// Copyright 2015 The Go Authors. All rights reserved. // Use of this source code is governed by a BSD-style // license that can be found in the LICENSE file. package runtime // 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 { 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 }