lib/Analysis/FlowSensitive/TypeErasedDataflowAnalysis.cpp - llvm-project/clang - Git at Google

 //===- 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 <memory>
 #include <utility>
 #include <vector>

 #include "clang/AST/DeclCXX.h"
 #include "clang/Analysis/Analyses/PostOrderCFGView.h"
 #include "clang/Analysis/CFG.h"
 #include "clang/Analysis/FlowSensitive/DataflowEnvironment.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/DenseSet.h"
 #include "llvm/ADT/None.h"
 #include "llvm/ADT/Optional.h"
 #include "llvm/Support/raw_ostream.h"

 namespace clang {
 namespace dataflow {

 /// 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 `BlockStates` that are set to
 ///   `llvm::None` represent basic blocks that are not evaluated yet.
 static TypeErasedDataflowAnalysisState computeBlockInputState(
     const ControlFlowContext &CFCtx,
     std::vector<llvm::Optional<TypeErasedDataflowAnalysisState>> &BlockStates,
     const CFGBlock &Block, const Environment &InitEnv,
     TypeErasedDataflowAnalysis &Analysis) {
   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 StmtBlock = CFCtx.getStmtToBlock().find(Block.getTerminatorStmt());
       assert(StmtBlock != CFCtx.getStmtToBlock().end());
       Preds.erase(StmtBlock->getSecond());
     }
   }

   llvm::Optional<TypeErasedDataflowAnalysisState> MaybeState;
   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 =
         BlockStates[Pred->getBlockID()];
     if (!MaybePredState.hasValue())
       continue;

     const TypeErasedDataflowAnalysisState &PredState =
         MaybePredState.getValue();
     if (MaybeState.hasValue()) {
       Analysis.joinTypeErased(MaybeState->Lattice, PredState.Lattice);
       MaybeState->Env.join(PredState.Env, Analysis);
     } else {
       MaybeState = PredState;
     }
   }
   if (!MaybeState.hasValue()) {
     // 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(), InitEnv);
   }
   return *MaybeState;
 }

 /// Transfers `State` by evaluating `CfgStmt` in the context of `Analysis`.
 /// `HandleTransferredStmt` (if provided) will be applied to `CfgStmt`, after it
 /// is evaluated.
 static void
 transferCFGStmt(const CFGStmt &CfgStmt, TypeErasedDataflowAnalysis &Analysis,
                 TypeErasedDataflowAnalysisState &State,
                 std::function<void(const CFGStmt &,
                                    const TypeErasedDataflowAnalysisState &)>
                     HandleTransferredStmt) {
   const Stmt *S = CfgStmt.getStmt();
   assert(S != nullptr);

   if (Analysis.applyBuiltinTransfer())
     transfer(*S, State.Env);
   Analysis.transferTypeErased(S, State.Lattice, State.Env);

   if (HandleTransferredStmt != nullptr)
     HandleTransferredStmt(CfgStmt, State);
 }

 /// Transfers `State` by evaluating `CfgInit`.
 static void transferCFGInitializer(const CFGInitializer &CfgInit,
                                    TypeErasedDataflowAnalysisState &State) {
   const auto &ThisLoc = *cast<AggregateStorageLocation>(
       State.Env.getThisPointeeStorageLocation());

   const CXXCtorInitializer *Initializer = CfgInit.getInitializer();
   assert(Initializer != nullptr);

   auto *InitStmt = Initializer->getInit();
   assert(InitStmt != nullptr);

   auto *InitStmtLoc =
       State.Env.getStorageLocation(*InitStmt, SkipPast::Reference);
   if (InitStmtLoc == nullptr)
     return;

   auto *InitStmtVal = State.Env.getValue(*InitStmtLoc);
   if (InitStmtVal == nullptr)
     return;

   const FieldDecl *Member = Initializer->getMember();
   assert(Member != nullptr);

   if (Member->getType()->isReferenceType()) {
     auto &MemberLoc = ThisLoc.getChild(*Member);
     State.Env.setValue(MemberLoc,
                        State.Env.takeOwnership(
                            std::make_unique<ReferenceValue>(*InitStmtLoc)));
   } else {
     auto &MemberLoc = ThisLoc.getChild(*Member);
     State.Env.setValue(MemberLoc, *InitStmtVal);
   }
 }

