[llvm] [ObjCARC] Delete empty autoreleasepools with no autoreleases in them (PR #144788)

[Jon Roelofs via llvm-commits]

================
@@ -2485,6 +2490,137 @@ bool ObjCARCOpt::run(Function &F, AAResults &AA) {
   return Changed;
 }
 
+/// Optimize autorelease pools by eliminating empty push/pop pairs.
+void ObjCARCOpt::OptimizeAutoreleasePools(Function &F) {
+  LLVM_DEBUG(dbgs() << "\n== ObjCARCOpt::OptimizeAutoreleasePools ==\n");
+
+  OptimizationRemarkEmitter ORE(&F);
+
+  // Track empty autorelease pool push/pop pairs
+  SmallVector<std::pair<CallInst *, CallInst *>, 4> EmptyPoolPairs;
+
+  // Process each basic block independently.
+  // TODO: Can we optimize inter-block autorelease pool pairs?
+  // This would involve tracking autorelease pool state across blocks.
+  for (BasicBlock &BB : F) {
+    // Use a stack to track nested autorelease pools
+    SmallVector<std::pair<CallInst *, bool>, 4>
+        PoolStack; // {push_inst, has_autorelease_in_scope}
+
+    for (Instruction &Inst : BB) {
+      ARCInstKind Class = GetBasicARCInstKind(&Inst);
+
+      switch (Class) {
+      case ARCInstKind::AutoreleasepoolPush: {
+        // Start tracking a new autorelease pool scope
+        auto *Push = cast<CallInst>(&Inst);
+        PoolStack.push_back(
+            {Push, false}); // {push_inst, has_autorelease_in_scope}
+        LLVM_DEBUG(dbgs() << "Found autorelease pool push: " << *Push << "\n");
+        break;
+      }
+
+      case ARCInstKind::AutoreleasepoolPop: {
+        auto *Pop = cast<CallInst>(&Inst);
+
+        if (PoolStack.empty())
+          break;
+
+        auto &TopPool = PoolStack.back();
+        CallInst *PendingPush = TopPool.first;
+        bool HasAutoreleaseInScope = TopPool.second;
+
+        // Pop the stack - remove this pool scope
+        PoolStack.pop_back();
+
+        // Early exit if this pop doesn't match the pending push
+        if (Pop->getArgOperand(0)->stripPointerCasts() != PendingPush)
+          break;
+
+        // Early exit if there were autoreleases in this scope
+        if (HasAutoreleaseInScope)
+          break;
+
+        // Optimize: eliminate this empty autorelease pool pair
+        ORE.emit([&]() {
+          return OptimizationRemark(DEBUG_TYPE, "AutoreleasePoolElimination",
+                                    PendingPush)
+                 << "eliminated empty autorelease pool pair";
+        });
+
+        // Store the pair for careful deletion later
+        EmptyPoolPairs.push_back({PendingPush, Pop});
+
+        ++NumNoops;
+        break;
+      }
+      case ARCInstKind::CallOrUser:
+      case ARCInstKind::Call:
+      case ARCInstKind::Autorelease:
+      case ARCInstKind::AutoreleaseRV:
+      case ARCInstKind::FusedRetainAutorelease:
+      case ARCInstKind::FusedRetainAutoreleaseRV:
+      case ARCInstKind::LoadWeak: {
+        // Track that we have autorelease calls in the current pool scope
+        if (!PoolStack.empty()) {
+          PoolStack.back().second = true; // Set has_autorelease_in_scope = true
+          LLVM_DEBUG(
+              dbgs()
+              << "Found autorelease or potiential autorelease in pool scope: "
+              << Inst << "\n");
+        }
+        break;
+      }
+
+      // Enumerate all remaining ARCInstKind cases explicitly
+      case ARCInstKind::Retain:
+      case ARCInstKind::RetainRV:
+      case ARCInstKind::UnsafeClaimRV:
+      case ARCInstKind::RetainBlock:
+      case ARCInstKind::Release:
+      case ARCInstKind::NoopCast:
+      case ARCInstKind::LoadWeakRetained:
+      case ARCInstKind::StoreWeak:
+      case ARCInstKind::InitWeak:
+      case ARCInstKind::MoveWeak:
+      case ARCInstKind::CopyWeak:
+      case ARCInstKind::DestroyWeak:
+      case ARCInstKind::StoreStrong:
+      case ARCInstKind::IntrinsicUser:
+      case ARCInstKind::User:
+      case ARCInstKind::None:
+        // These instruction kinds don't affect autorelease pool optimization
+        break;
+      }
+    }
+  }
+
+  // Handle empty pool pairs carefully to avoid use-after-delete
+  SmallVector<CallInst *, 8> DeadInsts;
+  for (auto &Pair : EmptyPoolPairs) {
+    CallInst *Push = Pair.first;
+    CallInst *Pop = Pair.second;
+
+    // Replace all uses of push with poison before deletion
+    Push->replaceAllUsesWith(PoisonValue::get(Push->getType()));
+
+    LLVM_DEBUG(dbgs() << "Erasing empty pool pair: " << *Push << " and " << *Pop
+                      << "\n");
+    DeadInsts.push_back(Pop);
+    DeadInsts.push_back(Push);
----------------
jroelofs wrote:

If you call eraseFromParent here, and set Changed to true at every iteration, you can get rid of the second if/loop.

https://github.com/llvm/llvm-project/pull/144788