cmd/compile: for tail calls in stubs, ensure args are alive

The generated code for interface stubs sometimes just messes
with a few of the args and then tail-calls to the target routine.
The args that aren't explicitly modified appear to not be used.
But they are used, by the thing we're tail calling.

Fixes #16016

Change-Id: Ib9b3a8311bb714a201daee002885fcb59e0463fa
Reviewed-on: https://go-review.googlesource.com/23960
Run-TryBot: David Chase <drchase@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>

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

@@ -577,6 +577,15 @@ func progeffects(prog *obj.Prog, vars []*Node, uevar bvec, varkill bvec, avarini
 
 		return
 	}
+	if prog.As == obj.AJMP && prog.To.Type == obj.TYPE_MEM && prog.To.Name == obj.NAME_EXTERN {
+		// This is a tail call. Ensure the arguments are still alive.
+		// See issue 16016.
+		for i, node := range vars {
+			if node.Class == PPARAM {
+				bvset(uevar, int32(i))
+			}
+		}
+	}
 
 	if prog.As == obj.ATEXT {
 		// A text instruction marks the entry point to a function and

test/fixedbugs/issue16016.go

@@ -0,0 +1,35 @@
+// 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.
+
+package main
+
+import "time"
+
+type T struct{}
+
+func (*T) Foo(vals []interface{}) {
+	switch v := vals[0].(type) {
+	case string:
+		_ = v
+	}
+}
+
+type R struct{ *T }
+
+type Q interface {
+	Foo([]interface{})
+}
+
+func main() {
+	var q Q = &R{&T{}}
+	for i := 0; i < 10000; i++ {
+		go func() {
+			defer q.Foo([]interface{}{"meow"})
+			time.Sleep(100 * time.Millisecond)
+		}()
+	}
+	time.Sleep(1 * time.Second)
+}