[MLIR][OpenMP] Make omp.distribute into a loop wrapper (#87239)

This patch updates the definition of `omp.distribute` to enforce the
restrictions of a wrapper operation.

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

@@ -840,7 +840,8 @@ def YieldOp : OpenMP_Op<"yield",
 //===----------------------------------------------------------------------===//
 def DistributeOp : OpenMP_Op<"distribute", [AttrSizedOperandSegments,
                              DeclareOpInterfaceMethods<LoopWrapperInterface>,
-                             RecursiveMemoryEffects]> {
+                             RecursiveMemoryEffects,
+                             SingleBlockImplicitTerminator<"TerminatorOp">]> {
   let summary = "distribute construct";
   let description = [{
     The distribute construct specifies that the iterations of one or more loops
@@ -855,15 +856,28 @@ def DistributeOp : OpenMP_Op<"distribute", [AttrSizedOperandSegments,
     The distribute loop construct specifies that the iterations of the loop(s)
     will be executed in parallel by threads in the current context. These
     iterations are spread across threads that already exist in the enclosing
-    region. The lower and upper bounds specify a half-open range: the
-    range includes the lower bound but does not include the upper bound. If the
-    `inclusive` attribute is specified then the upper bound is also included.
+    region.
+
+    The body region can contain a single block which must contain a single
+    operation and a terminator. The operation must be another compatible loop
+    wrapper or an `omp.loop_nest`.
 
     The `dist_schedule_static` attribute specifies the  schedule for this
     loop, determining how the loop is distributed across the parallel threads.
     The optional `schedule_chunk` associated with this determines further
     controls this distribution.
 
+    ```mlir
+    omp.distribute <clauses> {
+      omp.loop_nest (%i1, %i2) : index = (%c0, %c0) to (%c10, %c10) step (%c1, %c1) {
+        %a = load %arrA[%i1, %i2] : memref<?x?xf32>
+        %b = load %arrB[%i1, %i2] : memref<?x?xf32>
+        %sum = arith.addf %a, %b : f32
+        store %sum, %arrC[%i1, %i2] : memref<?x?xf32>
+        omp.yield
+      }
+    }
+    ```
     // TODO: private_var, firstprivate_var, lastprivate_var, collapse
   }];
   let arguments = (ins

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

@@ -1656,6 +1656,17 @@ LogicalResult DistributeOp::verify() {
     return emitError(
         "expected equal sizes for allocate and allocator variables");
 
+  if (!isWrapper())
+    return emitOpError() << "must be a loop wrapper";
+
+  if (LoopWrapperInterface nested = getNestedWrapper()) {
+    // Check for the allowed leaf constructs that may appear in a composite
+    // construct directly after DISTRIBUTE.
+    if (!isa<ParallelOp, SimdLoopOp>(nested))
+      return emitError() << "only supported nested wrappers are 'omp.parallel' "
+                            "and 'omp.simdloop'";
+  }
+
   return success();
 }
 

mlir/test/Dialect/OpenMP/invalid.mlir

@@ -1866,7 +1866,16 @@ func.func @omp_target_depend(%data_var: memref<i32>) {
 
 // -----
 
-func.func @omp_distribute(%data_var : memref<i32>) -> () {
+func.func @omp_distribute_schedule(%chunk_size : i32) -> () {
+  // expected-error @below {{op chunk size set without dist_schedule_static being present}}
+  "omp.distribute"(%chunk_size) <{operandSegmentSizes = array<i32: 1, 0, 0>}> ({
+      "omp.terminator"() : () -> ()
+    }) : (i32) -> ()
+}
+
+// -----
+
+func.func @omp_distribute_allocate(%data_var : memref<i32>) -> () {
   // expected-error @below {{expected equal sizes for allocate and allocator variables}}
   "omp.distribute"(%data_var) <{operandSegmentSizes = array<i32: 0, 1, 0>}> ({
       "omp.terminator"() : () -> ()
@@ -1875,6 +1884,29 @@ func.func @omp_distribute(%data_var : memref<i32>) -> () {
 
