libgo/go/runtime/mkfastlog2table.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 // +build ignore
   6
   7 // fastlog2Table contains log2 approximations for 5 binary digits.
   8 // This is used to implement fastlog2, which is used for heap sampling.
   9
  10 package main
  11
  12 import (
  13         "bytes"
  14         "fmt"
  15         "io/ioutil"
  16         "log"
  17         "math"
  18 )
  19
  20 func main() {
  21         var buf bytes.Buffer
  22
  23         fmt.Fprintln(&buf, "// AUTO-GENERATED by mkfastlog2table.go")
  24         fmt.Fprintln(&buf, "// Run go generate from src/runtime to update.")
  25         fmt.Fprintln(&buf, "// See mkfastlog2table.go for comments.")
  26         fmt.Fprintln(&buf)
  27         fmt.Fprintln(&buf, "package runtime")
  28         fmt.Fprintln(&buf)
  29         fmt.Fprintln(&buf, "const fastlogNumBits =", fastlogNumBits)
  30         fmt.Fprintln(&buf)
  31
  32         fmt.Fprintln(&buf, "var fastlog2Table = [1<<fastlogNumBits + 1]float64{")
  33         table := computeTable()
  34         for _, t := range table {
  35                 fmt.Fprintf(&buf, "\t%v,\n", t)
  36         }
  37         fmt.Fprintln(&buf, "}")
  38
  39         if err := ioutil.WriteFile("fastlog2table.go", buf.Bytes(), 0644); err != nil {
  40                 log.Fatalln(err)
  41         }
  42 }
  43
  44 const fastlogNumBits = 5
  45
  46 func computeTable() []float64 {
  47         fastlog2Table := make([]float64, 1<<fastlogNumBits+1)
  48         for i := 0; i <= (1 << fastlogNumBits); i++ {
  49                 fastlog2Table[i] = math.Log2(1.0 + float64(i)/(1<<fastlogNumBits))
  50         }
  51         return fastlog2Table
  52 }