[flang-commits] [flang] [Flang][MLIR][OpenMP] Create a deferred declare target marking process for Bridge.cpp (PR #78502)

[Sergio Afonso via flang-commits]

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+<!--===- docs/OpenMP-declare-target.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
+
+-->
+
+# Introduction to Declare Target
+
+In OpenMP `declare target` is a directive that can be applied to a function or variable (primarily global) to notate to the compiler that it should be generated in a particular devices environment. In essence whether something should be emitted for host or device, or both. An example of its usage for both data and functions can be seen below.
+
+```Fortran
+module test_0
+    integer :: sp = 0
+!$omp declare target link(sp)
+end module test_0
+
+program main
+    use test_0
+!$omp target map(tofrom:sp)
+    sp = 1
+!$omp end target
+end program
+```
+
+In the above example, we created a variable in a seperate module, mark it as `declare target` and then map it, embedding it into the device IR and assigning to it. 
+
+
+```Fortran
+function func_t_device() result(i)
+    !$omp declare target to(func_t_device) device_type(nohost)
+        INTEGER :: I
+        I = 1
+end function func_t_device
+
+program main
+!$omp target
+    call func_t_device()
+!$omp end target
+end program
+```
+
+In the above example, we are stating that a function is required on device utilising `declare target`, and that we will not be utilising it on host, so we are in theory free to remove or ignore it. A user could also in this case, leave off the `declare target` from the function and it would be implicitly marked `declare target any` (for both host and device), as it's been utilised within a target region.
+
+# Declare Target as represented in the OpenMP Dialect
+
+In the OpenMP Dialect `declare target` is not represented by a specific `operation` instead it's a OpenMP dialect specific `attribute` that can be applied to any operation in any dialect. This helps to simplify the utilisation of it, instead of replacing or modifying existing global or function `operations` in a dialect it applies to it as extra metadata that the lowering can use in different ways as is neccesary. 
+
+The `attribute` is composed of multiple fields representing the clauses you would find on the `declare target` directive i.e. device type (`nohost`, `any`, `host`) or the capture clause (`link` or `to`). A small example of `declare target` applied to an Fortran `real` can be found below:
+
+```
+fir.global internal @_QFEi {omp.declare_target = #omp.declaretarget<device_type = (any), capture_clause = (to)>} : f32 {
+    %0 = fir.undefined f32
+    fir.has_value %0 : f32
+}
+```
+
+This would look similar for function style `operations`.
+
+The application and access of this attribute is aided by an OpenMP Dialect MLIR Interface named `DeclareTargetInterface`, which can be utilised on operations to access the appropriate interface functions, e.g.:
+
+```C++
+auto declareTargetGlobal = llvm::dyn_cast<mlir::omp::DeclareTargetInterface>(Op.getOperation());
+declareTargetGlobal.isDeclareTarget();
+```
+
+# Declare Target Fortran OpenMP Lowering
+
+The initial lowering of `declare target` to MLIR for both use-cases is done inside of the usual OpenMP lowering in flang/lib/Lower/OpenMP.cpp, however some direct calls to `declare target` related functions from Flang's Bridge in flang/lib/Lower/Bridge.cpp are made.
----------------
skatrak wrote:

```suggestion
The initial lowering of `declare target` to MLIR for both use-cases is done inside of the usual OpenMP lowering in flang/lib/Lower/OpenMP.cpp. However, some direct calls to `declare target` related functions from Flang's Bridge in flang/lib/Lower/Bridge.cpp are made.
```

https://github.com/llvm/llvm-project/pull/78502