testing: don't measure cleanup time after B.Loop

B.Loop resets the timer on the first iteration so that setup code
isn't measured, but it currently leaves the timer running after the
last iteration, meaning that cleanup code will still be measured. Fix
this by stopping the timer when B.Loop returns false to indicate the
end of the benchmark.

Updates #61515

Change-Id: I0e0502cb2ce3c24cf872682b88d74e8be2c4529b
Reviewed-on: https://go-review.googlesource.com/c/go/+/635898
Reviewed-by: Junyang Shao <shaojunyang@google.com>
Auto-Submit: Austin Clements <austin@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>

src/testing/benchmark.go

@@ -366,6 +366,8 @@ func (b *B) ReportMetric(n float64, unit string) {
 func (b *B) stopOrScaleBLoop() bool {
 	timeElapsed := highPrecisionTimeSince(b.start)
 	if timeElapsed >= b.benchTime.d {
+		// Stop the timer so we don't count cleanup time
+		b.StopTimer()
 		return false
 	}
 	// Loop scaling
@@ -393,6 +395,7 @@ func (b *B) loopSlowPath() bool {
 			b.loopN++
 			return true
 		}
+		b.StopTimer()
 		return false
 	}
 	// Handles fixed time case
@@ -413,7 +416,8 @@ func (b *B) loopSlowPath() bool {
 //
 // Loop resets the benchmark timer the first time it is called in a benchmark,
 // so any setup performed prior to starting the benchmark loop does not count
-// toward the benchmark measurement.
+// toward the benchmark measurement. Likewise, when it returns false, it stops
+// the timer so cleanup code is not measured.
 //
 // The compiler never optimizes away calls to functions within the body of a
 // "for b.Loop() { ... }" loop. This prevents surprises that can otherwise occur

src/testing/loop_test.go

@@ -8,6 +8,7 @@ func TestBenchmarkBLoop(t *T) {
 	var initialStart highPrecisionTime
 	var firstStart highPrecisionTime
 	var lastStart highPrecisionTime
+	var runningEnd bool
 	runs := 0
 	iters := 0
 	finalBN := 0
@@ -22,6 +23,7 @@ func TestBenchmarkBLoop(t *T) {
 			iters++
 		}
 		finalBN = b.N
+		runningEnd = b.timerOn
 	})
 	// Verify that a b.Loop benchmark is invoked just once.
 	if runs != 1 {
@@ -46,6 +48,10 @@ func TestBenchmarkBLoop(t *T) {
 	if lastStart != firstStart {
 		t.Errorf("timer was reset during iteration")
 	}
+	// Verify that it stopped the timer after the last loop.
+	if runningEnd {
+		t.Errorf("timer was still running after last iteration")
+	}
 }
 
 // See also TestBenchmarkBLoop* in other files.