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llvm-project/llvm/lib/Transforms/ObjCARC/ObjCARCContract.cpp

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//===- ObjCARCContract.cpp - ObjC ARC Optimization ------------------------===//
//
// The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
/// \file
/// This file defines late ObjC ARC optimizations. ARC stands for Automatic
/// Reference Counting and is a system for managing reference counts for objects
/// in Objective C.
///
/// This specific file mainly deals with ``contracting'' multiple lower level
/// operations into singular higher level operations through pattern matching.
///
/// WARNING: This file knows about certain library functions. It recognizes them
/// by name, and hardwires knowledge of their semantics.
///
/// WARNING: This file knows about how certain Objective-C library functions are
/// used. Naive LLVM IR transformations which would otherwise be
/// behavior-preserving may break these assumptions.
///
//===----------------------------------------------------------------------===//
// TODO: ObjCARCContract could insert PHI nodes when uses aren't
// dominated by single calls.
#include "ObjCARC.h"
#include "ARCRuntimeEntryPoints.h"
#include "DependencyAnalysis.h"
#include "ProvenanceAnalysis.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/IR/Dominators.h"
#include "llvm/IR/InlineAsm.h"
#include "llvm/IR/Operator.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/raw_ostream.h"
using namespace llvm;
using namespace llvm::objcarc;
#define DEBUG_TYPE "objc-arc-contract"
STATISTIC(NumPeeps, "Number of calls peephole-optimized");
STATISTIC(NumStoreStrongs, "Number objc_storeStrong calls formed");
//===----------------------------------------------------------------------===//
// Declarations
//===----------------------------------------------------------------------===//
namespace {
/// \brief Late ARC optimizations
///
/// These change the IR in a way that makes it difficult to be analyzed by
/// ObjCARCOpt, so it's run late.
class ObjCARCContract : public FunctionPass {
bool Changed;
AliasAnalysis *AA;
DominatorTree *DT;
ProvenanceAnalysis PA;
ARCRuntimeEntryPoints EP;
/// A flag indicating whether this optimization pass should run.
bool Run;
/// The inline asm string to insert between calls and RetainRV calls to make
/// the optimization work on targets which need it.
const MDString *RVInstMarker;
/// The set of inserted objc_storeStrong calls. If at the end of walking the
/// function we have found no alloca instructions, these calls can be marked
/// "tail".
SmallPtrSet<CallInst *, 8> StoreStrongCalls;
/// Returns true if we eliminated Inst.
bool tryToPeepholeInstruction(Function &F, Instruction *Inst,
inst_iterator &Iter,
SmallPtrSetImpl<Instruction *> &DepInsts,
SmallPtrSetImpl<const BasicBlock *> &Visited,
bool &TailOkForStoreStrong);
bool optimizeRetainCall(Function &F, Instruction *Retain);
bool
contractAutorelease(Function &F, Instruction *Autorelease,
ARCInstKind Class,
SmallPtrSetImpl<Instruction *> &DependingInstructions,
SmallPtrSetImpl<const BasicBlock *> &Visited);
void tryToContractReleaseIntoStoreStrong(Instruction *Release,
inst_iterator &Iter);
void getAnalysisUsage(AnalysisUsage &AU) const override;
bool doInitialization(Module &M) override;
bool runOnFunction(Function &F) override;
public:
static char ID;
ObjCARCContract() : FunctionPass(ID) {
initializeObjCARCContractPass(*PassRegistry::getPassRegistry());
}
};
}
//===----------------------------------------------------------------------===//
// Implementation
//===----------------------------------------------------------------------===//
/// Turn objc_retain into objc_retainAutoreleasedReturnValue if the operand is a
/// return value. We do this late so we do not disrupt the dataflow analysis in
/// ObjCARCOpt.
bool ObjCARCContract::optimizeRetainCall(Function &F, Instruction *Retain) {
ImmutableCallSite CS(GetArgRCIdentityRoot(Retain));
const Instruction *Call = CS.getInstruction();
if (!Call)
return false;
if (Call->getParent() != Retain->getParent())
return false;
// Check that the call is next to the retain.
