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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 <optional>
#include <system_error>
#include <utility>
#include <vector>
#include "clang/AST/ASTDumper.h"
#include "clang/AST/DeclCXX.h"
#include "clang/AST/OperationKinds.h"
#include "clang/AST/StmtCXX.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/RecordOps.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/STLExtras.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/Error.h"
#define DEBUG_TYPE "clang-dataflow"
namespace clang {
namespace dataflow {
/// 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();
}
// A "backedge" node is a block introduced in the CFG exclusively to indicate a
// loop backedge. They are exactly identified by the presence of a non-null
// pointer to the entry block of the loop condition. Note that this is not
// necessarily the block with the loop statement as terminator, because
// short-circuit operators will result in multiple blocks encoding the loop
// condition, only one of which will contain the loop statement as terminator.
static bool isBackedgeNode(const CFGBlock &B) {
return B.getLoopTarget() != nullptr;
}
namespace {
/// Extracts the terminator's condition expression.
class TerminatorVisitor
: public ConstStmtVisitor<TerminatorVisitor, const Expr *> {
public:
TerminatorVisitor() = default;
const Expr *VisitIfStmt(const IfStmt *S) { return S->getCond(); }
const Expr *VisitWhileStmt(const WhileStmt *S) { return S->getCond(); }
const Expr *VisitDoStmt(const DoStmt *S) { return S->getCond(); }
const Expr *VisitForStmt(const ForStmt *S) { return S->getCond(); }
const Expr *VisitCXXForRangeStmt(const CXXForRangeStmt *) {
// Don't do anything special for CXXForRangeStmt, because the condition
// (being implicitly generated) isn't visible from the loop body.
return nullptr;
}
const Expr *VisitBinaryOperator(const BinaryOperator *S) {
assert(S->getOpcode() == BO_LAnd || S->getOpcode() == BO_LOr);
return S->getLHS();
}
const Expr *VisitConditionalOperator(const ConditionalOperator *S) {
return S->getCond();
}
};
/// Holds data structures required for running dataflow analysis.
struct AnalysisContext {
AnalysisContext(const AdornedCFG &ACFG, TypeErasedDataflowAnalysis &Analysis,
const Environment &InitEnv,
llvm::ArrayRef<std::optional<TypeErasedDataflowAnalysisState>>
BlockStates)
: ACFG(ACFG), Analysis(Analysis), InitEnv(InitEnv),
Log(*InitEnv.getDataflowAnalysisContext().getOptions().Log),
BlockStates(BlockStates) {
Log.beginAnalysis(ACFG, Analysis);
}
~AnalysisContext() { Log.endAnalysis(); }
/// Contains the CFG being analyzed.
const AdornedCFG &ACFG;
/// The analysis to be run.
TypeErasedDataflowAnalysis &Analysis;
/// Initial state to start the analysis.
const Environment &InitEnv;
Logger &Log;
/// Stores the state of a CFG block if it has been evaluated by the analysis.
/// The indices correspond to the block IDs.
llvm::ArrayRef<std::optional<TypeErasedDataflowAnalysisState>> BlockStates;
};
class PrettyStackTraceAnalysis : public llvm::PrettyStackTraceEntry {
public:
PrettyStackTraceAnalysis(const AdornedCFG &ACFG, const char *Message)
: ACFG(ACFG), Message(Message) {}
void print(raw_ostream &OS) const override {
OS << Message << "\n";
OS << "Decl:\n";
ACFG.getDecl().dump(OS);
OS << "CFG:\n";
ACFG.getCFG().print(OS, LangOptions(), false);
}
private:
const AdornedCFG &ACFG;
const char *Message;
};
class PrettyStackTraceCFGElement : public llvm::PrettyStackTraceEntry {
public:
PrettyStackTraceCFGElement(const CFGElement &Element, int BlockIdx,
int ElementIdx, const char *Message)
: Element(Element), BlockIdx(BlockIdx), ElementIdx(ElementIdx),
Message(Message) {}
void print(raw_ostream &OS) const override {
OS << Message << ": Element [B" << BlockIdx << "." << ElementIdx << "]\n";
if (auto Stmt = Element.getAs<CFGStmt>()) {
OS << "Stmt:\n";
ASTDumper Dumper(OS, false);
Dumper.Visit(Stmt->getStmt());
}
}
private:
const CFGElement ∈
int BlockIdx;
int ElementIdx;
const char *Message;
};
// Builds a joined TypeErasedDataflowAnalysisState from 0 or more sources,
// each of which may be owned (built as part of the join) or external (a
// reference to an Environment that will outlive the builder).
