[OpenMP] Add support for custom callback in AMDGPUStream (llvm#112785)

Summary:
We have the ability to schedule callbacks after certain events complete.
Currently we can register an arbitrary callback in CUDA, but can't in
AMDGPU. I am planning on using this support to move the RPC handling to
a separate thread, then using these callbacks to suspend / resume it
when no kernels are running. This is a preliminary patch to keep this
noise out of that one.

offload/plugins-nextgen/amdgpu/src/rtl.cpp

@@ -927,6 +927,8 @@ struct AMDGPUStreamTy {
     AMDGPUSignalManagerTy *SignalManager;
   };
 
+  using AMDGPUStreamCallbackTy = Error(void *Data);
+
   /// The stream is composed of N stream's slots. The struct below represents
   /// the fields of each slot. Each slot has a signal and an optional action
   /// function. When appending an HSA asynchronous operation to the stream, one
@@ -942,65 +944,82 @@ struct AMDGPUStreamTy {
     /// operation as input signal.
     AMDGPUSignalTy *Signal;
 
-    /// The action that must be performed after the operation's completion. Set
+    /// The actions that must be performed after the operation's completion. Set
     /// to nullptr when there is no action to perform.
-    Error (*ActionFunction)(void *);
+    llvm::SmallVector<AMDGPUStreamCallbackTy *> Callbacks;
 
     /// Space for the action's arguments. A pointer to these arguments is passed
     /// to the action function. Notice the space of arguments is limited.
-    union {
+    union ActionArgsTy {
       MemcpyArgsTy MemcpyArgs;
       ReleaseBufferArgsTy ReleaseBufferArgs;
       ReleaseSignalArgsTy ReleaseSignalArgs;
-    } ActionArgs;
+      void *CallbackArgs;
+    };
+
+    llvm::SmallVector<ActionArgsTy> ActionArgs;
 
     /// Create an empty slot.
-    StreamSlotTy() : Signal(nullptr), ActionFunction(nullptr) {}
+    StreamSlotTy() : Signal(nullptr), Callbacks({}), ActionArgs({}) {}
 
     /// Schedule a host memory copy action on the slot.
     Error schedHostMemoryCopy(void *Dst, const void *Src, size_t Size) {
-      ActionFunction = memcpyAction;
-      ActionArgs.MemcpyArgs = MemcpyArgsTy{Dst, Src, Size};
+      Callbacks.emplace_back(memcpyAction);
+      ActionArgs.emplace_back().MemcpyArgs = MemcpyArgsTy{Dst, Src, Size};
       return Plugin::success();
     }
 
     /// Schedule a release buffer action on the slot.
     Error schedReleaseBuffer(void *Buffer, AMDGPUMemoryManagerTy &Manager) {
-      ActionFunction = releaseBufferAction;
-      ActionArgs.ReleaseBufferArgs = ReleaseBufferArgsTy{Buffer, &Manager};
+      Callbacks.emplace_back(releaseBufferAction);
+      ActionArgs.emplace_back().ReleaseBufferArgs =
+          ReleaseBufferArgsTy{Buffer, &Manager};
       return Plugin::success();
     }
 
     /// Schedule a signal release action on the slot.
     Error schedReleaseSignal(AMDGPUSignalTy *SignalToRelease,
                              AMDGPUSignalManagerTy *SignalManager) {
-      ActionFunction = releaseSignalAction;
-      ActionArgs.ReleaseSignalArgs =
+      Callbacks.emplace_back(releaseSignalAction);
+      ActionArgs.emplace_back().ReleaseSignalArgs =
           ReleaseSignalArgsTy{SignalToRelease, SignalManager};
       return Plugin::success();
     }
 
+    /// Register a callback to be called on compleition
+    Error schedCallback(AMDGPUStreamCallbackTy *Func, void *Data) {
+      Callbacks.emplace_back(Func);
+      ActionArgs.emplace_back().CallbackArgs = Data;
+
+      return Plugin::success();
+    }
+
     // Perform the action if needed.
     Error performAction() {
-      if (!ActionFunction)
+      if (Callbacks.empty())
         return Plugin::success();
 
-      // Perform the action.
-      if (ActionFunction == memcpyAction) {
-        if (auto Err = memcpyAction(&ActionArgs))
-          return Err;
-      } else if (ActionFunction == releaseBufferAction) {
-        if (auto Err = releaseBufferAction(&ActionArgs))
-          return Err;
-      } else if (ActionFunction == releaseSignalAction) {
-        if (auto Err = releaseSignalAction(&ActionArgs))
-          return Err;
-      } else {
-        return Plugin::error("Unknown action function!");
+      assert(Callbacks.size() == ActionArgs.size() && "Size mismatch");
+      for (auto [Callback, ActionArg] : llvm::zip(Callbacks, ActionArgs)) {
+        // Perform the action.
+        if (Callback == memcpyAction) {
+          if (auto Err = memcpyAction(&ActionArg))
+            return Err;
+        } else if (Callback == releaseBufferAction) {
+          if (auto Err = releaseBufferAction(&ActionArg))
+            return Err;
+        } else if (Callback == releaseSignalAction) {
+          if (auto Err = releaseSignalAction(&ActionArg))
+            return Err;
+        } else if (Callback) {
+          if (auto Err = Callback(ActionArg.CallbackArgs))
+            return Err;
+        }
       }
 
       // Invalidate the action.
-      ActionFunction = nullptr;
+      Callbacks.clear();
+      ActionArgs.clear();
 
       return Plugin::success();
     }