BasicBlock::const_iterator I = ++Call->getIterator();
while (IsNoopInstruction(&*I))
++I;
if (&*I != Retain)
return false;
// Turn it to an objc_retainAutoreleasedReturnValue.
Changed = true;
++NumPeeps;
DEBUG(dbgs() << "Transforming objc_retain => "
"objc_retainAutoreleasedReturnValue since the operand is a "
"return value.\nOld: "<< *Retain << "\n");
// We do not have to worry about tail calls/does not throw since
// retain/retainRV have the same properties.
Constant *Decl = EP.get(ARCRuntimeEntryPointKind::RetainRV);
cast<CallInst>(Retain)->setCalledFunction(Decl);
DEBUG(dbgs() << "New: " << *Retain << "\n");
return true;
}
/// Merge an autorelease with a retain into a fused call.
bool ObjCARCContract::contractAutorelease(
Function &F, Instruction *Autorelease, ARCInstKind Class,
SmallPtrSetImpl<Instruction *> &DependingInstructions,
SmallPtrSetImpl<const BasicBlock *> &Visited) {
const Value *Arg = GetArgRCIdentityRoot(Autorelease);
// Check that there are no instructions between the retain and the autorelease
// (such as an autorelease_pop) which may change the count.
CallInst *Retain = nullptr;
if (Class == ARCInstKind::AutoreleaseRV)
FindDependencies(RetainAutoreleaseRVDep, Arg,
Autorelease->getParent(), Autorelease,
DependingInstructions, Visited, PA);
else
FindDependencies(RetainAutoreleaseDep, Arg,
Autorelease->getParent(), Autorelease,
DependingInstructions, Visited, PA);
Visited.clear();
if (DependingInstructions.size() != 1) {
DependingInstructions.clear();
return false;
}
Retain = dyn_cast_or_null<CallInst>(*DependingInstructions.begin());
DependingInstructions.clear();
if (!Retain || GetBasicARCInstKind(Retain) != ARCInstKind::Retain ||
GetArgRCIdentityRoot(Retain) != Arg)
return false;
Changed = true;
++NumPeeps;
DEBUG(dbgs() << " Fusing retain/autorelease!\n"
" Autorelease:" << *Autorelease << "\n"
" Retain: " << *Retain << "\n");
Constant *Decl = EP.get(Class == ARCInstKind::AutoreleaseRV
? ARCRuntimeEntryPointKind::RetainAutoreleaseRV
: ARCRuntimeEntryPointKind::RetainAutorelease);
Retain->setCalledFunction(Decl);
DEBUG(dbgs() << " New RetainAutorelease: " << *Retain << "\n");
EraseInstruction(Autorelease);
return true;
}
static StoreInst *findSafeStoreForStoreStrongContraction(LoadInst *Load,
Instruction *Release,
ProvenanceAnalysis &PA,
AliasAnalysis *AA) {
StoreInst *Store = nullptr;
bool SawRelease = false;
// Get the location associated with Load.
MemoryLocation Loc = MemoryLocation::get(Load);
auto *LocPtr = Loc.Ptr->stripPointerCasts();
// Walk down to find the store and the release, which may be in either order.
for (auto I = std::next(BasicBlock::iterator(Load)),
E = Load->getParent()->end();
I != E; ++I) {
// If we found the store we were looking for and saw the release,
// break. There is no more work to be done.
if (Store && SawRelease)
break;
// Now we know that we have not seen either the store or the release. If I
// is the release, mark that we saw the release and continue.
Instruction *Inst = &*I;
if (Inst == Release) {
SawRelease = true;
continue;
}
// Otherwise, we check if Inst is a "good" store. Grab the instruction class
// of Inst.
ARCInstKind Class = GetBasicARCInstKind(Inst);
// If Inst is an unrelated retain, we don't care about it.
