Analysis/FlowSensitive/DataflowEnvironment.cpp

//===-- DataflowEnvironment.cpp ---------------------------------*- C++ -*-===//
//
// 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 defines an Environment class that is used by dataflow analyses
//  that run over Control-Flow Graphs (CFGs) to keep track of the state of the
//  program at given program points.
//
//===----------------------------------------------------------------------===//

#include "clang/Analysis/FlowSensitive/DataflowEnvironment.h"
#include "clang/AST/Decl.h"
#include "clang/AST/DeclCXX.h"
#include "clang/AST/Type.h"
#include "clang/Analysis/FlowSensitive/DataflowLattice.h"
#include "clang/Analysis/FlowSensitive/Value.h"
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/DenseSet.h"
#include "llvm/Support/Casting.h"
#include "llvm/Support/ErrorHandling.h"
#include <cassert>
#include <memory>
#include <utility>

namespace clang {
namespace dataflow {

// FIXME: convert these to parameters of the analysis or environment. Current
// settings have been experimentaly validated, but only for a particular
// analysis.
static constexpr int MaxCompositeValueDepth = 3;
static constexpr int MaxCompositeValueSize = 1000;

/// Returns a map consisting of key-value entries that are present in both maps.
template <typename K, typename V>
llvm::DenseMap<K, V> intersectDenseMaps(const llvm::DenseMap<K, V> &Map1,
                                        const llvm::DenseMap<K, V> &Map2) {
  llvm::DenseMap<K, V> Result;
  for (auto &Entry : Map1) {
    auto It = Map2.find(Entry.first);
    if (It != Map2.end() && Entry.second == It->second)
      Result.insert({Entry.first, Entry.second});
  }
  return Result;
}

static bool areEquivalentIndirectionValues(Value *Val1, Value *Val2) {
  if (auto *IndVal1 = dyn_cast<ReferenceValue>(Val1)) {
    auto *IndVal2 = cast<ReferenceValue>(Val2);
    return &IndVal1->getReferentLoc() == &IndVal2->getReferentLoc();
  }
  if (auto *IndVal1 = dyn_cast<PointerValue>(Val1)) {
    auto *IndVal2 = cast<PointerValue>(Val2);
    return &IndVal1->getPointeeLoc() == &IndVal2->getPointeeLoc();
  }
  return false;
}

/// Returns true if and only if `Val1` is equivalent to `Val2`.
static bool equivalentValues(QualType Type, Value *Val1,
                             const Environment &Env1, Value *Val2,
                             const Environment &Env2,
                             Environment::ValueModel &Model) {
  return Val1 == Val2 || areEquivalentIndirectionValues(Val1, Val2) ||
         Model.compareEquivalent(Type, *Val1, Env1, *Val2, Env2);
}

/// Attempts to merge distinct values `Val1` and `Val2` in `Env1` and `Env2`,
/// respectively, of the same type `Type`. Merging generally produces a single
/// value that (soundly) approximates the two inputs, although the actual
/// meaning depends on `Model`.
static Value *mergeDistinctValues(QualType Type, Value *Val1,
                                  const Environment &Env1, Value *Val2,
                                  const Environment &Env2,
                                  Environment &MergedEnv,
                                  Environment::ValueModel &Model) {
  // Join distinct boolean values preserving information about the constraints
  // in the respective path conditions.
  //
  // FIXME: Does not work for backedges, since the two (or more) paths will not
  // have mutually exclusive conditions.
  if (auto *Expr1 = dyn_cast<BoolValue>(Val1)) {
    auto *Expr2 = cast<BoolValue>(Val2);
    auto &MergedVal = MergedEnv.makeAtomicBoolValue();
    MergedEnv.addToFlowCondition(MergedEnv.makeOr(
        MergedEnv.makeAnd(Env1.getFlowConditionToken(),
                          MergedEnv.makeIff(MergedVal, *Expr1)),
        MergedEnv.makeAnd(Env2.getFlowConditionToken(),
                          MergedEnv.makeIff(MergedVal, *Expr2))));
    return &MergedVal;
  }

  // FIXME: add unit tests that cover this statement.
  if (areEquivalentIndirectionValues(Val1, Val2)) {
    return Val1;
  }

