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 import "unsafe" 8 9 // fastlog2 implements a fast approximation to the base 2 log of a 10 // float64. This is used to compute a geometric distribution for heap 11 // sampling, without introducing dependencies into package math. This 12 // uses a very rough approximation using the float64 exponent and the 13 // first 25 bits of the mantissa. The top 5 bits of the mantissa are 14 // used to load limits from a table of constants and the rest are used 15 // to scale linearly between them. 16 func fastlog2(x float64) float64 { 17 const fastlogScaleBits = 20 18 const fastlogScaleRatio = 1.0 / (1 << fastlogScaleBits) 19 20 xBits := float64bits(x) 21 // Extract the exponent from the IEEE float64, and index a constant 22 // table with the first 10 bits from the mantissa. 23 xExp := int64((xBits>>52)&0x7FF) - 1023 24 xManIndex := (xBits >> (52 - fastlogNumBits)) % (1 << fastlogNumBits) 25 xManScale := (xBits >> (52 - fastlogNumBits - fastlogScaleBits)) % (1 << fastlogScaleBits) 26 27 low, high := fastlog2Table[xManIndex], fastlog2Table[xManIndex+1] 28 return float64(xExp) + low + (high-low)*float64(xManScale)*fastlogScaleRatio 29 } 30 31 // float64bits returns the IEEE 754 binary representation of f. 32 // Taken from math.Float64bits to avoid dependencies into package math. 33 func float64bits(f float64) uint64 { return *(*uint64)(unsafe.Pointer(&f)) } 34
