go/src/runtime/fastlog2.go

// fastlog2 implements a fast approximation to the base 2 log of a
// float64. This is used to compute a geometric distribution for heap
// sampling, without introducing dependencies into package math. This
// uses a very rough approximation using the float64 exponent and the
// first 25 bits of the mantissa. The top 5 bits of the mantissa are
// used to load limits from a table of constants and the rest are used
// to scale linearly between them.
func fastlog2(x float64) float64 { … }