1 //- CFLSteensAliasAnalysis.cpp - Unification-based Alias Analysis ---*- C++-*-//
3 // The LLVM Compiler Infrastructure
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
8 //===----------------------------------------------------------------------===//
10 // This file implements a CFL-base, summary-based alias analysis algorithm. It
11 // does not depend on types. The algorithm is a mixture of the one described in
12 // "Demand-driven alias analysis for C" by Xin Zheng and Radu Rugina, and "Fast
13 // algorithms for Dyck-CFL-reachability with applications to Alias Analysis" by
14 // Zhang Q, Lyu M R, Yuan H, and Su Z. -- to summarize the papers, we build a
15 // graph of the uses of a variable, where each node is a memory location, and
16 // each edge is an action that happened on that memory location. The "actions"
17 // can be one of Dereference, Reference, or Assign. The precision of this
18 // analysis is roughly the same as that of an one level context-sensitive
19 // Steensgaard's algorithm.
21 // Two variables are considered as aliasing iff you can reach one value's node
22 // from the other value's node and the language formed by concatenating all of
23 // the edge labels (actions) conforms to a context-free grammar.
25 // Because this algorithm requires a graph search on each query, we execute the
26 // algorithm outlined in "Fast algorithms..." (mentioned above)
27 // in order to transform the graph into sets of variables that may alias in
28 // ~nlogn time (n = number of variables), which makes queries take constant
30 //===----------------------------------------------------------------------===//
32 // N.B. AliasAnalysis as a whole is phrased as a FunctionPass at the moment, and
33 // CFLSteensAA is interprocedural. This is *technically* A Bad Thing, because
34 // FunctionPasses are only allowed to inspect the Function that they're being
35 // run on. Realistically, this likely isn't a problem until we allow
36 // FunctionPasses to run concurrently.
38 #include "llvm/Analysis/CFLSteensAliasAnalysis.h"
40 #include "StratifiedSets.h"
41 #include "llvm/ADT/DenseMap.h"
42 #include "llvm/ADT/None.h"
43 #include "llvm/ADT/Optional.h"
44 #include "llvm/Analysis/TargetLibraryInfo.h"
45 #include "llvm/IR/Constants.h"
46 #include "llvm/IR/Function.h"
47 #include "llvm/Pass.h"
48 #include "llvm/Support/Compiler.h"
49 #include "llvm/Support/Debug.h"
50 #include "llvm/Support/ErrorHandling.h"
51 #include "llvm/Support/raw_ostream.h"
58 using namespace llvm::cflaa;
60 #define DEBUG_TYPE "cfl-steens-aa"
62 CFLSteensAAResult::CFLSteensAAResult(const TargetLibraryInfo &TLI)
63 : AAResultBase(), TLI(TLI) {}
64 CFLSteensAAResult::CFLSteensAAResult(CFLSteensAAResult &&Arg)
65 : AAResultBase(std::move(Arg)), TLI(Arg.TLI) {}
66 CFLSteensAAResult::~CFLSteensAAResult() {}
68 /// Information we have about a function and would like to keep around.
69 class CFLSteensAAResult::FunctionInfo {
70 StratifiedSets<InstantiatedValue> Sets;
74 FunctionInfo(Function &Fn, const SmallVectorImpl<Value *> &RetVals,
75 StratifiedSets<InstantiatedValue> S);
77 const StratifiedSets<InstantiatedValue> &getStratifiedSets() const {
80 const AliasSummary &getAliasSummary() const { return Summary; }
83 /// Try to go from a Value* to a Function*. Never returns nullptr.
84 static Optional<Function *> parentFunctionOfValue(Value *);
86 const StratifiedIndex StratifiedLink::SetSentinel =
87 std::numeric_limits<StratifiedIndex>::max();
89 //===----------------------------------------------------------------------===//
90 // Function declarations that require types defined in the namespace above
91 //===----------------------------------------------------------------------===//
93 /// Determines whether it would be pointless to add the given Value to our sets.
94 static bool canSkipAddingToSets(Value *Val);
96 static Optional<Function *> parentFunctionOfValue(Value *Val) {
97 if (auto *Inst = dyn_cast<Instruction>(Val)) {
98 auto *Bb = Inst->getParent();
99 return Bb->getParent();
102 if (auto *Arg = dyn_cast<Argument>(Val))
103 return Arg->getParent();
107 static bool canSkipAddingToSets(Value *Val) {
108 // Constants can share instances, which may falsely unify multiple
110 // store i32* null, i32** %ptr1
111 // store i32* null, i32** %ptr2
112 // clearly ptr1 and ptr2 should not be unified into the same set, so
113 // we should filter out the (potentially shared) instance to
115 if (isa<Constant>(Val)) {
116 // TODO: Because all of these things are constant, we can determine whether
117 // the data is *actually* mutable at graph building time. This will probably
118 // come for free/cheap with offset awareness.
