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/InstIterator.h"
25#include "llvm/IR/Instructions.h"
26#include "llvm/IR/Module.h"
27#include "llvm/IR/PassManager.h"
28#include "llvm/InitializePasses.h"
29#include "llvm/Pass.h"
30#include "llvm/Support/CommandLine.h"
31
32using namespace llvm;
33
34#define DEBUG_TYPE "globalsmodref-aa"
35
36STATISTIC(NumNonAddrTakenGlobalVars,
37 "Number of global vars without address taken");
38STATISTIC(NumNonAddrTakenFunctions,"Number of functions without address taken");
39STATISTIC(NumNoMemFunctions, "Number of functions that do not access memory");
40STATISTIC(NumReadMemFunctions, "Number of functions that only read memory");
41STATISTIC(NumIndirectGlobalVars, "Number of indirect global objects");
42
43// An option to enable unsafe alias results from the GlobalsModRef analysis.
44// When enabled, GlobalsModRef will provide no-alias results which in extremely
45// rare cases may not be conservatively correct. In particular, in the face of
46// transforms which cause asymmetry between how effective getUnderlyingObject
47// is for two pointers, it may produce incorrect results.
48//
49// These unsafe results have been returned by GMR for many years without
50// causing significant issues in the wild and so we provide a mechanism to
51// re-enable them for users of LLVM that have a particular performance
52// sensitivity and no known issues. The option also makes it easy to evaluate
53// the performance impact of these results.
54static cl::opt<bool> EnableUnsafeGlobalsModRefAliasResults(
55 "enable-unsafe-globalsmodref-alias-results", cl::init(Val: false), cl::Hidden);
56
57/// The mod/ref information collected for a particular function.
58///
59/// We collect information about mod/ref behavior of a function here, both in
60/// general and as pertains to specific globals. We only have this detailed
61/// information when we know *something* useful about the behavior. If we
62/// saturate to fully general mod/ref, we remove the info for the function.
63class GlobalsAAResult::FunctionInfo {
64 typedef SmallDenseMap<const GlobalValue *, ModRefInfo, 16> GlobalInfoMapType;
65
66 /// Build a wrapper struct that has 8-byte alignment. All heap allocations
67 /// should provide this much alignment at least, but this makes it clear we
68 /// specifically rely on this amount of alignment.
69 struct alignas(8) AlignedMap {
70 AlignedMap() = default;
71 AlignedMap(const AlignedMap &Arg) = default;
72 GlobalInfoMapType Map;
73 };
74
75 /// Pointer traits for our aligned map.
76 struct AlignedMapPointerTraits {
77 static inline void *getAsVoidPointer(AlignedMap *P) { return P; }
78 static inline AlignedMap *getFromVoidPointer(void *P) {
79 return (AlignedMap *)P;
80 }
81 static constexpr int NumLowBitsAvailable = 3;
82 static_assert(alignof(AlignedMap) >= (1 << NumLowBitsAvailable),
83 "AlignedMap insufficiently aligned to have enough low bits.");
84 };
85
86 /// The bit that flags that this function may read any global. This is
87 /// chosen to mix together with ModRefInfo bits.
88 /// FIXME: This assumes ModRefInfo lattice will remain 4 bits!
89 /// FunctionInfo.getModRefInfo() masks out everything except ModRef so
90 /// this remains correct.
91 enum { MayReadAnyGlobal = 4 };
92
93 /// Checks to document the invariants of the bit packing here.
94 static_assert((MayReadAnyGlobal & static_cast<int>(ModRefInfo::ModRef)) == 0,
95 "ModRef and the MayReadAnyGlobal flag bits overlap.");
96 static_assert(((MayReadAnyGlobal | static_cast<int>(ModRefInfo::ModRef)) >>
97 AlignedMapPointerTraits::NumLowBitsAvailable) == 0,
98 "Insufficient low bits to store our flag and ModRef info.");
99
100public:
101 FunctionInfo() = default;
102 ~FunctionInfo() {
103 delete Info.getPointer();
104 }
105 // Spell out the copy ond move constructors and assignment operators to get
106 // deep copy semantics and correct move semantics in the face of the
107 // pointer-int pair.
108 FunctionInfo(const FunctionInfo &Arg)
109 : Info(nullptr, Arg.Info.getInt()) {
110 if (const auto *ArgPtr = Arg.Info.getPointer())
111 Info.setPointer(new AlignedMap(*ArgPtr));
112 }
113 FunctionInfo(FunctionInfo &&Arg)
114 : Info(Arg.Info.getPointer(), Arg.Info.getInt()) {
115 Arg.Info.setPointerAndInt(PtrVal: nullptr, IntVal: 0);
116 }
117 FunctionInfo &operator=(const FunctionInfo &RHS) {
118 delete Info.getPointer();
119 Info.setPointerAndInt(PtrVal: nullptr, IntVal: RHS.Info.getInt());
120 if (const auto *RHSPtr = RHS.Info.getPointer())
121 Info.setPointer(new AlignedMap(*RHSPtr));
122 return *this;
123 }
124 FunctionInfo &operator=(FunctionInfo &&RHS) {
125 delete Info.getPointer();
126 Info.setPointerAndInt(PtrVal: RHS.Info.getPointer(), IntVal: RHS.Info.getInt());
127 RHS.Info.setPointerAndInt(PtrVal: nullptr, IntVal: 0);
128 return *this;
129 }
130
131 /// This method clears MayReadAnyGlobal bit added by GlobalsAAResult to return
132 /// the corresponding ModRefInfo.
133 ModRefInfo globalClearMayReadAnyGlobal(int I) const {
134 return ModRefInfo(I & static_cast<int>(ModRefInfo::ModRef));
135 }
136
137 /// Returns the \c ModRefInfo info for this function.
138 ModRefInfo getModRefInfo() const {
139 return globalClearMayReadAnyGlobal(I: Info.getInt());
140 }
141
142 /// Adds new \c ModRefInfo for this function to its state.
