cmd/compile: omit write barrier when assigning global function

Currently we generate write barriers when the right side of an
assignment is a global function. This doesn't fall into the existing
case of storing an address of a global because we haven't lowered the
function to a pointer yet.

This write barrier is unnecessary, so eliminate it.

Fixes #13901.

Change-Id: Ibc10e00a8803db0fd75224b66ab94c3737842a79
Reviewed-on: https://go-review.googlesource.com/20772
Run-TryBot: Austin Clements <austin@google.com>
Reviewed-by: Keith Randall <khr@golang.org>

src/cmd/compile/internal/gc/walk.go

@@ -2140,6 +2140,12 @@ func needwritebarrier(l *Node, r *Node) bool {
 		return false
 	}
 
+	// No write barrier for storing global function, which is live
+	// no matter what.
+	if r.Op == ONAME && r.Class == PFUNC {
+		return false
+	}
+
 	// Otherwise, be conservative and use write barrier.
 	return true
 }

test/writebarrier.go

@@ -158,3 +158,13 @@ func t1(i interface{}) **int {
 	}
 	return nil
 }
+
+type T17 struct {
+	f func(*T17)
+}
+
+func f17(x *T17) {
+	// See golang.org/issue/13901
+	x.f = f17                      // no barrier
+	x.f = func(y *T17) { *y = *x } // ERROR "write barrier"
+}