| //===--- CaptureTracking.cpp - Determine whether a pointer is captured ----===// |
| // |
| // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions. |
| // See https://llvm.org/LICENSE.txt for license information. |
| // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception |
| // |
| //===----------------------------------------------------------------------===// |
| // |
| // This file contains routines that help determine which pointers are captured. |
| // A pointer value is captured if the function makes a copy of any part of the |
| // pointer that outlives the call. Not being captured means, more or less, that |
| // the pointer is only dereferenced and not stored in a global. Returning part |
| // of the pointer as the function return value may or may not count as capturing |
| // the pointer, depending on the context. |
| // |
| //===----------------------------------------------------------------------===// |
| |
| #include "llvm/Analysis/CaptureTracking.h" |
| #include "llvm/ADT/SmallSet.h" |
| #include "llvm/ADT/SmallVector.h" |
| #include "llvm/ADT/Statistic.h" |
| #include "llvm/Analysis/AliasAnalysis.h" |
| #include "llvm/Analysis/CFG.h" |
| #include "llvm/Analysis/ValueTracking.h" |
| #include "llvm/IR/Constants.h" |
| #include "llvm/IR/Dominators.h" |
| #include "llvm/IR/Instructions.h" |
| #include "llvm/IR/IntrinsicInst.h" |
| #include "llvm/Support/CommandLine.h" |
| |
| using namespace llvm; |
| |
| #define DEBUG_TYPE "capture-tracking" |
| |
| STATISTIC(NumCaptured, "Number of pointers maybe captured"); |
| STATISTIC(NumNotCaptured, "Number of pointers not captured"); |
| STATISTIC(NumCapturedBefore, "Number of pointers maybe captured before"); |
| STATISTIC(NumNotCapturedBefore, "Number of pointers not captured before"); |
| |
| /// The default value for MaxUsesToExplore argument. It's relatively small to |
| /// keep the cost of analysis reasonable for clients like BasicAliasAnalysis, |
| /// where the results can't be cached. |
| /// TODO: we should probably introduce a caching CaptureTracking analysis and |
| /// use it where possible. The caching version can use much higher limit or |
| /// don't have this cap at all. |
| static cl::opt<unsigned> |
| DefaultMaxUsesToExplore("capture-tracking-max-uses-to-explore", cl::Hidden, |
| cl::desc("Maximal number of uses to explore."), |
| cl::init(20)); |
| |
| unsigned llvm::getDefaultMaxUsesToExploreForCaptureTracking() { |
| return DefaultMaxUsesToExplore; |
| } |
| |
| CaptureTracker::~CaptureTracker() {} |
| |
| bool CaptureTracker::shouldExplore(const Use *U) { return true; } |
| |
| bool CaptureTracker::isDereferenceableOrNull(Value *O, const DataLayout &DL) { |
| // An inbounds GEP can either be a valid pointer (pointing into |
| // or to the end of an allocation), or be null in the default |
| // address space. So for an inbounds GEP there is no way to let |
| // the pointer escape using clever GEP hacking because doing so |
| // would make the pointer point outside of the allocated object |
| // and thus make the GEP result a poison value. Similarly, other |
| // dereferenceable pointers cannot be manipulated without producing |
| // poison. |
| if (auto *GEP = dyn_cast<GetElementPtrInst>(O)) |
| if (GEP->isInBounds()) |
| return true; |
| bool CanBeNull, CanBeFreed; |
| return O->getPointerDereferenceableBytes(DL, CanBeNull, CanBeFreed); |
| } |
| |
| namespace { |
| struct SimpleCaptureTracker : public CaptureTracker { |
| explicit SimpleCaptureTracker(bool ReturnCaptures) |
| : ReturnCaptures(ReturnCaptures), Captured(false) {} |
| |
