cmd/compile: mark live heap-allocated pparamout vars as needzero

If we don't mark them as needzero, we have a live pointer variable
containing possible garbage, which will baffle the GC.

Fixes #16249.

Change-Id: I7c423ceaca199ddd46fc2c23e5965e7973f07584
Reviewed-on: https://go-review.googlesource.com/24715
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

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

@@ -1181,6 +1181,7 @@ func livenessepilogue(lv *Liveness) {
 	if hasdefer {
 		for _, n := range lv.vars {
 			if n.IsOutputParamHeapAddr() {
+				n.Name.Needzero = true
 				xoffset := n.Xoffset + stkptrsize
 				onebitwalktype1(n.Type, &xoffset, ambig)
 			}

test/fixedbugs/issue16249.go

+// run
+
+// Copyright 2016 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Liveness calculations were wrong for a result parameter pushed onto
+// the heap in a function that used defer.  Program would crash with
+//     runtime: bad pointer in frame main.A at 0xc4201e6838: 0x1
+
+package main
+
+import "errors"
+
+var sink interface{}
+
+//go:noinline
+func f(err *error) {
+	if err != nil {
+		sink = err
+	}
+}
+
+//go:noinline
+func A(n, m int64) (res int64, err error) {
+	defer f(&err) // output parameter's address escapes to a defer.
+	if n < 0 {
+		err = errors.New("No negative")
+		return
+	}
+	if n <= 1 {
+		res = n
+		return
+	}
+	res = B(m) // This call to B drizzles a little junk on the stack.
+	res, err = A(n-1, m)
+	res++
+	return
+}
+
+// B does a little bit of recursion dribbling not-zero onto the stack.
+//go:noinline
+func B(n int64) (res int64) {
+	if n <= 1 { // Prefer to leave a 1 on the stack.
+		return n
+	}
+	return 1 + B(n-1)
+}
+
+func main() {
+	x, e := A(0, 0)
+	for j := 0; j < 4; j++ { // j controls amount of B's stack dribble
+		for i := 0; i < 1000; i++ { // try more and more recursion until stack growth occurs in newobject in prologue
+			x, e = A(int64(i), int64(j))
+		}
+	}
+	_, _ = x, e
+}