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//===- DataflowAnalysis.h ---------------------------------------*- 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 base types and functions for building dataflow analyses
// that run over Control-Flow Graphs (CFGs).
//
//===----------------------------------------------------------------------===//
#ifndef LLVM_CLANG_ANALYSIS_FLOWSENSITIVE_DATAFLOWANALYSIS_H
#define LLVM_CLANG_ANALYSIS_FLOWSENSITIVE_DATAFLOWANALYSIS_H
#include <iterator>
#include <optional>
#include <type_traits>
#include <utility>
#include <vector>
#include "clang/AST/ASTContext.h"
#include "clang/Analysis/CFG.h"
#include "clang/Analysis/FlowSensitive/AdornedCFG.h"
#include "clang/Analysis/FlowSensitive/DataflowEnvironment.h"
#include "clang/Analysis/FlowSensitive/DataflowLattice.h"
#include "clang/Analysis/FlowSensitive/MatchSwitch.h"
#include "clang/Analysis/FlowSensitive/TypeErasedDataflowAnalysis.h"
#include "clang/Analysis/FlowSensitive/WatchedLiteralsSolver.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/ADT/STLFunctionalExtras.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/Support/Errc.h"
#include "llvm/Support/Error.h"
namespace clang {
namespace dataflow {
/// Base class template for dataflow analyses built on a single lattice type.
///
/// Requirements:
///
/// `Derived` must be derived from a specialization of this class template and
/// must provide the following public members:
/// * `LatticeT initialElement()` - returns a lattice element that models the
/// initial state of a basic block;
/// * `void transfer(const CFGElement &, LatticeT &, Environment &)` - applies
/// the analysis transfer function for a given CFG element and lattice
/// element.
///
/// `Derived` can optionally provide the following members:
/// * `void transferBranch(bool Branch, const Stmt *Stmt, TypeErasedLattice &E,
/// Environment &Env)` - applies the analysis transfer
/// function for a given edge from a CFG block of a conditional statement.
///
/// `Derived` can optionally override the virtual functions in the
/// `Environment::ValueModel` interface (which is an indirect base class of
/// this class).
///
/// `LatticeT` is a bounded join-semilattice that is used by `Derived` and must
/// provide the following public members:
/// * `LatticeJoinEffect join(const LatticeT &)` - joins the object and the
/// argument by computing their least upper bound, modifies the object if
/// necessary, and returns an effect indicating whether any changes were
/// made to it;
/// FIXME: make it `static LatticeT join(const LatticeT&, const LatticeT&)`
/// * `bool operator==(const LatticeT &) const` - returns true if and only if
/// the object is equal to the argument.
///
/// `LatticeT` can optionally provide the following members:
/// * `LatticeJoinEffect widen(const LatticeT &Previous)` - replaces the
/// lattice element with an approximation that can reach a fixed point more
/// quickly than iterated application of the transfer function alone. The
/// previous value is provided to inform the choice of widened value. The
/// function must also serve as a comparison operation, by indicating whether
/// the widened value is equivalent to the previous value with the returned
/// `LatticeJoinEffect`.
template <typename Derived, typename LatticeT>
class DataflowAnalysis : public TypeErasedDataflowAnalysis {
public:
/// Bounded join-semilattice that is used in the analysis.
using Lattice = LatticeT;
explicit DataflowAnalysis(ASTContext &Context) : Context(Context) {}
explicit DataflowAnalysis(ASTContext &Context,
DataflowAnalysisOptions Options)
: TypeErasedDataflowAnalysis(Options), Context(Context) {}
ASTContext &getASTContext() final { return Context; }
TypeErasedLattice typeErasedInitialElement() final {
return {static_cast<Derived *>(this)->initialElement()};
}
TypeErasedLattice joinTypeErased(const TypeErasedLattice &E1,
const TypeErasedLattice &E2) final {
// FIXME: change the signature of join() to avoid copying here.
