1//===- GlobalsModRef.cpp - Simple Mod/Ref Analysis for Globals ------------===//
2//
3// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4// See https://llvm.org/LICENSE.txt for license information.
5// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6//
7//===----------------------------------------------------------------------===//
8//
9// This simple pass provides alias and mod/ref information for global values
10// that do not have their address taken, and keeps track of whether functions
11// read or write memory (are "pure"). For this simple (but very common) case,
12// we can provide pretty accurate and useful information.
13//
14//===----------------------------------------------------------------------===//
15
16#include "llvm/Analysis/GlobalsModRef.h"
17#include "llvm/ADT/SCCIterator.h"
18#include "llvm/ADT/SmallPtrSet.h"
19#include "llvm/ADT/Statistic.h"
20#include "llvm/Analysis/CallGraph.h"
21#include "llvm/Analysis/MemoryBuiltins.h"
22#include "llvm/Analysis/TargetLibraryInfo.h"
23#include "llvm/Analysis/ValueTracking.h"
24#include "llvm/IR/Constants.h"
25#include "llvm/IR/InstIterator.h"
26#include "llvm/IR/Instructions.h"
27#include "llvm/IR/Module.h"
28#include "llvm/IR/PassManager.h"
29#include "llvm/InitializePasses.h"
30#include "llvm/Pass.h"
31#include "llvm/Support/CommandLine.h"
32
33using namespace llvm;
34
35#define DEBUG_TYPE "globalsmodref-aa"
36
37STATISTIC(NumNonAddrTakenGlobalVars,
38 "Number of global vars without address taken");
39STATISTIC(NumNonAddrTakenFunctions,"Number of functions without address taken");
40STATISTIC(NumNoMemFunctions, "Number of functions that do not access memory");
41STATISTIC(NumReadMemFunctions, "Number of functions that only read memory");
42STATISTIC(NumIndirectGlobalVars, "Number of indirect global objects");
43
44// An option to enable unsafe alias results from the GlobalsModRef analysis.
45// When enabled, GlobalsModRef will provide no-alias results which in extremely
46// rare cases may not be conservatively correct. In particular, in the face of
47// transforms which cause asymmetry between how effective getUnderlyingObject
48// is for two pointers, it may produce incorrect results.
49//
50// These unsafe results have been returned by GMR for many years without
51// causing significant issues in the wild and so we provide a mechanism to
52// re-enable them for users of LLVM that have a particular performance
53// sensitivity and no known issues. The option also makes it easy to evaluate
54// the performance impact of these results.
55static cl::opt<bool> EnableUnsafeGlobalsModRefAliasResults(
56 "enable-unsafe-globalsmodref-alias-results", cl::init(Val: false), cl::Hidden);
57
58/// The mod/ref information collected for a particular function.
59///
60/// We collect information about mod/ref behavior of a function here, both in
61/// general and as pertains to specific globals. We only have this detailed
62/// information when we know *something* useful about the behavior. If we
63/// saturate to fully general mod/ref, we remove the info for the function.
64class GlobalsAAResult::FunctionInfo {
65 typedef SmallDenseMap<const GlobalValue *, ModRefInfo, 16> GlobalInfoMapType;
66
67 /// Build a wrapper struct that has 8-byte alignment. All heap allocations
68 /// should provide this much alignment at least, but this makes it clear we
69 /// specifically rely on this amount of alignment.
70 struct alignas(8) AlignedMap {
71 AlignedMap() = default;
72 AlignedMap(const AlignedMap &Arg) = default;
73 GlobalInfoMapType Map;
74 };
75
76 /// Pointer traits for our aligned map.
77 struct AlignedMapPointerTraits {
78 static inline void *getAsVoidPointer(AlignedMap *P) { return P; }
79 static inline AlignedMap *getFromVoidPointer(void *P) {
80 return (AlignedMap *)P;
81 }
82 static constexpr int NumLowBitsAvailable = 3;
83 static_assert(alignof(AlignedMap) >= (1 << NumLowBitsAvailable),
84 "AlignedMap insufficiently aligned to have enough low bits.");
85 };
86
87 /// The bit that flags that this function may read any global. This is
88 /// chosen to mix together with ModRefInfo bits.
89 /// FIXME: This assumes ModRefInfo lattice will remain 4 bits!
90 /// FunctionInfo.getModRefInfo() masks out everything except ModRef so
91 /// this remains correct.
92 enum { MayReadAnyGlobal = 4 };
93
94 /// Checks to document the invariants of the bit packing here.
95 static_assert((MayReadAnyGlobal & static_cast<int>(ModRefInfo::ModRef)) == 0,
96 "ModRef and the MayReadAnyGlobal flag bits overlap.");
97 static_assert(((MayReadAnyGlobal | static_cast<int>(ModRefInfo::ModRef)) >>
98 AlignedMapPointerTraits::NumLowBitsAvailable) == 0,
99 "Insufficient low bits to store our flag and ModRef info.");
100
101public:
102 FunctionInfo() = default;
103 ~FunctionInfo() {
104 delete Info.getPointer();
105 }
106 // Spell out the copy ond move constructors and assignment operators to get
107 // deep copy semantics and correct move semantics in the face of the
108 // pointer-int pair.
109 FunctionInfo(const FunctionInfo &Arg)
110 : Info(nullptr, Arg.Info.getInt()) {
111 if (const auto *ArgPtr = Arg.Info.getPointer())
112 Info.setPointer(new AlignedMap(*ArgPtr));
113 }
114 FunctionInfo(FunctionInfo &&Arg)
115 : Info(Arg.Info.getPointer(), Arg.Info.getInt()) {
116 Arg.Info.setPointerAndInt(PtrVal: nullptr, IntVal: 0);
117 }
118 FunctionInfo &operator=(const FunctionInfo &RHS) {
119 delete Info.getPointer();
120 Info.setPointerAndInt(PtrVal: nullptr, IntVal: RHS.Info.getInt());
121 if (const auto *RHSPtr = RHS.Info.getPointer())
122 Info.setPointer(new AlignedMap(*RHSPtr));
123 return *this;
124 }
125 FunctionInfo &operator=(FunctionInfo &&RHS) {
126 delete Info.getPointer();
127 Info.setPointerAndInt(PtrVal: RHS.Info.getPointer(), IntVal: RHS.Info.getInt());
128 RHS.Info.setPointerAndInt(PtrVal: nullptr, IntVal: 0);
129 return *this;
130 }
131
132 /// This method clears MayReadAnyGlobal bit added by GlobalsAAResult to return
133 /// the corresponding ModRefInfo.
