JS: rename UselessRangeCheck -> UselessComparisonTest

javascript/ql/src/Statements/UselessComparisonTest.ql

@@ -0,0 +1,60 @@
+/**
+ * @name Useless comparison test
+ * @description A comparison that always evaluates to true or always evaluates to false may
+ *              indicate faulty logic and dead code.
+ * @kind problem
+ * @problem.severity warning
+ * @id js/useless-range-check
+ * @tags correctness
+ * @precision high
+ */
+
+import javascript
+
+/**
+ * Holds if there are any contradictory guard nodes in `container`.
+ *
+ * We use this to restrict reachability analysis to a small set of containers.
+ */
+predicate hasContradictoryGuardNodes(StmtContainer container) {
+  exists (ConditionGuardNode guard |
+    RangeAnalysis::isContradictoryGuardNode(guard) and
+    container = guard.getContainer())
+}
+
+/**
+ * Holds if `block` is reachable and is in a container with contradictory guard nodes.
+ */
+predicate isReachable(BasicBlock block) {
+  exists (StmtContainer container |
+    hasContradictoryGuardNodes(container) and
+    block = container.getEntryBB())
+  or
+  isReachable(block.getAPredecessor()) and
+  not RangeAnalysis::isContradictoryGuardNode(block.getANode())
+}
+
+/**
+ * Holds if `block` is unreachable, but could be reached if `guard` was not contradictory.
+ */
+predicate isBlockedByContradictoryGuardNodes(BasicBlock block, ConditionGuardNode guard) {
+  RangeAnalysis::isContradictoryGuardNode(guard) and
+  isReachable(block.getAPredecessor()) and // the guard itself is reachable
+  block = guard.getBasicBlock()
+  or
+  isBlockedByContradictoryGuardNodes(block.getAPredecessor(), guard) and
+  not isReachable(block)
+}
+
+/**
+ * Holds if the given guard node is contradictory and causes an expression or statement to be unreachable.
+ */
+predicate isGuardNodeWithDeadCode(ConditionGuardNode guard) {
+  exists (BasicBlock block |
+    isBlockedByContradictoryGuardNodes(block, guard) and
+    block.getANode() instanceof ExprOrStmt)
+}
+
+from ConditionGuardNode guard
+where isGuardNodeWithDeadCode(guard)
+select guard.getTest(), "The condition '" + guard.getTest() + "' is always " + guard.getOutcome().booleanNot()