//
// TODO: This is one area where the optimization could be made more
// aggressive.
if (IsRetain(Class))
continue;
// If we have seen the store, but not the release...
if (Store) {
// We need to make sure that it is safe to move the release from its
// current position to the store. This implies proving that any
// instruction in between Store and the Release conservatively can not use
// the RCIdentityRoot of Release. If we can prove we can ignore Inst, so
// continue...
if (!CanUse(Inst, Load, PA, Class)) {
continue;
}
// Otherwise, be conservative and return nullptr.
return nullptr;
}
// Ok, now we know we have not seen a store yet. See if Inst can write to
// our load location, if it can not, just ignore the instruction.
if (!(AA->getModRefInfo(Inst, Loc) & MRI_Mod))
continue;
Store = dyn_cast<StoreInst>(Inst);
// If Inst can, then check if Inst is a simple store. If Inst is not a
// store or a store that is not simple, then we have some we do not
// understand writing to this memory implying we can not move the load
// over the write to any subsequent store that we may find.
if (!Store || !Store->isSimple())
return nullptr;
// Then make sure that the pointer we are storing to is Ptr. If so, we
// found our Store!
if (Store->getPointerOperand()->stripPointerCasts() == LocPtr)
continue;
// Otherwise, we have an unknown store to some other ptr that clobbers
// Loc.Ptr. Bail!
return nullptr;
}
// If we did not find the store or did not see the release, fail.
if (!Store || !SawRelease)
return nullptr;
// We succeeded!
return Store;
}
static Instruction *
findRetainForStoreStrongContraction(Value *New, StoreInst *Store,
Instruction *Release,
ProvenanceAnalysis &PA) {
// Walk up from the Store to find the retain.
BasicBlock::iterator I = Store->getIterator();
BasicBlock::iterator Begin = Store->getParent()->begin();
while (I != Begin && GetBasicARCInstKind(&*I) != ARCInstKind::Retain) {
Instruction *Inst = &*I;
// It is only safe to move the retain to the store if we can prove
// conservatively that nothing besides the release can decrement reference
// counts in between the retain and the store.
if (CanDecrementRefCount(Inst, New, PA) && Inst != Release)
return nullptr;
--I;
}
Instruction *Retain = &*I;
if (GetBasicARCInstKind(Retain) != ARCInstKind::Retain)
return nullptr;
if (GetArgRCIdentityRoot(Retain) != New)
return nullptr;
return Retain;
}
/// Attempt to merge an objc_release with a store, load, and objc_retain to form
/// an objc_storeStrong. An objc_storeStrong:
///
/// objc_storeStrong(i8** %old_ptr, i8* new_value)
///
/// is equivalent to the following IR sequence:
///
/// ; Load old value.
/// %old_value = load i8** %old_ptr (1)
///
/// ; Increment the new value and then release the old value. This must occur
/// ; in order in case old_value releases new_value in its destructor causing
/// ; us to potentially have a dangling ptr.
/// tail call i8* @objc_retain(i8* %new_value) (2)
/// tail call void @objc_release(i8* %old_value) (3)
///
/// ; Store the new_value into old_ptr
/// store i8* %new_value, i8** %old_ptr (4)
///
/// The safety of this optimization is based around the following
/// considerations:
///
/// 1. We are forming the store strong at the store. Thus to perform this
/// optimization it must be safe to move the retain, load, and release to
/// (4).
/// 2. We need to make sure that any re-orderings of (1), (2), (3), (4) are
/// safe.
void ObjCARCContract::tryToContractReleaseIntoStoreStrong(Instruction *Release,
inst_iterator &Iter) {
// See if we are releasing something that we just loaded.
auto *Load = dyn_cast<LoadInst>(GetArgRCIdentityRoot(Release));
if (!Load || !Load->isSimple())
return;
// For now, require everything to be in one basic block.
BasicBlock *BB = Release->getParent();
if (Load->getParent() != BB)
return;
// First scan down the BB from Load, looking for a store of the RCIdentityRoot
// of Load's
StoreInst *Store =
findSafeStoreForStoreStrongContraction(Load, Release, PA, AA);
// If we fail, bail.
if (!Store)
return;
// Then find what new_value's RCIdentity Root is.
Value *New = GetRCIdentityRoot(Store->getValueOperand());
// Then walk up the BB and look for a retain on New without any intervening
// instructions which conservatively might decrement ref counts.
