From 0a668c21618b59d2bcac27a5af83137e899fc6cb Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Ezgi=20=C3=87i=C3=A7ek?= <ezgi@fb.com>
Date: Tue, 7 Aug 2018 00:58:42 -0700
Subject: [PATCH] [DEAD_STORE] Capture `constexpr`s in lambdas

Reviewed By: jvillard

Differential Revision: D9179442

fbshipit-source-id: 8df2769c6
---
 infer/src/checkers/liveness.ml                | 24 ++++++++-
 .../cpp/liveness/dead_stores_constexpr.cpp    | 53 +++++++++++++++++++
 .../codetoanalyze/cpp/liveness/issues.exp     |  2 +
 3 files changed, 77 insertions(+), 2 deletions(-)
 create mode 100644 infer/tests/codetoanalyze/cpp/liveness/dead_stores_constexpr.cpp

diff --git a/infer/src/checkers/liveness.ml b/infer/src/checkers/liveness.ml
index a2876bf55..14433b14e 100644
--- a/infer/src/checkers/liveness.ml
+++ b/infer/src/checkers/liveness.ml
@@ -50,7 +50,27 @@ module TransferFunctions (CFG : ProcCfg.S) = struct
         add_live_actuals_ actuals live_acc
 
 
-  let exec_instr astate _ _ = function
+  (* In C++14, for a lambda at block scope, constexpr variables in the
+     reaching scope may be used inside the lambda, even if they are
+     not captured.  For further details:
+     https://timsong-cpp.github.io/cppwp/n4140/expr.prim.lambda#12 *)
+  let add_local_consts_for_lambdas pdesc call_exp astate =
+    let pname = Procdesc.get_proc_name pdesc in
+    match call_exp with
+    | Exp.Const (Cfun (Typ.Procname.ObjC_Cpp cpp_pname))
+      when Typ.Procname.ObjC_Cpp.is_cpp_lambda cpp_pname ->
+        Procdesc.get_locals pdesc
+        |> List.fold ~init:astate ~f:(fun acc var_data ->
+               let open ProcAttributes in
+               if Typ.is_const var_data.typ.quals then
+                 let const_local_var = Pvar.mk var_data.name pname |> Var.of_pvar in
+                 Domain.add const_local_var acc
+               else acc )
+    | _ ->
+        astate
+
+
+  let exec_instr astate {ProcData.pdesc} _ = function
     | Sil.Load (lhs_id, _, _, _) when Ident.is_none lhs_id ->
         (* dummy deref inserted by frontend--don't count as a read *)
         astate
@@ -68,7 +88,7 @@ module TransferFunctions (CFG : ProcCfg.S) = struct
         exp_add_live exp astate
     | Sil.Call ((ret_id, _), call_exp, actuals, _, _) ->
         Domain.remove (Var.of_id ret_id) astate |> exp_add_live call_exp
-        |> add_live_actuals actuals call_exp
+        |> add_live_actuals actuals call_exp |> add_local_consts_for_lambdas pdesc call_exp
     | Sil.Declare_locals _ | Remove_temps _ | Abstract _ | Nullify _ ->
         astate
 
diff --git a/infer/tests/codetoanalyze/cpp/liveness/dead_stores_constexpr.cpp b/infer/tests/codetoanalyze/cpp/liveness/dead_stores_constexpr.cpp
new file mode 100644
index 000000000..672f0eab7
--- /dev/null
+++ b/infer/tests/codetoanalyze/cpp/liveness/dead_stores_constexpr.cpp
@@ -0,0 +1,53 @@
+/*
+ * Copyright (c) 2016-present, Facebook, Inc.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+int foo(int a);
+
+// before, we were raising DEAD_STORE for x
+void capture_constexpr_good() {
+  constexpr int x = 10;
+  []() {
+    foo(x);
+    return;
+  }();
+}
+
+void call_it(void (*f)(void)) { f(); }
+
+void FP_capture_constexpr_good() {
+  constexpr int x = 1;
+  auto lambda = []() {
+    foo(x);
+    return;
+  };
+  call_it(lambda);
+}
+
+// we always assume const exprs are captured in lambdas
+void FN_capture_constexpr_good() {
+  constexpr int x = 10;
+  []() {
+    foo(0);
+    return;
+  }();
+}
+
+void FN_init_capture_reassign_bad() {
+  constexpr int i = 1; // this is a dead store
+  return [i = 1]() { return i; }();
+}
+
+// expected DEAD_STORE
+void capture_constexpr_bad() {
+  constexpr int x = 1;
+  foo(2);
+}
+
+// expected since dead stores to a "sentinel" value are ignored
+void capture_constexpr_sentinel_good() {
+  constexpr int x = 0;
+  []() { return; }();
+}
diff --git a/infer/tests/codetoanalyze/cpp/liveness/issues.exp b/infer/tests/codetoanalyze/cpp/liveness/issues.exp
index 2a8fd1d86..a1d027711 100644
--- a/infer/tests/codetoanalyze/cpp/liveness/issues.exp
+++ b/infer/tests/codetoanalyze/cpp/liveness/issues.exp
@@ -18,5 +18,7 @@ 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]