[mlir][Transforms] Dialect conversion: Align handling of dropped values

Handle dropped block arguments and dropped op results in the same way: build a source materialization (that may fold away if unused). This simplifies the code base a bit and makes it possible to merge `legalizeConvertedArgumentTypes` and `legalizeConvertedOpResultTypes` in a future commit. These two functions are almost doing the same thing now.

This commit also fixes a bug where circular materializations were built, e.g.:
```
%0 = "builtin.unrealized_conversion_cast"(%1) : (!a) -> !b
%1 = "builtin.unrealized_conversion_cast"(%0) : (!b) -> !a
// No further uses of %0, %1.
```

This happened when:
1. An op was erased. (No replacement values provided.)
2. A conversion pattern for another op builds a replacement value (first cast op) during `remapValues`, but that SSA value is not used during the pattern application.
3. During the finalization phase, `legalizeConvertedOpResultTypes` thinks that the erased op is alive because of the cast op that was built in Step 2. It builds a cast from that replacement value to the original type.
4. During the commit phase, all uses of the original op are repalced with the casted value produced in Step 3. We have generated circular IR.

mlir/lib/Transforms/Utils/DialectConversion.cpp

@@ -930,6 +930,8 @@ struct ConversionPatternRewriterImpl : public RewriterBase::Listener {
   /// to modify/access them is invalid rewriter API usage.
   SetVector<Operation *> replacedOps;
 
+  DenseSet<Operation *> unresolvedMaterializations;
+
   /// The current type converter, or nullptr if no type converter is currently
   /// active.
   const TypeConverter *currentTypeConverter = nullptr;
@@ -1055,6 +1057,7 @@ void UnresolvedMaterializationRewrite::rollback() {
     for (Value input : op->getOperands())
       rewriterImpl.mapping.erase(input);
   }
+  rewriterImpl.unresolvedMaterializations.erase(op);
   op->erase();
 }
 
@@ -1341,6 +1344,7 @@ Value ConversionPatternRewriterImpl::buildUnresolvedMaterialization(
   builder.setInsertionPoint(ip.getBlock(), ip.getPoint());
   auto convertOp =
       builder.create<UnrealizedConversionCastOp>(loc, outputType, inputs);
+  unresolvedMaterializations.insert(convertOp);
   appendRewrite<UnresolvedMaterializationRewrite>(convertOp, converter, kind);
   return convertOp.getResult(0);
 }
@@ -1379,9 +1383,21 @@ void ConversionPatternRewriterImpl::notifyOpReplaced(Operation *op,
   // Create mappings for each of the new result values.
   for (auto [newValue, result] : llvm::zip(newValues, op->getResults())) {
     if (!newValue) {
-      resultChanged = true;
-      continue;
+      // This result was dropped and no replacement value was provided.
+      if (unresolvedMaterializations.contains(op)) {
+        // Do not create another materializations if we are erasing a
+        // materialization.
+        resultChanged = true;
+        continue;
+      }
+
+      // Materialize a replacement value "out of thin air".
+      newValue = buildUnresolvedMaterialization(
+          MaterializationKind::Source, computeInsertPoint(result),
+          result.getLoc(), /*inputs=*/ValueRange(),
+          /*outputType=*/result.getType(), currentTypeConverter);
     }
+
     // Remap, and check for any result type changes.
     mapping.map(result, newValue);
     resultChanged |= (newValue.getType() != result.getType());
@@ -2359,11 +2375,6 @@ struct OperationConverter {
       ConversionPatternRewriterImpl &rewriterImpl,
       DenseMap<Value, SmallVector<Value>> &inverseMapping);
 
-  /// Legalize an operation result that was marked as "erased".
-  LogicalResult
-  legalizeErasedResult(Operation *op, OpResult result,
-                       ConversionPatternRewriterImpl &rewriterImpl);
-
   /// Dialect conversion configuration.
   ConversionConfig config;
 
@@ -2500,26 +2511,18 @@ LogicalResult OperationConverter::legalizeConvertedOpResultTypes(
       continue;
     Operation *op = opReplacement->getOperation();
     for (OpResult result : op->getResults()) {
-      Value newValue = rewriterImpl.mapping.lookupOrNull(result);
-
-      // If the operation result was replaced with null, all of the uses of this
-      // value should be replaced.
-      if (!newValue) {
-        if (failed(legalizeErasedResult(op, result, rewriterImpl)))
-          return failure();
+      // If the type of this op result changed and the result is still live,
+      // we need to materialize a conversion.
+      if (rewriterImpl.mapping.lookupOrNull(result, result.getType()))
         continue;
-      }
-
-      // Otherwise, check to see if the type of the result changed.
-      if (result.getType() == newValue.getType())
-        continue;
-
       Operation *liveUser =
           findLiveUserOfReplaced(result, rewriterImpl, inverseMapping);
       if (!liveUser)
         continue;
 
       // Legalize this result.
+      Value newValue = rewriterImpl.mapping.lookupOrNull(result);
+      assert(newValue && "replacement value not found");
       Value castValue = rewriterImpl.buildUnresolvedMaterialization(
           MaterializationKind::Source, computeInsertPoint(result), op->getLoc(),
           /*inputs=*/newValue, /*outputType=*/result.getType(),
@@ -2850,25 +2853,6 @@ LogicalResult OperationConverter::legalizeUnresolvedMaterializations(
   return success();
 }
 
-LogicalResult OperationConverter::legalizeErasedResult(
-    Operation *op, OpResult result,
-    ConversionPatternRewriterImpl &rewriterImpl) {
-  // If the operation result was replaced with null, all of the uses of this
-  // value should be replaced.
-  auto liveUserIt = llvm::find_if_not(result.getUsers(), [&](Operation *user) {
-    return rewriterImpl.isOpIgnored(user);
-  });
-  if (liveUserIt != result.user_end()) {
-    InFlightDiagnostic diag = op->emitError("failed to legalize operation '")
-                              << op->getName() << "' marked as erased";
-    diag.attachNote(liveUserIt->getLoc())
-        << "found live user of result #" << result.getResultNumber() << ": "
-        << *liveUserIt;
-    return failure();
-  }
-  return success();
-}
-
 //===----------------------------------------------------------------------===//
 // Reconcile Unrealized Casts
 //===----------------------------------------------------------------------===//

mlir/test/Transforms/test-legalize-erased-op-with-uses.mlir

@@ -3,8 +3,7 @@
 // Test that an error is emitted when an operation is marked as "erased", but
 // has users that live across the conversion.
 func.func @remove_all_ops(%arg0: i32) -> i32 {
-  // expected-error@below {{failed to legalize operation 'test.illegal_op_a' marked as erased}}
+  // expected-error@below {{failed to legalize unresolved materialization from () to 'i32' that remained live after conversion}}
   %0 = "test.illegal_op_a"() : () -> i32
-  // expected-note@below {{found live user of result #0: func.return %0 : i32}}
   return %0 : i32
 }