cmd/compile: Inconsistent behavior

[theanarkh]

Go version

go version go1.22.2 darwin/arm64

Output of go env in your module/workspace:

GO111MODULE=''
GOARCH='arm64'
GOBIN=''
GOEXE=''
GOEXPERIMENT=''
GOFLAGS=''
GOHOSTARCH='arm64'
GOHOSTOS='darwin'
GOINSECURE=''
GOOS='darwin'
GOTMPDIR=''
GOTOOLCHAIN='auto'
GOVCS=''
GOVERSION='go1.22.2'
GCCGO='gccgo'
AR='ar'
CC='clang'
CXX='clang++'
CGO_ENABLED='1'
GOWORK=''
CGO_CFLAGS='-O2 -g'
CGO_CPPFLAGS=''
CGO_CXXFLAGS='-O2 -g'
CGO_FFLAGS='-O2 -g'
CGO_LDFLAGS='-O2 -g'
PKG_CONFIG='pkg-config'
GOGCCFLAGS='-fPIC -arch arm64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -ffile-prefix-map=/var/folders/s8/7ynb7ztx01l2vgsdm0fty3t80000gn/T/go-build27692839=/tmp/go-build -gno-record-gcc-switches -fno-common'

What did you do?

package main

import (
	"fmt"
	"runtime"
	"strings"
)

func GetFuncName() string {
	pc := make([]uintptr, 10)
	runtime.Callers(2, pc)
	f := runtime.FuncForPC(pc[0])
	name := f.Name()
	res := strings.Split(name, ".")
	return res[len(res)-1]
}

func wrapper(name string) func() {
	return func() {
		// It should print "demo"
		fmt.Println("function name: ", name)
	}
}

func demo() {
	// name is "demo"
	name := GetFuncName()
	defer wrapper(name)()
}

func main() {
	demo()
}

What did you see happen?

run go build demo.go && ./demo, output: function name: wrapper
run go build -gcflags "-N" demo.go && ./demo, output: function name: demo.

What did you expect to see?

The output is always demo.

[randall77]

runtime.FuncForPC does not (and can't) handle inlining correctly. Use runtime.CallersFrames for optimization-independent traceback.
That said, it is very strange that wrapper appears as the output. It is not on the stack at all when runtime.Callers is invoked.

[theanarkh]

Thanks! runtime.CallersFrames works.

[fengyoulin]

For runtime.Callers(), the doc said:

Callers fills the slice pc with the return program counters of function invocations on the calling goroutine's stack.

So, when you call runtime.FuncForPC() you should decrement the PC by one. Like this:

f := runtime.FuncForPC(pc[0]-1)