1 files changed, 92 insertions, 0 deletions
diff --git a/libgo/go/big/calibrate_test.go b/libgo/go/big/calibrate_test.go
new file mode 100644
index 000000000..c6cd2e693
--- /dev/null
+++ b/libgo/go/big/calibrate_test.go
@@ -0,0 +1,92 @@
+// Copyright 2009 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.
+
+// This file prints execution times for the Mul benchmark
+// given different Karatsuba thresholds. The result may be
+// used to manually fine-tune the threshold constant. The
+// results are somewhat fragile; use repeated runs to get
+// a clear picture.
+
+// Usage: gotest -calibrate
+
+package big
+
+import (
+	"flag"
+	"fmt"
+	"testing"
+	"time"
+)
+
+
+var calibrate = flag.Bool("calibrate", false, "run calibration test")
+
+
+// measure returns the time to run f
+func measure(f func()) int64 {
+	const N = 100
+	start := time.Nanoseconds()
+	for i := N; i > 0; i-- {
+		f()
+	}
+	stop := time.Nanoseconds()
+	return (stop - start) / N
+}
+
+
+func computeThresholds() {
+	fmt.Printf("Multiplication times for varying Karatsuba thresholds\n")
+	fmt.Printf("(run repeatedly for good results)\n")
+
+	// determine Tk, the work load execution time using basic multiplication
+	karatsubaThreshold = 1e9 // disable karatsuba
+	Tb := measure(benchmarkMulLoad)
+	fmt.Printf("Tb = %dns\n", Tb)
+
+	// thresholds
+	n := 8 // any lower values for the threshold lead to very slow multiplies
+	th1 := -1
+	th2 := -1
+
+	var deltaOld int64
+	for count := -1; count != 0; count-- {
+		// determine Tk, the work load execution time using Karatsuba multiplication
+		karatsubaThreshold = n // enable karatsuba
+		Tk := measure(benchmarkMulLoad)
+
+		// improvement over Tb
+		delta := (Tb - Tk) * 100 / Tb
+
+		fmt.Printf("n = %3d  Tk = %8dns  %4d%%", n, Tk, delta)
+
+		// determine break-even point
+		if Tk < Tb && th1 < 0 {
+			th1 = n
+			fmt.Print("  break-even point")
+		}
+
+		// determine diminishing return
+		if 0 < delta && delta < deltaOld && th2 < 0 {
+			th2 = n
+			fmt.Print("  diminishing return")
+		}
+		deltaOld = delta
+
+		fmt.Println()
+
+		// trigger counter
+		if th1 >= 0 && th2 >= 0 && count < 0 {
+			count = 20 // this many extra measurements after we got both thresholds
+		}
+
+		n++
+	}
+}
+
+
+func TestCalibrate(t *testing.T) {
+	if *calibrate {
+		computeThresholds()
+	}
+}