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

[laszlokindrat]

Summary

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.

[lattner]

LG, thank you! Please add something to the comment above the loadedDialects field to indicate that it needs to be after the other operations for destruction order. Thanks!

[laszlokindrat]

Add comment re destruction order.

mlir/lib/IR/MLIRContext.cpp

#endif
   /// multi-threading is disabled, and the threadpool wasn't externally provided
   /// using `setThreadPool`, this will be nullptr.
   llvm::ThreadPool *threadPool = nullptr;
 
   /// In case where the thread pool is owned by the context, this ensures
   /// 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;
 
   /// This is a mapping from operation name to the operation info describing it.
   llvm::StringMap<std::unique_ptr<OperationName::Impl>> operations;
 
   /// A vector of operation info specifically for registered operations.
   llvm::StringMap<RegisteredOperationName> registeredOperations;
 
   /// This is a sorted container of registered operations for a deterministic
   /// 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;
 
   //===--------------------------------------------------------------------===//
   // Affine uniquing
   //===--------------------------------------------------------------------===//
 
   // Affine expression, map and integer set uniquing.