143 void addModRefInfo(ModRefInfo NewMRI) {
144 Info.setInt(Info.getInt() | static_cast<int>(NewMRI));
145 }
146
147 /// Returns whether this function may read any global variable, and we don't
148 /// know which global.
149 bool mayReadAnyGlobal() const { return Info.getInt() & MayReadAnyGlobal; }
150
151 /// Sets this function as potentially reading from any global.
152 void setMayReadAnyGlobal() { Info.setInt(Info.getInt() | MayReadAnyGlobal); }
153
154 /// Returns the \c ModRefInfo info for this function w.r.t. a particular
155 /// global, which may be more precise than the general information above.
156 ModRefInfo getModRefInfoForGlobal(const GlobalValue &GV) const {
157 ModRefInfo GlobalMRI =
158 mayReadAnyGlobal() ? ModRefInfo::Ref : ModRefInfo::NoModRef;
159 if (AlignedMap *P = Info.getPointer()) {
160 auto I = P->Map.find(Val: &GV);
161 if (I != P->Map.end())
162 GlobalMRI |= I->second;
163 }
164 return GlobalMRI;
165 }
166
167 /// Add mod/ref info from another function into ours, saturating towards
168 /// ModRef.
169 void addFunctionInfo(const FunctionInfo &FI) {
170 addModRefInfo(NewMRI: FI.getModRefInfo());
171
172 if (FI.mayReadAnyGlobal())
173 setMayReadAnyGlobal();
174
175 if (AlignedMap *P = FI.Info.getPointer())
176 for (const auto &G : P->Map)
177 addModRefInfoForGlobal(GV: *G.first, NewMRI: G.second);
178 }
179
180 void addModRefInfoForGlobal(const GlobalValue &GV, ModRefInfo NewMRI) {
181 AlignedMap *P = Info.getPointer();
182 if (!P) {
183 P = new AlignedMap();
184 Info.setPointer(P);
185 }
186 auto &GlobalMRI = P->Map[&GV];
187 GlobalMRI |= NewMRI;
188 }
189
190 /// Clear a global's ModRef info. Should be used when a global is being
191 /// deleted.
192 void eraseModRefInfoForGlobal(const GlobalValue &GV) {
193 if (AlignedMap *P = Info.getPointer())
194 P->Map.erase(Val: &GV);
195 }
196
197private:
198 /// All of the information is encoded into a single pointer, with a three bit
199 /// integer in the low three bits. The high bit provides a flag for when this
200 /// function may read any global. The low two bits are the ModRefInfo. And
201 /// the pointer, when non-null, points to a map from GlobalValue to
202 /// ModRefInfo specific to that GlobalValue.
203 PointerIntPair<AlignedMap *, 3, unsigned, AlignedMapPointerTraits> Info;
204};
205
206void GlobalsAAResult::DeletionCallbackHandle::deleted() {
207 Value *V = getValPtr();
208 if (auto *F = dyn_cast<Function>(Val: V))
209 GAR->FunctionInfos.erase(Val: F);
210
211 if (GlobalValue *GV = dyn_cast<GlobalValue>(Val: V)) {
212 if (GAR->NonAddressTakenGlobals.erase(Ptr: GV)) {
213 // This global might be an indirect global. If so, remove it and
214 // remove any AllocRelatedValues for it.
215 if (GAR->IndirectGlobals.erase(Ptr: GV)) {
216 // Remove any entries in AllocsForIndirectGlobals for this global.
217 for (auto I = GAR->AllocsForIndirectGlobals.begin(),
218 E = GAR->AllocsForIndirectGlobals.end();
219 I != E; ++I)
220 if (I->second == GV)
221 GAR->AllocsForIndirectGlobals.erase(I);
222 }
223
224 // Scan the function info we have collected and remove this global
225 // from all of them.
226 for (auto &FIPair : GAR->FunctionInfos)
227 FIPair.second.eraseModRefInfoForGlobal(GV: *GV);
228 }
229 }
230
231 // If this is an allocation related to an indirect global, remove it.
232 GAR->AllocsForIndirectGlobals.erase(Val: V);
233
234 // And clear out the handle.
235 setValPtr(nullptr);
236 GAR->Handles.erase(position: I);
237 // This object is now destroyed!
238}
239
240MemoryEffects GlobalsAAResult::getMemoryEffects(const Function *F) {
241 if (FunctionInfo *FI = getFunctionInfo(F))
242 return MemoryEffects(FI->getModRefInfo());
243
244 return MemoryEffects::unknown();
245}
246
247/// Returns the function info for the function, or null if we don't have
248/// anything useful to say about it.
249GlobalsAAResult::FunctionInfo *
250GlobalsAAResult::getFunctionInfo(const Function *F) {
251 auto I = FunctionInfos.find(Val: F);
252 if (I != FunctionInfos.end())
253 return &I->second;
254 return nullptr;
255}
256
257/// AnalyzeGlobals - Scan through the users of all of the internal
258/// GlobalValue's in the program. If none of them have their "address taken"
259/// (really, their address passed to something nontrivial), record this fact,
260/// and record the functions that they are used directly in.
261void GlobalsAAResult::AnalyzeGlobals(Module &M) {
262 SmallPtrSet<Function *, 32> TrackedFunctions;
263 for (Function &F : M)
264 if (F.hasLocalLinkage()) {
265 if (!AnalyzeUsesOfPointer(V: &F)) {
266 // Remember that we are tracking this global.