| void tooManyUses() override { Captured = true; } |
| |
| bool captured(const Use *U) override { |
| if (isa<ReturnInst>(U->getUser()) && !ReturnCaptures) |
| return false; |
| |
| Captured = true; |
| return true; |
| } |
| |
| bool ReturnCaptures; |
| |
| bool Captured; |
| }; |
| |
| /// Only find pointer captures which happen before the given instruction. Uses |
| /// the dominator tree to determine whether one instruction is before another. |
| /// Only support the case where the Value is defined in the same basic block |
| /// as the given instruction and the use. |
| struct CapturesBefore : public CaptureTracker { |
| |
| CapturesBefore(bool ReturnCaptures, const Instruction *I, |
| const DominatorTree *DT, bool IncludeI, const LoopInfo *LI) |
| : BeforeHere(I), DT(DT), ReturnCaptures(ReturnCaptures), |
| IncludeI(IncludeI), Captured(false), LI(LI) {} |
| |
| void tooManyUses() override { Captured = true; } |
| |
| bool isSafeToPrune(Instruction *I) { |
| if (BeforeHere == I) |
| return !IncludeI; |
| |
| // We explore this usage only if the usage can reach "BeforeHere". |
| // If use is not reachable from entry, there is no need to explore. |
| if (!DT->isReachableFromEntry(I->getParent())) |
| return true; |
| |
| // Check whether there is a path from I to BeforeHere. |
| return !isPotentiallyReachable(I, BeforeHere, nullptr, DT, LI); |
| } |
| |
| bool captured(const Use *U) override { |
| Instruction *I = cast<Instruction>(U->getUser()); |
| if (isa<ReturnInst>(I) && !ReturnCaptures) |
| return false; |
| |
| // Check isSafeToPrune() here rather than in shouldExplore() to avoid |
| // an expensive reachability query for every instruction we look at. |
| // Instead we only do one for actual capturing candidates. |
| if (isSafeToPrune(I)) |
| return false; |
| |
| Captured = true; |
| return true; |
| } |
| |
| const Instruction *BeforeHere; |
| const DominatorTree *DT; |
| |
| bool ReturnCaptures; |
| bool IncludeI; |
| |
| bool Captured; |
| |
| const LoopInfo *LI; |
| }; |
| |
| /// Find the 'earliest' instruction before which the pointer is known not to |
| /// be captured. Here an instruction A is considered earlier than instruction |
| /// B, if A dominates B. If 2 escapes do not dominate each other, the |
| /// terminator of the common dominator is chosen. If not all uses cannot be |
| /// analyzed, the earliest escape is set to the first instruction in the |
| /// function entry block. |
| // NOTE: Users have to make sure instructions compared against the earliest |
| // escape are not in a cycle. |
| struct EarliestCaptures : public CaptureTracker { |
| |
| EarliestCaptures(bool ReturnCaptures, Function &F, const DominatorTree &DT) |
| : DT(DT), ReturnCaptures(ReturnCaptures), Captured(false), F(F) {} |
| |
| void tooManyUses() override { |
| Captured = true; |
| EarliestCapture = &*F.getEntryBlock().begin(); |
| } |
| |
| bool captured(const Use *U) override { |
| Instruction *I = cast<Instruction>(U->getUser()); |
| if (isa<ReturnInst>(I) && !ReturnCaptures) |
| return false; |
| |
| if (!EarliestCapture) { |
| EarliestCapture = I; |
| } else if (EarliestCapture->getParent() == I->getParent()) { |
| if (I->comesBefore(EarliestCapture)) |
| EarliestCapture = I; |
| } else { |
| BasicBlock *CurrentBB = I->getParent(); |
| BasicBlock *EarliestBB = EarliestCapture->getParent(); |
| if (DT.dominates(EarliestBB, CurrentBB)) { |
| // EarliestCapture already comes before the current use. |
| } else if (DT.dominates(CurrentBB, EarliestBB)) { |
| EarliestCapture = I; |
| } else { |
| // Otherwise find the nearest common dominator and use its terminator. |