Lattice L1 = llvm::any_cast<const Lattice &>(E1.Value);
const Lattice &L2 = llvm::any_cast<const Lattice &>(E2.Value);
L1.join(L2);
return {std::move(L1)};
}
LatticeJoinEffect widenTypeErased(TypeErasedLattice &Current,
const TypeErasedLattice &Previous) final {
Lattice &C = llvm::any_cast<Lattice &>(Current.Value);
const Lattice &P = llvm::any_cast<const Lattice &>(Previous.Value);
return widenInternal(Rank0{}, C, P);
}
bool isEqualTypeErased(const TypeErasedLattice &E1,
const TypeErasedLattice &E2) final {
const Lattice &L1 = llvm::any_cast<const Lattice &>(E1.Value);
const Lattice &L2 = llvm::any_cast<const Lattice &>(E2.Value);
return L1 == L2;
}
void transferTypeErased(const CFGElement &Element, TypeErasedLattice &E,
Environment &Env) final {
Lattice &L = llvm::any_cast<Lattice &>(E.Value);
static_cast<Derived *>(this)->transfer(Element, L, Env);
}
void transferBranchTypeErased(bool Branch, const Stmt *Stmt,
TypeErasedLattice &E, Environment &Env) final {
transferBranchInternal(Rank0{}, *static_cast<Derived *>(this), Branch, Stmt,
E, Env);
}
private:
// These `Rank` structs are used for template metaprogramming to choose
// between overloads.
struct Rank1 {};
struct Rank0 : Rank1 {};
// The first-choice implementation: use `widen` when it is available.
template <typename T>
static auto widenInternal(Rank0, T &Current, const T &Prev)
-> decltype(Current.widen(Prev)) {
return Current.widen(Prev);
}
// The second-choice implementation: `widen` is unavailable. Widening is
// merged with equality checking, so when widening is unimplemented, we
// default to equality checking.
static LatticeJoinEffect widenInternal(Rank1, const Lattice &Current,
const Lattice &Prev) {
return Prev == Current ? LatticeJoinEffect::Unchanged
: LatticeJoinEffect::Changed;
}
// The first-choice implementation: `transferBranch` is implemented.
template <typename Analysis>
static auto transferBranchInternal(Rank0, Analysis &A, bool Branch,
const Stmt *Stmt, TypeErasedLattice &L,
Environment &Env)
-> std::void_t<decltype(A.transferBranch(
Branch, Stmt, std::declval<LatticeT &>(), Env))> {
A.transferBranch(Branch, Stmt, llvm::any_cast<Lattice &>(L.Value), Env);
}
// The second-choice implementation: `transferBranch` is unimplemented. No-op.
template <typename Analysis>
static void transferBranchInternal(Rank1, Analysis &A, bool, const Stmt *,
TypeErasedLattice &, Environment &) {}
ASTContext &Context;
};
// Model of the program at a given program point.
template <typename LatticeT> struct DataflowAnalysisState {
// Model of a program property.
LatticeT Lattice;
// Model of the state of the program (store and heap).
Environment Env;
};
/// A callback to be called with the state before or after visiting a CFG
/// element.
template <typename AnalysisT>
using CFGEltCallback = std::function<void(
const CFGElement &,
const DataflowAnalysisState<typename AnalysisT::Lattice> &)>;
/// A pair of callbacks to be called with the state before and after visiting a
/// CFG element.
/// Either or both of the callbacks may be null.
template <typename AnalysisT> struct CFGEltCallbacks {
CFGEltCallback<AnalysisT> Before;
CFGEltCallback<AnalysisT> After;
};
/// A callback for performing diagnosis on a CFG element, called with the state
/// before or after visiting that CFG element. Returns a list of diagnostics
/// to emit (if any).
template <typename AnalysisT, typename Diagnostic>
using DiagnosisCallback = llvm::function_ref<llvm::SmallVector<Diagnostic>(
const CFGElement &, ASTContext &,
const TransferStateForDiagnostics<typename AnalysisT::Lattice> &)>;
/// A pair of callbacks for performing diagnosis on a CFG element, called with
/// the state before and after visiting that CFG element.
/// Either or both of the callbacks may be null.
template <typename AnalysisT, typename Diagnostic> struct DiagnosisCallbacks {
DiagnosisCallback<AnalysisT, Diagnostic> Before;
DiagnosisCallback<AnalysisT, Diagnostic> After;
};
/// Default for the maximum number of SAT solver iterations during analysis.
inline constexpr std::int64_t kDefaultMaxSATIterations = 1'000'000'000;
/// Default for the maximum number of block visits during analysis.
inline constexpr std::int32_t kDefaultMaxBlockVisits = 20'000;
/// Performs dataflow analysis and returns a mapping from basic block IDs to
/// dataflow analysis states that model the respective basic blocks. The
/// returned vector, if any, will have the same size as the number of CFG
/// blocks, with indices corresponding to basic block IDs. Returns an error if
/// the dataflow analysis cannot be performed successfully. Otherwise, calls
/// `PostAnalysisCallbacks` on each CFG element with the final analysis results
/// before and after that program point.
