Is there a fast C or C++ standard library function for double precision inverse square root?
No. No, there isn't. Not in C++. Nope.
You can use this function for faster inverse square root computing
There's an article on wikipedia on how it works: https://en.wikipedia.org/wiki/Fast_inverse_square_root
Also there's a C version of this algorithm.
float invSqrt( float number ){
union {
float f;
uint32_t i;
} conv;
float x2;
const float threehalfs = 1.5F;
x2 = number * 0.5F;
conv.f = number;
conv.i = 0x5f3759df - ( conv.i >> 1 );
conv.f = conv.f * ( threehalfs - ( x2 * conv.f * conv.f ) );
return conv.f;
}