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

[Adrian Prantl via llvm-commits]

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@@ -2514,6 +2521,181 @@ void PrintCrashIRInstrumentation::registerCallbacks(
       });
 }
 
+void DroppedVariableStats::registerCallbacks(
+    PassInstrumentationCallbacks &PIC) {
+  if (!DroppedVarStats)
+    return;
+
+  PIC.registerBeforeNonSkippedPassCallback(
+      [this](StringRef P, Any IR) { return this->runBeforePass(P, IR); });
+  PIC.registerAfterPassCallback(
+      [this](StringRef P, Any IR, const PreservedAnalyses &PA) {
+        return this->runAfterPass(P, IR, PA);
+      });
+  PIC.registerAfterPassInvalidatedCallback(
+      [this](StringRef P, const PreservedAnalyses &PA) {
+        return this->runAfterPassInvalidated(P, PA);
+      });
+}
+
+void DroppedVariableStats::runBeforePass(StringRef PassID, Any IR) {
+  DebugVariablesStack.push_back(
+      {DenseMap<const Function *, DenseSet<VarID>>(),
+       DenseMap<const Function *, DenseSet<VarID>>()});
+  InlinedAts.push_back(DenseMap<StringRef, DenseMap<VarID, DILocation *>>());
+  if (auto *M = unwrapIR<Module>(IR))
+    return this->runOnModule(M, true);
+  if (auto *F = unwrapIR<Function>(IR))
+    return this->runOnFunction(F, true);
+  return;
+}
+
+void DroppedVariableStats::runOnFunction(const Function *F, bool Before) {
+  auto &DebugVariables = DebugVariablesStack.back();
+  auto &VarIDMap = (Before ? DebugVariables.DebugVariablesBefore
+                           : DebugVariables.DebugVariablesAfter);
+  auto &InlinedAtsMap = InlinedAts.back();
+  auto FuncName = F->getName();
+  if (Before)
+    InlinedAtsMap.try_emplace(FuncName, DenseMap<VarID, DILocation *>());
+  VarIDMap.try_emplace(F, DenseSet<VarID>());
+  auto &VarIDs = VarIDMap[F];
+  for (const auto &I : instructions(F)) {
+    for (DbgRecord &DR : I.getDbgRecordRange()) {
+      if (auto *Dbg = dyn_cast<DbgVariableRecord>(&DR)) {
+        auto *DbgVar = Dbg->getVariable();
+        auto DbgLoc = DR.getDebugLoc();
+        VarID Key{DbgVar->getScope(), DbgLoc->getInlinedAtScope(), DbgVar};
+        VarIDs.insert(Key);
+        if (Before)
+          InlinedAtsMap[FuncName].try_emplace(Key, DbgLoc.getInlinedAt());
+      }
+    }
+  }
+}
+
+void DroppedVariableStats::runOnModule(const Module *M, bool Before) {
+  for (auto &F : *M)
+    runOnFunction(&F, Before);
+}
+
+void DroppedVariableStats::removeVarFromAllSets(VarID Var, const Function *F) {
+  // Do not remove Var from the last element, it will be popped from the stack.
+  for (auto &DebugVariables : llvm::drop_end(DebugVariablesStack))
+    DebugVariables.DebugVariablesBefore[F].erase(Var);
+}
+
+void DroppedVariableStats::calculateDroppedVarStatsOnModule(
+    const Module *M, StringRef PassID, std::string FuncOrModName,
+    std::string PassLevel) {
+  for (auto &F : *M) {
+    calculateDroppedVarStatsOnFunction(&F, PassID, FuncOrModName, PassLevel);
+  }
+}
+
+void DroppedVariableStats::calculateDroppedVarStatsOnFunction(
+    const Function *F, StringRef PassID, std::string FuncOrModName,
+    std::string PassLevel) {
+  unsigned DroppedCount = 0;
+  auto FuncName = F->getName();
+  auto &DebugVariables = DebugVariablesStack.back();
+  auto &DebugVariablesBeforeMap = DebugVariables.DebugVariablesBefore[F];
+  auto &DebugVariablesAfterMap = DebugVariables.DebugVariablesAfter[F];
+  auto &InlinedAtsMap = InlinedAts.back()[FuncName];
+  // Find an Instruction that shares the same scope as the dropped #dbg_value or
+  // has a scope that is the child of the scope of the #dbg_value, and has an
+  // inlinedAt equal to the inlinedAt of the #dbg_value or it's inlinedAt chain
+  // contains the inlinedAt of the #dbg_value, if such an Instruction is found,
+  // debug information is dropped.
+  for (auto Var : DebugVariablesBeforeMap) {
+    if (!DebugVariablesAfterMap.contains(Var)) {
+      const auto *DbgValScope = std::get<0>(Var);
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adrian-prantl wrote:

If you can fit it within the 80 column limit, I would replace the autos here with the type names

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