///
/// `MaxBlockVisits` caps the number of block visits during analysis. See
/// `runTypeErasedDataflowAnalysis` for a full description. The default value is
/// essentially arbitrary -- large enough to accommodate what seems like any
/// reasonable CFG, but still small enough to limit the cost of hitting the
/// limit.
template <typename AnalysisT>
llvm::Expected<std::vector<
std::optional<DataflowAnalysisState<typename AnalysisT::Lattice>>>>
runDataflowAnalysis(const AdornedCFG &ACFG, AnalysisT &Analysis,
const Environment &InitEnv,
CFGEltCallbacks<AnalysisT> PostAnalysisCallbacks,
std::int32_t MaxBlockVisits = kDefaultMaxBlockVisits) {
CFGEltCallbacksTypeErased TypeErasedCallbacks;
if (PostAnalysisCallbacks.Before) {
TypeErasedCallbacks.Before =
[&PostAnalysisCallbacks](const CFGElement &Element,
const TypeErasedDataflowAnalysisState &State) {
auto *Lattice =
llvm::any_cast<typename AnalysisT::Lattice>(&State.Lattice.Value);
// FIXME: we should not be copying the environment here!
// Ultimately the `CFGEltCallback` only gets a const reference anyway.
PostAnalysisCallbacks.Before(
Element, DataflowAnalysisState<typename AnalysisT::Lattice>{
*Lattice, State.Env.fork()});
};
}
if (PostAnalysisCallbacks.After) {
TypeErasedCallbacks.After =
[&PostAnalysisCallbacks](const CFGElement &Element,
const TypeErasedDataflowAnalysisState &State) {
auto *Lattice =
llvm::any_cast<typename AnalysisT::Lattice>(&State.Lattice.Value);
// FIXME: we should not be copying the environment here!
// Ultimately the `CFGEltCallback` only gets a const reference anyway.
PostAnalysisCallbacks.After(
Element, DataflowAnalysisState<typename AnalysisT::Lattice>{
*Lattice, State.Env.fork()});
};
}
auto TypeErasedBlockStates = runTypeErasedDataflowAnalysis(
ACFG, Analysis, InitEnv, TypeErasedCallbacks, MaxBlockVisits);
if (!TypeErasedBlockStates)
return TypeErasedBlockStates.takeError();
std::vector<std::optional<DataflowAnalysisState<typename AnalysisT::Lattice>>>
BlockStates;
BlockStates.reserve(TypeErasedBlockStates->size());
llvm::transform(
std::move(*TypeErasedBlockStates), std::back_inserter(BlockStates),
[](auto &OptState) {
return llvm::transformOptional(
std::move(OptState), [](TypeErasedDataflowAnalysisState &&State) {
return DataflowAnalysisState<typename AnalysisT::Lattice>{
llvm::any_cast<typename AnalysisT::Lattice>(
std::move(State.Lattice.Value)),
std::move(State.Env)};
});
});
return std::move(BlockStates);
}
/// Overload that takes only one post-analysis callback, which is run on the
/// state after visiting the `CFGElement`. This is provided for backwards
/// compatibility; new callers should call the overload taking `CFGEltCallbacks`
/// instead.
template <typename AnalysisT>
llvm::Expected<std::vector<
std::optional<DataflowAnalysisState<typename AnalysisT::Lattice>>>>
runDataflowAnalysis(
const AdornedCFG &ACFG, AnalysisT &Analysis, const Environment &InitEnv,
CFGEltCallback<AnalysisT> PostAnalysisCallbackAfterElt = nullptr,
std::int32_t MaxBlockVisits = kDefaultMaxBlockVisits) {
return runDataflowAnalysis(ACFG, Analysis, InitEnv,
{nullptr, PostAnalysisCallbackAfterElt},
MaxBlockVisits);
}
// Create an analysis class that is derived from `DataflowAnalysis`. This is an
// SFINAE adapter that allows us to call two different variants of constructor
// (either with or without the optional `Environment` parameter).
// FIXME: Make all classes derived from `DataflowAnalysis` take an `Environment`
// parameter in their constructor so that we can get rid of this abomination.
template <typename AnalysisT>
auto createAnalysis(ASTContext &ASTCtx, Environment &Env)
-> decltype(AnalysisT(ASTCtx, Env)) {
return AnalysisT(ASTCtx, Env);
}
template <typename AnalysisT>
auto createAnalysis(ASTContext &ASTCtx, Environment &Env)
-> decltype(AnalysisT(ASTCtx)) {
return AnalysisT(ASTCtx);
}
/// Runs a dataflow analysis over the given function and then runs `Diagnoser`
/// over the results. Returns a list of diagnostics for `FuncDecl` or an
/// error. Currently, errors can occur (at least) because the analysis requires
/// too many iterations over the CFG or the SAT solver times out.
