[llvm] [CodeGen] Expose the extensibility of PassConfig to plugins (PR #139059)

[Stefan Gränitz via llvm-commits]

================
@@ -531,6 +532,39 @@ class LLVM_ABI TargetMachine {
   }
 };
 
+using PassConfigCallback =
+    std::function<void(TargetMachine &, PassManagerBase &, TargetPassConfig *)>;
+
+class TargetPassConfigCallbackRegistry : public PassConfigCallback {
+protected:
+  static SmallVector<PassConfigCallback *, 1> Callbacks;
+
+  explicit TargetPassConfigCallbackRegistry(PassConfigCallback &&C)
+      : PassConfigCallback(std::move(C)) {}
+
+public:
+  static void invokeCallbacks(TargetMachine &TM, PassManagerBase &PM,
+                              TargetPassConfig *PassConfig) {
+    for (const PassConfigCallback *C : Callbacks)
+      (*C)(TM, PM, PassConfig);
+  }
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weliveindetail wrote:

Do we need a registry? IIUC this could a simple freestanding function that is declared here and defined in the cpp (where you have the static member init right now). Maybe let the name express it, like `invokeGlobalTargetPassConfigCallbacks(...)`? We don't need the callbacks vector here in the header. It should be a global `ManagedStatic` then (instead of the static member init).

https://github.com/llvm/llvm-project/pull/139059