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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/DenseMap.h"
#include "llvm/ADT/None.h"
#include "llvm/ADT/Optional.h"
#include "llvm/Analysis/MemoryBuiltins.h"
#include "llvm/Analysis/TargetLibraryInfo.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/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(const TargetLibraryInfo &TLI)
: AAResultBase(), TLI(TLI) {}
CFLAAResult::CFLAAResult(CFLAAResult &&Arg)
: AAResultBase(std::move(Arg)), TLI(Arg.TLI) {}
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.
static bool getPossibleTargets(CallSite, SmallVectorImpl<Function *> &);
const StratifiedIndex StratifiedLink::SetSentinel =
std::numeric_limits<StratifiedIndex>::max();
namespace {
/// StratifiedInfo Attribute things.
typedef unsigned StratifiedAttr;
LLVM_CONSTEXPR unsigned MaxStratifiedAttrIndex = NumStratifiedAttrs;
LLVM_CONSTEXPR unsigned AttrEscapedIndex = 0;
LLVM_CONSTEXPR unsigned AttrUnknownIndex = 1;
LLVM_CONSTEXPR unsigned AttrGlobalIndex = 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 AttrEscaped = 1 << AttrEscapedIndex;
LLVM_CONSTEXPR StratifiedAttr AttrUnknown = 1 << AttrUnknownIndex;
LLVM_CONSTEXPR StratifiedAttr AttrGlobal = 1 << AttrGlobalIndex;
/// 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
};
/// The Program Expression Graph (PEG) of CFL analysis
class CFLGraph {
typedef Value *Node;
struct Edge {
StratifiedAttrs Attr;
EdgeType Type;
Node Other;
};
typedef std::vector<Edge> EdgeList;
typedef DenseMap<Node, EdgeList> NodeMap;
NodeMap NodeImpls;
// Gets the inverse of a given EdgeType.
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");
}
const EdgeList *getNode(Node N) const {
auto Itr = NodeImpls.find(N);
if (Itr == NodeImpls.end())
return nullptr;
return &Itr->second;
}
EdgeList *getNode(Node N) {
auto Itr = NodeImpls.find(N);
if (Itr == NodeImpls.end())
return nullptr;
return &Itr->second;
}
static Node nodeDeref(const NodeMap::value_type &P) { return P.first; }
typedef std::pointer_to_unary_function<const NodeMap::value_type &, Node>
NodeDerefFun;
public:
typedef EdgeList::const_iterator const_edge_iterator;
typedef mapped_iterator<NodeMap::const_iterator, NodeDerefFun>
const_node_iterator;
bool addNode(Node N) {
return NodeImpls.insert(std::make_pair(N, EdgeList())).second;
}
void addEdge(Node From, Node To, EdgeType Type,
StratifiedAttrs Attr = StratifiedAttrs()) {
auto FromEdges = getNode(From);
assert(FromEdges != nullptr);
auto ToEdges = getNode(To);
assert(ToEdges != nullptr);
FromEdges->push_back(Edge{Attr, Type, To});
ToEdges->push_back(Edge{Attr, flipWeight(Type), From});
}
iterator_range<const_edge_iterator> edgesFor(Node N) const {
auto Edges = getNode(N);
assert(Edges != nullptr);
return make_range(Edges->begin(), Edges->end());
}
iterator_range<const_node_iterator> nodes() const {
return make_range<const_node_iterator>(
map_iterator(NodeImpls.begin(), NodeDerefFun(nodeDeref)),
map_iterator(NodeImpls.end(), NodeDerefFun(nodeDeref)));
}
bool empty() const { return NodeImpls.empty(); }
std::size_t size() const { return NodeImpls.size(); }
};
/// Gets the edges our graph should have, based on an Instruction*
class GetEdgesVisitor : public InstVisitor<GetEdgesVisitor, void> {
CFLAAResult &AA;
const TargetLibraryInfo &TLI;
CFLGraph &Graph;
SmallPtrSetImpl<Value *> &Externals;
SmallPtrSetImpl<Value *> &Escapes;
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;
}
void addNode(Value *Val) {
if (!Graph.addNode(Val))
return;
if (isa<GlobalValue>(Val))
Externals.insert(Val);
