[MLIR][OpenMP] Improve omp.section block arguments handling

The `omp.section` operation is an outlier in that the block arguments it has
are defined by clauses on the required parent `omp.sections` operation.

This patch updates the definition of this operation introducing the
`BlockArgOpenMPOpInterface` to simplify the handling and verification of these
block arguments, implemented based on the parent `omp.sections`.

mlir/include/mlir/Dialect/OpenMP/OpenMPOps.td

@@ -207,8 +207,9 @@ def TeamsOp : OpenMP_Op<"teams", traits = [
 // 2.8.1 Sections Construct
 //===----------------------------------------------------------------------===//
 
-def SectionOp : OpenMP_Op<"section", [HasParent<"SectionsOp">],
-                          singleRegion = true> {
+def SectionOp : OpenMP_Op<"section", traits = [
+    BlockArgOpenMPOpInterface, HasParent<"SectionsOp">
+  ], singleRegion = true> {
   let summary = "section directive";
   let description = [{
     A section operation encloses a region which represents one section in a
@@ -218,6 +219,13 @@ def SectionOp : OpenMP_Op<"section", [HasParent<"SectionsOp">],
     operation. This is done to reflect situations where these block arguments
     represent variables private to each section.
   }];
+  let extraClassDeclaration = [{
+    // Override BlockArgOpenMPOpInterface methods based on the parent
+    // omp.sections operation. Only forward-declare here because SectionsOp is
+    // not completely defined at this point.
+    unsigned numPrivateBlockArgs();
+    unsigned numReductionBlockArgs();
+  }] # clausesExtraClassDeclaration;
   let assemblyFormat = "$region attr-dict";
 }
 

mlir/lib/Dialect/OpenMP/IR/OpenMPDialect.cpp

@@ -1844,6 +1844,18 @@ LogicalResult TeamsOp::verify() {
                                 getReductionByref());
 }
 
+//===----------------------------------------------------------------------===//
+// SectionOp
+//===----------------------------------------------------------------------===//
+
+unsigned SectionOp::numPrivateBlockArgs() {
+  return getParentOp().numPrivateBlockArgs();
+}
+
+unsigned SectionOp::numReductionBlockArgs() {
+  return getParentOp().numReductionBlockArgs();
+}
+
 //===----------------------------------------------------------------------===//
 // SectionsOp
 //===----------------------------------------------------------------------===//

mlir/test/Dialect/OpenMP/invalid.mlir

@@ -1584,6 +1584,31 @@ func.func @omp_sections() {
 
 // -----
 
+omp.declare_reduction @add_f32 : f32
+init {
+^bb0(%arg: f32):
+  %0 = arith.constant 0.0 : f32
+  omp.yield (%0 : f32)
+}
+combiner {
+^bb1(%arg0: f32, %arg1: f32):
+  %1 = arith.addf %arg0, %arg1 : f32
+  omp.yield (%1 : f32)
+}
+
+func.func @omp_sections(%x : !llvm.ptr) {
+  omp.sections reduction(@add_f32 %x -> %arg0 : !llvm.ptr) {
+    // expected-error @below {{op expected at least 1 entry block argument(s)}}
+    omp.section {
+      omp.terminator
+    }
+    omp.terminator
+  }
+  return
+}
+
+// -----
+
 func.func @omp_single(%data_var : memref<i32>) -> () {
   // expected-error @below {{expected equal sizes for allocate and allocator variables}}
   "omp.single" (%data_var) ({

mlir/test/Dialect/OpenMP/ops.mlir

@@ -1127,11 +1127,13 @@ func.func @sections_reduction() {
   omp.sections reduction(@add_f32 %0 -> %arg0 : !llvm.ptr) {
     // CHECK: omp.section
     omp.section {
+    ^bb0(%arg1 : !llvm.ptr):
       %1 = arith.constant 2.0 : f32
       omp.terminator
     }
     // CHECK: omp.section
     omp.section {
+    ^bb0(%arg1 : !llvm.ptr):
       %1 = arith.constant 3.0 : f32
       omp.terminator
     }
@@ -1148,11 +1150,13 @@ func.func @sections_reduction_byref() {
   omp.sections reduction(byref @add_f32 %0 -> %arg0 : !llvm.ptr) {
     // CHECK: omp.section
     omp.section {
+    ^bb0(%arg1 : !llvm.ptr):
       %1 = arith.constant 2.0 : f32
       omp.terminator
     }
     // CHECK: omp.section
     omp.section {
+    ^bb0(%arg1 : !llvm.ptr):
       %1 = arith.constant 3.0 : f32
       omp.terminator
     }
@@ -1246,10 +1250,12 @@ func.func @sections_reduction2() {
   // CHECK: omp.sections reduction(@add2_f32 %{{.+}} -> %{{.+}} : memref<1xf32>)
   omp.sections reduction(@add2_f32 %0 -> %arg0 : memref<1xf32>) {
     omp.section {
+    ^bb0(%arg1 : !llvm.ptr):
       %1 = arith.constant 2.0 : f32
       omp.terminator
     }
     omp.section {
+    ^bb0(%arg1 : !llvm.ptr):
       %1 = arith.constant 2.0 : f32
       omp.terminator
     }