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/ValueTracking.h"
23 #include "llvm/IR/DerivedTypes.h"
24 #include "llvm/IR/InstIterator.h"
25 #include "llvm/IR/Instructions.h"
26 #include "llvm/IR/IntrinsicInst.h"
27 #include "llvm/IR/Module.h"
28 #include "llvm/Pass.h"
29 #include "llvm/Support/CommandLine.h"
32 #define DEBUG_TYPE "globalsmodref-aa"
34 STATISTIC(NumNonAddrTakenGlobalVars,
35 "Number of global vars without address taken");
36 STATISTIC(NumNonAddrTakenFunctions,"Number of functions without address taken");
37 STATISTIC(NumNoMemFunctions, "Number of functions that do not access memory");
38 STATISTIC(NumReadMemFunctions, "Number of functions that only read memory");
39 STATISTIC(NumIndirectGlobalVars, "Number of indirect global objects");
41 // An option to enable unsafe alias results from the GlobalsModRef analysis.
42 // When enabled, GlobalsModRef will provide no-alias results which in extremely
43 // rare cases may not be conservatively correct. In particular, in the face of
44 // transforms which cause assymetry between how effective GetUnderlyingObject
45 // is for two pointers, it may produce incorrect results.
47 // These unsafe results have been returned by GMR for many years without
48 // causing significant issues in the wild and so we provide a mechanism to
49 // re-enable them for users of LLVM that have a particular performance
50 // sensitivity and no known issues. The option also makes it easy to evaluate
51 // the performance impact of these results.
52 static cl::opt<bool> EnableUnsafeGlobalsModRefAliasResults(
53 "enable-unsafe-globalsmodref-alias-results", cl::init(false), cl::Hidden);
55 /// The mod/ref information collected for a particular function.
57 /// We collect information about mod/ref behavior of a function here, both in
58 /// general and as pertains to specific globals. We only have this detailed
59 /// information when we know *something* useful about the behavior. If we
60 /// saturate to fully general mod/ref, we remove the info for the function.
61 class GlobalsModRef::FunctionInfo {
62 typedef SmallDenseMap<const GlobalValue *, ModRefInfo, 16> GlobalInfoMapType;
64 /// Build a wrapper struct that has 8-byte alignment. All heap allocations
65 /// should provide this much alignment at least, but this makes it clear we
66 /// specifically rely on this amount of alignment.
67 struct LLVM_ALIGNAS(8) AlignedMap {
69 AlignedMap(const AlignedMap &Arg) : Map(Arg.Map) {}
70 GlobalInfoMapType Map;
73 /// Pointer traits for our aligned map.
74 struct AlignedMapPointerTraits {
75 static inline void *getAsVoidPointer(AlignedMap *P) { return P; }
76 static inline AlignedMap *getFromVoidPointer(void *P) {
77 return (AlignedMap *)P;
79 enum { NumLowBitsAvailable = 3 };
80 static_assert(AlignOf<AlignedMap>::Alignment >= (1 << NumLowBitsAvailable),
81 "AlignedMap insufficiently aligned to have enough low bits.");
84 /// The bit that flags that this function may read any global. This is
85 /// chosen to mix together with ModRefInfo bits.
86 enum { MayReadAnyGlobal = 4 };
88 /// Checks to document the invariants of the bit packing here.
