Current File : //usr/local/go119/src/runtime/mkfastlog2table.go
// Copyright 2015 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
//go:build ignore
// fastlog2Table contains log2 approximations for 5 binary digits.
// This is used to implement fastlog2, which is used for heap sampling.
package main
import (
"bytes"
"fmt"
"log"
"math"
"os"
)
func main() {
var buf bytes.Buffer
fmt.Fprintln(&buf, "// Code generated by mkfastlog2table.go; DO NOT EDIT.")
fmt.Fprintln(&buf, "// Run go generate from src/runtime to update.")
fmt.Fprintln(&buf, "// See mkfastlog2table.go for comments.")
fmt.Fprintln(&buf)
fmt.Fprintln(&buf, "package runtime")
fmt.Fprintln(&buf)
fmt.Fprintln(&buf, "const fastlogNumBits =", fastlogNumBits)
fmt.Fprintln(&buf)
fmt.Fprintln(&buf, "var fastlog2Table = [1<<fastlogNumBits + 1]float64{")
table := computeTable()
for _, t := range table {
fmt.Fprintf(&buf, "\t%v,\n", t)
}
fmt.Fprintln(&buf, "}")
if err := os.WriteFile("fastlog2table.go", buf.Bytes(), 0644); err != nil {
log.Fatalln(err)
}
}
const fastlogNumBits = 5
func computeTable() []float64 {
fastlog2Table := make([]float64, 1<<fastlogNumBits+1)
for i := 0; i <= (1 << fastlogNumBits); i++ {
fastlog2Table[i] = log2(1.0 + float64(i)/(1<<fastlogNumBits))
}
return fastlog2Table
}
// log2 is a local copy of math.Log2 with an explicit float64 conversion
// to disable FMA. This lets us generate the same output on all platforms.
func log2(x float64) float64 {
frac, exp := math.Frexp(x)
// Make sure exact powers of two give an exact answer.
// Don't depend on Log(0.5)*(1/Ln2)+exp being exactly exp-1.
if frac == 0.5 {
return float64(exp - 1)
}
return float64(nlog(frac)*(1/math.Ln2)) + float64(exp)
}
// nlog is a local copy of math.Log with explicit float64 conversions
// to disable FMA. This lets us generate the same output on all platforms.
func nlog(x float64) float64 {
const (
Ln2Hi = 6.93147180369123816490e-01 /* 3fe62e42 fee00000 */
Ln2Lo = 1.90821492927058770002e-10 /* 3dea39ef 35793c76 */
L1 = 6.666666666666735130e-01 /* 3FE55555 55555593 */
L2 = 3.999999999940941908e-01 /* 3FD99999 9997FA04 */
L3 = 2.857142874366239149e-01 /* 3FD24924 94229359 */
L4 = 2.222219843214978396e-01 /* 3FCC71C5 1D8E78AF */
L5 = 1.818357216161805012e-01 /* 3FC74664 96CB03DE */
L6 = 1.531383769920937332e-01 /* 3FC39A09 D078C69F */
L7 = 1.479819860511658591e-01 /* 3FC2F112 DF3E5244 */
)
// special cases
switch {
case math.IsNaN(x) || math.IsInf(x, 1):
return x
case x < 0:
return math.NaN()
case x == 0:
return math.Inf(-1)
}
// reduce
f1, ki := math.Frexp(x)
if f1 < math.Sqrt2/2 {
f1 *= 2
ki--
}
f := f1 - 1
k := float64(ki)
// compute
s := float64(f / (2 + f))
s2 := float64(s * s)
s4 := float64(s2 * s2)
t1 := s2 * float64(L1+float64(s4*float64(L3+float64(s4*float64(L5+float64(s4*L7))))))
t2 := s4 * float64(L2+float64(s4*float64(L4+float64(s4*L6))))
R := float64(t1 + t2)
hfsq := float64(0.5 * f * f)
return float64(k*Ln2Hi) - ((hfsq - (float64(s*float64(hfsq+R)) + float64(k*Ln2Lo))) - f)
}
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