  // FIXME: Consider destroying `MergedValue` immediately if `ValueModel::merge`
  // returns false to avoid storing unneeded values in `DACtx`.
  if (Value *MergedVal = MergedEnv.createValue(Type))
    if (Model.merge(Type, *Val1, Env1, *Val2, Env2, *MergedVal, MergedEnv))
      return MergedVal;

  return nullptr;
}

/// Initializes a global storage value.
static void initGlobalVar(const VarDecl &D, Environment &Env) {
  if (!D.hasGlobalStorage() ||
      Env.getStorageLocation(D, SkipPast::None) != nullptr)
    return;

  auto &Loc = Env.createStorageLocation(D);
  Env.setStorageLocation(D, Loc);
  if (auto *Val = Env.createValue(D.getType()))
    Env.setValue(Loc, *Val);
}

/// Initializes a global storage value.
static void initGlobalVar(const Decl &D, Environment &Env) {
  if (auto *V = dyn_cast<VarDecl>(&D))
    initGlobalVar(*V, Env);
}

/// Initializes global storage values that are declared or referenced from
/// sub-statements of `S`.
// FIXME: Add support for resetting globals after function calls to enable
// the implementation of sound analyses.
static void initGlobalVars(const Stmt &S, Environment &Env) {
  for (auto *Child : S.children()) {
    if (Child != nullptr)
      initGlobalVars(*Child, Env);
  }

  if (auto *DS = dyn_cast<DeclStmt>(&S)) {
    if (DS->isSingleDecl()) {
      initGlobalVar(*DS->getSingleDecl(), Env);
    } else {
      for (auto *D : DS->getDeclGroup())
        initGlobalVar(*D, Env);
    }
  } else if (auto *E = dyn_cast<DeclRefExpr>(&S)) {
    initGlobalVar(*E->getDecl(), Env);
  } else if (auto *E = dyn_cast<MemberExpr>(&S)) {
    initGlobalVar(*E->getMemberDecl(), Env);
  }
}

Environment::Environment(DataflowAnalysisContext &DACtx)
    : DACtx(&DACtx), FlowConditionToken(&DACtx.makeFlowConditionToken()) {}

Environment::Environment(const Environment &Other)
    : DACtx(Other.DACtx), DeclToLoc(Other.DeclToLoc),
      ExprToLoc(Other.ExprToLoc), LocToVal(Other.LocToVal),
      MemberLocToStruct(Other.MemberLocToStruct),
      FlowConditionToken(&DACtx->forkFlowCondition(*Other.FlowConditionToken)) {
}

Environment &Environment::operator=(const Environment &Other) {
  Environment Copy(Other);
  *this = std::move(Copy);
  return *this;
}

Environment::Environment(DataflowAnalysisContext &DACtx,
                         const DeclContext &DeclCtx)
    : Environment(DACtx) {
  if (const auto *FuncDecl = dyn_cast<FunctionDecl>(&DeclCtx)) {
    assert(FuncDecl->getBody() != nullptr);
    initGlobalVars(*FuncDecl->getBody(), *this);
    for (const auto *ParamDecl : FuncDecl->parameters()) {
      assert(ParamDecl != nullptr);
      auto &ParamLoc = createStorageLocation(*ParamDecl);
      setStorageLocation(*ParamDecl, ParamLoc);
      if (Value *ParamVal = createValue(ParamDecl->getType()))
        setValue(ParamLoc, *ParamVal);
    }
  }

  if (const auto *MethodDecl = dyn_cast<CXXMethodDecl>(&DeclCtx)) {
    auto *Parent = MethodDecl->getParent();
    assert(Parent != nullptr);
    if (Parent->isLambda())
      MethodDecl = dyn_cast<CXXMethodDecl>(Parent->getDeclContext());

    if (MethodDecl && !MethodDecl->isStatic()) {
      QualType ThisPointeeType = MethodDecl->getThisObjectType();
      // FIXME: Add support for union types.
      if (!ThisPointeeType->isUnionType()) {
        auto &ThisPointeeLoc = createStorageLocation(ThisPointeeType);
        DACtx.setThisPointeeStorageLocation(ThisPointeeLoc);
        if (Value *ThisPointeeVal = createValue(ThisPointeeType))
          setValue(ThisPointeeLoc, *ThisPointeeVal);
      }
    }
  }
}

Environment Environment::pushCall(const CallExpr *Call) const {
  Environment Env(*this);