89 static_assert((MayReadAnyGlobal & MRI_ModRef) == 0,
90 "ModRef and the MayReadAnyGlobal flag bits overlap.");
91 static_assert(((MayReadAnyGlobal | MRI_ModRef) >>
92 AlignedMapPointerTraits::NumLowBitsAvailable) == 0,
93 "Insufficient low bits to store our flag and ModRef info.");
96 FunctionInfo() : Info() {}
98 delete Info.getPointer();
100 // Spell out the copy ond move constructors and assignment operators to get
101 // deep copy semantics and correct move semantics in the face of the
103 FunctionInfo(const FunctionInfo &Arg)
104 : Info(nullptr, Arg.Info.getInt()) {
105 if (const auto *ArgPtr = Arg.Info.getPointer())
106 Info.setPointer(new AlignedMap(*ArgPtr));
108 FunctionInfo(FunctionInfo &&Arg)
109 : Info(Arg.Info.getPointer(), Arg.Info.getInt()) {
110 Arg.Info.setPointerAndInt(nullptr, 0);
112 FunctionInfo &operator=(const FunctionInfo &RHS) {
113 delete Info.getPointer();
114 Info.setPointerAndInt(nullptr, RHS.Info.getInt());
115 if (const auto *RHSPtr = RHS.Info.getPointer())
116 Info.setPointer(new AlignedMap(*RHSPtr));
119 FunctionInfo &operator=(FunctionInfo &&RHS) {
120 delete Info.getPointer();
121 Info.setPointerAndInt(RHS.Info.getPointer(), RHS.Info.getInt());
122 RHS.Info.setPointerAndInt(nullptr, 0);
126 /// Returns the \c ModRefInfo info for this function.
127 ModRefInfo getModRefInfo() const {
128 return ModRefInfo(Info.getInt() & MRI_ModRef);
131 /// Adds new \c ModRefInfo for this function to its state.
132 void addModRefInfo(ModRefInfo NewMRI) {
133 Info.setInt(Info.getInt() | NewMRI);
136 /// Returns whether this function may read any global variable, and we don't
137 /// know which global.
138 bool mayReadAnyGlobal() const { return Info.getInt() & MayReadAnyGlobal; }
140 /// Sets this function as potentially reading from any global.
141 void setMayReadAnyGlobal() { Info.setInt(Info.getInt() | MayReadAnyGlobal); }
143 /// Returns the \c ModRefInfo info for this function w.r.t. a particular
144 /// global, which may be more precise than the general information above.
145 ModRefInfo getModRefInfoForGlobal(const GlobalValue &GV) const {
146 ModRefInfo GlobalMRI = mayReadAnyGlobal() ? MRI_Ref : MRI_NoModRef;
147 if (AlignedMap *P = Info.getPointer()) {
148 auto I = P->Map.find(&GV);
149 if (I != P->Map.end())
150 GlobalMRI = ModRefInfo(GlobalMRI | I->second);
155 /// Add mod/ref info from another function into ours, saturating towards
157 void addFunctionInfo(const FunctionInfo &FI) {
158 addModRefInfo(FI.getModRefInfo());
160 if (FI.mayReadAnyGlobal())
161 setMayReadAnyGlobal();
163 if (AlignedMap *P = FI.Info.getPointer())
164 for (const auto &G : P->Map)
165 addModRefInfoForGlobal(*G.first, G.second);
168 void addModRefInfoForGlobal(const GlobalValue &GV, ModRefInfo NewMRI) {
169 AlignedMap *P = Info.getPointer();
171 P = new AlignedMap();
174 auto &GlobalMRI = P->Map[&GV];
175 GlobalMRI = ModRefInfo(GlobalMRI | NewMRI);
178 /// Clear a global's ModRef info. Should be used when a global is being
180 void eraseModRefInfoForGlobal(const GlobalValue &GV) {
181 if (AlignedMap *P = Info.getPointer())
186 /// All of the information is encoded into a single pointer, with a three bit
187 /// integer in the low three bits. The high bit provides a flag for when this
188 /// function may read any global. The low two bits are the ModRefInfo. And
189 /// the pointer, when non-null, points to a map from GlobalValue to
190 /// ModRefInfo specific to that GlobalValue.
191 PointerIntPair<AlignedMap *, 3, unsigned, AlignedMapPointerTraits> Info;
194 void GlobalsModRef::DeletionCallbackHandle::deleted() {
195 Value *V = getValPtr();
196 if (auto *F = dyn_cast<Function>(V))
197 GMR.FunctionInfos.erase(F);
199 if (GlobalValue *GV = dyn_cast<GlobalValue>(V)) {
200 if (GMR.NonAddressTakenGlobals.erase(GV)) {
201 // This global might be an indirect global. If so, remove it and
202 // remove any AllocRelatedValues for it.
