1 //===- GlobalsModRef.cpp - Simple Mod/Ref Analysis for Globals ------------===//
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 simple pass provides alias and mod/ref information for global values
11 // that do not have their address taken, and keeps track of whether functions
12 // read or write memory (are "pure"). For this simple (but very common) case,
13 // we can provide pretty accurate and useful information.
15 //===----------------------------------------------------------------------===//
17 #include "llvm/Analysis/GlobalsModRef.h"
18 #include "llvm/ADT/SCCIterator.h"
19 #include "llvm/ADT/SmallPtrSet.h"
20 #include "llvm/ADT/Statistic.h"
21 #include "llvm/Analysis/MemoryBuiltins.h"
22 #include "llvm/Analysis/TargetLibraryInfo.h"
23 #include "llvm/Analysis/ValueTracking.h"
24 #include "llvm/IR/DerivedTypes.h"
25 #include "llvm/IR/InstIterator.h"
26 #include "llvm/IR/Instructions.h"
27 #include "llvm/IR/IntrinsicInst.h"
28 #include "llvm/IR/Module.h"
29 #include "llvm/Pass.h"
30 #include "llvm/Support/CommandLine.h"
33 #define DEBUG_TYPE "globalsmodref-aa"
35 STATISTIC(NumNonAddrTakenGlobalVars,
36 "Number of global vars without address taken");
37 STATISTIC(NumNonAddrTakenFunctions,"Number of functions without address taken");
38 STATISTIC(NumNoMemFunctions, "Number of functions that do not access memory");
39 STATISTIC(NumReadMemFunctions, "Number of functions that only read memory");
40 STATISTIC(NumIndirectGlobalVars, "Number of indirect global objects");
42 // An option to enable unsafe alias results from the GlobalsModRef analysis.
43 // When enabled, GlobalsModRef will provide no-alias results which in extremely
44 // rare cases may not be conservatively correct. In particular, in the face of
45 // transforms which cause assymetry between how effective GetUnderlyingObject
46 // is for two pointers, it may produce incorrect results.
48 // These unsafe results have been returned by GMR for many years without
49 // causing significant issues in the wild and so we provide a mechanism to
50 // re-enable them for users of LLVM that have a particular performance
51 // sensitivity and no known issues. The option also makes it easy to evaluate
52 // the performance impact of these results.
53 static cl::opt<bool> EnableUnsafeGlobalsModRefAliasResults(
54 "enable-unsafe-globalsmodref-alias-results", cl::init(false), cl::Hidden);
56 /// The mod/ref information collected for a particular function.
58 /// We collect information about mod/ref behavior of a function here, both in
59 /// general and as pertains to specific globals. We only have this detailed
60 /// information when we know *something* useful about the behavior. If we
61 /// saturate to fully general mod/ref, we remove the info for the function.
62 class GlobalsAAResult::FunctionInfo {
63 typedef SmallDenseMap<const GlobalValue *, ModRefInfo, 16> GlobalInfoMapType;
65 /// Build a wrapper struct that has 8-byte alignment. All heap allocations
66 /// should provide this much alignment at least, but this makes it clear we
67 /// specifically rely on this amount of alignment.
68 struct LLVM_ALIGNAS(8) AlignedMap {
70 AlignedMap(const AlignedMap &Arg) : Map(Arg.Map) {}
71 GlobalInfoMapType Map;
74 /// Pointer traits for our aligned map.
75 struct AlignedMapPointerTraits {
76 static inline void *getAsVoidPointer(AlignedMap *P) { return P; }
77 static inline AlignedMap *getFromVoidPointer(void *P) {
78 return (AlignedMap *)P;
80 enum { NumLowBitsAvailable = 3 };
81 static_assert(alignof(AlignedMap) >= (1 << NumLowBitsAvailable),
82 "AlignedMap insufficiently aligned to have enough low bits.");
85 /// The bit that flags that this function may read any global. This is
86 /// chosen to mix together with ModRefInfo bits.
87 /// FIXME: This assumes ModRefInfo lattice will remain 4 bits!
88 /// It overlaps with ModRefInfo::Must bit!
89 /// FunctionInfo.getModRefInfo() masks out everything except ModRef so
90 /// this remains correct, but the Must info is lost.
91 enum { MayReadAnyGlobal = 4 };
93 /// Checks to document the invariants of the bit packing here.