Instruction *Retain =
findRetainForStoreStrongContraction(New, Store, Release, PA);
// If we fail, bail.
if (!Retain)
return;
Changed = true;
++NumStoreStrongs;
DEBUG(
llvm::dbgs() << " Contracting retain, release into objc_storeStrong.\n"
<< " Old:\n"
<< " Store: " << *Store << "\n"
<< " Release: " << *Release << "\n"
<< " Retain: " << *Retain << "\n"
<< " Load: " << *Load << "\n");
LLVMContext &C = Release->getContext();
Type *I8X = PointerType::getUnqual(Type::getInt8Ty(C));
Type *I8XX = PointerType::getUnqual(I8X);
Value *Args[] = { Load->getPointerOperand(), New };
if (Args[0]->getType() != I8XX)
Args[0] = new BitCastInst(Args[0], I8XX, "", Store);
if (Args[1]->getType() != I8X)
Args[1] = new BitCastInst(Args[1], I8X, "", Store);
Constant *Decl = EP.get(ARCRuntimeEntryPointKind::StoreStrong);
CallInst *StoreStrong = CallInst::Create(Decl, Args, "", Store);
StoreStrong->setDoesNotThrow();
StoreStrong->setDebugLoc(Store->getDebugLoc());
// We can't set the tail flag yet, because we haven't yet determined
// whether there are any escaping allocas. Remember this call, so that
// we can set the tail flag once we know it's safe.
StoreStrongCalls.insert(StoreStrong);
DEBUG(llvm::dbgs() << " New Store Strong: " << *StoreStrong << "\n");
if (&*Iter == Retain) ++Iter;
if (&*Iter == Store) ++Iter;
Store->eraseFromParent();
Release->eraseFromParent();
EraseInstruction(Retain);
if (Load->use_empty())
Load->eraseFromParent();
}
bool ObjCARCContract::tryToPeepholeInstruction(
Function &F, Instruction *Inst, inst_iterator &Iter,
SmallPtrSetImpl<Instruction *> &DependingInsts,
SmallPtrSetImpl<const BasicBlock *> &Visited,
bool &TailOkForStoreStrongs) {
// Only these library routines return their argument. In particular,
// objc_retainBlock does not necessarily return its argument.
ARCInstKind Class = GetBasicARCInstKind(Inst);
switch (Class) {
case ARCInstKind::FusedRetainAutorelease:
case ARCInstKind::FusedRetainAutoreleaseRV:
return false;
case ARCInstKind::Autorelease:
case ARCInstKind::AutoreleaseRV:
return contractAutorelease(F, Inst, Class, DependingInsts, Visited);
case ARCInstKind::Retain:
// Attempt to convert retains to retainrvs if they are next to function
// calls.
if (!optimizeRetainCall(F, Inst))
return false;
// If we succeed in our optimization, fall through.
LLVM_FALLTHROUGH;
case ARCInstKind::RetainRV:
case ARCInstKind::ClaimRV: {
// If we're compiling for a target which needs a special inline-asm
// marker to do the return value optimization, insert it now.
if (!RVInstMarker)
return false;
BasicBlock::iterator BBI = Inst->getIterator();
BasicBlock *InstParent = Inst->getParent();
// Step up to see if the call immediately precedes the RV call.
// If it's an invoke, we have to cross a block boundary. And we have
// to carefully dodge no-op instructions.
do {
if (BBI == InstParent->begin()) {
BasicBlock *Pred = InstParent->getSinglePredecessor();
if (!Pred)
goto decline_rv_optimization;
BBI = Pred->getTerminator()->getIterator();
break;
}
--BBI;
} while (IsNoopInstruction(&*BBI));
if (&*BBI == GetArgRCIdentityRoot(Inst)) {
DEBUG(dbgs() << "Adding inline asm marker for the return value "
"optimization.\n");
Changed = true;
InlineAsm *IA = InlineAsm::get(
FunctionType::get(Type::getVoidTy(Inst->getContext()),
/*isVarArg=*/false),
RVInstMarker->getString(),
/*Constraints=*/"", /*hasSideEffects=*/true);
CallInst::Create(IA, "", Inst);
}
decline_rv_optimization:
return false;
}
case ARCInstKind::InitWeak: {
// objc_initWeak(p, null) => *p = null
CallInst *CI = cast<CallInst>(Inst);
if (IsNullOrUndef(CI->getArgOperand(1))) {
Value *Null =
ConstantPointerNull::get(cast<PointerType>(CI->getType()));
Changed = true;
new StoreInst(Null, CI->getArgOperand(0), CI);
DEBUG(dbgs() << "OBJCARCContract: Old = " << *CI << "\n"
<< " New = " << *Null << "\n");
CI->replaceAllUsesWith(Null);
CI->eraseFromParent();
}
return true;
}
case ARCInstKind::Release:
// Try to form an objc store strong from our release. If we fail, there is
// nothing further to do below, so continue.
