cmd/6g: fix componentgen for funarg structs.

Fixes #4518.

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/6932045

src/cmd/6g/cgen.c

@@ -1632,6 +1632,12 @@ componentgen(Node *nr, Node *nl)
 	case TSTRUCT:
 		loffset = nodl.xoffset;
 		roffset = nodr.xoffset;
+		// funarg structs may not begin at offset zero.
+		if(nl->type->etype == TSTRUCT && nl->type->funarg && nl->type->type)
+			loffset -= nl->type->type->width;
+		if(nr != N && nr->type->etype == TSTRUCT && nr->type->funarg && nr->type->type)
+			roffset -= nr->type->type->width;
+
 		for(t=nl->type->type; t; t=t->down) {
 			nodl.xoffset = loffset + t->width;
 			nodl.type = t->type;

test/fixedbugs/issue4518.go

@@ -0,0 +1,67 @@
+// run
+
+// Copyright 2012 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.
+
+// Issue 4518. In some circumstances "return F(...)"
+// where F has multiple returns is miscompiled by 6g due to
+// bold assumptions in componentgen.
+
+package main
+
+func DontInline() {}
+
+func F(e interface{}) (int, int) {
+	DontInline()
+	return 3, 7
+}
+
+func G() (int, int) {
+	DontInline()
+	return 3, 7
+}
+
+func bogus1(d interface{}) (int, int) {
+	switch {
+	default:
+		return F(d)
+	}
+	return 0, 0
+}
+
+func bogus2() (int, int) {
+	switch {
+	default:
+		return F(3)
+	}
+	return 0, 0
+}
+
+func bogus3(d interface{}) (int, int) {
+	switch {
+	default:
+		return G()
+	}
+	return 0, 0
+}
+
+func bogus4() (int, int) {
+	switch {
+	default:
+		return G()
+	}
+	return 0, 0
+}
+
+func check(a, b int) {
+	if a != 3 || b != 7 {
+		println(a, b)
+		panic("a != 3 || b != 7")
+	}
+}
+
+func main() {
+	check(bogus1(42))
+	check(bogus2())
+}