 TypeErasedDataflowAnalysisState transferBlock(
     const ControlFlowContext &CFCtx,
     std::vector<llvm::Optional<TypeErasedDataflowAnalysisState>> &BlockStates,
     const CFGBlock &Block, const Environment &InitEnv,
     TypeErasedDataflowAnalysis &Analysis,
     std::function<void(const CFGStmt &,
                        const TypeErasedDataflowAnalysisState &)>
         HandleTransferredStmt) {
   TypeErasedDataflowAnalysisState State =
       computeBlockInputState(CFCtx, BlockStates, Block, InitEnv, Analysis);
   for (const CFGElement &Element : Block) {
     switch (Element.getKind()) {
     case CFGElement::Statement:
       transferCFGStmt(*Element.getAs<CFGStmt>(), Analysis, State,
                       HandleTransferredStmt);
       break;
     case CFGElement::Initializer:
       if (Analysis.applyBuiltinTransfer())
         transferCFGInitializer(*Element.getAs<CFGInitializer>(), State);
       break;
     default:
       // FIXME: Evaluate other kinds of `CFGElement`.
       break;
     }
   }
   return State;
 }

 std::vector<llvm::Optional<TypeErasedDataflowAnalysisState>>
 runTypeErasedDataflowAnalysis(const ControlFlowContext &CFCtx,
                               TypeErasedDataflowAnalysis &Analysis,
                               const Environment &InitEnv) {
   PostOrderCFGView POV(&CFCtx.getCFG());
   ForwardDataflowWorklist Worklist(CFCtx.getCFG(), &POV);

   std::vector<llvm::Optional<TypeErasedDataflowAnalysisState>> BlockStates;
   BlockStates.resize(CFCtx.getCFG().size(), llvm::None);

   // 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);

   // 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: Set up statistics (see llvm/ADT/Statistic.h) to count average number
   // of iterations, number of functions that time out, etc.
   uint32_t Iterations = 0;
   static constexpr uint32_t MaxIterations = 1 << 16;
   while (const CFGBlock *Block = Worklist.dequeue()) {
     if (++Iterations > MaxIterations) {
       llvm::errs() << "Maximum number of iterations reached, giving up.\n";
       break;
     }

     const llvm::Optional<TypeErasedDataflowAnalysisState> &OldBlockState =
         BlockStates[Block->getBlockID()];
     TypeErasedDataflowAnalysisState NewBlockState =
         transferBlock(CFCtx, BlockStates, *Block, InitEnv, Analysis);

     if (OldBlockState.hasValue() &&
         Analysis.isEqualTypeErased(OldBlockState.getValue().Lattice,
                                    NewBlockState.Lattice) &&
         OldBlockState->Env == NewBlockState.Env) {
       // 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 `llvm::None` at this point) to also analyze dead code.

   return BlockStates;
 }

 } // namespace dataflow
 } // namespace clang
	//===- 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 <memory>
	#include <utility>
	#include <vector>

	#include "clang/AST/DeclCXX.h"
	#include "clang/Analysis/Analyses/PostOrderCFGView.h"
	#include "clang/Analysis/CFG.h"
	#include "clang/Analysis/FlowSensitive/DataflowEnvironment.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/DenseSet.h"
	#include "llvm/ADT/None.h"
	#include "llvm/ADT/Optional.h"
	#include "llvm/Support/raw_ostream.h"

	namespace clang {
	namespace dataflow {

	/// 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 `BlockStates` that are set to
	/// `llvm::None` represent basic blocks that are not evaluated yet.
	static TypeErasedDataflowAnalysisState computeBlockInputState(
	const ControlFlowContext &CFCtx,
	std::vector<llvm::Optional<TypeErasedDataflowAnalysisState>> &BlockStates,
	const CFGBlock &Block, const Environment &InitEnv,
	TypeErasedDataflowAnalysis &Analysis) {
	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 StmtBlock = CFCtx.getStmtToBlock().find(Block.getTerminatorStmt());
	assert(StmtBlock != CFCtx.getStmtToBlock().end());
	Preds.erase(StmtBlock->getSecond());
	}
	}

	llvm::Optional<TypeErasedDataflowAnalysisState> MaybeState;
	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 =
	BlockStates[Pred->getBlockID()];
	if (!MaybePredState.hasValue())
	continue;

	const TypeErasedDataflowAnalysisState &PredState =
	MaybePredState.getValue();
	if (MaybeState.hasValue()) {
	Analysis.joinTypeErased(MaybeState->Lattice, PredState.Lattice);
	MaybeState->Env.join(PredState.Env, Analysis);
	} else {
	MaybeState = PredState;
	}
	}
	if (!MaybeState.hasValue()) {
	// 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(), InitEnv);
	}
	return *MaybeState;
	}

