struct field reordering (minimizing padding)

compiler/types/types.odin

@@ -636,6 +636,43 @@ fields_for :: proc(store: ^Store, value: Type) -> []Field {
 	return store.fields[start:end]
 }
 
+field_layout_precedes :: proc(
+	store: ^Store,
+	value: Type,
+	left, right: int,
+	selected := target.DEFAULT,
+) -> bool {
+	fields := fields_for(store, value)
+	item, ok := node(store, value)
+	if !ok || item.kind != .Struct || item.c_layout ||
+	   left < 0 || left >= len(fields) || right < 0 || right >= len(fields) {
+		return left < right
+	}
+	left_alignment := alignment_of(fields[left].type, store, selected)
+	right_alignment := alignment_of(fields[right].type, store, selected)
+	return left_alignment > right_alignment ||
+	       (left_alignment == right_alignment && left < right)
+}
+
+physical_field_index :: proc(
+	store: ^Store,
+	value: Type,
+	logical_index: int,
+	selected := target.DEFAULT,
+) -> int {
+	fields := fields_for(store, value)
+	if logical_index < 0 || logical_index >= len(fields) {
+		return logical_index
+	}
+	result := 0
+	for _, other_index in fields {
+		if field_layout_precedes(store, value, other_index, logical_index, selected) {
+			result += 1
+		}
+	}
+	return result
+}
+
 params_for :: proc(store: ^Store, value: Type) -> []Field {
 	item, ok := node(store, value)
 	if !ok || item.kind != .Function {
@@ -1709,11 +1746,18 @@ size :: proc(value: Type, store: ^Store, selected := target.DEFAULT) -> u64 {
 		}
 		offset: u64
 		max_align: u64 = 1
-		for field in fields_for(store, value) {
-			field_align := u64(alignment_of(field.type, store, selected))
-			offset = (offset+field_align-1)/field_align*field_align
-			offset += size(field.type, store, selected)
-			max_align = max(max_align, field_align)
+		if !item.c_layout {
+			for field in fields_for(store, value) {
+				offset += size(field.type, store, selected)
+				max_align = max(max_align, u64(alignment_of(field.type, store, selected)))
+			}
+		} else {
+			for field in fields_for(store, value) {
+				field_align := u64(alignment_of(field.type, store, selected))
+				offset = (offset+field_align-1)/field_align*field_align
+				offset += size(field.type, store, selected)
+				max_align = max(max_align, field_align)
+			}
 		}
 		return (offset+max_align-1)/max_align*max_align
 	case .Union: