Add union designated initializer

parse.c

@@ -74,6 +74,10 @@ struct Initializer {
   // If it's an initializer for an aggregate type (e.g. array or struct),
   // `children` has initializers for its children.
   Initializer **children;
+
+  // Only one member can be initialized for a union.
+  // `mem` is used to clarify which member is initialized.
+  Member *mem;
 };
 
 // For local variable initializer.
@@ -917,6 +921,13 @@ static void designation(Token **rest, Token *tok, Initializer *init) {
     return;
   }
 
+  if (equal(tok, ".") && init->ty->kind == TY_UNION) {
+    Member *mem = struct_designator(&tok, tok, init->ty);
+    init->mem = mem;
+    designation(rest, tok, init->children[mem->idx]);
+    return;
+  }
+
   if (equal(tok, "."))
     error_tok(tok, "field name not in struct or union initializer");
 
@@ -1062,7 +1073,18 @@ static void struct_initializer2(Token **rest, Token *tok, Initializer *init, Mem
 
 static void union_initializer(Token **rest, Token *tok, Initializer *init) {
   // Unlike structs, union initializers take only one initializer,
-  // and that initializes the first union member.
+  // and that initializes the first union member by default.
+  // You can initialize other member using a designated initializer.
+  if (equal(tok, "{") && equal(tok->next, ".")) {
+    Member *mem = struct_designator(&tok, tok->next, init->ty);
+    init->mem = mem;
+    designation(&tok, tok, init->children[mem->idx]);
+    *rest = skip(tok, "}");
+    return;
+  }
+
+  init->mem = init->ty->members;
+
   if (equal(tok, "{")) {
     initializer2(&tok, tok->next, init->children[0]);
     consume(&tok, tok, ",");
@@ -1199,8 +1221,9 @@ static Node *create_lvar_init(Initializer *init, Type *ty, InitDesg *desg, Token
   }
 
   if (ty->kind == TY_UNION) {
-    InitDesg desg2 = {desg, 0, ty->members};
-    return create_lvar_init(init->children[0], ty->members->ty, &desg2, tok);
+    Member *mem = init->mem ? init->mem : ty->members;
+    InitDesg desg2 = {desg, 0, mem};
+    return create_lvar_init(init->children[mem->idx], mem->ty, &desg2, tok);
   }
 
   if (!init->expr)
@@ -1290,8 +1313,12 @@ write_gvar_data(Relocation *cur, Initializer *init, Type *ty, char *buf, int off
     return cur;
   }
 
-  if (ty->kind == TY_UNION)
-    return write_gvar_data(cur, init->children[0], ty->members->ty, buf, offset);
+  if (ty->kind == TY_UNION) {
+    if (!init->mem)
+      return cur;
+    return write_gvar_data(cur, init->children[init->mem->idx],
+                           init->mem->ty, buf, offset);
+  }
 
   if (!init->expr)
     return cur;

test/initializer.c

@@ -27,6 +27,8 @@ struct {int a[2];} g40[2] = {{1, 2}, 3, 4};
 struct {int a[2];} g41[2] = {1, 2, 3, 4};
 char g43[][4] = {'f', 'o', 'o', 0, 'b', 'a', 'r', 0};
 char *g44 = {"foo"};
+union { int a; char b[4]; } g50 = {.b[2]=0x12};
+union { int a; } g51[2] = {};
 
 typedef char T60[];
 T60 g60 = {1, 2, 3};
@@ -246,6 +248,13 @@ int main() {
   ASSERT(5, ((struct { int a,b,c; }){ .c=5 }).c);
   ASSERT(0, ((struct { int a,b,c; }){ .c=5 }).a);
 
+  ASSERT(0x00ff, ({ union { unsigned short a; char b[2]; } x={.b[0]=0xff}; x.a; }));
+  ASSERT(0xff00, ({ union { unsigned short a; char b[2]; } x={.b[1]=0xff}; x.a; }));
+
+  ASSERT(0x00120000, g50.a);
+  ASSERT(0, g51[0].a);
+  ASSERT(0, g51[1].a);
+
   printf("OK\n");
   return 0;
 }