  // FIXME: Currently this only works if the callee is never a method and the
  // same callee is never analyzed from multiple separate callsites. To
  // generalize this, we'll need to store a "context" field (probably a stack of
  // `const CallExpr *`s) in the `Environment`, and then change the
  // `DataflowAnalysisContext` class to hold a map from contexts to "frames",
  // where each frame stores its own version of what are currently the
  // `DeclToLoc`, `ExprToLoc`, and `ThisPointeeLoc` fields.

  const auto *FuncDecl = Call->getDirectCallee();
  assert(FuncDecl != nullptr);
  const auto *Body = FuncDecl->getBody();
  assert(Body != nullptr);
  // FIXME: In order to allow the callee to reference globals, we probably need
  // to call `initGlobalVars` here in some way.

  auto ParamIt = FuncDecl->param_begin();
  auto ParamEnd = FuncDecl->param_end();
  auto ArgIt = Call->arg_begin();
  auto ArgEnd = Call->arg_end();

  // FIXME: Parameters don't always map to arguments 1:1; examples include
  // overloaded operators implemented as member functions, and parameter packs.
  for (; ArgIt != ArgEnd; ++ParamIt, ++ArgIt) {
    assert(ParamIt != ParamEnd);

    const VarDecl *Param = *ParamIt;
    const Expr *Arg = *ArgIt;
    auto *ArgLoc = Env.getStorageLocation(*Arg, SkipPast::Reference);
    assert(ArgLoc != nullptr);
    Env.setStorageLocation(*Param, *ArgLoc);
  }

  return Env;
}

bool Environment::equivalentTo(const Environment &Other,
                               Environment::ValueModel &Model) const {
  assert(DACtx == Other.DACtx);

  if (DeclToLoc != Other.DeclToLoc)
    return false;

  if (ExprToLoc != Other.ExprToLoc)
    return false;

  // Compare the contents for the intersection of their domains.
  for (auto &Entry : LocToVal) {
    const StorageLocation *Loc = Entry.first;
    assert(Loc != nullptr);

    Value *Val = Entry.second;
    assert(Val != nullptr);

    auto It = Other.LocToVal.find(Loc);
    if (It == Other.LocToVal.end())
      continue;
    assert(It->second != nullptr);

    if (!equivalentValues(Loc->getType(), Val, *this, It->second, Other, Model))
      return false;
  }

  return true;
}

LatticeJoinEffect Environment::join(const Environment &Other,
                                    Environment::ValueModel &Model) {
  assert(DACtx == Other.DACtx);

  auto Effect = LatticeJoinEffect::Unchanged;

  Environment JoinedEnv(*DACtx);

  JoinedEnv.DeclToLoc = intersectDenseMaps(DeclToLoc, Other.DeclToLoc);
  if (DeclToLoc.size() != JoinedEnv.DeclToLoc.size())
    Effect = LatticeJoinEffect::Changed;

  JoinedEnv.ExprToLoc = intersectDenseMaps(ExprToLoc, Other.ExprToLoc);
  if (ExprToLoc.size() != JoinedEnv.ExprToLoc.size())
    Effect = LatticeJoinEffect::Changed;

  JoinedEnv.MemberLocToStruct =
      intersectDenseMaps(MemberLocToStruct, Other.MemberLocToStruct);
  if (MemberLocToStruct.size() != JoinedEnv.MemberLocToStruct.size())
    Effect = LatticeJoinEffect::Changed;

  // FIXME: set `Effect` as needed.
  JoinedEnv.FlowConditionToken = &DACtx->joinFlowConditions(
      *FlowConditionToken, *Other.FlowConditionToken);

  for (auto &Entry : LocToVal) {
    const StorageLocation *Loc = Entry.first;
    assert(Loc != nullptr);

    Value *Val = Entry.second;
    assert(Val != nullptr);

    auto It = Other.LocToVal.find(Loc);
    if (It == Other.LocToVal.end())
      continue;
    assert(It->second != nullptr);

    if (Val == It->second) {
      JoinedEnv.LocToVal.insert({Loc, Val});
      continue;
    }

    if (Value *MergedVal = mergeDistinctValues(
            Loc->getType(), Val, *this, It->second, Other, JoinedEnv, Model))
      JoinedEnv.LocToVal.insert({Loc, MergedVal});
  }
  if (LocToVal.size() != JoinedEnv.LocToVal.size())
    Effect = LatticeJoinEffect::Changed;

  *this = std::move(JoinedEnv);

  return Effect;
}

StorageLocation &Environment::createStorageLocation(QualType Type) {
  return DACtx->getStableStorageLocation(Type);
}