| auto *NearestCommonDom = |
| DT.findNearestCommonDominator(CurrentBB, EarliestBB); |
| EarliestCapture = NearestCommonDom->getTerminator(); |
| } |
| } |
| Captured = true; |
| |
| // Return false to continue analysis; we need to see all potential |
| // captures. |
| return false; |
| } |
| |
| Instruction *EarliestCapture = nullptr; |
| |
| const DominatorTree &DT; |
| |
| bool ReturnCaptures; |
| |
| bool Captured; |
| |
| Function &F; |
| }; |
| } |
| |
| /// PointerMayBeCaptured - Return true if this pointer value may be captured |
| /// by the enclosing function (which is required to exist). This routine can |
| /// be expensive, so consider caching the results. The boolean ReturnCaptures |
| /// specifies whether returning the value (or part of it) from the function |
| /// counts as capturing it or not. The boolean StoreCaptures specified whether |
| /// storing the value (or part of it) into memory anywhere automatically |
| /// counts as capturing it or not. |
| bool llvm::PointerMayBeCaptured(const Value *V, |
| bool ReturnCaptures, bool StoreCaptures, |
| unsigned MaxUsesToExplore) { |
| assert(!isa<GlobalValue>(V) && |
| "It doesn't make sense to ask whether a global is captured."); |
| |
| // TODO: If StoreCaptures is not true, we could do Fancy analysis |
| // to determine whether this store is not actually an escape point. |
| // In that case, BasicAliasAnalysis should be updated as well to |
| // take advantage of this. |
| (void)StoreCaptures; |
| |
| SimpleCaptureTracker SCT(ReturnCaptures); |
| PointerMayBeCaptured(V, &SCT, MaxUsesToExplore); |
| if (SCT.Captured) |
| ++NumCaptured; |
| else |
| ++NumNotCaptured; |
| return SCT.Captured; |
| } |
| |
| /// PointerMayBeCapturedBefore - Return true if this pointer value may be |
| /// captured by the enclosing function (which is required to exist). If a |
| /// DominatorTree is provided, only captures which happen before the given |
| /// instruction are considered. This routine can be expensive, so consider |
| /// caching the results. The boolean ReturnCaptures specifies whether |
| /// returning the value (or part of it) from the function counts as capturing |
| /// it or not. The boolean StoreCaptures specified whether storing the value |
| /// (or part of it) into memory anywhere automatically counts as capturing it |
| /// or not. |
| bool llvm::PointerMayBeCapturedBefore(const Value *V, bool ReturnCaptures, |
| bool StoreCaptures, const Instruction *I, |
| const DominatorTree *DT, bool IncludeI, |
| unsigned MaxUsesToExplore, |
| const LoopInfo *LI) { |
| assert(!isa<GlobalValue>(V) && |
| "It doesn't make sense to ask whether a global is captured."); |
| |
| if (!DT) |
| return PointerMayBeCaptured(V, ReturnCaptures, StoreCaptures, |
| MaxUsesToExplore); |
| |
| // TODO: See comment in PointerMayBeCaptured regarding what could be done |
| // with StoreCaptures. |
| |
| CapturesBefore CB(ReturnCaptures, I, DT, IncludeI, LI); |
| PointerMayBeCaptured(V, &CB, MaxUsesToExplore); |
| if (CB.Captured) |
| ++NumCapturedBefore; |
| else |
| ++NumNotCapturedBefore; |
| return CB.Captured; |
| } |
| |
| Instruction *llvm::FindEarliestCapture(const Value *V, Function &F, |
| bool ReturnCaptures, bool StoreCaptures, |
| const DominatorTree &DT, |
| unsigned MaxUsesToExplore) { |
| assert(!isa<GlobalValue>(V) && |
| "It doesn't make sense to ask whether a global is captured."); |
| |
| EarliestCaptures CB(ReturnCaptures, F, DT); |
| PointerMayBeCaptured(V, &CB, MaxUsesToExplore); |
| if (CB.Captured) |
| ++NumCapturedBefore; |
| else |
| ++NumNotCapturedBefore; |
| return CB.EarliestCapture; |
