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llvm-mirror/lib/Transforms/ObjCARC/ObjCARCContract.cpp
Chandler Carruth 2e896f4f08 [PM/AA] Extract the ModRef enums from the AliasAnalysis class in 
preparation for de-coupling the AA implementations.

In order to do this, they had to become fake-scoped using the
traditional LLVM pattern of a leading initialism. These can't be actual
scoped enumerations because they're bitfields and thus inherently we use
them as integers.

I've also renamed the behavior enums that are specific to reasoning
about the mod/ref behavior of functions when called. This makes it more
clear that they have a very narrow domain of applicability.

I think there is a significantly cleaner API for all of this, but
I don't want to try to do really substantive changes for now, I just
want to refactor the things away from analysis groups so I'm preserving
the exact original design and just cleaning up the names, style, and
lifting out of the class.

Differential Revision: http://reviews.llvm.org/D10564

llvm-svn: 242963
2015-07-22 23:15:57 +00:00

666 lines
25 KiB
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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 *RetainRVMarker;
/// 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;
++I;
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);
// 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() == Loc.Ptr)
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;
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 == 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.
// FALLTHROUGH
case ARCInstKind::RetainRV: {
// If we're compiling for a target which needs a special inline-asm
// marker to do the retainAutoreleasedReturnValue optimization,
// insert it now.
if (!RetainRVMarker)
return false;
BasicBlock::iterator BBI = Inst;
BasicBlock *InstParent = Inst->getParent();
// Step up to see if the call immediately precedes the RetainRV 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();
break;
}
--BBI;
} while (IsNoopInstruction(BBI));
if (&*BBI == GetArgRCIdentityRoot(Inst)) {
DEBUG(dbgs() << "Adding inline asm marker for "
"retainAutoreleasedReturnValue optimization.\n");
Changed = true;
InlineAsm *IA =
InlineAsm::get(FunctionType::get(Type::getVoidTy(Inst->getContext()),
/*isVarArg=*/false),
RetainRVMarker->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<AliasAnalysis>();
DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
PA.setAA(&getAnalysis<AliasAnalysis>());
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*.
Value *Arg = cast<CallInst>(Inst)->getArgOperand(0);
// TODO: Change this to a do-while.
for (;;) {
// If we're compiling bugpointed code, don't get in trouble.
if (!isa<Instruction>(Arg) && !isa<Argument>(Arg))
break;
// 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);
}
}
}
// 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)->mayBeOverridden())
Arg = cast<GlobalAlias>(Arg)->getAliasee();
else
break;
}
}
// 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_AG_DEPENDENCY(AliasAnalysis)
INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
INITIALIZE_PASS_END(ObjCARCContract, "objc-arc-contract",
"ObjC ARC contraction", false, false)
void ObjCARCContract::getAnalysisUsage(AnalysisUsage &AU) const {
AU.addRequired<AliasAnalysis>();
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 RetainRVMarker.
RetainRVMarker = 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)))
RetainRVMarker = S;
}
return false;
}
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