[mlir][Transforms] Add listener support to dialect conversion

Author: matthias-springer

mlir/include/mlir/Transforms/DialectConversion.h

@@ -1091,6 +1091,39 @@ struct ConversionConfig {
   /// IR during an analysis conversion and only pre-existing operations are
   /// added to the set.
   DenseSet<Operation *> *legalizableOps = nullptr;
+
+  /// An optional listener that is notified about all IR modifications in case
+  /// dialect conversion succeeds. If the dialect conversion fails and no IR
+  /// modifications are visible (i.e., they were all rolled back), no
+  /// notifications are sent.
+  ///
+  /// Note: Notifications are sent in a delayed fashion, when the dialect
+  /// conversion is guaranteed to succeed. At that point, some IR modifications
+  /// may already have been materialized. Consequently, operations/blocks that
+  /// are passed to listener callbacks should not be accessed. (Ops/blocks are
+  /// guaranteed to be valid pointers and accessing op names is allowed. But
+  /// there are no guarantees about the state of ops/blocks at the time that a
+  /// callback is triggered.)
+  ///
+  /// Example: Consider a dialect conversion a new op ("test.foo") is created
+  /// and inserted, and later moved to another block. (Moving ops also triggers
+  /// "notifyOperationInserted".)
+  ///
+  /// (1) notifyOperationInserted: "test.foo" (into block "b1")
+  /// (2) notifyOperationInserted: "test.foo" (moved to another block "b2")
+  ///
+  /// When querying "op->getBlock()" during the first "notifyOperationInserted",
+  /// "b2" would be returned because "moving an op" is a kind of rewrite that is
+  /// immediately performed by the dialect conversion (and rolled back upon
+  /// failure).
+  //
+  // Note: When receiving a "notifyBlockInserted"/"notifyOperationInserted"
+  // callback, the previous region/block is provided to the callback, but not
+  // the iterator pointing to the exact location within the region/block. That
+  // is because these notifications are sent with a delay (after the IR has
+  // already been modified) and iterators into past IR state cannot be
+  // represented at the moment.
+  RewriterBase::Listener *listener = nullptr;
 };
 
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