go/types: make array and struct size computations symetric

LGTM=adonovan
R=adonovan
CC=golang-codereviews
https://golang.org/cl/157110043

go/types/sizes.go

@@ -28,11 +28,12 @@ type Sizes interface {
 //	  specified size.
 //	- The size of strings and interfaces is 2*WordSize.
 //	- The size of slices is 3*WordSize.
-//	- The size of arrays is the aligned size (see below) of an element
-//        multiplied by the number of elements.
+//	- The size of an array of n elements corresponds to the size of
+//	  a struct of n consecutive fields of the array's element type.
 //      - The size of a struct is the offset of the last field plus that
-//	  field's size. If the struct is used in an array its size must
-//        first be aligned to a multiple of the struct's alignment.
+//	  field's size. As with all element types, if the struct is used
+//	  in an array its size must first be aligned to a multiple of the
+//	  struct's alignment.
 //	- All other types have size WordSize.
 //	- Arrays and structs are aligned per spec definition; all other
 //	  types are naturally aligned with a maximum alignment MaxAlign.
@@ -117,9 +118,13 @@ func (s *StdSizes) Sizeof(T Type) int64 {
 			return s.WordSize * 2
 		}
 	case *Array:
+		n := t.len
+		if n == 0 {
+			return 0
+		}
 		a := s.Alignof(t.elem)
 		z := s.Sizeof(t.elem)
-		return align(z, a) * t.len // may be 0
+		return align(z, a)*(n-1) + z
 	case *Slice:
 		return s.WordSize * 3
 	case *Struct:

go/types/testdata/builtins.src

@@ -649,6 +649,21 @@ type S2 struct{ // offset
 	*S1     //  0
 }               //  8
 
+type S3 struct { // offset
+	a int64  //  0
+	b int32  //  8
+}                // 12
+
+type S4 struct { // offset
+	S3       //  0
+	int32    // 12
+}                // 16
+
+type S5 struct {   // offset
+	a [3]int32 //  0
+	b int32    // 12
+}                  // 16
+
 func (S2) m() {}
 
 func Alignof1() {
@@ -780,6 +795,18 @@ func Sizeof1() {
 	var y2 S2
 	assert(unsafe.Sizeof(y2) == 8)
 
+	var y3 S3
+	assert(unsafe.Sizeof(y3) == 12)
+
+	var y4 S4
+	assert(unsafe.Sizeof(y4) == 16)
+
+	var y5 S5
+	assert(unsafe.Sizeof(y5) == 16)
+
+	var a3 [10]S3
+	assert(unsafe.Sizeof(a3) == 156)
+
 	// test case for issue 5670
 	type T struct {
 		a int32