267 NonAddressTakenGlobals.insert(Ptr: &F);
268 TrackedFunctions.insert(Ptr: &F);
269 Handles.emplace_front(args&: *this, args: &F);
270 Handles.front().I = Handles.begin();
271 ++NumNonAddrTakenFunctions;
272 } else
273 UnknownFunctionsWithLocalLinkage = true;
274 }
275
276 SmallPtrSet<Function *, 16> Readers, Writers;
277 for (GlobalVariable &GV : M.globals())
278 if (GV.hasLocalLinkage()) {
279 if (!AnalyzeUsesOfPointer(V: &GV, Readers: &Readers,
280 Writers: GV.isConstant() ? nullptr : &Writers)) {
281 // Remember that we are tracking this global, and the mod/ref fns
282 NonAddressTakenGlobals.insert(Ptr: &GV);
283 Handles.emplace_front(args&: *this, args: &GV);
284 Handles.front().I = Handles.begin();
285
286 for (Function *Reader : Readers) {
287 if (TrackedFunctions.insert(Ptr: Reader).second) {
288 Handles.emplace_front(args&: *this, args&: Reader);
289 Handles.front().I = Handles.begin();
290 }
291 FunctionInfos[Reader].addModRefInfoForGlobal(GV, NewMRI: ModRefInfo::Ref);
292 }
293
294 if (!GV.isConstant()) // No need to keep track of writers to constants
295 for (Function *Writer : Writers) {
296 if (TrackedFunctions.insert(Ptr: Writer).second) {
297 Handles.emplace_front(args&: *this, args&: Writer);
298 Handles.front().I = Handles.begin();
299 }
300 FunctionInfos[Writer].addModRefInfoForGlobal(GV, NewMRI: ModRefInfo::Mod);
301 }
302 ++NumNonAddrTakenGlobalVars;
303
304 // If this global holds a pointer type, see if it is an indirect global.
305 if (GV.getValueType()->isPointerTy() &&
306 AnalyzeIndirectGlobalMemory(GV: &GV))
307 ++NumIndirectGlobalVars;
308 }
309 Readers.clear();
310 Writers.clear();
311 }
312}
313
314/// AnalyzeUsesOfPointer - Look at all of the users of the specified pointer.
315/// If this is used by anything complex (i.e., the address escapes), return
316/// true. Also, while we are at it, keep track of those functions that read and
317/// write to the value.
318///
319/// If OkayStoreDest is non-null, stores into this global are allowed.
320bool GlobalsAAResult::AnalyzeUsesOfPointer(Value *V,
321 SmallPtrSetImpl<Function *> *Readers,
322 SmallPtrSetImpl<Function *> *Writers,
323 GlobalValue *OkayStoreDest) {
324 if (!V->getType()->isPointerTy())
325 return true;
326
327 for (Use &U : V->uses()) {
328 User *I = U.getUser();
329 if (LoadInst *LI = dyn_cast<LoadInst>(Val: I)) {
330 if (Readers)
331 Readers->insert(Ptr: LI->getParent()->getParent());
332 } else if (StoreInst *SI = dyn_cast<StoreInst>(Val: I)) {
333 if (V == SI->getOperand(i_nocapture: 1)) {
334 if (Writers)
335 Writers->insert(Ptr: SI->getParent()->getParent());
336 } else if (SI->getOperand(i_nocapture: 1) != OkayStoreDest) {
337 return true; // Storing the pointer
338 }
339 } else if (Operator::getOpcode(V: I) == Instruction::GetElementPtr) {
340 if (AnalyzeUsesOfPointer(V: I, Readers, Writers))
341 return true;
342 } else if (Operator::getOpcode(V: I) == Instruction::BitCast ||
343 Operator::getOpcode(V: I) == Instruction::AddrSpaceCast) {
344 if (AnalyzeUsesOfPointer(V: I, Readers, Writers, OkayStoreDest))
345 return true;
346 } else if (auto *Call = dyn_cast<CallBase>(Val: I)) {
347 if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(Val: I)) {
348 if (II->getIntrinsicID() == Intrinsic::threadlocal_address &&
349 V == II->getArgOperand(i: 0)) {
350 if (AnalyzeUsesOfPointer(V: II, Readers, Writers))
351 return true;
352 continue;
353 }
354 }
355 // Make sure that this is just the function being called, not that it is
356 // passing into the function.
357 if (Call->isDataOperand(U: &U)) {
358 // Detect calls to free.
359 if (Call->isArgOperand(U: &U) &&
360 getFreedOperand(CB: Call, TLI: &GetTLI(*Call->getFunction())) == U) {
361 if (Writers)
362 Writers->insert(Ptr: Call->getParent()->getParent());
363 } else {
364 // In general, we return true for unknown calls, but there are
365 // some simple checks that we can do for functions that
366 // will never call back into the module.
367 auto *F = Call->getCalledFunction();
368 // TODO: we should be able to remove isDeclaration() check
369 // and let the function body analysis check for captures,
370 // and collect the mod-ref effects. This information will
371 // be later propagated via the call graph.
372 if (!F || !F->isDeclaration())
373 return true;
374 // Note that the NoCallback check here is a little bit too
375 // conservative. If there are no captures of the global
376 // in the module, then this call may not be a capture even
377 // if it does not have NoCallback.
378 if (!Call->hasFnAttr(Kind: Attribute::NoCallback) ||
379 !Call->isArgOperand(U: &U) ||
380 !Call->doesNotCapture(OpNo: Call->getArgOperandNo(U: &U)))
381 return true;
382
383 // Conservatively, assume the call reads and writes the global.
384 // We could use memory attributes to make it more precise.
385 if (Readers)
386 Readers->insert(Ptr: Call->getParent()->getParent());
387 if (Writers)
388 Writers->insert(Ptr: Call->getParent()->getParent());
389 }
390 }
391 } else if (ICmpInst *ICI = dyn_cast<ICmpInst>(Val: I)) {
392 if (!isa<ConstantPointerNull>(Val: ICI->getOperand(i_nocapture: 1)))
393 return true; // Allow comparison against null.
394 } else if (Constant *C = dyn_cast<Constant>(Val: I)) {
395 // Ignore constants which don't have any live uses.
396 if (isa<GlobalValue>(Val: C) || C->isConstantUsed())
397 return true;
398 } else {
399 return true;
400 }
401 }
402
403 return false;
404}
405
406/// AnalyzeIndirectGlobalMemory - We found an non-address-taken global variable
407/// which holds a pointer type. See if the global always points to non-aliased
408/// heap memory: that is, all initializers of the globals store a value known
409/// to be obtained via a noalias return function call which have no other use.
410/// Further, all loads out of GV must directly use the memory, not store the
411/// pointer somewhere. If this is true, we consider the memory pointed to by
412/// GV to be owned by GV and can disambiguate other pointers from it.
