DeadCode: Use HoldsForAllInstances

Eliminate false positives where a line of code is used in some
copies (instances) but not others.

Author: lcartey

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

@@ -12,6 +12,7 @@
  */
 
 import cpp
+import codingstandards.cpp.alertreporting.HoldsForAllInstances
 import codingstandards.cpp.Customizations
 import codingstandards.cpp.Exclusions
 import codingstandards.cpp.deadcode.UselessAssignments
@@ -31,10 +32,6 @@ predicate isDeadOrUnreachableStmt(Stmt s) {
   s.getBasicBlock() = any(UnreachableBasicBlock ubb).getABasicBlock()
 }
 
-/**
- * Holds if the `Stmt` `s` is dead, i.e. could be executed, but its removal would not meaningfully
- * affect the program.
- */
 predicate isDeadStmt(Stmt s) {
   // A `DeclStmt` is dead code if:
   // - All the declarations are variable declarations
@@ -108,17 +105,33 @@ predicate isDeadStmt(Stmt s) {
   exists(TryStmt ts | s = ts and isDeadStmt(ts.getStmt()))
 }
 
-query predicate problems(Stmt s, string message) {
-  not isExcluded(s, getQuery()) and
+/**
+ * Holds if the `Stmt` `s` is dead, i.e. could be executed, but its removal would not meaningfully
+ * affect the program.
+ */
+class DeadStmtInstance extends Stmt {
+  DeadStmtInstance() {
+    isDeadStmt(this) and
+    // Exclude compiler generated statements
+    not this.isCompilerGenerated() and
+    // Exclude code fully generated by macros, because the code may be "live" in other expansions
+    not this.isInMacroExpansion() and
+    // MISRA defines dead code as an "_executed_ statement whose removal would not affect the program
+    // output". We therefore exclude unreachable statements as they are, by definition, not executed.
+    not this.getBasicBlock() = any(UnreachableBasicBlock ubb).getABasicBlock()
+  }
+}
+
+class DeadStmt = HoldsForAllInstances<DeadStmtInstance>::LogicalResultStmt;
+
+query predicate problems(DeadStmt s, string message) {
+  not isExcluded(s.getAStmtInstance(), getQuery()) and
   message = "This statement is dead code." and
-  isDeadStmt(s) and
   // Report only the highest level dead statement, to avoid over reporting
-  not isDeadStmt(s.getParentStmt()) and
-  // MISRA defines dead code as an "_executed_ statement whose removal would not affect the program
-  // output". We therefore exclude unreachable statements as they are, by definition, not executed.
-  not s.getBasicBlock() = any(UnreachableBasicBlock ubb).getABasicBlock() and
-  // Exclude code fully generated by macros, because the code may be "live" in other expansions
-  not s.isInMacroExpansion() and
-  // Exclude compiler generated statements
-  not s.isCompilerGenerated()
+  not exists(DeadStmt parent |
+    // All instances must share a dead statement parent for us to report the parent instead
+    forall(Stmt instance | instance = s.getAStmtInstance() |
+      parent.getAStmtInstance() = instance.getParentStmt()
+    )
+  )
 }