CaptureTracking.cpp

Go to the documentation of this file.

//===--- 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;
  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)
      : 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.
      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 (!BeforeHere->comesBefore(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;

      Captured = true;
      return true;
    }

    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);
  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) {
  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);
  PointerMayBeCaptured(V, &CB, MaxUsesToExplore);
  if (CB.Captured)
    ++NumCapturedBefore;
  else
    ++NumNotCapturedBefore;
  return CB.Captured;
}

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 a local allocation, check to see if it escapes.
  if (isa<AllocaInst>(V) || isNoAliasCall(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;
  }

  // If this is an argument that corresponds to a byval or noalias argument,
  // then it has not escaped before entering the function.  Check if it escapes
  // inside the function.
  if (const Argument *A = dyn_cast<Argument>(V))
    if (A->hasByValAttr() || A->hasNoAliasAttr()) {
      // Note even if the argument is marked nocapture, we still need to check
      // for copies made inside the function. The nocapture attribute only
      // specifies that there are no copies made that outlive the function.
      auto Ret = !PointerMayBeCaptured(V, false, /*StoreCaptures=*/true);
      if (IsCapturedCache)
        CacheIt->second = Ret;
      return Ret;
    }

  return false;
}