Interpret GCC's attribute __used__

The Linux kernel marks various otherwise static (i.e., file-local)
symbols using __attribute__((__used__)) to retain them. Not interpreting
this attribute makes linking wrongly discard code and objects.

regression/goto-instrument/gcc_attribute_used1/main.c

@@ -0,0 +1,10 @@
+#ifdef __GNUC__
+static int foo __attribute__((used)) = 42;
+#else
+int foo = 42;
+#endif
+
+int main()
+{
+  return 0;
+}

regression/goto-instrument/gcc_attribute_used1/test.desc

@@ -0,0 +1,9 @@
+CORE
+main.c
+
+^EXIT=0$
+^SIGNAL=0$
+^[[:space:]]*foo = 42;$
+--
+^warning: ignoring
+^CONVERSION ERROR$

src/ansi-c/ansi_c_convert_type.cpp

@@ -169,6 +169,8 @@ void ansi_c_convert_typet::read_rec(const typet &type)
     c_storage_spec.is_register=true;
   else if(type.id()==ID_weak)
     c_storage_spec.is_weak=true;
+  else if(type.id() == ID_used)
+    c_storage_spec.is_used = true;
   else if(type.id()==ID_auto)
   {
     // ignore

src/ansi-c/ansi_c_declaration.cpp

@@ -165,6 +165,10 @@ void ansi_c_declarationt::to_symbol(
         else  if(get_is_extern()) // traditional GCC
           symbol.is_file_local=true;
       }
+
+      // GCC __attribute__((__used__)) - do not treat those as file-local
+      if(get_is_used())
+        symbol.is_file_local = false;
     }
   }
   else // non-function
@@ -181,7 +185,7 @@ void ansi_c_declarationt::to_symbol(
     symbol.is_file_local=
       symbol.is_macro ||
       (!get_is_global() && !get_is_extern()) ||
-      (get_is_global() && get_is_static()) ||
+      (get_is_global() && get_is_static() && !get_is_used()) ||
       symbol.is_parameter;
   }
 }

src/ansi-c/ansi_c_declaration.h

@@ -195,6 +195,16 @@ class ansi_c_declarationt:public exprt
     set(ID_is_weak, is_weak);
   }
 
+  bool get_is_used() const
+  {
+    return get_bool(ID_is_used);
+  }
+
+  void set_is_used(bool is_used)
+  {
+    set(ID_is_used, is_used);
+  }
+
   void to_symbol(
     const ansi_c_declaratort &,
     symbolt &symbol) const;

src/ansi-c/c_storage_spec.cpp

@@ -32,6 +32,8 @@ void c_storage_spect::read(const typet &type)
     is_register=true;
   else if(type.id()==ID_weak)
     is_weak=true;
+  else if(type.id() == ID_used)
+    is_used = true;
   else if(type.id()==ID_auto)
   {
     // ignore

src/ansi-c/c_storage_spec.h

@@ -35,13 +35,14 @@ class c_storage_spect
     is_register=false;
     is_inline=false;
     is_weak=false;
+    is_used = false;
     alias.clear();
     asm_label.clear();
     section.clear();
   }
 
   bool is_typedef, is_extern, is_static, is_register,
-       is_inline, is_thread_local, is_weak;
+       is_inline, is_thread_local, is_weak, is_used;
 
   // __attribute__((alias("foo")))
   irep_idt alias;
@@ -59,6 +60,7 @@ class c_storage_spect
            is_thread_local==other.is_thread_local &&
            is_inline==other.is_inline &&
            is_weak==other.is_weak &&
+           is_used == other.is_used &&
            alias==other.alias &&
            asm_label==other.asm_label &&
            section==other.section;
@@ -78,6 +80,7 @@ class c_storage_spect
     is_inline       |=other.is_inline;
     is_thread_local |=other.is_thread_local;
     is_weak         |=other.is_weak;
+    is_used         |=other.is_used;
     if(alias.empty())
       alias=other.alias;
     if(asm_label.empty())

src/ansi-c/c_typecheck_base.cpp

@@ -688,6 +688,7 @@ void c_typecheck_baset::typecheck_declaration(
       declaration.set_is_register(full_spec.is_register);
       declaration.set_is_typedef(full_spec.is_typedef);
       declaration.set_is_weak(full_spec.is_weak);
+      declaration.set_is_used(full_spec.is_used);
 
       symbolt symbol;
       declaration.to_symbol(*d_it, symbol);

src/ansi-c/parser.y

@@ -149,6 +149,7 @@ extern char *yyansi_ctext;
 %token TOK_GCC_ATTRIBUTE_CONSTRUCTOR "constructor"
 %token TOK_GCC_ATTRIBUTE_DESTRUCTOR "destructor"
 %token TOK_GCC_ATTRIBUTE_FALLTHROUGH "fallthrough"
+%token TOK_GCC_ATTRIBUTE_USED "used"
 %token TOK_GCC_LABEL   "__label__"
 %token TOK_MSC_ASM     "__asm"
 %token TOK_MSC_BASED   "__based"
@@ -1544,6 +1545,8 @@ gcc_type_attribute:
         { $$=$1; set($$, ID_constructor); }
         | TOK_GCC_ATTRIBUTE_DESTRUCTOR
         { $$=$1; set($$, ID_destructor); }
+        | TOK_GCC_ATTRIBUTE_USED
+        { $$=$1; set($$, ID_used); }
         ;
 
 gcc_attribute:

src/ansi-c/scanner.l

@@ -1582,6 +1582,9 @@ __decltype          { if(PARSER.cpp98 &&
 
 "fallthrough"       { BEGIN(GCC_ATTRIBUTE3); loc(); return TOK_GCC_ATTRIBUTE_FALLTHROUGH; }
 
+"used" |
+"__used__"          { BEGIN(GCC_ATTRIBUTE3); loc(); return TOK_GCC_ATTRIBUTE_USED; }
+
 {ws}                { /* ignore */ }
 {newline}           { /* ignore */ }
 {identifier}        { BEGIN(GCC_ATTRIBUTE4); }

src/util/irep_ids.def

@@ -561,6 +561,8 @@ IREP_ID_ONE(noreturn)
 IREP_ID_TWO(C_noreturn, #noreturn)
 IREP_ID_ONE(weak)
 IREP_ID_ONE(is_weak)
+IREP_ID_ONE(used)
+IREP_ID_ONE(is_used)
 IREP_ID_TWO(C_spec_loop_invariant, #spec_loop_invariant)
 IREP_ID_TWO(C_spec_requires, #spec_requires)
 IREP_ID_TWO(C_spec_ensures, #spec_ensures)