413bool GlobalsAAResult::AnalyzeIndirectGlobalMemory(GlobalVariable *GV) {
414 // Keep track of values related to the allocation of the memory, f.e. the
415 // value produced by the noalias call and any casts.
416 std::vector<Value *> AllocRelatedValues;
417
418 // If the initializer is a valid pointer, bail.
419 if (Constant *C = GV->getInitializer())
420 if (!C->isNullValue())
421 return false;
422
423 // Walk the user list of the global. If we find anything other than a direct
424 // load or store, bail out.
425 for (User *U : GV->users()) {
426 if (LoadInst *LI = dyn_cast<LoadInst>(Val: U)) {
427 // The pointer loaded from the global can only be used in simple ways:
428 // we allow addressing of it and loading storing to it. We do *not* allow
429 // storing the loaded pointer somewhere else or passing to a function.
430 if (AnalyzeUsesOfPointer(V: LI))
431 return false; // Loaded pointer escapes.
432 // TODO: Could try some IP mod/ref of the loaded pointer.
433 } else if (StoreInst *SI = dyn_cast<StoreInst>(Val: U)) {
434 // Storing the global itself.
435 if (SI->getOperand(i_nocapture: 0) == GV)
436 return false;
437
438 // If storing the null pointer, ignore it.
439 if (isa<ConstantPointerNull>(Val: SI->getOperand(i_nocapture: 0)))
440 continue;
441
442 // Check the value being stored.
443 Value *Ptr = getUnderlyingObject(V: SI->getOperand(i_nocapture: 0));
444
445 if (!isNoAliasCall(V: Ptr))
446 return false; // Too hard to analyze.
447
448 // Analyze all uses of the allocation. If any of them are used in a
449 // non-simple way (e.g. stored to another global) bail out.
450 if (AnalyzeUsesOfPointer(V: Ptr, /*Readers*/ nullptr, /*Writers*/ nullptr,
451 OkayStoreDest: GV))
452 return false; // Loaded pointer escapes.
453
454 // Remember that this allocation is related to the indirect global.
455 AllocRelatedValues.push_back(x: Ptr);
456 } else {
457 // Something complex, bail out.
458 return false;
459 }
460 }
461
462 // Okay, this is an indirect global. Remember all of the allocations for
463 // this global in AllocsForIndirectGlobals.
464 while (!AllocRelatedValues.empty()) {
465 AllocsForIndirectGlobals[AllocRelatedValues.back()] = GV;
466 Handles.emplace_front(args&: *this, args&: AllocRelatedValues.back());
467 Handles.front().I = Handles.begin();
468 AllocRelatedValues.pop_back();
469 }
470 IndirectGlobals.insert(Ptr: GV);
471 Handles.emplace_front(args&: *this, args&: GV);
472 Handles.front().I = Handles.begin();
473 return true;
474}
475
476void GlobalsAAResult::CollectSCCMembership(CallGraph &CG) {
477 // We do a bottom-up SCC traversal of the call graph. In other words, we
478 // visit all callees before callers (leaf-first).
479 unsigned SCCID = 0;
480 for (scc_iterator<CallGraph *> I = scc_begin(G: &CG); !I.isAtEnd(); ++I) {
481 const std::vector<CallGraphNode *> &SCC = *I;
482 assert(!SCC.empty() && "SCC with no functions?");
483
484 for (auto *CGN : SCC)
485 if (Function *F = CGN->getFunction())
486 FunctionToSCCMap[F] = SCCID;
487 ++SCCID;
488 }
489}
490
491/// AnalyzeCallGraph - At this point, we know the functions where globals are
492/// immediately stored to and read from. Propagate this information up the call
493/// graph to all callers and compute the mod/ref info for all memory for each
494/// function.
495void GlobalsAAResult::AnalyzeCallGraph(CallGraph &CG, Module &M) {
496 // We do a bottom-up SCC traversal of the call graph. In other words, we
497 // visit all callees before callers (leaf-first).
498 for (scc_iterator<CallGraph *> I = scc_begin(G: &CG); !I.isAtEnd(); ++I) {
499 const std::vector<CallGraphNode *> &SCC = *I;
500 assert(!SCC.empty() && "SCC with no functions?");
501
502 Function *F = SCC[0]->getFunction();
503
504 if (!F || !F->isDefinitionExact()) {
505 // Calls externally or not exact - can't say anything useful. Remove any
506 // existing function records (may have been created when scanning
507 // globals).
508 for (auto *Node : SCC)
509 FunctionInfos.erase(Val: Node->getFunction());
510 continue;
511 }
512
513 FunctionInfo &FI = FunctionInfos[F];
514 Handles.emplace_front(args&: *this, args&: F);
515 Handles.front().I = Handles.begin();
516 bool KnowNothing = false;
517
518 // Intrinsics, like any other synchronizing function, can make effects
519 // of other threads visible. Without nosync we know nothing really.
520 // Similarly, if `nocallback` is missing the function, or intrinsic,
521 // can call into the module arbitrarily. If both are set the function
522 // has an effect but will not interact with accesses of internal
523 // globals inside the module. We are conservative here for optnone
524 // functions, might not be necessary.
525 auto MaySyncOrCallIntoModule = [](const Function &F) {
526 return !F.isDeclaration() || !F.hasNoSync() ||
527 !F.hasFnAttribute(Kind: Attribute::NoCallback);
528 };
529
530 // Collect the mod/ref properties due to called functions. We only compute
531 // one mod-ref set.
532 for (unsigned i = 0, e = SCC.size(); i != e && !KnowNothing; ++i) {
533 if (!F) {
534 KnowNothing = true;
535 break;
536 }
537
538 if (F->isDeclaration() || F->hasOptNone()) {
539 // Try to get mod/ref behaviour from function attributes.
540 if (F->doesNotAccessMemory()) {
541 // Can't do better than that!
