[Offload] Introduce ATTACH map-type support for pointer attachment. (#149036)

This patch introduces libomptarget support for the ATTACH map-type,
which can be used to implement OpenMP conditional compliant pointer
attachment, based on whether the pointer/pointee is newly mapped on a
given construct.

For example, for the following:

```c
  int *p;
  #pragma omp target enter data map(p[1:10])
```

The following maps can be emitted by clang:
```
  (A)
  &p[0], &p[1], 10 * sizeof(p[1]), TO | FROM
  &p, &p[1], sizeof(p), ATTACH
```

Without this map-type, these two possible maps could be emitted by
clang:
```
  (B)
  &p[0], &p[1], 10 * sizeof(p[1]), TO | FROM

  (C)
  &p, &p[1], 10 * sizeof(p[1]), TO | FROM | PTR_AND_OBJ
````

(B) does not perform any pointer attachment, while (C) also maps the
pointer p, which are both incorrect.

In terms of implementation, maps with the ATTACH map-type are handled
after all other maps have been processed, as it requires knowledge of
which new allocations happened as part of the construct. As per OpenMP
5.0, an attachment should happen only when either the pointer or the
pointee was newly mapped while handling the construct.

Maps with ATTACH map-type-bit do not increase/decrease the ref-count.

With OpenMP 6.1, `attach(always/never)` can be used to force/prevent
attachment. For `attach(always)`, the compiler will insert the ALWAYS
map-type, which would let libomptarget bypass the check about one of the
pointer/pointee being new. With `attach(never)`, the ATTACH map will not
be emitted at all.

The size argument of the ATTACH map-type can specify values greater than
`sizeof(void*)` which can be used to support pointer attachment on
Fortran descriptors. Note that this also requires shadow-pointer
tracking to also support them. That has not been implemented in this
patch.

This was worked upon in coordination with Ravi Narayanaswamy, who has
since retired. Happy retirement, Ravi!

---------

Co-authored-by: Alex Duran <alejandro.duran@intel.com>

offload/include/OpenMP/Mapping.h

@@ -417,12 +417,42 @@ struct MapperComponentsTy {
 typedef void (*MapperFuncPtrTy)(void *, void *, void *, int64_t, int64_t,
                                 void *);
 
+/// Structure to store information about a single ATTACH map entry.
+struct AttachMapInfo {
+  void *PointerBase;
+  void *PointeeBegin;
+  int64_t PointerSize;
+  int64_t MapType;
+  map_var_info_t Pointername;
+
+  AttachMapInfo(void *PointerBase, void *PointeeBegin, int64_t Size,
+                int64_t Type, map_var_info_t Name)
+      : PointerBase(PointerBase), PointeeBegin(PointeeBegin), PointerSize(Size),
+        MapType(Type), Pointername(Name) {}
+};
+
+/// Structure to track ATTACH entries and new allocations across recursive calls
+/// (for handling mappers) to targetDataBegin for a given construct.
+struct AttachInfoTy {
+  /// ATTACH map entries for deferred processing.
+  llvm::SmallVector<AttachMapInfo> AttachEntries;
+
+  /// Key: host pointer, Value: allocation size.
+  llvm::DenseMap<void *, int64_t> NewAllocations;
+
+  AttachInfoTy() = default;
+
+  // Delete copy constructor and copy assignment operator to prevent copying
+  AttachInfoTy(const AttachInfoTy &) = delete;
+  AttachInfoTy &operator=(const AttachInfoTy &) = delete;
+};
+
 // Function pointer type for targetData* functions (targetDataBegin,
 // targetDataEnd and targetDataUpdate).
 typedef int (*TargetDataFuncPtrTy)(ident_t *, DeviceTy &, int32_t, void **,
                                    void **, int64_t *, int64_t *,
                                    map_var_info_t *, void **, AsyncInfoTy &,
-                                   bool);
+                                   AttachInfoTy *, bool);
 
 void dumpTargetPointerMappings(const ident_t *Loc, DeviceTy &Device,
                                bool toStdOut = false);
@@ -431,20 +461,26 @@ int targetDataBegin(ident_t *Loc, DeviceTy &Device, int32_t ArgNum,
                     void **ArgsBase, void **Args, int64_t *ArgSizes,
                     int64_t *ArgTypes, map_var_info_t *ArgNames,
                     void **ArgMappers, AsyncInfoTy &AsyncInfo,
+                    AttachInfoTy *AttachInfo = nullptr,
                     bool FromMapper = false);
 
 int targetDataEnd(ident_t *Loc, DeviceTy &Device, int32_t ArgNum,
                   void **ArgBases, void **Args, int64_t *ArgSizes,
                   int64_t *ArgTypes, map_var_info_t *ArgNames,
                   void **ArgMappers, AsyncInfoTy &AsyncInfo,
-                  bool FromMapper = false);
+                  AttachInfoTy *AttachInfo = nullptr, bool FromMapper = false);
 
 int targetDataUpdate(ident_t *Loc, DeviceTy &Device, int32_t ArgNum,
                      void **ArgsBase, void **Args, int64_t *ArgSizes,
                      int64_t *ArgTypes, map_var_info_t *ArgNames,
                      void **ArgMappers, AsyncInfoTy &AsyncInfo,
+                     AttachInfoTy *AttachInfo = nullptr,
                      bool FromMapper = false);
 
