[flang] [llvm] [flang][OpenMP] Decompose compound constructs, do recursive lowering (PR #90098)

[Kareem Ergawy via llvm-commits]

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+//===- ConstructDecompositionT.h -- Decomposing compound constructs -------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+// Given a compound construct with a set of clauses, generate the list of
+// constituent leaf constructs, each with a list of clauses that apply to it.
+//
+// Note: Clauses that are not originally present, but that are implied by the
+// OpenMP spec are materialized, and are present in the output.
+//
+// Note: Composite constructs will also be broken up into leaf constructs.
+// If composite constructs require processing as a whole, the lists of clauses
+// for each leaf constituent should be concatenated.
+//===----------------------------------------------------------------------===//
+#ifndef LLVM_FRONTEND_OPENMP_CONSTRUCTDECOMPOSITIONT_H
+#define LLVM_FRONTEND_OPENMP_CONSTRUCTDECOMPOSITIONT_H
+
+#include "llvm/ADT/ArrayRef.h"
+#include "llvm/ADT/DenseMap.h"
+#include "llvm/ADT/DenseSet.h"
+#include "llvm/ADT/STLExtras.h"
+#include "llvm/ADT/SmallVector.h"
+#include "llvm/ADT/iterator_range.h"
+#include "llvm/Frontend/OpenMP/ClauseT.h"
+#include "llvm/Frontend/OpenMP/OMP.h"
+
+#include <iterator>
+#include <list>
+#include <optional>
+#include <tuple>
+#include <type_traits>
+#include <utility>
+#include <variant>
+
+static inline llvm::ArrayRef<llvm::omp::Directive> getWorksharing() {
+  static llvm::omp::Directive worksharing[] = {
+      llvm::omp::Directive::OMPD_do,     llvm::omp::Directive::OMPD_for,
+      llvm::omp::Directive::OMPD_scope,  llvm::omp::Directive::OMPD_sections,
+      llvm::omp::Directive::OMPD_single, llvm::omp::Directive::OMPD_workshare,
+  };
+  return worksharing;
+}
+
+static inline llvm::ArrayRef<llvm::omp::Directive> getWorksharingLoop() {
+  static llvm::omp::Directive worksharingLoop[] = {
+      llvm::omp::Directive::OMPD_do,
+      llvm::omp::Directive::OMPD_for,
+  };
+  return worksharingLoop;
+}
+
+namespace detail {
+template <typename Container, typename Predicate>
+typename std::remove_reference_t<Container>::iterator
+find_unique(Container &&container, Predicate &&pred) {
+  auto first = std::find_if(container.begin(), container.end(), pred);
+  if (first == container.end())
+    return first;
+  auto second = std::find_if(std::next(first), container.end(), pred);
+  if (second == container.end())
+    return first;
+  return container.end();
----------------
ergawy wrote:

I think this should work and might be a bit easier to read:
```suggestion
  std::count_if(container.begin(), container.end(), pred) == 1 ? first : container.end();
```

https://github.com/llvm/llvm-project/pull/90098