203 if (GMR.IndirectGlobals.erase(GV)) {
204 // Remove any entries in AllocsForIndirectGlobals for this global.
205 for (auto I = GMR.AllocsForIndirectGlobals.begin(),
206 E = GMR.AllocsForIndirectGlobals.end();
209 GMR.AllocsForIndirectGlobals.erase(I);
212 // Scan the function info we have collected and remove this global
214 for (auto &FIPair : GMR.FunctionInfos)
215 FIPair.second.eraseModRefInfoForGlobal(*GV);
219 // If this is an allocation related to an indirect global, remove it.
220 GMR.AllocsForIndirectGlobals.erase(V);
222 // And clear out the handle.
224 GMR.Handles.erase(I);
225 // This object is now destroyed!
228 char GlobalsModRef::ID = 0;
229 INITIALIZE_AG_PASS_BEGIN(GlobalsModRef, AliasAnalysis, "globalsmodref-aa",
230 "Simple mod/ref analysis for globals", false, true,
232 INITIALIZE_PASS_DEPENDENCY(CallGraphWrapperPass)
233 INITIALIZE_AG_PASS_END(GlobalsModRef, AliasAnalysis, "globalsmodref-aa",
234 "Simple mod/ref analysis for globals", false, true,
237 Pass *llvm::createGlobalsModRefPass() { return new GlobalsModRef(); }
239 GlobalsModRef::GlobalsModRef() : ModulePass(ID) {
240 initializeGlobalsModRefPass(*PassRegistry::getPassRegistry());
243 FunctionModRefBehavior GlobalsModRef::getModRefBehavior(const Function *F) {
244 FunctionModRefBehavior Min = FMRB_UnknownModRefBehavior;
246 if (FunctionInfo *FI = getFunctionInfo(F)) {
247 if (FI->getModRefInfo() == MRI_NoModRef)
248 Min = FMRB_DoesNotAccessMemory;
249 else if ((FI->getModRefInfo() & MRI_Mod) == 0)
250 Min = FMRB_OnlyReadsMemory;
253 return FunctionModRefBehavior(AliasAnalysis::getModRefBehavior(F) & Min);
256 FunctionModRefBehavior GlobalsModRef::getModRefBehavior(ImmutableCallSite CS) {
257 FunctionModRefBehavior Min = FMRB_UnknownModRefBehavior;
259 if (const Function *F = CS.getCalledFunction())
260 if (FunctionInfo *FI = getFunctionInfo(F)) {
261 if (FI->getModRefInfo() == MRI_NoModRef)
262 Min = FMRB_DoesNotAccessMemory;
263 else if ((FI->getModRefInfo() & MRI_Mod) == 0)
264 Min = FMRB_OnlyReadsMemory;
267 return FunctionModRefBehavior(AliasAnalysis::getModRefBehavior(CS) & Min);
270 /// Returns the function info for the function, or null if we don't have
271 /// anything useful to say about it.
272 GlobalsModRef::FunctionInfo *GlobalsModRef::getFunctionInfo(const Function *F) {
273 auto I = FunctionInfos.find(F);
274 if (I != FunctionInfos.end())
279 /// AnalyzeGlobals - Scan through the users of all of the internal
280 /// GlobalValue's in the program. If none of them have their "address taken"
281 /// (really, their address passed to something nontrivial), record this fact,
282 /// and record the functions that they are used directly in.
283 void GlobalsModRef::AnalyzeGlobals(Module &M) {
284 SmallPtrSet<Function *, 64> TrackedFunctions;
285 for (Function &F : M)
286 if (F.hasLocalLinkage())
287 if (!AnalyzeUsesOfPointer(&F)) {
288 // Remember that we are tracking this global.
