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 } 53
