[dead store] Do not report dead stores on constexpr

Summary: We report dead store false positives in template arguments when constexpr is used. To remove the false positives, with the expense of some false negatives, we do not report dead stores on constexpr anymore.

Reviewed By: mbouaziz

Differential Revision: D9608095

fbshipit-source-id: 91b0c71c4

infer/src/IR/ProcAttributes.ml

@@ -41,7 +41,8 @@ let string_of_var_attribute = function Modify_in_block -> "<Modify_in_block>"
 
 let var_attribute_equal = [%compare.equal: var_attribute]
 
-type var_data = {name: Mangled.t; typ: Typ.t; attributes: var_attribute list} [@@deriving compare]
+type var_data = {name: Mangled.t; typ: Typ.t; attributes: var_attribute list; is_constexpr: bool}
+[@@deriving compare]
 
 let pp_var_data fmt {name; typ; attributes} =
   F.fprintf fmt "@[<h>{ name=@ %a;@ typ=@ %a" Mangled.pp name (Typ.pp_full Pp.text) typ ;

infer/src/IR/ProcAttributes.mli

@@ -23,7 +23,7 @@ type var_attribute = Modify_in_block
 val var_attribute_equal : var_attribute -> var_attribute -> bool
 (** Equality for var_attribute *)
 
-type var_data = {name: Mangled.t; typ: Typ.t; attributes: var_attribute list}
+type var_data = {name: Mangled.t; typ: Typ.t; attributes: var_attribute list; is_constexpr: bool}
 
 type t =
   { access: PredSymb.access  (** visibility access *)

infer/src/checkers/liveness.ml

@@ -170,11 +170,17 @@ let checker {Callbacks.tenv; summary; proc_desc} : Summary.t =
     | _ ->
         false
   in
+  let locals = Procdesc.get_locals proc_desc in
+  let is_constexpr pvar =
+    List.find locals ~f:(fun local_data ->
+        Mangled.equal (Pvar.get_name pvar) local_data.ProcAttributes.name )
+    |> 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
+      ( 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

infer/src/clang/cTrans.ml

@@ -210,7 +210,9 @@ module CTrans_funct (F : CModule_type.CFrontend) : CModule_type.CTranslation = s
     let context = trans_state.context in
     let procdesc = context.CContext.procdesc in
     let pvar, typ = mk_temp_sil_var_for_expr context.CContext.tenv procdesc var_name expr_info in
-    let var_data = ProcAttributes.{name= Pvar.get_name pvar; typ; attributes= []} in
+    let var_data =
+      ProcAttributes.{name= Pvar.get_name pvar; typ; attributes= []; is_constexpr= false}
+    in
     Procdesc.append_locals procdesc [var_data] ;
     (Exp.Lvar pvar, typ)
 
@@ -246,7 +248,7 @@ module CTrans_funct (F : CModule_type.CFrontend) : CModule_type.CTranslation = s
               let procdesc = trans_state.context.CContext.procdesc in
               let pvar = mk_temp_sil_var procdesc "__temp_return_" in
               let var_data : ProcAttributes.var_data =
-                {name= Pvar.get_name pvar; typ= return_type; attributes= []}
+                {name= Pvar.get_name pvar; typ= return_type; attributes= []; is_constexpr= false}
               in
               Procdesc.append_locals procdesc [var_data] ;
               Exp.Lvar pvar
@@ -1136,7 +1138,7 @@ module CTrans_funct (F : CModule_type.CFrontend) : CModule_type.CTranslation = s
           let pvar = Pvar.mk_tmp "__temp_construct_" (Procdesc.get_proc_name procdesc) in
           let class_type = CType_decl.get_type_from_expr_info ei context.CContext.tenv in
           let var_data : ProcAttributes.var_data =
-            {name= Pvar.get_name pvar; typ= class_type; attributes= []}
+            {name= Pvar.get_name pvar; typ= class_type; attributes= []; is_constexpr= false}
           in
           Procdesc.append_locals procdesc [var_data] ;
           (Exp.Lvar pvar, class_type)
@@ -1541,7 +1543,10 @@ module CTrans_funct (F : CModule_type.CFrontend) : CModule_type.CTranslation = s
         in
         Procdesc.node_set_succs_exn context.procdesc join_node succ_nodes [] ;
         let pvar = mk_temp_sil_var procdesc "SIL_temp_conditional___" in
-        let var_data = ProcAttributes.{name= Pvar.get_name pvar; typ= var_typ; attributes= []} in
+        let var_data =
+          ProcAttributes.
+            {name= Pvar.get_name pvar; typ= var_typ; attributes= []; is_constexpr= false}
+        in
         Procdesc.append_locals procdesc [var_data] ;
         let continuation' = mk_cond_continuation trans_state.continuation in
         let trans_state' = {trans_state with continuation= continuation'; succ_nodes= []} in
@@ -2903,7 +2908,9 @@ module CTrans_funct (F : CModule_type.CFrontend) : CModule_type.CTranslation = s
     let var_exp_typ = (Exp.Lvar pvar, typ_tmp) in
     let res_trans = init_expr_trans trans_state var_exp_typ stmt_info (Some temp_exp) in
     let _, typ = res_trans.return in
-    let var_data = ProcAttributes.{name= Pvar.get_name pvar; typ; attributes= []} in
+    let var_data =
+      ProcAttributes.{name= Pvar.get_name pvar; typ; attributes= []; is_constexpr= false}
+    in
     Procdesc.append_locals procdesc [var_data] ;
     res_trans
 

