[clang] ignore `__attribute__((unused))` variable initialisations

Summary: When a `VarDecl` has the attribute `unused` then do not assign its initialisation result to the corresponding variable.

Reviewed By: ngorogiannis

Differential Revision: D13974497

fbshipit-source-id: 28029f995

infer/src/checkers/liveness.ml

@@ -172,11 +172,8 @@ let checker {Callbacks.tenv; summary; proc_desc} : Summary.t =
     |> Option.exists ~f:(fun local -> local.ProcAttributes.is_constexpr)
   in
   let should_report pvar typ live_vars captured_by_ref_vars =
-    (* T32000971: a temporay fix for dead store false positives until we support __unused__ attribute *)
-    let is_unused_tmp_var pvar = String.equal "__tmp" (Pvar.to_string pvar) in
     not
       ( Pvar.is_frontend_tmp pvar || Pvar.is_return pvar || Pvar.is_global pvar || is_constexpr pvar
-      || is_unused_tmp_var pvar
       || VarSet.mem (Var.of_pvar pvar) captured_by_ref_vars
       || Domain.mem (Var.of_pvar pvar) live_vars
       || Procdesc.is_captured_pvar proc_desc pvar

infer/src/clang/cTrans.ml

@@ -2220,7 +2220,8 @@ module CTrans_funct (F : CModule_type.CFrontend) : CModule_type.CTranslation = s
     |> mk_trans_result ret_exp_typ
 
 
-  and init_expr_trans trans_state var_exp_typ ?qual_type var_stmt_info init_expr_opt =
+  and init_expr_trans ?(is_var_unused = false) trans_state var_exp_typ ?qual_type var_stmt_info
+      init_expr_opt =
     match init_expr_opt with
     | None -> (
       match
@@ -2254,8 +2255,9 @@ module CTrans_funct (F : CModule_type.CFrontend) : CModule_type.CTranslation = s
         in
         let assign_trans_control_opt =
           if
-            (* variable might be initialized already - do nothing in that case*)
-            List.exists ~f:(Exp.equal var_exp) res_trans_ie.control.initd_exps
+            is_var_unused
+            || (* variable might be initialized already - do nothing in that case*)
+               List.exists ~f:(Exp.equal var_exp) res_trans_ie.control.initd_exps
           then None
           else
             let sil_e1', ie_typ = res_trans_ie.return in
@@ -2292,18 +2294,21 @@ module CTrans_funct (F : CModule_type.CFrontend) : CModule_type.CTranslation = s
     let context = trans_state.context in
     let procdesc = context.CContext.procdesc in
     let procname = Procdesc.get_proc_name procdesc in
-    let do_var_dec var_decl qual_type vdi next_node =
+    let do_var_dec ~is_var_unused var_decl qual_type vdi next_node =
       let pvar = CVar_decl.sil_var_of_decl context var_decl procname in
       let typ = CType_decl.qual_type_to_sil_type context.CContext.tenv qual_type in
       CVar_decl.add_var_to_locals procdesc var_decl typ pvar ;
       let trans_state' = {trans_state with succ_nodes= next_node} in
-      init_expr_trans trans_state' (Exp.Lvar pvar, typ) ~qual_type stmt_info
+      init_expr_trans ~is_var_unused trans_state' (Exp.Lvar pvar, typ) ~qual_type stmt_info
         vdi.Clang_ast_t.vdi_init_expr
     in
+    let has_unused_attr attributes =
+      List.exists attributes ~f:(function Clang_ast_t.UnusedAttr _ -> true | _ -> false)
+    in
     let rec aux : decl list -> trans_result option = function
       | [] ->
           None
-      | (VarDecl (_, _, qt, vdi) as var_decl) :: var_decls' ->
+      | (VarDecl ({di_attributes}, _, qt, vdi) as var_decl) :: var_decls' ->
           (* Var are defined when procdesc is created, here we only take care of initialization *)
           let res_trans_tl = aux var_decls' in
           let root_nodes_tl, instrs_tl, initd_exps_tl =
@@ -2313,7 +2318,8 @@ module CTrans_funct (F : CModule_type.CFrontend) : CModule_type.CTranslation = s
             | Some {control= {root_nodes; instrs; initd_exps}} ->
                 (root_nodes, instrs, initd_exps)
           in
-          let res_trans_tmp = do_var_dec var_decl qt vdi root_nodes_tl in
+          let is_var_unused = has_unused_attr di_attributes in
+          let res_trans_tmp = do_var_dec ~is_var_unused var_decl qt vdi root_nodes_tl in
           (* keep the last return and leaf_nodes from the list *)
           let return, leaf_nodes =
             match res_trans_tl with

infer/tests/codetoanalyze/cpp/liveness/dead_stores.cpp

@@ -484,18 +484,17 @@ class Exceptions {
 
 void init_in_binop_bad(int x) { x = -x & ~int{0}; }
 
-void FN_unused_tmp_bad() {
-  // T32000971
-  int __tmp = 1;
-}
+void unused_tmp_bad() { int __tmp = 1; }
 
 #define UNUSED(a) __typeof__(&a) __attribute__((unused)) __tmp = &a;
 
-void unused_attribute_ok() {
+void unused_attribute_tmp_ok() {
   int x;
   UNUSED(x);
 }
 
+void unused_attribute_ok() { int __attribute__((unused)) x = 42; }
+
 struct ChainedCalls {
   ChainedCalls chained(int i);
 };

infer/tests/codetoanalyze/cpp/liveness/issues.exp

@@ -18,6 +18,7 @@ codetoanalyze/cpp/liveness/dead_stores.cpp, dead_stores::plus_plus1_bad, 2, DEAD
 codetoanalyze/cpp/liveness/dead_stores.cpp, dead_stores::plus_plus2_bad, 2, DEAD_STORE, no_bucket, ERROR, [Write of unused value]
 codetoanalyze/cpp/liveness/dead_stores.cpp, dead_stores::plus_plus3_bad, 2, DEAD_STORE, no_bucket, ERROR, [Write of unused value]
 codetoanalyze/cpp/liveness/dead_stores.cpp, dead_stores::reassign_param_bad, 0, DEAD_STORE, no_bucket, ERROR, [Write of unused value]
+codetoanalyze/cpp/liveness/dead_stores.cpp, dead_stores::unused_tmp_bad, 0, DEAD_STORE, no_bucket, ERROR, [Write of unused value]
 codetoanalyze/cpp/liveness/dead_stores.cpp, dead_stores::use_then_dead_bad, 3, DEAD_STORE, no_bucket, ERROR, [Write of unused value]
 codetoanalyze/cpp/liveness/dead_stores_constexpr.cpp, capture_const_bad, 1, DEAD_STORE, no_bucket, ERROR, [Write of unused value]
 codetoanalyze/cpp/liveness/non_type_template_param.cpp, X<3>_isZeroBad, 1, DEAD_STORE, no_bucket, ERROR, [Write of unused value]