[llvm] Add a pass to collect dropped variable statistics (PR #102233)

[Adrian Prantl via llvm-commits]

================
@@ -578,6 +579,83 @@ class PrintCrashIRInstrumentation {
   static void SignalHandler(void *);
 };
 
+/// A class to collect and print dropped debug information variable statistics.
+/// After every LLVM IR pass is run, it will print how many #dbg_values were
+/// dropped due to that pass.
+class DroppedVariableStats {
+public:
+  DroppedVariableStats(bool DroppedVarStatsEnabled) {
+    if (DroppedVarStatsEnabled)
+      llvm::outs()
+          << "Pass Level, Pass Name, Num of Dropped Variables, Func or "
+             "Module Name\n";
+  };
+  // We intend this to be unique per-compilation, thus no copies.
+  DroppedVariableStats(const DroppedVariableStats &) = delete;
+  void operator=(const DroppedVariableStats &) = delete;
+
+  void registerCallbacks(PassInstrumentationCallbacks &PIC);
+  void runBeforePass(StringRef PassID, Any IR);
+  void runAfterPass(StringRef PassID, Any IR, const PreservedAnalyses &PA);
+  void runAfterPassInvalidated(StringRef PassID, const PreservedAnalyses &PA);
+  bool getPassDroppedVariables() { return PassDroppedVariables; }
+
+private:
+  bool PassDroppedVariables = false;
+  /// VarID is a unique key that represents a #dbg_value
----------------
adrian-prantl wrote:

```suggestion
  /// A unique key that represents a #dbg_value.
```

https://github.com/llvm/llvm-project/pull/102233