testing: fix Benchmark() to start at 1 iteration, not 100

The run1 call removed in golang.org/cl/36990 was necessary to
initialize the duration of the benchmark. With it gone, the math in
launch() starts from 100. This doesn't work out well for second-long
benchmark methods. Put it back.

Updates #18815

Change-Id: I461f3466c805d0c61124a2974662f7ad45335794
Reviewed-on: https://go-review.googlesource.com/37530
Run-TryBot: Heschi Kreinick <heschi@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Marcel van Lohuizen <mpvl@golang.org>

src/testing/benchmark.go

@@ -657,7 +657,10 @@ func Benchmark(f func(b *B)) BenchmarkResult {
 		benchFunc: f,
 		benchTime: *benchTime,
 	}
-	return b.run()
+	if b.run1() {
+		b.run()
+	}
+	return b.result
 }
 
 type discard struct{}

src/testing/sub_test.go

@@ -7,6 +7,7 @@ package testing
 import (
 	"bytes"
 	"fmt"
+	"os"
 	"regexp"
 	"runtime"
 	"strings"
@@ -530,6 +531,16 @@ func TestBenchmarkOutput(t *T) {
 	Benchmark(func(b *B) {})
 }
 
+func TestBenchmarkStartsFrom1(t *T) {
+	var first = true
+	Benchmark(func(b *B) {
+		if first && b.N != 1 {
+			panic(fmt.Sprintf("Benchmark() first N=%v; want 1", b.N))
+		}
+		first = false
+	})
+}
+
 func TestParallelSub(t *T) {
 	c := make(chan int)
 	block := make(chan int)
@@ -591,6 +602,7 @@ func TestBenchmark(t *T) {
 	res := Benchmark(func(b *B) {
 		for i := 0; i < 5; i++ {
 			b.Run("", func(b *B) {
+				fmt.Fprintf(os.Stderr, "b.N: %v\n", b.N)
 				for i := 0; i < b.N; i++ {
 					time.Sleep(time.Millisecond)
 				}