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 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 dependences 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 dependences into package math.
  33 func float64bits(f float64) uint64 { return *(*uint64)(unsafe.Pointer(&f)) }