94 static_assert((MayReadAnyGlobal & static_cast<int>(ModRefInfo::MustModRef)) ==
96 "ModRef and the MayReadAnyGlobal flag bits overlap.");
97 static_assert(((MayReadAnyGlobal |
98 static_cast<int>(ModRefInfo::MustModRef)) >>
99 AlignedMapPointerTraits::NumLowBitsAvailable) == 0,
100 "Insufficient low bits to store our flag and ModRef info.");
103 FunctionInfo() : Info() {}
105 delete Info.getPointer();
107 // Spell out the copy ond move constructors and assignment operators to get
108 // deep copy semantics and correct move semantics in the face of the
110 FunctionInfo(const FunctionInfo &Arg)
111 : Info(nullptr, Arg.Info.getInt()) {
112 if (const auto *ArgPtr = Arg.Info.getPointer())
113 Info.setPointer(new AlignedMap(*ArgPtr));
115 FunctionInfo(FunctionInfo &&Arg)
116 : Info(Arg.Info.getPointer(), Arg.Info.getInt()) {
117 Arg.Info.setPointerAndInt(nullptr, 0);
119 FunctionInfo &operator=(const FunctionInfo &RHS) {
120 delete Info.getPointer();
121 Info.setPointerAndInt(nullptr, RHS.Info.getInt());
122 if (const auto *RHSPtr = RHS.Info.getPointer())
123 Info.setPointer(new AlignedMap(*RHSPtr));
126 FunctionInfo &operator=(FunctionInfo &&RHS) {
127 delete Info.getPointer();
128 Info.setPointerAndInt(RHS.Info.getPointer(), RHS.Info.getInt());
129 RHS.Info.setPointerAndInt(nullptr, 0);
133 /// This method clears MayReadAnyGlobal bit added by GlobalsAAResult to return
134 /// the corresponding ModRefInfo. It must align in functionality with
136 ModRefInfo globalClearMayReadAnyGlobal(int I) const {
137 return ModRefInfo((I & static_cast<int>(ModRefInfo::ModRef)) |
138 static_cast<int>(ModRefInfo::NoModRef));
141 /// Returns the \c ModRefInfo info for this function.
142 ModRefInfo getModRefInfo() const {
143 return globalClearMayReadAnyGlobal(Info.getInt());
146 /// Adds new \c ModRefInfo for this function to its state.
147 void addModRefInfo(ModRefInfo NewMRI) {
148 Info.setInt(Info.getInt() | static_cast<int>(setMust(NewMRI)));
151 /// Returns whether this function may read any global variable, and we don't
152 /// know which global.
153 bool mayReadAnyGlobal() const { return Info.getInt() & MayReadAnyGlobal; }
155 /// Sets this function as potentially reading from any global.
156 void setMayReadAnyGlobal() { Info.setInt(Info.getInt() | MayReadAnyGlobal); }
158 /// Returns the \c ModRefInfo info for this function w.r.t. a particular
159 /// global, which may be more precise than the general information above.
160 ModRefInfo getModRefInfoForGlobal(const GlobalValue &GV) const {
161 ModRefInfo GlobalMRI =
162 mayReadAnyGlobal() ? ModRefInfo::Ref : ModRefInfo::NoModRef;
163 if (AlignedMap *P = Info.getPointer()) {
164 auto I = P->Map.find(&GV);
165 if (I != P->Map.end())
166 GlobalMRI = unionModRef(GlobalMRI, I->second);
171 /// Add mod/ref info from another function into ours, saturating towards
173 void addFunctionInfo(const FunctionInfo &FI) {
174 addModRefInfo(FI.getModRefInfo());
176 if (FI.mayReadAnyGlobal())
177 setMayReadAnyGlobal();
179 if (AlignedMap *P = FI.Info.getPointer())
180 for (const auto &G : P->Map)
181 addModRefInfoForGlobal(*G.first, G.second);
184 void addModRefInfoForGlobal(const GlobalValue &GV, ModRefInfo NewMRI) {
185 AlignedMap *P = Info.getPointer();
187 P = new AlignedMap();
190 auto &GlobalMRI = P->Map[&GV];
191 GlobalMRI = unionModRef(GlobalMRI, NewMRI);
194 /// Clear a global's ModRef info. Should be used when a global is being
196 void eraseModRefInfoForGlobal(const GlobalValue &GV) {
197 if (AlignedMap *P = Info.getPointer())
202 /// All of the information is encoded into a single pointer, with a three bit
203 /// integer in the low three bits. The high bit provides a flag for when this
204 /// function may read any global. The low two bits are the ModRefInfo. And
205 /// the pointer, when non-null, points to a map from GlobalValue to
206 /// ModRefInfo specific to that GlobalValue.
207 PointerIntPair<AlignedMap *, 3, unsigned, AlignedMapPointerTraits> Info;
210 void GlobalsAAResult::DeletionCallbackHandle::deleted() {
211 Value *V = getValPtr();
212 if (auto *F = dyn_cast<Function>(V))
213 GAR->FunctionInfos.erase(F);
215 if (GlobalValue *GV = dyn_cast<GlobalValue>(V)) {
216 if (GAR->NonAddressTakenGlobals.erase(GV)) {
217 // This global might be an indirect global. If so, remove it and
218 // remove any AllocRelatedValues for it.
219 if (GAR->IndirectGlobals.erase(GV)) {
220 // Remove any entries in AllocsForIndirectGlobals for this global.
221 for (auto I = GAR->AllocsForIndirectGlobals.begin(),
222 E = GAR->AllocsForIndirectGlobals.end();
225 GAR->AllocsForIndirectGlobals.erase(I);
228 // Scan the function info we have collected and remove this global
230 for (auto &FIPair : GAR->FunctionInfos)
231 FIPair.second.eraseModRefInfoForGlobal(*GV);
235 // If this is an allocation related to an indirect global, remove it.
236 GAR->AllocsForIndirectGlobals.erase(V);
238 // And clear out the handle.
240 GAR->Handles.erase(I);
241 // This object is now destroyed!