StorageLocation &Environment::createStorageLocation(const VarDecl &D) {
  // Evaluated declarations are always assigned the same storage locations to
  // ensure that the environment stabilizes across loop iterations. Storage
  // locations for evaluated declarations are stored in the analysis context.
  return DACtx->getStableStorageLocation(D);
}

StorageLocation &Environment::createStorageLocation(const Expr &E) {
  // Evaluated expressions are always assigned the same storage locations to
  // ensure that the environment stabilizes across loop iterations. Storage
  // locations for evaluated expressions are stored in the analysis context.
  return DACtx->getStableStorageLocation(E);
}

void Environment::setStorageLocation(const ValueDecl &D, StorageLocation &Loc) {
  assert(DeclToLoc.find(&D) == DeclToLoc.end());
  DeclToLoc[&D] = &Loc;
}

StorageLocation *Environment::getStorageLocation(const ValueDecl &D,
                                                 SkipPast SP) const {
  auto It = DeclToLoc.find(&D);
  return It == DeclToLoc.end() ? nullptr : &skip(*It->second, SP);
}

void Environment::setStorageLocation(const Expr &E, StorageLocation &Loc) {
  const Expr &CanonE = ignoreCFGOmittedNodes(E);
  assert(ExprToLoc.find(&CanonE) == ExprToLoc.end());
  ExprToLoc[&CanonE] = &Loc;
}

StorageLocation *Environment::getStorageLocation(const Expr &E,
                                                 SkipPast SP) const {
  // FIXME: Add a test with parens.
  auto It = ExprToLoc.find(&ignoreCFGOmittedNodes(E));
  return It == ExprToLoc.end() ? nullptr : &skip(*It->second, SP);
}

StorageLocation *Environment::getThisPointeeStorageLocation() const {
  return DACtx->getThisPointeeStorageLocation();
}

PointerValue &Environment::getOrCreateNullPointerValue(QualType PointeeType) {
  return DACtx->getOrCreateNullPointerValue(PointeeType);
}

void Environment::setValue(const StorageLocation &Loc, Value &Val) {
  LocToVal[&Loc] = &Val;

  if (auto *StructVal = dyn_cast<StructValue>(&Val)) {
    auto &AggregateLoc = *cast<AggregateStorageLocation>(&Loc);

    const QualType Type = AggregateLoc.getType();
    assert(Type->isStructureOrClassType());

    for (const FieldDecl *Field : getObjectFields(Type)) {
      assert(Field != nullptr);
      StorageLocation &FieldLoc = AggregateLoc.getChild(*Field);
      MemberLocToStruct[&FieldLoc] = std::make_pair(StructVal, Field);
      if (auto *FieldVal = StructVal->getChild(*Field))
        setValue(FieldLoc, *FieldVal);
    }
  }

  auto It = MemberLocToStruct.find(&Loc);
  if (It != MemberLocToStruct.end()) {
    // `Loc` is the location of a struct member so we need to also update the
    // value of the member in the corresponding `StructValue`.

    assert(It->second.first != nullptr);
    StructValue &StructVal = *It->second.first;

    assert(It->second.second != nullptr);
    const ValueDecl &Member = *It->second.second;

    StructVal.setChild(Member, Val);
  }
}

Value *Environment::getValue(const StorageLocation &Loc) const {
  auto It = LocToVal.find(&Loc);
  return It == LocToVal.end() ? nullptr : It->second;
}

Value *Environment::getValue(const ValueDecl &D, SkipPast SP) const {
  auto *Loc = getStorageLocation(D, SP);
  if (Loc == nullptr)
    return nullptr;
  return getValue(*Loc);
}

Value *Environment::getValue(const Expr &E, SkipPast SP) const {
  auto *Loc = getStorageLocation(E, SP);
  if (Loc == nullptr)
    return nullptr;
  return getValue(*Loc);
}

Value *Environment::createValue(QualType Type) {
  llvm::DenseSet<QualType> Visited;
  int CreatedValuesCount = 0;
  Value *Val = createValueUnlessSelfReferential(Type, Visited, /*Depth=*/0,
                                                CreatedValuesCount);
  if (CreatedValuesCount > MaxCompositeValueSize) {
    llvm::errs() << "Attempting to initialize a huge value of type: " << Type
                 << '\n';
  }
  return Val;
}

