[flang][acc] Use non-boxed value for assumed-size in acc data clause

Assumed-size arrays end up looking as follows at the HLFIR level:
```
  func.func @_QPsub(%arg0: !fir.ref<!fir.array<?xf64>> {fir.bindc_name =
"arr"}) {
    ...
    %1 = fir.shape %c-1 : (index) -> !fir.shape<1>
    %2:2 = hlfir.declare %arg0(%1) dummy_scope %0 {uniq_name =
"_QFsubEarr"} : (!fir.ref<!fir.array<?xf64>>, !fir.shape<1>,
!fir.dscope) -> (!fir.box<!fir.array<?xf64>>,
!fir.ref<!fir.array<?xf64>>)
```

The declare operation produces an entity with Fortran properties
(wrapped via a box) or the raw data pointer. The current acc lowering
uses the box value.

During ConvertHLFIRtoFIR, this leads to a forced materialization of
descriptor even though the descriptor itself does not hold useful
extent information (it holds -1). Other operations such as those that
index the array access the raw pointer directly and thus do not force
materialization of descriptor.

Since there is nothing useful in descriptor and since at end of the
day the acc dialect can accept a pointer to the data, make it
consistent. This is useful property because without any acc data
clauses, an assumed-size array becomes live-in via raw pointer
typically. Thus, the materialization of descriptor is not something
acc lowering forces.

Author: Razvan Lupusoru

flang/lib/Lower/OpenACC.cpp

@@ -369,6 +369,55 @@ getSymbolFromAccObject(const Fortran::parser::AccObject &accObject) {
   llvm::report_fatal_error("Could not find symbol");
 }
 
+static mlir::Value getBaseAddr(Fortran::semantics::Symbol &symbol,
+                               const fir::factory::AddrAndBoundsInfo &info) {
+  if (Fortran::semantics::IsAssumedSizeArray(symbol)) {
+    // Assumed-size arrays in FIR are represented as:
+    //  func.func @func(%arg0: !fir.ref<!fir.array<?xf64>> {fir.bindc_name = "arr"}) {
+    //    %arr:2 = hlfir.declare %arg0(%shape) ... -> (!fir.box<!fir.array<?xf64>>, !fir.ref<!fir.array<?xf64>>)
+    // The `rawInput` refers to the #1 output of the `hlfir.declare` operation.
+    // This is preferred since the Fortran variable properties does not contain
+    // any useful size information.
+    return info.rawInput;
+  }
+
+  if (Fortran::semantics::IsOptional(symbol)) {
+    // When there is an optional argument for which there is a possibility
+    // to create a descriptor, pick the rawInput instead. This is done to
+    // avoid materializing the descriptor which leads to following pattern
+    // generated at the FIR level which adds an extra indirection that makes
+    // recovering original variable not evident.
+    // This is the pattern we want to avoid to be generated:
+    //  %1 = fir.declare %arg0 ... {fortran_attrs = #fir.var_attrs<optional>, uniq_name = "_QFsub1Eassumedshapeoptarr"} : (!fir.box<!fir.array<?xf32>>, !fir.dscope) -> !fir.box<!fir.array<?xf32>>
+    //  %2 = fir.is_present %1 : (!fir.box<!fir.array<?xf32>>) -> i1
+    //  %3 = fir.if %2 -> (!fir.box<!fir.array<?xf32>>) {
+    //    %5 = fir.rebox %1 : (!fir.box<!fir.array<?xf32>>) -> !fir.box<!fir.array<?xf32>>
+    //    fir.result %5 : !fir.box<!fir.array<?xf32>>
+    //  } else {
+    //    %5 = fir.absent !fir.box<!fir.array<?xf32>>
+    //    fir.result %5 : !fir.box<!fir.array<?xf32>>
+    //  }
+    //  %4 = acc.copyin var(%3 : !fir.box<!fir.array<?xf32>>) ...
+    //
+    // Instead by picking the rawInput we get the following pattern:
+    //  %1 = fir.declare %arg0 ... {fortran_attrs = #fir.var_attrs<optional>, uniq_name = "_QFsub1Eassumedshapeoptarr"} : (!fir.box<!fir.array<?xf32>>, !fir.dscope) -> !fir.box<!fir.array<?xf32>>
+    //  %2 = acc.copyin var(%2 : !fir.box<!fir.array<?xf32>>) ...
+    if (fir::unwrapRefType(info.addr.getType()) !=
+        fir::unwrapRefType(info.rawInput.getType())) {
+      return info.rawInput;
+    }
+  }
+
+  // The `addr` field refers to the address of the Fortran entity, but with the
+  // ssa value that when lowered to FIR will include the tied Fortran variable
+  // properties. Additionally, in cases where `unwrapFirBox` is requested,
+  // it refers to the address of the data (either result of fir.box_addr or
+  // result of `fir.if` in case of optional).
+  // Therefore, use the processed address in all cases by default unless it was
+  // deemed through the earlier checks in this routine that it is not useful.
+  return info.addr;
+}
+
 template <typename Op>
 static void
 genDataOperandOperations(const Fortran::parser::AccObjectList &objectList,
@@ -399,13 +448,7 @@ genDataOperandOperations(const Fortran::parser::AccObjectList &objectList,
             /*genDefaultBounds=*/generateDefaultBounds);
     LLVM_DEBUG(llvm::dbgs() << __func__ << "\n"; info.dump(llvm::dbgs()));
 
-    // If the input value is optional and is not a descriptor, we use the
-    // rawInput directly.
-    mlir::Value baseAddr = ((fir::unwrapRefType(info.addr.getType()) !=
-                             fir::unwrapRefType(info.rawInput.getType())) &&
-                            info.isPresent)
-                               ? info.rawInput
-                               : info.addr;
+    mlir::Value baseAddr = getBaseAddr(symbol, info);
     Op op = createDataEntryOp<Op>(
         builder, operandLocation, baseAddr, asFortran, bounds, structured,
         implicit, dataClause, baseAddr.getType(), async, asyncDeviceTypes,

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

@@ -92,8 +92,7 @@ subroutine acc_undefined_extent(a)
 ! CHECK: %[[DIMS0:.*]]:3 = fir.box_dims %[[DECL_ARG0]]#0, %c0{{.*}} : (!fir.box<!fir.array<?xf32>>, index) -> (index, index, index)
 ! CHECK: %[[UB:.*]] = arith.subi %[[DIMS0]]#1, %c1{{.*}} : index
 ! CHECK: %[[BOUND:.*]] = acc.bounds lowerbound(%c0{{.*}} : index) upperbound(%[[UB]] : index) extent(%[[DIMS0]]#1 : index) stride(%[[DIMS0]]#2 : index) startIdx(%c1{{.*}} : index) {strideInBytes = true}
-! CHECK: %[[ADDR:.*]] = fir.box_addr %[[DECL_ARG0]]#0 : (!fir.box<!fir.array<?xf32>>) -> !fir.ref<!fir.array<?xf32>>
-! CHECK: %[[PRESENT:.*]] = acc.present varPtr(%[[ADDR]] : !fir.ref<!fir.array<?xf32>>) bounds(%[[BOUND]]) -> !fir.ref<!fir.array<?xf32>> {name = "a"}
+! CHECK: %[[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>>)
 
   subroutine acc_multi_strides(a)