testing: b.StopTimer breaks b.Loop #72922
Description
Go version
go version go1.24.1 linux/amd64
Output of go env in your module/workspace:
AR='ar'
CC='gcc'
CGO_CFLAGS='-O2 -g'
CGO_CPPFLAGS=''
CGO_CXXFLAGS='-O2 -g'
CGO_ENABLED='1'
CGO_FFLAGS='-O2 -g'
CGO_LDFLAGS='-O2 -g'
CXX='g++'
GCCGO='gccgo'
GO111MODULE=''
GOAMD64='v1'
GOARCH='amd64'
GOAUTH='netrc'
GOBIN=''
GOCACHE='/home/dominikh/.cache/go-build'
GOCACHEPROG=''
GODEBUG=''
GOENV='/home/dominikh/.config/go/env'
GOEXE=''
GOEXPERIMENT=''
GOFIPS140='off'
GOFLAGS=''
GOGCCFLAGS='-fPIC -m64 -pthread -Wl,--no-gc-sections -fmessage-length=0 -ffile-prefix-map=/tmp/go-build1859758891=/tmp/go-build -gno-record-gcc-switches'
GOHOSTARCH='amd64'
GOHOSTOS='linux'
GOINSECURE=''
GOMOD='/home/dominikh/prj/src/example.com/go.mod'
GOMODCACHE='/home/dominikh/prj/pkg/mod'
GONOPROXY=''
GONOSUMDB=''
GOOS='linux'
GOPATH='/home/dominikh/prj'
GOPRIVATE=''
GOPROXY='https://proxy.golang.org,direct'
GOROOT='/home/dominikh/prj/go'
GOSUMDB='sum.golang.org'
GOTELEMETRY='on'
GOTELEMETRYDIR='/home/dominikh/.config/go/telemetry'
GOTMPDIR=''
GOTOOLCHAIN='auto'
GOTOOLDIR='/home/dominikh/prj/go/pkg/tool/linux_amd64'
GOVCS=''
GOVERSION='go1.24.1'
GOWORK=''
PKG_CONFIG='pkg-config'What did you do?
Consider the following benchmark function:
func BenchmarkFoo(b *testing.B) {
for b.Loop() {
println("iter")
b.StopTimer()
// expensive per-iteration setup work
b.StartTimer()
time.Sleep(100 * time.Millisecond)
}
}
Run with go test -bench=. -benchtime=1s
b.Loop never returns false and the benchmark never terminates.
In B.stopOrScaleBLoop we use b.start to compute how long we took to iterate b.N times. However, calling B.StartTimer sets b.start to the current time.
None of the documentation says that using b.StopTimer/b.StartTimer is forbidden when using b.Loop.