542 } else if (F->onlyReadsMemory()) {
543 FI.addModRefInfo(NewMRI: ModRefInfo::Ref);
544 if (!F->onlyAccessesArgMemory() && MaySyncOrCallIntoModule(*F))
545 // This function might call back into the module and read a global -
546 // consider every global as possibly being read by this function.
547 FI.setMayReadAnyGlobal();
548 } else {
549 FI.addModRefInfo(NewMRI: ModRefInfo::ModRef);
550 if (!F->onlyAccessesArgMemory())
551 FI.setMayReadAnyGlobal();
552 if (MaySyncOrCallIntoModule(*F)) {
553 KnowNothing = true;
554 break;
555 }
556 }
557 continue;
558 }
559
560 for (CallGraphNode::iterator CI = SCC[i]->begin(), E = SCC[i]->end();
561 CI != E && !KnowNothing; ++CI)
562 if (Function *Callee = CI->second->getFunction()) {
563 if (FunctionInfo *CalleeFI = getFunctionInfo(F: Callee)) {
564 // Propagate function effect up.
565 FI.addFunctionInfo(FI: *CalleeFI);
566 } else {
567 // Can't say anything about it. However, if it is inside our SCC,
568 // then nothing needs to be done.
569 CallGraphNode *CalleeNode = CG[Callee];
570 if (!is_contained(Range: SCC, Element: CalleeNode))
571 KnowNothing = true;
572 }
573 } else {
574 KnowNothing = true;
575 }
576 }
577
578 // If we can't say anything useful about this SCC, remove all SCC functions
579 // from the FunctionInfos map.
580 if (KnowNothing) {
581 for (auto *Node : SCC)
582 FunctionInfos.erase(Val: Node->getFunction());
583 continue;
584 }
585
586 // Scan the function bodies for explicit loads or stores.
587 for (auto *Node : SCC) {
588 if (isModAndRefSet(MRI: FI.getModRefInfo()))
589 break; // The mod/ref lattice saturates here.
590
591 // Don't prove any properties based on the implementation of an optnone
592 // function. Function attributes were already used as a best approximation
593 // above.
594 if (Node->getFunction()->hasOptNone())
595 continue;
596
597 for (Instruction &I : instructions(F: Node->getFunction())) {
598 if (isModAndRefSet(MRI: FI.getModRefInfo()))
599 break; // The mod/ref lattice saturates here.
600
601 // We handle calls specially because the graph-relevant aspects are
602 // handled above.
603 if (isa<CallBase>(Val: &I))
604 continue;
605
606 // All non-call instructions we use the primary predicates for whether
607 // they read or write memory.
608 if (I.mayReadFromMemory())
609 FI.addModRefInfo(NewMRI: ModRefInfo::Ref);
610 if (I.mayWriteToMemory())
611 FI.addModRefInfo(NewMRI: ModRefInfo::Mod);
612 }
613 }
614
615 if (!isModSet(MRI: FI.getModRefInfo()))
616 ++NumReadMemFunctions;
617 if (!isModOrRefSet(MRI: FI.getModRefInfo()))
618 ++NumNoMemFunctions;
619
620 // Finally, now that we know the full effect on this SCC, clone the
621 // information to each function in the SCC.
622 // FI is a reference into FunctionInfos, so copy it now so that it doesn't
623 // get invalidated if DenseMap decides to re-hash.
624 FunctionInfo CachedFI = FI;
625 for (unsigned i = 1, e = SCC.size(); i != e; ++i)
626 FunctionInfos[SCC[i]->getFunction()] = CachedFI;
627 }
628}
629
630// GV is a non-escaping global. V is a pointer address that has been loaded from.
631// If we can prove that V must escape, we can conclude that a load from V cannot
632// alias GV.
633static bool isNonEscapingGlobalNoAliasWithLoad(const GlobalValue *GV,
634 const Value *V,
635 int &Depth,
636 const DataLayout &DL) {
637 SmallPtrSet<const Value *, 8> Visited;
638 SmallVector<const Value *, 8> Inputs;
639 Visited.insert(Ptr: V);
640 Inputs.push_back(Elt: V);
641 do {
642 const Value *Input = Inputs.pop_back_val();
643
644 if (isa<GlobalValue>(Val: Input) || isa<Argument>(Val: Input) || isa<CallInst>(Val: Input) ||
645 isa<InvokeInst>(Val: 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.
649 //
650 // (Transitive) loads from a global are also safe - if this aliased
651 // another global, its address would escape, so no alias.
652 continue;
653
654 // Recurse through a limited number of selects, loads and PHIs. This is an
655 // arbitrary depth of 4, lower numbers could be used to fix compile time
656 // issues if needed, but this is generally expected to be only be important
657 // for small depths.
658 if (++Depth > 4)
659 return false;
660
661 if (auto *LI = dyn_cast<LoadInst>(Val: Input)) {
662 Inputs.push_back(Elt: getUnderlyingObject(V: LI->getPointerOperand()));
663 continue;
664 }
665 if (auto *SI = dyn_cast<SelectInst>(Val: Input)) {
666 const Value *LHS = getUnderlyingObject(V: SI->getTrueValue());
667 const Value *RHS = getUnderlyingObject(V: SI->getFalseValue());
668 if (Visited.insert(Ptr: LHS).second)
669 Inputs.push_back(Elt: LHS);
670 if (Visited.insert(Ptr: RHS).second)
671 Inputs.push_back(Elt: RHS);
672 continue;
673 }
674 if (auto *PN = dyn_cast<PHINode>(Val: Input)) {
675 for (const Value *Op : PN->incoming_values()) {
676 Op = getUnderlyingObject(V: Op);
677 if (Visited.insert(Ptr: Op).second)
678 Inputs.push_back(Elt: Op);
679 }
680 continue;
681 }
682
683 return false;
684 } while (!Inputs.empty());
685
686 // All inputs were known to be no-alias.
