testing: allow manual timer control in testing.B.Loop

Fixes #72922

Change-Id: I56610d2d11d151a8f95b6434bbedbfcd5c11c317
Reviewed-on: https://go-review.googlesource.com/c/go/+/658975
LUCI-TryBot-Result: Go LUCI <[email protected]>
Commit-Queue: Junyang Shao <[email protected]>
Reviewed-by: Austin Clements <[email protected]>

Author: JunyangShao

src/testing/benchmark.go

@@ -368,16 +368,16 @@ func (b *B) ReportMetric(n float64, unit string) {
 }
 
 func (b *B) stopOrScaleBLoop() bool {
-	timeElapsed := highPrecisionTimeSince(b.start)
-	if timeElapsed >= b.benchTime.d {
+	t := b.Elapsed()
+	if t >= b.benchTime.d {
 		// Stop the timer so we don't count cleanup time
 		b.StopTimer()
 		return false
 	}
 	// Loop scaling
 	goalns := b.benchTime.d.Nanoseconds()
 	prevIters := int64(b.N)
-	b.N = predictN(goalns, prevIters, timeElapsed.Nanoseconds(), prevIters)
+	b.N = predictN(goalns, prevIters, t.Nanoseconds(), prevIters)
 	b.loopN++
 	return true
 }

src/testing/loop_test.go

@@ -7,7 +7,7 @@ package testing
 func TestBenchmarkBLoop(t *T) {
 	var initialStart highPrecisionTime
 	var firstStart highPrecisionTime
-	var lastStart highPrecisionTime
+	var scaledStart highPrecisionTime
 	var runningEnd bool
 	runs := 0
 	iters := 0
@@ -19,7 +19,9 @@ func TestBenchmarkBLoop(t *T) {
 			if iters == 0 {
 				firstStart = b.start
 			}
-			lastStart = b.start
+			if iters == 1 {
+				scaledStart = b.start
+			}
 			iters++
 		}
 		finalBN = b.N
@@ -45,8 +47,8 @@ func TestBenchmarkBLoop(t *T) {
 	if firstStart == initialStart {
 		t.Errorf("b.Loop did not reset the timer")
 	}
-	if lastStart != firstStart {
-		t.Errorf("timer was reset during iteration")
+	if scaledStart != firstStart {
+		t.Errorf("b.Loop stops and restarts the timer during iteration")
 	}
 	// Verify that it stopped the timer after the last loop.
 	if runningEnd {