+// Process deferred ATTACH map entries collected during targetDataBegin.
+int processAttachEntries(DeviceTy &Device, AttachInfoTy &AttachInfo,
+                         AsyncInfoTy &AsyncInfo);
+
 struct MappingInfoTy {
   MappingInfoTy(DeviceTy &Device) : Device(Device) {}
 

offload/include/device.h

@@ -98,6 +98,10 @@ struct DeviceTy {
   int32_t dataExchange(void *SrcPtr, DeviceTy &DstDev, void *DstPtr,
                        int64_t Size, AsyncInfoTy &AsyncInfo);
 
+  // Insert a data fence between previous data operations and the following
+  // operations if necessary for the device.
+  int32_t dataFence(AsyncInfoTy &AsyncInfo);
+
   /// Notify the plugin about a new mapping starting at the host address
   /// \p HstPtr and \p Size bytes.
   int32_t notifyDataMapped(void *HstPtr, int64_t Size);

offload/include/omptarget.h

@@ -77,6 +77,9 @@ enum tgt_map_type {
   // the structured region
   // This is an OpenMP extension for the sake of OpenACC support.
   OMP_TGT_MAPTYPE_OMPX_HOLD = 0x2000,
+  // Attach pointer and pointee, after processing all other maps.
+  // Applicable to map-entering directives. Does not change ref-count.
+  OMP_TGT_MAPTYPE_ATTACH = 0x4000,
   // descriptor for non-contiguous target-update
   OMP_TGT_MAPTYPE_NON_CONTIG = 0x100000000000,
   // member of struct, member given by [16 MSBs] - 1

offload/libomptarget/device.cpp

@@ -191,6 +191,10 @@ int32_t DeviceTy::dataExchange(void *SrcPtr, DeviceTy &DstDev, void *DstPtr,
                                   DstPtr, Size, AsyncInfo);
 }
 
+int32_t DeviceTy::dataFence(AsyncInfoTy &AsyncInfo) {
+  return RTL->data_fence(RTLDeviceID, AsyncInfo);
+}
+
 int32_t DeviceTy::notifyDataMapped(void *HstPtr, int64_t Size) {
   DP("Notifying about new mapping: HstPtr=" DPxMOD ", Size=%" PRId64 "\n",
      DPxPTR(HstPtr), Size);

offload/libomptarget/interface.cpp

@@ -30,6 +30,7 @@
 #include <cstdint>
 #include <cstdio>
 #include <cstdlib>
+#include <memory>
 
 #ifdef OMPT_SUPPORT
 using namespace llvm::omp::target::ompt;
@@ -165,12 +166,24 @@ targetData(ident_t *Loc, int64_t DeviceId, int32_t ArgNum, void **ArgsBase,
                                              OMPT_GET_RETURN_ADDRESS);)
 
   int Rc = OFFLOAD_SUCCESS;
+
+  // Only allocate AttachInfo for targetDataBegin
+  std::unique_ptr<AttachInfoTy> AttachInfo;
+  if (TargetDataFunction == targetDataBegin)
+    AttachInfo = std::make_unique<AttachInfoTy>();
+
   Rc = TargetDataFunction(Loc, *DeviceOrErr, ArgNum, ArgsBase, Args, ArgSizes,
                           ArgTypes, ArgNames, ArgMappers, AsyncInfo,
-                          false /*FromMapper=*/);
+                          AttachInfo.get(), /*FromMapper=*/false);
 
