cmd/compile: not inlined closures can't inline inner closures

[DmitriyMV]

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

$ go version
go version go1.15 darwin/amd64

Does this issue reproduce with the latest release?

Yes.

What did you do?

Wrote code:

package go_closures

func callInt(val int) int {
	getInt := func() int { return val }
	return callFn(getInt)
}

func callFn(getInt func() int) int {
	outer := func() int { return getInt() }
	return globalFunc(outer)
}

var globalFunc = func(f func() int) int {
	return f()
}

What did you expect to see?

getInt is inlined into outer. outer escapes to the heap.

What did you see instead?

getInt isn't inlined into outer. Both outer and getInt escape to the heap.

[DmitriyMV]

The main problem here is that getInt closure escapes when it can be folded into outer (even with call).

[mdempsky]

The issue here is we don't currently support inlining functions that contain closures. We'd first need to inline callFn into callInt to be able to inline getInt into outer.

Escape analysis is a red herring here. Escape analysis runs after inlining and doesn't affect inlining decisions.

[mdempsky]

/cc @danscales , who's working on making functions that contain closures inlinable

[danscales]

Yes, I will look at this bug, after I submit the base change that allows inlining of functions with closures.

[DmitriyMV]

Still reproduces on 1.17:

package main

import (
	"fmt"
	"testing"
)

func callInt(val int) int {
	getInt := func() int { return val }
	return callFn(getInt)
}

func callFn(getInt func() int) int {
	outer := func() int { return getInt() }
	return globalFunc(outer)
}

var globalFunc = func(f func() int) int {
	return f()
}

func main() {
	result := testing.Benchmark(Bench)
	fmt.Println("result.AllocsPerOp()", result.AllocsPerOp())
}

func Bench(b *testing.B) {
	for i := 0; i < b.N; i++ {
		callInt(i)
	}
}

[danscales]

Thank you, will take a look if any improvement is possible for 1.18.

[DmitriyMV]

Looks like it's fixed on Go 1.23 - there is now only two allocations for variable and for function literal.