simplifying symbol table scopes

Linker cannot register extern import symbols initially because
the import object files are not available until all other object files are
resolved. To work around this, we create a stub weak symbol and assign
it to each import symbol. This allows linker to proceed. Once the import
object files are generated, linker replaces the stub symbol with
the actual ones from the import objects.

src/linker/lnk.c

@@ -462,6 +462,12 @@ lnk_make_null_obj(Arena *arena)
 {
   COFF_ObjWriter *obj_writer = coff_obj_writer_alloc(0,COFF_MachineType_Unknown);
 
+  // make import stub
+  {
+    COFF_ObjSymbol *tag = coff_obj_writer_push_symbol_abs(obj_writer, str8_lit("RAD_IMPORT_STUB_NULL"), 0, COFF_SymStorageClass_Static);
+    coff_obj_writer_push_symbol_weak(obj_writer, str8_lit(LNK_IMPORT_STUB), COFF_WeakExt_AntiDependency, tag);
+  }
+
   // push .debug$T sections with null leaf
   String8 null_debug_data;
   {
@@ -1026,10 +1032,6 @@ THREAD_POOL_TASK_FUNC(lnk_undef_symbol_finder)
     if (has_defn) {
       continue;
     }
-    LNK_Symbol *has_import = lnk_symbol_table_search(task->symtab, LNK_SymbolScope_Import, symbol->name);
-    if (has_import) {
-      continue;
-    }
     
     LNK_Symbol *member_symbol = lnk_symbol_table_search(task->symtab, LNK_SymbolScope_Lib, symbol->name);
     if (member_symbol) {
@@ -1059,10 +1061,6 @@ THREAD_POOL_TASK_FUNC(lnk_weak_symbol_finder)
         continue;
       }
     }
-    LNK_Symbol *has_import = lnk_symbol_table_search(task->symtab, LNK_SymbolScope_Import, symbol->name);
-    if (has_import) {
-      continue;
-    }
  
     LNK_Symbol *member_symbol = 0;
     {
@@ -1296,15 +1294,16 @@ lnk_build_link_context(TP_Context *tp, TP_Arena *tp_arena, LNK_Config *config)
           }
 
           // was import already created?
-          if (lnk_symbol_table_search(symtab, LNK_SymbolScope_Import, import_header.func_name)) {
+          if (lnk_symbol_table_search(symtab, LNK_SymbolScope_Defined, import_header.func_name)) {
             continue;
           }
 
           // create import stubs (later replaced with acutal imports generated by linker)
-          LNK_Symbol *thunk_symbol = lnk_make_import_symbol(scratch.arena, import_header.func_name, input->data.coff_import);
-          LNK_Symbol *imp_symbol = lnk_make_import_symbol(scratch.arena, push_str8f(scratch.arena, "__imp_%S", import_header.func_name), input->data.coff_import);
-          lnk_symbol_table_push(symtab, LNK_SymbolScope_Import, thunk_symbol);
-          lnk_symbol_table_push(symtab, LNK_SymbolScope_Import, imp_symbol);
+          LNK_Symbol *import_stub  = lnk_symbol_table_search(symtab, LNK_SymbolScope_Defined, str8_lit(LNK_IMPORT_STUB));
+          LNK_Symbol *thunk_symbol = lnk_make_defined_symbol(scratch.arena, import_header.func_name, import_stub->u.defined.obj, import_stub->u.defined.symbol_idx);
+          LNK_Symbol *imp_symbol   = lnk_make_defined_symbol(scratch.arena, push_str8f(scratch.arena, "__imp_%S", import_header.func_name), import_stub->u.defined.obj, import_stub->u.defined.symbol_idx);
+          lnk_symbol_table_push(symtab, LNK_SymbolScope_Defined, thunk_symbol);
+          lnk_symbol_table_push(symtab, LNK_SymbolScope_Defined, imp_symbol);
 
           // pick imports hash table
           HashTable *imports_ht;

src/linker/lnk.h

@@ -15,6 +15,7 @@ typedef struct LNK_LinkContext
 
 // -- Image --------------------------------------------------------------------
 
+#define LNK_IMPORT_STUB "*** RAD_IMPORT_STUB ***"
 #define LNK_REMOVED_SECTION_NUMBER_32 (U32)-3
 #define LNK_REMOVED_SECTION_NUMBER_16 (U16)-3
 

src/linker/lnk_symbol_table.c

@@ -21,15 +21,6 @@ lnk_make_lib_symbol(Arena *arena, String8 name, struct LNK_Lib *lib, U64 member_
   return symbol;
 }
 
-internal LNK_Symbol *
-lnk_make_import_symbol(Arena *arena, String8 name, String8 import_header)
-{
-  LNK_Symbol *symbol = push_array(arena, LNK_Symbol, 1);
-  symbol->name                = name;
-  symbol->u.imp.import_header = import_header;
-  return symbol;
-}
-
 internal LNK_Symbol *
 lnk_make_undefined_symbol(Arena *arena, String8 name, struct LNK_Obj *obj)
 {
@@ -299,9 +290,6 @@ lnk_can_replace_symbol(LNK_SymbolScope scope, LNK_Symbol *dst, LNK_Symbol *src)
       lnk_error(LNK_Error_InvalidPath, "unable to find a suitable replacement logic for symbol combination");
     }
   } break;
-  case LNK_SymbolScope_Import: {
-    lnk_error_multiply_defined_symbol(dst, src);
-  } break;
   case LNK_SymbolScope_Lib: {
     // link.exe picks symbol from lib that is discovered first
     can_replace = lnk_symbol_lib_is_before(src, dst);
@@ -344,10 +332,6 @@ lnk_on_symbol_replace(LNK_SymbolScope scope, LNK_Symbol *dst, LNK_Symbol *src)
     }
 #endif
   } break;
-  case LNK_SymbolScope_Import: {
-    // illegal to replace imports
-    InvalidPath;
-  } break;
   case LNK_SymbolScope_Lib: {
     // nothing to replace
   } break;

src/linker/lnk_symbol_table.h

@@ -8,7 +8,6 @@
 typedef enum
 {
   LNK_SymbolScope_Defined,
-  LNK_SymbolScope_Import,
   LNK_SymbolScope_Lib,
   LNK_SymbolScope_Count
 } LNK_SymbolScope;
@@ -30,18 +29,12 @@ typedef struct LNK_SymbolUndefined
   struct LNK_Obj *obj;
 } LNK_SymbolUndefined;
 
-typedef struct LNK_SymbolImport
-{
-  String8 import_header;
-} LNK_SymbolImport;
-
 typedef struct LNK_Symbol
 {
   String8 name;
   union {
     LNK_SymbolDefined   defined;
     LNK_SymbolLib       lib;
-    LNK_SymbolImport    imp;
     LNK_SymbolUndefined undef;
   } u;
 } LNK_Symbol;
@@ -120,7 +113,6 @@ typedef struct
 internal LNK_Symbol * lnk_make_defined_symbol(Arena *arena, String8 name, struct LNK_Obj *obj, U32 symbol_idx);
 internal LNK_Symbol * lnk_make_lib_symbol(Arena *arena, String8 name, struct LNK_Lib *lib, U64 member_offset);
 internal LNK_Symbol * lnk_make_undefined_symbol(Arena *arena, String8 name, struct LNK_Obj *obj);
-internal LNK_Symbol * lnk_make_import_symbol(Arena *arena, String8 name, String8 import_header);
 
 // --- Symbol Containers ------------------------------------------------------