tryToContractReleaseIntoStoreStrong(Inst, Iter);
return true;
case ARCInstKind::User:
// Be conservative if the function has any alloca instructions.
// Technically we only care about escaping alloca instructions,
// but this is sufficient to handle some interesting cases.
if (isa<AllocaInst>(Inst))
TailOkForStoreStrongs = false;
return true;
case ARCInstKind::IntrinsicUser:
// Remove calls to @clang.arc.use(...).
Inst->eraseFromParent();
return true;
default:
return true;
}
}
//===----------------------------------------------------------------------===//
// Top Level Driver
//===----------------------------------------------------------------------===//
bool ObjCARCContract::runOnFunction(Function &F) {
if (!EnableARCOpts)
return false;
// If nothing in the Module uses ARC, don't do anything.
if (!Run)
return false;
Changed = false;
AA = &getAnalysis<AAResultsWrapperPass>().getAAResults();
DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
PA.setAA(&getAnalysis<AAResultsWrapperPass>().getAAResults());
DEBUG(llvm::dbgs() << "**** ObjCARC Contract ****\n");
// Track whether it's ok to mark objc_storeStrong calls with the "tail"
// keyword. Be conservative if the function has variadic arguments.
// It seems that functions which "return twice" are also unsafe for the
// "tail" argument, because they are setjmp, which could need to
// return to an earlier stack state.
bool TailOkForStoreStrongs =
!F.isVarArg() && !F.callsFunctionThatReturnsTwice();
// For ObjC library calls which return their argument, replace uses of the
// argument with uses of the call return value, if it dominates the use. This
// reduces register pressure.
SmallPtrSet<Instruction *, 4> DependingInstructions;
SmallPtrSet<const BasicBlock *, 4> Visited;
for (inst_iterator I = inst_begin(&F), E = inst_end(&F); I != E;) {
Instruction *Inst = &*I++;
DEBUG(dbgs() << "Visiting: " << *Inst << "\n");
// First try to peephole Inst. If there is nothing further we can do in
// terms of undoing objc-arc-expand, process the next inst.
if (tryToPeepholeInstruction(F, Inst, I, DependingInstructions, Visited,
TailOkForStoreStrongs))
continue;
// Otherwise, try to undo objc-arc-expand.
// Don't use GetArgRCIdentityRoot because we don't want to look through bitcasts
// and such; to do the replacement, the argument must have type i8*.
// Function for replacing uses of Arg dominated by Inst.
auto ReplaceArgUses = [Inst, this](Value *Arg) {
// If we're compiling bugpointed code, don't get in trouble.
if (!isa<Instruction>(Arg) && !isa<Argument>(Arg))
return;
// Look through the uses of the pointer.
for (Value::use_iterator UI = Arg->use_begin(), UE = Arg->use_end();
UI != UE; ) {
// Increment UI now, because we may unlink its element.
Use &U = *UI++;
unsigned OperandNo = U.getOperandNo();
// If the call's return value dominates a use of the call's argument
// value, rewrite the use to use the return value. We check for
// reachability here because an unreachable call is considered to
// trivially dominate itself, which would lead us to rewriting its
// argument in terms of its return value, which would lead to
// infinite loops in GetArgRCIdentityRoot.
if (DT->isReachableFromEntry(U) && DT->dominates(Inst, U)) {
Changed = true;
Instruction *Replacement = Inst;
Type *UseTy = U.get()->getType();
if (PHINode *PHI = dyn_cast<PHINode>(U.getUser())) {
// For PHI nodes, insert the bitcast in the predecessor block.
