[mlir] Print aliases for recursive types (#110346)

We're already keeping track of the alias depth to ensure that aliases
are printed before they're referenced. For recursive types, we can
additionally track whether an alias has been printed and only reference
it if so, to lift the restrictions on not printing aliases inside
mutable types.

Author: smeenai

mlir/lib/IR/AsmPrinter.cpp

@@ -545,6 +545,10 @@ class SymbolAlias {
   bool isType : 1;
   /// A flag indicating whether this alias may be deferred or not.
   bool isDeferrable : 1;
+
+public:
+  /// Used to avoid printing incomplete aliases for recursive types.
+  bool isPrinted = false;
 };
 
 /// This class represents a utility that initializes the set of attribute and
@@ -1222,6 +1226,8 @@ LogicalResult AliasState::getAlias(Type ty, raw_ostream &os) const {
   const auto *it = attrTypeToAlias.find(ty.getAsOpaquePointer());
   if (it == attrTypeToAlias.end())
     return failure();
+  if (!it->second.isPrinted)
+    return failure();
 
   it->second.print(os);
   return success();
@@ -1238,12 +1244,9 @@ void AliasState::printAliases(AsmPrinter::Impl &p, NewLineCounter &newLine,
     p.getStream() << " = ";
 
     if (alias.isTypeAlias()) {
-      // TODO: Support nested aliases in mutable types.
       Type type = Type::getFromOpaquePointer(opaqueSymbol);
-      if (type.hasTrait<TypeTrait::IsMutable>())
-        p.getStream() << type;
-      else
-        p.printTypeImpl(type);
+      p.printTypeImpl(type);
+      alias.isPrinted = true;
     } else {
       // TODO: Support nested aliases in mutable attributes.
       Attribute attr = Attribute::getFromOpaquePointer(opaqueSymbol);

mlir/test/IR/recursive-type.mlir

@@ -2,7 +2,10 @@
 
 // CHECK: !testrec = !test.test_rec<type_to_alias, test_rec<type_to_alias>>
 // CHECK: ![[$NAME:.*]] = !test.test_rec_alias<name, !test.test_rec_alias<name>>
+// CHECK: ![[$NAME5:.*]] = !test.test_rec_alias<name5, !test.test_rec_alias<name3, !test.test_rec_alias<name4, !test.test_rec_alias<name5>>>>
 // CHECK: ![[$NAME2:.*]] = !test.test_rec_alias<name2, tuple<!test.test_rec_alias<name2>, i32>>
+// CHECK: ![[$NAME4:.*]] = !test.test_rec_alias<name4, !name5_>
+// CHECK: ![[$NAME3:.*]] = !test.test_rec_alias<name3, !name4_>
 
 // CHECK-LABEL: @roundtrip
 func.func @roundtrip() {
@@ -24,6 +27,14 @@ func.func @roundtrip() {
   // CHECK: () -> ![[$NAME2]]
   "test.dummy_op_for_roundtrip"() : () -> !test.test_rec_alias<name2, tuple<!test.test_rec_alias<name2>, i32>>
   "test.dummy_op_for_roundtrip"() : () -> !test.test_rec_alias<name2, tuple<!test.test_rec_alias<name2>, i32>>
+
+  // Mutual recursion.
+  // CHECK: () -> ![[$NAME3]]
+  // CHECK: () -> ![[$NAME4]]
+  // CHECK: () -> ![[$NAME5]]
+  "test.dummy_op_for_roundtrip"() : () -> !test.test_rec_alias<name3, !test.test_rec_alias<name4, !test.test_rec_alias<name5, !test.test_rec_alias<name3>>>>
+  "test.dummy_op_for_roundtrip"() : () -> !test.test_rec_alias<name4, !test.test_rec_alias<name5, !test.test_rec_alias<name3, !test.test_rec_alias<name4>>>>
+  "test.dummy_op_for_roundtrip"() : () -> !test.test_rec_alias<name5, !test.test_rec_alias<name3, !test.test_rec_alias<name4, !test.test_rec_alias<name5>>>>
   return
 }