cmd/cgo: fix default alignment for empty structs

Fixes #5242.

LGTM=iant
R=iant
CC=golang-codereviews
https://golang.org/cl/125120043

misc/cgo/test/cgo_test.go

@@ -53,5 +53,6 @@ func Test5986(t *testing.T)                { test5986(t) }
 func Test7665(t *testing.T)                { test7665(t) }
 func TestNaming(t *testing.T)              { testNaming(t) }
 func Test7560(t *testing.T)                { test7560(t) }
+func Test5242(t *testing.T)                { test5242(t) }
 
 func BenchmarkCgoCall(b *testing.B) { benchCgoCall(b) }

misc/cgo/test/issue5242.go

+// Copyright 2014 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 5242.  Cgo incorrectly computed the alignment of structs
+// with no Go accessible fields as 0, and then panicked on
+// modulo-by-zero computations.
+
+package cgotest
+
+/*
+typedef struct {
+} foo;
+
+typedef struct {
+	int x : 1;
+} bar;
+
+int issue5242(foo f, bar b) {
+	return 5242;
+}
+*/
+import "C"
+
+import "testing"
+
+func test5242(t *testing.T) {
+	if got := C.issue5242(C.foo{}, C.bar{}); got != 5242 {
+		t.Errorf("got %v", got)
+	}
+}

src/cmd/cgo/gcc.go

@@ -1534,6 +1534,9 @@ func (c *typeConv) pad(fld []*ast.Field, size int64) []*ast.Field {
 
 // Struct conversion: return Go and (6g) C syntax for type.
 func (c *typeConv) Struct(dt *dwarf.StructType, pos token.Pos) (expr *ast.StructType, csyntax string, align int64) {
+	// Minimum alignment for a struct is 1 byte.
+	align = 1
+
 	var buf bytes.Buffer
 	buf.WriteString("struct {")
 	fld := make([]*ast.Field, 0, 2*len(dt.Field)+1) // enough for padding around every field