[OpenMP] Avoid checking parent reference count in targetDataEnd

[jdenny]

Summary

The patch has the following benefits:

• Eliminates a lock/unlock of the data mapping table.
• Clarifies the logic that determines whether a struct member's device-to-host transfer occurs. The old logic, which checks the parent struct's reference count, is a leftover from back when we had a different map interface (as pointed out at https://reviews.llvm.org/D104924#2846972).

[tianshilei1992]

I'm also very curious about that. If and only if the parent struct's reference count 1.

[grokos]

The new code is equivalent to the old one but clearer and with one fewer lock. Checking the parent struct is a leftover from back when we had a different map interface.

[jdenny]

In D104924#2846972, @grokos wrote:

The new code is equivalent to the old one but clearer and with one fewer lock. Checking the parent struct is a leftover from back when we had a different map interface.

Thanks! I'm going to integrate this comment into the commit log.

I'll soon propose a similar patch for targetDataBegin.

[jdenny]

Should I try to land this patch now? It's been accepted, but the similar D105121 has not and seemed to stir more controversy.

[jdenny]

Rebased.

openmp/libomptarget/src/omptarget.cpp

for (int32_t I = ArgNum - 1; I >= 0; --I) {
     if ((ArgTypes[I] & OMP_TGT_MAPTYPE_FROM) || DelEntry) {
       // Move data back to the host
       if (ArgTypes[I] & OMP_TGT_MAPTYPE_FROM) {
         bool Always = ArgTypes[I] & OMP_TGT_MAPTYPE_ALWAYS;
         bool CopyMember = false;
         if (!(PM->RTLs.RequiresFlags & OMP_REQ_UNIFIED_SHARED_MEMORY) ||
             HasCloseModifier) {
           if ((ArgTypes[I] & OMP_TGT_MAPTYPE_MEMBER_OF) &&
-              !(ArgTypes[I] & OMP_TGT_MAPTYPE_PTR_AND_OBJ)) {
-            // Copy data only if the "parent" struct has RefCount==1.
-            int32_t ParentIdx = getParentIndex(ArgTypes[I]);
-            uint64_t ParentRC = Device.getMapEntryRefCnt(Args[ParentIdx]);
-            assert(ParentRC > 0 && "parent struct not found");
-            if (ParentRC == 1)
-              CopyMember = true;
-          }
+              !(ArgTypes[I] & OMP_TGT_MAPTYPE_PTR_AND_OBJ))
+            CopyMember = IsLast;
         }
 
         if ((DelEntry || Always || CopyMember) &&
             !(PM->RTLs.RequiresFlags & OMP_REQ_UNIFIED_SHARED_MEMORY &&
               TgtPtrBegin == HstPtrBegin)) {
           DP("Moving %" PRId64 " bytes (tgt:" DPxMOD ") -> (hst:" DPxMOD ")\n",
              DataSize, DPxPTR(TgtPtrBegin), DPxPTR(HstPtrBegin));
           Ret = Device.retrieveData(HstPtrBegin, TgtPtrBegin, DataSize,