// Avoids unneccesary copies of the environment.
class JoinedStateBuilder {
AnalysisContext &AC;
Environment::ExprJoinBehavior JoinBehavior;
std::vector<const TypeErasedDataflowAnalysisState *> All;
std::deque<TypeErasedDataflowAnalysisState> Owned;
TypeErasedDataflowAnalysisState
join(const TypeErasedDataflowAnalysisState &L,
const TypeErasedDataflowAnalysisState &R) {
return {AC.Analysis.joinTypeErased(L.Lattice, R.Lattice),
Environment::join(L.Env, R.Env, AC.Analysis, JoinBehavior)};
}
public:
JoinedStateBuilder(AnalysisContext &AC,
Environment::ExprJoinBehavior JoinBehavior)
: AC(AC), JoinBehavior(JoinBehavior) {}
void addOwned(TypeErasedDataflowAnalysisState State) {
Owned.push_back(std::move(State));
All.push_back(&Owned.back());
}
void addUnowned(const TypeErasedDataflowAnalysisState &State) {
All.push_back(&State);
}
TypeErasedDataflowAnalysisState take() && {
if (All.empty())
// FIXME: Consider passing `Block` to Analysis.typeErasedInitialElement
// to enable building analyses like computation of dominators that
// initialize the state of each basic block differently.
return {AC.Analysis.typeErasedInitialElement(), AC.InitEnv.fork()};
if (All.size() == 1)
// Join the environment with itself so that we discard expression state if
// desired.
// FIXME: We could consider writing special-case code for this that only
// does the discarding, but it's not clear if this is worth it.
return {All[0]->Lattice, Environment::join(All[0]->Env, All[0]->Env,
AC.Analysis, JoinBehavior)};
auto Result = join(*All[0], *All[1]);
for (unsigned I = 2; I < All.size(); ++I)
Result = join(Result, *All[I]);
return Result;
}
};
} // namespace
static const Expr *getTerminatorCondition(const Stmt *TerminatorStmt) {
return TerminatorStmt == nullptr ? nullptr
: TerminatorVisitor().Visit(TerminatorStmt);
}
/// 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) {
std::vector<const CFGBlock *> Preds(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() != nullptr &&
Block.succ_begin()->getReachableBlock()->hasNoReturnElement()) {
auto &StmtToBlock = AC.ACFG.getStmtToBlock();
auto StmtBlock = StmtToBlock.find(Block.getTerminatorStmt());
assert(StmtBlock != StmtToBlock.end());
llvm::erase(Preds, StmtBlock->getSecond());
}
}
// If any of the predecessor blocks contains an expression consumed in a
// different block, we need to keep expression state.
// Note that in this case, we keep expression state for all predecessors,
// rather than only those predecessors that actually contain an expression
// consumed in a different block. While this is potentially suboptimal, it's
// actually likely, if we have control flow within a full expression, that
// all predecessors have expression state consumed in a different block.
Environment::ExprJoinBehavior JoinBehavior = Environment::DiscardExprState;
for (const CFGBlock *Pred : Preds) {
if (Pred && AC.ACFG.containsExprConsumedInDifferentBlock(*Pred)) {
JoinBehavior = Environment::KeepExprState;
break;
}
}
JoinedStateBuilder Builder(AC, JoinBehavior);
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 std::optional<TypeErasedDataflowAnalysisState> &MaybePredState =
AC.BlockStates[Pred->getBlockID()];
if (!MaybePredState)
continue;
const TypeErasedDataflowAnalysisState &PredState = *MaybePredState;
const Expr *Cond = getTerminatorCondition(Pred->getTerminatorStmt());
if (Cond == nullptr) {
Builder.addUnowned(PredState);
continue;
}
bool BranchVal = blockIndexInPredecessor(*Pred, Block) == 0;
// `transferBranch` may need to mutate the environment to describe the
// dynamic effect of the terminator for a given branch. Copy now.