-  if (Rc == OFFLOAD_SUCCESS)
-    Rc = AsyncInfo.synchronize();
+  if (Rc == OFFLOAD_SUCCESS) {
+    // Process deferred ATTACH entries BEFORE synchronization
+    if (AttachInfo && !AttachInfo->AttachEntries.empty())
+      Rc = processAttachEntries(*DeviceOrErr, *AttachInfo, AsyncInfo);
+
+    if (Rc == OFFLOAD_SUCCESS)
+      Rc = AsyncInfo.synchronize();
+  }
 
   handleTargetOutcome(Rc == OFFLOAD_SUCCESS, Loc);
 }

offload/libomptarget/omptarget.cpp

@@ -293,7 +293,8 @@ void targetUnlockExplicit(void *HostPtr, int DeviceNum, const char *Name) {
 int targetDataMapper(ident_t *Loc, DeviceTy &Device, void *ArgBase, void *Arg,
                      int64_t ArgSize, int64_t ArgType, map_var_info_t ArgNames,
                      void *ArgMapper, AsyncInfoTy &AsyncInfo,
-                     TargetDataFuncPtrTy TargetDataFunction) {
+                     TargetDataFuncPtrTy TargetDataFunction,
+                     AttachInfoTy *AttachInfo = nullptr) {
   DP("Calling the mapper function " DPxMOD "\n", DPxPTR(ArgMapper));
 
   // The mapper function fills up Components.
@@ -324,17 +325,178 @@ int targetDataMapper(ident_t *Loc, DeviceTy &Device, void *ArgBase, void *Arg,
                               MapperArgsBase.data(), MapperArgs.data(),
                               MapperArgSizes.data(), MapperArgTypes.data(),
                               MapperArgNames.data(), /*arg_mappers*/ nullptr,
-                              AsyncInfo, /*FromMapper=*/true);
+                              AsyncInfo, AttachInfo, /*FromMapper=*/true);
 
