
If so, is this a missed optimization opportunity?

Toshi