	/// Transfers `State` by evaluating `CfgStmt` in the context of `Analysis`.
	/// `HandleTransferredStmt` (if provided) will be applied to `CfgStmt`, after it
	/// is evaluated.
	static void
	transferCFGStmt(const CFGStmt &CfgStmt, TypeErasedDataflowAnalysis &Analysis,
	TypeErasedDataflowAnalysisState &State,
	std::function<void(const CFGStmt &,
	const TypeErasedDataflowAnalysisState &)>
	HandleTransferredStmt) {
	const Stmt *S = CfgStmt.getStmt();
	assert(S != nullptr);

	if (Analysis.applyBuiltinTransfer())
	transfer(*S, State.Env);
	Analysis.transferTypeErased(S, State.Lattice, State.Env);

	if (HandleTransferredStmt != nullptr)
	HandleTransferredStmt(CfgStmt, State);
	}

	/// Transfers `State` by evaluating `CfgInit`.
	static void transferCFGInitializer(const CFGInitializer &CfgInit,
	TypeErasedDataflowAnalysisState &State) {
	const auto &ThisLoc = *cast<AggregateStorageLocation>(
	State.Env.getThisPointeeStorageLocation());

	const CXXCtorInitializer *Initializer = CfgInit.getInitializer();
	assert(Initializer != nullptr);

	auto *InitStmt = Initializer->getInit();
	assert(InitStmt != nullptr);

	auto *InitStmtLoc =
	State.Env.getStorageLocation(*InitStmt, SkipPast::Reference);
	if (InitStmtLoc == nullptr)
	return;

	auto InitStmtVal = State.Env.getValue(InitStmtLoc);
	if (InitStmtVal == nullptr)
	return;

	const FieldDecl *Member = Initializer->getMember();
	assert(Member != nullptr);

	if (Member->getType()->isReferenceType()) {
	auto &MemberLoc = ThisLoc.getChild(*Member);
	State.Env.setValue(MemberLoc,
	State.Env.takeOwnership(
	std::make_unique<ReferenceValue>(*InitStmtLoc)));
	} else {
	auto &MemberLoc = ThisLoc.getChild(*Member);
	State.Env.setValue(MemberLoc, *InitStmtVal);
	}
	}

	TypeErasedDataflowAnalysisState transferBlock(
	const ControlFlowContext &CFCtx,
	std::vector<llvm::Optional<TypeErasedDataflowAnalysisState>> &BlockStates,
	const CFGBlock &Block, const Environment &InitEnv,
	TypeErasedDataflowAnalysis &Analysis,
	std::function<void(const CFGStmt &,
	const TypeErasedDataflowAnalysisState &)>
	HandleTransferredStmt) {
	TypeErasedDataflowAnalysisState State =
	computeBlockInputState(CFCtx, BlockStates, Block, InitEnv, Analysis);
	for (const CFGElement &Element : Block) {
	switch (Element.getKind()) {
	case CFGElement::Statement:
	transferCFGStmt(*Element.getAs<CFGStmt>(), Analysis, State,
	HandleTransferredStmt);
	break;
	case CFGElement::Initializer:
	if (Analysis.applyBuiltinTransfer())
	transferCFGInitializer(*Element.getAs<CFGInitializer>(), State);
	break;
	default:
	// FIXME: Evaluate other kinds of `CFGElement`.
	break;
	}
	}
	return State;
	}

	std::vector<llvm::Optional<TypeErasedDataflowAnalysisState>>
	runTypeErasedDataflowAnalysis(const ControlFlowContext &CFCtx,
	TypeErasedDataflowAnalysis &Analysis,
	const Environment &InitEnv) {
	PostOrderCFGView POV(&CFCtx.getCFG());
	ForwardDataflowWorklist Worklist(CFCtx.getCFG(), &POV);

	std::vector<llvm::Optional<TypeErasedDataflowAnalysisState>> BlockStates;
	BlockStates.resize(CFCtx.getCFG().size(), llvm::None);

	// 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);

	// 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: Set up statistics (see llvm/ADT/Statistic.h) to count average number
	// of iterations, number of functions that time out, etc.
	uint32_t Iterations = 0;
	static constexpr uint32_t MaxIterations = 1 << 16;
	while (const CFGBlock *Block = Worklist.dequeue()) {
	if (++Iterations > MaxIterations) {
	llvm::errs() << "Maximum number of iterations reached, giving up.\n";
	break;
	}

	const llvm::Optional<TypeErasedDataflowAnalysisState> &OldBlockState =
	BlockStates[Block->getBlockID()];
	TypeErasedDataflowAnalysisState NewBlockState =
	transferBlock(CFCtx, BlockStates, *Block, InitEnv, Analysis);

	if (OldBlockState.hasValue() &&
	Analysis.isEqualTypeErased(OldBlockState.getValue().Lattice,
	NewBlockState.Lattice) &&
	OldBlockState->Env == NewBlockState.Env) {
	// 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 `llvm::None` at this point) to also analyze dead code.

	return BlockStates;
	}

	} // namespace dataflow
	} // namespace clang