cmd/compile: similar returns not optimized

[marigonzes]

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

$ go version
go version devel +323212b Sun Jun 9 16:23:11 2019 +0000 linux/amd64

Does this issue reproduce with the latest release?

Yes.

What did you do?

I compiled the following function:

func f1(x int, b bool) int {
	if b {
		return x
	}
	return x
}

What did you expect to see?

I expected it to be compiled to a move and a return, given that the return value is always the same.

What did you see instead?

Instead, it was compiled to:

        movblzx "".b+16(SP), AX
        testb   AL, AL
        jeq     f1_pc20
        movq    "".x+8(SP), AX
        movq    AX, "".~r2+24(SP)
        ret
f1_pc20:
        movq    "".x+8(SP), AX
        movq    AX, "".~r2+24(SP)
        ret

I think the compiler should recognize both cases to be the same.

Note: if this function is called from another one, the optimization is applied. For example:

func f2(x int, b bool) int {
    return f1(x, b)
}

is compiled to

        movq    "".x+8(SP), AX
        movq    AX, "".~r2+24(SP)
        ret

[randall77]

Yep, this is a TODO. I even have a CL for it: https://go-review.googlesource.com/c/go/+/33634
I never got around to actually finishing it up.

[MichaelTJones]

Is the underlying issue different than the split/common approach in your CL? That seems a late stage remedy to what should already have been rewritten away by a "Moot Code Elimination" pass.