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TypeErasedDataflowAnalysis.cpp
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TypeErasedDataflowAnalysis.cpp
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//===- TypeErasedDataflowAnalysis.cpp -------------------------------------===//
//
// 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 type-erased base types and functions for building dataflow
// analyses that run over Control-Flow Graphs (CFGs).
//
//===----------------------------------------------------------------------===//
#include <algorithm>
#include <memory>
#include <system_error>
#include <utility>
#include <vector>
#include "clang/AST/DeclCXX.h"
#include "clang/AST/OperationKinds.h"
#include "clang/AST/StmtVisitor.h"
#include "clang/Analysis/Analyses/PostOrderCFGView.h"
#include "clang/Analysis/CFG.h"
#include "clang/Analysis/FlowSensitive/DataflowEnvironment.h"
#include "clang/Analysis/FlowSensitive/DataflowLattice.h"
#include "clang/Analysis/FlowSensitive/DataflowWorklist.h"
#include "clang/Analysis/FlowSensitive/Transfer.h"
#include "clang/Analysis/FlowSensitive/TypeErasedDataflowAnalysis.h"
#include "clang/Analysis/FlowSensitive/Value.h"
#include "llvm/ADT/ArrayRef.h"
#include "llvm/ADT/DenseSet.h"
#include "llvm/ADT/Optional.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/Support/Error.h"
#include "llvm/Support/ErrorHandling.h"
namespace clang {
namespace dataflow {
class StmtToEnvMapImpl : public StmtToEnvMap {
public:
StmtToEnvMapImpl(
const ControlFlowContext &CFCtx,
llvm::ArrayRef<llvm::Optional<TypeErasedDataflowAnalysisState>>
BlockToState)
: CFCtx(CFCtx), BlockToState(BlockToState) {}
const Environment *getEnvironment(const Stmt &S) const override {
auto BlockIt = CFCtx.getStmtToBlock().find(&ignoreCFGOmittedNodes(S));
assert(BlockIt != CFCtx.getStmtToBlock().end());
const auto &State = BlockToState[BlockIt->getSecond()->getBlockID()];
assert(State);
return &State->Env;
}
private:
const ControlFlowContext &CFCtx;
llvm::ArrayRef<llvm::Optional<TypeErasedDataflowAnalysisState>> BlockToState;
};
/// Returns the index of `Block` in the successors of `Pred`.
static int blockIndexInPredecessor(const CFGBlock &Pred,
const CFGBlock &Block) {
auto BlockPos = llvm::find_if(
Pred.succs(), [&Block](const CFGBlock::AdjacentBlock &Succ) {
return Succ && Succ->getBlockID() == Block.getBlockID();
});
return BlockPos - Pred.succ_begin();
}
static bool isLoopHead(const CFGBlock &B) {
if (const auto *T = B.getTerminatorStmt())
switch (T->getStmtClass()) {
case Stmt::WhileStmtClass:
case Stmt::DoStmtClass:
case Stmt::ForStmtClass:
return true;
default:
return false;
}
return false;
}
// The return type of the visit functions in TerminatorVisitor. The first
// element represents the terminator expression (that is the conditional
// expression in case of a path split in the CFG). The second element
// represents whether the condition was true or false.
using TerminatorVisitorRetTy = std::pair<const Expr *, bool>;
/// Extends the flow condition of an environment based on a terminator
/// statement.
