[llvm] [OFFLOAD] Add support for indexed per-thread containers (PR #164263)

[Kevin Sala Penades via llvm-commits]

================
@@ -14,26 +14,106 @@
 #define OFFLOAD_PERTHREADTABLE_H
 
 #include <list>
+#include <llvm/ADT/SmallVector.h>
+#include <llvm/Support/Error.h>
 #include <memory>
 #include <mutex>
+#include <type_traits>
+
+template <typename ObjectType> struct PerThread {
+  struct PerThreadData {
+    std::unique_ptr<ObjectType> ThreadEntry;
+  };
+
+  std::mutex Mutex;
+  llvm::SmallVector<std::shared_ptr<PerThreadData>> ThreadDataList;
+
+  // define default constructors, disable copy and move constructors
+  PerThread() = default;
+  PerThread(const PerThread &) = delete;
+  PerThread(PerThread &&) = delete;
+  PerThread &operator=(const PerThread &) = delete;
+  PerThread &operator=(PerThread &&) = delete;
+  ~PerThread() {
+    assert(Mutex.try_lock() &&
+           "Cannot be deleted while other threads are adding entries");
----------------
kevinsala wrote:

It's undefined behavior to destroy the mutex while it's locked:
> The behavior of a program is undefined if a mutex is destroyed while still owned by any threads, or a thread terminates while owning a mutex.

Given that there is no function to test the mutex, you may use:

```cpp
assert(M.try_lock() && (M.unlock(), true) && "Cannot ...");
```

https://github.com/llvm/llvm-project/pull/164263