From 8dd6235f4d3827ab72c39ad3daeeb17d2738bc85 Mon Sep 17 00:00:00 2001
From: Rui Ueyama <ruiu@cs.stanford.edu>
Date: Thu, 27 Aug 2020 23:36:12 +0900
Subject: [PATCH] Add bitfield

---
 chibicc.h       |  5 +++++
 codegen.c       | 33 ++++++++++++++++++++++++++++++++-
 parse.c         | 32 +++++++++++++++++++++++++++-----
 test/bitfield.c | 25 +++++++++++++++++++++++++
 4 files changed, 89 insertions(+), 6 deletions(-)
 create mode 100644 test/bitfield.c

diff --git a/chibicc.h b/chibicc.h
index 8afcd4b..dfea0b1 100644
--- a/chibicc.h
+++ b/chibicc.h
@@ -309,6 +309,11 @@ struct Member {
   int idx;
   int align;
   int offset;
+
+  // Bitfield
+  bool is_bitfield;
+  int bit_offset;
+  int bit_width;
 };
 
 extern Type *ty_void;
diff --git a/codegen.c b/codegen.c
index ea8fdef..4b4c68b 100644
--- a/codegen.c
+++ b/codegen.c
@@ -578,10 +578,23 @@ static void gen_expr(Node *node) {
     return;
   }
   case ND_VAR:
-  case ND_MEMBER:
     gen_addr(node);
     load(node->ty);
     return;
+  case ND_MEMBER: {
+    gen_addr(node);
+    load(node->ty);
+
+    Member *mem = node->member;
+    if (mem->is_bitfield) {
+      println("  shl $%d, %%rax", 64 - mem->bit_width - mem->bit_offset);
+      if (mem->ty->is_unsigned)
+        println("  shr $%d, %%rax", 64 - mem->bit_width);
+      else
+        println("  sar $%d, %%rax", 64 - mem->bit_width);
+    }
+    return;
+  }
   case ND_DEREF:
     gen_expr(node->lhs);
     load(node->ty);
@@ -593,6 +606,24 @@ static void gen_expr(Node *node) {
     gen_addr(node->lhs);
     push();
     gen_expr(node->rhs);
+
+    if (node->lhs->kind == ND_MEMBER && node->lhs->member->is_bitfield) {
+      // If the lhs is a bitfield, we need to read the current value
+      // from memory and merge it with a new value.
+      Member *mem = node->lhs->member;
+      println("  mov %%rax, %%rdi");
+      println("  and $%ld, %%rdi", (1L << mem->bit_width) - 1);
+      println("  shl $%d, %%rdi", mem->bit_offset);
+
+      println("  mov (%%rsp), %%rax");
+      load(mem->ty);
+
+      long mask = ((1L << mem->bit_width) - 1) << mem->bit_offset;
+      println("  mov $%ld, %%r9", ~mask);
+      println("  and %%r9, %%rax");
+      println("  or %%rdi, %%rax");
+    }
+
     store(node->ty);
     return;
   case ND_STMT_EXPR:
diff --git a/parse.c b/parse.c
index 6d2ed0f..4421f96 100644
--- a/parse.c
+++ b/parse.c
@@ -160,6 +160,10 @@ static bool is_function(Token *tok);
 static Token *function(Token *tok, Type *basety, VarAttr *attr);
 static Token *global_variable(Token *tok, Type *basety, VarAttr *attr);
 
+static int align_down(int n, int align) {
+  return align_to(n - align + 1, align);
+}
+
 static void enter_scope(void) {
   Scope *sc = calloc(1, sizeof(Scope));
   sc->next = scope;
@@ -2004,6 +2008,12 @@ static void struct_members(Token **rest, Token *tok, Type *ty) {
       mem->name = mem->ty->name;
       mem->idx = idx++;
       mem->align = attr.align ? attr.align : mem->ty->align;
+
+      if (consume(&tok, tok, ":")) {
+        mem->is_bitfield = true;
+        mem->bit_width = const_expr(&tok, tok);
+      }
+
       cur = cur->next = mem;
     }
   }
@@ -2072,16 +2082,28 @@ static Type *struct_decl(Token **rest, Token *tok) {
     return ty;
 
   // Assign offsets within the struct to members.
-  int offset = 0;
+  int bits = 0;
+
   for (Member *mem = ty->members; mem; mem = mem->next) {
-    offset = align_to(offset, mem->align);
-    mem->offset = offset;
-    offset += mem->ty->size;
+    if (mem->is_bitfield) {
+      int sz = mem->ty->size;
+      if (bits / (sz * 8) != (bits + mem->bit_width - 1) / (sz * 8))
+        bits = align_to(bits, sz * 8);
+
+      mem->offset = align_down(bits / 8, sz);
+      mem->bit_offset = bits % (sz * 8);
+      bits += mem->bit_width;
+    } else {
+      bits = align_to(bits, mem->align * 8);
+      mem->offset = bits / 8;
+      bits += mem->ty->size * 8;
+    }
 
     if (ty->align < mem->align)
       ty->align = mem->align;
   }
-  ty->size = align_to(offset, ty->align);
+
+  ty->size = align_to(bits, ty->align * 8) / 8;
   return ty;
 }
 
diff --git a/test/bitfield.c b/test/bitfield.c
new file mode 100644
index 0000000..9a63acc
--- /dev/null
+++ b/test/bitfield.c
@@ -0,0 +1,25 @@
+#include "test.h"
+
+int main() {
+  ASSERT(4, sizeof(struct {int x:1; }));
+  ASSERT(8, sizeof(struct {long x:1; }));
+
+  struct bit1 {
+    short a;
+    char b;
+    int c : 2;
+    int d : 3;
+    int e : 3;
+  };
+
+  ASSERT(4, sizeof(struct bit1));
+  ASSERT(1, ({ struct bit1 x; x.a=1; x.b=2; x.c=3; x.d=4; x.e=5; x.a; }));
+  ASSERT(1, ({ struct bit1 x={1,2,3,4,5}; x.a; }));
+  ASSERT(2, ({ struct bit1 x={1,2,3,4,5}; x.b; }));
+  ASSERT(-1, ({ struct bit1 x={1,2,3,4,5}; x.c; }));
+  ASSERT(-4, ({ struct bit1 x={1,2,3,4,5}; x.d; }));
+  ASSERT(-3, ({ struct bit1 x={1,2,3,4,5}; x.e; }));
+
+  printf("OK\n");
+  return 0;
+}
-- 
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