Source file src/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, "// Code generated by mkfastlog2table.go; DO NOT EDIT.")
    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	

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