gc: fix zero-length struct eval

Fixes #2232.

R=ken2
CC=golang-dev
https://golang.org/cl/4960054

src/cmd/5g/cgen.c

@@ -1201,8 +1201,6 @@ sgen(Node *n, Node *res, int32 w)
 		dump("r", n);
 		dump("res", res);
 	}
-	if(w == 0)
-		return;
 	if(w < 0)
 		fatal("sgen copy %d", w);
 	if(n->ullman >= UINF && res->ullman >= UINF)
@@ -1210,6 +1208,15 @@ sgen(Node *n, Node *res, int32 w)
 	if(n->type == T)
 		fatal("sgen: missing type");
 
+	if(w == 0) {
+		// evaluate side effects only.
+		regalloc(&dst, types[tptr], N);
+		agen(res, &dst);
+		agen(n, &dst);
+		regfree(&dst);
+		return;
+	}
+
 	// determine alignment.
 	// want to avoid unaligned access, so have to use
 	// smaller operations for less aligned types.

src/cmd/6g/cgen.c

@@ -1029,11 +1029,9 @@ sgen(Node *n, Node *ns, int32 w)
 		dump("r", n);
 		dump("res", ns);
 	}
-	if(w == 0)
-		return;
-	if(n->ullman >= UINF && ns->ullman >= UINF) {
+
+	if(n->ullman >= UINF && ns->ullman >= UINF)
 		fatal("sgen UINF");
-	}
 
 	if(w < 0)
 		fatal("sgen copy %d", w);
@@ -1041,6 +1039,15 @@ sgen(Node *n, Node *ns, int32 w)
 	if(w == 16)
 		if(componentgen(n, ns))
 			return;
+	
+	if(w == 0) {
+		// evaluate side effects only
+		regalloc(&nodr, types[tptr], N);
+		agen(ns, &nodr);
+		agen(n, &nodr);
+		regfree(&nodr);
+		return;
+	}
 
 	// offset on the stack
 	osrc = stkof(n);

src/cmd/8g/cgen.c

@@ -1136,15 +1136,20 @@ sgen(Node *n, Node *res, int32 w)
 		dump("r", n);
 		dump("res", res);
 	}
-	if(w == 0)
-		return;
-	if(n->ullman >= UINF && res->ullman >= UINF) {
+	if(n->ullman >= UINF && res->ullman >= UINF)
 		fatal("sgen UINF");
-	}
 
 	if(w < 0)
 		fatal("sgen copy %d", w);
 
+	if(w == 0) {
+		// evaluate side effects only.
+		tempname(&tdst, types[tptr]);
+		agen(res, &tdst);
+		agen(n, &tdst);
+		return;
+	}
+
 	// offset on the stack
 	osrc = stkof(n);
 	odst = stkof(res);

test/struct0.go

@@ -0,0 +1,34 @@
+// $G $D/$F.go && $L $F.$A && ./$A.out
+
+// Copyright 2011 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.
+
+// zero length structs.
+// used to not be evaluated.
+// issue 2232.
+
+package main
+
+func recv(c chan interface{}) struct{} {
+	return (<-c).(struct{})
+}
+
+var m = make(map[interface{}]int)
+
+func recv1(c chan interface{}) {
+	defer rec()
+	m[(<-c).(struct{})] = 0
+}
+
+func rec() {
+	recover()
+}
+
+func main() {
+	c := make(chan interface{})
+	go recv(c)
+	c <- struct{}{}
+	go recv1(c)
+	c <- struct{}{}
+}