From 85e46b1071b54649740b35df939f32ed188c0e13 Mon Sep 17 00:00:00 2001
From: Rui Ueyama <ruiu@cs.stanford.edu>
Date: Fri, 4 Sep 2020 17:29:15 +0900
Subject: [PATCH] Add tentative definition

---
 chibicc.h        |  1 +
 codegen.c        |  5 +++++
 parse.c          | 31 +++++++++++++++++++++++++++++++
 test/common      |  2 ++
 test/commonsym.c | 19 +++++++++++++++++++
 5 files changed, 58 insertions(+)
 create mode 100644 test/commonsym.c

diff --git a/chibicc.h b/chibicc.h
index c01e339..0f133b7 100644
--- a/chibicc.h
+++ b/chibicc.h
@@ -140,6 +140,7 @@ struct Obj {
   bool is_static;
 
   // Global variable
+  bool is_tentative;
   char *init_data;
   Relocation *rel;
 
diff --git a/codegen.c b/codegen.c
index 0612118..2066802 100644
--- a/codegen.c
+++ b/codegen.c
@@ -1136,6 +1136,11 @@ static void emit_data(Obj *prog) {
       ? MAX(16, var->align) : var->align;
     println("  .align %d", align);
 
+    if (var->is_tentative) {
+      println("  .comm %s, %d, %d", var->name, var->ty->size, align);
+      continue;
+    }
+
     if (var->init_data) {
       println("  .data");
       println("%s:", var->name);
diff --git a/parse.c b/parse.c
index a03e8c1..96286e1 100644
--- a/parse.c
+++ b/parse.c
@@ -2947,6 +2947,8 @@ static Token *global_variable(Token *tok, Type *basety, VarAttr *attr) {
 
     if (equal(tok, "="))
       gvar_initializer(&tok, tok->next, var);
+    else if (!attr->is_extern)
+      var->is_tentative = true;
   }
   return tok;
 }
@@ -2962,6 +2964,33 @@ static bool is_function(Token *tok) {
   return ty->kind == TY_FUNC;
 }
 
+// Remove redundant tentative definitions.
+static void scan_globals(void) {
+  Obj head;
+  Obj *cur = &head;
+
+  for (Obj *var = globals; var; var = var->next) {
+    if (!var->is_tentative) {
+      cur = cur->next = var;
+      continue;
+    }
+
+    // Find another definition of the same identifier.
+    Obj *var2 = globals;
+    for (; var2; var2 = var2->next)
+      if (var != var2 && var2->is_definition && !strcmp(var->name, var2->name))
+        break;
+
+    // If there's another definition, the tentative definition
+    // is redundant
+    if (!var2)
+      cur = cur->next = var;
+  }
+
+  cur->next = NULL;
+  globals = head.next;
+}
+
 // program = (typedef | function-definition | global-variable)*
 Obj *parse(Token *tok) {
   globals = NULL;
@@ -2990,5 +3019,7 @@ Obj *parse(Token *tok) {
     if (var->is_root)
       mark_live(var);
 
+  // Remove redundant tentative definitions.
+  scan_globals();
   return globals;
 }
diff --git a/test/common b/test/common
index c60a42a..9676755 100644
--- a/test/common
+++ b/test/common
@@ -17,6 +17,8 @@ int *ext2 = &ext1;
 int ext3 = 7;
 int ext_fn1(int x) { return x; }
 int ext_fn2(int x) { return x; }
+int common_ext2 = 3;
+static int common_local;
 
 int false_fn() { return 512; }
 int true_fn() { return 513; }
diff --git a/test/commonsym.c b/test/commonsym.c
new file mode 100644
index 0000000..7599527
--- /dev/null
+++ b/test/commonsym.c
@@ -0,0 +1,19 @@
+#include "test.h"
+
+int x;
+int x = 5;
+int y = 7;
+int y;
+int common_ext1;
+int common_ext2;
+static int common_local;
+
+int main() {
+  ASSERT(5, x);
+  ASSERT(7, y);
+  ASSERT(0, common_ext1);
+  ASSERT(3, common_ext2);
+
+  printf("OK\n");
+  return 0;
+}