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llvm-project/llvm/lib/Analysis/CFLAliasAnalysis.cpp

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//===- CFLAliasAnalysis.cpp - CFL-Based Alias Analysis Implementation ------==//
//
// The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This file implements a CFL-based context-insensitive alias analysis
// algorithm. It does not depend on types. The algorithm is a mixture of the one
// described in "Demand-driven alias analysis for C" by Xin Zheng and Radu
// Rugina, and "Fast algorithms for Dyck-CFL-reachability with applications to
// Alias Analysis" by Zhang Q, Lyu M R, Yuan H, and Su Z. -- to summarize the
// papers, we build a graph of the uses of a variable, where each node is a
// memory location, and each edge is an action that happened on that memory
// location. The "actions" can be one of Dereference, Reference, or Assign.
//
// Two variables are considered as aliasing iff you can reach one value's node
// from the other value's node and the language formed by concatenating all of
// the edge labels (actions) conforms to a context-free grammar.
//
// Because this algorithm requires a graph search on each query, we execute the
// algorithm outlined in "Fast algorithms..." (mentioned above)
// in order to transform the graph into sets of variables that may alias in
// ~nlogn time (n = number of variables), which makes queries take constant
// time.
//===----------------------------------------------------------------------===//
// N.B. AliasAnalysis as a whole is phrased as a FunctionPass at the moment, and
// CFLAA is interprocedural. This is *technically* A Bad Thing, because
// FunctionPasses are only allowed to inspect the Function that they're being
// run on. Realistically, this likely isn't a problem until we allow
// FunctionPasses to run concurrently.
#include "llvm/Analysis/CFLAliasAnalysis.h"
#include "StratifiedSets.h"
#include "llvm/ADT/BitVector.h"
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/None.h"
#include "llvm/ADT/Optional.h"
#include "llvm/IR/Constants.h"
#include "llvm/IR/Function.h"
#include "llvm/IR/InstVisitor.h"
#include "llvm/IR/Instructions.h"
#include "llvm/Pass.h"
#include "llvm/Support/Allocator.h"
#include "llvm/Support/Compiler.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/ErrorHandling.h"
#include "llvm/Support/raw_ostream.h"
#include <algorithm>
#include <cassert>
#include <memory>
#include <tuple>
using namespace llvm;
#define DEBUG_TYPE "cfl-aa"
CFLAAResult::CFLAAResult() : AAResultBase() {}
CFLAAResult::CFLAAResult(CFLAAResult &&Arg) : AAResultBase(std::move(Arg)) {}
CFLAAResult::~CFLAAResult() {}
/// Information we have about a function and would like to keep around.
struct CFLAAResult::FunctionInfo {
StratifiedSets<Value *> Sets;
// Lots of functions have < 4 returns. Adjust as necessary.
SmallVector<Value *, 4> ReturnedValues;
FunctionInfo(StratifiedSets<Value *> &&S, SmallVector<Value *, 4> &&RV)
: Sets(std::move(S)), ReturnedValues(std::move(RV)) {}
};
/// Try to go from a Value* to a Function*. Never returns nullptr.
static Optional<Function *> parentFunctionOfValue(Value *);
/// Returns possible functions called by the Inst* into the given
/// SmallVectorImpl. Returns true if targets found, false otherwise. This is
/// templated so we can use it with CallInsts and InvokeInsts.
template <typename Inst>
static bool getPossibleTargets(Inst *, SmallVectorImpl<Function *> &);
/// Some instructions need to have their users tracked. Instructions like
/// `add` require you to get the users of the Instruction* itself, other
/// instructions like `store` require you to get the users of the first
/// operand. This function gets the "proper" value to track for each
/// type of instruction we support.
static Optional<Value *> getTargetValue(Instruction *);
/// Determines whether or not we an instruction is useless to us (e.g.
/// FenceInst)
static bool hasUsefulEdges(Instruction *);
const StratifiedIndex StratifiedLink::SetSentinel =
std::numeric_limits<StratifiedIndex>::max();
namespace {
/// StratifiedInfo Attribute things.
typedef unsigned StratifiedAttr;
LLVM_CONSTEXPR unsigned MaxStratifiedAttrIndex = NumStratifiedAttrs;
LLVM_CONSTEXPR unsigned AttrAllIndex = 0;
LLVM_CONSTEXPR unsigned AttrGlobalIndex = 1;
LLVM_CONSTEXPR unsigned AttrUnknownIndex = 2;
LLVM_CONSTEXPR unsigned AttrFirstArgIndex = 3;
LLVM_CONSTEXPR unsigned AttrLastArgIndex = MaxStratifiedAttrIndex;
LLVM_CONSTEXPR unsigned AttrMaxNumArgs = AttrLastArgIndex - AttrFirstArgIndex;
LLVM_CONSTEXPR StratifiedAttr AttrNone = 0;
LLVM_CONSTEXPR StratifiedAttr AttrUnknown = 1 << AttrUnknownIndex;
LLVM_CONSTEXPR StratifiedAttr AttrAll = ~AttrNone;
/// StratifiedSets call for knowledge of "direction", so this is how we
/// represent that locally.
enum class Level { Same, Above, Below };
/// Edges can be one of four "weights" -- each weight must have an inverse
/// weight (Assign has Assign; Reference has Dereference).
enum class EdgeType {
/// The weight assigned when assigning from or to a value. For example, in:
/// %b = getelementptr %a, 0
/// ...The relationships are %b assign %a, and %a assign %b. This used to be
/// two edges, but having a distinction bought us nothing.
