[flang][openacc] Set extent to 0 when it is undefined (llvm#71108)

Set upperbound to lowerbound value and extent to zero when extent is
undefined.

Author: clementval

flang/lib/Lower/DirectivesCommon.h

@@ -632,11 +632,18 @@ genBaseBoundsOps(fir::FirOpBuilder &builder, mlir::Location loc,
   for (std::size_t dim = 0; dim < dataExv.rank(); ++dim) {
     mlir::Value baseLb =
         fir::factory::readLowerBound(builder, loc, dataExv, dim, one);
+    mlir::Value zero = builder.createIntegerConstant(loc, idxTy, 0);
+    mlir::Value ub;
+    mlir::Value lb = zero;
     mlir::Value ext = fir::factory::readExtent(builder, loc, dataExv, dim);
-    mlir::Value lb = builder.createIntegerConstant(loc, idxTy, 0);
+    if (mlir::isa<fir::UndefOp>(ext.getDefiningOp())) {
+      ext = zero;
+      ub = lb;
+    } else {
+      // ub = extent - 1
+      ub = builder.create<mlir::arith::SubIOp>(loc, ext, one);
+    }
 
-    // ub = extent - 1
-    mlir::Value ub = builder.create<mlir::arith::SubIOp>(loc, ext, one);
     mlir::Value bound =
         builder.create<BoundsOp>(loc, boundTy, lb, ub, ext, one, false, baseLb);
     bounds.push_back(bound);
@@ -738,7 +745,7 @@ genBoundsOps(fir::FirOpBuilder &builder, mlir::Location loc,
           const auto &strideExpr{std::get<2>(triplet->t)};
           if (strideExpr) {
             mlir::emitError(loc, "stride cannot be specified on "
-                                 "an OpenMP array section");
+                                 "an array section");
             break;
           }
         }

flang/test/Lower/OpenACC/acc-bounds.f90

@@ -86,4 +86,20 @@ subroutine acc_derived_type_component_allocatable_array()
 ! CHECK: return
 ! CHECK: }
 
+  subroutine acc_undefined_extent(a)
+    real, dimension(1:*) :: a
+
+    !$acc kernels present(a)
+    !$acc end kernels
+  end subroutine
+! CHECK-LABEL: func.func @_QMopenacc_boundsPacc_undefined_extent(
+! CHECK-SAME: %[[ARG0:.*]]: !fir.ref<!fir.array<?xf32>> {fir.bindc_name = "a"}) {
+! HLFIR: %[[DECL_ARG0:.*]]:2 = hlfir.declare %[[ARG0]](%{{.*}}) {uniq_name = "_QMopenacc_boundsFacc_undefined_extentEa"} : (!fir.ref<!fir.array<?xf32>>, !fir.shape<1>) -> (!fir.box<!fir.array<?xf32>>, !fir.ref<!fir.array<?xf32>>)
+! CHECK: %[[ONE:.*]] = arith.constant 1 : index
+! CHECK: %[[ZERO:.*]] = arith.constant 0 : index
+! CHECK: %[[BOUND:.*]] = acc.bounds lowerbound(%[[ZERO]] : index) upperbound(%[[ZERO]] : index) extent(%[[ZERO]] : index) stride(%[[ONE]] : index) startIdx(%[[ONE]] : index)
+! FIR:   %[[PRESENT:.*]] = acc.present varPtr(%[[ARG0]] : !fir.ref<!fir.array<?xf32>>) bounds(%[[BOUND]]) -> !fir.ref<!fir.array<?xf32>> {name = "a"}
+! HLFIR: %[[PRESENT:.*]] = acc.present varPtr(%[[DECL_ARG0]]#1 : !fir.ref<!fir.array<?xf32>>) bounds(%[[BOUND]]) -> !fir.ref<!fir.array<?xf32>> {name = "a"}
+! CHECK: acc.kernels dataOperands(%[[PRESENT]] : !fir.ref<!fir.array<?xf32>>)
+
 end module