[flang][OpenMP][HLFIR] Support vector subscripted array sections for DEPEND (#133892)

The OpenMP runtime needs the base address of the array section to
identify the dependency.

If we just put the vector subscript through the usual HLFIR expression
lowering, that would generate a new contiguous array representing the
values of the elements in the array which was sectioned. We cannot use
addresses from this array because these addresses would not match
dependencies on the original array. For example

```
integer :: array(1024)
integer :: indices(2)

indices(1) = 1
indices(2) = 100

!$omp task depend(out: array(1:512))
!$omp end task

!$omp task depend(in: array(indices))
!$omp end task
```

This requires taking the lowering path previously only used for ordered
assignments to get the address of the elements in the original array
which were indexed. This is done using `hlfir.elemental_addr`. e.g.
```
array(indices) = 2
```

`hlfir.elemental_addr` is awkward to use because it (by design) doesn't
return something like `!hlfir.expr<>` (like `hlfir.elemental`) and so it
can't have a generic lowering: each place it is used has to carefully
inline the contents of the operation and extract the needed address.

For this reason, `hlfir.elemental_addr` is not allowed outside of these
ordered assignments. In this commit I ignore this restriction so that I
can use `hlfir.elemental_addr` to lower the OpenMP DEPEND clause (this
works because the operation is inlined and removed before the verifier
runs).

One alternative solution would have been to provide my own more limited
re-implementation of `HlfirDesignatorBuilder` which skipped
`hlfir::elemental_addr`, instead inlining its body directly at the
current insertion point applying indices only for the first element.
This would have been difficult to maintain because designation in
Fortran is complex.

Author: tblah

flang/lib/Lower/OpenMP/ClauseProcessor.cpp

@@ -889,13 +889,62 @@ bool ClauseProcessor::processDepend(lower::SymMap &symMap,
       } else if (evaluate::IsArrayElement(*object.ref())) {
         // Array Section
         SomeExpr expr = *object.ref();
-        if (isVectorSubscript(expr))
-          TODO(converter.getCurrentLocation(),
-               "Vector subscripted array section for task dependency");
 
-        hlfir::EntityWithAttributes entity = convertExprToHLFIR(
-            converter.getCurrentLocation(), converter, expr, symMap, stmtCtx);
-        dependVar = entity.getBase();
+        if (isVectorSubscript(expr)) {
+          // OpenMP needs the address of the first indexed element (required by
+          // the standard to be the lowest index) to identify the dependency. We
+          // don't need an accurate length for the array section because the
+          // OpenMP standard forbids overlapping array sections.
+
+          // Get a hlfir.elemental_addr op describing the address of the value
+          // indexed from the original array.
+          // Note: the hlfir.elemental_addr op verifier requires it to be inside
+          // of a hlfir.region_assign op. This is because the only place in base
+          // Fortran where you need the address of a vector subscript would be
+          // in an assignment operation. We are not doing an assignment here
+          // but we do want the address (without having to duplicate all of
+          // Fortran designation lowering!). This operation is never seen by the
+          // verifier because it is immediately inlined.
+          hlfir::ElementalAddrOp addrOp =
+              convertVectorSubscriptedExprToElementalAddr(
+                  converter.getCurrentLocation(), converter, expr, symMap,
+                  stmtCtx);
+          if (!addrOp.getCleanup().empty())
+            TODO(converter.getCurrentLocation(),
+                 "Vector subscript in DEPEND clause requring a cleanup region");
+
+          // hlfir.elemental_addr doesn't have a normal lowering because it
+          // can't return a value. Instead we need to inline it here using
+          // values for the first element. Similar to hlfir::inlineElementalOp.
+
+          mlir::Value one = builder.createIntegerConstant(
+              converter.getCurrentLocation(), builder.getIndexType(), 1);
+          mlir::SmallVector<mlir::Value> oneBasedIndices;
+          oneBasedIndices.resize(addrOp.getIndices().size(), one);
+
+          mlir::IRMapping mapper;
+          mapper.map(addrOp.getIndices(), oneBasedIndices);
+          assert(addrOp.getElementalRegion().hasOneBlock());
+          mlir::Operation *newOp;
+          for (mlir::Operation &op :
+               addrOp.getElementalRegion().back().getOperations())
+            newOp = builder.clone(op, mapper);
+          auto yield = mlir::cast<hlfir::YieldOp>(newOp);
+
+          addrOp->erase();
+
+          if (!yield.getCleanup().empty())
+            TODO(converter.getCurrentLocation(),
+                 "Vector subscript in DEPEND clause requring element cleanup");
+
+          dependVar = yield.getEntity();
+          yield->erase();
+        } else {
+          // Ordinary array section e.g. A(1:512:2)
+          hlfir::EntityWithAttributes entity = convertExprToHLFIR(
+              converter.getCurrentLocation(), converter, expr, symMap, stmtCtx);
+          dependVar = entity.getBase();
+        }
       } else {
         semantics::Symbol *sym = object.sym();
         dependVar = converter.getSymbolAddress(*sym);

