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

[Jon Roelofs via llvm-commits]

================
@@ -2485,6 +2490,202 @@ 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) || isa<BitCastInst>(U)) {
+      Value *CastResult = dyn_cast<Value>(U);
+      if (CastResult &&
+          checkLeadsToTarget(CastResult, 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
+        CallInst *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: {
+        CallInst *Pop = cast<CallInst>(&Inst);
+
+        if (!PoolStack.empty()) {
+          auto &TopPool = PoolStack.back();
+          CallInst *PendingPush = TopPool.first;
+          bool HasAutoreleaseInScope = TopPool.second;
+
+          // Check if this pop matches the pending push by comparing the token
+          Value *PopArg = Pop->getArgOperand(0);
+          bool IsMatchingPop = (PopArg == PendingPush);
+
+          // Also handle pointer casts by stripping them
+          if (!IsMatchingPop) {
+            Value *StrippedPopArg = PopArg->stripPointerCasts();
+            IsMatchingPop = (StrippedPopArg == PendingPush);
+          }
+
+          // Before adding to EmptyPoolPairs, verify ALL uses of the push
+          // eventually lead to this specific pop instruction
+          bool SafeToOptimize = true;
+          if (IsMatchingPop) {
+            for (User *U : PendingPush->users()) {
+              // Check if this use eventually leads to our pop instruction
+              bool LeadsToExpectedPop = false;
+
+              if (U == Pop) {
+                LeadsToExpectedPop = true;
+              } else {
+                // For pointer casts, check if they eventually lead to the pop
+                // (up to 3 levels deep)
+                Value *CastResult = dyn_cast<Value>(U);
+                if (CastResult && isa<CastInst>(U)) {
+                  SmallPtrSet<Value *, 8> Visited;
+                  if (checkLeadsToTarget(CastResult, Pop, 3, Visited)) {
+                    LeadsToExpectedPop = true;
+                  }
+                }
+              }
+
+              if (!LeadsToExpectedPop) {
+                SafeToOptimize = false;
+                LLVM_DEBUG(dbgs()
+                           << "Unsafe to optimize: push has unexpected use: "
+                           << *U << "\n");
+                break;
+              }
+            }
+          }
+
+          if (IsMatchingPop && !HasAutoreleaseInScope && SafeToOptimize) {
----------------
jroelofs wrote:

recommend jump-threading this if into the preceding one to get rid of the `IsMatchingPop &&`

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