289 NonAddressTakenGlobals.insert(&F);
290 TrackedFunctions.insert(&F);
291 Handles.emplace_front(*this, &F);
292 Handles.front().I = Handles.begin();
293 ++NumNonAddrTakenFunctions;
296 SmallPtrSet<Function *, 64> Readers, Writers;
297 for (GlobalVariable &GV : M.globals())
298 if (GV.hasLocalLinkage()) {
299 if (!AnalyzeUsesOfPointer(&GV, &Readers,
300 GV.isConstant() ? nullptr : &Writers)) {
301 // Remember that we are tracking this global, and the mod/ref fns
302 NonAddressTakenGlobals.insert(&GV);
303 Handles.emplace_front(*this, &GV);
304 Handles.front().I = Handles.begin();
306 for (Function *Reader : Readers) {
307 if (TrackedFunctions.insert(Reader).second) {
308 Handles.emplace_front(*this, Reader);
309 Handles.front().I = Handles.begin();
311 FunctionInfos[Reader].addModRefInfoForGlobal(GV, MRI_Ref);
314 if (!GV.isConstant()) // No need to keep track of writers to constants
315 for (Function *Writer : Writers) {
316 if (TrackedFunctions.insert(Writer).second) {
317 Handles.emplace_front(*this, Writer);
318 Handles.front().I = Handles.begin();
320 FunctionInfos[Writer].addModRefInfoForGlobal(GV, MRI_Mod);
322 ++NumNonAddrTakenGlobalVars;
324 // If this global holds a pointer type, see if it is an indirect global.
325 if (GV.getType()->getElementType()->isPointerTy() &&
326 AnalyzeIndirectGlobalMemory(&GV))
327 ++NumIndirectGlobalVars;
334 /// AnalyzeUsesOfPointer - Look at all of the users of the specified pointer.
335 /// If this is used by anything complex (i.e., the address escapes), return
336 /// true. Also, while we are at it, keep track of those functions that read and
337 /// write to the value.
339 /// If OkayStoreDest is non-null, stores into this global are allowed.
340 bool GlobalsModRef::AnalyzeUsesOfPointer(Value *V,
341 SmallPtrSetImpl<Function *> *Readers,
342 SmallPtrSetImpl<Function *> *Writers,
343 GlobalValue *OkayStoreDest) {
344 if (!V->getType()->isPointerTy())
347 for (Use &U : V->uses()) {
348 User *I = U.getUser();
349 if (LoadInst *LI = dyn_cast<LoadInst>(I)) {
351 Readers->insert(LI->getParent()->getParent());
352 } else if (StoreInst *SI = dyn_cast<StoreInst>(I)) {
353 if (V == SI->getOperand(1)) {
355 Writers->insert(SI->getParent()->getParent());
356 } else if (SI->getOperand(1) != OkayStoreDest) {
357 return true; // Storing the pointer
359 } else if (Operator::getOpcode(I) == Instruction::GetElementPtr) {
360 if (AnalyzeUsesOfPointer(I, Readers, Writers))
362 } else if (Operator::getOpcode(I) == Instruction::BitCast) {
363 if (AnalyzeUsesOfPointer(I, Readers, Writers, OkayStoreDest))
365 } else if (auto CS = CallSite(I)) {
366 // Make sure that this is just the function being called, not that it is
367 // passing into the function.
368 if (!CS.isCallee(&U)) {
369 // Detect calls to free.
370 if (isFreeCall(I, TLI)) {
372 Writers->insert(CS->getParent()->getParent());
374 return true; // Argument of an unknown call.
377 } else if (ICmpInst *ICI = dyn_cast<ICmpInst>(I)) {
378 if (!isa<ConstantPointerNull>(ICI->getOperand(1)))
379 return true; // Allow comparison against null.
388 /// AnalyzeIndirectGlobalMemory - We found an non-address-taken global variable
389 /// which holds a pointer type. See if the global always points to non-aliased
390 /// heap memory: that is, all initializers of the globals are allocations, and
391 /// those allocations have no use other than initialization of the global.
392 /// Further, all loads out of GV must directly use the memory, not store the
393 /// pointer somewhere. If this is true, we consider the memory pointed to by
394 /// GV to be owned by GV and can disambiguate other pointers from it.
395 bool GlobalsModRef::AnalyzeIndirectGlobalMemory(GlobalValue *GV) {
396 // Keep track of values related to the allocation of the memory, f.e. the
397 // value produced by the malloc call and any casts.