244 FunctionModRefBehavior GlobalsAAResult::getModRefBehavior(const Function *F) {
245 FunctionModRefBehavior Min = FMRB_UnknownModRefBehavior;
247 if (FunctionInfo *FI = getFunctionInfo(F)) {
248 if (!isModOrRefSet(FI->getModRefInfo()))
249 Min = FMRB_DoesNotAccessMemory;
250 else if (!isModSet(FI->getModRefInfo()))
251 Min = FMRB_OnlyReadsMemory;
254 return FunctionModRefBehavior(AAResultBase::getModRefBehavior(F) & Min);
257 FunctionModRefBehavior
258 GlobalsAAResult::getModRefBehavior(ImmutableCallSite CS) {
259 FunctionModRefBehavior Min = FMRB_UnknownModRefBehavior;
261 if (!CS.hasOperandBundles())
262 if (const Function *F = CS.getCalledFunction())
263 if (FunctionInfo *FI = getFunctionInfo(F)) {
264 if (!isModOrRefSet(FI->getModRefInfo()))
265 Min = FMRB_DoesNotAccessMemory;
266 else if (!isModSet(FI->getModRefInfo()))
267 Min = FMRB_OnlyReadsMemory;
270 return FunctionModRefBehavior(AAResultBase::getModRefBehavior(CS) & Min);
273 /// Returns the function info for the function, or null if we don't have
274 /// anything useful to say about it.
275 GlobalsAAResult::FunctionInfo *
276 GlobalsAAResult::getFunctionInfo(const Function *F) {
277 auto I = FunctionInfos.find(F);
278 if (I != FunctionInfos.end())
283 /// AnalyzeGlobals - Scan through the users of all of the internal
284 /// GlobalValue's in the program. If none of them have their "address taken"
285 /// (really, their address passed to something nontrivial), record this fact,
286 /// and record the functions that they are used directly in.
287 void GlobalsAAResult::AnalyzeGlobals(Module &M) {
288 SmallPtrSet<Function *, 32> TrackedFunctions;
289 for (Function &F : M)
290 if (F.hasLocalLinkage())
291 if (!AnalyzeUsesOfPointer(&F)) {
292 // Remember that we are tracking this global.
293 NonAddressTakenGlobals.insert(&F);
294 TrackedFunctions.insert(&F);
295 Handles.emplace_front(*this, &F);
296 Handles.front().I = Handles.begin();
297 ++NumNonAddrTakenFunctions;
300 SmallPtrSet<Function *, 16> Readers, Writers;
301 for (GlobalVariable &GV : M.globals())
302 if (GV.hasLocalLinkage()) {
303 if (!AnalyzeUsesOfPointer(&GV, &Readers,
304 GV.isConstant() ? nullptr : &Writers)) {
305 // Remember that we are tracking this global, and the mod/ref fns
306 NonAddressTakenGlobals.insert(&GV);
307 Handles.emplace_front(*this, &GV);
308 Handles.front().I = Handles.begin();
310 for (Function *Reader : Readers) {
311 if (TrackedFunctions.insert(Reader).second) {
312 Handles.emplace_front(*this, Reader);
313 Handles.front().I = Handles.begin();
315 FunctionInfos[Reader].addModRefInfoForGlobal(GV, ModRefInfo::Ref);
318 if (!GV.isConstant()) // No need to keep track of writers to constants
319 for (Function *Writer : Writers) {
320 if (TrackedFunctions.insert(Writer).second) {
321 Handles.emplace_front(*this, Writer);
322 Handles.front().I = Handles.begin();
324 FunctionInfos[Writer].addModRefInfoForGlobal(GV, ModRefInfo::Mod);
326 ++NumNonAddrTakenGlobalVars;
328 // If this global holds a pointer type, see if it is an indirect global.
329 if (GV.getValueType()->isPointerTy() &&
330 AnalyzeIndirectGlobalMemory(&GV))
331 ++NumIndirectGlobalVars;
338 /// AnalyzeUsesOfPointer - Look at all of the users of the specified pointer.
339 /// If this is used by anything complex (i.e., the address escapes), return
340 /// true. Also, while we are at it, keep track of those functions that read and
341 /// write to the value.
343 /// If OkayStoreDest is non-null, stores into this global are allowed.
344 bool GlobalsAAResult::AnalyzeUsesOfPointer(Value *V,
345 SmallPtrSetImpl<Function *> *Readers,
346 SmallPtrSetImpl<Function *> *Writers,
347 GlobalValue *OkayStoreDest) {
348 if (!V->getType()->isPointerTy())
351 for (Use &U : V->uses()) {
352 User *I = U.getUser();
353 if (LoadInst *LI = dyn_cast<LoadInst>(I)) {
355 Readers->insert(LI->getParent()->getParent());
356 } else if (StoreInst *SI = dyn_cast<StoreInst>(I)) {
357 if (V == SI->getOperand(1)) {
359 Writers->insert(SI->getParent()->getParent());
360 } else if (SI->getOperand(1) != OkayStoreDest) {
361 return true; // Storing the pointer
363 } else if (Operator::getOpcode(I) == Instruction::GetElementPtr) {
364 if (AnalyzeUsesOfPointer(I, Readers, Writers))
366 } else if (Operator::getOpcode(I) == Instruction::BitCast) {
367 if (AnalyzeUsesOfPointer(I, Readers, Writers, OkayStoreDest))
369 } else if (auto CS = CallSite(I)) {
370 // Make sure that this is just the function being called, not that it is
371 // passing into the function.
