testing: separate b.Loop counter from b.N

Currently, b.Loop uses b.N as the iteration count target. However,
since it updates the target as it goes, the behavior is quite
different from a b.N-style benchmark. To avoid user confusion, this CL
gives b.Loop a separate, unexported iteration count target. It ensures
b.N is 0 within the b.Loop loop to help catch misuses, and commits the
final iteration count to b.N only once the loop is done (as the
documentation states "After Loop returns false, b.N contains the total
number of iterations that ran, so the benchmark may use b.N to compute
other average metrics.")

Since there are now two variables used by b.Loop, we put them in an
unnamed struct. Also, we rename b.loopN to b.loop.i because this
variable tracks the current iteration index (conventionally "i"), not
the target (conventionally "n").

Unfortunately, a simple renaming causes B.Loop to be too large for the
inliner. Thus, we make one simplification to B.Loop to keep it under
the threshold. We're about to lean into that simplification anyway in
a follow-up CL, so this is just temporary.

Prep for #72933 and #72971.

Change-Id: Ide1c4f1b9ca37f300f3beb0e60ba6202331b183e
Reviewed-on: https://go-review.googlesource.com/c/go/+/659655
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Junyang Shao <shaojunyang@google.com>
Auto-Submit: Austin Clements <austin@google.com>

1 files changed, 12 insertions, 0 deletions

diff --git a/src/testing/loop_test.go b/src/testing/loop_test.go
index 781a8566e8..7a42919643 100644
--- a/src/testing/loop_test.go
+++ b/src/testing/loop_test.go
@@ -11,6 +11,8 @@ func TestBenchmarkBLoop(t *T) {
 	var runningEnd bool
 	runs := 0
 	iters := 0
+	firstBN := 0
+	restBN := 0
 	finalBN := 0
 	bRet := Benchmark(func(b *B) {
 		initialStart = b.start
@@ -18,6 +20,9 @@ func TestBenchmarkBLoop(t *T) {
 		for b.Loop() {
 			if iters == 0 {
 				firstStart = b.start
+				firstBN = b.N
+			} else {
+				restBN = max(restBN, b.N)
 			}
 			if iters == 1 {
 				scaledStart = b.start
@@ -39,6 +44,13 @@ func TestBenchmarkBLoop(t *T) {
 	if finalBN != iters || bRet.N != iters {
 		t.Errorf("benchmark iterations mismatch: %d loop iterations, final b.N=%d, bRet.N=%d", iters, finalBN, bRet.N)
 	}
+	// Verify that b.N was 0 inside the loop
+	if firstBN != 0 {
+		t.Errorf("want b.N == 0 on first iteration, got %d", firstBN)
+	}
+	if restBN != 0 {
+		t.Errorf("want b.N == 0 on subsequent iterations, got %d", restBN)
+	}
 	// Make sure the benchmark ran for an appropriate amount of time.
 	if bRet.T < benchTime.d {
 		t.Fatalf("benchmark ran for %s, want >= %s", bRet.T, benchTime.d)