testing: fix fractional ns/op printing

CL 166717 changed the way ns/op benchmark results were printed and
inadvertently rounded all ns/op results down to an integer, even if
they were small enough to print with digits after the decimal place.
For example, prior to this change, we got output like

    BenchmarkFast-8       380491575	 3.12 ns/op

CL 166717 changed this to

    BenchmarkFast-8       380491575      3.00 ns/op

This had the further side-effect that ns/op values between 0 and 1
would not be printed at all because they would be rounded down to 0.

This CL fixes this by always recomputing the float64 value of ns/op
instead of using the int64 truncation from BenchmarkResult.NsPerOp.

Fixes #30997. Fixes #31005.

Change-Id: I21f73b9d5cc5ad41e7ff535675d07ca00051ecd7
Reviewed-on: https://go-review.googlesource.com/c/go/+/168937Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>

src/testing/benchmark.go

@@ -406,9 +406,14 @@ func (r BenchmarkResult) String() string {
 	buf := new(strings.Builder)
 	fmt.Fprintf(buf, "%8d", r.N)
 
-	if ns := r.NsPerOp(); ns != 0 {
+	// Get ns/op as a float.
+	ns, ok := r.Extra["ns/op"]
+	if !ok {
+		ns = float64(r.T.Nanoseconds()) / float64(r.N)
+	}
+	if ns != 0 {
 		buf.WriteByte('\t')
-		prettyPrint(buf, float64(ns), "ns/op")
+		prettyPrint(buf, ns, "ns/op")
 	}
 
 	if mbs := r.mbPerSec(); mbs != 0 {

src/testing/benchmark_test.go

@@ -12,6 +12,7 @@ import (
 	"sync/atomic"
 	"testing"
 	"text/template"
+	"time"
 )
 
 var prettyPrintTests = []struct {
@@ -40,6 +41,32 @@ func TestPrettyPrint(t *testing.T) {
 	}
 }
 
+func TestResultString(t *testing.T) {
+	// Test fractional ns/op handling
+	r := testing.BenchmarkResult{
+		N: 100,
+		T: 240 * time.Nanosecond,
+	}
+	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 {
+		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 {
+		t.Errorf("String: expected %q, actual %q", want, got)
+	}
+
+	// Test 0 ns/op
+	r.T = 0
+	if want, got := "     100", r.String(); want != got {
+		t.Errorf("String: expected %q, actual %q", want, got)
+	}
+}
+
 func TestRunParallel(t *testing.T) {
 	testing.Benchmark(func(b *testing.B) {
 		procs := uint32(0)