[mlir][OpFormatGen] Add support for eliding UnitAttr when used to anchor an optional group

Unit attributes are given meaning by their existence, and thus have no meaningful value beyond "is it present". As such, in the format of an operation unit attributes are generally used to guard the printing of other elements and aren't generally printed themselves; as the presence of the group when parsing means that the unit attribute should be added. This revision adds support to the declarative format for eliding unit attributes in situations where they anchor an optional group, but aren't the first element.

For example,
```
let assemblyFormat = "(`is_optional` $unit_attr^)? attr-dict";
```

would print `foo.op is_optional` when $unit_attr is present, instead of the current `foo.op is_optional unit`.

Differential Revision: https://reviews.llvm.org/D84577

mlir/docs/OpDefinitions.md

@@ -733,7 +733,7 @@ information. An optional group is defined by wrapping a set of elements within
 An example of an operation with an optional group is `std.return`, which has a
 variadic number of operands.
 
-```
+```tablegen
 def ReturnOp : ... {
   let arguments = (ins Variadic<AnyType>:$operands);
 
@@ -743,6 +743,36 @@ def ReturnOp : ... {
 }
 ```
 
+##### Unit Attributes
+
+In MLIR, the [`unit` Attribute](LangRef.md#unit-attribute) is special in that it
+only has one possible value, i.e. it derives meaning from its existence. When a
+unit attribute is used to anchor an optional group and is not the first element
+of the group, the presence of the unit attribute can be directly correlated with
+the presence of the optional group itself. As such, in these situations the unit
+attribute will not be printed or present in the output and will be automatically
+inferred when parsing by the presence of the optional group itself.
+
+For example, the following operation:
+
+```tablegen
+def FooOp : ... {
+  let arguments = (ins UnitAttr:$is_read_only);
+
+  let assemblyFormat = "attr-dict (`is_read_only` $is_read_only^)?";
+}
+```
+
+would be formatted as such:
+
+```mlir
+// When the unit attribute is present:
+foo.op is_read_only
+
+// When the unit attribute is not present:
+foo.op
+```
+
 #### Requirements
 
 The format specification has a certain set of requirements that must be adhered

mlir/test/lib/Dialect/Test/TestOps.td

@@ -1391,6 +1391,17 @@ def FormatInferVariadicTypeFromNonVariadic
   let assemblyFormat = "$operands attr-dict `:` type($result)";
 }
 
+def FormatOptionalUnitAttr : TEST_Op<"format_optional_unit_attribute"> {
+  let arguments = (ins UnitAttr:$is_optional);
+  let assemblyFormat = "(`is_optional` $is_optional^)? attr-dict";
+}
+
+def FormatOptionalUnitAttrNoElide
+    : TEST_Op<"format_optional_unit_attribute_no_elide"> {
+  let arguments = (ins UnitAttr:$is_optional);
+  let assemblyFormat = "($is_optional^)? attr-dict";
+}
+
 //===----------------------------------------------------------------------===//
 // AllTypesMatch type inference
 //===----------------------------------------------------------------------===//

mlir/test/mlir-tblgen/op-format.mlir

@@ -82,6 +82,20 @@ test.format_operand_e_op %i64, %memref : i64, memref<1xf64>
 
 }) { arg_names = ["i", "j", "k"] } : () -> ()
 
+//===----------------------------------------------------------------------===//
+// Format optional attributes
+//===----------------------------------------------------------------------===//
+
+// CHECK: test.format_optional_unit_attribute is_optional
+test.format_optional_unit_attribute is_optional
+
+// CHECK: test.format_optional_unit_attribute
+// CHECK-NOT: is_optional
+test.format_optional_unit_attribute
+
+// CHECK: test.format_optional_unit_attribute_no_elide unit
+test.format_optional_unit_attribute_no_elide unit
+
 //===----------------------------------------------------------------------===//
 // Format optional operands and results
 //===----------------------------------------------------------------------===//

mlir/tools/mlir-tblgen/OpFormatGen.cpp

@@ -107,6 +107,11 @@ struct AttributeVariable
     Optional<Type> attrType = var->attr.getValueType();
     return attrType ? attrType->getBuilderCall() : llvm::None;
   }
+
+  /// Return if this attribute refers to a UnitAttr.
+  bool isUnitAttr() const {
+    return var->attr.getBaseAttr().getAttrDefName() == "UnitAttr";
+  }
 };
 
 /// This class represents a variable that refers to an operand argument.
@@ -645,9 +650,23 @@ static void genElementParser(Element *element, OpMethodBody &body,
       body << "  if (!" << opVar->getVar()->name << "Operands.empty()) {\n";
     }
 
+    // If the anchor is a unit attribute, we don't need to print it. When
+    // parsing, we will add this attribute if this group is present.
+    Element *elidedAnchorElement = nullptr;
+    auto *anchorAttr = dyn_cast<AttributeVariable>(optional->getAnchor());
+    if (anchorAttr && anchorAttr != firstElement && anchorAttr->isUnitAttr()) {
+      elidedAnchorElement = anchorAttr;
+
+      // Add the anchor unit attribute to the operation state.
+      body << "    result.addAttribute(\"" << anchorAttr->getVar()->name
+           << "\", parser.getBuilder().getUnitAttr());\n";
+    }
+
     // Generate the rest of the elements normally.
-    for (auto &childElement : llvm::drop_begin(elements, 1))
-      genElementParser(&childElement, body, attrTypeCtx);
+    for (Element &childElement : llvm::drop_begin(elements, 1)) {
+      if (&childElement != elidedAnchorElement)
+        genElementParser(&childElement, body, attrTypeCtx);
+    }
     body << "  }\n";
 
     /// Literals.
@@ -1058,10 +1077,23 @@ static void genElementPrinter(Element *element, OpMethodBody &body,
            << cast<AttributeVariable>(anchor)->getVar()->name << "\")) {\n";
     }
 
+    // If the anchor is a unit attribute, we don't need to print it. When
+    // parsing, we will add this attribute if this group is present.
+    auto elements = optional->getElements();
+    Element *elidedAnchorElement = nullptr;
+    auto *anchorAttr = dyn_cast<AttributeVariable>(anchor);
+    if (anchorAttr && anchorAttr != &*elements.begin() &&
+        anchorAttr->isUnitAttr()) {
+      elidedAnchorElement = anchorAttr;
+    }
+
     // Emit each of the elements.
-    for (Element &childElement : optional->getElements())
-      genElementPrinter(&childElement, body, fmt, op, shouldEmitSpace,
-                        lastWasPunctuation);
+    for (Element &childElement : elements) {
+      if (&childElement != elidedAnchorElement) {
+        genElementPrinter(&childElement, body, fmt, op, shouldEmitSpace,
+                          lastWasPunctuation);
+      }
+    }
     body << "  }\n";
     return;
   }