else if (auto CExpr = dyn_cast<ConstantExpr>(Val))
if (hasUsefulEdges(CExpr))
visitConstantExpr(CExpr);
}
void addEdge(Value *From, Value *To, EdgeType Type, StratifiedAttrs Attr) {
addNode(From);
if (To != From)
addNode(To);
Graph.addEdge(From, To, Type, Attr);
}
public:
GetEdgesVisitor(CFLAAResult &AA, const TargetLibraryInfo &TLI,
CFLGraph &Graph, SmallPtrSetImpl<Value *> &Externals,
SmallPtrSetImpl<Value *> &Escapes)
: AA(AA), TLI(TLI), Graph(Graph), Externals(Externals), Escapes(Escapes) {
}
void visitInstruction(Instruction &) {
llvm_unreachable("Unsupported instruction encountered");
}
void visitPtrToIntInst(PtrToIntInst &Inst) {
auto *Ptr = Inst.getOperand(0);
addEdge(Ptr, &Inst, EdgeType::Assign, AttrEscaped);
}
void visitIntToPtrInst(IntToPtrInst &Inst) {
auto *Ptr = &Inst;
addEdge(Ptr, Ptr, EdgeType::Assign, AttrUnknown);
}
void visitCastInst(CastInst &Inst) {
auto *Src = Inst.getOperand(0);
addEdge(Src, &Inst, EdgeType::Assign, AttrNone);
}
void visitBinaryOperator(BinaryOperator &Inst) {
auto *Op1 = Inst.getOperand(0);
auto *Op2 = Inst.getOperand(1);
addEdge(Op1, &Inst, EdgeType::Assign, AttrNone);
addEdge(Op2, &Inst, EdgeType::Assign, AttrNone);
}
void visitAtomicCmpXchgInst(AtomicCmpXchgInst &Inst) {
auto *Ptr = Inst.getPointerOperand();
auto *Val = Inst.getNewValOperand();
addEdge(Ptr, Val, EdgeType::Dereference, AttrNone);
}
void visitAtomicRMWInst(AtomicRMWInst &Inst) {
auto *Ptr = Inst.getPointerOperand();
auto *Val = Inst.getValOperand();
addEdge(Ptr, Val, EdgeType::Dereference, AttrNone);
}
void visitPHINode(PHINode &Inst) {
for (Value *Val : Inst.incoming_values())
addEdge(Val, &Inst, EdgeType::Assign, AttrNone);
}
void visitGetElementPtrInst(GetElementPtrInst &Inst) {
auto *Op = Inst.getPointerOperand();
addEdge(Op, &Inst, 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();
auto *FalseVal = Inst.getFalseValue();
addEdge(TrueVal, &Inst, EdgeType::Assign, AttrNone);
addEdge(FalseVal, &Inst, EdgeType::Assign, AttrNone);
}
void visitAllocaInst(AllocaInst &Inst) { Graph.addNode(&Inst); }
void visitLoadInst(LoadInst &Inst) {
auto *Ptr = Inst.getPointerOperand();
auto *Val = &Inst;
addEdge(Val, Ptr, EdgeType::Reference, AttrNone);
}
void visitStoreInst(StoreInst &Inst) {
auto *Ptr = Inst.getPointerOperand();
auto *Val = Inst.getValueOperand();
addEdge(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;
addEdge(Val, Val, EdgeType::Assign, AttrUnknown);
}
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;
}
// 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;
};
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<Edge, 8> Output;
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, Externals});
}
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});
}
}
}
// Commit all edges in Output to CFLGraph
for (const auto &Edge : Output)
addEdge(Edge.From, Edge.To, Edge.Weight, Edge.AdditionalAttrs);
return true;
}
void visitCallSite(CallSite CS) {
auto Inst = CS.getInstruction();
// Make sure all arguments and return value are added to the graph first
for (Value *V : CS.args())
addNode(V);
if (!Inst->getType()->isVoidTy())
addNode(Inst);
// Check if Inst is a call to a library function that allocates/deallocates
// on the heap. Those kinds of functions do not introduce any aliases.
// TODO: address other common library functions such as realloc(), strdup(),
// etc.
if (isMallocLikeFn(Inst, &TLI) || isCallocLikeFn(Inst, &TLI) ||
isFreeCall(Inst, &TLI))
return;
// TODO: Add support for noalias args/all the other fun function attributes
// that we can tack on.
