[OpenMP][MLIR] Preserve to/from flags in mapper base entry for mappers (llvm#159799)

With declare mapper, the parent base entry was emitted as `TARGET_PARAM`
only. The mapper received a map-type without `to/from`, causing
components to degrade to `alloc`-only (no copies), breaking allocatable
payload mapping. This PR preserves the map-type bits from the parent.

This fixes llvm#156466.

Author: TIFitis

mlir/lib/Target/LLVMIR/Dialect/OpenMP/OpenMPToLLVMIRTranslation.cpp

@@ -4089,11 +4089,30 @@ static llvm::omp::OpenMPOffloadMappingFlags mapParentWithMembers(
   assert(!ompBuilder.Config.isTargetDevice() &&
          "function only supported for host device codegen");
 
-  // Map the first segment of our structure
-  combinedInfo.Types.emplace_back(
+  // Map the first segment of the parent. If a user-defined mapper is attached,
+  // include the parent's to/from-style bits (and common modifiers) in this
+  // base entry so the mapper receives correct copy semantics via its 'type'
+  // parameter. Also keep TARGET_PARAM when required for kernel arguments.
+  llvm::omp::OpenMPOffloadMappingFlags baseFlag =
       isTargetParams
           ? llvm::omp::OpenMPOffloadMappingFlags::OMP_MAP_TARGET_PARAM
-          : llvm::omp::OpenMPOffloadMappingFlags::OMP_MAP_NONE);
+          : llvm::omp::OpenMPOffloadMappingFlags::OMP_MAP_NONE;
+
+  // Detect if this mapping uses a user-defined mapper.
+  bool hasUserMapper = mapData.Mappers[mapDataIndex] != nullptr;
+  if (hasUserMapper) {
+    using mapFlags = llvm::omp::OpenMPOffloadMappingFlags;
+    // Preserve relevant map-type bits from the parent clause. These include
+    // the copy direction (TO/FROM), as well as commonly used modifiers that
+    // should be visible to the mapper for correct behaviour.
+    mapFlags parentFlags = mapData.Types[mapDataIndex];
+    mapFlags preserve = mapFlags::OMP_MAP_TO | mapFlags::OMP_MAP_FROM |
+                        mapFlags::OMP_MAP_ALWAYS | mapFlags::OMP_MAP_CLOSE |
+                        mapFlags::OMP_MAP_PRESENT | mapFlags::OMP_MAP_OMPX_HOLD;
+    baseFlag |= (parentFlags & preserve);
+  }
+
+  combinedInfo.Types.emplace_back(baseFlag);
   combinedInfo.DevicePointers.emplace_back(
       mapData.DevicePointers[mapDataIndex]);
   combinedInfo.Mappers.emplace_back(mapData.Mappers[mapDataIndex]);

offload/test/offloading/fortran/target-declare-mapper-allocatable.f90

@@ -0,0 +1,48 @@
+! This test validates that declare mapper for a derived type with an
+! allocatable component preserves TO/FROM semantics for the component,
+! ensuring the payload is copied back to the host on target exit.
+
+! REQUIRES: flang, amdgpu
+
+! RUN: %libomptarget-compile-fortran-run-and-check-generic
+
+program target_declare_mapper_allocatable
+  implicit none
+
+  type :: real_t
+    real, allocatable :: real_arr(:)
+  end type real_t
+
+  ! Map the allocatable array payload via a named mapper.
+  !$omp declare mapper (xyz : real_t :: t) map(tofrom: t%real_arr)
+
+  type(real_t) :: r
+  integer :: i
+  logical :: ok
+
+  allocate(r%real_arr(10))
+  r%real_arr = 1.0
+
+  !$omp target map(mapper(xyz), tofrom: r)
+    do i = 1, size(r%real_arr)
+      r%real_arr(i) = 3.0
+    end do
+  !$omp end target
+
+  ok = .true.
+  do i = 1, size(r%real_arr)
+    if (r%real_arr(i) /= 3.0) ok = .false.
+  end do
+  if (ok) then
+    print *, "Test passed!"
+  else
+    print *, "Test failed!"
+    do i = 1, size(r%real_arr)
+      print *, r%real_arr(i)
+    end do
+  end if
+
+  deallocate(r%real_arr)
+end program target_declare_mapper_allocatable
+
+! CHECK: Test passed!