class TerminatorVisitor
: public ConstStmtVisitor<TerminatorVisitor, TerminatorVisitorRetTy> {
public:
TerminatorVisitor(const StmtToEnvMap &StmtToEnv, Environment &Env,
int BlockSuccIdx, TransferOptions TransferOpts)
: StmtToEnv(StmtToEnv), Env(Env),
BlockSuccIdx(BlockSuccIdx), TransferOpts(TransferOpts) {}
TerminatorVisitorRetTy VisitIfStmt(const IfStmt *S) {
auto *Cond = S->getCond();
assert(Cond != nullptr);
return extendFlowCondition(*Cond);
}
TerminatorVisitorRetTy VisitWhileStmt(const WhileStmt *S) {
auto *Cond = S->getCond();
assert(Cond != nullptr);
return extendFlowCondition(*Cond);
}
TerminatorVisitorRetTy VisitDoStmt(const DoStmt *S) {
auto *Cond = S->getCond();
assert(Cond != nullptr);
return extendFlowCondition(*Cond);
}
TerminatorVisitorRetTy VisitForStmt(const ForStmt *S) {
auto *Cond = S->getCond();
if (Cond != nullptr)
return extendFlowCondition(*Cond);
return {nullptr, false};
}
TerminatorVisitorRetTy VisitBinaryOperator(const BinaryOperator *S) {
assert(S->getOpcode() == BO_LAnd || S->getOpcode() == BO_LOr);
auto *LHS = S->getLHS();
assert(LHS != nullptr);
return extendFlowCondition(*LHS);
}
TerminatorVisitorRetTy
VisitConditionalOperator(const ConditionalOperator *S) {
auto *Cond = S->getCond();
assert(Cond != nullptr);
return extendFlowCondition(*Cond);
}
private:
TerminatorVisitorRetTy extendFlowCondition(const Expr &Cond) {
// The terminator sub-expression might not be evaluated.
if (Env.getStorageLocation(Cond, SkipPast::None) == nullptr)
transfer(StmtToEnv, Cond, Env, TransferOpts);
// FIXME: The flow condition must be an r-value, so `SkipPast::None` should
// suffice.
auto *Val =
cast_or_null<BoolValue>(Env.getValue(Cond, SkipPast::Reference));
// Value merging depends on flow conditions from different environments
// being mutually exclusive -- that is, they cannot both be true in their
// entirety (even if they may share some clauses). So, we need *some* value
// for the condition expression, even if just an atom.
if (Val == nullptr) {
// FIXME: Consider introducing a helper for this get-or-create pattern.
auto *Loc = Env.getStorageLocation(Cond, SkipPast::None);
if (Loc == nullptr) {
Loc = &Env.createStorageLocation(Cond);
Env.setStorageLocation(Cond, *Loc);
}
Val = &Env.makeAtomicBoolValue();
Env.setValue(*Loc, *Val);
}
bool ConditionValue = true;
// The condition must be inverted for the successor that encompasses the
// "else" branch, if such exists.
if (BlockSuccIdx == 1) {
Val = &Env.makeNot(*Val);
ConditionValue = false;
}
Env.addToFlowCondition(*Val);
return {&Cond, ConditionValue};
}
const StmtToEnvMap &StmtToEnv;
Environment &Env;
int BlockSuccIdx;
TransferOptions TransferOpts;
};
/// Holds data structures required for running dataflow analysis.
struct AnalysisContext {
AnalysisContext(
const ControlFlowContext &CFCtx, TypeErasedDataflowAnalysis &Analysis,
const Environment &InitEnv,
llvm::ArrayRef<llvm::Optional<TypeErasedDataflowAnalysisState>>
BlockStates)
: CFCtx(CFCtx), Analysis(Analysis), InitEnv(InitEnv),
BlockStates(BlockStates) {}
/// Contains the CFG being analyzed.
const ControlFlowContext &CFCtx;
/// The analysis to be run.
TypeErasedDataflowAnalysis &Analysis;
/// Initial state to start the analysis.
const Environment &InitEnv;
/// Stores the state of a CFG block if it has been evaluated by the analysis.
/// The indices correspond to the block IDs.
llvm::ArrayRef<llvm::Optional<TypeErasedDataflowAnalysisState>> BlockStates;
};
/// Computes the input state for a given basic block by joining the output
/// states of its predecessors.
