| //===--- CaptureTracking.cpp - Determine whether a pointer is captured ----===// |
| // |
| // The LLVM Compiler Infrastructure |
| // |
| // This file is distributed under the University of Illinois Open Source |
| // License. See LICENSE.TXT for details. |
| // |
| //===----------------------------------------------------------------------===// |
| // |
| // 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/ADT/SmallSet.h" |
| #include "llvm/ADT/SmallVector.h" |
| #include "llvm/Analysis/AliasAnalysis.h" |
| #include "llvm/Analysis/CFG.h" |
| #include "llvm/Analysis/CaptureTracking.h" |
| #include "llvm/IR/CallSite.h" |
| #include "llvm/IR/Constants.h" |
| #include "llvm/IR/Dominators.h" |
| #include "llvm/IR/Instructions.h" |
| |
| using namespace llvm; |
| |
| CaptureTracker::~CaptureTracker() {} |
| |
| bool CaptureTracker::shouldExplore(const Use *U) { return true; } |
| |
| 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; |
| }; |
| |
| struct NumberedInstCache { |
| SmallDenseMap<const Instruction *, unsigned, 32> NumberedInsts; |
| BasicBlock::const_iterator LastInstFound; |
| unsigned LastInstPos; |
| const BasicBlock *BB; |
| |
| NumberedInstCache(const BasicBlock *BasicB) : LastInstPos(0), BB(BasicB) { |
| LastInstFound = BB->end(); |
| } |
| |
| /// \brief Find the first instruction 'A' or 'B' in 'BB'. Number out |
| /// instruction while walking 'BB'. |
| const Instruction *find(const Instruction *A, const Instruction *B) { |
| const Instruction *Inst = nullptr; |
| assert(!(LastInstFound == BB->end() && LastInstPos != 0) && |
| "Instruction supposed to be in NumberedInsts"); |
| |
| // Start the search with the instruction found in the last lookup round. |
| auto II = BB->begin(); |
| auto IE = BB->end(); |
| if (LastInstFound != IE) |
| II = std::next(LastInstFound); |
| |
| // Number all instructions up to the point where we find 'A' or 'B'. |
| for (++LastInstPos; II != IE; ++II, ++LastInstPos) { |
| Inst = cast<Instruction>(II); |
| NumberedInsts[Inst] = LastInstPos; |
| if (Inst == A || Inst == B) |
| break; |
| } |
| |
| assert(II != IE && "Instruction not found?"); |
| LastInstFound = II; |
| return Inst; |
| } |
| |
| /// \brief Find out whether 'A' dominates 'B', meaning whether 'A' |
| /// comes before 'B' in 'BB'. This is a simplification that considers |
| /// cached instruction positions and ignores other basic blocks, being |
| /// only relevant to compare relative instructions positions inside 'BB'. |
| bool dominates(const Instruction *A, const Instruction *B) { |
| assert(A->getParent() == B->getParent() && |
| "Instructions must be in the same basic block!"); |
| |
| unsigned NA = NumberedInsts.lookup(A); |
| unsigned NB = NumberedInsts.lookup(B); |
| if (NA && NB) |
| return NA < NB; |
| if (NA) |
| return true; |
| if (NB) |
| return false; |
| |
| return A == find(A, B); |
| } |
| }; |
| |
| /// 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, DominatorTree *DT, |
| bool IncludeI) |
| : LocalInstCache(I->getParent()), BeforeHere(I), DT(DT), |
| ReturnCaptures(ReturnCaptures), IncludeI(IncludeI), Captured(false) {} |
| |
| void tooManyUses() override { Captured = true; } |
| |
| bool isSafeToPrune(Instruction *I) { |
| BasicBlock *BB = I->getParent(); |
| // 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 (BeforeHere != I && !DT->isReachableFromEntry(BB)) |
| return true; |
| |
| // Compute the case where both instructions are inside the same basic |
| // block. Since instructions in the same BB as BeforeHere are numbered in |
| // 'LocalInstCache', avoid using 'dominates' and 'isPotentiallyReachable' |
| // which are very expensive for large basic blocks. |
| if (BB == BeforeHere->getParent()) { |
| // 'I' dominates 'BeforeHere' => not safe to prune. |
| // |
| // The value defined by an invoke dominates an instruction only if it |
| // dominates every instruction in UseBB. A PHI is dominated only if |
| // the instruction dominates every possible use in the UseBB. Since |
| // UseBB == BB, avoid pruning. |
| if (isa<InvokeInst>(BeforeHere) || isa<PHINode>(I) || I == BeforeHere) |
| return false; |
| if (!LocalInstCache.dominates(BeforeHere, I)) |
| return false; |
| |
| // 'BeforeHere' comes before 'I', it's safe to prune if we also |
| // guarantee that 'I' never reaches 'BeforeHere' through a back-edge or |
| // by its successors, i.e, prune if: |
| // |
| // (1) BB is an entry block or have no sucessors. |
| // (2) There's no path coming back through BB sucessors. |
| if (BB == &BB->getParent()->getEntryBlock() || |