   return Rc;
 }
 
+/// Utility function to perform a pointer attachment operation.
+///
+/// For something like:
+/// ```cpp
+///  int *p;
+///  ...
+///  #pragma omp target enter data map(to:p[10:10])
+/// ```
+///
+/// for which the attachment operation gets represented using:
+/// ```
+///   &p, &p[10], sizeof(p), ATTACH
+/// ```
+///
+/// (Hst|Tgt)PtrAddr   represents &p
+/// (Hst|Tgt)PteeBase  represents &p[0]
+/// (Hst|Tgt)PteeBegin represents &p[10]
+///
+/// This function first computes the expected TgtPteeBase using:
+///   `<Select>TgtPteeBase = TgtPteeBegin - (HstPteeBegin - HstPteeBase)`
+///
+/// and then attaches TgtPteeBase to TgtPtrAddr.
+///
+/// \p HstPtrSize represents the size of the pointer p. For C/C++, this
+/// should be same as "sizeof(void*)" (say 8).
+///
+/// However, for Fortran, pointers/allocatables, which are also eligible for
+/// "pointer-attachment", may be implemented using descriptors that contain the
+/// address of the pointee in the first 8 bytes, but also contain other
+/// information such as lower-bound/upper-bound etc in their subsequent fields.
+///
+/// For example, for the following:
+/// ```fortran
+///   integer, allocatable :: x(:)
+///   integer, pointer :: p(:)
+///   ...
+///   p => x(10: 19)
+///   ...
+///   !$omp target enter data map(to:p(:))
+/// ```
+///
+/// The map should trigger a pointer-attachment (assuming the pointer-attachment
+/// conditions as noted on processAttachEntries are met) between the descriptor
+/// for p, and its pointee data.
+///
+/// Since only the first 8 bytes of the descriptor contain the address of the
+/// pointee, an attachment operation on device descriptors involves:
+/// * Setting the first 8 bytes of the device descriptor to point the device
+/// address of the pointee.
+/// * Copying the remaining information about bounds/offset etc. from the host
+/// descriptor to the device descriptor.
+///
+/// The function also handles pointer-attachment portion of PTR_AND_OBJ maps,
+/// like:
+/// ```
+///   &p, &p[10], 10 * sizeof(p[10]), PTR_AND_OBJ
+/// ```
+/// by using `sizeof(void*)` as \p HstPtrSize.
+static int performPointerAttachment(DeviceTy &Device, AsyncInfoTy &AsyncInfo,
+                                    void **HstPtrAddr, void *HstPteeBase,
+                                    void *HstPteeBegin, void **TgtPtrAddr,
+                                    void *TgtPteeBegin, int64_t HstPtrSize,
+                                    TargetPointerResultTy &PtrTPR) {
+  assert(PtrTPR.getEntry() &&
+         "Need a valid pointer entry to perform pointer-attachment");
+
+  int64_t VoidPtrSize = sizeof(void *);
+  assert(HstPtrSize >= VoidPtrSize && "PointerSize is too small");
+
+  uint64_t Delta = reinterpret_cast<uint64_t>(HstPteeBegin) -
+                   reinterpret_cast<uint64_t>(HstPteeBase);
+  void *TgtPteeBase = reinterpret_cast<void *>(
+      reinterpret_cast<uint64_t>(TgtPteeBegin) - Delta);
+
+  // Add shadow pointer tracking
+  // TODO: Support shadow-tracking of larger than VoidPtrSize pointers,
+  // to support restoration of Fortran descriptors. Currently, this check
+  // would return false, even if the host Fortran descriptor had been
+  // updated since its previous map, and we should have updated its
+  // device counterpart. e.g.
+  //
+  //   !$omp target enter data map(x(1:100)) !             (1)
+  //   p => x(10: 19)
+  //   !$omp target enter data map(p, p(:)) !              (2)
+  //   p => x(5: 9)
+  //   !$omp target enter data map(attach(always): p(:)) ! (3)
+  //
+  // While PtrAddr(&desc_p) and PteeBase(&p(1)) are same for (2) and (3), the
+  // pointer attachment for (3) needs to update the bounds information
+  // in the descriptor of p on device.
+  if (!PtrTPR.getEntry()->addShadowPointer(
+          ShadowPtrInfoTy{HstPtrAddr, HstPteeBase, TgtPtrAddr, TgtPteeBase})) {
+    DP("Pointer " DPxMOD " is already attached to " DPxMOD "\n",
+       DPxPTR(TgtPtrAddr), DPxPTR(TgtPteeBase));
+    return OFFLOAD_SUCCESS;
+  }
+
+  DP("Update pointer (" DPxMOD ") -> [" DPxMOD "]\n", DPxPTR(TgtPtrAddr),
+     DPxPTR(TgtPteeBase));
+
+  // Lambda to handle submitData result and perform final steps.
+  auto HandleSubmitResult = [&](int SubmitResult) -> int {
+    if (SubmitResult != OFFLOAD_SUCCESS) {
+      REPORT("Failed to update pointer on device.\n");
+      return OFFLOAD_FAIL;
+    }
+
+    if (PtrTPR.getEntry()->addEventIfNecessary(Device, AsyncInfo) !=
+        OFFLOAD_SUCCESS)
+      return OFFLOAD_FAIL;
+
+    return OFFLOAD_SUCCESS;
+  };
+
+  bool IsPtrAFortranDescriptor = HstPtrSize > VoidPtrSize;
+  if (!IsPtrAFortranDescriptor) {
+    // For "regular" pointers, we can use the VoidPtrLocation from AsyncInfo as
+    // the buffer space for the submission.
+    void *&BufferElement = AsyncInfo.getVoidPtrLocation();
+    BufferElement = TgtPteeBase;
+
+    // Submit the updated pointer value to device
+    return HandleSubmitResult(Device.submitData(
+        TgtPtrAddr, &BufferElement, VoidPtrSize, AsyncInfo, PtrTPR.getEntry()));
+  }
+
+  // For larger "pointers" (like Fortran's descriptors), we create a dynamic
+  // buffer, which will be eventually destroyed by AsyncInfo's post-processing
+  // callback.
+  char *DataBuffer = new char[HstPtrSize];
+
+  // For such descriptors, to the first VoidPtrSize bytes, we store the
+  // pointee's device address.
+  std::memcpy(DataBuffer, &TgtPteeBase, sizeof(void *));
+
+  // And to the remaining bytes, we copy the remaining contents of the host
+  // descriptor after the initial VoidPtrSize bytes.
+  uint64_t HstDescriptorFieldsSize = HstPtrSize - VoidPtrSize;
+  void *HstDescriptorFieldsAddr =
+      reinterpret_cast<char *>(HstPtrAddr) + VoidPtrSize;
+  std::memcpy(DataBuffer + VoidPtrSize, HstDescriptorFieldsAddr,
+              HstDescriptorFieldsSize);
+
+  DP("Updating %" PRId64 " bytes of descriptor (" DPxMOD ") (pointer + %" PRId64
+     " additional bytes from host descriptor " DPxMOD ")\n",
+     HstPtrSize, DPxPTR(TgtPtrAddr), HstDescriptorFieldsSize,
+     DPxPTR(HstDescriptorFieldsAddr));
+
+  // Submit the entire buffer to device
+  int SubmitResult = Device.submitData(TgtPtrAddr, DataBuffer, HstPtrSize,
+                                       AsyncInfo, PtrTPR.getEntry());
+
+  AsyncInfo.addPostProcessingFunction([DataBuffer]() -> int {
+    delete[] DataBuffer;
+    return OFFLOAD_SUCCESS;
+  });
+  return HandleSubmitResult(SubmitResult);
+}
+
 /// Internal function to do the mapping and transfer the data to the device
 int targetDataBegin(ident_t *Loc, DeviceTy &Device, int32_t ArgNum,
                     void **ArgsBase, void **Args, int64_t *ArgSizes,
                     int64_t *ArgTypes, map_var_info_t *ArgNames,
                     void **ArgMappers, AsyncInfoTy &AsyncInfo,
-                    bool FromMapper) {
+                    AttachInfoTy *AttachInfo, bool FromMapper) {
+  assert(AttachInfo && "AttachInfo must be available for targetDataBegin for "
+                       "handling ATTACH map-types.");
   // process each input.
   for (int32_t I = 0; I < ArgNum; ++I) {
     // Ignore private variables and arrays - there is no mapping for them.
@@ -352,7 +514,7 @@ int targetDataBegin(ident_t *Loc, DeviceTy &Device, int32_t ArgNum,
       map_var_info_t ArgName = (!ArgNames) ? nullptr : ArgNames[I];
       int Rc = targetDataMapper(Loc, Device, ArgsBase[I], Args[I], ArgSizes[I],
                                 ArgTypes[I], ArgName, ArgMappers[I], AsyncInfo,
-                                targetDataBegin);
+                                targetDataBegin, AttachInfo);
 
