[llvm] Introduce CovMap in ObjectYAML (PR #127432)

[Jessica Paquette via llvm-commits]

================
@@ -0,0 +1,385 @@
+//===- CovMap.h - ObjectYAML Interface for coverage map ---------*- C++ -*-===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+//
+// - llvm::coverage::yaml
+//
+//   Describes binary file formats and YAML structures of coverage map.
+//
+// - llvm::yaml
+//
+//   Attachments for YAMLTraits.
+//
+// - llvm::covmap
+//
+//   Provides YAML encoder and decoder for coverage map.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_OBJECTYAML_COVMAP_H
+#define LLVM_OBJECTYAML_COVMAP_H
+
+#include "llvm/ADT/ArrayRef.h"
+#include "llvm/ADT/DenseMap.h"
+#include "llvm/ADT/StringRef.h"
+#include "llvm/ObjectYAML/ELFYAML.h"
+#include "llvm/Support/Error.h"
+#include "llvm/Support/YAMLTraits.h"
+#include <cstdint>
+#include <memory>
+#include <optional>
+#include <variant>
+
+namespace llvm {
+class InstrProfSymtab;
+class raw_ostream;
+} // namespace llvm
+
+namespace llvm::coverage::yaml {
+
+/// This works like vector container but can be replaced with
+/// MutableArrayRef. See also SequenceTraits<VectorOrRef>.
+template <typename T, typename Vec = std::vector<T>> class VectorOrRef {
+  using Ref = MutableArrayRef<T>;
+
+  /// Holds vector type initially.
+  std::variant<Vec, Ref> Array;
+
+public:
+  // FIXME: Iterator impl is minimal easy.
+  using iterator = T *;
+
+  iterator begin() {
+    if (auto *V = std::get_if<Vec>(&Array))
+      return &V->front();
+    else
+      return &std::get<Ref>(Array).front();
+  }
+
+  iterator end() {
+    if (auto *V = std::get_if<Vec>(&Array))
+      return &V->back() + 1;
+    else
+      return &std::get<Ref>(Array).back() + 1;
+  }
+
+  size_t size() const {
+    if (const auto *V = std::get_if<Vec>(&Array))
+      return V->size();
+    else
+      return std::get<Ref>(Array).size();
+  }
+
+  T &operator[](int Idx) {
+    if (auto *V = std::get_if<Vec>(&Array))
+      return (*V)[Idx];
+    else
----------------
ornata wrote:

https://llvm.org/docs/CodingStandards.html#don-t-use-else-after-a-return

prefer 

```
if ()
  return ...
return ...
```

https://github.com/llvm/llvm-project/pull/127432