687 return true;
688}
689
690// There are particular cases where we can conclude no-alias between
691// a non-addr-taken global and some other underlying object. Specifically,
692// a non-addr-taken global is known to not be escaped from any function. It is
693// also incorrect for a transformation to introduce an escape of a global in
694// a way that is observable when it was not there previously. One function
695// being transformed to introduce an escape which could possibly be observed
696// (via loading from a global or the return value for example) within another
697// function is never safe. If the observation is made through non-atomic
698// operations on different threads, it is a data-race and UB. If the
699// observation is well defined, by being observed the transformation would have
700// changed program behavior by introducing the observed escape, making it an
701// invalid transform.
702//
703// This property does require that transformations which *temporarily* escape
704// a global that was not previously escaped, prior to restoring it, cannot rely
705// on the results of GMR::alias. This seems a reasonable restriction, although
706// currently there is no way to enforce it. There is also no realistic
707// optimization pass that would make this mistake. The closest example is
708// a transformation pass which does reg2mem of SSA values but stores them into
709// global variables temporarily before restoring the global variable's value.
710// This could be useful to expose "benign" races for example. However, it seems
711// reasonable to require that a pass which introduces escapes of global
712// variables in this way to either not trust AA results while the escape is
713// active, or to be forced to operate as a module pass that cannot co-exist
714// with an alias analysis such as GMR.
715bool GlobalsAAResult::isNonEscapingGlobalNoAlias(const GlobalValue *GV,
716 const Value *V,
717 const Instruction *CtxI) {
718 // In order to know that the underlying object cannot alias the
719 // non-addr-taken global, we must know that it would have to be an escape.
720 // Thus if the underlying object is a function argument, a load from
721 // a global, or the return of a function, it cannot alias. We can also
722 // recurse through PHI nodes and select nodes provided all of their inputs
723 // resolve to one of these known-escaping roots.
724
725 // A non-addr-taken global cannot alias with any non-pointer value.
726 // Check this early and exit.
727 if (!V->getType()->isPointerTy())
728 return true;
729
730 SmallPtrSet<const Value *, 8> Visited;
731 SmallVector<const Value *, 8> Inputs;
732 Visited.insert(Ptr: V);
733 Inputs.push_back(Elt: V);
734 int Depth = 0;
735 do {
736 const Value *Input = Inputs.pop_back_val();
737
738 if (auto *InputGV = dyn_cast<GlobalValue>(Val: Input)) {
739 // If one input is the very global we're querying against, then we can't
740 // conclude no-alias.
741 if (InputGV == GV)
742 return false;
743
744 // Distinct GlobalVariables never alias, unless overriden or zero-sized.
745 // FIXME: The condition can be refined, but be conservative for now.
746 auto *GVar = dyn_cast<GlobalVariable>(Val: GV);
747 auto *InputGVar = dyn_cast<GlobalVariable>(Val: InputGV);
748 if (GVar && InputGVar &&
749 !GVar->isDeclaration() && !InputGVar->isDeclaration() &&
750 !GVar->isInterposable() && !InputGVar->isInterposable()) {
751 Type *GVType = GVar->getInitializer()->getType();
752 Type *InputGVType = InputGVar->getInitializer()->getType();
753 if (GVType->isSized() && InputGVType->isSized() &&
754 (DL.getTypeAllocSize(Ty: GVType) > 0) &&
755 (DL.getTypeAllocSize(Ty: InputGVType) > 0))
756 continue;
757 }
758
759 // Conservatively return false, even though we could be smarter
760 // (e.g. look through GlobalAliases).
761 return false;
762 }
763
764 if (isa<Argument>(Val: Input) || isa<CallInst>(Val: Input) ||
765 isa<InvokeInst>(Val: Input)) {
766 // Arguments to functions or returns from functions are inherently
767 // escaping, so we can immediately classify those as not aliasing any
768 // non-addr-taken globals.
769 continue;
770 }
771
772 if (CtxI)
773 if (auto *CPN = dyn_cast<ConstantPointerNull>(Val: Input)) {
774 // Null pointer cannot alias with a non-addr-taken global.
775 const Function *F = CtxI->getFunction();
776 if (!NullPointerIsDefined(F, AS: CPN->getType()->getAddressSpace()))
777 continue;
778 }
779
780 // Recurse through a limited number of selects, loads and PHIs. This is an
781 // arbitrary depth of 4, lower numbers could be used to fix compile time
782 // issues if needed, but this is generally expected to be only be important
783 // for small depths.
784 if (++Depth > 4)
785 return false;
786
787 if (auto *LI = dyn_cast<LoadInst>(Val: Input)) {
788 // A pointer loaded from a global would have been captured, and we know
789 // that the global is non-escaping, so no alias.
790 const Value *Ptr = getUnderlyingObject(V: LI->getPointerOperand());
791 if (isNonEscapingGlobalNoAliasWithLoad(GV, V: Ptr, Depth, DL))
792 // The load does not alias with GV.
793 continue;
794 // Otherwise, a load could come from anywhere, so bail.
795 return false;
796 }
797 if (auto *SI = dyn_cast<SelectInst>(Val: Input)) {
798 const Value *LHS = getUnderlyingObject(V: SI->getTrueValue());
799 const Value *RHS = getUnderlyingObject(V: SI->getFalseValue());
800 if (Visited.insert(Ptr: LHS).second)
801 Inputs.push_back(Elt: LHS);
802 if (Visited.insert(Ptr: RHS).second)
803 Inputs.push_back(Elt: RHS);
804 continue;
805 }
806 if (auto *PN = dyn_cast<PHINode>(Val: Input)) {
807 for (const Value *Op : PN->incoming_values()) {
808 Op = getUnderlyingObject(V: Op);
809 if (Visited.insert(Ptr: Op).second)
810 Inputs.push_back(Elt: Op);
811 }
812 continue;
813 }
814
815 // FIXME: It would be good to handle other obvious no-alias cases here, but
816 // it isn't clear how to do so reasonably without building a small version
817 // of BasicAA into this code.
818 return false;
819 } while (!Inputs.empty());
820
821 // If all the inputs to V were definitively no-alias, then V is no-alias.
822 return true;
823}
824
825bool GlobalsAAResult::invalidate(Module &, const PreservedAnalyses &PA,
826 ModuleAnalysisManager::Invalidator &) {
827 // Check whether the analysis has been explicitly invalidated. Otherwise, it's
828 // stateless and remains preserved.