flang/test/Lower/OpenMP/Todo/depend-clause-vector-subscript-array-section.f90

@@ -49,3 +49,36 @@ subroutine assumedShape(array)
 ! CHECK:           }
 ! CHECK:           return
 ! CHECK:         }
+
+subroutine vectorSubscriptArraySection(array, indices)
+  integer :: array(:)
+  integer :: indices(:)
+
+  !$omp task depend (in: array(indices))
+  !$omp end task
+end subroutine
+! CHECK-LABEL:   func.func @_QPvectorsubscriptarraysection(
+! CHECK-SAME:                                              %[[VAL_0:[0-9]+|[a-zA-Z$._-][a-zA-Z0-9$._-]*]]: !fir.box<!fir.array<?xi32>> {fir.bindc_name = "array"},
+! CHECK-SAME:                                              %[[VAL_1:[0-9]+|[a-zA-Z$._-][a-zA-Z0-9$._-]*]]: !fir.box<!fir.array<?xi32>> {fir.bindc_name = "indices"}) {
+! CHECK:           %[[VAL_2:.*]] = fir.dummy_scope : !fir.dscope
+! CHECK:           %[[VAL_3:.*]]:2 = hlfir.declare %[[VAL_0]] dummy_scope %[[VAL_2]] {uniq_name = "_QFvectorsubscriptarraysectionEarray"} : (!fir.box<!fir.array<?xi32>>, !fir.dscope) -> (!fir.box<!fir.array<?xi32>>, !fir.box<!fir.array<?xi32>>)
+! CHECK:           %[[VAL_4:.*]]:2 = hlfir.declare %[[VAL_1]] dummy_scope %[[VAL_2]] {uniq_name = "_QFvectorsubscriptarraysectionEindices"} : (!fir.box<!fir.array<?xi32>>, !fir.dscope) -> (!fir.box<!fir.array<?xi32>>, !fir.box<!fir.array<?xi32>>)
+! CHECK:           %[[VAL_5:.*]] = arith.constant 0 : index
+! CHECK:           %[[VAL_6:.*]]:3 = fir.box_dims %[[VAL_4]]#0, %[[VAL_5]] : (!fir.box<!fir.array<?xi32>>, index) -> (index, index, index)
+! CHECK:           %[[VAL_7:.*]] = fir.shape %[[VAL_6]]#1 : (index) -> !fir.shape<1>
+! CHECK:           %[[VAL_8:.*]] = hlfir.elemental %[[VAL_7]] unordered : (!fir.shape<1>) -> !hlfir.expr<?xi64> {
+! CHECK:           ^bb0(%[[VAL_9:.*]]: index):
+! CHECK:             %[[VAL_10:.*]] = hlfir.designate %[[VAL_4]]#0 (%[[VAL_9]])  : (!fir.box<!fir.array<?xi32>>, index) -> !fir.ref<i32>
+! CHECK:             %[[VAL_11:.*]] = fir.load %[[VAL_10]] : !fir.ref<i32>
+! CHECK:             %[[VAL_12:.*]] = fir.convert %[[VAL_11]] : (i32) -> i64
+! CHECK:             hlfir.yield_element %[[VAL_12]] : i64
+! CHECK:           }
+! CHECK:           %[[VAL_13:.*]] = arith.constant 1 : index
+! CHECK:           %[[VAL_14:.*]] = hlfir.apply %[[VAL_8]], %[[VAL_13]] : (!hlfir.expr<?xi64>, index) -> i64
+! CHECK:           %[[VAL_15:.*]] = hlfir.designate %[[VAL_3]]#0 (%[[VAL_14]])  : (!fir.box<!fir.array<?xi32>>, i64) -> !fir.ref<i32>
+! CHECK:           omp.task depend(taskdependin -> %[[VAL_15]] : !fir.ref<i32>) {
+! CHECK:             omp.terminator
+! CHECK:           }
+! CHECK:           hlfir.destroy %[[VAL_8]] : !hlfir.expr<?xi64>
+! CHECK:           return
+! CHECK:         }