CodeGen: Don't measure the sizes of union members

In general, we can't tell which member is active in a union so it is not
safe to try and measure any of them.

Explicitly set the alignment of unions (and structs/classes) in CodeGen
if it is available, as the C++ compiler can no longer infer it from the
members.

.circleci/config.yml

@@ -20,21 +20,21 @@ workflows:
             - build-gcc
           oid_test_args: "-ftype-graph"
           tests_regex: "OidIntegration\\..*"
-          exclude_regex: ".*inheritance_polymorphic.*|.*pointers_incomplete_containing_struct|.*arrays_member_int0|OidIntegration.unions.*|.*thrift_unions_dynamic_.*"
+          exclude_regex: ".*inheritance_polymorphic.*|.*pointers_incomplete_containing_struct|.*arrays_member_int0"
       - test:
           name: test-typed-data-segment-gcc
           requires:
             - build-gcc
           oid_test_args: "-ftyped-data-segment"
           tests_regex: "OidIntegration\\..*"
-          exclude_regex: ".*inheritance_polymorphic.*|.*pointers.*|.*arrays_member_int0|.*cycles_.*|OidIntegration.unions.*|.*thrift_unions_dynamic_.*"
+          exclude_regex: ".*inheritance_polymorphic.*|.*pointers.*|.*arrays_member_int0|.*cycles_.*"
       - test:
           name: test-tree-builder-type-checking-gcc
           requires:
             - build-gcc
           oid_test_args: "-ftree-builder-type-checking"
           tests_regex: "OidIntegration\\..*"
-          exclude_regex: ".*inheritance_polymorphic.*|.*pointers.*|.*arrays_member_int0|.*cycles_.*|OidIntegration.unions.*|.*thrift_unions_dynamic_.*"
+          exclude_regex: ".*inheritance_polymorphic.*|.*pointers.*|.*arrays_member_int0|.*cycles_.*"
       - coverage:
           name: coverage
           requires:
@@ -68,7 +68,7 @@ workflows:
           oid_test_args: "-ftype-graph"
           tests_regex: "OidIntegration\\..*"
           # Tests disabled due to bad DWARF generated by the old clang compiler in CI
-          exclude_regex: ".*inheritance_polymorphic.*|.*pointers_incomplete_containing_struct|.*arrays_member_int0|.*fbstring.*|.*std_string_*|.*multi_arg_tb_.*|.*ignored_a|OidIntegration.unions.*|.*thrift_unions_dynamic_.*"
+          exclude_regex: ".*inheritance_polymorphic.*|.*pointers_incomplete_containing_struct|.*arrays_member_int0|.*fbstring.*|.*std_string_*|.*multi_arg_tb_.*|.*ignored_a"
 
 executors:
   ubuntu-docker:

oi/CodeGen.cpp

@@ -251,6 +251,13 @@ void genDefsClass(const Class& c, std::string& code) {
     code += "__attribute__((__packed__)) ";
   }
 
+  if (c.kind() == Class::Kind::Union) {
+    // Need to specify alignment manually for unions as their members have been
+    // removed. It would be nice to do this for all types, but our alignment
+    // information is not complete, so it would result in some errors.
+    code += "alignas(" + std::to_string(c.align()) + ") ";
+  }
+
   code += c.name() + " {\n";
   for (const auto& mem : c.members) {
     code += "  " + mem.type().name() + " " + mem.name;

oi/type_graph/RemoveMembers.cpp

@@ -41,6 +41,12 @@ void RemoveMembers::accept(Type& type) {
 }
 
 void RemoveMembers::visit(Class& c) {
+  if (c.kind() == Class::Kind::Union) {
+    // In general, we can't tell which member is active in a union, so it is not
+    // safe to try and measure any of them.
+    c.members.clear();
+  }
+
   for (const auto& param : c.templateParams) {
     accept(param.type());
   }

oi/type_graph/RemoveMembers.h

@@ -28,6 +28,8 @@ namespace type_graph {
  *
  * Removes members as requested by the [[codegen.ignore]] section in the OI
  * config.
+ *
+ * Removes members from unions as we have no way of telling which one is active.
  */
 class RemoveMembers : public RecursiveVisitor {
  public:

test/test_codegen.cpp

@@ -201,9 +201,8 @@ TEST(CodeGenTest, UnionMembersAlignment) {
 )",
                 R"(
 [0] Union: MyUnion_0 (size: 8, align: 8)
-      Member: a_0 (offset: 0, align: 1)
+      Member: __oi_padding_0 (offset: 0)
+[1]     Array: (length: 8)
           Primitive: int8_t
-      Member: b_1 (offset: 8, align: 8)
-        Primitive: int64_t
 )");
 }

test/test_remove_members.cpp

@@ -185,3 +185,16 @@ TEST(RemoveMembersTest, RecurseClassChild) {
             Primitive: int32_t
 )");
 }
+
+TEST(RemoveMembersTest, Union) {
+  test(RemoveMembers::createPass({}), R"(
+[0] Union: MyUnion (size: 4)
+      Member: a (offset: 0)
+        Primitive: int32_t
+      Member: b (offset: 0)
+        Primitive: int32_t
+)",
+       R"(
+[0] Union: MyUnion (size: 4)
+)");
+}