testing: always ReadMemStats before first benchmark run

If the only way the user indicates they want alloc stats shown
is via ReportAllocs, we don't know that until benchFunc is run.
Therefore, StopTimer's ReadMemStats will return incorrect data
for single cycle runs since there's no counterpart ReadMemStats from
StartTimer that initializes alloc stats.

It appears that this bug was introduced by CL 46612,
"testing: only call ReadMemStats if necessary when benchmarking"

Fixes #20590

Change-Id: I3b5ef91677823f4b98011880a3be15423baf7e33
Reviewed-on: https://go-review.googlesource.com/46612Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/testing/benchmark.go

@@ -73,11 +73,9 @@ type B struct {
 // a call to StopTimer.
 func (b *B) StartTimer() {
 	if !b.timerOn {
-		if *benchmarkMemory || b.showAllocResult {
-			runtime.ReadMemStats(&memStats)
-			b.startAllocs = memStats.Mallocs
-			b.startBytes = memStats.TotalAlloc
-		}
+		runtime.ReadMemStats(&memStats)
+		b.startAllocs = memStats.Mallocs
+		b.startBytes = memStats.TotalAlloc
 		b.start = time.Now()
 		b.timerOn = true
 	}

src/testing/sub_test.go

@@ -540,6 +540,16 @@ func TestBenchmarkStartsFrom1(t *T) {
 	})
 }
 
+func TestBenchmarkReadMemStatsBeforeFirstRun(t *T) {
+	var first = true
+	Benchmark(func(b *B) {
+		if first && (b.startAllocs == 0 || b.startBytes == 0) {
+			panic(fmt.Sprintf("ReadMemStats not called before first run"))
+		}
+		first = false
+	})
+}
+
 func TestParallelSub(t *T) {
 	c := make(chan int)
 	block := make(chan int)