cmd/compile: avoidable construction of stack objects

[mdempsky]

Compiling the program below with -live shows that x gets turned into a stack object:

package p

var m map[int16]bool

func f() {
        var x struct { p *int; i int16 }
        x.i = g()
        m[x.i] = true
        h()
        j(x.i)
}

func g() int16
func h()
func j(int16)

This is because the assignment m[x.i] = true passes x.i by reference, causing x to get marked as addrtaken.

Note that the logically equivalent program that replaces m[x.i] = true with tmp := x.i; m[tmp] = true does not create a stack object for x.

[mdempsky]

Slightly more realistic test case. x here should not require a stack object either:

package p

var m map[interface{}]bool

func f() {
	x := g()
	m[x] = true
	h()
}

func g() interface{}
func h()

(Edit: Actually, it's an autotmp copy of x that gets the stack object, not x itself. But my larger point is that f shouldn't need stack objects at all, since we statically know the lifetime of all variables there.)

[gopherbot]

Change https://golang.org/cl/284412 mentions this issue: [dev.regabi] cmd/compile: convert OPANIC argument to interface{} during typecheck