SmallVector<Function *, 4> Targets;
if (getPossibleTargets(CS, Targets))
if (tryInterproceduralAnalysis(Targets, Inst, CS.args()))
return;
// Because the function is opaque, we need to note that anything
// could have happened to the arguments (unless the function is marked
// readonly or readnone), and that the result could alias just about
// anything, too (unless the result is marked noalias).
if (!CS.onlyReadsMemory())
for (Value *V : CS.args()) {
Escapes.insert(V);
}
if (!Inst->getType()->isVoidTy()) {
auto *Fn = CS.getCalledFunction();
if (Fn == nullptr || !Fn->doesNotAlias(0))
addEdge(Inst, Inst, EdgeType::Assign, AttrUnknown);
}
}
/// 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;
addEdge(Val, Ptr, EdgeType::Reference, AttrNone);
}
void visitInsertElementInst(InsertElementInst &Inst) {
auto *Vec = Inst.getOperand(0);
auto *Val = Inst.getOperand(1);
addEdge(Vec, &Inst, EdgeType::Assign, AttrNone);
addEdge(&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
addEdge(&Inst, &Inst, EdgeType::Assign, AttrUnknown);
}
void visitInsertValueInst(InsertValueInst &Inst) {
auto *Agg = Inst.getOperand(0);
auto *Val = Inst.getOperand(1);
addEdge(Agg, &Inst, EdgeType::Assign, AttrNone);
addEdge(&Inst, Val, EdgeType::Dereference, AttrNone);
}
void visitExtractValueInst(ExtractValueInst &Inst) {
auto *Ptr = Inst.getAggregateOperand();
addEdge(&Inst, Ptr, EdgeType::Reference, AttrNone);
}
void visitShuffleVectorInst(ShuffleVectorInst &Inst) {
auto *From1 = Inst.getOperand(0);
auto *From2 = Inst.getOperand(1);
addEdge(From1, &Inst, EdgeType::Assign, AttrNone);
addEdge(From2, &Inst, 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"
}
}
};
class CFLGraphBuilder {
// Input of the builder
CFLAAResult &Analysis;
const TargetLibraryInfo &TLI;
// Output of the builder
CFLGraph Graph;
SmallVector<Value *, 4> ReturnedValues;
// Auxiliary structures used by the builder
SmallPtrSet<Value *, 8> ExternalValues;
SmallPtrSet<Value *, 8> EscapedValues;
// Helper functions
// Determines whether or not we an instruction is useless to us (e.g.
// FenceInst)
static bool hasUsefulEdges(Instruction *Inst) {
bool IsNonInvokeTerminator =
isa<TerminatorInst>(Inst) && !isa<InvokeInst>(Inst);
return !isa<CmpInst>(Inst) && !isa<FenceInst>(Inst) &&
!IsNonInvokeTerminator;
}
void addArgumentToGraph(Argument &Arg) {
Graph.addNode(&Arg);
ExternalValues.insert(&Arg);
}
// 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.
void addInstructionToGraph(Instruction &Inst) {
// 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;
GetEdgesVisitor(Analysis, TLI, Graph, ExternalValues, EscapedValues)
.visit(Inst);
}
// Builds the graph needed for constructing the StratifiedSets for the given
// function
void buildGraphFrom(Function &Fn) {
for (auto &Bb : Fn.getBasicBlockList())
for (auto &Inst : Bb.getInstList())
addInstructionToGraph(Inst);
for (auto &Arg : Fn.args())
addArgumentToGraph(Arg);
}
public:
CFLGraphBuilder(CFLAAResult &Analysis, const TargetLibraryInfo &TLI,
Function &Fn)
: Analysis(Analysis), TLI(TLI) {
buildGraphFrom(Fn);
}
const CFLGraph &getCFLGraph() { return Graph; }
SmallVector<Value *, 4> takeReturnValues() {
return std::move(ReturnedValues);
}
const SmallPtrSet<Value *, 8> &getExternalValues() { return ExternalValues; }
const SmallPtrSet<Value *, 8> &getEscapedValues() { return EscapedValues; }
};
}
//===----------------------------------------------------------------------===//
// Function declarations that require types defined in the namespace above
//===----------------------------------------------------------------------===//
/// Given a StratifiedAttrs, returns true if it marks the corresponding values
/// as globals or arguments
static bool isGlobalOrArgAttr(StratifiedAttrs Attr);
/// Given a StratifiedAttrs, returns true if the corresponding values come from
/// an unknown source (such as opaque memory or an integer cast)
static bool isUnknownAttr(StratifiedAttrs Attr);
/// Given an argument number, returns the appropriate StratifiedAttr to set.
static StratifiedAttr argNumberToAttr(unsigned ArgNum);
/// Given a Value, potentially return which StratifiedAttr it maps to.
static Optional<StratifiedAttr> valueToAttr(Value *Val);
/// Gets the "Level" that one should travel in StratifiedSets
/// given an EdgeType.
