[clang] [flang] [llvm] [mlir] [OpenMP][flang] Add initial support for by-ref reductions on the GPU (PR #165714)

[Kareem Ergawy via cfe-commits]

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@@ -2591,8 +2597,49 @@ void OpenMPIRBuilder::emitReductionListCopy(
     // Now that all active lanes have read the element in the
     // Reduce list, shuffle over the value from the remote lane.
     if (ShuffleInElement) {
-      shuffleAndStore(AllocaIP, SrcElementAddr, DestElementAddr, RI.ElementType,
-                      RemoteLaneOffset, ReductionArrayTy);
+      Type *ShuffleType = RI.ElementType;
+      Value *ShuffleSrcAddr = SrcElementAddr;
+      Value *ShuffleDestAddr = DestElementAddr;
+      Value *Zero = ConstantInt::get(Builder.getInt32Ty(), 0);
+      AllocaInst *LocalStorage = nullptr;
+
+      if (IsByRefElem) {
+        assert(RI.ByRefElementType && "Expected by-ref element type to be set");
+        assert(RI.ByRefAllocatedType &&
+               "Expected by-ref allocated type to be set");
+        // For by-ref reductions, we need to copy from the remote lane the
+        // actual value of the partial reduction computed by that remote lane;
+        // rather than, for example, a pointer to that data or, even worse, a
+        // pointer to the descriptor of the by-ref reduction element.
+        ShuffleType = RI.ByRefElementType;
+
+        ShuffleSrcAddr = Builder.CreateGEP(RI.ByRefAllocatedType,
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ergawy wrote:

This is somewhat leaking the Fortran descriptor `struct` to the OMP IR builder. I think we can abstract this away somehow but I do not know if it will worth it.

https://github.com/llvm/llvm-project/pull/165714