[llvm] [ObjCARC] Delete empty autoreleasepools with no autoreleases in them (PR #144788)
via llvm-commits
llvm-commits at lists.llvm.org
Wed Jun 18 16:27:12 PDT 2025
https://github.com/AZero13 updated https://github.com/llvm/llvm-project/pull/144788
>From 797ffa6cc4b7d38b76c736a5ad649ee76f41b95d Mon Sep 17 00:00:00 2001
From: Rose <gfunni234 at gmail.com>
Date: Wed, 18 Jun 2025 16:05:44 -0400
Subject: [PATCH] [ObjCARC] Delete empty autoreleasepools with no autoreleases
in them
Erase empty autorelease pools that have no autorelease in them
---
.../ObjCARC/ARCRuntimeEntryPoints.h | 16 ++
llvm/lib/Transforms/ObjCARC/ObjCARCOpts.cpp | 212 +++++++++++++++++-
.../ObjCARC/test_autorelease_pool.ll | 106 +++++++++
3 files changed, 329 insertions(+), 5 deletions(-)
create mode 100644 llvm/test/Transforms/ObjCARC/test_autorelease_pool.ll
diff --git a/llvm/lib/Transforms/ObjCARC/ARCRuntimeEntryPoints.h b/llvm/lib/Transforms/ObjCARC/ARCRuntimeEntryPoints.h
index 3fa844eda21cf..6135c7b938a3e 100644
--- a/llvm/lib/Transforms/ObjCARC/ARCRuntimeEntryPoints.h
+++ b/llvm/lib/Transforms/ObjCARC/ARCRuntimeEntryPoints.h
@@ -46,6 +46,8 @@ enum class ARCRuntimeEntryPointKind {
UnsafeClaimRV,
RetainAutorelease,
RetainAutoreleaseRV,
+ AutoreleasePoolPush,
+ AutoreleasePoolPop,
};
/// Declarations for ObjC runtime functions and constants. These are initialized
@@ -67,6 +69,8 @@ class ARCRuntimeEntryPoints {
UnsafeClaimRV = nullptr;
RetainAutorelease = nullptr;
RetainAutoreleaseRV = nullptr;
+ AutoreleasePoolPush = nullptr;
+ AutoreleasePoolPop = nullptr;
}
Function *get(ARCRuntimeEntryPointKind kind) {
@@ -101,6 +105,12 @@ class ARCRuntimeEntryPoints {
case ARCRuntimeEntryPointKind::RetainAutoreleaseRV:
return getIntrinsicEntryPoint(RetainAutoreleaseRV,
Intrinsic::objc_retainAutoreleaseReturnValue);
+ case ARCRuntimeEntryPointKind::AutoreleasePoolPush:
+ return getIntrinsicEntryPoint(AutoreleasePoolPush,
+ Intrinsic::objc_autoreleasePoolPush);
+ case ARCRuntimeEntryPointKind::AutoreleasePoolPop:
+ return getIntrinsicEntryPoint(AutoreleasePoolPop,
+ Intrinsic::objc_autoreleasePoolPop);
}
llvm_unreachable("Switch should be a covered switch.");
@@ -143,6 +153,12 @@ class ARCRuntimeEntryPoints {
/// Declaration for objc_retainAutoreleaseReturnValue().
Function *RetainAutoreleaseRV = nullptr;
+ /// Declaration for objc_autoreleasePoolPush().
+ Function *AutoreleasePoolPush = nullptr;
+
+ /// Declaration for objc_autoreleasePoolPop().
+ Function *AutoreleasePoolPop = nullptr;
+
Function *getIntrinsicEntryPoint(Function *&Decl, Intrinsic::ID IntID) {
if (Decl)
return Decl;
diff --git a/llvm/lib/Transforms/ObjCARC/ObjCARCOpts.cpp b/llvm/lib/Transforms/ObjCARC/ObjCARCOpts.cpp
index 5eb3f51d38945..d50292f5b43fc 100644
--- a/llvm/lib/Transforms/ObjCARC/ObjCARCOpts.cpp
+++ b/llvm/lib/Transforms/ObjCARC/ObjCARCOpts.cpp
@@ -39,6 +39,7 @@
#include "llvm/Analysis/ObjCARCAnalysisUtils.h"
#include "llvm/Analysis/ObjCARCInstKind.h"
#include "llvm/Analysis/ObjCARCUtil.h"
+#include "llvm/Analysis/OptimizationRemarkEmitter.h"
#include "llvm/IR/BasicBlock.h"
#include "llvm/IR/CFG.h"
#include "llvm/IR/Constant.h"
@@ -132,11 +133,8 @@ static const Value *FindSingleUseIdentifiedObject(const Value *Arg) {
//
// The second retain and autorelease can be deleted.
