go: testing: benchmark iteration reports incorrectly

What version of Go are you using (go version)?

$ go version
go version devel +8266570ba7 Fri Sep 25 19:10:05 2020 +0000 darwin/amd64

Does this issue reproduce with the latest release?

Yes. 1.15.2.

What operating system and processor architecture are you using (go env)?

$ go env
GO111MODULE=""
GOARCH="amd64"
GOBIN=""
GOCACHE="/Users/changkun/Library/Caches/go-build"
GOENV="/Users/changkun/Library/Application Support/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOINSECURE=""
GOMODCACHE="/Users/changkun/go/pkg/mod"
GONOPROXY=""
GONOSUMDB=""
GOOS="darwin"
GOPATH="/Users/changkun/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/Users/changkun/dev/godev/go-gerrit"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/Users/changkun/dev/godev/go-gerrit/pkg/tool/darwin_amd64"
GCCGO="gccgo"
AR="ar"
CC="clang"
CXX="clang++"
CGO_ENABLED="1"
GOMOD="/Users/changkun/dev/godev/go-gerrit/src/go.mod"
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/jd/vym7tt9s2_379d4ccjcj1xrw0000gn/T/go-build637901099=/tmp/go-build -gno-record-gcc-switches -fno-common"

What did you do?

According to the doc:

The benchmark function must run the target code b.N times. During benchmark execution, b.N is adjusted until the benchmark function lasts long enough to be timed reliably. The output

BenchmarkRandInt-8   	68453040	        17.8 ns/op

means that the loop ran 68453040 times at a speed of 17.8 ns per loop.

But the following test fails and the benchmark runs the target code more than b.N times.

// main_test.go
func TestResultIteration(t *testing.T) {
	var v int32
	r := testing.Benchmark(func(b *testing.B) {
		for i := 0; i < b.N; i++ {
			atomic.AddInt32(&v, 1)
		}
	})

	if r.N != int(v) {
		t.Fatalf("incorrect iteration count, want %d got %d", v, r.N)
	}
}

What did you expect to see?

PASS

What did you see instead?

=== RUN   TestResultIteration
    main_test.go:27: incorrect iteration count, want 359815341 got 258805240
--- FAIL: TestResultIteration (1.67s)
FAIL