cmd/compile: support inlining non-escaping closures

[josharian]

Split out from #9337 and #15537.

[rogpeppe]

I'd very much like to see this happen. An idiom that's useful for dealing with lots of returns from a function with long-winded return values is this:

func f() (int, int, error) {
	fail := func(err error) (int, int, error) {
		return 0, 0, err
	}
	if somethingWrong {
		return fail(someError)
	}
	return foo, bar, nil
}

It would be great if this generated exactly the same code as the equivalent:

func f() (int, int, error) {
	if somethingWrong {
		return 0, 0, someError
	}
	return foo, bar, nil
}

[gopherbot]

Change https://golang.org/cl/65071 mentions this issue: cmd/compile: inline closures whenever possible

[huguesb]

Taking a stab at this. I have a CL that can handle the basic test case above, with serious limitations/bugs.

I'm not sure what's going on with captured variables (getting invariant violations in the SSA phase) and would appreciate some guidance from someone more familiar with the compiler.

Also, there doesn't seem to be any easy way to tell if a variable is assigned more than once. Am I missing something?

[gopherbot]

Change https://golang.org/cl/72490 mentions this issue: cmd/compile: inline closures with captures

[gopherbot]

Change https://golang.org/cl/75770 mentions this issue: cmd/compile: fix reassignment check

[gopherbot]

Change https://golang.org/cl/83538 mentions this issue: cmd/compile: remove broken inlining accounting code

[thepudds]

Hi @rogpeppe, is your specific request in #15561 (comment) addressed at this point, or still pending?

For your example, it looks like fail gets inlined, but f does not (cannot inline f: unhandled op CLOSURE), but I'm not sure which piece you personally were focused on in your example.

Separately, I'm not sure sure if this overall issue is about addressing inlining fail in this example, or f, or both. Said another way, I am not sure if this issue should still be open, vs. marked as resolved.

func f() (int, int, error) {
	fail := func(err error) (int, int, error) {
		return 0, 0, err
	}
	if somethingWrong {
		return fail(someError)
	}
	return foo, bar, nil
}

(And sorry, I just looked at a bunch of examples, so non-trivial chance I might just be confused here).

[gopherbot]

Change https://golang.org/cl/260018 mentions this issue: sort: inline search convenience wrappers

[DmitriyMV]

This appears to be fixed on the latest Go https://go.godbolt.org/z/MdGb366c5

@josharian @rogpeppe am I correct?