       if (Rc != OFFLOAD_SUCCESS) {
         REPORT("Call to targetDataBegin via targetDataMapper for custom mapper"
@@ -369,6 +531,18 @@ int targetDataBegin(ident_t *Loc, DeviceTy &Device, int32_t ArgNum,
     int64_t DataSize = ArgSizes[I];
     map_var_info_t HstPtrName = (!ArgNames) ? nullptr : ArgNames[I];
 
+    // ATTACH map-types are supposed to be handled after all mapping for the
+    // construct is done. Defer their processing.
+    if (ArgTypes[I] & OMP_TGT_MAPTYPE_ATTACH) {
+      AttachInfo->AttachEntries.emplace_back(
+          /*PointerBase=*/HstPtrBase, /*PointeeBegin=*/HstPtrBegin,
+          /*PointerSize=*/DataSize, /*MapType=*/ArgTypes[I],
+          /*PointeeName=*/HstPtrName);
+
+      DP("Deferring ATTACH map-type processing for argument %d\n", I);
+      continue;
+    }
+
     // Adjust for proper alignment if this is a combined entry (for structs).
     // Look at the next argument - if that is MEMBER_OF this one, then this one
     // is a combined entry.
@@ -434,13 +608,18 @@ int targetDataBegin(ident_t *Loc, DeviceTy &Device, int32_t ArgNum,
                                   : "device failure or illegal mapping");
         return OFFLOAD_FAIL;
       }
+
+      // Track new allocation, for eventual use in attachment decision-making.
+      if (PointerTpr.Flags.IsNewEntry && !IsHostPtr)
+        AttachInfo->NewAllocations[HstPtrBase] = sizeof(void *);
+
       DP("There are %zu bytes allocated at target address " DPxMOD " - is%s new"
          "\n",
          sizeof(void *), DPxPTR(PointerTgtPtrBegin),
          (PointerTpr.Flags.IsNewEntry ? "" : " not"));
       PointerHstPtrBegin = HstPtrBase;
       // modify current entry.
-      HstPtrBase = *(void **)HstPtrBase;
+      HstPtrBase = *reinterpret_cast<void **>(HstPtrBase);
       // No need to update pointee ref count for the first element of the
       // subelement that comes from mapper.
       UpdateRef =
@@ -464,6 +643,11 @@ int targetDataBegin(ident_t *Loc, DeviceTy &Device, int32_t ArgNum,
                                 : "device failure or illegal mapping");
       return OFFLOAD_FAIL;
     }
+
+    // Track new allocation, for eventual use in attachment decision-making.
+    if (TPR.Flags.IsNewEntry && !IsHostPtr && TgtPtrBegin)
+      AttachInfo->NewAllocations[HstPtrBegin] = DataSize;
+
     DP("There are %" PRId64 " bytes allocated at target address " DPxMOD
        " - is%s new\n",
        DataSize, DPxPTR(TgtPtrBegin), (TPR.Flags.IsNewEntry ? "" : " not"));
@@ -476,30 +660,13 @@ int targetDataBegin(ident_t *Loc, DeviceTy &Device, int32_t ArgNum,
     }
 
