[mlir][OpenACC] add unstructured attributes for acc.loop with early exits (#164990)

"!$acc loop" directive may be placed above loops with early exits.

Currently flang lowers loop with early exits to explicit control flow
(this may be revisited when MLIR allows early exits in structured
region). The acc loop directive cannot simply be ignored in such case in
lowering because it may hold data clauses that should be applied when
reaching that point.

This patch adds an "unstructured" attribute to acc.loop to support that
case.
An acc.loop with such attributes may hold data operands but must have no
controls. It is expected that the loop logic is implemented in its body
in a way that the acc dialect may not understand.

Such acc.loop is just a container and the loop with early exit will be
executed sequentially.

Author: jeanPerier

mlir/include/mlir/Dialect/OpenACC/OpenACCOps.td

@@ -2557,6 +2557,12 @@ def OpenACC_LoopOp : OpenACC_Op<"loop",
     device-type-aware getter methods. When modifying these operands, the
     corresponding `device_type` attributes must be updated to maintain
     consistency between operands and their target device types.
+
+    The `unstructured` attribute indicates that the loops inside the OpenACC
+    construct contain early exits and cannot be lowered to structured MLIR
+    operations. When this flag is set, the acc.loop should have no induction
+    variables and the loop must be implemented via explicit control flow
+    inside its body.
   }];
 
   let arguments = (ins
@@ -2590,7 +2596,8 @@ def OpenACC_LoopOp : OpenACC_Op<"loop",
       OptionalAttr<SymbolRefArrayAttr>:$firstprivatizationRecipes,
       Variadic<AnyType>:$reductionOperands,
       OptionalAttr<SymbolRefArrayAttr>:$reductionRecipes,
-      OptionalAttr<OpenACC_CombinedConstructsAttr>:$combined
+      OptionalAttr<OpenACC_CombinedConstructsAttr>:$combined,
+      UnitAttr:$unstructured
   );
 
   let results = (outs Variadic<AnyType>:$results);

mlir/lib/Dialect/OpenACC/IR/OpenACC.cpp

@@ -3068,8 +3068,12 @@ LogicalResult acc::LoopOp::verify() {
   if (getRegion().empty())
     return emitError("expected non-empty body.");
 
-  // When it is container-like - it is expected to hold a loop-like operation.
-  if (isContainerLike()) {
+  if (getUnstructured()) {
+    if (!isContainerLike())
+      return emitError(
+          "unstructured acc.loop must not have induction variables");
+  } else if (isContainerLike()) {
+    // When it is container-like - it is expected to hold a loop-like operation.
     // Obtain the maximum collapse count - we use this to check that there
     // are enough loops contained.
     uint64_t collapseCount = getCollapseValue().value_or(1);

mlir/test/Dialect/OpenACC/invalid.mlir

@@ -492,6 +492,15 @@ func.func @fct1(%0 : !llvm.ptr) -> () {
 
 // -----
 
+%i1 = arith.constant 1 : i32
+%i2 = arith.constant 10 : i32
+// expected-error@+1 {{unstructured acc.loop must not have induction variables}}
+acc.loop control(%iv : i32) = (%i1 : i32) to (%i2 : i32) step (%i1 : i32) {
+  acc.yield
+} attributes {independent = [#acc.device_type<none>], unstructured}
+
+// -----
+
 // expected-error@+1 {{expect at least one of num, dim or static values}}
 acc.loop gang({}) {
   "test.openacc_dummy_op"() : () -> ()

mlir/test/Dialect/OpenACC/ops.mlir

@@ -2143,6 +2143,20 @@ func.func @acc_loop_container() {
 
 // -----
 
+func.func @acc_unstructured_loop() {
+  acc.loop {
+    acc.yield
+  } attributes {independent = [#acc.device_type<none>], unstructured}
+  return
+}
+
+// CHECK-LABEL: func.func @acc_unstructured_loop
+// CHECK:       acc.loop
+// CHECK:         acc.yield
+// CHECK:       } attributes {independent = [#acc.device_type<none>], unstructured}
+
+// -----
+
 // Test private recipe with data bounds for array slicing
 acc.private.recipe @privatization_memref_slice : memref<10x10xf32> init {
 ^bb0(%arg0: memref<10x10xf32>, %bounds0: !acc.data_bounds_ty, %bounds1: !acc.data_bounds_ty):