398 std::vector<Value *> AllocRelatedValues;
400 // Walk the user list of the global. If we find anything other than a direct
401 // load or store, bail out.
402 for (User *U : GV->users()) {
403 if (LoadInst *LI = dyn_cast<LoadInst>(U)) {
404 // The pointer loaded from the global can only be used in simple ways:
405 // we allow addressing of it and loading storing to it. We do *not* allow
406 // storing the loaded pointer somewhere else or passing to a function.
407 if (AnalyzeUsesOfPointer(LI))
408 return false; // Loaded pointer escapes.
409 // TODO: Could try some IP mod/ref of the loaded pointer.
410 } else if (StoreInst *SI = dyn_cast<StoreInst>(U)) {
411 // Storing the global itself.
412 if (SI->getOperand(0) == GV)
415 // If storing the null pointer, ignore it.
416 if (isa<ConstantPointerNull>(SI->getOperand(0)))
419 // Check the value being stored.
420 Value *Ptr = GetUnderlyingObject(SI->getOperand(0),
421 GV->getParent()->getDataLayout());
423 if (!isAllocLikeFn(Ptr, TLI))
424 return false; // Too hard to analyze.
426 // Analyze all uses of the allocation. If any of them are used in a
427 // non-simple way (e.g. stored to another global) bail out.
428 if (AnalyzeUsesOfPointer(Ptr, /*Readers*/ nullptr, /*Writers*/ nullptr,
430 return false; // Loaded pointer escapes.
432 // Remember that this allocation is related to the indirect global.
433 AllocRelatedValues.push_back(Ptr);
435 // Something complex, bail out.
440 // Okay, this is an indirect global. Remember all of the allocations for
441 // this global in AllocsForIndirectGlobals.
442 while (!AllocRelatedValues.empty()) {
443 AllocsForIndirectGlobals[AllocRelatedValues.back()] = GV;
444 Handles.emplace_front(*this, AllocRelatedValues.back());
445 Handles.front().I = Handles.begin();
446 AllocRelatedValues.pop_back();
448 IndirectGlobals.insert(GV);
449 Handles.emplace_front(*this, GV);
450 Handles.front().I = Handles.begin();
454 /// AnalyzeCallGraph - At this point, we know the functions where globals are
455 /// immediately stored to and read from. Propagate this information up the call
456 /// graph to all callers and compute the mod/ref info for all memory for each
458 void GlobalsModRef::AnalyzeCallGraph(CallGraph &CG, Module &M) {
459 // We do a bottom-up SCC traversal of the call graph. In other words, we
460 // visit all callees before callers (leaf-first).
461 for (scc_iterator<CallGraph *> I = scc_begin(&CG); !I.isAtEnd(); ++I) {
462 const std::vector<CallGraphNode *> &SCC = *I;
463 assert(!SCC.empty() && "SCC with no functions?");
465 if (!SCC[0]->getFunction()) {
466 // Calls externally - can't say anything useful. Remove any existing
467 // function records (may have been created when scanning globals).
468 for (auto *Node : SCC)
469 FunctionInfos.erase(Node->getFunction());
473 FunctionInfo &FI = FunctionInfos[SCC[0]->getFunction()];
474 bool KnowNothing = false;
476 // Collect the mod/ref properties due to called functions. We only compute
478 for (unsigned i = 0, e = SCC.size(); i != e && !KnowNothing; ++i) {
479 Function *F = SCC[i]->getFunction();
485 if (F->isDeclaration()) {
486 // Try to get mod/ref behaviour from function attributes.
487 if (F->doesNotAccessMemory()) {
488 // Can't do better than that!
489 } else if (F->onlyReadsMemory()) {
490 FI.addModRefInfo(MRI_Ref);
491 if (!F->isIntrinsic())
492 // This function might call back into the module and read a global -
493 // consider every global as possibly being read by this function.
494 FI.setMayReadAnyGlobal();
496 FI.addModRefInfo(MRI_ModRef);
497 // Can't say anything useful unless it's an intrinsic - they don't
498 // read or write global variables of the kind considered here.
