testing: increase benchmark output to four significant figures

Currently, the benchmark output from the testing package prints small
values with three significant figures. This means it can only
distinguish 1 part in 100, or a 1% error, which can be enough to throw
off further analysis of the output. This CL increases it to four
significant figures. For time values, at least, anything beyond four
significant figures is almost certainly noise.

Fixes #34626.

Change-Id: I3bcf305427130026276e6a4c78167989319f280c
Reviewed-on: https://go-review.googlesource.com/c/go/+/267102
Trust: Austin Clements <austin@google.com>
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Michael Pratt <mpratt@google.com>

2 files changed, 23 insertions, 18 deletions

diff --git a/src/testing/benchmark.go b/src/testing/benchmark.go
index 1b81ec3a2d..a8f75e9712 100644
--- a/src/testing/benchmark.go
+++ b/src/testing/benchmark.go
@@ -451,23 +451,27 @@ func (r BenchmarkResult) String() string {
 
 func prettyPrint(w io.Writer, x float64, unit string) {
 	// Print all numbers with 10 places before the decimal point
-	// and small numbers with three sig figs.
+	// and small numbers with four sig figs. Field widths are
+	// chosen to fit the whole part in 10 places while aligning
+	// the decimal point of all fractional formats.
 	var format string
 	switch y := math.Abs(x); {
-	case y == 0 || y >= 99.95:
+	case y == 0 || y >= 999.95:
 		format = "%10.0f %s"
-	case y >= 9.995:
+	case y >= 99.995:
 		format = "%12.1f %s"
-	case y >= 0.9995:
+	case y >= 9.9995:
 		format = "%13.2f %s"
-	case y >= 0.09995:
+	case y >= 0.99995:
 		format = "%14.3f %s"
-	case y >= 0.009995:
+	case y >= 0.099995:
 		format = "%15.4f %s"
-	case y >= 0.0009995:
+	case y >= 0.0099995:
 		format = "%16.5f %s"
-	default:
+	case y >= 0.00099995:
 		format = "%17.6f %s"
+	default:
+		format = "%18.7f %s"
 	}
 	fmt.Fprintf(w, format, x, unit)
 }
diff --git a/src/testing/benchmark_test.go b/src/testing/benchmark_test.go
index 1434c2613f..4c1cbd1933 100644
--- a/src/testing/benchmark_test.go
+++ b/src/testing/benchmark_test.go
@@ -22,13 +22,14 @@ var prettyPrintTests = []struct {
 	{0, "         0 x"},
 	{1234.1, "      1234 x"},
 	{-1234.1, "     -1234 x"},
-	{99.950001, "       100 x"},
-	{99.949999, "        99.9 x"},
-	{9.9950001, "        10.0 x"},
-	{9.9949999, "         9.99 x"},
-	{-9.9949999, "        -9.99 x"},
-	{0.0099950001, "         0.0100 x"},
-	{0.0099949999, "         0.00999 x"},
+	{999.950001, "      1000 x"},
+	{999.949999, "       999.9 x"},
+	{99.9950001, "       100.0 x"},
+	{99.9949999, "        99.99 x"},
+	{-99.9949999, "       -99.99 x"},
+	{0.000999950001, "         0.001000 x"},
+	{0.000999949999, "         0.0009999 x"}, // smallest case
+	{0.0000999949999, "         0.0001000 x"},
 }
 
 func TestPrettyPrint(t *testing.T) {
@@ -50,13 +51,13 @@ func TestResultString(t *testing.T) {
 	if r.NsPerOp() != 2 {
 		t.Errorf("NsPerOp: expected 2, actual %v", r.NsPerOp())
 	}
-	if want, got := "     100\t         2.40 ns/op", r.String(); want != got {
+	if want, got := "     100\t         2.400 ns/op", r.String(); want != got {
 		t.Errorf("String: expected %q, actual %q", want, got)
 	}
 
 	// Test sub-1 ns/op (issue #31005)
 	r.T = 40 * time.Nanosecond
-	if want, got := "     100\t         0.400 ns/op", r.String(); want != got {
+	if want, got := "     100\t         0.4000 ns/op", r.String(); want != got {
 		t.Errorf("String: expected %q, actual %q", want, got)
 	}
 
@@ -130,7 +131,7 @@ func TestReportMetric(t *testing.T) {
 	}
 	// Test stringing.
 	res.N = 1 // Make the output stable
-	want := "       1\t     12345 ns/op\t         0.200 frobs/op"
+	want := "       1\t     12345 ns/op\t         0.2000 frobs/op"
 	if want != res.String() {
 		t.Errorf("expected %q, actual %q", want, res.String())
 	}