372 if (CS.isDataOperand(&U)) {
373 // Detect calls to free.
374 if (CS.isArgOperand(&U) && isFreeCall(I, &TLI)) {
376 Writers->insert(CS->getParent()->getParent());
378 return true; // Argument of an unknown call.
381 } else if (ICmpInst *ICI = dyn_cast<ICmpInst>(I)) {
382 if (!isa<ConstantPointerNull>(ICI->getOperand(1)))
383 return true; // Allow comparison against null.
384 } else if (Constant *C = dyn_cast<Constant>(I)) {
385 // Ignore constants which don't have any live uses.
386 if (isa<GlobalValue>(C) || C->isConstantUsed())
396 /// AnalyzeIndirectGlobalMemory - We found an non-address-taken global variable
397 /// which holds a pointer type. See if the global always points to non-aliased
398 /// heap memory: that is, all initializers of the globals are allocations, and
399 /// those allocations have no use other than initialization of the global.
400 /// Further, all loads out of GV must directly use the memory, not store the
401 /// pointer somewhere. If this is true, we consider the memory pointed to by
402 /// GV to be owned by GV and can disambiguate other pointers from it.
403 bool GlobalsAAResult::AnalyzeIndirectGlobalMemory(GlobalVariable *GV) {
404 // Keep track of values related to the allocation of the memory, f.e. the
405 // value produced by the malloc call and any casts.
406 std::vector<Value *> AllocRelatedValues;
408 // If the initializer is a valid pointer, bail.
409 if (Constant *C = GV->getInitializer())
410 if (!C->isNullValue())
413 // Walk the user list of the global. If we find anything other than a direct
414 // load or store, bail out.
415 for (User *U : GV->users()) {
416 if (LoadInst *LI = dyn_cast<LoadInst>(U)) {
417 // The pointer loaded from the global can only be used in simple ways:
418 // we allow addressing of it and loading storing to it. We do *not* allow
419 // storing the loaded pointer somewhere else or passing to a function.
420 if (AnalyzeUsesOfPointer(LI))
421 return false; // Loaded pointer escapes.
422 // TODO: Could try some IP mod/ref of the loaded pointer.
423 } else if (StoreInst *SI = dyn_cast<StoreInst>(U)) {
424 // Storing the global itself.
425 if (SI->getOperand(0) == GV)
428 // If storing the null pointer, ignore it.
429 if (isa<ConstantPointerNull>(SI->getOperand(0)))
432 // Check the value being stored.
433 Value *Ptr = GetUnderlyingObject(SI->getOperand(0),
434 GV->getParent()->getDataLayout());
436 if (!isAllocLikeFn(Ptr, &TLI))
437 return false; // Too hard to analyze.
439 // Analyze all uses of the allocation. If any of them are used in a
440 // non-simple way (e.g. stored to another global) bail out.
441 if (AnalyzeUsesOfPointer(Ptr, /*Readers*/ nullptr, /*Writers*/ nullptr,
443 return false; // Loaded pointer escapes.
445 // Remember that this allocation is related to the indirect global.
446 AllocRelatedValues.push_back(Ptr);
448 // Something complex, bail out.
453 // Okay, this is an indirect global. Remember all of the allocations for
454 // this global in AllocsForIndirectGlobals.
455 while (!AllocRelatedValues.empty()) {
456 AllocsForIndirectGlobals[AllocRelatedValues.back()] = GV;
457 Handles.emplace_front(*this, AllocRelatedValues.back());
458 Handles.front().I = Handles.begin();
459 AllocRelatedValues.pop_back();
461 IndirectGlobals.insert(GV);
462 Handles.emplace_front(*this, GV);
463 Handles.front().I = Handles.begin();
467 void GlobalsAAResult::CollectSCCMembership(CallGraph &CG) {
468 // We do a bottom-up SCC traversal of the call graph. In other words, we
469 // visit all callees before callers (leaf-first).
471 for (scc_iterator<CallGraph *> I = scc_begin(&CG); !I.isAtEnd(); ++I) {
472 const std::vector<CallGraphNode *> &SCC = *I;
473 assert(!SCC.empty() && "SCC with no functions?");
475 for (auto *CGN : SCC)
476 if (Function *F = CGN->getFunction())
477 FunctionToSCCMap[F] = SCCID;
482 /// AnalyzeCallGraph - At this point, we know the functions where globals are
483 /// immediately stored to and read from. Propagate this information up the call
484 /// graph to all callers and compute the mod/ref info for all memory for each
486 void GlobalsAAResult::AnalyzeCallGraph(CallGraph &CG, Module &M) {
487 // We do a bottom-up SCC traversal of the call graph. In other words, we
488 // visit all callees before callers (leaf-first).