499 KnowNothing = !F->isIntrinsic();
504 for (CallGraphNode::iterator CI = SCC[i]->begin(), E = SCC[i]->end();
505 CI != E && !KnowNothing; ++CI)
506 if (Function *Callee = CI->second->getFunction()) {
507 if (FunctionInfo *CalleeFI = getFunctionInfo(Callee)) {
508 // Propagate function effect up.
509 FI.addFunctionInfo(*CalleeFI);
511 // Can't say anything about it. However, if it is inside our SCC,
512 // then nothing needs to be done.
513 CallGraphNode *CalleeNode = CG[Callee];
514 if (std::find(SCC.begin(), SCC.end(), CalleeNode) == SCC.end())
522 // If we can't say anything useful about this SCC, remove all SCC functions
523 // from the FunctionInfos map.
525 for (auto *Node : SCC)
526 FunctionInfos.erase(Node->getFunction());
530 // Scan the function bodies for explicit loads or stores.
531 for (auto *Node : SCC) {
532 if (FI.getModRefInfo() == MRI_ModRef)
533 break; // The mod/ref lattice saturates here.
534 for (Instruction &I : instructions(Node->getFunction())) {
535 if (FI.getModRefInfo() == MRI_ModRef)
536 break; // The mod/ref lattice saturates here.
538 // We handle calls specially because the graph-relevant aspects are
540 if (auto CS = CallSite(&I)) {
541 if (isAllocationFn(&I, TLI) || isFreeCall(&I, TLI)) {
542 // FIXME: It is completely unclear why this is necessary and not
543 // handled by the above graph code.
544 FI.addModRefInfo(MRI_ModRef);
545 } else if (Function *Callee = CS.getCalledFunction()) {
546 // The callgraph doesn't include intrinsic calls.
547 if (Callee->isIntrinsic()) {
548 FunctionModRefBehavior Behaviour =
549 AliasAnalysis::getModRefBehavior(Callee);
550 FI.addModRefInfo(ModRefInfo(Behaviour & MRI_ModRef));
556 // All non-call instructions we use the primary predicates for whether
557 // thay read or write memory.
558 if (I.mayReadFromMemory())
559 FI.addModRefInfo(MRI_Ref);
560 if (I.mayWriteToMemory())
561 FI.addModRefInfo(MRI_Mod);
565 if ((FI.getModRefInfo() & MRI_Mod) == 0)
566 ++NumReadMemFunctions;
567 if (FI.getModRefInfo() == MRI_NoModRef)
570 // Finally, now that we know the full effect on this SCC, clone the
571 // information to each function in the SCC.
572 for (unsigned i = 1, e = SCC.size(); i != e; ++i)
573 FunctionInfos[SCC[i]->getFunction()] = FI;
577 // There are particular cases where we can conclude no-alias between
578 // a non-addr-taken global and some other underlying object. Specifically,
579 // a non-addr-taken global is known to not be escaped from any function. It is
580 // also incorrect for a transformation to introduce an escape of a global in
581 // a way that is observable when it was not there previously. One function
582 // being transformed to introduce an escape which could possibly be observed
583 // (via loading from a global or the return value for example) within another
584 // function is never safe. If the observation is made through non-atomic
585 // operations on different threads, it is a data-race and UB. If the
586 // observation is well defined, by being observed the transformation would have
587 // changed program behavior by introducing the observed escape, making it an
588 // invalid transform.
590 // This property does require that transformations which *temporarily* escape
591 // a global that was not previously escaped, prior to restoring it, cannot rely
592 // on the results of GMR::alias. This seems a reasonable restriction, although
593 // currently there is no way to enforce it. There is also no realistic
594 // optimization pass that would make this mistake. The closest example is
595 // a transformation pass which does reg2mem of SSA values but stores them into
596 // global variables temporarily before restoring the global variable's value.
597 // This could be useful to expose "benign" races for example. However, it seems
598 // reasonable to require that a pass which introduces escapes of global
599 // variables in this way to either not trust AA results while the escape is
600 // active, or to be forced to operate as a module pass that cannot co-exist
601 // with an alias analysis such as GMR.
