libgo/go/runtime/fastlog2.go

   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 }