///
/// Requirements:
///
/// All predecessors of `Block` except those with loop back edges must have
/// already been transferred. States in `AC.BlockStates` that are set to
/// `std::nullopt` represent basic blocks that are not evaluated yet.
static TypeErasedDataflowAnalysisState
computeBlockInputState(const CFGBlock &Block, AnalysisContext &AC) {
llvm::DenseSet<const CFGBlock *> Preds;
Preds.insert(Block.pred_begin(), Block.pred_end());
if (Block.getTerminator().isTemporaryDtorsBranch()) {
// This handles a special case where the code that produced the CFG includes
// a conditional operator with a branch that constructs a temporary and
// calls a destructor annotated as noreturn. The CFG models this as follows:
//
// B1 (contains the condition of the conditional operator) - succs: B2, B3
// B2 (contains code that does not call a noreturn destructor) - succs: B4
// B3 (contains code that calls a noreturn destructor) - succs: B4
// B4 (has temporary destructor terminator) - succs: B5, B6
// B5 (noreturn block that is associated with the noreturn destructor call)
// B6 (contains code that follows the conditional operator statement)
//
// The first successor (B5 above) of a basic block with a temporary
// destructor terminator (B4 above) is the block that evaluates the
// destructor. If that block has a noreturn element then the predecessor
// block that constructed the temporary object (B3 above) is effectively a
// noreturn block and its state should not be used as input for the state
// of the block that has a temporary destructor terminator (B4 above). This
// holds regardless of which branch of the ternary operator calls the
// noreturn destructor. However, it doesn't cases where a nested ternary
// operator includes a branch that contains a noreturn destructor call.
//
// See `NoreturnDestructorTest` for concrete examples.
if (Block.succ_begin()->getReachableBlock()->hasNoReturnElement()) {
auto &StmtToBlock = AC.CFCtx.getStmtToBlock();
auto StmtBlock = StmtToBlock.find(Block.getTerminatorStmt());
assert(StmtBlock != StmtToBlock.end());
Preds.erase(StmtBlock->getSecond());
}
}
llvm::Optional<TypeErasedDataflowAnalysisState> MaybeState;
auto &Analysis = AC.Analysis;
auto BuiltinTransferOpts = Analysis.builtinTransferOptions();
for (const CFGBlock *Pred : Preds) {
// Skip if the `Block` is unreachable or control flow cannot get past it.
if (!Pred || Pred->hasNoReturnElement())
continue;
// Skip if `Pred` was not evaluated yet. This could happen if `Pred` has a
// loop back edge to `Block`.
const llvm::Optional<TypeErasedDataflowAnalysisState> &MaybePredState =
AC.BlockStates[Pred->getBlockID()];
if (!MaybePredState)
continue;
TypeErasedDataflowAnalysisState PredState = *MaybePredState;
if (BuiltinTransferOpts) {
if (const Stmt *PredTerminatorStmt = Pred->getTerminatorStmt()) {
const StmtToEnvMapImpl StmtToEnv(AC.CFCtx, AC.BlockStates);
auto [Cond, CondValue] =
TerminatorVisitor(StmtToEnv, PredState.Env,
blockIndexInPredecessor(*Pred, Block),
*BuiltinTransferOpts)
.Visit(PredTerminatorStmt);
if (Cond != nullptr)
// FIXME: Call transferBranchTypeErased even if BuiltinTransferOpts
// are not set.
Analysis.transferBranchTypeErased(CondValue, Cond, PredState.Lattice,
PredState.Env);
}
}
if (MaybeState) {
Analysis.joinTypeErased(MaybeState->Lattice, PredState.Lattice);
MaybeState->Env.join(PredState.Env, Analysis);
} else {
MaybeState = std::move(PredState);
}
}
if (!MaybeState) {
// FIXME: Consider passing `Block` to `Analysis.typeErasedInitialElement()`
// to enable building analyses like computation of dominators that
// initialize the state of each basic block differently.