Assign,
/// The edge used when we have an edge going from some handle to a Value.
/// Examples of this include:
/// %b = load %a (%b Dereference %a)
/// %b = extractelement %a, 0 (%a Dereference %b)
Dereference,
/// The edge used when our edge goes from a value to a handle that may have
/// contained it at some point. Examples:
/// %b = load %a (%a Reference %b)
/// %b = extractelement %a, 0 (%b Reference %a)
Reference
};
// \brief Encodes the notion of a "use"
struct Edge {
/// Which value the edge is coming from
Value *From;
/// Which value the edge is pointing to
Value *To;
/// Edge weight
EdgeType Weight;
/// Whether we aliased any external values along the way that may be
/// invisible to the analysis (i.e. landingpad for exceptions, calls for
/// interprocedural analysis, etc.)
StratifiedAttrs AdditionalAttrs;
Edge(Value *From, Value *To, EdgeType W, StratifiedAttrs A)
: From(From), To(To), Weight(W), AdditionalAttrs(A) {}
};
/// Gets the edges our graph should have, based on an Instruction*
class GetEdgesVisitor : public InstVisitor<GetEdgesVisitor, void> {
CFLAAResult &AA;
SmallVectorImpl<Edge> &Output;
public:
GetEdgesVisitor(CFLAAResult &AA, SmallVectorImpl<Edge> &Output)
: AA(AA), Output(Output) {}
void visitInstruction(Instruction &) {
llvm_unreachable("Unsupported instruction encountered");
}
void visitPtrToIntInst(PtrToIntInst &Inst) {
auto *Ptr = Inst.getOperand(0);
Output.push_back(Edge(Ptr, Ptr, EdgeType::Assign, AttrUnknown));
}
void visitIntToPtrInst(IntToPtrInst &Inst) {
auto *Ptr = &Inst;
Output.push_back(Edge(Ptr, Ptr, EdgeType::Assign, AttrUnknown));
}
void visitCastInst(CastInst &Inst) {
Output.push_back(
Edge(&Inst, Inst.getOperand(0), EdgeType::Assign, AttrNone));
}
void visitBinaryOperator(BinaryOperator &Inst) {
auto *Op1 = Inst.getOperand(0);
auto *Op2 = Inst.getOperand(1);
Output.push_back(Edge(&Inst, Op1, EdgeType::Assign, AttrNone));
Output.push_back(Edge(&Inst, Op2, EdgeType::Assign, AttrNone));
}
void visitAtomicCmpXchgInst(AtomicCmpXchgInst &Inst) {
auto *Ptr = Inst.getPointerOperand();
auto *Val = Inst.getNewValOperand();
Output.push_back(Edge(Ptr, Val, EdgeType::Dereference, AttrNone));
}
void visitAtomicRMWInst(AtomicRMWInst &Inst) {
auto *Ptr = Inst.getPointerOperand();
auto *Val = Inst.getValOperand();
Output.push_back(Edge(Ptr, Val, EdgeType::Dereference, AttrNone));
}
void visitPHINode(PHINode &Inst) {
for (Value *Val : Inst.incoming_values())
Output.push_back(Edge(&Inst, Val, EdgeType::Assign, AttrNone));
}
void visitGetElementPtrInst(GetElementPtrInst &Inst) {
auto *Op = Inst.getPointerOperand();
Output.push_back(Edge(&Inst, Op, EdgeType::Assign, AttrNone));
for (auto I = Inst.idx_begin(), E = Inst.idx_end(); I != E; ++I)
Output.push_back(Edge(&Inst, *I, EdgeType::Assign, AttrNone));
}
void visitSelectInst(SelectInst &Inst) {
// Condition is not processed here (The actual statement producing
// the condition result is processed elsewhere). For select, the
// condition is evaluated, but not loaded, stored, or assigned
// simply as a result of being the condition of a select.
auto *TrueVal = Inst.getTrueValue();
Output.push_back(Edge(&Inst, TrueVal, EdgeType::Assign, AttrNone));
auto *FalseVal = Inst.getFalseValue();
Output.push_back(Edge(&Inst, FalseVal, EdgeType::Assign, AttrNone));
}
void visitAllocaInst(AllocaInst &) {}
void visitLoadInst(LoadInst &Inst) {
auto *Ptr = Inst.getPointerOperand();
auto *Val = &Inst;
Output.push_back(Edge(Val, Ptr, EdgeType::Reference, AttrNone));
}
void visitStoreInst(StoreInst &Inst) {
auto *Ptr = Inst.getPointerOperand();
auto *Val = Inst.getValueOperand();
Output.push_back(Edge(Ptr, Val, EdgeType::Dereference, AttrNone));
}
void visitVAArgInst(VAArgInst &Inst) {
// We can't fully model va_arg here. For *Ptr = Inst.getOperand(0), it does
// two things:
// 1. Loads a value from *((T*)*Ptr).
// 2. Increments (stores to) *Ptr by some target-specific amount.