134 ModRefInfo globalClearMayReadAnyGlobal(int I) const {
135 return ModRefInfo(I & static_cast<int>(ModRefInfo::ModRef));
136 }
137
138 /// Returns the \c ModRefInfo info for this function.
139 ModRefInfo getModRefInfo() const {
140 return globalClearMayReadAnyGlobal(I: Info.getInt());
141 }
142
143 /// Adds new \c ModRefInfo for this function to its state.
144 void addModRefInfo(ModRefInfo NewMRI) {
145 Info.setInt(Info.getInt() | static_cast<int>(NewMRI));
146 }
147
148 /// Returns whether this function may read any global variable, and we don't
149 /// know which global.
150 bool mayReadAnyGlobal() const { return Info.getInt() & MayReadAnyGlobal; }
151
152 /// Sets this function as potentially reading from any global.
153 void setMayReadAnyGlobal() { Info.setInt(Info.getInt() | MayReadAnyGlobal); }
154
155 /// Returns the \c ModRefInfo info for this function w.r.t. a particular
156 /// global, which may be more precise than the general information above.
157 ModRefInfo getModRefInfoForGlobal(const GlobalValue &GV) const {
158 ModRefInfo GlobalMRI =
159 mayReadAnyGlobal() ? ModRefInfo::Ref : ModRefInfo::NoModRef;
160 if (AlignedMap *P = Info.getPointer()) {
161 auto I = P->Map.find(Val: &GV);
162 if (I != P->Map.end())
163 GlobalMRI |= I->second;
164 }
165 return GlobalMRI;
166 }
167
168 /// Add mod/ref info from another function into ours, saturating towards
169 /// ModRef.
170 void addFunctionInfo(const FunctionInfo &FI) {
171 addModRefInfo(NewMRI: FI.getModRefInfo());
172
173 if (FI.mayReadAnyGlobal())
174 setMayReadAnyGlobal();
175
176 if (AlignedMap *P = FI.Info.getPointer())
177 for (const auto &G : P->Map)
178 addModRefInfoForGlobal(GV: *G.first, NewMRI: G.second);
179 }
180
181 void addModRefInfoForGlobal(const GlobalValue &GV, ModRefInfo NewMRI) {
182 AlignedMap *P = Info.getPointer();
183 if (!P) {
184 P = new AlignedMap();
185 Info.setPointer(P);
186 }
187 auto &GlobalMRI = P->Map[&GV];
188 GlobalMRI |= NewMRI;
189 }
190
191 /// Clear a global's ModRef info. Should be used when a global is being
192 /// deleted.
193 void eraseModRefInfoForGlobal(const GlobalValue &GV) {
194 if (AlignedMap *P = Info.getPointer())
195 P->Map.erase(Val: &GV);
196 }
197
198private:
199 /// All of the information is encoded into a single pointer, with a three bit
200 /// integer in the low three bits. The high bit provides a flag for when this
201 /// function may read any global. The low two bits are the ModRefInfo. And
202 /// the pointer, when non-null, points to a map from GlobalValue to
203 /// ModRefInfo specific to that GlobalValue.
204 PointerIntPair<AlignedMap *, 3, unsigned, AlignedMapPointerTraits> Info;
205};
206
207void GlobalsAAResult::DeletionCallbackHandle::deleted() {
208 Value *V = getValPtr();
209 if (auto *F = dyn_cast<Function>(Val: V))
210 GAR->FunctionInfos.erase(Val: F);
211
212 if (GlobalValue *GV = dyn_cast<GlobalValue>(Val: V)) {
213 if (GAR->NonAddressTakenGlobals.erase(Ptr: GV)) {
214 // This global might be an indirect global. If so, remove it and
215 // remove any AllocRelatedValues for it.
216 if (GAR->IndirectGlobals.erase(Ptr: GV)) {
217 // Remove any entries in AllocsForIndirectGlobals for this global.
218 GAR->AllocsForIndirectGlobals.remove_if(
219 Pred: [GV](const auto &Entry) { return Entry.second == GV; });
220 }
221
222 // Scan the function info we have collected and remove this global
223 // from all of them.
224 for (auto &FIPair : GAR->FunctionInfos)
225 FIPair.second.eraseModRefInfoForGlobal(GV: *GV);
226 }
227 }
228
229 // If this is an allocation related to an indirect global, remove it.
230 GAR->AllocsForIndirectGlobals.erase(Val: V);
231
232 // And clear out the handle.
233 setValPtr(nullptr);
234 GAR->Handles.erase(position: I);
235 // This object is now destroyed!
236}
237
238MemoryEffects GlobalsAAResult::getMemoryEffects(const Function *F) {
239 if (FunctionInfo *FI = getFunctionInfo(F))
240 return MemoryEffects(FI->getModRefInfo());
241
242 return MemoryEffects::unknown();
243}
244
245/// Returns the function info for the function, or null if we don't have
246/// anything useful to say about it.
247GlobalsAAResult::FunctionInfo *
248GlobalsAAResult::getFunctionInfo(const Function *F) {
249 auto I = FunctionInfos.find(Val: F);
250 if (I != FunctionInfos.end())
251 return &I->second;
252 return nullptr;
253}
254
255/// AnalyzeGlobals - Scan through the users of all of the internal
256/// GlobalValue's in the program. If none of them have their "address taken"
257/// (really, their address passed to something nontrivial), record this fact,
258/// and record the functions that they are used directly in.
259void GlobalsAAResult::AnalyzeGlobals(Module &M) {
260 SmallPtrSet<Function *, 32> TrackedFunctions;
261 for (Function &F : M)
262 if (F.hasLocalLinkage()) {
263 if (!AnalyzeUsesOfPointer(V: &F)) {
264 // Remember that we are tracking this global.