MaybeState.emplace(Analysis.typeErasedInitialElement(), AC.InitEnv);
}
return *MaybeState;
}
/// Built-in transfer function for `CFGStmt`.
void builtinTransferStatement(const CFGStmt &Elt,
TypeErasedDataflowAnalysisState &InputState,
AnalysisContext &AC) {
const Stmt *S = Elt.getStmt();
assert(S != nullptr);
transfer(StmtToEnvMapImpl(AC.CFCtx, AC.BlockStates), *S, InputState.Env,
*AC.Analysis.builtinTransferOptions());
}
/// Built-in transfer function for `CFGInitializer`.
void builtinTransferInitializer(const CFGInitializer &Elt,
TypeErasedDataflowAnalysisState &InputState) {
const CXXCtorInitializer *Init = Elt.getInitializer();
assert(Init != nullptr);
auto &Env = InputState.Env;
const auto &ThisLoc =
*cast<AggregateStorageLocation>(Env.getThisPointeeStorageLocation());
const FieldDecl *Member = Init->getMember();
if (Member == nullptr)
// Not a field initializer.
return;
auto *InitStmt = Init->getInit();
assert(InitStmt != nullptr);
auto *InitStmtLoc = Env.getStorageLocation(*InitStmt, SkipPast::Reference);
if (InitStmtLoc == nullptr)
return;
auto *InitStmtVal = Env.getValue(*InitStmtLoc);
if (InitStmtVal == nullptr)
return;
if (Member->getType()->isReferenceType()) {
auto &MemberLoc = ThisLoc.getChild(*Member);
Env.setValue(MemberLoc, Env.takeOwnership(std::make_unique<ReferenceValue>(
*InitStmtLoc)));
} else {
auto &MemberLoc = ThisLoc.getChild(*Member);
Env.setValue(MemberLoc, *InitStmtVal);
}
}
void builtinTransfer(const CFGElement &Elt,
TypeErasedDataflowAnalysisState &State,
AnalysisContext &AC) {
switch (Elt.getKind()) {
case CFGElement::Statement:
builtinTransferStatement(Elt.castAs<CFGStmt>(), State, AC);
break;
case CFGElement::Initializer:
builtinTransferInitializer(Elt.castAs<CFGInitializer>(), State);
break;
default:
// FIXME: Evaluate other kinds of `CFGElement`.
break;
}
}
/// Transfers `State` by evaluating each element in the `Block` based on the
/// `AC.Analysis` specified.
///
/// Built-in transfer functions (if the option for `ApplyBuiltinTransfer` is set
/// by the analysis) will be applied to the element before evaluation by the
/// user-specified analysis.
/// `PostVisitCFG` (if provided) will be applied to the element after evaluation
/// by the user-specified analysis.
TypeErasedDataflowAnalysisState
transferCFGBlock(const CFGBlock &Block, AnalysisContext &AC,
std::function<void(const CFGElement &,
const TypeErasedDataflowAnalysisState &)>
PostVisitCFG = nullptr) {
auto State = computeBlockInputState(Block, AC);
for (const auto &Element : Block) {
// Built-in analysis
if (AC.Analysis.builtinTransferOptions()) {
builtinTransfer(Element, State, AC);
}
// User-provided analysis
AC.Analysis.transferTypeErased(&Element, State.Lattice, State.Env);
// Post processing
if (PostVisitCFG) {
PostVisitCFG(Element, State);
}
}
return State;
}
TypeErasedDataflowAnalysisState transferBlock(
const ControlFlowContext &CFCtx,
llvm::ArrayRef<llvm::Optional<TypeErasedDataflowAnalysisState>> BlockStates,
const CFGBlock &Block, const Environment &InitEnv,
TypeErasedDataflowAnalysis &Analysis,
std::function<void(const CFGElement &,
const TypeErasedDataflowAnalysisState &)>
PostVisitCFG) {
AnalysisContext AC(CFCtx, Analysis, InitEnv, BlockStates);
return transferCFGBlock(Block, AC, PostVisitCFG);
}
llvm::Expected<std::vector<llvm::Optional<TypeErasedDataflowAnalysisState>>>
runTypeErasedDataflowAnalysis(
const ControlFlowContext &CFCtx, TypeErasedDataflowAnalysis &Analysis,
const Environment &InitEnv,
std::function<void(const CFGElement &,
const TypeErasedDataflowAnalysisState &)>
PostVisitCFG) {
PostOrderCFGView POV(&CFCtx.getCFG());
ForwardDataflowWorklist Worklist(CFCtx.getCFG(), &POV);
std::vector<llvm::Optional<TypeErasedDataflowAnalysisState>> BlockStates(
CFCtx.getCFG().size(), std::nullopt);
// The entry basic block doesn't contain statements so it can be skipped.