// For now, we'll handle this like a landingpad instruction (by placing the
// result in its own group, and having that group alias externals).
auto *Val = &Inst;
Output.push_back(Edge(Val, Val, EdgeType::Assign, AttrAll));
}
static bool isFunctionExternal(Function *Fn) {
return Fn->isDeclaration() || !Fn->hasLocalLinkage();
}
/// Gets whether the sets at Index1 above, below, or equal to the sets at
/// Index2. Returns None if they are not in the same set chain.
static Optional<Level> getIndexRelation(const StratifiedSets<Value *> &Sets,
StratifiedIndex Index1,
StratifiedIndex Index2) {
if (Index1 == Index2)
return Level::Same;
const auto *Current = &Sets.getLink(Index1);
while (Current->hasBelow()) {
if (Current->Below == Index2)
return Level::Below;
Current = &Sets.getLink(Current->Below);
}
Current = &Sets.getLink(Index1);
while (Current->hasAbove()) {
if (Current->Above == Index2)
return Level::Above;
Current = &Sets.getLink(Current->Above);
}
return None;
}
bool
tryInterproceduralAnalysis(const SmallVectorImpl<Function *> &Fns,
Value *FuncValue,
const iterator_range<User::op_iterator> &Args) {
const unsigned ExpectedMaxArgs = 8;
const unsigned MaxSupportedArgs = 50;
assert(Fns.size() > 0);
// This algorithm is n^2, so an arbitrary upper-bound of 50 args was
// selected, so it doesn't take too long in insane cases.
if (std::distance(Args.begin(), Args.end()) > (int)MaxSupportedArgs)
return false;
// Exit early if we'll fail anyway
for (auto *Fn : Fns) {
if (isFunctionExternal(Fn) || Fn->isVarArg())
return false;
auto &MaybeInfo = AA.ensureCached(Fn);
if (!MaybeInfo.hasValue())
return false;
}
SmallVector<Value *, ExpectedMaxArgs> Arguments(Args.begin(), Args.end());
SmallVector<StratifiedInfo, ExpectedMaxArgs> Parameters;
for (auto *Fn : Fns) {
auto &Info = *AA.ensureCached(Fn);
auto &Sets = Info.Sets;
auto &RetVals = Info.ReturnedValues;
Parameters.clear();
for (auto &Param : Fn->args()) {
auto MaybeInfo = Sets.find(&Param);
// Did a new parameter somehow get added to the function/slip by?
if (!MaybeInfo.hasValue())
return false;
Parameters.push_back(*MaybeInfo);
}
// Adding an edge from argument -> return value for each parameter that
// may alias the return value
for (unsigned I = 0, E = Parameters.size(); I != E; ++I) {
auto &ParamInfo = Parameters[I];
auto &ArgVal = Arguments[I];
bool AddEdge = false;
StratifiedAttrs Externals;
for (unsigned X = 0, XE = RetVals.size(); X != XE; ++X) {
auto MaybeInfo = Sets.find(RetVals[X]);
if (!MaybeInfo.hasValue())
return false;
auto &RetInfo = *MaybeInfo;
auto RetAttrs = Sets.getLink(RetInfo.Index).Attrs;
auto ParamAttrs = Sets.getLink(ParamInfo.Index).Attrs;
auto MaybeRelation =
getIndexRelation(Sets, ParamInfo.Index, RetInfo.Index);
if (MaybeRelation.hasValue()) {
AddEdge = true;
Externals |= RetAttrs | ParamAttrs;
}
}
if (AddEdge)
Output.push_back(Edge(FuncValue, ArgVal, EdgeType::Assign,
StratifiedAttrs().flip()));
}
if (Parameters.size() != Arguments.size())
return false;
/// Adding edges between arguments for arguments that may end up aliasing
/// each other. This is necessary for functions such as
/// void foo(int** a, int** b) { *a = *b; }
/// (Technically, the proper sets for this would be those below
/// Arguments[I] and Arguments[X], but our algorithm will produce
/// extremely similar, and equally correct, results either way)
for (unsigned I = 0, E = Arguments.size(); I != E; ++I) {
auto &MainVal = Arguments[I];
auto &MainInfo = Parameters[I];
auto &MainAttrs = Sets.getLink(MainInfo.Index).Attrs;
for (unsigned X = I + 1; X != E; ++X) {
auto &SubInfo = Parameters[X];
auto &SubVal = Arguments[X];
auto &SubAttrs = Sets.getLink(SubInfo.Index).Attrs;
auto MaybeRelation =
getIndexRelation(Sets, MainInfo.Index, SubInfo.Index);
if (!MaybeRelation.hasValue())
continue;
auto NewAttrs = SubAttrs | MainAttrs;
Output.push_back(Edge(MainVal, SubVal, EdgeType::Assign, NewAttrs));
}
}
}
return true;
}
template <typename InstT> void visitCallLikeInst(InstT &Inst) {
// TODO: Add support for noalias args/all the other fun function attributes
// that we can tack on.
SmallVector<Function *, 4> Targets;
if (getPossibleTargets(&Inst, Targets)) {
if (tryInterproceduralAnalysis(Targets, &Inst, Inst.arg_operands()))
return;
// Cleanup from interprocedural analysis
Output.clear();
}
// Because the function is opaque, we need to note that anything
// could have happened to the arguments, and that the result could alias
// just about anything, too.
