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

[Jon Roelofs via llvm-commits]

================
@@ -2485,6 +2490,169 @@ bool ObjCARCOpt::run(Function &F, AAResults &AA) {
   return Changed;
 }
 
+/// Helper function to recursively check if a value eventually leads to the
+/// target instruction through pointer casts and uses, up to a specified depth.
+static bool checkLeadsToTarget(Value *Val, User *Target, unsigned MaxDepth,
+                               SmallPtrSet<Value *, 8> &Visited) {
+  if (MaxDepth == 0)
+    return false;
+
+  // Avoid infinite recursion by tracking visited values
+  if (!Visited.insert(Val).second)
+    return false;
+
+  for (User *U : Val->users()) {
+    if (U == Target)
+      return true;
+
+    // For pointer casts, recursively check their users
+    if (isa<CastInst>(U)) {
+      if (checkLeadsToTarget(U, Target, MaxDepth - 1, Visited)) {
+        return true;
+      }
+    }
+  }
+
+  return false;
+}
+
+/// 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
+        LLVM_DEBUG(dbgs() << "Eliminating empty autorelease pool pair: "
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jroelofs wrote:

I think the remake should be emitted here.

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