M0-1-9: Fix dead-code false positive when constexpr integer is used in array size

change_notes/2024-09-17-fix-fp-678-m0-1-9.md

@@ -0,0 +1,2 @@
+- `M0-1-9` - `DeadCode.qll`:
+  - Fixes #678. Remove dead code false positive when integer constant expression is used to define the size of an array.

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

@@ -18,12 +18,6 @@ import cpp
 import codingstandards.cpp.autosar
 import codingstandards.cpp.deadcode.UnusedVariables
 
-/** Gets the constant value of a constexpr/const variable. */
-private string getConstExprValue(Variable v) {
-  result = v.getInitializer().getExpr().getValue() and
-  (v.isConst() or v.isConstexpr())
-}
-
 // This predicate is similar to getUseCount for M0-1-4 except that it also
 // considers static_asserts. This was created to cater for M0-1-3 specifically
 // and hence, doesn't attempt to reuse the M0-1-4 specific predicate
@@ -44,11 +38,7 @@ int getUseCountConservatively(Variable 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)
-      )
+      countUsesInLocalArraySize(v)
 }
 
 from PotentiallyUnusedLocalVariable v

cpp/common/src/codingstandards/cpp/deadcode/UnusedVariables.qll

@@ -150,3 +150,21 @@ predicate maybeACompileTimeTemplateArgument(Variable v) {
     )
   )
 }
+
+/** Gets the constant value of a constexpr/const variable. */
+string getConstExprValue(Variable v) {
+  result = v.getInitializer().getExpr().getValue() and
+  (v.isConst() or v.isConstexpr())
+}
+
+/**
+ * Counts uses of `Variable` v in a local array of size `n`
+ */
+int countUsesInLocalArraySize(Variable v) {
+  result =
+    count(ArrayType at, LocalVariable arrayVariable |
+      arrayVariable.getType().resolveTypedefs() = at and
+      v.(PotentiallyUnusedLocalVariable).getFunction() = arrayVariable.getFunction() and
+      at.getArraySize().toString() = getConstExprValue(v)
+    )
+}

cpp/common/src/codingstandards/cpp/rules/deadcode/DeadCode.qll

@@ -16,6 +16,7 @@ import codingstandards.cpp.Customizations
 import codingstandards.cpp.Exclusions
 import codingstandards.cpp.deadcode.UselessAssignments
 import codingstandards.cpp.deadcode.UnreachableCode
+import codingstandards.cpp.deadcode.UnusedVariables
 
 abstract class DeadCodeSharedQuery extends Query { }
 
@@ -39,6 +40,7 @@ predicate isDeadStmt(Stmt s) {
   // - All the declarations are variable declarations
   // - None of those variables are ever accessed in non-dead code
   // - The initializers for each of the variables are pure
+  // - It isn't constexpr and used to declare an array size
   exists(DeclStmt ds |
     ds = s and
     // Use forex so that we don't flag "fake" generated `DeclStmt`s (e.g. those generated by the
@@ -50,7 +52,8 @@ predicate isDeadStmt(Stmt s) {
         not exists(VariableAccess va |
           va.getTarget() = v and
           not isDeadOrUnreachableStmt(va.getEnclosingStmt())
-        )
+        ) and
+        not (countUsesInLocalArraySize(v) > 0)
       )
     )
   )

cpp/common/test/rules/deadcode/DeadCode.expected

@@ -12,3 +12,6 @@
 | test.cpp:72:3:73:3 | try { ... } | This statement is dead code. |
 | test.cpp:73:17:74:3 | { ... } | This statement is dead code. |
 | test.cpp:79:17:80:3 | { ... } | This statement is dead code. |
+| test.cpp:85:3:85:43 | declaration | This statement is dead code. |
+| test.cpp:87:3:87:30 | declaration | This statement is dead code. |
+| test.cpp:90:3:90:50 | declaration | This statement is dead code. |

cpp/common/test/rules/deadcode/test.cpp

@@ -81,5 +81,13 @@ int test_dead_code(int x) {
 
   static_assert(1); // COMPLIANT
 
-  return live5 + live6; // COMPLIANT
-}
+  constexpr int constexpr_array_size{6};    // COMPLIANT
+  int unused_array[constexpr_array_size]{}; // NON_COMPLIANT
+
+  constexpr int unused_int{2}; // NON_COMPLIANT
+
+  constexpr int constexpr_used_array[]{3, 4, 5};   // COMPLIANT
+  constexpr int constexpr_unused_array[]{0, 1, 2}; // NON_COMPLIANT
+
+  return live5 + live6 + constexpr_used_array[1]; // COMPLIANT
+}