| !BB->getTerminator()->getNumSuccessors()) |
| return true; |
| |
| SmallVector<BasicBlock*, 32> Worklist; |
| Worklist.append(succ_begin(BB), succ_end(BB)); |
| if (!isPotentiallyReachableFromMany(Worklist, BB, DT)) |
| return true; |
| |
| return false; |
| } |
| |
| // If the value is defined in the same basic block as use and BeforeHere, |
| // there is no need to explore the use if BeforeHere dominates use. |
| // Check whether there is a path from I to BeforeHere. |
| if (BeforeHere != I && DT->dominates(BeforeHere, I) && |
| !isPotentiallyReachable(I, BeforeHere, DT)) |
| return true; |
| |
| return false; |
| } |
| |
| bool shouldExplore(const Use *U) override { |
| Instruction *I = cast<Instruction>(U->getUser()); |
| |
| if (BeforeHere == I && !IncludeI) |
| return false; |
| |
| if (isSafeToPrune(I)) |
| return false; |
| |
| return true; |
| } |
| |
| bool captured(const Use *U) override { |
| if (isa<ReturnInst>(U->getUser()) && !ReturnCaptures) |
| return false; |
| |
| if (!shouldExplore(U)) |
| return false; |
| |
| Captured = true; |
| return true; |
| } |
| |
| NumberedInstCache LocalInstCache; |
| const Instruction *BeforeHere; |
| DominatorTree *DT; |
| |
| bool ReturnCaptures; |
| bool IncludeI; |
| |
| bool Captured; |
| }; |
| } |
| |
| /// 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) { |
| 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); |
| 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, |
| DominatorTree *DT, bool IncludeI) { |
| assert(!isa<GlobalValue>(V) && |
| "It doesn't make sense to ask whether a global is captured."); |
| |
| if (!DT) |
| return PointerMayBeCaptured(V, ReturnCaptures, StoreCaptures); |
| |
| // TODO: See comment in PointerMayBeCaptured regarding what could be done |
| // with StoreCaptures. |
| |
| CapturesBefore CB(ReturnCaptures, I, DT, IncludeI); |
| PointerMayBeCaptured(V, &CB); |
| return CB.Captured; |
| } |
| |
| /// TODO: Write a new FunctionPass AliasAnalysis so that it can keep |
| /// a cache. Then we can move the code from BasicAliasAnalysis into |
| /// that path, and remove this threshold. |
| static int const Threshold = 20; |
| |
| void llvm::PointerMayBeCaptured(const Value *V, CaptureTracker *Tracker) { |
| assert(V->getType()->isPointerTy() && "Capture is for pointers only!"); |
| SmallVector<const Use *, Threshold> Worklist; |
| SmallSet<const Use *, Threshold> Visited; |
| int 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++ >= Threshold) |
| return Tracker->tooManyUses(); |
| |
| if (!Tracker->shouldExplore(&U)) continue; |
| Visited.insert(&U); |
| Worklist.push_back(&U); |
| } |
| |
| while (!Worklist.empty()) { |
| const Use *U = Worklist.pop_back_val(); |
| Instruction *I = cast<Instruction>(U->getUser()); |
| V = U->get(); |
| |
| switch (I->getOpcode()) { |
| case Instruction::Call: |
| case Instruction::Invoke: { |
| CallSite CS(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 (CS.onlyReadsMemory() && CS.doesNotThrow() && I->getType()->isVoidTy()) |
| break; |
| |
| // 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). |
| CallSite::arg_iterator B = CS.arg_begin(), E = CS.arg_end(); |
| for (CallSite::arg_iterator A = B; A != E; ++A) |
| if (A->get() == V && !CS.doesNotCapture(A - B)) |
| // The parameter is not marked 'nocapture' - captured. |
| if (Tracker->captured(U)) |
| return; |
| break; |
| } |
| case Instruction::Load: |
| // Loading from a pointer does not cause it to be captured. |
| break; |
| case Instruction::VAArg: |
| // "va-arg" from a pointer does not cause it to be captured. |
| break; |
| case Instruction::Store: |
| if (V == I->getOperand(0)) |
| // Stored the pointer - conservatively assume it may be captured. |
| if (Tracker->captured(U)) |
| return; |
| // Storing to the pointee does not cause the pointer to be captured. |
| 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. |
| Count = 0; |
| for (Use &UU : I->uses()) { |
| // If there are lots of uses, conservatively say that the value |
| // is captured to avoid taking too much compile time. |
| if (Count++ >= Threshold) |
| return Tracker->tooManyUses(); |
| |
| if (Visited.insert(&UU).second) |
| if (Tracker->shouldExplore(&UU)) |
| Worklist.push_back(&UU); |
| } |
| break; |
| case Instruction::ICmp: |
| // 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 (ConstantPointerNull *CPN = |
| dyn_cast<ConstantPointerNull>(I->getOperand(1))) |
| if (CPN->getType()->getAddressSpace() == 0) |
| if (isNoAliasCall(V->stripPointerCasts())) |
| 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. |
| } |