265 NonAddressTakenGlobals.insert(Ptr: &F);
266 TrackedFunctions.insert(Ptr: &F);
267 Handles.emplace_front(args&: *this, args: &F);
268 Handles.front().I = Handles.begin();
269 ++NumNonAddrTakenFunctions;
270 } else
271 UnknownFunctionsWithLocalLinkage = true;
272 }
273
274 SmallPtrSet<Function *, 16> Readers, Writers;
275 for (GlobalVariable &GV : M.globals())
276 if (GV.hasLocalLinkage()) {
277 if (!AnalyzeUsesOfPointer(V: &GV, Readers: &Readers,
278 Writers: GV.isConstant() ? nullptr : &Writers)) {
279 // Remember that we are tracking this global, and the mod/ref fns
280 NonAddressTakenGlobals.insert(Ptr: &GV);
281 Handles.emplace_front(args&: *this, args: &GV);
282 Handles.front().I = Handles.begin();
283
284 for (Function *Reader : Readers) {
285 if (TrackedFunctions.insert(Ptr: Reader).second) {
286 Handles.emplace_front(args&: *this, args&: Reader);
287 Handles.front().I = Handles.begin();
288 }
289 FunctionInfos[Reader].addModRefInfoForGlobal(GV, NewMRI: ModRefInfo::Ref);
290 }
291
292 if (!GV.isConstant()) // No need to keep track of writers to constants
293 for (Function *Writer : Writers) {
294 if (TrackedFunctions.insert(Ptr: Writer).second) {
295 Handles.emplace_front(args&: *this, args&: Writer);
296 Handles.front().I = Handles.begin();
297 }
298 FunctionInfos[Writer].addModRefInfoForGlobal(GV, NewMRI: ModRefInfo::Mod);
299 }
300 ++NumNonAddrTakenGlobalVars;
301
302 // If this global holds a pointer type, see if it is an indirect global.
303 if (GV.getValueType()->isPointerTy() &&
304 AnalyzeIndirectGlobalMemory(GV: &GV))
305 ++NumIndirectGlobalVars;
306 }
307 Readers.clear();
308 Writers.clear();
309 }
310}
311
312/// AnalyzeUsesOfPointer - Look at all of the users of the specified pointer.
313/// If this is used by anything complex (i.e., the address escapes), return
314/// true. Also, while we are at it, keep track of those functions that read and
315/// write to the value.
316///
317/// If OkayStoreDest is non-null, stores into this global are allowed.
318bool GlobalsAAResult::AnalyzeUsesOfPointer(Value *V,
319 SmallPtrSetImpl<Function *> *Readers,
320 SmallPtrSetImpl<Function *> *Writers,
321 GlobalValue *OkayStoreDest) {
322 if (!V->getType()->isPointerTy())
323 return true;
324
325 for (Use &U : V->uses()) {
326 User *I = U.getUser();
327 if (LoadInst *LI = dyn_cast<LoadInst>(Val: I)) {
328 if (Readers)
329 Readers->insert(Ptr: LI->getParent()->getParent());
330 } else if (StoreInst *SI = dyn_cast<StoreInst>(Val: I)) {
331 if (V == SI->getOperand(i_nocapture: 1)) {
332 if (Writers)
333 Writers->insert(Ptr: SI->getParent()->getParent());
334 } else if (SI->getOperand(i_nocapture: 1) != OkayStoreDest) {
335 return true; // Storing the pointer
336 }
337 } else if (Operator::getOpcode(V: I) == Instruction::GetElementPtr) {
338 if (AnalyzeUsesOfPointer(V: I, Readers, Writers))
339 return true;
340 } else if (Operator::getOpcode(V: I) == Instruction::BitCast ||
341 Operator::getOpcode(V: I) == Instruction::AddrSpaceCast) {
342 if (AnalyzeUsesOfPointer(V: I, Readers, Writers, OkayStoreDest))
343 return true;
344 } else if (auto *Call = dyn_cast<CallBase>(Val: I)) {
345 if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(Val: I)) {
346 if (II->getIntrinsicID() == Intrinsic::threadlocal_address &&
347 V == II->getArgOperand(i: 0)) {
348 if (AnalyzeUsesOfPointer(V: II, Readers, Writers))
349 return true;
350 continue;
351 }
352 }
353 // Make sure that this is just the function being called, not that it is
354 // passing into the function.
355 if (Call->isDataOperand(U: &U)) {
356 // Detect calls to free.
357 if (Call->isArgOperand(U: &U) &&
358 getFreedOperand(CB: Call, TLI: &GetTLI(*Call->getFunction())) == U) {
359 if (Writers)
360 Writers->insert(Ptr: Call->getParent()->getParent());
361 } else {
362 // In general, we return true for unknown calls, but there are
363 // some simple checks that we can do for functions that
364 // will never call back into the module.
365 auto *F = Call->getCalledFunction();
366 // TODO: we should be able to remove isDeclaration() check
367 // and let the function body analysis check for captures,
368 // and collect the mod-ref effects. This information will
369 // be later propagated via the call graph.
370 if (!F || !F->isDeclaration())
371 return true;
372 // Note that the NoCallback check here is a little bit too
373 // conservative. If there are no captures of the global
374 // in the module, then this call may not be a capture even
375 // if it does not have NoCallback.
376 if (!Call->hasFnAttr(Kind: Attribute::NoCallback) ||
377 !Call->isArgOperand(U: &U) ||
378 !Call->doesNotCapture(OpNo: Call->getArgOperandNo(U: &U)))
379 return true;
380
381 // Conservatively, assume the call reads and writes the global.
382 // We could use memory attributes to make it more precise.
383 if (Readers)
384 Readers->insert(Ptr: Call->getParent()->getParent());
385 if (Writers)
386 Writers->insert(Ptr: Call->getParent()->getParent());
387 }
388 }
389 } else if (ICmpInst *ICI = dyn_cast<ICmpInst>(Val: I)) {
390 if (!isa<ConstantPointerNull>(Val: ICI->getOperand(i_nocapture: 1)))
391 return true; // Allow comparison against null.
392 } else if (Constant *C = dyn_cast<Constant>(Val: I)) {
393 // Ignore constants which don't have any live uses.