-// TODO: It should be possible to delete
-// objc_autoreleasePoolPush and objc_autoreleasePoolPop
-// pairs if nothing is actually autoreleased between them. Also, autorelease
-// calls followed by objc_autoreleasePoolPop calls (perhaps in ObjC++ code
-// after inlining) can be turned into plain release calls.
+// TODO: Autorelease calls followed by objc_autoreleasePoolPop calls (perhaps in
+// ObjC++ code after inlining) can be turned into plain release calls.
// TODO: Critical-edge splitting. If the optimial insertion point is
// a critical edge, the current algorithm has to fail, because it doesn't
@@ -566,6 +564,8 @@ class ObjCARCOpt {
void OptimizeReturns(Function &F);
+ void OptimizeAutoreleasePools(Function &F);
+
template <typename PredicateT>
static void cloneOpBundlesIf(CallBase *CI,
SmallVectorImpl<OperandBundleDef> &OpBundles,
@@ -2473,6 +2473,11 @@ bool ObjCARCOpt::run(Function &F, AAResults &AA) {
(1 << unsigned(ARCInstKind::AutoreleaseRV))))
OptimizeReturns(F);
+ // Optimizations for autorelease pools.
+ if (UsedInThisFunction & ((1 << unsigned(ARCInstKind::AutoreleasepoolPush)) |
+ (1 << unsigned(ARCInstKind::AutoreleasepoolPop))))
+ OptimizeAutoreleasePools(F);
+
// Gather statistics after optimization.
#ifndef NDEBUG
if (AreStatisticsEnabled()) {
@@ -2485,6 +2490,203 @@ 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) {
+ LLVM_DEBUG(dbgs() << "Eliminating empty autorelease pool pair: "
+ << *PendingPush << " and " << *Pop << "\n");
+
+ // Store the pair for careful deletion later
+ EmptyPoolPairs.push_back({PendingPush, Pop});
+
+ ++NumNoops;
+ }
+
+ // Pop the stack - remove this pool scope
+ PoolStack.pop_back();
+ }
+ break;
+ }
+ case ARCInstKind::CallOrUser:
+ case ARCInstKind::Call:
+ case ARCInstKind::Autorelease:
+ case ARCInstKind::AutoreleaseRV: {
+ // 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::FusedRetainAutorelease:
+ case ARCInstKind::FusedRetainAutoreleaseRV:
+ case ARCInstKind::LoadWeakRetained:
+ case ARCInstKind::StoreWeak:
+ case ARCInstKind::InitWeak:
+ case ARCInstKind::LoadWeak:
+ 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);
+ }
+
+ // Remove the pairs - set Changed = true when we actually delete instructions
+ if (!DeadInsts.empty()) {
+ Changed = true;
+ for (CallInst *DeadInst : DeadInsts) {
+ LLVM_DEBUG(dbgs() << "Erasing dead instruction: " << *DeadInst << "\n");
+ DeadInst->eraseFromParent();
+ }
+
+ // Emit optimization remark for eliminated pool pairs
+ ORE.emit([&]() {
+ return OptimizationRemark(DEBUG_TYPE, "AutoreleasePoolElimination", &F)
+ << "eliminated " << ore::NV("NumPairs", EmptyPoolPairs.size())
+ << " empty autorelease pool pair(s)";
+ });
+ }
+}
+
/// @}
///
diff --git a/llvm/test/Transforms/ObjCARC/test_autorelease_pool.ll b/llvm/test/Transforms/ObjCARC/test_autorelease_pool.ll
new file mode 100644
index 0000000000000..f1388a003ea01
--- /dev/null
+++ b/llvm/test/Transforms/ObjCARC/test_autorelease_pool.ll
@@ -0,0 +1,106 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 5
+; Test for autorelease pool optimizations
+; RUN: opt -passes=objc-arc < %s -S | FileCheck %s
+
+declare ptr @llvm.objc.autoreleasePoolPush()
+declare void @llvm.objc.autoreleasePoolPop(ptr)
+declare ptr @llvm.objc.autorelease(ptr)
+declare ptr @llvm.objc.retain(ptr)
+declare ptr @create_object()
+declare void @use_object(ptr)
+declare ptr @object_with_thing()
+declare void @opaque_callee()
+
+; Test 1: Empty autorelease pool should be eliminated
+define void @test_empty_pool() {
