[MLIR][OpenMP][Docs] Document clause-based op representation (NFC) (llvm#107234)

This patch documents the clause-based op represetation discussed in
[this
RFC](https://discourse.llvm.org/t/rfc-clause-based-representation-of-openmp-dialect-operations/79053).

Author: skatrak

mlir/docs/Dialects/OpenMPDialect/_index.md

@@ -41,3 +41,199 @@ mapping information is named `MapInfoOp` / `omp.map.info`. The same rules are
 followed if multiple operations are created for different variants of the same
 directive, e.g. `atomic` becomes `Atomic{Read,Write,Update,Capture}Op` /
 `omp.atomic.{read,write,update,capture}`.
+
+## Clause-Based Operation Definition
+
+One main feature of the OpenMP specification is that, even though the set of
+clauses that could be applied to a given directive is independent from other
+directives, these clauses can generally apply to multiple directives. Since
+clauses usually define which arguments the corresponding MLIR operation takes,
+it is possible (and preferred) to define OpenMP dialect operations based on the
+list of clauses taken by the corresponding directive. This makes it simpler to
+keep their representation consistent across operations and minimizes redundancy
+in the dialect.
+
+To achieve this, the base `OpenMP_Clause` tablegen class has been created. It is
+intended to be used to create clause definitions that can be then attached to
+multiple `OpenMP_Op` definitions, resulting in the latter inheriting by default
+all properties defined by clauses attached, similarly to the trait mechanism.
+This mechanism is implemented in
+[OpenMPOpBase.td](https://github.com/llvm/llvm-project/blob/main/mlir/include/mlir/Dialect/OpenMP/OpenMPOpBase.td).
+
+### Adding a Clause
+
+OpenMP clause definitions are located in
+[OpenMPClauses.td](https://github.com/llvm/llvm-project/blob/main/mlir/include/mlir/Dialect/OpenMP/OpenMPClauses.td).
+For each clause, an `OpenMP_Clause` subclass and a definition based on it must
+be created. The subclass must take a `bit` template argument for each of the
+properties it can populate on associated `OpenMP_Op`s. These must be forwarded
+to the base class. The definition must be an instantiation of the base class
+where all these template arguments are set to `false`. The definition's name
+must be `OpenMP_<Name>Clause`, whereas its base class' must be
+`OpenMP_<Name>ClauseSkip`. Following this pattern makes it possible to
+optionally skip the inheritance of some properties when defining operations:
+[more info](#overriding-clause-inherited-properties).
+
+Clauses can define the following properties:
+  - `list<Traits> traits`: To be used when having a certain clause always
+implies some op trait, like the `map` clause and the `MapClauseOwningInterface`.
+  - `dag(ins) arguments`: Mandatory property holding values and attributes
+used to represent the clause. Argument names use snake_case and should contain
+the clause name to avoid name clashes between clauses. Variadic arguments
+(non-attributes) must contain the "_vars" suffix.
+  - `string {req,opt}AssemblyFormat`: Optional formatting strings to produce
+custom human-friendly printers and parsers for arguments associated with the
+clause. It will be combined with assembly formats for other clauses as explained
+[below](#adding-an-operation).
+  - `string description`: Optional description text to describe the clause and
+its representation.
+  - `string extraClassDeclaration`: Optional C++ declarations to be added to
+operation classes including the clause.
+
+For example:
+
+```tablegen
+class OpenMP_ExampleClauseSkip<
+    bit traits = false, bit arguments = false, bit assemblyFormat = false,
+    bit description = false, bit extraClassDeclaration = false
+  > : OpenMP_Clause<traits, arguments, assemblyFormat, description,
+                    extraClassDeclaration> {
+  let arguments = (ins
+    Optional<AnyType>:$example_var
+  );
+
+  let optAssemblyFormat = [{
+    `example` `(` $example_var `:` type($example_var) `)`
+  }];
+
+  let description = [{
+    The `example_var` argument defines the variable to which the EXAMPLE clause
+    applies.
+  }];
+}
+
+def OpenMP_ExampleClause : OpenMP_ExampleClauseSkip<>;
+```
+
+### Adding an Operation
+
+Operations in the OpenMP dialect, located in
+[OpenMPOps.td](https://github.com/llvm/llvm-project/blob/main/mlir/include/mlir/Dialect/OpenMP/OpenMPOps.td),
+can be defined like any other regular operation by just specifying a `mnemonic`
+and optional list of `traits` when inheriting from `OpenMP_Op`, and then
+defining the expected `description`, `arguments`, etc. properties inside of its
+body. However, in most cases, basing the operation definition on its list of
+accepted clauses is significantly simpler because some of the properties can
+just be inherited from these clauses.
+
+In general, the way to achieve this is to specify, in addition to the `mnemonic`
+and optional list of `traits`, a list of `clauses` where all the applicable
+`OpenMP_<Name>Clause` definitions are added. Then, the only properties that
+would have to be defined in the operation's body are the `summary` and
+`description`. For the latter, only the operation itself would have to be
