Merge pull request #17988 from owen-mc/java/fix-unreachable-blocks-in-const-switch-stmt

Java: fix unreachable basic blocks in const switch stmt

Author: owen-mc

java/ql/lib/change-notes/2024-11-14-unreachable-basic-block-in-constant-switch-statement.md

@@ -0,0 +1,4 @@
+---
+category: minorAnalysis
+---
+* In a switch statement with a constant switch expression, all non-matching cases were being marked as unreachable, including those that can be reached by falling through from the matching case. This has now been fixed.

java/ql/lib/semmle/code/java/controlflow/UnreachableBlocks.qll

@@ -217,13 +217,15 @@ class UnreachableBasicBlock extends BasicBlock {
     not this instanceof CatchClause
     or
     // Switch statements with a constant comparison expression may have unreachable cases.
-    exists(ConstSwitchStmt constSwitchStmt, BasicBlock failingCaseBlock |
-      failingCaseBlock = constSwitchStmt.getAFailingCase().getBasicBlock()
-    |
+    exists(ConstSwitchStmt constSwitchStmt, BasicBlock unreachableCaseBlock |
+      // Not accessible from the switch expression
+      unreachableCaseBlock = constSwitchStmt.getAFailingCase().getBasicBlock() and
       // Not accessible from the successful case
-      not constSwitchStmt.getMatchingCase().getBasicBlock().getABBSuccessor*() = failingCaseBlock and
-      // Blocks dominated by the failing case block are unreachable
-      constSwitchStmt.getAFailingCase().getBasicBlock().bbDominates(this)
+      not constSwitchStmt.getMatchingCase().getBasicBlock().getABBSuccessor*() =
+        unreachableCaseBlock
+    |
+      // Blocks dominated by an unreachable case block are unreachable
+      unreachableCaseBlock.bbDominates(this)
     )
   }
 }

java/ql/test/library-tests/unreachableblocks/UnreachableBlocks.expected

@@ -3,7 +3,6 @@
 | unreachableblocks/Unreachable.java:12:22:14:3 | { ... } |
 | unreachableblocks/Unreachable.java:17:3:17:9 | case ... |
 | unreachableblocks/Unreachable.java:19:3:19:9 | case ... |
-| unreachableblocks/Unreachable.java:22:3:22:9 | case ... |
 | unreachableblocks/Unreachable.java:24:3:24:9 | case ... |
 | unreachableblocks/Unreachable.java:26:3:26:10 | case ... |
 | unreachableblocks/Unreachable.java:27:3:27:10 | default |