// The goal of the loop is in part to unify many Values into one set, so we
// don't care if the function is void there.
for (Value *V : Inst.arg_operands())
Output.push_back(Edge(&Inst, V, EdgeType::Assign, AttrAll));
if (Inst.getNumArgOperands() == 0 &&
Inst.getType() != Type::getVoidTy(Inst.getContext()))
Output.push_back(Edge(&Inst, &Inst, EdgeType::Assign, AttrAll));
}
void visitCallInst(CallInst &Inst) { visitCallLikeInst(Inst); }
void visitInvokeInst(InvokeInst &Inst) { visitCallLikeInst(Inst); }
/// Because vectors/aggregates are immutable and unaddressable, there's
/// nothing we can do to coax a value out of them, other than calling
/// Extract{Element,Value}. We can effectively treat them as pointers to
/// arbitrary memory locations we can store in and load from.
void visitExtractElementInst(ExtractElementInst &Inst) {
auto *Ptr = Inst.getVectorOperand();
auto *Val = &Inst;
Output.push_back(Edge(Val, Ptr, EdgeType::Reference, AttrNone));
}
void visitInsertElementInst(InsertElementInst &Inst) {
auto *Vec = Inst.getOperand(0);
auto *Val = Inst.getOperand(1);
Output.push_back(Edge(&Inst, Vec, EdgeType::Assign, AttrNone));
Output.push_back(Edge(&Inst, Val, EdgeType::Dereference, AttrNone));
}
void visitLandingPadInst(LandingPadInst &Inst) {
// Exceptions come from "nowhere", from our analysis' perspective.
// So we place the instruction its own group, noting that said group may
// alias externals
Output.push_back(Edge(&Inst, &Inst, EdgeType::Assign, AttrAll));
}
void visitInsertValueInst(InsertValueInst &Inst) {
auto *Agg = Inst.getOperand(0);
auto *Val = Inst.getOperand(1);
Output.push_back(Edge(&Inst, Agg, EdgeType::Assign, AttrNone));
Output.push_back(Edge(&Inst, Val, EdgeType::Dereference, AttrNone));
}
void visitExtractValueInst(ExtractValueInst &Inst) {
auto *Ptr = Inst.getAggregateOperand();
Output.push_back(Edge(&Inst, Ptr, EdgeType::Reference, AttrNone));
}
void visitShuffleVectorInst(ShuffleVectorInst &Inst) {
auto *From1 = Inst.getOperand(0);
auto *From2 = Inst.getOperand(1);
Output.push_back(Edge(&Inst, From1, EdgeType::Assign, AttrNone));
Output.push_back(Edge(&Inst, From2, EdgeType::Assign, AttrNone));
}
void visitConstantExpr(ConstantExpr *CE) {
switch (CE->getOpcode()) {
default:
llvm_unreachable("Unknown instruction type encountered!");
// Build the switch statement using the Instruction.def file.
#define HANDLE_INST(NUM, OPCODE, CLASS) \
case Instruction::OPCODE: \
visit##OPCODE(*(CLASS *)CE); \
break;
#include "llvm/IR/Instruction.def"
}
}
};
/// For a given instruction, we need to know which Value* to get the
/// users of in order to build our graph. In some cases (i.e. add),
/// we simply need the Instruction*. In other cases (i.e. store),
/// finding the users of the Instruction* is useless; we need to find
/// the users of the first operand. This handles determining which
/// value to follow for us.
///
/// Note: we *need* to keep this in sync with GetEdgesVisitor. Add
/// something to GetEdgesVisitor, add it here -- remove something from
/// GetEdgesVisitor, remove it here.
class GetTargetValueVisitor
: public InstVisitor<GetTargetValueVisitor, Value *> {
public:
Value *visitInstruction(Instruction &Inst) { return &Inst; }
Value *visitStoreInst(StoreInst &Inst) { return Inst.getPointerOperand(); }
Value *visitAtomicCmpXchgInst(AtomicCmpXchgInst &Inst) {
return Inst.getPointerOperand();
}
Value *visitAtomicRMWInst(AtomicRMWInst &Inst) {
return Inst.getPointerOperand();
}
Value *visitInsertElementInst(InsertElementInst &Inst) {
return Inst.getOperand(0);
}
Value *visitInsertValueInst(InsertValueInst &Inst) {
return Inst.getAggregateOperand();
}
};
/// Set building requires a weighted bidirectional graph.
template <typename EdgeTypeT> class WeightedBidirectionalGraph {
public:
typedef std::size_t Node;
private:
const static Node StartNode = Node(0);
struct Edge {
EdgeTypeT Weight;
Node Other;
Edge(const EdgeTypeT &W, const Node &N) : Weight(W), Other(N) {}
bool operator==(const Edge &E) const {
return Weight == E.Weight && Other == E.Other;
}
bool operator!=(const Edge &E) const { return !operator==(E); }
};
struct NodeImpl {
std::vector<Edge> Edges;
};
std::vector<NodeImpl> NodeImpls;
bool inbounds(Node NodeIndex) const { return NodeIndex < NodeImpls.size(); }
const NodeImpl &getNode(Node N) const { return NodeImpls[N]; }
NodeImpl &getNode(Node N) { return NodeImpls[N]; }
public:
/// \brief Iterator for edges. Because this graph is bidirected, we don't
/// allow modification of the edges using this iterator. Additionally, the
/// iterator becomes invalid if you add edges to or from the node you're
/// getting the edges of.
