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 // Check the pointer operand use of the store.
332 if (&U == &SI->getOperandUse(i: 1)) {
333 if (Writers)
334 Writers->insert(Ptr: SI->getParent()->getParent());
335 } else if (SI->getOperand(i_nocapture: 1) != OkayStoreDest) {
336 return true; // Storing the pointer
337 }
338 } else if (Operator::getOpcode(V: I) == Instruction::GetElementPtr) {
339 if (AnalyzeUsesOfPointer(V: I, Readers, Writers))
340 return true;
341 } else if (Operator::getOpcode(V: I) == Instruction::BitCast ||
342 Operator::getOpcode(V: I) == Instruction::AddrSpaceCast) {
343 if (AnalyzeUsesOfPointer(V: I, Readers, Writers, OkayStoreDest))
344 return true;
345 } else if (auto *Call = dyn_cast<CallBase>(Val: I)) {
346 if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(Val: I)) {
347 if (II->getIntrinsicID() == Intrinsic::threadlocal_address &&
348 V == II->getArgOperand(i: 0)) {
349 if (AnalyzeUsesOfPointer(V: II, Readers, Writers))
350 return true;
351 continue;
352 }
353 }
354 // Make sure that this is just the function being called, not that it is
355 // passing into the function.
356 if (Call->isDataOperand(U: &U)) {
357 // Detect calls to free.
358 if (Call->isArgOperand(U: &U) &&
359 getFreedOperand(CB: Call, TLI: &GetTLI(*Call->getFunction())) == U) {
360 if (Writers)
361 Writers->insert(Ptr: Call->getParent()->getParent());
362 } else {
363 // In general, we return true for unknown calls, but there are
364 // some simple checks that we can do for functions that
365 // will never call back into the module.
366 auto *F = Call->getCalledFunction();
367 // TODO: we should be able to remove isDeclaration() check
368 // and let the function body analysis check for captures,
369 // and collect the mod-ref effects. This information will
370 // be later propagated via the call graph.
371 if (!F || !F->isDeclaration())
372 return true;
373 // Note that the NoCallback check here is a little bit too
374 // conservative. If there are no captures of the global
375 // in the module, then this call may not be a capture even
376 // if it does not have NoCallback.
377 if (!Call->hasFnAttr(Kind: Attribute::NoCallback) ||
378 !Call->isArgOperand(U: &U) ||
379 !Call->doesNotCapture(OpNo: Call->getArgOperandNo(U: &U)))
380 return true;
381
382 // Conservatively, assume the call reads and writes the global.
383 // We could use memory attributes to make it more precise.
384 if (Readers)
385 Readers->insert(Ptr: Call->getParent()->getParent());
386 if (Writers)
387 Writers->insert(Ptr: Call->getParent()->getParent());
388 }
389 }
390 } else if (ICmpInst *ICI = dyn_cast<ICmpInst>(Val: I)) {
391 if (!isa<ConstantPointerNull>(Val: ICI->getOperand(i_nocapture: 1)))
392 return true; // Allow comparison against null.
393 } else if (Constant *C = dyn_cast<Constant>(Val: I)) {
394 // Ignore constants which don't have any live uses.
395 if (isa<GlobalValue>(Val: C) || C->isConstantUsed())
396 return true;
397 } else {
398 return true;
399 }
400 }
401
402 return false;
403}
404
405/// AnalyzeIndirectGlobalMemory - We found an non-address-taken global variable
406/// which holds a pointer type. See if the global always points to non-aliased
407/// heap memory: that is, all initializers of the globals store a value known
408/// to be obtained via a noalias return function call which have no other use.
409/// Further, all loads out of GV must directly use the memory, not store the
410/// pointer somewhere. If this is true, we consider the memory pointed to by
411/// GV to be owned by GV and can disambiguate other pointers from it.
412bool GlobalsAAResult::AnalyzeIndirectGlobalMemory(GlobalVariable *GV) {
413 // Keep track of values related to the allocation of the memory, f.e. the
414 // value produced by the noalias call and any casts.
415 std::vector<Value *> AllocRelatedValues;
416
417 // If the initializer is a non-null pointer, bail.
418 if (Constant *C = GV->getInitializer())
419 if (!isa<ConstantPointerNull>(Val: C))
420 return false;
421
422 // Walk the user list of the global. If we find anything other than a direct
423 // load or store, bail out.
424 for (User *U : GV->users()) {
425 if (LoadInst *LI = dyn_cast<LoadInst>(Val: U)) {
426 // The pointer loaded from the global can only be used in simple ways:
427 // we allow addressing of it and loading storing to it. We do *not* allow
428 // storing the loaded pointer somewhere else or passing to a function.
429 if (AnalyzeUsesOfPointer(V: LI))
430 return false; // Loaded pointer escapes.
