1 // Copyright 2015 The Go Authors. All rights reserved. 2 // Use of this source code is governed by a BSD-style 3 // license that can be found in the LICENSE file. 4 5 package runtime 6 7 // fastlog2 implements a fast approximation to the base 2 log of a 8 // float64. This is used to compute a geometric distribution for heap 9 // sampling, without introducing dependencies into package math. This 10 // uses a very rough approximation using the float64 exponent and the 11 // first 25 bits of the mantissa. The top 5 bits of the mantissa are 12 // used to load limits from a table of constants and the rest are used 13 // to scale linearly between them. 14 func fastlog2(x float64) float64 { 15 const fastlogScaleBits = 20 16 const fastlogScaleRatio = 1.0 / (1 << fastlogScaleBits) 17 18 xBits := float64bits(x) 19 // Extract the exponent from the IEEE float64, and index a constant 20 // table with the first 10 bits from the mantissa. 21 xExp := int64((xBits>>52)&0x7FF) - 1023 22 xManIndex := (xBits >> (52 - fastlogNumBits)) % (1 << fastlogNumBits) 23 xManScale := (xBits >> (52 - fastlogNumBits - fastlogScaleBits)) % (1 << fastlogScaleBits) 24 25 low, high := fastlog2Table[xManIndex], fastlog2Table[xManIndex+1] 26 return float64(xExp) + low + (high-low)*float64(xManScale)*fastlogScaleRatio 27 } 28