394 if (isa<GlobalValue>(Val: C) || C->isConstantUsed())
395 return true;
396 } else {
397 return true;
398 }
399 }
400
401 return false;
402}
403
404/// AnalyzeIndirectGlobalMemory - We found an non-address-taken global variable
405/// which holds a pointer type. See if the global always points to non-aliased
406/// heap memory: that is, all initializers of the globals store a value known
407/// to be obtained via a noalias return function call which have no other use.
408/// Further, all loads out of GV must directly use the memory, not store the
409/// pointer somewhere. If this is true, we consider the memory pointed to by
410/// GV to be owned by GV and can disambiguate other pointers from it.
411bool GlobalsAAResult::AnalyzeIndirectGlobalMemory(GlobalVariable *GV) {
412 // Keep track of values related to the allocation of the memory, f.e. the
413 // value produced by the noalias call and any casts.
414 std::vector<Value *> AllocRelatedValues;
415
416 // If the initializer is a non-null pointer, bail.
417 if (Constant *C = GV->getInitializer())
418 if (!isa<ConstantPointerNull>(Val: C))
419 return false;
420
421 // Walk the user list of the global. If we find anything other than a direct
422 // load or store, bail out.
423 for (User *U : GV->users()) {
424 if (LoadInst *LI = dyn_cast<LoadInst>(Val: U)) {
425 // The pointer loaded from the global can only be used in simple ways:
426 // we allow addressing of it and loading storing to it. We do *not* allow
427 // storing the loaded pointer somewhere else or passing to a function.
428 if (AnalyzeUsesOfPointer(V: LI))
429 return false; // Loaded pointer escapes.
430 // TODO: Could try some IP mod/ref of the loaded pointer.
431 } else if (StoreInst *SI = dyn_cast<StoreInst>(Val: U)) {
432 // Storing the global itself.
433 if (SI->getOperand(i_nocapture: 0) == GV)
434 return false;
435
436 // If storing the null pointer, ignore it.
437 if (isa<ConstantPointerNull>(Val: SI->getOperand(i_nocapture: 0)))
438 continue;
439
440 // Check the value being stored.
441 Value *Ptr = getUnderlyingObject(V: SI->getOperand(i_nocapture: 0));
442
443 if (!isNoAliasCall(V: Ptr))
444 return false; // Too hard to analyze.
445
446 // Analyze all uses of the allocation. If any of them are used in a
447 // non-simple way (e.g. stored to another global) bail out.
448 if (AnalyzeUsesOfPointer(V: Ptr, /*Readers*/ nullptr, /*Writers*/ nullptr,
449 OkayStoreDest: GV))
450 return false; // Loaded pointer escapes.
451
452 // Remember that this allocation is related to the indirect global.
453 AllocRelatedValues.push_back(x: Ptr);
454 } else {
455 // Something complex, bail out.
456 return false;
457 }
458 }
459
460 // Okay, this is an indirect global. Remember all of the allocations for
461 // this global in AllocsForIndirectGlobals.
462 while (!AllocRelatedValues.empty()) {
463 AllocsForIndirectGlobals[AllocRelatedValues.back()] = GV;
464 Handles.emplace_front(args&: *this, args&: AllocRelatedValues.back());
465 Handles.front().I = Handles.begin();
466 AllocRelatedValues.pop_back();
467 }
468 IndirectGlobals.insert(Ptr: GV);
469 Handles.emplace_front(args&: *this, args&: GV);
470 Handles.front().I = Handles.begin();
471 return true;
472}
473
474void GlobalsAAResult::CollectSCCMembership(CallGraph &CG) {
475 // We do a bottom-up SCC traversal of the call graph. In other words, we
476 // visit all callees before callers (leaf-first).
477 unsigned SCCID = 0;
478 for (scc_iterator<CallGraph *> I = scc_begin(G: &CG); !I.isAtEnd(); ++I) {
479 const std::vector<CallGraphNode *> &SCC = *I;
480 assert(!SCC.empty() && "SCC with no functions?");
481
482 for (auto *CGN : SCC)
483 if (Function *F = CGN->getFunction())
484 FunctionToSCCMap[F] = SCCID;
485 ++SCCID;
486 }
487}
488
489/// AnalyzeCallGraph - At this point, we know the functions where globals are
490/// immediately stored to and read from. Propagate this information up the call
491/// graph to all callers and compute the mod/ref info for all memory for each
492/// function.
493void GlobalsAAResult::AnalyzeCallGraph(CallGraph &CG, Module &M) {
494 // We do a bottom-up SCC traversal of the call graph. In other words, we
495 // visit all callees before callers (leaf-first).
496 for (scc_iterator<CallGraph *> I = scc_begin(G: &CG); !I.isAtEnd(); ++I) {
497 const std::vector<CallGraphNode *> &SCC = *I;
498 assert(!SCC.empty() && "SCC with no functions?");
499
500 Function *F = SCC[0]->getFunction();
501
502 if (!F || !F->isDefinitionExact()) {
503 // Calls externally or not exact - can't say anything useful. Remove any
504 // existing function records (may have been created when scanning
505 // globals).
506 for (auto *Node : SCC)
507 FunctionInfos.erase(Val: Node->getFunction());
508 continue;
509 }
510
511 FunctionInfo &FI = FunctionInfos[F];
512 Handles.emplace_front(args&: *this, args&: F);
513 Handles.front().I = Handles.begin();
514 bool KnowNothing = false;
515
516 // Intrinsics, like any other synchronizing function, can make effects
517 // of other threads visible. Without nosync we know nothing really.
518 // Similarly, if `nocallback` is missing the function, or intrinsic,
519 // can call into the module arbitrarily. If both are set the function
520 // has an effect but will not interact with accesses of internal
521 // globals inside the module. We are conservative here for optnone
522 // functions, might not be necessary.
523 auto MaySyncOrCallIntoModule = [](const Function &F) {
524 return !F.isDeclaration() || !F.hasNoSync() ||
525 !F.hasFnAttribute(Kind: Attribute::NoCallback);
526 };
527
528 // Collect the mod/ref properties due to called functions. We only compute
529 // one mod-ref set.