829 auto PAC = PA.getChecker<GlobalsAA>();
830 return !PAC.preservedWhenStateless();
831}
832
833/// alias - If one of the pointers is to a global that we are tracking, and the
834/// other is some random pointer, we know there cannot be an alias, because the
835/// address of the global isn't taken.
836AliasResult GlobalsAAResult::alias(const MemoryLocation &LocA,
837 const MemoryLocation &LocB,
838 AAQueryInfo &AAQI, const Instruction *CtxI) {
839 // Get the base object these pointers point to.
840 const Value *UV1 =
841 getUnderlyingObject(V: LocA.Ptr->stripPointerCastsForAliasAnalysis());
842 const Value *UV2 =
843 getUnderlyingObject(V: LocB.Ptr->stripPointerCastsForAliasAnalysis());
844
845 // If either of the underlying values is a global, they may be non-addr-taken
846 // globals, which we can answer queries about.
847 const GlobalValue *GV1 = dyn_cast<GlobalValue>(Val: UV1);
848 const GlobalValue *GV2 = dyn_cast<GlobalValue>(Val: UV2);
849 if (GV1 || GV2) {
850 // If the global's address is taken, pretend we don't know it's a pointer to
851 // the global.
852 if (GV1 && !NonAddressTakenGlobals.count(Ptr: GV1))
853 GV1 = nullptr;
854 if (GV2 && !NonAddressTakenGlobals.count(Ptr: GV2))
855 GV2 = nullptr;
856
857 // If the two pointers are derived from two different non-addr-taken
858 // globals we know these can't alias.
859 if (GV1 && GV2 && GV1 != GV2)
860 return AliasResult::NoAlias;
861
862 // If one is and the other isn't, it isn't strictly safe but we can fake
863 // this result if necessary for performance. This does not appear to be
864 // a common problem in practice.
865 if (EnableUnsafeGlobalsModRefAliasResults)
866 if ((GV1 || GV2) && GV1 != GV2)
867 return AliasResult::NoAlias;
868
869 // Check for a special case where a non-escaping global can be used to
870 // conclude no-alias.
871 if ((GV1 || GV2) && GV1 != GV2) {
872 const GlobalValue *GV = GV1 ? GV1 : GV2;
873 const Value *UV = GV1 ? UV2 : UV1;
874 if (isNonEscapingGlobalNoAlias(GV, V: UV, CtxI))
875 return AliasResult::NoAlias;
876 }
877
878 // Otherwise if they are both derived from the same addr-taken global, we
879 // can't know the two accesses don't overlap.
880 }
881
882 // These pointers may be based on the memory owned by an indirect global. If
883 // so, we may be able to handle this. First check to see if the base pointer
884 // is a direct load from an indirect global.
885 GV1 = GV2 = nullptr;
886 if (const LoadInst *LI = dyn_cast<LoadInst>(Val: UV1))
887 if (GlobalVariable *GV = dyn_cast<GlobalVariable>(Val: LI->getOperand(i_nocapture: 0)))
888 if (IndirectGlobals.count(Ptr: GV))
889 GV1 = GV;
890 if (const LoadInst *LI = dyn_cast<LoadInst>(Val: UV2))
891 if (const GlobalVariable *GV = dyn_cast<GlobalVariable>(Val: LI->getOperand(i_nocapture: 0)))
892 if (IndirectGlobals.count(Ptr: GV))
893 GV2 = GV;
894
895 // These pointers may also be from an allocation for the indirect global. If
896 // so, also handle them.
897 if (!GV1)
898 GV1 = AllocsForIndirectGlobals.lookup(Val: UV1);
899 if (!GV2)
900 GV2 = AllocsForIndirectGlobals.lookup(Val: UV2);
901
902 // Now that we know whether the two pointers are related to indirect globals,
903 // use this to disambiguate the pointers. If the pointers are based on
904 // different indirect globals they cannot alias.
905 if (GV1 && GV2 && GV1 != GV2)
906 return AliasResult::NoAlias;
907
908 // If one is based on an indirect global and the other isn't, it isn't
909 // strictly safe but we can fake this result if necessary for performance.
910 // This does not appear to be a common problem in practice.
911 if (EnableUnsafeGlobalsModRefAliasResults)
912 if ((GV1 || GV2) && GV1 != GV2)
913 return AliasResult::NoAlias;
914
915 return AliasResult::MayAlias;
916}
917
918ModRefInfo GlobalsAAResult::getModRefInfoForArgument(const CallBase *Call,
919 const GlobalValue *GV,
920 AAQueryInfo &AAQI) {
921 if (Call->doesNotAccessMemory())
922 return ModRefInfo::NoModRef;
923 ModRefInfo ConservativeResult =
924 Call->onlyReadsMemory() ? ModRefInfo::Ref : ModRefInfo::ModRef;
925
926 // Iterate through all the arguments to the called function. If any argument
927 // is based on GV, return the conservative result.
928 for (const auto &A : Call->args()) {
929 SmallVector<const Value*, 4> Objects;
930 getUnderlyingObjects(V: A, Objects);
931
932 // All objects must be identified.
933 if (!all_of(Range&: Objects, P: isIdentifiedObject) &&
934 // Try ::alias to see if all objects are known not to alias GV.
935 !all_of(Range&: Objects, P: [&](const Value *V) {
936 return this->alias(LocA: MemoryLocation::getBeforeOrAfter(Ptr: V),
937 LocB: MemoryLocation::getBeforeOrAfter(Ptr: GV), AAQI,
938 CtxI: Call) == AliasResult::NoAlias;
939 }))
940 return ConservativeResult;
941
942 if (is_contained(Range&: Objects, Element: GV))
943 return ConservativeResult;
944 }
945
946 // We identified all objects in the argument list, and none of them were GV.
947 return ModRefInfo::NoModRef;
948}
949
950ModRefInfo GlobalsAAResult::getModRefInfo(const CallBase *Call,
951 const MemoryLocation &Loc,
952 AAQueryInfo &AAQI) {
953 ModRefInfo Known = ModRefInfo::ModRef;
954
955 // If we are asking for mod/ref info of a direct call with a pointer to a
956 // global we are tracking, return information if we have it.