static Level directionOfEdgeType(EdgeType);
/// 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;
}
static bool getPossibleTargets(CallSite CS,
SmallVectorImpl<Function *> &Output) {
if (auto *Fn = CS.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 bool isGlobalOrArgAttr(StratifiedAttrs Attr) {
return Attr.reset(AttrEscapedIndex).reset(AttrUnknownIndex).any();
}
static bool isUnknownAttr(StratifiedAttrs Attr) {
return Attr.test(AttrUnknownIndex);
}
static Optional<StratifiedAttr> valueToAttr(Value *Val) {
if (isa<GlobalValue>(Val))
return AttrGlobal;
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 argNumberToAttr(Arg->getArgNo());
return None;
}
static StratifiedAttr argNumberToAttr(unsigned ArgNum) {
if (ArgNum >= AttrMaxNumArgs)
return AttrUnknown;
return 1 << (ArgNum + AttrFirstArgIndex);
}
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 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) {
CFLGraphBuilder GraphBuilder(*this, TLI, *Fn);
StratifiedSetsBuilder<Value *> SetBuilder;
auto &Graph = GraphBuilder.getCFLGraph();
SmallVector<Value *, 16> Worklist;
for (auto Node : Graph.nodes())
Worklist.push_back(Node);
while (!Worklist.empty()) {
auto *CurValue = Worklist.pop_back_val();
SetBuilder.add(CurValue);
if (canSkipAddingToSets(CurValue))
continue;
for (const auto &Edge : Graph.edgesFor(CurValue)) {
auto Label = Edge.Type;
auto *OtherValue = Edge.Other;
if (canSkipAddingToSets(OtherValue))
continue;
bool Added;
switch (directionOfEdgeType(Label)) {
case Level::Above:
Added = SetBuilder.addAbove(CurValue, OtherValue);
break;
case Level::Below:
Added = SetBuilder.addBelow(CurValue, OtherValue);
break;
case Level::Same:
Added = SetBuilder.addWith(CurValue, OtherValue);
break;
}
auto Aliasing = Edge.Attr;
SetBuilder.noteAttributes(CurValue, Aliasing);
SetBuilder.noteAttributes(OtherValue, Aliasing);
if (Added)
Worklist.push_back(OtherValue);
}
}
// Special handling for globals and arguments
for (auto *External : GraphBuilder.getExternalValues()) {
SetBuilder.add(External);
auto Attr = valueToAttr(External);
if (Attr.hasValue()) {
SetBuilder.noteAttributes(External, *Attr);
SetBuilder.addAttributesBelow(External, AttrUnknown);
}
}
for (auto *Escape : GraphBuilder.getEscapedValues()) {
SetBuilder.add(Escape);
SetBuilder.noteAttributes(Escape, AttrEscaped);
SetBuilder.addAttributesBelow(Escape, AttrUnknown);
}
return FunctionInfo(SetBuilder.build(), GraphBuilder.takeReturnValues());
}
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");
// Note that we can't do Cache[Fn] = buildSetsFrom(Fn) here: the function call
// may get evaluated after operator[], potentially triggering a DenseMap
// resize and invalidating the reference returned by operator[]
auto FunInfo = buildSetsFrom(Fn);
Cache[Fn] = std::move(FunInfo);
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;
// If both values are local (meaning the corresponding set has attribute
// AttrNone or AttrEscaped), then we know that CFLAA fully models them: they
// may-alias each other if and only if they are in the same set
// If at least one value is non-local (meaning it either is global/argument or
// it comes from unknown sources like integer cast), the situation becomes a
// bit more interesting. We follow three general rules described below:
// - Non-local values may alias each other
// - AttrNone values do not alias any non-local values
// - AttrEscaped do not alias globals/arguments, but they may alias
// AttrUnknown values
if (SetA.Index == SetB.Index)
return MayAlias;
if (AttrsA.none() || AttrsB.none())
return NoAlias;
if (isUnknownAttr(AttrsA) || isUnknownAttr(AttrsB))
return MayAlias;
if (isGlobalOrArgAttr(AttrsA) && isGlobalOrArgAttr(AttrsB))
return MayAlias;
return NoAlias;
}
char CFLAA::PassID;
CFLAAResult CFLAA::run(Function &F, AnalysisManager<Function> &AM) {
return CFLAAResult(AM.getResult<TargetLibraryAnalysis>(F));
}
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());
}
void CFLAAWrapperPass::initializePass() {
auto &TLIWP = getAnalysis<TargetLibraryInfoWrapperPass>();
Result.reset(new CFLAAResult(TLIWP.getTLI()));
}
void CFLAAWrapperPass::getAnalysisUsage(AnalysisUsage &AU) const {
AU.setPreservesAll();
AU.addRequired<TargetLibraryInfoWrapperPass>();
}
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