+defined, and the description for its clause-inherited arguments is appended
+through the inherited `clausesDescription` property.
+
+If the operation is intended to have a single region, this is better achieved by
+setting the `singleRegion=true` template argument of `OpenMP_Op` rather manually
+populating the `regions` property of the operation, because that way the default
+`assemblyFormat` is also updated correspondingly.
+
+For example:
+
+```tablegen
+def ExampleOp : OpenMP_Op<"example", traits = [
+    AttrSizedOperandSegments, ...
+  ], clauses = [
+    OpenMP_AlignedClause, OpenMP_IfClause, OpenMP_LinearClause, ...
+  ], singleRegion = true> {
+  let summary = "example construct";
+  let description = [{
+    The example construct represents...
+  }] # clausesDescription;
+}
+```
+
+This is possible because the `arguments`, `assemblyFormat` and
+`extraClassDeclaration` properties of the operation are by default
+populated by concatenating the corresponding properties of the clauses on the
+list. In the case of the `assemblyFormat`, this involves combining the
+`reqAssemblyFormat` and the `optAssemblyFormat` properties. The
+`reqAssemblyFormat` of all clauses is concatenated first and separated using
+spaces, whereas the `optAssemblyFormat` is wrapped in an `oilist()` and
+interleaved with "|" instead of spaces. The resulting `assemblyFormat` contains
+the required assembly format strings, followed by the optional assembly format
+strings, optionally the `$region` and the `attr-dict`.
+
+### Overriding Clause-Inherited Properties
+
+Although the clause-based definition of operations can greatly reduce work, it's
+also somewhat restrictive, since there may be some situations where only part of
+the operation definition can be automated in that manner. For a fine-grained
+control over properties inherited from each clause two features are available:
+
+  - Inhibition of properties. By using `OpenMP_<Name>ClauseSkip` tablegen
+classes, the list of properties copied from the clause to the operation can be
+selected. For example, `OpenMP_IfClauseSkip<assemblyFormat = true>` would result
+in every property defined for the `OpenMP_IfClause` except for the
+`assemblyFormat` being used to initially populate the properties of the
+operation.
+  - Augmentation of properties. There are times when there is a need to add to
+a clause-populated operation property. Instead of overriding the property in the
+definition of the operation and having to manually replicate what would
+otherwise be automatically populated before adding to it, some internal
+properties are defined to hold this default value: `clausesArgs`,
+`clausesAssemblyFormat`, `clauses{Req,Opt}AssemblyFormat` and
+`clausesExtraClassDeclaration`.
+
+In the following example, assuming both the `OpenMP_InReductionClause` and the
+`OpenMP_ReductionClause` define a `getReductionVars` extra class declaration,
+we skip the conflicting `extraClassDeclaration`s inherited by both clauses and
+provide another implementation, without having to also re-define other
+declarations inherited from the `OpenMP_AllocateClause`:
+
+```tablegen
+def ExampleOp : OpenMP_Op<"example", traits = [
+    AttrSizedOperandSegments, ...
+  ], clauses = [
+    OpenMP_AllocateClause,
+    OpenMP_InReductionClauseSkip<extraClassDeclaration = true>,
+    OpenMP_ReductionClauseSkip<extraClassDeclaration = true>
+  ], singleRegion = true> {
+  let summary = "example construct";
+  let description = [{
+    This operation represents...
+  }] # clausesDescription;
+
+  // Override the clause-populated extraClassDeclaration and add the default
+  // back via appending clausesExtraClassDeclaration to it. This has the effect
+  // of adding one declaration. Since this property is skipped for the
+  // InReduction and Reduction clauses, clausesExtraClassDeclaration won't
+  // incorporate the definition of this property for these clauses.
+  let extraClassDeclaration = [{
+    SmallVector<Value> getReductionVars() {
+      // Concatenate inReductionVars and reductionVars and return the result...
+    }
+  }] # clausesExtraClassDeclaration;
+}
+```
+
+These features are intended for complex edge cases, but an effort should be made
+to avoid having to use them, since they may introduce inconsistencies and
+complexity to the dialect.
+
+### Tablegen Verification Pass
+
+As a result of the implicit way in which fundamental properties of MLIR
+operations are populated following this approach, and the ability to override
+them, forgetting to append clause-inherited values might result in hard to debug
+tablegen errors.
+
+For this reason, the `-verify-openmp-ops` tablegen pseudo-backend was created.
+It runs before any other tablegen backends are triggered for the
+[OpenMPOps.td](https://github.com/llvm/llvm-project/blob/main/mlir/include/mlir/Dialect/OpenMP/OpenMPOps.td)
+file and warns any time a property defined for a clause is not found in the
+corresponding operation, except if it is explicitly skipped as described
+[above](#overriding-clause-inherited-properties). This way, in case of a later
+tablegen failure while processing OpenMP dialect operations, earlier messages
+triggered by that pass can point to a likely solution.