     if (ArgTypes[I] & OMP_TGT_MAPTYPE_PTR_AND_OBJ && !IsHostPtr) {
-
-      uint64_t Delta = (uint64_t)HstPtrBegin - (uint64_t)HstPtrBase;
-      void *ExpectedTgtPtrBase = (void *)((uint64_t)TgtPtrBegin - Delta);
-
-      if (PointerTpr.getEntry()->addShadowPointer(ShadowPtrInfoTy{
-              (void **)PointerHstPtrBegin, HstPtrBase,
-              (void **)PointerTgtPtrBegin, ExpectedTgtPtrBase})) {
-        DP("Update pointer (" DPxMOD ") -> [" DPxMOD "]\n",
-           DPxPTR(PointerTgtPtrBegin), DPxPTR(TgtPtrBegin));
-
-        void *&TgtPtrBase = AsyncInfo.getVoidPtrLocation();
-        TgtPtrBase = ExpectedTgtPtrBase;
-
-        int Ret =
-            Device.submitData(PointerTgtPtrBegin, &TgtPtrBase, sizeof(void *),
-                              AsyncInfo, PointerTpr.getEntry());
-        if (Ret != OFFLOAD_SUCCESS) {
-          REPORT("Copying data to device failed.\n");
-          return OFFLOAD_FAIL;
-        }
-        if (PointerTpr.getEntry()->addEventIfNecessary(Device, AsyncInfo) !=
-            OFFLOAD_SUCCESS)
-          return OFFLOAD_FAIL;
-      }
+      int Ret = performPointerAttachment(
+          Device, AsyncInfo, reinterpret_cast<void **>(PointerHstPtrBegin),
+          HstPtrBase, HstPtrBegin,
+          reinterpret_cast<void **>(PointerTgtPtrBegin), TgtPtrBegin,
+          sizeof(void *), PointerTpr);
+      if (Ret != OFFLOAD_SUCCESS)
+        return OFFLOAD_FAIL;
     }
 
     // Check if variable can be used on the device:
@@ -515,6 +682,189 @@ int targetDataBegin(ident_t *Loc, DeviceTy &Device, int32_t ArgNum,
   return OFFLOAD_SUCCESS;
 }
 