489 for (scc_iterator<CallGraph *> I = scc_begin(&CG); !I.isAtEnd(); ++I) {
490 const std::vector<CallGraphNode *> &SCC = *I;
491 assert(!SCC.empty() && "SCC with no functions?");
493 Function *F = SCC[0]->getFunction();
495 if (!F || !F->isDefinitionExact()) {
496 // Calls externally or not exact - can't say anything useful. Remove any
497 // existing function records (may have been created when scanning
499 for (auto *Node : SCC)
500 FunctionInfos.erase(Node->getFunction());
504 FunctionInfo &FI = FunctionInfos[F];
505 Handles.emplace_front(*this, F);
506 Handles.front().I = Handles.begin();
507 bool KnowNothing = false;
509 // Collect the mod/ref properties due to called functions. We only compute
511 for (unsigned i = 0, e = SCC.size(); i != e && !KnowNothing; ++i) {
517 if (F->isDeclaration() || F->hasFnAttribute(Attribute::OptimizeNone)) {
518 // Try to get mod/ref behaviour from function attributes.
519 if (F->doesNotAccessMemory()) {
520 // Can't do better than that!
521 } else if (F->onlyReadsMemory()) {
522 FI.addModRefInfo(ModRefInfo::Ref);
523 if (!F->isIntrinsic() && !F->onlyAccessesArgMemory())
524 // This function might call back into the module and read a global -
525 // consider every global as possibly being read by this function.
526 FI.setMayReadAnyGlobal();
528 FI.addModRefInfo(ModRefInfo::ModRef);
529 // Can't say anything useful unless it's an intrinsic - they don't
530 // read or write global variables of the kind considered here.
531 KnowNothing = !F->isIntrinsic();
536 for (CallGraphNode::iterator CI = SCC[i]->begin(), E = SCC[i]->end();
537 CI != E && !KnowNothing; ++CI)
538 if (Function *Callee = CI->second->getFunction()) {
539 if (FunctionInfo *CalleeFI = getFunctionInfo(Callee)) {
540 // Propagate function effect up.
541 FI.addFunctionInfo(*CalleeFI);
543 // Can't say anything about it. However, if it is inside our SCC,
544 // then nothing needs to be done.
545 CallGraphNode *CalleeNode = CG[Callee];
546 if (!is_contained(SCC, CalleeNode))
554 // If we can't say anything useful about this SCC, remove all SCC functions
555 // from the FunctionInfos map.
557 for (auto *Node : SCC)
558 FunctionInfos.erase(Node->getFunction());
562 // Scan the function bodies for explicit loads or stores.
563 for (auto *Node : SCC) {
564 if (isModAndRefSet(FI.getModRefInfo()))
565 break; // The mod/ref lattice saturates here.
567 // Don't prove any properties based on the implementation of an optnone
568 // function. Function attributes were already used as a best approximation
570 if (Node->getFunction()->hasFnAttribute(Attribute::OptimizeNone))
573 for (Instruction &I : instructions(Node->getFunction())) {
574 if (isModAndRefSet(FI.getModRefInfo()))
575 break; // The mod/ref lattice saturates here.
577 // We handle calls specially because the graph-relevant aspects are
579 if (auto CS = CallSite(&I)) {
580 if (isAllocationFn(&I, &TLI) || isFreeCall(&I, &TLI)) {
581 // FIXME: It is completely unclear why this is necessary and not
582 // handled by the above graph code.
583 FI.addModRefInfo(ModRefInfo::ModRef);
584 } else if (Function *Callee = CS.getCalledFunction()) {
585 // The callgraph doesn't include intrinsic calls.
586 if (Callee->isIntrinsic()) {
587 FunctionModRefBehavior Behaviour =
588 AAResultBase::getModRefBehavior(Callee);
589 FI.addModRefInfo(createModRefInfo(Behaviour));
595 // All non-call instructions we use the primary predicates for whether
596 // thay read or write memory.
597 if (I.mayReadFromMemory())
598 FI.addModRefInfo(ModRefInfo::Ref);
599 if (I.mayWriteToMemory())
600 FI.addModRefInfo(ModRefInfo::Mod);
604 if (!isModSet(FI.getModRefInfo()))
605 ++NumReadMemFunctions;
606 if (!isModOrRefSet(FI.getModRefInfo()))
609 // Finally, now that we know the full effect on this SCC, clone the
610 // information to each function in the SCC.
611 // FI is a reference into FunctionInfos, so copy it now so that it doesn't
612 // get invalidated if DenseMap decides to re-hash.
613 FunctionInfo CachedFI = FI;
614 for (unsigned i = 1, e = SCC.size(); i != e; ++i)
615 FunctionInfos[SCC[i]->getFunction()] = CachedFI;
619 // GV is a non-escaping global. V is a pointer address that has been loaded from.
620 // If we can prove that V must escape, we can conclude that a load from V cannot
622 static bool isNonEscapingGlobalNoAliasWithLoad(const GlobalValue *GV,
625 const DataLayout &DL) {
626 SmallPtrSet<const Value *, 8> Visited;
627 SmallVector<const Value *, 8> Inputs;
631 const Value *Input = Inputs.pop_back_val();
633 if (isa<GlobalValue>(Input) || isa<Argument>(Input) || isa<CallInst>(Input) ||
634 isa<InvokeInst>(Input))
635 // Arguments to functions or returns from functions are inherently
636 // escaping, so we can immediately classify those as not aliasing any
637 // non-addr-taken globals.
