[mlir] Change the order of members in MLIRContext to ensure dialects are destroyed first

Currently, the dialects precede the registered operations in the context object, which means that the latter is destroyed first. At the same time, Operation::~Operation dereferences the registered operation when destroying properties, which can cause use-after-free (e.g. if a dialect owns an op). This patch fixes that by changing the order of the members so that dialects come after registered operations.

Differential Revision: https://reviews.llvm.org/D151440

Author: Laszlo Kindrat

mlir/lib/IR/MLIRContext.cpp

@@ -175,11 +175,6 @@ class MLIRContextImpl {
   /// destruction with the context.
   std::unique_ptr<llvm::ThreadPool> ownedThreadPool;
 
-  /// This is a list of dialects that are created referring to this context.
-  /// The MLIRContext owns the objects.
-  DenseMap<StringRef, std::unique_ptr<Dialect>> loadedDialects;
-  DialectRegistry dialectsRegistry;
-
   /// An allocator used for AbstractAttribute and AbstractType objects.
   llvm::BumpPtrAllocator abstractDialectSymbolAllocator;
 
@@ -193,6 +188,12 @@ class MLIRContextImpl {
   /// and efficient `getRegisteredOperations` implementation.
   SmallVector<RegisteredOperationName, 0> sortedRegisteredOperations;
 
+  /// This is a list of dialects that are created referring to this context.
+  /// The MLIRContext owns the objects. These need to be declared after the
+  /// registered operations to ensure correct destruction order.
+  DenseMap<StringRef, std::unique_ptr<Dialect>> loadedDialects;
+  DialectRegistry dialectsRegistry;
+
   /// A mutex used when accessing operation information.
   llvm::sys::SmartRWMutex<true> operationInfoMutex;