unsigned ValNo = PHINode::getIncomingValueNumForOperand(OperandNo);
BasicBlock *BB = PHI->getIncomingBlock(ValNo);
if (Replacement->getType() != UseTy)
Replacement = new BitCastInst(Replacement, UseTy, "",
&BB->back());
// While we're here, rewrite all edges for this PHI, rather
// than just one use at a time, to minimize the number of
// bitcasts we emit.
for (unsigned i = 0, e = PHI->getNumIncomingValues(); i != e; ++i)
if (PHI->getIncomingBlock(i) == BB) {
// Keep the UI iterator valid.
if (UI != UE &&
&PHI->getOperandUse(
PHINode::getOperandNumForIncomingValue(i)) == &*UI)
++UI;
PHI->setIncomingValue(i, Replacement);
}
} else {
if (Replacement->getType() != UseTy)
Replacement = new BitCastInst(Replacement, UseTy, "",
cast<Instruction>(U.getUser()));
U.set(Replacement);
}
}
}
};
Value *Arg = cast<CallInst>(Inst)->getArgOperand(0);
Value *OrigArg = Arg;
// TODO: Change this to a do-while.
for (;;) {
ReplaceArgUses(Arg);
// If Arg is a no-op casted pointer, strip one level of casts and iterate.
if (const BitCastInst *BI = dyn_cast<BitCastInst>(Arg))
Arg = BI->getOperand(0);
else if (isa<GEPOperator>(Arg) &&
cast<GEPOperator>(Arg)->hasAllZeroIndices())
Arg = cast<GEPOperator>(Arg)->getPointerOperand();
else if (isa<GlobalAlias>(Arg) &&
!cast<GlobalAlias>(Arg)->isInterposable())
Arg = cast<GlobalAlias>(Arg)->getAliasee();
else
break;
}
// Replace bitcast users of Arg that are dominated by Inst.
SmallVector<BitCastInst *, 2> BitCastUsers;
// Add all bitcast users of the function argument first.
for (User *U : OrigArg->users())
if (auto *BC = dyn_cast<BitCastInst>(U))
BitCastUsers.push_back(BC);
// Replace the bitcasts with the call return. Iterate until list is empty.
while (!BitCastUsers.empty()) {
auto *BC = BitCastUsers.pop_back_val();
for (User *U : BC->users())
if (auto *B = dyn_cast<BitCastInst>(U))
BitCastUsers.push_back(B);
ReplaceArgUses(BC);
}
}
// If this function has no escaping allocas or suspicious vararg usage,
// objc_storeStrong calls can be marked with the "tail" keyword.
if (TailOkForStoreStrongs)
for (CallInst *CI : StoreStrongCalls)
CI->setTailCall();
StoreStrongCalls.clear();
return Changed;
}
//===----------------------------------------------------------------------===//
// Misc Pass Manager
//===----------------------------------------------------------------------===//
char ObjCARCContract::ID = 0;
INITIALIZE_PASS_BEGIN(ObjCARCContract, "objc-arc-contract",
"ObjC ARC contraction", false, false)
INITIALIZE_PASS_DEPENDENCY(AAResultsWrapperPass)
INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
INITIALIZE_PASS_END(ObjCARCContract, "objc-arc-contract",
"ObjC ARC contraction", false, false)
void ObjCARCContract::getAnalysisUsage(AnalysisUsage &AU) const {
AU.addRequired<AAResultsWrapperPass>();
AU.addRequired<DominatorTreeWrapperPass>();
AU.setPreservesCFG();
}
Pass *llvm::createObjCARCContractPass() { return new ObjCARCContract(); }
bool ObjCARCContract::doInitialization(Module &M) {
// If nothing in the Module uses ARC, don't do anything.
Run = ModuleHasARC(M);
if (!Run)
return false;
EP.init(&M);
// Initialize RVInstMarker.
RVInstMarker = nullptr;
if (NamedMDNode *NMD =
M.getNamedMetadata("clang.arc.retainAutoreleasedReturnValueMarker"))
if (NMD->getNumOperands() == 1) {
const MDNode *N = NMD->getOperand(0);
if (N->getNumOperands() == 1)
if (const MDString *S = dyn_cast<MDString>(N->getOperand(0)))
RVInstMarker = S;
}
return false;
}
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