639 // (Transitive) loads from a global are also safe - if this aliased
640 // another global, its address would escape, so no alias.
643 // Recurse through a limited number of selects, loads and PHIs. This is an
644 // arbitrary depth of 4, lower numbers could be used to fix compile time
645 // issues if needed, but this is generally expected to be only be important
650 if (auto *LI = dyn_cast<LoadInst>(Input)) {
651 Inputs.push_back(GetUnderlyingObject(LI->getPointerOperand(), DL));
654 if (auto *SI = dyn_cast<SelectInst>(Input)) {
655 const Value *LHS = GetUnderlyingObject(SI->getTrueValue(), DL);
656 const Value *RHS = GetUnderlyingObject(SI->getFalseValue(), DL);
657 if (Visited.insert(LHS).second)
658 Inputs.push_back(LHS);
659 if (Visited.insert(RHS).second)
660 Inputs.push_back(RHS);
663 if (auto *PN = dyn_cast<PHINode>(Input)) {
664 for (const Value *Op : PN->incoming_values()) {
665 Op = GetUnderlyingObject(Op, DL);
666 if (Visited.insert(Op).second)
667 Inputs.push_back(Op);
673 } while (!Inputs.empty());
675 // All inputs were known to be no-alias.
679 // There are particular cases where we can conclude no-alias between
680 // a non-addr-taken global and some other underlying object. Specifically,
681 // a non-addr-taken global is known to not be escaped from any function. It is
682 // also incorrect for a transformation to introduce an escape of a global in
683 // a way that is observable when it was not there previously. One function
684 // being transformed to introduce an escape which could possibly be observed
685 // (via loading from a global or the return value for example) within another
686 // function is never safe. If the observation is made through non-atomic
687 // operations on different threads, it is a data-race and UB. If the
688 // observation is well defined, by being observed the transformation would have
689 // changed program behavior by introducing the observed escape, making it an
690 // invalid transform.
692 // This property does require that transformations which *temporarily* escape
693 // a global that was not previously escaped, prior to restoring it, cannot rely
694 // on the results of GMR::alias. This seems a reasonable restriction, although
695 // currently there is no way to enforce it. There is also no realistic
696 // optimization pass that would make this mistake. The closest example is
697 // a transformation pass which does reg2mem of SSA values but stores them into
698 // global variables temporarily before restoring the global variable's value.
699 // This could be useful to expose "benign" races for example. However, it seems
700 // reasonable to require that a pass which introduces escapes of global
701 // variables in this way to either not trust AA results while the escape is
702 // active, or to be forced to operate as a module pass that cannot co-exist
703 // with an alias analysis such as GMR.
704 bool GlobalsAAResult::isNonEscapingGlobalNoAlias(const GlobalValue *GV,
706 // In order to know that the underlying object cannot alias the
707 // non-addr-taken global, we must know that it would have to be an escape.
708 // Thus if the underlying object is a function argument, a load from
709 // a global, or the return of a function, it cannot alias. We can also
710 // recurse through PHI nodes and select nodes provided all of their inputs
711 // resolve to one of these known-escaping roots.
712 SmallPtrSet<const Value *, 8> Visited;
713 SmallVector<const Value *, 8> Inputs;
718 const Value *Input = Inputs.pop_back_val();
720 if (auto *InputGV = dyn_cast<GlobalValue>(Input)) {
721 // If one input is the very global we're querying against, then we can't
722 // conclude no-alias.
726 // Distinct GlobalVariables never alias, unless overriden or zero-sized.
727 // FIXME: The condition can be refined, but be conservative for now.
728 auto *GVar = dyn_cast<GlobalVariable>(GV);
729 auto *InputGVar = dyn_cast<GlobalVariable>(InputGV);
730 if (GVar && InputGVar &&
731 !GVar->isDeclaration() && !InputGVar->isDeclaration() &&
732 !GVar->isInterposable() && !InputGVar->isInterposable()) {
733 Type *GVType = GVar->getInitializer()->getType();
734 Type *InputGVType = InputGVar->getInitializer()->getType();
735 if (GVType->isSized() && InputGVType->isSized() &&
736 (DL.getTypeAllocSize(GVType) > 0) &&
737 (DL.getTypeAllocSize(InputGVType) > 0))
741 // Conservatively return false, even though we could be smarter
742 // (e.g. look through GlobalAliases).
746 if (isa<Argument>(Input) || isa<CallInst>(Input) ||
747 isa<InvokeInst>(Input)) {
748 // Arguments to functions or returns from functions are inherently
749 // escaping, so we can immediately classify those as not aliasing any
750 // non-addr-taken globals.
754 // Recurse through a limited number of selects, loads and PHIs. This is an
755 // arbitrary depth of 4, lower numbers could be used to fix compile time
756 // issues if needed, but this is generally expected to be only be important
761 if (auto *LI = dyn_cast<LoadInst>(Input)) {
762 // A pointer loaded from a global would have been captured, and we know
763 // that the global is non-escaping, so no alias.