119 bool CanStoreMutableData = isa<GlobalValue>(Val) ||
120 isa<ConstantExpr>(Val) ||
121 isa<ConstantAggregate>(Val);
122 return !CanStoreMutableData;
128 CFLSteensAAResult::FunctionInfo::FunctionInfo(
129 Function &Fn, const SmallVectorImpl<Value *> &RetVals,
130 StratifiedSets<InstantiatedValue> S)
131 : Sets(std::move(S)) {
132 // Historically, an arbitrary upper-bound of 50 args was selected. We may want
133 // to remove this if it doesn't really matter in practice.
134 if (Fn.arg_size() > MaxSupportedArgsInSummary)
137 DenseMap<StratifiedIndex, InterfaceValue> InterfaceMap;
139 // Our intention here is to record all InterfaceValues that share the same
140 // StratifiedIndex in RetParamRelations. For each valid InterfaceValue, we
141 // have its StratifiedIndex scanned here and check if the index is presented
142 // in InterfaceMap: if it is not, we add the correspondence to the map;
143 // otherwise, an aliasing relation is found and we add it to
144 // RetParamRelations.
146 auto AddToRetParamRelations = [&](unsigned InterfaceIndex,
147 StratifiedIndex SetIndex) {
150 InterfaceValue CurrValue{InterfaceIndex, Level};
152 auto Itr = InterfaceMap.find(SetIndex);
153 if (Itr != InterfaceMap.end()) {
154 if (CurrValue != Itr->second)
155 Summary.RetParamRelations.push_back(
156 ExternalRelation{CurrValue, Itr->second, UnknownOffset});
160 auto &Link = Sets.getLink(SetIndex);
161 InterfaceMap.insert(std::make_pair(SetIndex, CurrValue));
162 auto ExternalAttrs = getExternallyVisibleAttrs(Link.Attrs);
163 if (ExternalAttrs.any())
164 Summary.RetParamAttributes.push_back(
165 ExternalAttribute{CurrValue, ExternalAttrs});
167 if (!Link.hasBelow())
171 SetIndex = Link.Below;
175 // Populate RetParamRelations for return values
176 for (auto *RetVal : RetVals) {
177 assert(RetVal != nullptr);
178 assert(RetVal->getType()->isPointerTy());
179 auto RetInfo = Sets.find(InstantiatedValue{RetVal, 0});
180 if (RetInfo.hasValue())
181 AddToRetParamRelations(0, RetInfo->Index);
184 // Populate RetParamRelations for parameters
186 for (auto &Param : Fn.args()) {
187 if (Param.getType()->isPointerTy()) {
188 auto ParamInfo = Sets.find(InstantiatedValue{&Param, 0});
189 if (ParamInfo.hasValue())
190 AddToRetParamRelations(I + 1, ParamInfo->Index);
196 // Builds the graph + StratifiedSets for a function.
197 CFLSteensAAResult::FunctionInfo CFLSteensAAResult::buildSetsFrom(Function *Fn) {
198 CFLGraphBuilder<CFLSteensAAResult> GraphBuilder(*this, TLI, *Fn);
199 StratifiedSetsBuilder<InstantiatedValue> SetBuilder;
201 // Add all CFLGraph nodes and all Dereference edges to StratifiedSets
202 auto &Graph = GraphBuilder.getCFLGraph();
203 for (const auto &Mapping : Graph.value_mappings()) {
204 auto Val = Mapping.first;
205 if (canSkipAddingToSets(Val))
207 auto &ValueInfo = Mapping.second;
209 assert(ValueInfo.getNumLevels() > 0);
210 SetBuilder.add(InstantiatedValue{Val, 0});
211 SetBuilder.noteAttributes(InstantiatedValue{Val, 0},
212 ValueInfo.getNodeInfoAtLevel(0).Attr);
213 for (unsigned I = 0, E = ValueInfo.getNumLevels() - 1; I < E; ++I) {
214 SetBuilder.add(InstantiatedValue{Val, I + 1});
215 SetBuilder.noteAttributes(InstantiatedValue{Val, I + 1},
216 ValueInfo.getNodeInfoAtLevel(I + 1).Attr);
217 SetBuilder.addBelow(InstantiatedValue{Val, I},
218 InstantiatedValue{Val, I + 1});
222 // Add all assign edges to StratifiedSets
223 for (const auto &Mapping : Graph.value_mappings()) {
224 auto Val = Mapping.first;
225 if (canSkipAddingToSets(Val))
227 auto &ValueInfo = Mapping.second;
229 for (unsigned I = 0, E = ValueInfo.getNumLevels(); I < E; ++I) {
230 auto Src = InstantiatedValue{Val, I};
231 for (auto &Edge : ValueInfo.getNodeInfoAtLevel(I).Edges)
232 SetBuilder.addWith(Src, Edge.Other);
236 return FunctionInfo(*Fn, GraphBuilder.getReturnValues(), SetBuilder.build());
239 void CFLSteensAAResult::scan(Function *Fn) {
240 auto InsertPair = Cache.insert(std::make_pair(Fn, Optional<FunctionInfo>()));
242 assert(InsertPair.second &&