431 // TODO: Could try some IP mod/ref of the loaded pointer.
432 } else if (StoreInst *SI = dyn_cast<StoreInst>(Val: U)) {
433 // Storing the global itself.
434 if (SI->getOperand(i_nocapture: 0) == GV)
435 return false;
436
437 // If storing the null pointer, ignore it.
438 if (isa<ConstantPointerNull>(Val: SI->getOperand(i_nocapture: 0)))
439 continue;
440
441 // Check the value being stored.
442 Value *Ptr = getUnderlyingObject(V: SI->getOperand(i_nocapture: 0));
443
444 if (!isNoAliasCall(V: Ptr))
445 return false; // Too hard to analyze.
446
447 // Analyze all uses of the allocation. If any of them are used in a
448 // non-simple way (e.g. stored to another global) bail out.
449 if (AnalyzeUsesOfPointer(V: Ptr, /*Readers*/ nullptr, /*Writers*/ nullptr,
450 OkayStoreDest: GV))
451 return false; // Loaded pointer escapes.
452
453 // Remember that this allocation is related to the indirect global.
454 AllocRelatedValues.push_back(x: Ptr);
455 } else {
456 // Something complex, bail out.
457 return false;
458 }
459 }
460
461 // Okay, this is an indirect global. Remember all of the allocations for
462 // this global in AllocsForIndirectGlobals.
463 while (!AllocRelatedValues.empty()) {
464 AllocsForIndirectGlobals[AllocRelatedValues.back()] = GV;
465 Handles.emplace_front(args&: *this, args&: AllocRelatedValues.back());
466 Handles.front().I = Handles.begin();
467 AllocRelatedValues.pop_back();
468 }
469 IndirectGlobals.insert(Ptr: GV);
470 Handles.emplace_front(args&: *this, args&: GV);
471 Handles.front().I = Handles.begin();
472 return true;
473}
474
475void GlobalsAAResult::CollectSCCMembership(CallGraph &CG) {
476 // We do a bottom-up SCC traversal of the call graph. In other words, we
477 // visit all callees before callers (leaf-first).
478 unsigned SCCID = 0;
479 for (scc_iterator<CallGraph *> I = scc_begin(G: &CG); !I.isAtEnd(); ++I) {
480 const std::vector<CallGraphNode *> &SCC = *I;
481 assert(!SCC.empty() && "SCC with no functions?");
482
483 for (auto *CGN : SCC)
484 if (Function *F = CGN->getFunction())
485 FunctionToSCCMap[F] = SCCID;
486 ++SCCID;
487 }
488}
489
490/// AnalyzeCallGraph - At this point, we know the functions where globals are
491/// immediately stored to and read from. Propagate this information up the call
492/// graph to all callers and compute the mod/ref info for all memory for each
493/// function.
494void GlobalsAAResult::AnalyzeCallGraph(CallGraph &CG, Module &M) {
495 // We do a bottom-up SCC traversal of the call graph. In other words, we
496 // visit all callees before callers (leaf-first).
497 for (scc_iterator<CallGraph *> I = scc_begin(G: &CG); !I.isAtEnd(); ++I) {
498 const std::vector<CallGraphNode *> &SCC = *I;
499 assert(!SCC.empty() && "SCC with no functions?");
500
501 Function *FirstF = SCC[0]->getFunction();
502
503 if (!FirstF || !FirstF->isDefinitionExact()) {
504 // Calls externally or not exact - can't say anything useful. Remove any
505 // existing function records (may have been created when scanning
506 // globals).
507 for (auto *Node : SCC)
508 FunctionInfos.erase(Val: Node->getFunction());
509 continue;
510 }
511
512 FunctionInfo &FI = FunctionInfos[FirstF];
513 Handles.emplace_front(args&: *this, args&: FirstF);
514 Handles.front().I = Handles.begin();
515 bool KnowNothing = false;
516
517 // Intrinsics, like any other synchronizing function, can make effects
518 // of other threads visible. Without nosync we know nothing really.
519 // Similarly, if `nocallback` is missing the function, or intrinsic,
520 // can call into the module arbitrarily. If both are set the function
521 // has an effect but will not interact with accesses of internal
522 // globals inside the module. We are conservative here for optnone
523 // functions, might not be necessary.
524 auto MaySyncOrCallIntoModule = [](const Function &F) {
525 return !F.isDeclaration() || !F.hasNoSync() ||
526 !F.hasFnAttribute(Kind: Attribute::NoCallback);
527 };
528
529 // Collect the mod/ref properties due to called functions. We only compute
530 // one mod-ref set.
531 for (unsigned i = 0, e = SCC.size(); i != e && !KnowNothing; ++i) {
532 Function *F = SCC[i]->getFunction();
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->getPointerType()->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