602 bool GlobalsModRef::isNonEscapingGlobalNoAlias(const GlobalValue *GV,
604 // In order to know that the underlying object cannot alias the
605 // non-addr-taken global, we must know that it would have to be an escape.
606 // Thus if the underlying object is a function argument, a load from
607 // a global, or the return of a function, it cannot alias. We can also
608 // recurse through PHI nodes and select nodes provided all of their inputs
609 // resolve to one of these known-escaping roots.
610 SmallPtrSet<const Value *, 8> Visited;
611 SmallVector<const Value *, 8> Inputs;
616 const Value *Input = Inputs.pop_back_val();
618 if (auto *InputGV = dyn_cast<GlobalValue>(Input)) {
619 // If one input is the very global we're querying against, then we can't
620 // conclude no-alias.
624 // Distinct GlobalVariables never alias, unless overriden or zero-sized.
625 // FIXME: The condition can be refined, but be conservative for now.
626 auto *GVar = dyn_cast<GlobalVariable>(GV);
627 auto *InputGVar = dyn_cast<GlobalVariable>(InputGV);
628 if (GVar && InputGVar &&
629 !GVar->isDeclaration() && !InputGVar->isDeclaration() &&
630 !GVar->mayBeOverridden() && !InputGVar->mayBeOverridden()) {
631 Type *GVType = GVar->getInitializer()->getType();
632 Type *InputGVType = InputGVar->getInitializer()->getType();
633 if (GVType->isSized() && InputGVType->isSized() &&
634 (DL->getTypeAllocSize(GVType) > 0) &&
635 (DL->getTypeAllocSize(InputGVType) > 0))
639 // Conservatively return false, even though we could be smarter
640 // (e.g. look through GlobalAliases).
644 if (isa<Argument>(Input) || isa<CallInst>(Input) ||
645 isa<InvokeInst>(Input)) {
646 // Arguments to functions or returns from functions are inherently
647 // escaping, so we can immediately classify those as not aliasing any
648 // non-addr-taken globals.
651 if (auto *LI = dyn_cast<LoadInst>(Input)) {
652 // A pointer loaded from a global would have been captured, and we know
653 // that the global is non-escaping, so no alias.
654 if (isa<GlobalValue>(GetUnderlyingObject(LI->getPointerOperand(), *DL)))
657 // Otherwise, a load could come from anywhere, so bail.
661 // Recurse through a limited number of selects and PHIs. This is an
662 // arbitrary depth of 4, lower numbers could be used to fix compile time
663 // issues if needed, but this is generally expected to be only be important
667 if (auto *SI = dyn_cast<SelectInst>(Input)) {
668 const Value *LHS = GetUnderlyingObject(SI->getTrueValue(), *DL);
669 const Value *RHS = GetUnderlyingObject(SI->getFalseValue(), *DL);
670 if (Visited.insert(LHS).second)
671 Inputs.push_back(LHS);
672 if (Visited.insert(RHS).second)
673 Inputs.push_back(RHS);
676 if (auto *PN = dyn_cast<PHINode>(Input)) {
677 for (const Value *Op : PN->incoming_values()) {
678 Op = GetUnderlyingObject(Op, *DL);
679 if (Visited.insert(Op).second)
680 Inputs.push_back(Op);
685 // FIXME: It would be good to handle other obvious no-alias cases here, but
686 // it isn't clear how to do so reasonbly without building a small version
687 // of BasicAA into this code. We could recurse into AliasAnalysis::alias
688 // here but that seems likely to go poorly as we're inside the
689 // implementation of such a query. Until then, just conservatievly retun
692 } while (!Inputs.empty());
694 // If all the inputs to V were definitively no-alias, then V is no-alias.
698 /// alias - If one of the pointers is to a global that we are tracking, and the
699 /// other is some random pointer, we know there cannot be an alias, because the
700 /// address of the global isn't taken.
701 AliasResult GlobalsModRef::alias(const MemoryLocation &LocA,
702 const MemoryLocation &LocB) {
703 // Get the base object these pointers point to.