Value *Environment::createValueUnlessSelfReferential(
    QualType Type, llvm::DenseSet<QualType> &Visited, int Depth,
    int &CreatedValuesCount) {
  assert(!Type.isNull());

  // Allow unlimited fields at depth 1; only cap at deeper nesting levels.
  if ((Depth > 1 && CreatedValuesCount > MaxCompositeValueSize) ||
      Depth > MaxCompositeValueDepth)
    return nullptr;

  if (Type->isBooleanType()) {
    CreatedValuesCount++;
    return &makeAtomicBoolValue();
  }

  if (Type->isIntegerType()) {
    CreatedValuesCount++;
    return &takeOwnership(std::make_unique<IntegerValue>());
  }

  if (Type->isReferenceType()) {
    CreatedValuesCount++;
    QualType PointeeType = Type->castAs<ReferenceType>()->getPointeeType();
    auto &PointeeLoc = createStorageLocation(PointeeType);

    if (Visited.insert(PointeeType.getCanonicalType()).second) {
      Value *PointeeVal = createValueUnlessSelfReferential(
          PointeeType, Visited, Depth, CreatedValuesCount);
      Visited.erase(PointeeType.getCanonicalType());

      if (PointeeVal != nullptr)
        setValue(PointeeLoc, *PointeeVal);
    }

    return &takeOwnership(std::make_unique<ReferenceValue>(PointeeLoc));
  }

  if (Type->isPointerType()) {
    CreatedValuesCount++;
    QualType PointeeType = Type->castAs<PointerType>()->getPointeeType();
    auto &PointeeLoc = createStorageLocation(PointeeType);

    if (Visited.insert(PointeeType.getCanonicalType()).second) {
      Value *PointeeVal = createValueUnlessSelfReferential(
          PointeeType, Visited, Depth, CreatedValuesCount);
      Visited.erase(PointeeType.getCanonicalType());

      if (PointeeVal != nullptr)
        setValue(PointeeLoc, *PointeeVal);
    }

    return &takeOwnership(std::make_unique<PointerValue>(PointeeLoc));
  }

  if (Type->isStructureOrClassType()) {
    CreatedValuesCount++;
    // FIXME: Initialize only fields that are accessed in the context that is
    // being analyzed.
    llvm::DenseMap<const ValueDecl *, Value *> FieldValues;
    for (const FieldDecl *Field : getObjectFields(Type)) {
      assert(Field != nullptr);

      QualType FieldType = Field->getType();
      if (Visited.contains(FieldType.getCanonicalType()))
        continue;

      Visited.insert(FieldType.getCanonicalType());
      if (auto *FieldValue = createValueUnlessSelfReferential(
              FieldType, Visited, Depth + 1, CreatedValuesCount))
        FieldValues.insert({Field, FieldValue});
      Visited.erase(FieldType.getCanonicalType());
    }

    return &takeOwnership(
        std::make_unique<StructValue>(std::move(FieldValues)));
  }

  return nullptr;
}

StorageLocation &Environment::skip(StorageLocation &Loc, SkipPast SP) const {
  switch (SP) {
  case SkipPast::None:
    return Loc;
  case SkipPast::Reference:
    // References cannot be chained so we only need to skip past one level of
    // indirection.
    if (auto *Val = dyn_cast_or_null<ReferenceValue>(getValue(Loc)))
      return Val->getReferentLoc();
    return Loc;
  case SkipPast::ReferenceThenPointer:
    StorageLocation &LocPastRef = skip(Loc, SkipPast::Reference);
    if (auto *Val = dyn_cast_or_null<PointerValue>(getValue(LocPastRef)))
      return Val->getPointeeLoc();
    return LocPastRef;
  }
  llvm_unreachable("bad SkipPast kind");
}

const StorageLocation &Environment::skip(const StorageLocation &Loc,
                                         SkipPast SP) const {
  return skip(*const_cast<StorageLocation *>(&Loc), SP);
}

void Environment::addToFlowCondition(BoolValue &Val) {
  DACtx->addFlowConditionConstraint(*FlowConditionToken, Val);
}

bool Environment::flowConditionImplies(BoolValue &Val) const {
  return DACtx->flowConditionImplies(*FlowConditionToken, Val);
}

void Environment::dump() const {
  DACtx->dumpFlowCondition(*FlowConditionToken);
}

} // namespace dataflow
} // namespace clang