constexpr 问题，为什么这两个不同的程序在 g++ 中运行的时间如此不同?

I'm using gcc 4.6.1 and am getting some interesting behavior involving calling a constexpr function. This program runs just fine and straight away prints out 12200160415121876738.

#include <iostream>

extern const unsigned long joe;

constexpr unsigned long fib(unsigned long int x)
{
   return (x <= 1) ? 1 : (fib(x - 1) + fib(x - 2));
}

const unsigned long joe = fib(92);

int main()
{
   ::std::cout << "Here I am!
";
   ::std::cout << joe << '
';
   return 0;
}

这个程序需要很长时间才能运行，我从来没有耐心等待它打印出一个值:

#include <iostream>

constexpr unsigned long fib(unsigned long int x)
{
   return (x <= 1) ? 1 : (fib(x - 1) + fib(x - 2));
}

int main()
{
   ::std::cout << "Here I am!
";
   ::std::cout << fib(92) << '
';
   return 0;
}