cmd/compile/internal/gc: add nil check for closure call on wasm

This commit adds an explicit nil check for closure calls on wasm,
so calling a nil func causes a proper panic instead of crashing on the
WebAssembly level.

Change-Id: I6246844f316677976cdd420618be5664444c25ae
Reviewed-on: https://go-review.googlesource.com/127759
Run-TryBot: Richard Musiol <neelance@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

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

@@ -3515,6 +3515,10 @@ func (s *state) call(n *Node, k callKind) *ssa.Value {
 			break
 		}
 		closure = s.expr(fn)
+		if thearch.LinkArch.Family == sys.Wasm {
+			// TODO(neelance): On other architectures this should be eliminated by the optimization steps
+			s.nilCheck(closure)
+		}
 	case OCALLMETH:
 		if fn.Op != ODOTMETH {
 			Fatalf("OCALLMETH: n.Left not an ODOTMETH: %v", fn)

test/closure4.go

@@ -0,0 +1,21 @@
+// run
+
+// Copyright 2018 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.
+
+// Check that calling a nil func causes a proper panic.
+
+package main
+
+func main() {
+	defer func() {
+		err := recover()
+		if err == nil {
+			panic("panic expected")
+		}
+	}()
+
+	var f func()
+	f()
+}