704 const Value *UV1 = GetUnderlyingObject(LocA.Ptr, *DL);
705 const Value *UV2 = GetUnderlyingObject(LocB.Ptr, *DL);
707 // If either of the underlying values is a global, they may be non-addr-taken
708 // globals, which we can answer queries about.
709 const GlobalValue *GV1 = dyn_cast<GlobalValue>(UV1);
710 const GlobalValue *GV2 = dyn_cast<GlobalValue>(UV2);
712 // If the global's address is taken, pretend we don't know it's a pointer to
714 if (GV1 && !NonAddressTakenGlobals.count(GV1))
716 if (GV2 && !NonAddressTakenGlobals.count(GV2))
719 // If the two pointers are derived from two different non-addr-taken
720 // globals we know these can't alias.
721 if (GV1 && GV2 && GV1 != GV2)
724 // If one is and the other isn't, it isn't strictly safe but we can fake
725 // this result if necessary for performance. This does not appear to be
726 // a common problem in practice.
727 if (EnableUnsafeGlobalsModRefAliasResults)
728 if ((GV1 || GV2) && GV1 != GV2)
731 // Check for a special case where a non-escaping global can be used to
732 // conclude no-alias.
733 if ((GV1 || GV2) && GV1 != GV2) {
734 const GlobalValue *GV = GV1 ? GV1 : GV2;
735 const Value *UV = GV1 ? UV2 : UV1;
736 if (isNonEscapingGlobalNoAlias(GV, UV))
740 // Otherwise if they are both derived from the same addr-taken global, we
741 // can't know the two accesses don't overlap.
744 // These pointers may be based on the memory owned by an indirect global. If
745 // so, we may be able to handle this. First check to see if the base pointer
746 // is a direct load from an indirect global.
748 if (const LoadInst *LI = dyn_cast<LoadInst>(UV1))
749 if (GlobalVariable *GV = dyn_cast<GlobalVariable>(LI->getOperand(0)))
750 if (IndirectGlobals.count(GV))
752 if (const LoadInst *LI = dyn_cast<LoadInst>(UV2))
753 if (const GlobalVariable *GV = dyn_cast<GlobalVariable>(LI->getOperand(0)))
754 if (IndirectGlobals.count(GV))
757 // These pointers may also be from an allocation for the indirect global. If
758 // so, also handle them.
760 GV1 = AllocsForIndirectGlobals.lookup(UV1);
762 GV2 = AllocsForIndirectGlobals.lookup(UV2);
764 // Now that we know whether the two pointers are related to indirect globals,
765 // use this to disambiguate the pointers. If the pointers are based on
766 // different indirect globals they cannot alias.
767 if (GV1 && GV2 && GV1 != GV2)
770 // If one is based on an indirect global and the other isn't, it isn't
771 // strictly safe but we can fake this result if necessary for performance.
772 // This does not appear to be a common problem in practice.
773 if (EnableUnsafeGlobalsModRefAliasResults)
774 if ((GV1 || GV2) && GV1 != GV2)
777 return AliasAnalysis::alias(LocA, LocB);
780 ModRefInfo GlobalsModRef::getModRefInfo(ImmutableCallSite CS,
781 const MemoryLocation &Loc) {
782 unsigned Known = MRI_ModRef;
784 // If we are asking for mod/ref info of a direct call with a pointer to a
785 // global we are tracking, return information if we have it.
786 const DataLayout &DL = CS.getCaller()->getParent()->getDataLayout();
787 if (const GlobalValue *GV =
788 dyn_cast<GlobalValue>(GetUnderlyingObject(Loc.Ptr, DL)))
789 if (GV->hasLocalLinkage())
790 if (const Function *F = CS.getCalledFunction())
791 if (NonAddressTakenGlobals.count(GV))
792 if (const FunctionInfo *FI = getFunctionInfo(F))
793 Known = FI->getModRefInfoForGlobal(*GV);
795 if (Known == MRI_NoModRef)
796 return MRI_NoModRef; // No need to query other mod/ref analyses
797 return ModRefInfo(Known & AliasAnalysis::getModRefInfo(CS, Loc));