530 for (unsigned i = 0, e = SCC.size(); i != e && !KnowNothing; ++i) {
531 if (!F) {
532 KnowNothing = true;
533 break;
534 }
535
536 if (F->isDeclaration() || F->hasOptNone()) {
537 // Try to get mod/ref behaviour from function attributes.
538 if (F->doesNotAccessMemory()) {
539 // Can't do better than that!
540 } else if (F->onlyReadsMemory()) {
541 FI.addModRefInfo(NewMRI: ModRefInfo::Ref);
542 if (!F->onlyAccessesArgMemory() && MaySyncOrCallIntoModule(*F))
543 // This function might call back into the module and read a global -
544 // consider every global as possibly being read by this function.
545 FI.setMayReadAnyGlobal();
546 } else {
547 FI.addModRefInfo(NewMRI: ModRefInfo::ModRef);
548 if (!F->onlyAccessesArgMemory())
549 FI.setMayReadAnyGlobal();
550 if (MaySyncOrCallIntoModule(*F)) {
551 KnowNothing = true;
552 break;
553 }
554 }
555 continue;
556 }
557
558 for (CallGraphNode::iterator CI = SCC[i]->begin(), E = SCC[i]->end();
559 CI != E && !KnowNothing; ++CI)
560 if (Function *Callee = CI->second->getFunction()) {
561 if (FunctionInfo *CalleeFI = getFunctionInfo(F: Callee)) {
562 // Propagate function effect up.
563 FI.addFunctionInfo(FI: *CalleeFI);
564 } else {
565 // Can't say anything about it. However, if it is inside our SCC,
566 // then nothing needs to be done.
567 CallGraphNode *CalleeNode = CG[Callee];
568 if (!is_contained(Range: SCC, Element: CalleeNode))
569 KnowNothing = true;
570 }
571 } else {
572 KnowNothing = true;
573 }
574 }
575
576 // If we can't say anything useful about this SCC, remove all SCC functions
577 // from the FunctionInfos map.
578 if (KnowNothing) {
579 for (auto *Node : SCC)
580 FunctionInfos.erase(Val: Node->getFunction());
581 continue;
582 }
583
584 // Scan the function bodies for explicit loads or stores.
585 for (auto *Node : SCC) {
586 if (isModAndRefSet(MRI: FI.getModRefInfo()))
587 break; // The mod/ref lattice saturates here.
588
589 // Don't prove any properties based on the implementation of an optnone
590 // function. Function attributes were already used as a best approximation
591 // above.
592 if (Node->getFunction()->hasOptNone())
593 continue;
594
595 for (Instruction &I : instructions(F: Node->getFunction())) {
596 if (isModAndRefSet(MRI: FI.getModRefInfo()))
597 break; // The mod/ref lattice saturates here.
598
599 // We handle calls specially because the graph-relevant aspects are
600 // handled above.
601 if (isa<CallBase>(Val: &I))
602 continue;
603
604 // All non-call instructions we use the primary predicates for whether
605 // they read or write memory.
606 if (I.mayReadFromMemory())
607 FI.addModRefInfo(NewMRI: ModRefInfo::Ref);
608 if (I.mayWriteToMemory())
609 FI.addModRefInfo(NewMRI: ModRefInfo::Mod);
610 }
611 }
612
613 if (!isModSet(MRI: FI.getModRefInfo()))
614 ++NumReadMemFunctions;
615 if (!isModOrRefSet(MRI: FI.getModRefInfo()))
616 ++NumNoMemFunctions;
617
618 // Finally, now that we know the full effect on this SCC, clone the
619 // information to each function in the SCC.
620 // FI is a reference into FunctionInfos, so copy it now so that it doesn't
621 // get invalidated if DenseMap decides to re-hash.
622 FunctionInfo CachedFI = FI;
623 for (unsigned i = 1, e = SCC.size(); i != e; ++i)
624 FunctionInfos[SCC[i]->getFunction()] = CachedFI;
625 }
626}
627
628// GV is a non-escaping global. V is a pointer address that has been loaded from.
629// If we can prove that V must escape, we can conclude that a load from V cannot
630// alias GV.
631static bool isNonEscapingGlobalNoAliasWithLoad(const GlobalValue *GV,
632 const Value *V,
633 int &Depth,
634 const DataLayout &DL) {
635 SmallPtrSet<const Value *, 8> Visited;
636 SmallVector<const Value *, 8> Inputs;
637 Visited.insert(Ptr: V);
638 Inputs.push_back(Elt: V);
639 do {
640 const Value *Input = Inputs.pop_back_val();
641
642 if (isa<GlobalValue>(Val: Input) || isa<Argument>(Val: Input) || isa<CallInst>(Val: Input) ||
643 isa<InvokeInst>(Val: Input))
644 // Arguments to functions or returns from functions are inherently
645 // escaping, so we can immediately classify those as not aliasing any
646 // non-addr-taken globals.
647 //
648 // (Transitive) loads from a global are also safe - if this aliased
649 // another global, its address would escape, so no alias.
650 continue;
651
652 // Recurse through a limited number of selects, loads and PHIs. This is an
653 // arbitrary depth of 4, lower numbers could be used to fix compile time
654 // issues if needed, but this is generally expected to be only be important
655 // for small depths.
656 if (++Depth > 4)
657 return false;
658
659 if (auto *LI = dyn_cast<LoadInst>(Val: Input)) {
660 Inputs.push_back(Elt: getUnderlyingObject(V: LI->getPointerOperand()));
661 continue;
662 }
663 if (auto *SI = dyn_cast<SelectInst>(Val: Input)) {
664 const Value *LHS = getUnderlyingObject(V: SI->getTrueValue());
665 const Value *RHS = getUnderlyingObject(V: SI->getFalseValue());
666 if (Visited.insert(Ptr: LHS).second)
667 Inputs.push_back(Elt: LHS);
668 if (Visited.insert(Ptr: RHS).second)
669 Inputs.push_back(Elt: RHS);
670 continue;
671 }
672 if (auto *PN = dyn_cast<PHINode>(Val: Input)) {
673 for (const Value *Op : PN->incoming_values()) {
674 Op = getUnderlyingObject(V: Op);
675 if (Visited.insert(Ptr: Op).second)
676 Inputs.push_back(Elt: Op);
677 }
678 continue;
679 }
680
681 return false;
682 } while (!Inputs.empty());
683
684 // All inputs were known to be no-alias.
