feat(cxx_indexer): prepare to eliminate simple aliases (#5535)

kythe/cxx/indexer/cxx/IndexerASTHooks.cc

@@ -568,6 +568,32 @@ bool IndexerASTVisitor::declDominatesPrunableSubtree(const clang::Decl* Decl) {
   return getAllParents()->DeclDominatesPrunableSubtree(Decl);
 }
 
+const clang::Decl* IndexerASTVisitor::GetInfluencedDeclFromLValueHead(
+    const clang::Stmt* head) {
+  if (head == nullptr) return nullptr;
+  head = SkipTrivialAliasing(head);
+  if (auto* expr = llvm::dyn_cast_or_null<clang::DeclRefExpr>(head);
+      expr != nullptr && expr->getFoundDecl() != nullptr &&
+      (expr->getFoundDecl()->getKind() == clang::Decl::Kind::Var ||
+       expr->getFoundDecl()->getKind() == clang::Decl::Kind::ParmVar)) {
+    return expr->getFoundDecl();
+  }
+  if (auto* expr = llvm::dyn_cast_or_null<clang::MemberExpr>(head);
+      expr != nullptr) {
+    if (auto* member = expr->getMemberDecl(); member != nullptr) {
+      return member;
+    }
+  }
+  return nullptr;
+}
+
+const clang::Stmt* IndexerASTVisitor::SkipTrivialAliasing(
+    const clang::Stmt* stmt) {
+  // TODO(zarko): the actual implementation.
+  if (stmt == nullptr) return nullptr;
+  return stmt;
+}
+
 bool IndexerASTVisitor::IsDefinition(const clang::VarDecl* VD) {
   if (const auto* PVD = dyn_cast<clang::ParmVarDecl>(VD)) {
     // For parameters, we want to report them as definitions iff they're

kythe/cxx/indexer/cxx/IndexerASTHooks.h

@@ -740,6 +740,22 @@ class IndexerASTVisitor : public RecursiveTypeVisitor<IndexerASTVisitor> {
   GraphObserver& Observer;
   clang::ASTContext& Context;
 
+  /// \return the `Decl` that is the target of influence by an lexpression with
+  /// head `head`, or null. For example, in `foo[x].bar(y).z`, the target of
+  /// influence is the member decl for `z`.
+  const clang::Decl* GetInfluencedDeclFromLValueHead(const clang::Stmt* head);
+
+  /// \brief Eliminates the trivial introduction of aliasing.
+  ///
+  /// In some situations (and modulo undefined behavior), the expression
+  /// `*(&foo)` can be reduced to `foo`. `foo` may be a simple DeclRefExpr, but
+  /// it could also be a MemberExpr that has alias semantics tied to another
+  /// field or some piece of generated code.
+  ///
+  /// \return the head of a trivial alias/dealias operation or `stmt` if the
+  /// operation failed to find one.
+  const clang::Stmt* SkipTrivialAliasing(const clang::Stmt* stmt);
+
   /// \brief The result of calling into the lexer.
   enum class LexerResult {
     Failure,  ///< The operation failed.
@@ -990,14 +1006,17 @@ class IndexerASTVisitor : public RecursiveTypeVisitor<IndexerASTVisitor> {
   /// \brief Marks that `stmt` was used as a write target.
   /// \return `stmt` as passed.
   const clang::Stmt* UsedAsWrite(const clang::Stmt* stmt) {
-    if (stmt != nullptr) use_kinds_[stmt] = GraphObserver::UseKind::kWrite;
+    if (stmt != nullptr)
+      use_kinds_[SkipTrivialAliasing(stmt)] = GraphObserver::UseKind::kWrite;
     return stmt;
   }
 
   /// \brief Marks that `stmt` was used as a read+write target.
   /// \return `stmt` as passed.
   const clang::Stmt* UsedAsReadWrite(const clang::Stmt* stmt) {
-    if (stmt != nullptr) use_kinds_[stmt] = GraphObserver::UseKind::kReadWrite;
+    if (stmt != nullptr)
+      use_kinds_[SkipTrivialAliasing(stmt)] =
+          GraphObserver::UseKind::kReadWrite;
     return stmt;
   }
 

kythe/cxx/indexer/cxx/clang_utils.cc

@@ -168,21 +168,4 @@ const clang::Stmt* FindLValueHead(const clang::Stmt* stmt) {
       return nullptr;
   }
 }
-
-const clang::Decl* GetInfluencedDeclFromLValueHead(const clang::Stmt* head) {
-  if (head == nullptr) return nullptr;
-  if (auto* expr = llvm::dyn_cast_or_null<clang::DeclRefExpr>(head);
-      expr != nullptr && expr->getFoundDecl() != nullptr &&
-      (expr->getFoundDecl()->getKind() == clang::Decl::Kind::Var ||
-       expr->getFoundDecl()->getKind() == clang::Decl::Kind::ParmVar)) {
-    return expr->getFoundDecl();
-  }
-  if (auto* expr = llvm::dyn_cast_or_null<clang::MemberExpr>(head);
-      expr != nullptr) {
-    if (auto* member = expr->getMemberDecl(); member != nullptr) {
-      return member;
-    }
-  }
-  return nullptr;
-}
 }  // namespace kythe