764 const Value *Ptr = GetUnderlyingObject(LI->getPointerOperand(), DL);
765 if (isNonEscapingGlobalNoAliasWithLoad(GV, Ptr, Depth, DL))
766 // The load does not alias with GV.
768 // Otherwise, a load could come from anywhere, so bail.
771 if (auto *SI = dyn_cast<SelectInst>(Input)) {
772 const Value *LHS = GetUnderlyingObject(SI->getTrueValue(), DL);
773 const Value *RHS = GetUnderlyingObject(SI->getFalseValue(), DL);
774 if (Visited.insert(LHS).second)
775 Inputs.push_back(LHS);
776 if (Visited.insert(RHS).second)
777 Inputs.push_back(RHS);
780 if (auto *PN = dyn_cast<PHINode>(Input)) {
781 for (const Value *Op : PN->incoming_values()) {
782 Op = GetUnderlyingObject(Op, DL);
783 if (Visited.insert(Op).second)
784 Inputs.push_back(Op);
789 // FIXME: It would be good to handle other obvious no-alias cases here, but
790 // it isn't clear how to do so reasonbly without building a small version
791 // of BasicAA into this code. We could recurse into AAResultBase::alias
792 // here but that seems likely to go poorly as we're inside the
793 // implementation of such a query. Until then, just conservatievly retun
796 } while (!Inputs.empty());
798 // If all the inputs to V were definitively no-alias, then V is no-alias.
802 /// alias - If one of the pointers is to a global that we are tracking, and the
803 /// other is some random pointer, we know there cannot be an alias, because the
804 /// address of the global isn't taken.
805 AliasResult GlobalsAAResult::alias(const MemoryLocation &LocA,
806 const MemoryLocation &LocB) {
807 // Get the base object these pointers point to.
808 const Value *UV1 = GetUnderlyingObject(LocA.Ptr, DL);
809 const Value *UV2 = GetUnderlyingObject(LocB.Ptr, DL);
811 // If either of the underlying values is a global, they may be non-addr-taken
812 // globals, which we can answer queries about.
813 const GlobalValue *GV1 = dyn_cast<GlobalValue>(UV1);
814 const GlobalValue *GV2 = dyn_cast<GlobalValue>(UV2);
816 // If the global's address is taken, pretend we don't know it's a pointer to
818 if (GV1 && !NonAddressTakenGlobals.count(GV1))
820 if (GV2 && !NonAddressTakenGlobals.count(GV2))
823 // If the two pointers are derived from two different non-addr-taken
824 // globals we know these can't alias.
825 if (GV1 && GV2 && GV1 != GV2)
828 // If one is and the other isn't, it isn't strictly safe but we can fake
829 // this result if necessary for performance. This does not appear to be
830 // a common problem in practice.
831 if (EnableUnsafeGlobalsModRefAliasResults)
832 if ((GV1 || GV2) && GV1 != GV2)
835 // Check for a special case where a non-escaping global can be used to
836 // conclude no-alias.
837 if ((GV1 || GV2) && GV1 != GV2) {
838 const GlobalValue *GV = GV1 ? GV1 : GV2;
839 const Value *UV = GV1 ? UV2 : UV1;
840 if (isNonEscapingGlobalNoAlias(GV, UV))
844 // Otherwise if they are both derived from the same addr-taken global, we
845 // can't know the two accesses don't overlap.
848 // These pointers may be based on the memory owned by an indirect global. If
849 // so, we may be able to handle this. First check to see if the base pointer
850 // is a direct load from an indirect global.
852 if (const LoadInst *LI = dyn_cast<LoadInst>(UV1))
853 if (GlobalVariable *GV = dyn_cast<GlobalVariable>(LI->getOperand(0)))
854 if (IndirectGlobals.count(GV))
856 if (const LoadInst *LI = dyn_cast<LoadInst>(UV2))
857 if (const GlobalVariable *GV = dyn_cast<GlobalVariable>(LI->getOperand(0)))
858 if (IndirectGlobals.count(GV))
861 // These pointers may also be from an allocation for the indirect global. If
862 // so, also handle them.
864 GV1 = AllocsForIndirectGlobals.lookup(UV1);
866 GV2 = AllocsForIndirectGlobals.lookup(UV2);
868 // Now that we know whether the two pointers are related to indirect globals,
869 // use this to disambiguate the pointers. If the pointers are based on
870 // different indirect globals they cannot alias.
871 if (GV1 && GV2 && GV1 != GV2)
874 // If one is based on an indirect global and the other isn't, it isn't
875 // strictly safe but we can fake this result if necessary for performance.
876 // This does not appear to be a common problem in practice.
877 if (EnableUnsafeGlobalsModRefAliasResults)
878 if ((GV1 || GV2) && GV1 != GV2)
881 return AAResultBase::alias(LocA, LocB);
884 ModRefInfo GlobalsAAResult::getModRefInfoForArgument(ImmutableCallSite CS,
885 const GlobalValue *GV) {
886 if (CS.doesNotAccessMemory())
887 return ModRefInfo::NoModRef;
888 ModRefInfo ConservativeResult =
889 CS.onlyReadsMemory() ? ModRefInfo::Ref : ModRefInfo::ModRef;
891 // Iterate through all the arguments to the called function. If any argument
892 // is based on GV, return the conservative result.
