C/C++ overlay: use new discard strategy

cpp/ql/lib/semmle/code/cpp/internal/Overlay.qll

@@ -1,122 +1,61 @@
-/**
- * Defines entity discard predicates for C++ overlay analysis.
- */
-
 private import OverlayXml
 
 /**
  * Holds always for the overlay variant and never for the base variant.
- * This local predicate is used to define local predicates that behave
- * differently for the base and overlay variant.
  */
 overlay[local]
 predicate isOverlay() { databaseMetadata("isOverlay", "true") }
 
-overlay[local]
-private string getLocationFilePath(@location_default loc) {
-  exists(@file file | locations_default(loc, file, _, _, _, _) | files(file, result))
-}
-
 /**
- * Gets the file path for an element with a single location.
+ * Holds if TRAP file or tag `t` is reachable from a source file named
+ * `source_file` in the given variant (base or overlay).
  */
 overlay[local]
-private string getSingleLocationFilePath(@element e) {
-  exists(@location_default loc |
-    var_decls(e, _, _, _, loc)
-    or
-    fun_decls(e, _, _, _, loc)
-    or
-    type_decls(e, _, loc)
-    or
-    namespace_decls(e, _, loc, _)
-    or
-    macroinvocations(e, _, loc, _)
-    or
-    preprocdirects(e, _, loc)
-    or
-    diagnostics(e, _, _, _, _, loc)
-    or
-    usings(e, _, loc, _)
-    or
-    static_asserts(e, _, _, loc, _)
-    or
-    derivations(e, _, _, _, loc)
-    or
-    frienddecls(e, _, _, loc)
-    or
-    comments(e, _, loc)
-    or
-    exprs(e, _, loc)
-    or
-    stmts(e, _, loc)
-    or
-    initialisers(e, _, _, loc)
-    or
-    attributes(e, _, _, _, loc)
-    or
-    attribute_args(e, _, _, _, loc)
-    or
-    namequalifiers(e, _, _, loc)
-    or
-    enumconstants(e, _, _, _, _, loc)
-    or
-    type_mentions(e, _, loc, _)
-    or
-    lambda_capture(e, _, _, _, _, _, loc)
-    or
-    concept_templates(e, _, loc)
-  |
-    result = getLocationFilePath(loc)
+private predicate locally_reachable_trap_or_tag(boolean is_overlay, string source_file, @trap_or_tag t) {
+  exists(@source_file sf, string source_file_raw, @trap trap |
+    (if isOverlay() then is_overlay = true else is_overlay = false) and
+    source_file_uses_trap(sf, trap) and
+    source_file_name(sf, source_file_raw) and
+    source_file = source_file_raw.replaceAll("\\", "/") and
+    (t = trap or trap_uses_tag(trap, t))
   )
 }
 
 /**
- * Gets the file path for an element with potentially multiple locations.
+ * Holds if element `e` is defined in TRAP file or tag `t` in the given
+ * variant (base or overlay).
  */
 overlay[local]
-private string getMultiLocationFilePath(@element e) {
-  exists(@location_default loc |
-    var_decls(_, e, _, _, loc)
-    or
-    fun_decls(_, e, _, _, loc)
-    or
-    type_decls(_, e, loc)
-    or
-    namespace_decls(_, e, loc, _)
-  |
-    result = getLocationFilePath(loc)
-  )
+private predicate locally_in_trap_or_tag(boolean is_overlay, @element e, @trap_or_tag t) {
+  (if isOverlay() then is_overlay = true else is_overlay = false) and
+  in_trap_or_tag(e, t)
 }
 
 /**
- * A local helper predicate that holds in the base variant and never in the
- * overlay variant.
- */
-overlay[local]
-private predicate isBase() { not isOverlay() }
-
-/**
- * Holds if `path` was extracted in the overlay database.
- */
-overlay[local]
-private predicate overlayHasFile(string path) {
-  isOverlay() and
-  files(_, path) and
-  path != ""
-}
-
-/**
- * Discards an element from the base variant if:
- * - It has a single location in a file extracted in the overlay, or
- * - All of its locations are in files extracted in the overlay.
+ * Holds if element `e` from the base variant should be discarded because
+ * it has been redefined or is no longer reachable in the overlay.
  */
 overlay[discard_entity]
-private predicate discardElement(@element e) {
-  isBase() and
-  (
-    overlayHasFile(getSingleLocationFilePath(e))
-    or
-    forex(string path | path = getMultiLocationFilePath(e) | overlayHasFile(path))
+private predicate discard_element(@element e) {
+  // If we don't have any knowledge about what TRAP file something
+  // is in, then we don't want to discard it, so we only consider
+  // entities that are known to be in a base TRAP file.
+  locally_in_trap_or_tag(false, e, _) and
+  // Anything that is reachable from an overlay source file should
+  // not be discarded.
+  not exists(@trap_or_tag t | locally_in_trap_or_tag(true, e, t) |
+    locally_reachable_trap_or_tag(true, _, t)
+  ) and
+  // Finally, we have to make sure that base shouldn't retain it.
+  // If it is reachable from a base source file, then that is
+  // sufficient unless either the base source file has changed (in
+  // particular, been deleted) or the overlay has redefined the TRAP
+  // file it is in.
+  forall(@trap_or_tag t, string source_file |
+    locally_in_trap_or_tag(false, e, t) and
+    locally_reachable_trap_or_tag(false, source_file, t)
+  |
+    overlayChangedFiles(source_file) or
+    locally_reachable_trap_or_tag(true, _, t)
   )
 }