You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: fastlog2.go
    path of file: ./repos/under-the-hood/gosrc/runtime
    
    the code of the file until where you have to start completion: // Copyright 2015 The Go Authors. A
    
func fastlog2(x float64) float64 {
	const fastlogScaleBits = 20
	const fastlogScaleRatio = 1.0 / (1 << fastlogScaleBits)

	xBits := float64bits(x)
	// Extract the exponent from the IEEE float64, and index a constant
	// table with the first 10 bits from the mantissa.
	xExp := int64((xBits>>52)&0x7FF) - 1023
	xManIndex := (xBits >> (52 - fastlogNumBits)) % (1 << fastlogNumBits)
	xManScale := (xBits >> (52 - fastlogNumBits - fastlogScaleBits)) % (1 << fastlogScaleBits)

	low, high := fastlog2Table[xManIndex], fastlog2Table[xManIndex+1]
	return float64(xExp) + low + (high-low)*float64(xManScale)*fastlogScaleRatio
}