893 for (auto &A : CS.args()) {
894 SmallVector<Value*, 4> Objects;
895 GetUnderlyingObjects(A, Objects, DL);
897 // All objects must be identified.
898 if (!all_of(Objects, isIdentifiedObject) &&
899 // Try ::alias to see if all objects are known not to alias GV.
900 !all_of(Objects, [&](Value *V) {
901 return this->alias(MemoryLocation(V), MemoryLocation(GV)) == NoAlias;
903 return ConservativeResult;
905 if (is_contained(Objects, GV))
906 return ConservativeResult;
909 // We identified all objects in the argument list, and none of them were GV.
910 return ModRefInfo::NoModRef;
913 ModRefInfo GlobalsAAResult::getModRefInfo(ImmutableCallSite CS,
914 const MemoryLocation &Loc) {
915 ModRefInfo Known = ModRefInfo::ModRef;
917 // If we are asking for mod/ref info of a direct call with a pointer to a
918 // global we are tracking, return information if we have it.
919 if (const GlobalValue *GV =
920 dyn_cast<GlobalValue>(GetUnderlyingObject(Loc.Ptr, DL)))
921 if (GV->hasLocalLinkage())
922 if (const Function *F = CS.getCalledFunction())
923 if (NonAddressTakenGlobals.count(GV))
924 if (const FunctionInfo *FI = getFunctionInfo(F))
925 Known = unionModRef(FI->getModRefInfoForGlobal(*GV),
926 getModRefInfoForArgument(CS, GV));
928 if (!isModOrRefSet(Known))
929 return ModRefInfo::NoModRef; // No need to query other mod/ref analyses
930 return intersectModRef(Known, AAResultBase::getModRefInfo(CS, Loc));
933 GlobalsAAResult::GlobalsAAResult(const DataLayout &DL,
934 const TargetLibraryInfo &TLI)
935 : AAResultBase(), DL(DL), TLI(TLI) {}
937 GlobalsAAResult::GlobalsAAResult(GlobalsAAResult &&Arg)
938 : AAResultBase(std::move(Arg)), DL(Arg.DL), TLI(Arg.TLI),
939 NonAddressTakenGlobals(std::move(Arg.NonAddressTakenGlobals)),
940 IndirectGlobals(std::move(Arg.IndirectGlobals)),
941 AllocsForIndirectGlobals(std::move(Arg.AllocsForIndirectGlobals)),
942 FunctionInfos(std::move(Arg.FunctionInfos)),
943 Handles(std::move(Arg.Handles)) {
944 // Update the parent for each DeletionCallbackHandle.
945 for (auto &H : Handles) {
946 assert(H.GAR == &Arg);
951 GlobalsAAResult::~GlobalsAAResult() {}
953 /*static*/ GlobalsAAResult
954 GlobalsAAResult::analyzeModule(Module &M, const TargetLibraryInfo &TLI,
956 GlobalsAAResult Result(M.getDataLayout(), TLI);
958 // Discover which functions aren't recursive, to feed into AnalyzeGlobals.
959 Result.CollectSCCMembership(CG);
961 // Find non-addr taken globals.
962 Result.AnalyzeGlobals(M);
965 Result.AnalyzeCallGraph(CG, M);
970 AnalysisKey GlobalsAA::Key;
972 GlobalsAAResult GlobalsAA::run(Module &M, ModuleAnalysisManager &AM) {
973 return GlobalsAAResult::analyzeModule(M,
974 AM.getResult<TargetLibraryAnalysis>(M),
975 AM.getResult<CallGraphAnalysis>(M));
978 char GlobalsAAWrapperPass::ID = 0;
979 INITIALIZE_PASS_BEGIN(GlobalsAAWrapperPass, "globals-aa",
980 "Globals Alias Analysis", false, true)
981 INITIALIZE_PASS_DEPENDENCY(CallGraphWrapperPass)
982 INITIALIZE_PASS_DEPENDENCY(TargetLibraryInfoWrapperPass)
983 INITIALIZE_PASS_END(GlobalsAAWrapperPass, "globals-aa",
984 "Globals Alias Analysis", false, true)
986 ModulePass *llvm::createGlobalsAAWrapperPass() {
987 return new GlobalsAAWrapperPass();
990 GlobalsAAWrapperPass::GlobalsAAWrapperPass() : ModulePass(ID) {
991 initializeGlobalsAAWrapperPassPass(*PassRegistry::getPassRegistry());
994 bool GlobalsAAWrapperPass::runOnModule(Module &M) {
995 Result.reset(new GlobalsAAResult(GlobalsAAResult::analyzeModule(
996 M, getAnalysis<TargetLibraryInfoWrapperPass>().getTLI(),
997 getAnalysis<CallGraphWrapperPass>().getCallGraph())));
1001 bool GlobalsAAWrapperPass::doFinalization(Module &M) {
1006 void GlobalsAAWrapperPass::getAnalysisUsage(AnalysisUsage &AU) const {
1007 AU.setPreservesAll();
1008 AU.addRequired<CallGraphWrapperPass>();
1009 AU.addRequired<TargetLibraryInfoWrapperPass>();