685 return true;
686}
687
688// There are particular cases where we can conclude no-alias between
689// a non-addr-taken global and some other underlying object. Specifically,
690// a non-addr-taken global is known to not be escaped from any function. It is
691// also incorrect for a transformation to introduce an escape of a global in
692// a way that is observable when it was not there previously. One function
693// being transformed to introduce an escape which could possibly be observed
694// (via loading from a global or the return value for example) within another
695// function is never safe. If the observation is made through non-atomic
696// operations on different threads, it is a data-race and UB. If the
697// observation is well defined, by being observed the transformation would have
698// changed program behavior by introducing the observed escape, making it an
699// invalid transform.
700//
701// This property does require that transformations which *temporarily* escape
702// a global that was not previously escaped, prior to restoring it, cannot rely
703// on the results of GMR::alias. This seems a reasonable restriction, although
704// currently there is no way to enforce it. There is also no realistic
705// optimization pass that would make this mistake. The closest example is
706// a transformation pass which does reg2mem of SSA values but stores them into
707// global variables temporarily before restoring the global variable's value.
708// This could be useful to expose "benign" races for example. However, it seems
709// reasonable to require that a pass which introduces escapes of global
710// variables in this way to either not trust AA results while the escape is
711// active, or to be forced to operate as a module pass that cannot co-exist
712// with an alias analysis such as GMR.
713bool GlobalsAAResult::isNonEscapingGlobalNoAlias(const GlobalValue *GV,
714 const Value *V,
715 const Instruction *CtxI) {
716 // In order to know that the underlying object cannot alias the
717 // non-addr-taken global, we must know that it would have to be an escape.
718 // Thus if the underlying object is a function argument, a load from
719 // a global, or the return of a function, it cannot alias. We can also
720 // recurse through PHI nodes and select nodes provided all of their inputs
721 // resolve to one of these known-escaping roots.
722
723 // A non-addr-taken global cannot alias with any non-pointer value.
724 // Check this early and exit.
725 if (!V->getType()->isPointerTy())
726 return true;
727
728 SmallPtrSet<const Value *, 8> Visited;
729 SmallVector<const Value *, 8> Inputs;
730 Visited.insert(Ptr: V);
731 Inputs.push_back(Elt: V);
732 int Depth = 0;
733 do {
734 const Value *Input = Inputs.pop_back_val();
735
736 if (auto *InputGV = dyn_cast<GlobalValue>(Val: Input)) {
737 // If one input is the very global we're querying against, then we can't
738 // conclude no-alias.
739 if (InputGV == GV)
740 return false;
741
742 // Distinct GlobalVariables never alias, unless overriden or zero-sized.
743 // FIXME: The condition can be refined, but be conservative for now.
744 auto *GVar = dyn_cast<GlobalVariable>(Val: GV);
745 auto *InputGVar = dyn_cast<GlobalVariable>(Val: InputGV);
746 if (GVar && InputGVar &&
747 !GVar->isDeclaration() && !InputGVar->isDeclaration() &&
748 !GVar->isInterposable() && !InputGVar->isInterposable()) {
749 Type *GVType = GVar->getInitializer()->getType();
750 Type *InputGVType = InputGVar->getInitializer()->getType();
751 if (GVType->isSized() && InputGVType->isSized() &&
752 (DL.getTypeAllocSize(Ty: GVType) > 0) &&
753 (DL.getTypeAllocSize(Ty: InputGVType) > 0))
754 continue;
755 }
756
757 // Conservatively return false, even though we could be smarter
758 // (e.g. look through GlobalAliases).
759 return false;
760 }
761
762 if (isa<Argument>(Val: Input) || isa<CallInst>(Val: Input) ||
763 isa<InvokeInst>(Val: Input)) {
764 // Arguments to functions or returns from functions are inherently
765 // escaping, so we can immediately classify those as not aliasing any
766 // non-addr-taken globals.
767 continue;
768 }
769
770 if (CtxI)
771 if (auto *CPN = dyn_cast<ConstantPointerNull>(Val: Input)) {
772 // Null pointer cannot alias with a non-addr-taken global.
773 const Function *F = CtxI->getFunction();
774 if (!NullPointerIsDefined(F, AS: CPN->getPointerType()->getAddressSpace()))
775 continue;
776 }
777
778 // Recurse through a limited number of selects, loads and PHIs. This is an
779 // arbitrary depth of 4, lower numbers could be used to fix compile time
780 // issues if needed, but this is generally expected to be only be important
781 // for small depths.
782 if (++Depth > 4)
783 return false;
784
785 if (auto *LI = dyn_cast<LoadInst>(Val: Input)) {
786 // A pointer loaded from a global would have been captured, and we know
787 // that the global is non-escaping, so no alias.
788 const Value *Ptr = getUnderlyingObject(V: LI->getPointerOperand());
789 if (isNonEscapingGlobalNoAliasWithLoad(GV, V: Ptr, Depth, DL))
790 // The load does not alias with GV.
791 continue;
792 // Otherwise, a load could come from anywhere, so bail.
793 return false;
794 }
795 if (auto *SI = dyn_cast<SelectInst>(Val: Input)) {
796 const Value *LHS = getUnderlyingObject(V: SI->getTrueValue());
797 const Value *RHS = getUnderlyingObject(V: SI->getFalseValue());
798 if (Visited.insert(Ptr: LHS).second)
799 Inputs.push_back(Elt: LHS);
800 if (Visited.insert(Ptr: RHS).second)
801 Inputs.push_back(Elt: RHS);
802 continue;
803 }
804 if (auto *PN = dyn_cast<PHINode>(Val: Input)) {
805 for (const Value *Op : PN->incoming_values()) {
806 Op = getUnderlyingObject(V: Op);
807 if (Visited.insert(Ptr: Op).second)
808 Inputs.push_back(Elt: Op);
809 }
810 continue;
811 }
812
813 // FIXME: It would be good to handle other obvious no-alias cases here, but
814 // it isn't clear how to do so reasonably without building a small version
815 // of BasicAA into this code.