struct EdgeIterator : public std::iterator<std::forward_iterator_tag,
std::tuple<EdgeTypeT, Node *>> {
EdgeIterator(const typename std::vector<Edge>::const_iterator &Iter)
: Current(Iter) {}
EdgeIterator(NodeImpl &Impl) : Current(Impl.begin()) {}
EdgeIterator &operator++() {
++Current;
return *this;
}
EdgeIterator operator++(int) {
EdgeIterator Copy(Current);
operator++();
return Copy;
}
std::tuple<EdgeTypeT, Node> &operator*() {
Store = std::make_tuple(Current->Weight, Current->Other);
return Store;
}
bool operator==(const EdgeIterator &Other) const {
return Current == Other.Current;
}
bool operator!=(const EdgeIterator &Other) const {
return !operator==(Other);
}
private:
typename std::vector<Edge>::const_iterator Current;
std::tuple<EdgeTypeT, Node> Store;
};
/// Wrapper for EdgeIterator with begin()/end() calls.
struct EdgeIterable {
EdgeIterable(const std::vector<Edge> &Edges)
: BeginIter(Edges.begin()), EndIter(Edges.end()) {}
EdgeIterator begin() { return EdgeIterator(BeginIter); }
EdgeIterator end() { return EdgeIterator(EndIter); }
private:
typename std::vector<Edge>::const_iterator BeginIter;
typename std::vector<Edge>::const_iterator EndIter;
};
// ----- Actual graph-related things ----- //
WeightedBidirectionalGraph() {}
WeightedBidirectionalGraph(WeightedBidirectionalGraph<EdgeTypeT> &&Other)
: NodeImpls(std::move(Other.NodeImpls)) {}
WeightedBidirectionalGraph<EdgeTypeT> &
operator=(WeightedBidirectionalGraph<EdgeTypeT> &&Other) {
NodeImpls = std::move(Other.NodeImpls);
return *this;
}
Node addNode() {
auto Index = NodeImpls.size();
auto NewNode = Node(Index);
NodeImpls.push_back(NodeImpl());
return NewNode;
}
void addEdge(Node From, Node To, const EdgeTypeT &Weight,
const EdgeTypeT &ReverseWeight) {
assert(inbounds(From));
assert(inbounds(To));
auto &FromNode = getNode(From);
auto &ToNode = getNode(To);
FromNode.Edges.push_back(Edge(Weight, To));
ToNode.Edges.push_back(Edge(ReverseWeight, From));
}
iterator_range<EdgeIterator> edgesFor(const Node &N) const {
const auto &Node = getNode(N);
return make_range(EdgeIterator(Node.Edges.begin()),
EdgeIterator(Node.Edges.end()));
}
bool empty() const { return NodeImpls.empty(); }
std::size_t size() const { return NodeImpls.size(); }
/// Gets an arbitrary node in the graph as a starting point for traversal.
Node getEntryNode() {
assert(inbounds(StartNode));
return StartNode;
}
};
typedef WeightedBidirectionalGraph<std::pair<EdgeType, StratifiedAttrs>> GraphT;
typedef DenseMap<Value *, GraphT::Node> NodeMapT;
}
//===----------------------------------------------------------------------===//
// Function declarations that require types defined in the namespace above
//===----------------------------------------------------------------------===//
/// Given an argument number, returns the appropriate Attr index to set.
static StratifiedAttr argNumberToAttrIndex(unsigned ArgNum);
/// Given a Value, potentially return which AttrIndex it maps to.
static Optional<StratifiedAttr> valueToAttrIndex(Value *Val);
/// Gets the inverse of a given EdgeType.
static EdgeType flipWeight(EdgeType Initial);
/// Gets edges of the given Instruction*, writing them to the SmallVector*.
static void argsToEdges(CFLAAResult &, Instruction *, SmallVectorImpl<Edge> &);
/// Gets edges of the given ConstantExpr*, writing them to the SmallVector*.
static void argsToEdges(CFLAAResult &, ConstantExpr *, SmallVectorImpl<Edge> &);
/// Gets the "Level" that one should travel in StratifiedSets
/// given an EdgeType.
static Level directionOfEdgeType(EdgeType);
/// Builds the graph needed for constructing the StratifiedSets for the
/// given function
static void buildGraphFrom(CFLAAResult &, Function *,
SmallVectorImpl<Value *> &, NodeMapT &, GraphT &);
/// Gets the edges of a ConstantExpr as if it was an Instruction. This function
/// also acts on any nested ConstantExprs, adding the edges of those to the
/// given SmallVector as well.
static void constexprToEdges(CFLAAResult &, ConstantExpr &,
SmallVectorImpl<Edge> &);
/// Given an Instruction, this will add it to the graph, along with any
/// Instructions that are potentially only available from said Instruction
/// For example, given the following line:
/// %0 = load i16* getelementptr ([1 x i16]* @a, 0, 0), align 2
/// addInstructionToGraph would add both the `load` and `getelementptr`
/// instructions to the graph appropriately.