+; CHECK-LABEL: define void @test_empty_pool() {
+; CHECK-NEXT: ret void
+;
+ %pool = call ptr @llvm.objc.autoreleasePoolPush()
+ call void @llvm.objc.autoreleasePoolPop(ptr %pool)
+ ret void
+}
+
+; Test 2: Pool with only release should be removed
+define void @test_autorelease_to_release() {
+; CHECK-LABEL: define void @test_autorelease_to_release() {
+; CHECK-NEXT: [[OBJ:%.*]] = call ptr @create_object()
+; CHECK-NEXT: call void @llvm.objc.release(ptr [[OBJ]]) #[[ATTR0:[0-9]+]], !clang.imprecise_release [[META0:![0-9]+]]
+; CHECK-NEXT: ret void
+;
+ %obj = call ptr @create_object()
+ %pool = call ptr @llvm.objc.autoreleasePoolPush()
+ call ptr @llvm.objc.autorelease(ptr %obj)
+ call void @llvm.objc.autoreleasePoolPop(ptr %pool)
+ ret void
+}
+
+; Test 3: Pool with autoreleases should not be optimized
+define void @test_multiple_autoreleases() {
+; CHECK-LABEL: define void @test_multiple_autoreleases() {
+; CHECK-NEXT: [[OBJ1:%.*]] = call ptr @create_object()
+; CHECK-NEXT: [[OBJ2:%.*]] = call ptr @create_object()
+; CHECK-NEXT: [[POOL:%.*]] = call ptr @llvm.objc.autoreleasePoolPush() #[[ATTR0]]
+; CHECK-NEXT: call void @use_object(ptr [[OBJ1]])
+; CHECK-NEXT: [[TMP1:%.*]] = call ptr @llvm.objc.autorelease(ptr [[OBJ1]]) #[[ATTR0]]
+; CHECK-NEXT: call void @use_object(ptr [[OBJ2]])
+; CHECK-NEXT: [[TMP2:%.*]] = call ptr @llvm.objc.autorelease(ptr [[OBJ2]]) #[[ATTR0]]
+; CHECK-NEXT: call void @llvm.objc.autoreleasePoolPop(ptr [[POOL]]) #[[ATTR0]]
+; CHECK-NEXT: ret void
+;
+ %obj1 = call ptr @create_object()
+ %obj2 = call ptr @create_object()
+ %pool = call ptr @llvm.objc.autoreleasePoolPush()
+ call void @use_object(ptr %obj1)
+ call ptr @llvm.objc.autorelease(ptr %obj1)
+ call void @use_object(ptr %obj2)
+ call ptr @llvm.objc.autorelease(ptr %obj2)
+ call void @llvm.objc.autoreleasePoolPop(ptr %pool)
+ ret void
+}
+
+; Test 4: Pool with calls should not be optimized
+define void @test_calls() {
+; CHECK-LABEL: define void @test_calls() {
+; CHECK-NEXT: [[POOL:%.*]] = call ptr @llvm.objc.autoreleasePoolPush() #[[ATTR0]]
+; CHECK-NEXT: [[OBJ1:%.*]] = call ptr @object_with_thing()
+; CHECK-NEXT: call void @use_object(ptr [[OBJ1]])
+; CHECK-NEXT: call void @llvm.objc.autoreleasePoolPop(ptr [[POOL]]) #[[ATTR0]]
+; CHECK-NEXT: ret void
+;
+ %pool = call ptr @llvm.objc.autoreleasePoolPush()
+ %obj1 = call ptr @object_with_thing()
+ call void @use_object(ptr %obj1)
+ call void @llvm.objc.autoreleasePoolPop(ptr %pool)
+ ret void
+}
+
+; Test 5: Pool with opaque call should not be optimized
+define void @test_opaque_call() {
+; CHECK-LABEL: define void @test_opaque_call() {
+; CHECK-NEXT: [[POOL:%.*]] = call ptr @llvm.objc.autoreleasePoolPush() #[[ATTR0]]
+; CHECK-NEXT: call void @opaque_callee()
+; CHECK-NEXT: call void @llvm.objc.autoreleasePoolPop(ptr [[POOL]]) #[[ATTR0]]
+; CHECK-NEXT: ret void
+;
+ %pool = call ptr @llvm.objc.autoreleasePoolPush()
+ call void @opaque_callee()
+ call void @llvm.objc.autoreleasePoolPop(ptr %pool)
+ ret void
+}
+
+; Test 6: Nested empty pools should be eliminated
+define void @test_nested_empty_pools() {
+; CHECK-LABEL: define void @test_nested_empty_pools() {
+; CHECK-NEXT: ret void
+;
+ %pool1 = call ptr @llvm.objc.autoreleasePoolPush()
+ %pool2 = call ptr @llvm.objc.autoreleasePoolPush()
+ call void @llvm.objc.autoreleasePoolPop(ptr %pool2)
+ call void @llvm.objc.autoreleasePoolPop(ptr %pool1)
+ ret void
+}
+
+;.
+; CHECK: [[META0]] = !{}
+;.
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