816 return false;
817 } while (!Inputs.empty());
818
819 // If all the inputs to V were definitively no-alias, then V is no-alias.
820 return true;
821}
822
823bool GlobalsAAResult::invalidate(Module &, const PreservedAnalyses &PA,
824 ModuleAnalysisManager::Invalidator &) {
825 // Check whether the analysis has been explicitly invalidated. Otherwise, it's
826 // stateless and remains preserved.
827 auto PAC = PA.getChecker<GlobalsAA>();
828 return !PAC.preservedWhenStateless();
829}
830
831/// alias - If one of the pointers is to a global that we are tracking, and the
832/// other is some random pointer, we know there cannot be an alias, because the
833/// address of the global isn't taken.
834AliasResult GlobalsAAResult::alias(const MemoryLocation &LocA,
835 const MemoryLocation &LocB,
836 AAQueryInfo &AAQI, const Instruction *CtxI) {
837 // Get the base object these pointers point to.
838 const Value *UV1 =
839 getUnderlyingObject(V: LocA.Ptr->stripPointerCastsForAliasAnalysis());
840 const Value *UV2 =
841 getUnderlyingObject(V: LocB.Ptr->stripPointerCastsForAliasAnalysis());
842
843 // If either of the underlying values is a global, they may be non-addr-taken
844 // globals, which we can answer queries about.
845 const GlobalValue *GV1 = dyn_cast<GlobalValue>(Val: UV1);
846 const GlobalValue *GV2 = dyn_cast<GlobalValue>(Val: UV2);
847 if (GV1 || GV2) {
848 // If the global's address is taken, pretend we don't know it's a pointer to
849 // the global.
850 if (GV1 && !NonAddressTakenGlobals.count(Ptr: GV1))
851 GV1 = nullptr;
852 if (GV2 && !NonAddressTakenGlobals.count(Ptr: GV2))
853 GV2 = nullptr;
854
855 // If the two pointers are derived from two different non-addr-taken
856 // globals we know these can't alias.
857 if (GV1 && GV2 && GV1 != GV2)
858 return AliasResult::NoAlias;
859
860 // If one is and the other isn't, it isn't strictly safe but we can fake
861 // this result if necessary for performance. This does not appear to be
862 // a common problem in practice.
863 if (EnableUnsafeGlobalsModRefAliasResults)
864 if ((GV1 || GV2) && GV1 != GV2)
865 return AliasResult::NoAlias;
866
867 // Check for a special case where a non-escaping global can be used to
868 // conclude no-alias.
869 if ((GV1 || GV2) && GV1 != GV2) {
870 const GlobalValue *GV = GV1 ? GV1 : GV2;
871 const Value *UV = GV1 ? UV2 : UV1;
872 if (isNonEscapingGlobalNoAlias(GV, V: UV, CtxI))
873 return AliasResult::NoAlias;
874 }
875
876 // Otherwise if they are both derived from the same addr-taken global, we
877 // can't know the two accesses don't overlap.
878 }
879
880 // These pointers may be based on the memory owned by an indirect global. If
881 // so, we may be able to handle this. First check to see if the base pointer
882 // is a direct load from an indirect global.
883 GV1 = GV2 = nullptr;
884 if (const LoadInst *LI = dyn_cast<LoadInst>(Val: UV1))
885 if (GlobalVariable *GV = dyn_cast<GlobalVariable>(Val: LI->getOperand(i_nocapture: 0)))
886 if (IndirectGlobals.count(Ptr: GV))
887 GV1 = GV;
888 if (const LoadInst *LI = dyn_cast<LoadInst>(Val: UV2))
889 if (const GlobalVariable *GV = dyn_cast<GlobalVariable>(Val: LI->getOperand(i_nocapture: 0)))
890 if (IndirectGlobals.count(Ptr: GV))
891 GV2 = GV;
892
893 // These pointers may also be from an allocation for the indirect global. If
894 // so, also handle them.
895 if (!GV1)
896 GV1 = AllocsForIndirectGlobals.lookup(Val: UV1);
897 if (!GV2)
898 GV2 = AllocsForIndirectGlobals.lookup(Val: UV2);
899
900 // Now that we know whether the two pointers are related to indirect globals,
901 // use this to disambiguate the pointers. If the pointers are based on
902 // different indirect globals they cannot alias.
903 if (GV1 && GV2 && GV1 != GV2)
904 return AliasResult::NoAlias;
905
906 // If one is based on an indirect global and the other isn't, it isn't
907 // strictly safe but we can fake this result if necessary for performance.
908 // This does not appear to be a common problem in practice.
909 if (EnableUnsafeGlobalsModRefAliasResults)
910 if ((GV1 || GV2) && GV1 != GV2)
911 return AliasResult::NoAlias;
912
913 return AliasResult::MayAlias;
914}
915
916ModRefInfo GlobalsAAResult::getModRefInfoForArgument(const CallBase *Call,
917 const GlobalValue *GV,
918 AAQueryInfo &AAQI) {
919 if (Call->doesNotAccessMemory())
920 return ModRefInfo::NoModRef;
921 ModRefInfo ConservativeResult =
922 Call->onlyReadsMemory() ? ModRefInfo::Ref : ModRefInfo::ModRef;
923
924 // Iterate through all the arguments to the called function. If any argument
925 // is based on GV, return the conservative result.
926 for (const auto &A : Call->args()) {
927 SmallVector<const Value*, 4> Objects;
928 getUnderlyingObjects(V: A, Objects);
929
930 // All objects must be identified.
931 if (!all_of(Range&: Objects, P: isIdentifiedObject) &&
932 // Try ::alias to see if all objects are known not to alias GV.
933 !all_of(Range&: Objects, P: [&](const Value *V) {
934 return this->alias(LocA: MemoryLocation::getBeforeOrAfter(Ptr: V),
935 LocB: MemoryLocation::getBeforeOrAfter(Ptr: GV), AAQI,
936 CtxI: Call) == AliasResult::NoAlias;
937 }))
938 return ConservativeResult;
939
940 if (is_contained(Range&: Objects, Element: GV))
941 return ConservativeResult;
942 }
943
944 // We identified all objects in the argument list, and none of them were GV.