static void addInstructionToGraph(CFLAAResult &, Instruction &,
SmallVectorImpl<Value *> &, NodeMapT &,
GraphT &);
/// Determines whether it would be pointless to add the given Value to our sets.
static bool canSkipAddingToSets(Value *Val);
static Optional<Function *> parentFunctionOfValue(Value *Val) {
if (auto *Inst = dyn_cast<Instruction>(Val)) {
auto *Bb = Inst->getParent();
return Bb->getParent();
}
if (auto *Arg = dyn_cast<Argument>(Val))
return Arg->getParent();
return None;
}
template <typename Inst>
static bool getPossibleTargets(Inst *Call,
SmallVectorImpl<Function *> &Output) {
if (auto *Fn = Call->getCalledFunction()) {
Output.push_back(Fn);
return true;
}
// TODO: If the call is indirect, we might be able to enumerate all potential
// targets of the call and return them, rather than just failing.
return false;
}
static Optional<Value *> getTargetValue(Instruction *Inst) {
GetTargetValueVisitor V;
return V.visit(Inst);
}
static bool hasUsefulEdges(Instruction *Inst) {
bool IsNonInvokeTerminator =
isa<TerminatorInst>(Inst) && !isa<InvokeInst>(Inst);
return !isa<CmpInst>(Inst) && !isa<FenceInst>(Inst) && !IsNonInvokeTerminator;
}
static bool hasUsefulEdges(ConstantExpr *CE) {
// ConstantExpr doesn't have terminators, invokes, or fences, so only needs
// to check for compares.
return CE->getOpcode() != Instruction::ICmp &&
CE->getOpcode() != Instruction::FCmp;
}
static Optional<StratifiedAttr> valueToAttrIndex(Value *Val) {
if (isa<GlobalValue>(Val))
return AttrGlobalIndex;
if (auto *Arg = dyn_cast<Argument>(Val))
// Only pointer arguments should have the argument attribute,
// because things can't escape through scalars without us seeing a
// cast, and thus, interaction with them doesn't matter.
if (!Arg->hasNoAliasAttr() && Arg->getType()->isPointerTy())
return argNumberToAttrIndex(Arg->getArgNo());
return None;
}
static StratifiedAttr argNumberToAttrIndex(unsigned ArgNum) {
if (ArgNum >= AttrMaxNumArgs)
return AttrAllIndex;
return ArgNum + AttrFirstArgIndex;
}
static EdgeType flipWeight(EdgeType Initial) {
switch (Initial) {
case EdgeType::Assign:
return EdgeType::Assign;
case EdgeType::Dereference:
return EdgeType::Reference;
case EdgeType::Reference:
return EdgeType::Dereference;
}
llvm_unreachable("Incomplete coverage of EdgeType enum");
}
static void argsToEdges(CFLAAResult &Analysis, Instruction *Inst,
SmallVectorImpl<Edge> &Output) {
assert(hasUsefulEdges(Inst) &&
"Expected instructions to have 'useful' edges");
GetEdgesVisitor v(Analysis, Output);
v.visit(Inst);
}
static void argsToEdges(CFLAAResult &Analysis, ConstantExpr *CE,
SmallVectorImpl<Edge> &Output) {
assert(hasUsefulEdges(CE) && "Expected constant expr to have 'useful' edges");
GetEdgesVisitor v(Analysis, Output);
v.visitConstantExpr(CE);
}
static Level directionOfEdgeType(EdgeType Weight) {
switch (Weight) {
case EdgeType::Reference:
return Level::Above;
case EdgeType::Dereference:
return Level::Below;
case EdgeType::Assign:
return Level::Same;
}
llvm_unreachable("Incomplete switch coverage");
}
static void constexprToEdges(CFLAAResult &Analysis,
ConstantExpr &CExprToCollapse,
SmallVectorImpl<Edge> &Results) {
SmallVector<ConstantExpr *, 4> Worklist;
Worklist.push_back(&CExprToCollapse);
SmallVector<Edge, 8> ConstexprEdges;
SmallPtrSet<ConstantExpr *, 4> Visited;
while (!Worklist.empty()) {
auto *CExpr = Worklist.pop_back_val();
if (!hasUsefulEdges(CExpr))
continue;
ConstexprEdges.clear();
argsToEdges(Analysis, CExpr, ConstexprEdges);
for (auto &Edge : ConstexprEdges) {
if (auto *Nested = dyn_cast<ConstantExpr>(Edge.From))
if (Visited.insert(Nested).second)
Worklist.push_back(Nested);
if (auto *Nested = dyn_cast<ConstantExpr>(Edge.To))
if (Visited.insert(Nested).second)
Worklist.push_back(Nested);
}
Results.append(ConstexprEdges.begin(), ConstexprEdges.end());
}
}
static void addInstructionToGraph(CFLAAResult &Analysis, Instruction &Inst,
SmallVectorImpl<Value *> &ReturnedValues,
NodeMapT &Map, GraphT &Graph) {
const auto findOrInsertNode = [&Map, &Graph](Value *Val) {
auto Pair = Map.insert(std::make_pair(Val, GraphT::Node()));
auto &Iter = Pair.first;
if (Pair.second) {
auto NewNode = Graph.addNode();
Iter->second = NewNode;
}
return Iter->second;
};
// We don't want the edges of most "return" instructions, but we *do* want
// to know what can be returned.