const CFGBlock &Entry = CFCtx.getCFG().getEntry();
BlockStates[Entry.getBlockID()] = {Analysis.typeErasedInitialElement(),
InitEnv};
Worklist.enqueueSuccessors(&Entry);
AnalysisContext AC(CFCtx, Analysis, InitEnv, BlockStates);
// Bugs in lattices and transfer functions can prevent the analysis from
// converging. To limit the damage (infinite loops) that these bugs can cause,
// limit the number of iterations.
// FIXME: Consider making the maximum number of iterations configurable.
// FIXME: Consider restricting the number of backedges followed, rather than
// iterations.
// FIXME: Set up statistics (see llvm/ADT/Statistic.h) to count average number
// of iterations, number of functions that time out, etc.
static constexpr uint32_t MaxAverageVisitsPerBlock = 4;
static constexpr uint32_t AbsoluteMaxIterations = 1 << 16;
const uint32_t RelativeMaxIterations =
MaxAverageVisitsPerBlock * BlockStates.size();
const uint32_t MaxIterations =
std::min(RelativeMaxIterations, AbsoluteMaxIterations);
uint32_t Iterations = 0;
while (const CFGBlock *Block = Worklist.dequeue()) {
if (++Iterations > MaxIterations) {
return llvm::createStringError(std::errc::timed_out,
"maximum number of iterations reached");
}
const llvm::Optional<TypeErasedDataflowAnalysisState> &OldBlockState =
BlockStates[Block->getBlockID()];
TypeErasedDataflowAnalysisState NewBlockState =
transferCFGBlock(*Block, AC);
if (OldBlockState) {
if (isLoopHead(*Block)) {
LatticeJoinEffect Effect1 = Analysis.widenTypeErased(
NewBlockState.Lattice, OldBlockState->Lattice);
LatticeJoinEffect Effect2 =
NewBlockState.Env.widen(OldBlockState->Env, Analysis);
if (Effect1 == LatticeJoinEffect::Unchanged &&
Effect2 == LatticeJoinEffect::Unchanged)
// The state of `Block` didn't change from widening so there's no need
// to revisit its successors.
continue;
} else if (Analysis.isEqualTypeErased(OldBlockState->Lattice,
NewBlockState.Lattice) &&
OldBlockState->Env.equivalentTo(NewBlockState.Env, Analysis)) {
// The state of `Block` didn't change after transfer so there's no need
// to revisit its successors.
continue;
}
}
BlockStates[Block->getBlockID()] = std::move(NewBlockState);
// Do not add unreachable successor blocks to `Worklist`.
if (Block->hasNoReturnElement())
continue;
Worklist.enqueueSuccessors(Block);
}
// FIXME: Consider evaluating unreachable basic blocks (those that have a
// state set to `std::nullopt` at this point) to also analyze dead code.
if (PostVisitCFG) {
for (const CFGBlock *Block : CFCtx.getCFG()) {
// Skip blocks that were not evaluated.
if (!BlockStates[Block->getBlockID()])
continue;
transferCFGBlock(*Block, AC, PostVisitCFG);
}
}
return BlockStates;
}
} // namespace dataflow
} // namespace clang