[MLIR][OpenMP] Document entry block argument-defining clauses (NFC) (#109811)

This patch adds general information on the proposed approach to unify
the handling and representation of clauses that define entry block
arguments attached to operations that accept them.

Author: skatrak

mlir/docs/Dialects/OpenMPDialect/_index.md

@@ -132,7 +132,8 @@ and optional list of `traits`, a list of `clauses` where all the applicable
 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.
+through the inherited `clausesDescription` property. By convention, the list of
+clauses for an operation must be specified in alphabetical order.
 
 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
@@ -285,7 +286,76 @@ argument's type:
   specific `mlir::Attribute` subclass) will be used instead.
   - Other attribute types will be represented with their `storageType`.
 - It will create `<Name>Operands` structure for each operation, which is an
-empty structure subclassing all operand structures defined for the corresponding `OpenMP_Op`'s clauses.
+empty structure subclassing all operand structures defined for the corresponding
+`OpenMP_Op`'s clauses.
+
+### Entry Block Argument-Defining Clauses
+
+In their MLIR representation, certain OpenMP clauses introduce a mapping between
+values defined outside the operation they are applied to and entry block
+arguments for the region of that MLIR operation. This enables, for example, the
+introduction of private copies of the same underlying variable defined outside
+the MLIR operation the clause is attached to. Currently, clauses with this
+property can be classified into three main categories:
+  - Map-like clauses: `map`, `use_device_addr` and `use_device_ptr`.
+  - Reduction-like clauses: `in_reduction`, `reduction` and `task_reduction`.
+  - Privatization clauses: `private`.
+
+All three kinds of entry block argument-defining clauses use a similar custom
+assembly format representation, only differing based on the different pieces of
+information attached to each kind. Below, one example of each is shown:
+
+```mlir
+omp.target map_entries(%x -> %x.m, %y -> %y.m : !llvm.ptr, !llvm.ptr) {
+  // Use %x.m, %y.m in place of %x and %y...
+}
+
+omp.wsloop reduction(@add.i32 %x -> %x.r, byref @add.f32 %y -> %y.r : !llvm.ptr, !llvm.ptr) {
+  // Use %x.r, %y.r in place of %x and %y...
+}
+
+omp.parallel private(@x.privatizer %x -> %x.p, @y.privatizer %y -> %y.p : !llvm.ptr, !llvm.ptr) {
+  // Use %x.p, %y.p in place of %x and %y...
+}
+```
+
+As a consequence of parsing and printing the operation's first region entry
+block argument names together with the custom assembly format of these clauses,
+entry block arguments (i.e. the `^bb0(...):` line) must not be explicitly
+defined for these operations. Additionally, it is not possible to implement this
+feature while allowing each clause to be independently parsed and printed,
+because they need to be printed/parsed together with the corresponding
+operation's first region. They must have a well-defined ordering in which
+multiple of these clauses are specified for a given operation, as well.
+
+The parsing/printing of these clauses together with the region provides the
+ability to define entry block arguments directly after the `->`. Forcing a
+specific ordering between these clauses makes the block argument ordering
+well-defined, which is the property used to easily match each clause with the
+entry block arguments defined by it.
+
+Custom printers and parsers for operation regions based on the entry block
+argument-defining clauses they take are implemented based on the
+`{parse,print}BlockArgRegion` functions, which take care of the sorting and
+formatting of each kind of clause, minimizing code duplication resulting from
+this approach. One example of the custom assembly format of an operation taking
+the `private` and `reduction` clauses is the following:
+
+```tablegen
+let assemblyFormat = clausesAssemblyFormat # [{
+  custom<PrivateReductionRegion>($region, $private_vars, type($private_vars),
+      $private_syms, $reduction_vars, type($reduction_vars), $reduction_byref,
+      $reduction_syms) attr-dict
+}];
+```
+
+The `BlockArgOpenMPOpInterface` has been introduced to simplify the addition and
+handling of these kinds of clauses. It holds `num<ClauseName>BlockArgs()`
+functions that by default return 0, to be overriden by each clause through the
+`extraClassDeclaration` property. Based on these functions and the expected
+alphabetical sorting between entry block argument-defining clauses, it
+implements `get<ClauseName>BlockArgs()` functions that are the intended method
+of accessing clause-defined block arguments.
 
 ## Loop-Associated Directives