[Mlir-commits] [mlir] [flang] [openmp] [Flang][OpenMP] Initial mapping of Fortran pointers and allocatables for target devices (PR #71766)

[Kiran Chandramohan]

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+<!--===- docs/OpenMP-descriptor-management.md
+
+   Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+   See https://llvm.org/LICENSE.txt for license information.
+   SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+-->
+
+# OpenMP dialect: Fortran descriptor type mapping for offload
+
+The descriptor mapping for OpenMP currently works differently to the planned direction for OpenACC, however, 
+it is possible and would likely be ideal to align the method with OpenACC in the future. However, at least 
+currently the OpenMP specification is less descriptive and has less stringent rules around descriptor based
+types so does not require as complex a set of descriptor management rules (although, in certain cases 
+for the interim adopting OpenACC's rules where it makes sense could be useful).
+
+The initial method for mapping Fortran types tied to descriptors for OpenMP offloading is to treat these types 
+as a special case of OpenMP record type (C/C++ structure/class, Fortran derived type etc.) mapping as far as the 
+runtime is concerned. Where the box (descriptor information) is the holding container and the underlying 
+data pointer is contained within the container, and we must generate explicit maps for both the pointer member and
+the container. As an example, a small C++ program that is equivalent to the concept described:
+
+```C++
+struct mock_descriptor {
+  long int x;
+  std::byte x1, x2, x3, x4;
+  void *pointer;
+  long int lx[1][3];
+};
+
+int main() {
+mock_descriptor data;
+#pragma omp target map(tofrom: data, data.pointer[:upper_bound])
+{
+    do something... 
+}
+
+ return 0;
+}
+```
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kiranchandramohan wrote:

Or alternatively specify that the example covers a C++ structure that looks like the descriptor used by Flang.

https://github.com/llvm/llvm-project/pull/71766