if (isa<ReturnInst>(&Inst))
ReturnedValues.push_back(&Inst);
if (!hasUsefulEdges(&Inst))
return;
SmallVector<Edge, 8> Edges;
argsToEdges(Analysis, &Inst, Edges);
// In the case of an unused alloca (or similar), edges may be empty. Note
// that it exists so we can potentially answer NoAlias.
if (Edges.empty()) {
auto MaybeVal = getTargetValue(&Inst);
assert(MaybeVal.hasValue());
auto *Target = *MaybeVal;
findOrInsertNode(Target);
return;
}
auto addEdgeToGraph = [&](const Edge &E) {
auto To = findOrInsertNode(E.To);
auto From = findOrInsertNode(E.From);
auto FlippedWeight = flipWeight(E.Weight);
auto Attrs = E.AdditionalAttrs;
Graph.addEdge(From, To, std::make_pair(E.Weight, Attrs),
std::make_pair(FlippedWeight, Attrs));
};
SmallVector<ConstantExpr *, 4> ConstantExprs;
for (const Edge &E : Edges) {
addEdgeToGraph(E);
if (auto *Constexpr = dyn_cast<ConstantExpr>(E.To))
ConstantExprs.push_back(Constexpr);
if (auto *Constexpr = dyn_cast<ConstantExpr>(E.From))
ConstantExprs.push_back(Constexpr);
}
for (ConstantExpr *CE : ConstantExprs) {
Edges.clear();
constexprToEdges(Analysis, *CE, Edges);
std::for_each(Edges.begin(), Edges.end(), addEdgeToGraph);
}
}
static void buildGraphFrom(CFLAAResult &Analysis, Function *Fn,
SmallVectorImpl<Value *> &ReturnedValues,
NodeMapT &Map, GraphT &Graph) {
// (N.B. We may remove graph construction entirely, because it doesn't really
// buy us much.)
for (auto &Bb : Fn->getBasicBlockList())
for (auto &Inst : Bb.getInstList())
addInstructionToGraph(Analysis, Inst, ReturnedValues, Map, Graph);
}
static bool canSkipAddingToSets(Value *Val) {
// Constants can share instances, which may falsely unify multiple
// sets, e.g. in
// store i32* null, i32** %ptr1
// store i32* null, i32** %ptr2
// clearly ptr1 and ptr2 should not be unified into the same set, so
// we should filter out the (potentially shared) instance to
// i32* null.
if (isa<Constant>(Val)) {
// TODO: Because all of these things are constant, we can determine whether
// the data is *actually* mutable at graph building time. This will probably
// come for free/cheap with offset awareness.
bool CanStoreMutableData = isa<GlobalValue>(Val) ||
isa<ConstantExpr>(Val) ||
isa<ConstantAggregate>(Val);
return !CanStoreMutableData;
}
return false;
}
// Builds the graph + StratifiedSets for a function.
CFLAAResult::FunctionInfo CFLAAResult::buildSetsFrom(Function *Fn) {
NodeMapT Map;
GraphT Graph;
SmallVector<Value *, 4> ReturnedValues;
buildGraphFrom(*this, Fn, ReturnedValues, Map, Graph);
DenseMap<GraphT::Node, Value *> NodeValueMap;
NodeValueMap.reserve(Map.size());
for (const auto &Pair : Map)
NodeValueMap.insert(std::make_pair(Pair.second, Pair.first));
const auto findValueOrDie = [&NodeValueMap](GraphT::Node Node) {
auto ValIter = NodeValueMap.find(Node);
assert(ValIter != NodeValueMap.end());
return ValIter->second;
};
StratifiedSetsBuilder<Value *> Builder;
SmallVector<GraphT::Node, 16> Worklist;
for (auto &Pair : Map) {
Worklist.clear();
auto *Value = Pair.first;
Builder.add(Value);
auto InitialNode = Pair.second;
Worklist.push_back(InitialNode);
while (!Worklist.empty()) {
auto Node = Worklist.pop_back_val();
auto *CurValue = findValueOrDie(Node);
if (canSkipAddingToSets(CurValue))
continue;
Optional<StratifiedAttr> MaybeCurIndex = valueToAttrIndex(CurValue);
if (MaybeCurIndex)
Builder.noteAttributes(CurValue, *MaybeCurIndex);
for (const auto &EdgeTuple : Graph.edgesFor(Node)) {
auto Weight = std::get<0>(EdgeTuple);
auto Label = Weight.first;
auto &OtherNode = std::get<1>(EdgeTuple);
auto *OtherValue = findValueOrDie(OtherNode);
if (canSkipAddingToSets(OtherValue))
continue;
bool Added;
switch (directionOfEdgeType(Label)) {
case Level::Above:
Added = Builder.addAbove(CurValue, OtherValue);
break;
case Level::Below:
Added = Builder.addBelow(CurValue, OtherValue);
break;
case Level::Same:
Added = Builder.addWith(CurValue, OtherValue);
break;
}
auto Aliasing = Weight.second;
if (MaybeCurIndex)
Aliasing.set(*MaybeCurIndex);
if (auto MaybeOtherIndex = valueToAttrIndex(OtherValue))
Aliasing.set(*MaybeOtherIndex);
Builder.noteAttributes(CurValue, Aliasing);
Builder.noteAttributes(OtherValue, Aliasing);
if (Added)
Worklist.push_back(OtherNode);
}
}
}
// There are times when we end up with parameters not in our graph (i.e. if
// it's only used as the condition of a branch). Other bits of code depend on
// things that were present during construction being present in the graph.
