cmd/compile: missed phi elim opportunity using result of function call

[josharian]

This is a reasonably common piece of code:

func isClosed(c chan struct{}) bool {
	select {
	case <-c:
		return true
	default:
		return false
	}
}

The core of it compiles to:

CALL	runtime.selectnbrecv(SB)
CMPB	16(SP), $0
JEQ	74
MOVB	$1, "".~r1+48(SP)
RET
MOVB	$0, "".~r1+48(SP)
RET

We should be able to use the result directly rather than branching.

[dmitshur]

CC @randall77, @mdempsky via owners.

[mdempsky]

I don't think it's specific to function calls. Even a function like:

func f(x bool) bool {
	if x {
		return true
	}
	return false
}

gets compiled with a branch.

Off hand, I don't know how to solve this. I suspect it needs a generalized form of common-tail merging.