built-in functions

[Bruno Schneider <boschneider@xxxxxxxxx>]

I'm trying to compile C++ code with the built-in "add with oveflow
check" functions[1]. But the compiler complains the built-ins have not
been declared. I'm using g++ 4.9.2.

Attached is some minimal code to test. Can anybody help?

[1] https://gcc.gnu.org/onlinedocs/gcc/Integer-Overflow-Builtins.html

-- 
Bruno Schneider

#include <iostream>
#include <limits>
using namespace std;
int main()
{
    unsigned int a = numeric_limits<unsigned int>::max();
    unsigned int b = 1;
    unsigned int c = a + b;
    if (__builtin_uadd_overflow(a, b, &c))
        cout << "overflow\n";
    else
        cout << a << " + " << b << " = " << c << "\n";
    return 0;
}