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Diffstat (limited to 'llvm/lib/Analysis/CaptureTracking.cpp')
| -rw-r--r-- | llvm/lib/Analysis/CaptureTracking.cpp | 389 |
1 files changed, 389 insertions, 0 deletions
diff --git a/llvm/lib/Analysis/CaptureTracking.cpp b/llvm/lib/Analysis/CaptureTracking.cpp new file mode 100644 index 000000000000..20e2f06540a3 --- /dev/null +++ b/llvm/lib/Analysis/CaptureTracking.cpp @@ -0,0 +1,389 @@ +//===--- 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/Analysis/AliasAnalysis.h" +#include "llvm/Analysis/CFG.h" +#include "llvm/Analysis/OrderedBasicBlock.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" + +using namespace llvm; + +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; + return O->getPointerDereferenceableBytes(DL, CanBeNull); +} + +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, OrderedBasicBlock *IC) + : OrderedBB(IC), 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 + // 'OrderedBB', 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 (!OrderedBB->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 successors. + // (2) There's no path coming back through BB successors. + if (BB == &BB->getParent()->getEntryBlock() || + !BB->getTerminator()->getNumSuccessors()) + return true; + + SmallVector<BasicBlock*, 32> Worklist; + Worklist.append(succ_begin(BB), succ_end(BB)); + return !isPotentiallyReachableFromMany(Worklist, BB, nullptr, DT); + } + + // 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, nullptr, 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; + } + + OrderedBasicBlock *OrderedBB; + const Instruction *BeforeHere; + const 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, + 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); + 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. A ordered basic block \p OBB can be used in order to speed up +/// queries about relative order among instructions in the same basic block. +bool llvm::PointerMayBeCapturedBefore(const Value *V, bool ReturnCaptures, + bool StoreCaptures, const Instruction *I, + const DominatorTree *DT, bool IncludeI, + OrderedBasicBlock *OBB, + unsigned MaxUsesToExplore) { + assert(!isa<GlobalValue>(V) && + "It doesn't make sense to ask whether a global is captured."); + bool UseNewOBB = OBB == nullptr; + + if (!DT) + return PointerMayBeCaptured(V, ReturnCaptures, StoreCaptures, + MaxUsesToExplore); + if (UseNewOBB) + OBB = new OrderedBasicBlock(I->getParent()); + + // TODO: See comment in PointerMayBeCaptured regarding what could be done + // with StoreCaptures. + + CapturesBefore CB(ReturnCaptures, I, DT, IncludeI, OBB); + PointerMayBeCaptured(V, &CB, MaxUsesToExplore); + + if (UseNewOBB) + delete OBB; + return CB.Captured; +} + +void llvm::PointerMayBeCaptured(const Value *V, CaptureTracker *Tracker, + unsigned MaxUsesToExplore) { + assert(V->getType()->isPointerTy() && "Capture is for pointers only!"); + SmallVector<const Use *, DefaultMaxUsesToExplore> Worklist; + SmallSet<const Use *, DefaultMaxUsesToExplore> 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) + return Tracker->tooManyUses(); + if (!Visited.insert(&U).second) + continue; + if (!Tracker->shouldExplore(&U)) + continue; + Worklist.push_back(&U); + } + }; + AddUses(V); + + 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: { + 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)) { + AddUses(Call); + 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). + for (auto IdxOpPair : enumerate(Call->data_ops())) { + int Idx = IdxOpPair.index(); + Value *A = IdxOpPair.value(); + if (A == V && !Call->doesNotCapture(Idx)) + // 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 (V == I->getOperand(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 (ARMWI->getValOperand() == V || 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 (ACXI->getCompareOperand() == V || ACXI->getNewValOperand() == V || + 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. + AddUses(I); + break; + case Instruction::ICmp: { + unsigned Idx = (I->getOperand(0) == V) ? 0 : 1; + 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(V->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. +} |