243 "Trying to scan a function that has already been cached");
245 // Note that we can't do Cache[Fn] = buildSetsFrom(Fn) here: the function call
246 // may get evaluated after operator[], potentially triggering a DenseMap
247 // resize and invalidating the reference returned by operator[]
248 auto FunInfo = buildSetsFrom(Fn);
249 Cache[Fn] = std::move(FunInfo);
251 Handles.push_front(FunctionHandle(Fn, this));
254 void CFLSteensAAResult::evict(Function *Fn) { Cache.erase(Fn); }
256 /// Ensures that the given function is available in the cache, and returns the
258 const Optional<CFLSteensAAResult::FunctionInfo> &
259 CFLSteensAAResult::ensureCached(Function *Fn) {
260 auto Iter = Cache.find(Fn);
261 if (Iter == Cache.end()) {
263 Iter = Cache.find(Fn);
264 assert(Iter != Cache.end());
265 assert(Iter->second.hasValue());
270 const AliasSummary *CFLSteensAAResult::getAliasSummary(Function &Fn) {
271 auto &FunInfo = ensureCached(&Fn);
272 if (FunInfo.hasValue())
273 return &FunInfo->getAliasSummary();
278 AliasResult CFLSteensAAResult::query(const MemoryLocation &LocA,
279 const MemoryLocation &LocB) {
280 auto *ValA = const_cast<Value *>(LocA.Ptr);
281 auto *ValB = const_cast<Value *>(LocB.Ptr);
283 if (!ValA->getType()->isPointerTy() || !ValB->getType()->isPointerTy())
286 Function *Fn = nullptr;
287 auto MaybeFnA = parentFunctionOfValue(ValA);
288 auto MaybeFnB = parentFunctionOfValue(ValB);
289 if (!MaybeFnA.hasValue() && !MaybeFnB.hasValue()) {
290 // The only times this is known to happen are when globals + InlineAsm are
293 << "CFLSteensAA: could not extract parent function information.\n");
297 if (MaybeFnA.hasValue()) {
299 assert((!MaybeFnB.hasValue() || *MaybeFnB == *MaybeFnA) &&
300 "Interprocedural queries not supported");
305 assert(Fn != nullptr);
306 auto &MaybeInfo = ensureCached(Fn);
307 assert(MaybeInfo.hasValue());
309 auto &Sets = MaybeInfo->getStratifiedSets();
310 auto MaybeA = Sets.find(InstantiatedValue{ValA, 0});
311 if (!MaybeA.hasValue())
314 auto MaybeB = Sets.find(InstantiatedValue{ValB, 0});
315 if (!MaybeB.hasValue())
320 auto AttrsA = Sets.getLink(SetA.Index).Attrs;
321 auto AttrsB = Sets.getLink(SetB.Index).Attrs;
323 // If both values are local (meaning the corresponding set has attribute
324 // AttrNone or AttrEscaped), then we know that CFLSteensAA fully models them:
325 // they may-alias each other if and only if they are in the same set.
326 // If at least one value is non-local (meaning it either is global/argument or
327 // it comes from unknown sources like integer cast), the situation becomes a
328 // bit more interesting. We follow three general rules described below:
329 // - Non-local values may alias each other
330 // - AttrNone values do not alias any non-local values
331 // - AttrEscaped do not alias globals/arguments, but they may alias
332 // AttrUnknown values
333 if (SetA.Index == SetB.Index)
335 if (AttrsA.none() || AttrsB.none())
337 if (hasUnknownOrCallerAttr(AttrsA) || hasUnknownOrCallerAttr(AttrsB))
339 if (isGlobalOrArgAttr(AttrsA) && isGlobalOrArgAttr(AttrsB))
344 AnalysisKey CFLSteensAA::Key;
346 CFLSteensAAResult CFLSteensAA::run(Function &F, FunctionAnalysisManager &AM) {
347 return CFLSteensAAResult(AM.getResult<TargetLibraryAnalysis>(F));
350 char CFLSteensAAWrapperPass::ID = 0;
351 INITIALIZE_PASS(CFLSteensAAWrapperPass, "cfl-steens-aa",
352 "Unification-Based CFL Alias Analysis", false, true)
354 ImmutablePass *llvm::createCFLSteensAAWrapperPass() {
355 return new CFLSteensAAWrapperPass();
358 CFLSteensAAWrapperPass::CFLSteensAAWrapperPass() : ImmutablePass(ID) {
359 initializeCFLSteensAAWrapperPassPass(*PassRegistry::getPassRegistry());
362 void CFLSteensAAWrapperPass::initializePass() {
363 auto &TLIWP = getAnalysis<TargetLibraryInfoWrapperPass>();
364 Result.reset(new CFLSteensAAResult(TLIWP.getTLI()));
367 void CFLSteensAAWrapperPass::getAnalysisUsage(AnalysisUsage &AU) const {
368 AU.setPreservesAll();
369 AU.addRequired<TargetLibraryInfoWrapperPass>();