945 return ModRefInfo::NoModRef;
946}
947
948ModRefInfo GlobalsAAResult::getModRefInfo(const CallBase *Call,
949 const MemoryLocation &Loc,
950 AAQueryInfo &AAQI) {
951 ModRefInfo Known = ModRefInfo::ModRef;
952
953 // If we are asking for mod/ref info of a direct call with a pointer to a
954 // global we are tracking, return information if we have it.
955 if (const GlobalValue *GV =
956 dyn_cast<GlobalValue>(Val: getUnderlyingObject(V: Loc.Ptr)))
957 // If GV is internal to this IR and there is no function with local linkage
958 // that has had their address taken, keep looking for a tighter ModRefInfo.
959 if (GV->hasLocalLinkage() && !UnknownFunctionsWithLocalLinkage)
960 if (const Function *F = Call->getCalledFunction())
961 if (NonAddressTakenGlobals.count(Ptr: GV))
962 if (const FunctionInfo *FI = getFunctionInfo(F))
963 Known = FI->getModRefInfoForGlobal(GV: *GV) |
964 getModRefInfoForArgument(Call, GV, AAQI);
965
966 return Known;
967}
968
969GlobalsAAResult::GlobalsAAResult(
970 const DataLayout &DL,
971 std::function<const TargetLibraryInfo &(Function &F)> GetTLI)
972 : DL(DL), GetTLI(std::move(GetTLI)) {}
973
974GlobalsAAResult::GlobalsAAResult(GlobalsAAResult &&Arg)
975 : AAResultBase(std::move(Arg)), DL(Arg.DL), GetTLI(std::move(Arg.GetTLI)),
976 NonAddressTakenGlobals(std::move(Arg.NonAddressTakenGlobals)),
977 IndirectGlobals(std::move(Arg.IndirectGlobals)),
978 AllocsForIndirectGlobals(std::move(Arg.AllocsForIndirectGlobals)),
979 FunctionInfos(std::move(Arg.FunctionInfos)),
980 Handles(std::move(Arg.Handles)) {
981 // Update the parent for each DeletionCallbackHandle.
982 for (auto &H : Handles) {
983 assert(H.GAR == &Arg);
984 H.GAR = this;
985 }
986}
987
988GlobalsAAResult::~GlobalsAAResult() = default;
989
990/*static*/ GlobalsAAResult GlobalsAAResult::analyzeModule(
991 Module &M, std::function<const TargetLibraryInfo &(Function &F)> GetTLI,
992 CallGraph &CG) {
993 GlobalsAAResult Result(M.getDataLayout(), GetTLI);
994
995 // Discover which functions aren't recursive, to feed into AnalyzeGlobals.
996 Result.CollectSCCMembership(CG);
997
998 // Find non-addr taken globals.
999 Result.AnalyzeGlobals(M);
1000
1001 // Propagate on CG.
1002 Result.AnalyzeCallGraph(CG, M);
1003
1004 return Result;
1005}
1006
1007AnalysisKey GlobalsAA::Key;
1008
1009GlobalsAAResult GlobalsAA::run(Module &M, ModuleAnalysisManager &AM) {
1010 FunctionAnalysisManager &FAM =
1011 AM.getResult<FunctionAnalysisManagerModuleProxy>(IR&: M).getManager();
1012 auto GetTLI = [&FAM](Function &F) -> TargetLibraryInfo & {
1013 return FAM.getResult<TargetLibraryAnalysis>(IR&: F);
1014 };
1015 return GlobalsAAResult::analyzeModule(M, GetTLI,
1016 CG&: AM.getResult<CallGraphAnalysis>(IR&: M));
1017}
1018
1019PreservedAnalyses RecomputeGlobalsAAPass::run(Module &M,
1020 ModuleAnalysisManager &AM) {
1021 if (auto *G = AM.getCachedResult<GlobalsAA>(IR&: M)) {
1022 auto &CG = AM.getResult<CallGraphAnalysis>(IR&: M);
1023 G->NonAddressTakenGlobals.clear();
1024 G->UnknownFunctionsWithLocalLinkage = false;
1025 G->IndirectGlobals.clear();
1026 G->AllocsForIndirectGlobals.clear();
1027 G->FunctionInfos.clear();
1028 G->FunctionToSCCMap.clear();
1029 G->Handles.clear();
1030 G->CollectSCCMembership(CG);
1031 G->AnalyzeGlobals(M);
1032 G->AnalyzeCallGraph(CG, M);
1033 }
1034 return PreservedAnalyses::all();
1035}
1036
1037char GlobalsAAWrapperPass::ID = 0;
1038INITIALIZE_PASS_BEGIN(GlobalsAAWrapperPass, "globals-aa",
1039 "Globals Alias Analysis", false, true)
1040INITIALIZE_PASS_DEPENDENCY(CallGraphWrapperPass)
1041INITIALIZE_PASS_DEPENDENCY(TargetLibraryInfoWrapperPass)
1042INITIALIZE_PASS_END(GlobalsAAWrapperPass, "globals-aa",
1043 "Globals Alias Analysis", false, true)
1044
1045ModulePass *llvm::createGlobalsAAWrapperPass() {
1046 return new GlobalsAAWrapperPass();
1047}
1048
1049GlobalsAAWrapperPass::GlobalsAAWrapperPass() : ModulePass(ID) {}
1050
1051bool GlobalsAAWrapperPass::runOnModule(Module &M) {
1052 auto GetTLI = [this](Function &F) -> TargetLibraryInfo & {
1053 return this->getAnalysis<TargetLibraryInfoWrapperPass>().getTLI(F);
1054 };
1055 Result.reset(p: new GlobalsAAResult(GlobalsAAResult::analyzeModule(
1056 M, GetTLI, CG&: getAnalysis<CallGraphWrapperPass>().getCallGraph())));
1057 return false;
1058}
1059
1060bool GlobalsAAWrapperPass::doFinalization(Module &M) {
1061 Result.reset();
1062 return false;
1063}
1064
1065void GlobalsAAWrapperPass::getAnalysisUsage(AnalysisUsage &AU) const {
1066 AU.setPreservesAll();
1067 AU.addRequired<CallGraphWrapperPass>();
1068 AU.addRequired<TargetLibraryInfoWrapperPass>();
1069}
1070