testing: Wrong number of allocations

[illiafox]

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

$ go version
go version go1.18.3 linux/amd64

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

go env Output

$ go env
GO111MODULE=""
GOARCH="amd64"
GOBIN=""
GOCACHE="/home/illia/.cache/go-build"
GOENV="/home/illia/.config/go/env"
GOEXE=""
GOEXPERIMENT=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOINSECURE=""
GOMODCACHE="/home/illia/go/pkg/mod"
GONOPROXY=""
GONOSUMDB=""
GOOS="linux"
GOPATH="/home/illia/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/usr/local/go"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/usr/local/go/pkg/tool/linux_amd64"
GOVCS=""
GOVERSION="go1.18.3"
GCCGO="gccgo"
GOAMD64="v1"
AR="ar"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD="/dev/null"
GOWORK=""
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 -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build2942373244=/tmp/go-build -gno-record-gcc-switches"

What did you do?

func ToBytes(s string) []byte {
	ptr := unsafe.Pointer(&s)
	slice := *(*[]byte)(ptr)
	return slice[:(*reflect.SliceHeader)(ptr).Len]
}

func BenchmarkToBytes(b *testing.B) {
	for i := 0; i < b.N; i++ {
		ToBytes("Hello, world!")
	}
}

func main() {
	alloc := testing.AllocsPerRun(1, func() {
		ToBytes("Hello, world!")
	})
	fmt.Println("Total allocations:", alloc)
}

go run .
go test -bench=BenchmarkToBytes -count=1 

What did you expect to see?

Same numbers of the allocations

What did you see instead?

$ go run .
Total allocations: 0

$ go test -bench=BenchmarkToBytes -count=1
goos: linux
goarch: amd64
cpu: Intel(R) Core(TM) i5-2540M CPU @ 2.60GHz
BenchmarkToBytes-4      1000000000               0.6898 ns/op

[mengzhuo]

The 0.6898 ns/op means times of operation instead of allocation.
Are you missing the benchmark with --benchmem?

[illiafox]

$ go test -bench=BenchmarkToBytes -count=1 -benchmem                                                                                                           
goos: linux
goarch: amd64
cpu: Intel(R) Core(TM) i5-2540M CPU @ 2.60GHz
BenchmarkToBytes-4      1000000000               0.6601 ns/op          0 B/op          0 allocs/op

oops, i thought that 1000000 is a number of allocations