TypeErasedDataflowAnalysisState Copy = MaybePredState->fork();
if (AC.Analysis.builtinOptions()) {
auto *CondVal = Copy.Env.get<BoolValue>(*Cond);
// In transferCFGBlock(), we ensure that we always have a `Value`
// for the terminator condition, so assert this. We consciously
// assert ourselves instead of asserting via `cast()` so that we get
// a more meaningful line number if the assertion fails.
assert(CondVal != nullptr);
BoolValue *AssertedVal =
BranchVal ? CondVal : &Copy.Env.makeNot(*CondVal);
Copy.Env.assume(AssertedVal->formula());
}
AC.Analysis.transferBranchTypeErased(BranchVal, Cond, Copy.Lattice,
Copy.Env);
Builder.addOwned(std::move(Copy));
}
return std::move(Builder).take();
}
/// Built-in transfer function for `CFGStmt`.
static void
builtinTransferStatement(unsigned CurBlockID, const CFGStmt &Elt,
TypeErasedDataflowAnalysisState &InputState,
AnalysisContext &AC) {
const Stmt *S = Elt.getStmt();
assert(S != nullptr);
transfer(StmtToEnvMap(AC.ACFG, AC.BlockStates, CurBlockID, InputState), *S,
InputState.Env);
}
/// Built-in transfer function for `CFGInitializer`.
static void
builtinTransferInitializer(const CFGInitializer &Elt,
TypeErasedDataflowAnalysisState &InputState) {
const CXXCtorInitializer *Init = Elt.getInitializer();
assert(Init != nullptr);
auto &Env = InputState.Env;
auto &ThisLoc = *Env.getThisPointeeStorageLocation();
if (!Init->isAnyMemberInitializer())
// FIXME: Handle base initialization
return;
auto *InitExpr = Init->getInit();
assert(InitExpr != nullptr);
const FieldDecl *Member = nullptr;
RecordStorageLocation *ParentLoc = &ThisLoc;
StorageLocation *MemberLoc = nullptr;
if (Init->isMemberInitializer()) {
Member = Init->getMember();
MemberLoc = ThisLoc.getChild(*Member);
} else {
IndirectFieldDecl *IndirectField = Init->getIndirectMember();
assert(IndirectField != nullptr);
MemberLoc = &ThisLoc;
for (const auto *I : IndirectField->chain()) {
Member = cast<FieldDecl>(I);
ParentLoc = cast<RecordStorageLocation>(MemberLoc);
MemberLoc = ParentLoc->getChild(*Member);
}
}
assert(Member != nullptr);
// FIXME: Instead of these case distinctions, we would ideally want to be able
// to simply use `Environment::createObject()` here, the same way that we do
// this in `TransferVisitor::VisitInitListExpr()`. However, this would require
// us to be able to build a list of fields that we then use to initialize an
// `RecordStorageLocation` -- and the problem is that, when we get here,
// the `RecordStorageLocation` already exists. We should explore if there's
// anything that we can do to change this.
if (Member->getType()->isReferenceType()) {
auto *InitExprLoc = Env.getStorageLocation(*InitExpr);
if (InitExprLoc == nullptr)
return;
ParentLoc->setChild(*Member, InitExprLoc);
} else if (auto *InitExprVal = Env.getValue(*InitExpr)) {
assert(MemberLoc != nullptr);
if (Member->getType()->isRecordType()) {
auto *InitValStruct = cast<RecordValue>(InitExprVal);
// FIXME: Rather than performing a copy here, we should really be
// initializing the field in place. This would require us to propagate the
// storage location of the field to the AST node that creates the
// `RecordValue`.
copyRecord(InitValStruct->getLoc(),
*cast<RecordStorageLocation>(MemberLoc), Env);
} else {
Env.setValue(*MemberLoc, *InitExprVal);
}
}
}
static void builtinTransfer(unsigned CurBlockID, const CFGElement &Elt,
TypeErasedDataflowAnalysisState &State,
AnalysisContext &AC) {
switch (Elt.getKind()) {
case CFGElement::Statement:
builtinTransferStatement(CurBlockID, Elt.castAs<CFGStmt>(), State, AC);
break;
case CFGElement::Initializer:
builtinTransferInitializer(Elt.castAs<CFGInitializer>(), State);
break;
case CFGElement::LifetimeEnds:
// Removing declarations when their lifetime ends serves two purposes:
// - Eliminate unnecessary clutter from `Environment::DeclToLoc`
// - Allow us to assert that, when joining two `Environment`s, the two
// `DeclToLoc` maps never contain entries that map the same declaration to
// different storage locations.
if (const ValueDecl *VD = Elt.castAs<CFGLifetimeEnds>().getVarDecl())
State.Env.removeDecl(*VD);
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.
static TypeErasedDataflowAnalysisState
transferCFGBlock(const CFGBlock &Block, AnalysisContext &AC,
std::function<void(const CFGElement &,
const TypeErasedDataflowAnalysisState &)>
PostVisitCFG = nullptr) {
AC.Log.enterBlock(Block, PostVisitCFG != nullptr);
auto State = computeBlockInputState(Block, AC);
AC.Log.recordState(State);
int ElementIdx = 1;
for (const auto &Element : Block) {
PrettyStackTraceCFGElement CrashInfo(Element, Block.getBlockID(),
ElementIdx++, "transferCFGBlock");
AC.Log.enterElement(Element);
// Built-in analysis
if (AC.Analysis.builtinOptions()) {
builtinTransfer(Block.getBlockID(), Element, State, AC);
}
// User-provided analysis
AC.Analysis.transferTypeErased(Element, State.Lattice, State.Env);
// Post processing
if (PostVisitCFG) {
PostVisitCFG(Element, State);
}
AC.Log.recordState(State);
}
// If we have a terminator, evaluate its condition.
