M0-1-3: Consider local variable usages in array size and template function arguments

change_notes/2024-08-06-fix-fp-658-M0-1-3.md

@@ -0,0 +1,2 @@
+- `M0-1-3` - `UnusedLocalVariable.ql`:
+  - Fixes #658. Considers usage of const/constexpr variables in array size and function parameters that are used in arguments of template functions.

cpp/autosar/src/rules/M0-1-3/UnusedLocalVariable.ql

@@ -18,10 +18,10 @@ import cpp
 import codingstandards.cpp.autosar
 import codingstandards.cpp.deadcode.UnusedVariables
 
-/** Gets the constant value of a constexpr variable. */
+/** Gets the constant value of a constexpr/const variable. */
 private string getConstExprValue(Variable v) {
   result = v.getInitializer().getExpr().getValue() and
-  v.isConstexpr()
+  (v.isConst() or v.isConstexpr())
 }
 
 // This predicate is similar to getUseCount for M0-1-4 except that it also
@@ -41,13 +41,28 @@ int getUseCountConservatively(Variable v) {
       ) +
       // For static asserts too, check if there is a child which has the same value
       // as the constexpr variable.
-      count(StaticAssert s | s.getCondition().getAChild*().getValue() = getConstExprValue(v))
+      count(StaticAssert s | s.getCondition().getAChild*().getValue() = getConstExprValue(v)) +
+      // In case an array type uses a constant in the same scope as the constexpr variable,
+      // consider it as used.
+      count(ArrayType at, LocalVariable arrayVariable |
+        arrayVariable.getType().resolveTypedefs() = at and
+        v.(PotentiallyUnusedLocalVariable).getFunction() = arrayVariable.getFunction() and
+        at.getArraySize().toString() = getConstExprValue(v)
+      )
 }
 
 from PotentiallyUnusedLocalVariable v
 where
   not isExcluded(v, DeadCodePackage::unusedLocalVariableQuery()) and
   // Local variable is never accessed
   not exists(v.getAnAccess()) and
+  // Sometimes multiple objects representing the same entities are created in
+  // the AST. Check if those are not accessed as well. Refer issue #658
+  not exists(LocalScopeVariable another |
+    another.getDefinitionLocation() = v.getDefinitionLocation() and
+    another.hasName(v.getName()) and
+    exists(another.getAnAccess()) and
+    another != v
+  ) and
   getUseCountConservatively(v) = 0
 select v, "Local variable '" + v.getName() + "' in '" + v.getFunction().getName() + "' is not used."

cpp/autosar/test/rules/M0-1-3/UnusedLocalVariable.expected

@@ -1,7 +1,6 @@
 | test.cpp:7:7:7:7 | y | Local variable 'y' in 'test_simple' is not used. |
-| test.cpp:14:13:14:13 | y | Local variable 'y' in 'test_const' is not used. |
-| test.cpp:17:7:17:7 | z | Local variable 'z' in 'test_const' is not used. |
-| test.cpp:23:5:23:5 | t | Local variable 't' in 'f1' is not used. |
-| test.cpp:23:5:23:5 | t | Local variable 't' in 'f1' is not used. |
-| test.cpp:44:6:44:6 | a | Local variable 'a' in 'test_side_effect_init' is not used. |
-| test.cpp:91:5:91:5 | t | Local variable 't' in 'template_function' is not used. |
+| test.cpp:15:7:15:7 | z | Local variable 'z' in 'test_const' is not used. |
+| test.cpp:21:5:21:5 | t | Local variable 't' in 'f1' is not used. |
+| test.cpp:21:5:21:5 | t | Local variable 't' in 'f1' is not used. |
+| test.cpp:42:6:42:6 | a | Local variable 'a' in 'test_side_effect_init' is not used. |
+| test.cpp:89:5:89:5 | t | Local variable 't' in 'template_function' is not used. |

cpp/autosar/test/rules/M0-1-3/test.cpp

@@ -11,9 +11,7 @@ int test_simple() {
 
 int test_const() {
   const int x = 1; // COMPLIANT - used below
-  const int y = 2; // COMPLIANT[FALSE_POSITIVE] - used in array initialization,
-                   // but the database does not contain sufficient information
-                   // for this case
+  const int y = 2; // COMPLIANT - used in array initialization,
   int z[y];        // NON_COMPLIANT - never used
   return x;
 }
@@ -98,4 +96,20 @@ class ClassT {
   void test() {}
 };
 
-void test_template_function() { template_function<ClassT>(); }
+void test_template_function() { template_function<ClassT>(); }
+
+int foo() {
+  constexpr int arrayDim = 10; // COMPLIANT - used in array size below
+  static int array[arrayDim]{};
+  return array[4];
+}
+
+template <typename T> static T another_templ_function() { return T(); }
+
+template <typename T, typename First, typename... Rest>
+static T another_templ_function(const First &first, const Rest &...rest) {
+  return first +
+         another_templ_function<T>(rest...); // COMPLIANT - 'rest' is used here
+}
+
+static int templ_fnc2() { return another_templ_function<int>(1, 2, 3, 4, 5); }