+/// Process deferred ATTACH map entries collected during targetDataBegin.
+///
+/// From OpenMP's perspective, when mapping something that has a base pointer,
+/// such as:
+/// ```cpp
+///   int *p;
+///   #pragma omp enter target data map(to: p[10:20])
+/// ```
+///
+/// a pointer-attachment between p and &p[10] should occur if both p and
+/// p[10] are present on the device after doing all allocations for all maps
+/// on the construct, and one of the following is true:
+///
+/// * The pointer p was newly allocated while handling the construct
+/// * The pointee p[10:20] was newly allocated while handling the construct
+/// * attach(always) map-type modifier was specified (OpenMP 6.1)
+///
+/// That's why we collect all attach entries and new memory allocations during
+/// targetDataBegin, and use that information to make the decision of whether
+/// to perform a pointer-attachment or not here, after maps have been handled.
+///
+/// Additionally, once we decide that a pointer-attachment should be performed,
+/// we need to make sure that it happens after any previously submitted data
+/// transfers have completed, to avoid the possibility of the pending transfers
+/// clobbering the attachment. For example:
+///
+/// ```cpp
+///   int *p = ...;
+///   int **pp = &p;
+///   map(to: pp[0], p[0])
+/// ```
+///
+/// Which would be represented by:
+/// ```
+/// &pp[0], &pp[0], sizeof(pp[0]), TO (1)
+/// &p[0], &p[0], sizeof(p[0]), TO    (2)
+///
+/// &pp, &pp[0], sizeof(pp), ATTACH   (3)
+/// &p, &p[0], sizeof(p), ATTACH      (4)
+/// ```
+///
+/// (4) and (1) are both trying to modify the device memory corresponding to
+/// `&p`. So, if we decide that (4) should do an attachment, we also need to
+/// ensure that (4) happens after (1) is complete.
+///
+/// For this purpose, we insert a data_fence before the first
+/// pointer-attachment, (3), to ensure that all pending transfers finish first.
+int processAttachEntries(DeviceTy &Device, AttachInfoTy &AttachInfo,
+                         AsyncInfoTy &AsyncInfo) {
+  // Report all tracked allocations from both main loop and ATTACH processing
+  if (!AttachInfo.NewAllocations.empty()) {
+    DP("Tracked %u total new allocations:\n",
+       (unsigned)AttachInfo.NewAllocations.size());
+    for (const auto &Alloc : AttachInfo.NewAllocations) {
+      DP("  Host ptr: " DPxMOD ", Size: %" PRId64 " bytes\n",
+         DPxPTR(Alloc.first), Alloc.second);
+    }
+  }
+
+  if (AttachInfo.AttachEntries.empty())
+    return OFFLOAD_SUCCESS;
+
+  DP("Processing %zu deferred ATTACH map entries\n",
+     AttachInfo.AttachEntries.size());
+
+  int Ret = OFFLOAD_SUCCESS;
+  bool IsFirstPointerAttachment = true;
+  for (size_t EntryIdx = 0; EntryIdx < AttachInfo.AttachEntries.size();
+       ++EntryIdx) {
+    const auto &AttachEntry = AttachInfo.AttachEntries[EntryIdx];
+
+    void **HstPtr = reinterpret_cast<void **>(AttachEntry.PointerBase);
+
+    void *HstPteeBase = *HstPtr;
+    void *HstPteeBegin = AttachEntry.PointeeBegin;
+
+    int64_t PtrSize = AttachEntry.PointerSize;
+    int64_t MapType = AttachEntry.MapType;
+
+    DP("Processing ATTACH entry %zu: HstPtr=" DPxMOD ", HstPteeBegin=" DPxMOD
+       ", Size=%" PRId64 ", Type=0x%" PRIx64 "\n",
+       EntryIdx, DPxPTR(HstPtr), DPxPTR(HstPteeBegin), PtrSize, MapType);
+
+    const bool IsAttachAlways = MapType & OMP_TGT_MAPTYPE_ALWAYS;
+
+    // Lambda to check if a pointer was newly allocated
+    auto WasNewlyAllocated = [&](void *Ptr, const char *PtrName) {
+      bool IsNewlyAllocated =
+          llvm::any_of(AttachInfo.NewAllocations, [&](const auto &Alloc) {
+            void *AllocPtr = Alloc.first;
+            int64_t AllocSize = Alloc.second;
+            return Ptr >= AllocPtr &&
+                   Ptr < reinterpret_cast<void *>(
+                             reinterpret_cast<char *>(AllocPtr) + AllocSize);
+          });
+      DP("Attach %s " DPxMOD " was newly allocated: %s\n", PtrName, DPxPTR(Ptr),
+         IsNewlyAllocated ? "yes" : "no");
+      return IsNewlyAllocated;
+    };
+
+    // Only process ATTACH if either the pointee or the pointer was newly
+    // allocated, or the ALWAYS flag is set.
+    if (!IsAttachAlways && !WasNewlyAllocated(HstPteeBegin, "pointee") &&
+        !WasNewlyAllocated(HstPtr, "pointer")) {
+      DP("Skipping ATTACH entry %zu: neither pointer nor pointee was newly "
+         "allocated and no ALWAYS flag\n",
+         EntryIdx);
+      continue;
+    }
+
+    // Lambda to perform target pointer lookup and validation
+    auto LookupTargetPointer =
+        [&](void *Ptr, int64_t Size,
+            const char *PtrType) -> std::optional<TargetPointerResultTy> {
+      // ATTACH map-type does not change ref-count, or do any allocation
+      // We just need to do a lookup for the pointer/pointee.
+      TargetPointerResultTy TPR = Device.getMappingInfo().getTgtPtrBegin(
+          Ptr, Size, /*UpdateRefCount=*/false,
+          /*UseHoldRefCount=*/false, /*MustContain=*/true);
+
+      DP("Attach %s lookup - IsPresent=%s, IsHostPtr=%s\n", PtrType,
+         TPR.isPresent() ? "yes" : "no",
+         TPR.Flags.IsHostPointer ? "yes" : "no");
+
+      if (!TPR.isPresent()) {
+        DP("Skipping ATTACH entry %zu: %s not present on device\n", EntryIdx,
+           PtrType);
+        return std::nullopt;
+      }
+      if (TPR.Flags.IsHostPointer) {
+        DP("Skipping ATTACH entry %zu: device version of the %s is a host "
+           "pointer.\n",
+           EntryIdx, PtrType);
+        return std::nullopt;
+      }
+
+      return TPR;
+    };
+
+    // Get device version of the pointee (e.g., &p[10]) first, as we can
+    // release its TPR after extracting the pointer value.
+    void *TgtPteeBegin = [&]() -> void * {
+      if (auto PteeTPROpt = LookupTargetPointer(HstPteeBegin, 0, "pointee"))
+        return PteeTPROpt->TargetPointer;
+      return nullptr;
+    }();
+
+    if (!TgtPteeBegin)
+      continue;
+
+    // Get device version of the pointer (e.g., &p) next. We need to keep its
+    // TPR for use in shadow-pointer handling during pointer-attachment.
+    auto PtrTPROpt = LookupTargetPointer(HstPtr, PtrSize, "pointer");
+    if (!PtrTPROpt)
+      continue;
+    TargetPointerResultTy &PtrTPR = *PtrTPROpt;
+    void **TgtPtrBase = reinterpret_cast<void **>(PtrTPR.TargetPointer);
+
+    // Insert a data-fence before the first pointer-attachment.
+    if (IsFirstPointerAttachment) {
+      IsFirstPointerAttachment = false;
+      DP("Inserting a data fence before the first pointer attachment.\n");
+      Ret = Device.dataFence(AsyncInfo);
+      if (Ret != OFFLOAD_SUCCESS) {
+        REPORT("Failed to insert data fence.\n");
+        return OFFLOAD_FAIL;
+      }
+    }
+
+    // Do the pointer-attachment, i.e. update the device pointer to point to
+    // device pointee.
+    Ret = performPointerAttachment(Device, AsyncInfo, HstPtr, HstPteeBase,
+                                   HstPteeBegin, TgtPtrBase, TgtPteeBegin,
+                                   PtrSize, PtrTPR);
+    if (Ret != OFFLOAD_SUCCESS)
+      return OFFLOAD_FAIL;
+
+    DP("ATTACH entry %zu processed successfully\n", EntryIdx);
+  }
+
+  return OFFLOAD_SUCCESS;
+}
+
 namespace {
 /// This structure contains information to deallocate a target pointer, aka.
 /// used to fix up the shadow map and potentially delete the entry from the
@@ -624,7 +974,8 @@ postProcessingTargetDataEnd(DeviceTy *Device,
 int targetDataEnd(ident_t *Loc, DeviceTy &Device, int32_t ArgNum,
                   void **ArgBases, void **Args, int64_t *ArgSizes,
                   int64_t *ArgTypes, map_var_info_t *ArgNames,
-                  void **ArgMappers, AsyncInfoTy &AsyncInfo, bool FromMapper) {
+                  void **ArgMappers, AsyncInfoTy &AsyncInfo,
+                  AttachInfoTy *AttachInfo, bool FromMapper) {
   int Ret = OFFLOAD_SUCCESS;
   auto *PostProcessingPtrs = new SmallVector<PostProcessingInfo>();
   // process each input.
@@ -635,6 +986,14 @@ int targetDataEnd(ident_t *Loc, DeviceTy &Device, int32_t ArgNum,
         (ArgTypes[I] & OMP_TGT_MAPTYPE_PRIVATE))
       continue;
 