957 if (const GlobalValue *GV =
958 dyn_cast<GlobalValue>(Val: getUnderlyingObject(V: Loc.Ptr)))
959 // If GV is internal to this IR and there is no function with local linkage
960 // that has had their address taken, keep looking for a tighter ModRefInfo.
961 if (GV->hasLocalLinkage() && !UnknownFunctionsWithLocalLinkage)
962 if (const Function *F = Call->getCalledFunction())
963 if (NonAddressTakenGlobals.count(Ptr: GV))
964 if (const FunctionInfo *FI = getFunctionInfo(F))
965 Known = FI->getModRefInfoForGlobal(GV: *GV) |
966 getModRefInfoForArgument(Call, GV, AAQI);
967
968 return Known;
969}
970
971GlobalsAAResult::GlobalsAAResult(
972 const DataLayout &DL,
973 std::function<const TargetLibraryInfo &(Function &F)> GetTLI)
974 : DL(DL), GetTLI(std::move(GetTLI)) {}
975
976GlobalsAAResult::GlobalsAAResult(GlobalsAAResult &&Arg)
977 : AAResultBase(std::move(Arg)), DL(Arg.DL), GetTLI(std::move(Arg.GetTLI)),
978 NonAddressTakenGlobals(std::move(Arg.NonAddressTakenGlobals)),
979 IndirectGlobals(std::move(Arg.IndirectGlobals)),
980 AllocsForIndirectGlobals(std::move(Arg.AllocsForIndirectGlobals)),
981 FunctionInfos(std::move(Arg.FunctionInfos)),
982 Handles(std::move(Arg.Handles)) {
983 // Update the parent for each DeletionCallbackHandle.
984 for (auto &H : Handles) {
985 assert(H.GAR == &Arg);
986 H.GAR = this;
987 }
988}
989
990GlobalsAAResult::~GlobalsAAResult() = default;
991
992/*static*/ GlobalsAAResult GlobalsAAResult::analyzeModule(
993 Module &M, std::function<const TargetLibraryInfo &(Function &F)> GetTLI,
994 CallGraph &CG) {
995 GlobalsAAResult Result(M.getDataLayout(), GetTLI);
996
997 // Discover which functions aren't recursive, to feed into AnalyzeGlobals.
998 Result.CollectSCCMembership(CG);
999
1000 // Find non-addr taken globals.
1001 Result.AnalyzeGlobals(M);
1002
1003 // Propagate on CG.
1004 Result.AnalyzeCallGraph(CG, M);
1005
1006 return Result;
1007}
1008
1009AnalysisKey GlobalsAA::Key;
1010
1011GlobalsAAResult GlobalsAA::run(Module &M, ModuleAnalysisManager &AM) {
1012 FunctionAnalysisManager &FAM =
1013 AM.getResult<FunctionAnalysisManagerModuleProxy>(IR&: M).getManager();
1014 auto GetTLI = [&FAM](Function &F) -> TargetLibraryInfo & {
1015 return FAM.getResult<TargetLibraryAnalysis>(IR&: F);
1016 };
1017 return GlobalsAAResult::analyzeModule(M, GetTLI,
1018 CG&: AM.getResult<CallGraphAnalysis>(IR&: M));
1019}
1020
1021PreservedAnalyses RecomputeGlobalsAAPass::run(Module &M,
1022 ModuleAnalysisManager &AM) {
1023 if (auto *G = AM.getCachedResult<GlobalsAA>(IR&: M)) {
1024 auto &CG = AM.getResult<CallGraphAnalysis>(IR&: M);
1025 G->NonAddressTakenGlobals.clear();
1026 G->UnknownFunctionsWithLocalLinkage = false;
1027 G->IndirectGlobals.clear();
1028 G->AllocsForIndirectGlobals.clear();
1029 G->FunctionInfos.clear();
1030 G->FunctionToSCCMap.clear();
1031 G->Handles.clear();
1032 G->CollectSCCMembership(CG);
1033 G->AnalyzeGlobals(M);
1034 G->AnalyzeCallGraph(CG, M);
1035 }
1036 return PreservedAnalyses::all();
1037}
1038
1039char GlobalsAAWrapperPass::ID = 0;
1040INITIALIZE_PASS_BEGIN(GlobalsAAWrapperPass, "globals-aa",
1041 "Globals Alias Analysis", false, true)
1042INITIALIZE_PASS_DEPENDENCY(CallGraphWrapperPass)
1043INITIALIZE_PASS_DEPENDENCY(TargetLibraryInfoWrapperPass)
1044INITIALIZE_PASS_END(GlobalsAAWrapperPass, "globals-aa",
1045 "Globals Alias Analysis", false, true)
1046
1047ModulePass *llvm::createGlobalsAAWrapperPass() {
1048 return new GlobalsAAWrapperPass();
1049}
1050
1051GlobalsAAWrapperPass::GlobalsAAWrapperPass() : ModulePass(ID) {}
1052
1053bool GlobalsAAWrapperPass::runOnModule(Module &M) {
1054 auto GetTLI = [this](Function &F) -> TargetLibraryInfo & {
1055 return this->getAnalysis<TargetLibraryInfoWrapperPass>().getTLI(F);
1056 };
1057 Result.reset(p: new GlobalsAAResult(GlobalsAAResult::analyzeModule(
1058 M, GetTLI, CG&: getAnalysis<CallGraphWrapperPass>().getCallGraph())));
1059 return false;
1060}
1061
1062bool GlobalsAAWrapperPass::doFinalization(Module &M) {
1063 Result.reset();
1064 return false;
1065}
1066
1067void GlobalsAAWrapperPass::getAnalysisUsage(AnalysisUsage &AU) const {
1068 AU.setPreservesAll();
1069 AU.addRequired<CallGraphWrapperPass>();
1070 AU.addRequired<TargetLibraryInfoWrapperPass>();
1071}
1072