| } |
| |
| void llvm::PointerMayBeCaptured(const Value *V, CaptureTracker *Tracker, |
| unsigned MaxUsesToExplore) { |
| assert(V->getType()->isPointerTy() && "Capture is for pointers only!"); |
| if (MaxUsesToExplore == 0) |
| MaxUsesToExplore = DefaultMaxUsesToExplore; |
| |
| SmallVector<const Use *, 20> Worklist; |
| Worklist.reserve(getDefaultMaxUsesToExploreForCaptureTracking()); |
| SmallSet<const Use *, 20> Visited; |
| |
| auto AddUses = [&](const Value *V) { |
| unsigned Count = 0; |
| for (const Use &U : V->uses()) { |
| // If there are lots of uses, conservatively say that the value |
| // is captured to avoid taking too much compile time. |
| if (Count++ >= MaxUsesToExplore) { |
| Tracker->tooManyUses(); |
| return false; |
| } |
| if (!Visited.insert(&U).second) |
| continue; |
| if (!Tracker->shouldExplore(&U)) |
| continue; |
| Worklist.push_back(&U); |
| } |
| return true; |
| }; |
| if (!AddUses(V)) |
| return; |
| |
| while (!Worklist.empty()) { |
| const Use *U = Worklist.pop_back_val(); |
| Instruction *I = cast<Instruction>(U->getUser()); |
| |
| switch (I->getOpcode()) { |
| case Instruction::Call: |
| case Instruction::Invoke: { |
| auto *Call = cast<CallBase>(I); |
| // Not captured if the callee is readonly, doesn't return a copy through |
| // its return value and doesn't unwind (a readonly function can leak bits |
| // by throwing an exception or not depending on the input value). |
| if (Call->onlyReadsMemory() && Call->doesNotThrow() && |
| Call->getType()->isVoidTy()) |
| break; |
| |
| // The pointer is not captured if returned pointer is not captured. |
| // NOTE: CaptureTracking users should not assume that only functions |
| // marked with nocapture do not capture. This means that places like |
| // getUnderlyingObject in ValueTracking or DecomposeGEPExpression |
| // in BasicAA also need to know about this property. |
| if (isIntrinsicReturningPointerAliasingArgumentWithoutCapturing(Call, |
| true)) { |
| if (!AddUses(Call)) |
| return; |
| break; |
| } |
| |
| // Volatile operations effectively capture the memory location that they |
| // load and store to. |
| if (auto *MI = dyn_cast<MemIntrinsic>(Call)) |
| if (MI->isVolatile()) |
| if (Tracker->captured(U)) |
| return; |
| |
| // Not captured if only passed via 'nocapture' arguments. Note that |
| // calling a function pointer does not in itself cause the pointer to |
| // be captured. This is a subtle point considering that (for example) |
| // the callee might return its own address. It is analogous to saying |
| // that loading a value from a pointer does not cause the pointer to be |
| // captured, even though the loaded value might be the pointer itself |
| // (think of self-referential objects). |
| if (Call->isDataOperand(U) && |
| !Call->doesNotCapture(Call->getDataOperandNo(U))) { |
| // The parameter is not marked 'nocapture' - captured. |
| if (Tracker->captured(U)) |
| return; |
| } |
| break; |
| } |
| case Instruction::Load: |
| // Volatile loads make the address observable. |
| if (cast<LoadInst>(I)->isVolatile()) |
| if (Tracker->captured(U)) |
| return; |
| break; |
| case Instruction::VAArg: |
| // "va-arg" from a pointer does not cause it to be captured. |
| break; |
| case Instruction::Store: |
| // Stored the pointer - conservatively assume it may be captured. |
| // Volatile stores make the address observable. |
| if (U->getOperandNo() == 0 || cast<StoreInst>(I)->isVolatile()) |
| if (Tracker->captured(U)) |
| return; |
| break; |
| case Instruction::AtomicRMW: { |
| // atomicrmw conceptually includes both a load and store from |