 // -----
 
+func.func @omp_distribute_wrapper() -> () {
+  // expected-error @below {{op must be a loop wrapper}}
+  "omp.distribute"() ({
+      %0 = arith.constant 0 : i32
+      "omp.terminator"() : () -> ()
+    }) : () -> ()
+}
+
+// -----
+
+func.func @omp_distribute_nested_wrapper(%data_var : memref<i32>) -> () {
+  // expected-error @below {{only supported nested wrappers are 'omp.parallel' and 'omp.simdloop'}}
+  "omp.distribute"() ({
+      "omp.wsloop"() ({
+        %0 = arith.constant 0 : i32
+        "omp.terminator"() : () -> ()
+      }) : () -> ()
+      "omp.terminator"() : () -> ()
+    }) : () -> ()
+}
+
+// -----
+
 omp.private {type = private} @x.privatizer : i32 alloc {
 ^bb0(%arg0: i32):
   %0 = arith.constant 0.0 : f32

mlir/test/Dialect/OpenMP/ops.mlir

@@ -559,30 +559,54 @@ func.func @omp_simdloop_pretty_multiple(%lb1 : index, %ub1 : index, %step1 : ind
 }
 
 // CHECK-LABEL: omp_distribute
-func.func @omp_distribute(%chunk_size : i32, %data_var : memref<i32>) -> () {
+func.func @omp_distribute(%chunk_size : i32, %data_var : memref<i32>, %arg0 : i32) -> () {
   // CHECK: omp.distribute
   "omp.distribute" () ({
-    omp.terminator
+    "omp.loop_nest" (%arg0, %arg0, %arg0) ({
+    ^bb0(%iv: i32):
+      "omp.yield"() : () -> ()
+    }) : (i32, i32, i32) -> ()
+    "omp.terminator"() : () -> ()
   }) {} : () -> ()
   // CHECK: omp.distribute
   omp.distribute {
-    omp.terminator
+    omp.loop_nest (%iv) : i32 = (%arg0) to (%arg0) step (%arg0) {
+      omp.yield
+    }
   }
   // CHECK: omp.distribute dist_schedule_static
   omp.distribute dist_schedule_static {
-    omp.terminator
+    omp.loop_nest (%iv) : i32 = (%arg0) to (%arg0) step (%arg0) {
+      omp.yield
+    }
   }
   // CHECK: omp.distribute dist_schedule_static chunk_size(%{{.+}} : i32)
   omp.distribute dist_schedule_static chunk_size(%chunk_size : i32) {
-    omp.terminator
+    omp.loop_nest (%iv) : i32 = (%arg0) to (%arg0) step (%arg0) {
+      omp.yield
+    }
   }
   // CHECK: omp.distribute order(concurrent)
   omp.distribute order(concurrent) {
-    omp.terminator
+    omp.loop_nest (%iv) : i32 = (%arg0) to (%arg0) step (%arg0) {
+      omp.yield
+    }
   }
   // CHECK: omp.distribute allocate(%{{.+}} : memref<i32> -> %{{.+}} : memref<i32>)
   omp.distribute allocate(%data_var : memref<i32> -> %data_var : memref<i32>) {
-    omp.terminator
+    omp.loop_nest (%iv) : i32 = (%arg0) to (%arg0) step (%arg0) {
+      omp.yield
+    }
+  }
+  // CHECK: omp.distribute
+  omp.distribute {
+    // TODO Remove induction variables from omp.simdloop.
+    omp.simdloop for (%iv) : i32 = (%arg0) to (%arg0) step (%arg0) {
+      omp.loop_nest (%iv2) : i32 = (%arg0) to (%arg0) step (%arg0) {
+        omp.yield
+      }
+      omp.yield
+    }
   }
 return
 }