+    // Ignore ATTACH entries - they should only be honored on map-entering
+    // directives. They may be encountered here while handling the "end" part of
+    // "#pragma omp target".
+    if (ArgTypes[I] & OMP_TGT_MAPTYPE_ATTACH) {
+      DP("Ignoring ATTACH entry %d in targetDataEnd\n", I);
+      continue;
+    }
+
     if (ArgMappers && ArgMappers[I]) {
       // Instead of executing the regular path of targetDataEnd, call the
       // targetDataMapper variant which will call targetDataEnd again
@@ -900,7 +1259,8 @@ static int getNonContigMergedDimension(__tgt_target_non_contig *NonContig,
 int targetDataUpdate(ident_t *Loc, DeviceTy &Device, int32_t ArgNum,
                      void **ArgsBase, void **Args, int64_t *ArgSizes,
                      int64_t *ArgTypes, map_var_info_t *ArgNames,
-                     void **ArgMappers, AsyncInfoTy &AsyncInfo, bool) {
+                     void **ArgMappers, AsyncInfoTy &AsyncInfo,
+                     AttachInfoTy *AttachInfo, bool FromMapper) {
   // process each input.
   for (int32_t I = 0; I < ArgNum; ++I) {
     if ((ArgTypes[I] & OMP_TGT_MAPTYPE_LITERAL) ||
@@ -1213,13 +1573,27 @@ static int processDataBefore(ident_t *Loc, int64_t DeviceId, void *HostPtr,
   if (!DeviceOrErr)
     FATAL_MESSAGE(DeviceId, "%s", toString(DeviceOrErr.takeError()).c_str());
 
+  // Create AttachInfo for tracking any ATTACH entries, or new-allocations
+  // when handling the "begin" mapping for a target constructs.
+  AttachInfoTy AttachInfo;
+
   int Ret = targetDataBegin(Loc, *DeviceOrErr, ArgNum, ArgBases, Args, ArgSizes,
-                            ArgTypes, ArgNames, ArgMappers, AsyncInfo);
+                            ArgTypes, ArgNames, ArgMappers, AsyncInfo,
+                            &AttachInfo, false /*FromMapper=*/);
   if (Ret != OFFLOAD_SUCCESS) {
     REPORT("Call to targetDataBegin failed, abort target.\n");
     return OFFLOAD_FAIL;
   }
 
+  // Process collected ATTACH entries
+  if (!AttachInfo.AttachEntries.empty()) {
+    Ret = processAttachEntries(*DeviceOrErr, AttachInfo, AsyncInfo);
+    if (Ret != OFFLOAD_SUCCESS) {
+      REPORT("Failed to process ATTACH entries.\n");
+      return OFFLOAD_FAIL;
+    }
+  }
+
   // List of (first-)private arrays allocated for this target region
   SmallVector<int> TgtArgsPositions(ArgNum, -1);