infer/src/clang/cVar_decl.ml

@@ -69,7 +69,13 @@ let add_var_to_locals procdesc var_decl typ pvar =
   | VarDecl (decl_info, _, _, vdi) ->
       if not vdi.Clang_ast_t.vdi_is_global then
         let attributes = get_var_attribute decl_info in
-        let var_data : ProcAttributes.var_data = {name= Pvar.get_name pvar; typ; attributes} in
+        let is_constexpr =
+          vdi.Clang_ast_t.vdi_is_const_expr
+          || (Typ.is_const typ.Typ.quals && vdi.Clang_ast_t.vdi_is_init_expr_cxx11_constant)
+        in
+        let var_data : ProcAttributes.var_data =
+          {name= Pvar.get_name pvar; typ; attributes; is_constexpr}
+        in
         Procdesc.append_locals procdesc [var_data]
   | _ ->
       assert false

infer/src/java/jTrans.ml

@@ -417,7 +417,7 @@ let create_cm_procdesc source_file program linereader icfg cm proc_name =
     let locals_ = translate_locals program tenv formals bytecode jbir_code in
     let locals =
       List.map locals_ ~f:(fun (name, typ) ->
-          ({name; typ; attributes= []} : ProcAttributes.var_data) )
+          ({name; typ; attributes= []; is_constexpr= false} : ProcAttributes.var_data) )
     in
     let method_annotation = JAnnotation.translate_method cm.Javalib.cm_annotations in
     let proc_attributes =

infer/tests/codetoanalyze/cpp/liveness/dead_stores_constexpr.cpp

@@ -41,7 +41,7 @@ void FN_init_capture_reassign_bad() {
 }
 
 // expected DEAD_STORE
-void capture_constexpr_bad() {
+void FN_capture_constexpr_bad() {
   constexpr int x = 1;
   foo(2);
 }

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

@@ -18,7 +18,5 @@ codetoanalyze/cpp/liveness/dead_stores.cpp, dead_stores::plus_plus2_bad, 2, DEAD
 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::use_then_dead_bad, 3, DEAD_STORE, no_bucket, ERROR, [Write of unused value]
-codetoanalyze/cpp/liveness/dead_stores_constexpr.cpp, FP_capture_constexpr_good, 1, DEAD_STORE, no_bucket, ERROR, [Write of unused value]
-codetoanalyze/cpp/liveness/dead_stores_constexpr.cpp, capture_constexpr_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]
 codetoanalyze/cpp/liveness/non_type_template_param.cpp, instanciateTemplateBad, 3, DEAD_STORE, no_bucket, ERROR, [Write of unused value]

infer/tests/codetoanalyze/cpp/liveness/non_type_template_param.cpp

@@ -20,3 +20,9 @@ int instanciateTemplateBad() {
   int unused = 1;
   return 0;
 }
+
+void instanciateTemplateConstOk() {
+  const int foo = 7;
+  X<foo> x;
+  x.isZeroBad();
+}