testing: Update testing.B.Loop to save benchmark results.

This is fixing some the missing logic of CL 627755.

Change-Id: I35acffef7299331fce21da4a80a26185b2e909f4
Reviewed-on: https://go-review.googlesource.com/c/go/+/630455
Commit-Queue: Junyang Shao <shaojunyang@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

3 files changed, 42 insertions, 31 deletions

diff --git a/src/testing/benchmark.go b/src/testing/benchmark.go
index db0aec5100..67a5abccb4 100644
--- a/src/testing/benchmark.go
+++ b/src/testing/benchmark.go
@@ -230,13 +230,12 @@ func (b *B) run1() bool {
 	b.mu.RLock()
 	finished := b.finished
 	b.mu.RUnlock()
-	// b.Loop() does its own ramp-up so we just need to run it once.
-	if b.hasSub.Load() || finished || b.loopN != 0 {
+	if b.hasSub.Load() || finished {
 		tag := "BENCH"
 		if b.skipped {
 			tag = "SKIP"
 		}
-		if b.chatty != nil && (len(b.output) > 0 || finished || b.loopN != 0) {
+		if b.chatty != nil && (len(b.output) > 0 || finished) {
 			b.trimOutput()
 			fmt.Fprintf(b.w, "%s--- %s: %s\n%s", b.chatty.prefix(), tag, b.name, b.output)
 		}
@@ -304,28 +303,32 @@ func (b *B) launch() {
 		b.signal <- true
 	}()
 
-	// Run the benchmark for at least the specified amount of time.
-	if b.benchTime.n > 0 {
-		// We already ran a single iteration in run1.
-		// If -benchtime=1x was requested, use that result.
-		// See https://golang.org/issue/32051.
-		if b.benchTime.n > 1 {
-			b.runN(b.benchTime.n)
-		}
-	} else {
-		d := b.benchTime.d
-		for n := int64(1); !b.failed && b.duration < d && n < 1e9; {
-			last := n
-			// Predict required iterations.
-			goalns := d.Nanoseconds()
-			prevIters := int64(b.N)
-			prevns := b.duration.Nanoseconds()
-			if prevns <= 0 {
-				// Round up, to avoid div by zero.
-				prevns = 1
+	// b.Loop does its own ramp-up logic so we just need to run it once.
+	// If b.loopN is non zero, it means b.Loop has already run.
+	if b.loopN == 0 {
+		// Run the benchmark for at least the specified amount of time.
+		if b.benchTime.n > 0 {
+			// We already ran a single iteration in run1.
+			// If -benchtime=1x was requested, use that result.
+			// See https://golang.org/issue/32051.
+			if b.benchTime.n > 1 {
+				b.runN(b.benchTime.n)
+			}
+		} else {
+			d := b.benchTime.d
+			for n := int64(1); !b.failed && b.duration < d && n < 1e9; {
+				last := n
+				// Predict required iterations.
+				goalns := d.Nanoseconds()
+				prevIters := int64(b.N)
+				prevns := b.duration.Nanoseconds()
+				if prevns <= 0 {
+					// Round up, to avoid div by zero.
+					prevns = 1
+				}
+				n = int64(predictN(goalns, prevIters, prevns, last))
+				b.runN(int(n))
 			}
-			n = int64(predictN(goalns, prevIters, prevns, last))
-			b.runN(int(n))
 		}
 	}
 	b.result = BenchmarkResult{b.N, b.duration, b.bytes, b.netAllocs, b.netBytes, b.extra}
diff --git a/src/testing/benchmark_test.go b/src/testing/benchmark_test.go
index 01bb695726..1f55fa5060 100644
--- a/src/testing/benchmark_test.go
+++ b/src/testing/benchmark_test.go
@@ -127,23 +127,26 @@ func TestRunParallelSkipNow(t *testing.T) {
 	})
 }
 
-func TestLoopEqualsRangeOverBN(t *testing.T) {
+func TestBLoopHasResults(t *testing.T) {
 	// Verify that b.N and the b.Loop() iteration count match.
-	var nIterated, nInfered int
-	testing.Benchmark(func(b *testing.B) {
+	var nIterated int
+	bRet := testing.Benchmark(func(b *testing.B) {
 		i := 0
 		for b.Loop() {
 			i++
 		}
 		nIterated = i
-		nInfered = b.N
 	})
-	if nIterated != nInfered {
-		t.Fatalf("Iteration of the two different benchmark loop flavor differs, got %d iterations want %d", nIterated, nInfered)
-	}
 	if nIterated == 0 {
 		t.Fatalf("Iteration count zero")
 	}
+	if bRet.N != nIterated {
+		t.Fatalf("Benchmark result N incorrect, got %d want %d", bRet.N, nIterated)
+	}
+	// We only need to check duration to make sure benchmark result is written.
+	if bRet.T == 0 {
+		t.Fatalf("Benchmark result duration unset")
+	}
 }
 
 func ExampleB_RunParallel() {
diff --git a/src/testing/testing_test.go b/src/testing/testing_test.go
index 4bf6378782..1e32a1e615 100644
--- a/src/testing/testing_test.go
+++ b/src/testing/testing_test.go
@@ -989,6 +989,11 @@ func TestBenchmarkBLoopIterationCorrect(t *testing.T) {
 	if c != want {
 		t.Errorf("got %d loop rampup; want %d", c, want)
 	}
+
+	re := regexp.MustCompile(`BenchmarkBLoopPrint-[0-9]+\s*2\s*[0-9]+\s*ns/op.*`)
+	if !re.Match(out) {
+		t.Errorf("Missing benchmark output")
+	}
 }
 
 func TestBenchmarkBNIterationCorrect(t *testing.T) {