From 76654319325efadf9ef19f4ce181397ad53f6914 Mon Sep 17 00:00:00 2001
From: nsfisis <nsfisis@gmail.com>
Date: Sun, 15 Mar 2026 12:35:04 +0900
Subject: feat: partially implement bit-fields

---
 src/ast.c   | 27 +++++++++++++++++++++++----
 src/ast.h   |  3 +++
 src/parse.c | 43 +++++++++++++++++++++++++++++++------------
 3 files changed, 57 insertions(+), 16 deletions(-)

(limited to 'src')

diff --git a/src/ast.c b/src/ast.c
index 790eb81..4d30a66 100644
--- a/src/ast.c
+++ b/src/ast.c
@@ -662,8 +662,15 @@ int type_sizeof_struct(Type* ty) {
 
     for (int i = 0; i < members_of(ty)->as.list->len; ++i) {
         AstNode* member = &members_of(ty)->as.list->items[i];
-        int size = type_sizeof(member->ty);
-        int align = type_alignof(member->ty);
+        int size, align;
+        if (member->as.struct_member->is_bitfield) {
+            // TODO
+            size = member->as.struct_member->bitfield_width / 8;
+            align = 1;
+        } else {
+            size = type_sizeof(member->ty);
+            align = type_alignof(member->ty);
+        }
 
         next_offset = to_aligned(next_offset, align);
         next_offset += size;
@@ -734,11 +741,23 @@ int type_offsetof(Type* ty, const char* name) {
 
     for (int i = 0; i < members_of(ty)->as.list->len; ++i) {
         AstNode* member = &members_of(ty)->as.list->items[i];
-        int size = type_sizeof(member->ty);
-        int align = type_alignof(member->ty);
+        int size, align;
+        if (member->as.struct_member->is_bitfield) {
+            // TODO
+            size = member->as.struct_member->bitfield_width / 8;
+            align = 1;
+        } else {
+            size = type_sizeof(member->ty);
+            align = type_alignof(member->ty);
+        }
 
         next_offset = to_aligned(next_offset, align);
         if (strcmp(member->as.struct_member->name, name) == 0) {
+            if (member->as.struct_member->is_bitfield) {
+                // C23: 7.21.4
+                // if the specified member is a bit-field, the behavior is undefined.
+                fatal_error("type_offsetof: the result of offsetof(bit-field) is undefined");
+            }
             return next_offset;
         }
         next_offset += size;
diff --git a/src/ast.h b/src/ast.h
index f1405c6..f12fd83 100644
--- a/src/ast.h
+++ b/src/ast.h
@@ -311,6 +311,9 @@ typedef struct {
 
 typedef struct {
     const char* name;
+    bool is_bitfield;
+    int bitfield_offset;
+    int bitfield_width;
 } StructMemberNode;
 
 typedef struct {
diff --git a/src/parse.c b/src/parse.c
index 12d4fbf..308277f 100644
--- a/src/parse.c
+++ b/src/parse.c
@@ -1898,7 +1898,6 @@ static AstNode* parse_member_declaration_list(Parser* p) {
 static AstNode* parse_member_declaration(Parser* p) {
     Type* base_ty = parse_specifier_qualifier_list(p);
 
-    AstNode* decls = NULL;
     if (consume_token_if(p, TokenKind_semicolon)) {
         char* name = generate_anonymous_name(p);
         AstNode* decls = ast_new_list(1);
@@ -1909,16 +1908,8 @@ static AstNode* parse_member_declaration(Parser* p) {
         ast_append(decls, member);
         return decls;
     }
-    decls = parse_member_declarator_list(p, base_ty);
-
+    AstNode* decls = parse_member_declarator_list(p, base_ty);
     expect(p, TokenKind_semicolon);
-
-    for (int i = 0; i < decls->as.list->len; i++) {
-        const char* member_name = decls->as.list->items[i].as.declarator->name;
-        decls->as.list->items[i].kind = AstNodeKind_struct_member;
-        decls->as.list->items[i].as.struct_member = calloc(1, sizeof(StructMemberNode));
-        decls->as.list->items[i].as.struct_member->name = member_name;
-    }
     return decls;
 }
 
@@ -2104,9 +2095,37 @@ static AstNode* parse_member_declarator_list(Parser* p, Type* base_ty) {
 
 // member-declarator:
 //     declarator
-//     TODO declarator? ':' constant-expr
+//     declarator ':' constant-expr
+//     TODO ':' constant-expr
 static AstNode* parse_member_declarator(Parser* p, Type* base_ty) {
-    return parse_declarator(p, base_ty);
+    AstNode* decl = parse_declarator(p, base_ty);
+
+    const char* name = decl->as.declarator->name;
+    decl->kind = AstNodeKind_struct_member;
+    decl->as.struct_member = calloc(1, sizeof(StructMemberNode));
+    decl->as.struct_member->name = name;
+
+    if (consume_token_if(p, TokenKind_colon)) {
+        AstNode* bit_width_node = parse_constant_expr(p);
+        InitData* bit_width_evaluated = eval_init_expr(bit_width_node, type_new(TypeKind_int));
+        if (bit_width_evaluated->len != 1)
+            fatal_error("parse_member_declarator: invalid bit-field");
+        if (bit_width_evaluated->blocks[0].kind != InitDataBlockKind_bytes)
+            fatal_error("parse_member_declarator: invalid bit-field");
+        if (bit_width_evaluated->blocks[0].as.bytes.len != sizeof(int))
+            fatal_error("parse_member_declarator: invalid bit-field");
+        int bit_width;
+        memcpy(&bit_width, bit_width_evaluated->blocks[0].as.bytes.buf, sizeof(int));
+        if (bit_width < 0)
+            fatal_error("parse_member_declarator: invalid bit-field");
+        if (bit_width % 8 != 0)
+            fatal_error("parse_member_declarator: unimplemented");
+        decl->as.struct_member->is_bitfield = true;
+        decl->as.struct_member->bitfield_offset = -1; // TODO
+        decl->as.struct_member->bitfield_width = bit_width;
+    }
+
+    return decl;
 }
 
 // enum-specifier:
-- 
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