// This `Expr` may not appear as a `CFGElement` anywhere else, and it's
// important that we evaluate it here (rather than while processing the
// terminator) so that we put the corresponding value in the right
// environment.
if (const Expr *TerminatorCond =
dyn_cast_or_null<Expr>(Block.getTerminatorCondition())) {
if (State.Env.getValue(*TerminatorCond) == nullptr)
// FIXME: This only runs the builtin transfer, not the analysis-specific
// transfer. Fixing this isn't trivial, as the analysis-specific transfer
// takes a `CFGElement` as input, but some expressions only show up as a
// terminator condition, but not as a `CFGElement`. The condition of an if
// statement is one such example.
transfer(StmtToEnvMap(AC.ACFG, AC.BlockStates, Block.getBlockID(), State),
*TerminatorCond, State.Env);
// If the transfer function didn't produce a value, create an atom so that
// we have *some* value for the condition expression. This ensures that
// when we extend the flow condition, it actually changes.
if (State.Env.getValue(*TerminatorCond) == nullptr)
State.Env.setValue(*TerminatorCond, State.Env.makeAtomicBoolValue());
AC.Log.recordState(State);
}
return State;
}
llvm::Expected<std::vector<std::optional<TypeErasedDataflowAnalysisState>>>
runTypeErasedDataflowAnalysis(
const AdornedCFG &ACFG, TypeErasedDataflowAnalysis &Analysis,
const Environment &InitEnv,
std::function<void(const CFGElement &,
const TypeErasedDataflowAnalysisState &)>
PostVisitCFG,
std::int32_t MaxBlockVisits) {
PrettyStackTraceAnalysis CrashInfo(ACFG, "runTypeErasedDataflowAnalysis");
std::optional<Environment> MaybeStartingEnv;
if (InitEnv.callStackSize() == 1) {
MaybeStartingEnv = InitEnv.fork();
MaybeStartingEnv->initialize();
}
const Environment &StartingEnv =
MaybeStartingEnv ? *MaybeStartingEnv : InitEnv;
const clang::CFG &CFG = ACFG.getCFG();
PostOrderCFGView POV(&CFG);
ForwardDataflowWorklist Worklist(CFG, &POV);
std::vector<std::optional<TypeErasedDataflowAnalysisState>> BlockStates(
CFG.size());
// The entry basic block doesn't contain statements so it can be skipped.
const CFGBlock &Entry = CFG.getEntry();
BlockStates[Entry.getBlockID()] = {Analysis.typeErasedInitialElement(),
StartingEnv.fork()};
Worklist.enqueueSuccessors(&Entry);
AnalysisContext AC(ACFG, Analysis, StartingEnv, BlockStates);
std::int32_t BlockVisits = 0;
while (const CFGBlock *Block = Worklist.dequeue()) {
LLVM_DEBUG(llvm::dbgs()
<< "Processing Block " << Block->getBlockID() << "\n");
if (++BlockVisits > MaxBlockVisits) {
return llvm::createStringError(std::errc::timed_out,
"maximum number of blocks processed");
}
const std::optional<TypeErasedDataflowAnalysisState> &OldBlockState =
BlockStates[Block->getBlockID()];
TypeErasedDataflowAnalysisState NewBlockState =
transferCFGBlock(*Block, AC);
LLVM_DEBUG({
llvm::errs() << "New Env:\n";
NewBlockState.Env.dump();
});
if (OldBlockState) {
LLVM_DEBUG({
llvm::errs() << "Old Env:\n";
OldBlockState->Env.dump();
});
if (isBackedgeNode(*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.
AC.Log.blockConverged();
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.
AC.Log.blockConverged();
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 : ACFG.getCFG()) {
// Skip blocks that were not evaluated.
if (!BlockStates[Block->getBlockID()])
continue;
transferCFGBlock(*Block, AC, PostVisitCFG);
}
}
return std::move(BlockStates);
}
} // namespace dataflow
} // namespace clang
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