[mlir][Transforms] Dialect conversion: Simplify handling of dropped arguments

This commit simplifies the handling of dropped arguments and updates some dialect conversion documentation that is outdated.

When converting a block signature, a `BlockTypeConversionRewrite` object and potentially multiple `ReplaceBlockArgRewrite` are created. During the "commit" phase, uses of the old block arguments are replaced with the new block arguments, but the old implementation was written in an inconsistent way: some block arguments were replaced in `BlockTypeConversionRewrite::commit` and some were replaced in `ReplaceBlockArgRewrite::commit`. The new `BlockTypeConversionRewrite::commit` implementation is much simpler and no longer modifies any IR; that is done only in `ReplaceBlockArgRewrite` now. The `ConvertedArgInfo` data structure is no longer needed.

To that end, materializations of dropped arguments are now built in `applySignatureConversion` instead of `materializeLiveConversions`; the latter function no longer has to deal with dropped arguments.

Other minor improvements:
- Improve variable name: `origOutputType` -> `origArgType`. Add an assertion to check that this field is only used for argument materializations.
- Add more comments to `applySignatureConversion`.

Note: Error messages around failed materializations for dropped basic block arguments changed slightly. That is because those materializations are now built in `legalizeUnresolvedMaterialization` instead of `legalizeConvertedArgumentTypes`.

Author: matthias-springer

mlir/docs/DialectConversion.md

@@ -246,6 +246,13 @@ depending on the situation.
 
     -   An argument materialization is used when converting the type of a block
         argument during a [signature conversion](#region-signature-conversion).
+        The new block argument types are specified in a `SignatureConversion`
+        object. An original block argument can be converted into multiple
+        block arguments, which is not supported everywhere in the dialect
+        conversion. (E.g., adaptors support only a single replacement value for
+        each original value.) Therefore, an argument materialization is used to
+        convert potentially multiple new block arguments back into a single SSA
+        value.
 
 *   Source Materialization
 
@@ -259,6 +266,9 @@ depending on the situation.
         *   When a block argument has been converted to a different type, but
             the original argument still has users that will remain live after
             the conversion process has finished.
+        *   When a block argument has been dropped, but the argument still has
+            users that will remain live after the conversion process has
+            finished.
         *   When the result type of an operation has been converted to a
             different type, but the original result still has users that will
             remain live after the conversion process is finished.
@@ -330,34 +340,37 @@ class TypeConverter {
 
   /// Register a materialization function, which must be convertible to the
   /// following form:
-  ///   `Optional<Value> (OpBuilder &, T, ValueRange, Location)`,
-  ///   where `T` is any subclass of `Type`.
-  /// This function is responsible for creating an operation, using the
-  /// OpBuilder and Location provided, that "converts" a range of values into a
-  /// single value of the given type `T`. It must return a Value of the
-  /// converted type on success, an `std::nullopt` if it failed but other
-  /// materialization can be attempted, and `nullptr` on unrecoverable failure.
-  /// It will only be called for (sub)types of `T`.
+  ///   `std::optional<Value>(OpBuilder &, T, ValueRange, Location)`,
+  /// where `T` is any subclass of `Type`. This function is responsible for
+  /// creating an operation, using the OpBuilder and Location provided, that
+  /// "casts" a range of values into a single value of the given type `T`. It
+  /// must return a Value of the converted type on success, an `std::nullopt` if
+  /// it failed but other materialization can be attempted, and `nullptr` on
+  /// unrecoverable failure. It will only be called for (sub)types of `T`.
+  /// Materialization functions must be provided when a type conversion may
+  /// persist after the conversion has finished.
   ///
   /// This method registers a materialization that will be called when
-  /// converting an illegal block argument type, to a legal type.
+  /// converting potentially multiple replacement block arguments (of a single
+  /// original block argument), to a single SSA value with a legal type.
   template <typename FnT,
             typename T = typename llvm::function_traits<FnT>::template arg_t<1>>
   void addArgumentMaterialization(FnT &&callback) {
     argumentMaterializations.emplace_back(
         wrapMaterialization<T>(std::forward<FnT>(callback)));
   }
   /// This method registers a materialization that will be called when
-  /// converting a legal type to an illegal source type. This is used when
-  /// conversions to an illegal type must persist beyond the main conversion.
+  /// converting a legal replacement value back to an illegal source type.
+  /// This is used when some uses of the original, illegal value must persist
+  /// beyond the main conversion.
   template <typename FnT,
             typename T = typename llvm::function_traits<FnT>::template arg_t<1>>
   void addSourceMaterialization(FnT &&callback) {
     sourceMaterializations.emplace_back(
         wrapMaterialization<T>(std::forward<FnT>(callback)));
   }
   /// This method registers a materialization that will be called when
-  /// converting type from an illegal, or source, type to a legal type.
+  /// converting an illegal (source) value to a legal (target) type.
   template <typename FnT,
             typename T = typename llvm::function_traits<FnT>::template arg_t<1>>
   void addTargetMaterialization(FnT &&callback) {

mlir/include/mlir/Transforms/DialectConversion.h

@@ -181,24 +181,26 @@ class TypeConverter {
   /// persist after the conversion has finished.
   ///
   /// This method registers a materialization that will be called when
-  /// converting an illegal block argument type, to a legal type.
+  /// converting potentially multiple replacement block arguments (of a single
+  /// original block argument), to a single SSA value with a legal type.
   template <typename FnT, typename T = typename llvm::function_traits<
                               std::decay_t<FnT>>::template arg_t<1>>
   void addArgumentMaterialization(FnT &&callback) {
     argumentMaterializations.emplace_back(
         wrapMaterialization<T>(std::forward<FnT>(callback)));
   }
   /// This method registers a materialization that will be called when
-  /// converting a legal type to an illegal source type. This is used when
-  /// conversions to an illegal type must persist beyond the main conversion.
+  /// converting a legal replacement value back to an illegal source type.
+  /// This is used when some uses of the original, illegal value must persist
+  /// beyond the main conversion.
   template <typename FnT, typename T = typename llvm::function_traits<
                               std::decay_t<FnT>>::template arg_t<1>>
   void addSourceMaterialization(FnT &&callback) {
     sourceMaterializations.emplace_back(
         wrapMaterialization<T>(std::forward<FnT>(callback)));
   }
   /// This method registers a materialization that will be called when
-  /// converting type from an illegal, or source, type to a legal type.
+  /// converting an illegal (source) value to a legal (target) type.
   template <typename FnT, typename T = typename llvm::function_traits<
                               std::decay_t<FnT>>::template arg_t<1>>
   void addTargetMaterialization(FnT &&callback) {