| // the same location. |
| // As with a store, the location being accessed is not captured, |
| // but the value being stored is. |
| // Volatile stores make the address observable. |
| auto *ARMWI = cast<AtomicRMWInst>(I); |
| if (U->getOperandNo() == 1 || ARMWI->isVolatile()) |
| if (Tracker->captured(U)) |
| return; |
| break; |
| } |
| case Instruction::AtomicCmpXchg: { |
| // cmpxchg conceptually includes both a load and store from |
| // the same location. |
| // As with a store, the location being accessed is not captured, |
| // but the value being stored is. |
| // Volatile stores make the address observable. |
| auto *ACXI = cast<AtomicCmpXchgInst>(I); |
| if (U->getOperandNo() == 1 || U->getOperandNo() == 2 || |
| ACXI->isVolatile()) |
| if (Tracker->captured(U)) |
| return; |
| break; |
| } |
| case Instruction::BitCast: |
| case Instruction::GetElementPtr: |
| case Instruction::PHI: |
| case Instruction::Select: |
| case Instruction::AddrSpaceCast: |
| // The original value is not captured via this if the new value isn't. |
| if (!AddUses(I)) |
| return; |
| break; |
| case Instruction::ICmp: { |
| unsigned Idx = U->getOperandNo(); |
| unsigned OtherIdx = 1 - Idx; |
| if (auto *CPN = dyn_cast<ConstantPointerNull>(I->getOperand(OtherIdx))) { |
| // Don't count comparisons of a no-alias return value against null as |
| // captures. This allows us to ignore comparisons of malloc results |
| // with null, for example. |
| if (CPN->getType()->getAddressSpace() == 0) |
| if (isNoAliasCall(U->get()->stripPointerCasts())) |
| break; |
| if (!I->getFunction()->nullPointerIsDefined()) { |
| auto *O = I->getOperand(Idx)->stripPointerCastsSameRepresentation(); |
| // Comparing a dereferenceable_or_null pointer against null cannot |
| // lead to pointer escapes, because if it is not null it must be a |
| // valid (in-bounds) pointer. |
| if (Tracker->isDereferenceableOrNull(O, I->getModule()->getDataLayout())) |
| break; |
| } |
| } |
| // Comparison against value stored in global variable. Given the pointer |
| // does not escape, its value cannot be guessed and stored separately in a |
| // global variable. |
| auto *LI = dyn_cast<LoadInst>(I->getOperand(OtherIdx)); |
| if (LI && isa<GlobalVariable>(LI->getPointerOperand())) |
| break; |
| // Otherwise, be conservative. There are crazy ways to capture pointers |
| // using comparisons. |
| if (Tracker->captured(U)) |
| return; |
| break; |
| } |
| default: |
| // Something else - be conservative and say it is captured. |
| if (Tracker->captured(U)) |
| return; |
| break; |
| } |
| } |
| |
| // All uses examined. |
| } |
| |
| bool llvm::isNonEscapingLocalObject( |
| const Value *V, SmallDenseMap<const Value *, bool, 8> *IsCapturedCache) { |
| SmallDenseMap<const Value *, bool, 8>::iterator CacheIt; |
| if (IsCapturedCache) { |
| bool Inserted; |
| std::tie(CacheIt, Inserted) = IsCapturedCache->insert({V, false}); |
| if (!Inserted) |
| // Found cached result, return it! |
| return CacheIt->second; |
| } |
| |
| // If this is an identified function-local object, check to see if it escapes. |
| if (isIdentifiedFunctionLocal(V)) { |
| // Set StoreCaptures to True so that we can assume in our callers that the |
| // pointer is not the result of a load instruction. Currently |
| // PointerMayBeCaptured doesn't have any special analysis for the |
| // StoreCaptures=false case; if it did, our callers could be refined to be |
| // more precise. |
| auto Ret = !PointerMayBeCaptured(V, false, /*StoreCaptures=*/true); |
| if (IsCapturedCache) |
| CacheIt->second = Ret; |
| return Ret; |
| } |
| |
| return false; |
| } |
