testing: Wrong number of allocations #54041
Description
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