What is the fastest way to get the value of π?

Question

I\'m looking for the fastest way to obtain the value of π, as a personal challenge. More specifically, I\'m using ways that don\'t involve using #define constan

Answer 1

Brent's method posted above by Chris is very good; Brent generally is a giant in the field of arbitrary-precision arithmetic.

If all you want is the Nth digit, the famous BBP formula is useful in hex