///
/// The default value of `MaxSATIterations` was chosen based on the following
/// observations:
/// - Non-pathological calls to the solver typically require only a few hundred
/// iterations.
/// - This limit is still low enough to keep runtimes acceptable (on typical
/// machines) in cases where we hit the limit.
///
/// `MaxBlockVisits` caps the number of block visits during analysis. See
/// `runDataflowAnalysis` for a full description and explanation of the default
/// value.
template <typename AnalysisT, typename Diagnostic>
llvm::Expected<llvm::SmallVector<Diagnostic>>
diagnoseFunction(const FunctionDecl &FuncDecl, ASTContext &ASTCtx,
DiagnosisCallbacks<AnalysisT, Diagnostic> Diagnoser,
std::int64_t MaxSATIterations = kDefaultMaxSATIterations,
std::int32_t MaxBlockVisits = kDefaultMaxBlockVisits) {
llvm::Expected<AdornedCFG> Context = AdornedCFG::build(FuncDecl);
if (!Context)
return Context.takeError();
auto Solver = std::make_unique<WatchedLiteralsSolver>(MaxSATIterations);
DataflowAnalysisContext AnalysisContext(*Solver);
Environment Env(AnalysisContext, FuncDecl);
AnalysisT Analysis = createAnalysis<AnalysisT>(ASTCtx, Env);
llvm::SmallVector<Diagnostic> Diagnostics;
CFGEltCallbacksTypeErased PostAnalysisCallbacks;
if (Diagnoser.Before) {
PostAnalysisCallbacks.Before =
[&ASTCtx, &Diagnoser,
&Diagnostics](const CFGElement &Elt,
const TypeErasedDataflowAnalysisState &State) mutable {
auto EltDiagnostics = Diagnoser.Before(
Elt, ASTCtx,
TransferStateForDiagnostics<typename AnalysisT::Lattice>(
llvm::any_cast<const typename AnalysisT::Lattice &>(
State.Lattice.Value),
State.Env));
llvm::move(EltDiagnostics, std::back_inserter(Diagnostics));
};
}
if (Diagnoser.After) {
PostAnalysisCallbacks.After =
[&ASTCtx, &Diagnoser,
&Diagnostics](const CFGElement &Elt,
const TypeErasedDataflowAnalysisState &State) mutable {
auto EltDiagnostics = Diagnoser.After(
Elt, ASTCtx,
TransferStateForDiagnostics<typename AnalysisT::Lattice>(
llvm::any_cast<const typename AnalysisT::Lattice &>(
State.Lattice.Value),
State.Env));
llvm::move(EltDiagnostics, std::back_inserter(Diagnostics));
};
}
if (llvm::Error Err =
runTypeErasedDataflowAnalysis(*Context, Analysis, Env,
PostAnalysisCallbacks, MaxBlockVisits)
.takeError())
return std::move(Err);
if (Solver->reachedLimit())
return llvm::createStringError(llvm::errc::interrupted,
"SAT solver timed out");
return Diagnostics;
}
/// Overload that takes only one diagnosis callback, which is run on the state
/// after visiting the `CFGElement`. This is provided for backwards
/// compatibility; new callers should call the overload taking
/// `DiagnosisCallbacks` instead.
template <typename AnalysisT, typename Diagnostic>
llvm::Expected<llvm::SmallVector<Diagnostic>>
diagnoseFunction(const FunctionDecl &FuncDecl, ASTContext &ASTCtx,
DiagnosisCallback<AnalysisT, Diagnostic> Diagnoser,
std::int64_t MaxSATIterations = kDefaultMaxSATIterations,
std::int32_t MaxBlockVisits = kDefaultMaxBlockVisits) {
DiagnosisCallbacks<AnalysisT, Diagnostic> Callbacks = {nullptr, Diagnoser};
return diagnoseFunction(FuncDecl, ASTCtx, Callbacks, MaxSATIterations,
MaxBlockVisits);
}
/// Abstract base class for dataflow "models": reusable analysis components that
/// model a particular aspect of program semantics in the `Environment`. For
/// example, a model may capture a type and its related functions.
class DataflowModel : public Environment::ValueModel {
public:
/// Return value indicates whether the model processed the `Element`.
virtual bool transfer(const CFGElement &Element, Environment &Env) = 0;
};
} // namespace dataflow
} // namespace clang
#endif // LLVM_CLANG_ANALYSIS_FLOWSENSITIVE_DATAFLOWANALYSIS_H
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