// So, we add all present arguments here.
for (auto &Arg : Fn->args()) {
if (!Builder.add(&Arg))
continue;
auto Attrs = valueToAttrIndex(&Arg);
if (Attrs.hasValue())
Builder.noteAttributes(&Arg, *Attrs);
}
return FunctionInfo(Builder.build(), std::move(ReturnedValues));
}
void CFLAAResult::scan(Function *Fn) {
auto InsertPair = Cache.insert(std::make_pair(Fn, Optional<FunctionInfo>()));
(void)InsertPair;
assert(InsertPair.second &&
"Trying to scan a function that has already been cached");
Cache[Fn] = buildSetsFrom(Fn);
Handles.push_front(FunctionHandle(Fn, this));
}
void CFLAAResult::evict(Function *Fn) { Cache.erase(Fn); }
/// Ensures that the given function is available in the cache, and returns the
/// entry.
const Optional<CFLAAResult::FunctionInfo> &
CFLAAResult::ensureCached(Function *Fn) {
auto Iter = Cache.find(Fn);
if (Iter == Cache.end()) {
scan(Fn);
Iter = Cache.find(Fn);
assert(Iter != Cache.end());
assert(Iter->second.hasValue());
}
return Iter->second;
}
AliasResult CFLAAResult::query(const MemoryLocation &LocA,
const MemoryLocation &LocB) {
auto *ValA = const_cast<Value *>(LocA.Ptr);
auto *ValB = const_cast<Value *>(LocB.Ptr);
Function *Fn = nullptr;
auto MaybeFnA = parentFunctionOfValue(ValA);
auto MaybeFnB = parentFunctionOfValue(ValB);
if (!MaybeFnA.hasValue() && !MaybeFnB.hasValue()) {
// The only times this is known to happen are when globals + InlineAsm are
// involved
DEBUG(dbgs() << "CFLAA: could not extract parent function information.\n");
return MayAlias;
}
if (MaybeFnA.hasValue()) {
Fn = *MaybeFnA;
assert((!MaybeFnB.hasValue() || *MaybeFnB == *MaybeFnA) &&
"Interprocedural queries not supported");
} else {
Fn = *MaybeFnB;
}
assert(Fn != nullptr);
auto &MaybeInfo = ensureCached(Fn);
assert(MaybeInfo.hasValue());
auto &Sets = MaybeInfo->Sets;
auto MaybeA = Sets.find(ValA);
if (!MaybeA.hasValue())
return MayAlias;
auto MaybeB = Sets.find(ValB);
if (!MaybeB.hasValue())
return MayAlias;
auto SetA = *MaybeA;
auto SetB = *MaybeB;
auto AttrsA = Sets.getLink(SetA.Index).Attrs;
auto AttrsB = Sets.getLink(SetB.Index).Attrs;
// Stratified set attributes are used as markets to signify whether a member
// of a StratifiedSet (or a member of a set above the current set) has
// interacted with either arguments or globals. "Interacted with" meaning its
// value may be different depending on the value of an argument or global. The
// thought behind this is that, because arguments and globals may alias each
// other, if AttrsA and AttrsB have touched args/globals, we must
// conservatively say that they alias. However, if at least one of the sets
// has no values that could legally be altered by changing the value of an
// argument or global, then we don't have to be as conservative.
if (AttrsA.any() && AttrsB.any())
return MayAlias;
// We currently unify things even if the accesses to them may not be in
// bounds, so we can't return partial alias here because we don't know whether
// the pointer is really within the object or not.
// e.g. Given an out of bounds GEP and an alloca'd pointer, we may unify the
// two. We can't return partial alias for this case. Since we do not currently
// track enough information to differentiate.
return SetA.Index == SetB.Index ? MayAlias : NoAlias;
}
char CFLAA::PassID;
CFLAAResult CFLAA::run(Function &F, AnalysisManager<Function> &AM) {
return CFLAAResult();
}
char CFLAAWrapperPass::ID = 0;
INITIALIZE_PASS(CFLAAWrapperPass, "cfl-aa", "CFL-Based Alias Analysis", false,
true)
ImmutablePass *llvm::createCFLAAWrapperPass() { return new CFLAAWrapperPass(); }
CFLAAWrapperPass::CFLAAWrapperPass() : ImmutablePass(ID) {
initializeCFLAAWrapperPassPass(*PassRegistry::getPassRegistry());
}
bool CFLAAWrapperPass::doInitialization(Module &M) {
Result.reset(new CFLAAResult());
return false;
}
bool CFLAAWrapperPass::doFinalization(Module &M) {
Result.reset();
return false;
}
void CFLAAWrapperPass::getAnalysisUsage(AnalysisUsage &AU) const {
AU.setPreservesAll();
}
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