cmd/compile: stores to unused variables on the stack not eliminated

[mundaym]

I am trying to replace math.Float64bits and friends with register to register moves and noticed that stores to variables on the stack aren't eliminated when their all their uses have been optimized away. Is there an existing issue for this?

For example, on tip (6e9ec14):

package main

var Sink int

func main() {
	x := 1
	Sink = *(&x)
}

Compiles to:

   	00000 (/tmp/main.go:5)	TEXT	"".main(SB)
   	00001 (/tmp/main.go:5)	FUNCDATA	$0, gclocals·33cdeccccebe80329f1fdbee7f5874cb(SB)
   	00002 (/tmp/main.go:5)	FUNCDATA	$1, gclocals·33cdeccccebe80329f1fdbee7f5874cb(SB)
v7	00003 (/tmp/main.go:6)	MOVQ	$1, "".x-8(SP)
v13	00004 (/tmp/main.go:7)	MOVQ	$1, "".Sink(SB)
b1	00005 (/tmp/main.go:8)	RET
   	00006 (<unknown line number>)	END

The MOVQ $1, "".x-8(SP) could be eliminated by the compiler but isn't.

[randall77]

Kind of a dup of #13095

[gopherbot]

CL https://golang.org/cl/38746 mentions this issue.