1 //===--- CaptureTracking.cpp - Determine whether a pointer is captured ----===// 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 file contains routines that help determine which pointers are captured. 10 // A pointer value is captured if the function makes a copy of any part of the 11 // pointer that outlives the call. Not being captured means, more or less, that 12 // the pointer is only dereferenced and not stored in a global. Returning part 13 // of the pointer as the function return value may or may not count as capturing 14 // the pointer, depending on the context. 15 // 16 //===----------------------------------------------------------------------===// 17 18 #include "llvm/Analysis/CaptureTracking.h" 19 #include "llvm/ADT/SmallPtrSet.h" 20 #include "llvm/ADT/SmallSet.h" 21 #include "llvm/ADT/SmallVector.h" 22 #include "llvm/ADT/Statistic.h" 23 #include "llvm/Analysis/AliasAnalysis.h" 24 #include "llvm/Analysis/CFG.h" 25 #include "llvm/Analysis/ValueTracking.h" 26 #include "llvm/IR/Constants.h" 27 #include "llvm/IR/Dominators.h" 28 #include "llvm/IR/Instructions.h" 29 #include "llvm/IR/IntrinsicInst.h" 30 #include "llvm/Support/CommandLine.h" 31 32 using namespace llvm; 33 34 #define DEBUG_TYPE "capture-tracking" 35 36 STATISTIC(NumCaptured, "Number of pointers maybe captured"); 37 STATISTIC(NumNotCaptured, "Number of pointers not captured"); 38 STATISTIC(NumCapturedBefore, "Number of pointers maybe captured before"); 39 STATISTIC(NumNotCapturedBefore, "Number of pointers not captured before"); 40 41 /// The default value for MaxUsesToExplore argument. It's relatively small to 42 /// keep the cost of analysis reasonable for clients like BasicAliasAnalysis, 43 /// where the results can't be cached. 44 /// TODO: we should probably introduce a caching CaptureTracking analysis and 45 /// use it where possible. The caching version can use much higher limit or 46 /// don't have this cap at all. 47 static cl::opt<unsigned> 48 DefaultMaxUsesToExplore("capture-tracking-max-uses-to-explore", cl::Hidden, 49 cl::desc("Maximal number of uses to explore."), 50 cl::init(100)); 51 52 unsigned llvm::getDefaultMaxUsesToExploreForCaptureTracking() { 53 return DefaultMaxUsesToExplore; 54 } 55 56 CaptureTracker::~CaptureTracker() = default; 57 58 bool CaptureTracker::shouldExplore(const Use *U) { return true; } 59 60 bool CaptureTracker::isDereferenceableOrNull(Value *O, const DataLayout &DL) { 61 // An inbounds GEP can either be a valid pointer (pointing into 62 // or to the end of an allocation), or be null in the default 63 // address space. So for an inbounds GEP there is no way to let 64 // the pointer escape using clever GEP hacking because doing so 65 // would make the pointer point outside of the allocated object 66 // and thus make the GEP result a poison value. Similarly, other 67 // dereferenceable pointers cannot be manipulated without producing 68 // poison. 69 if (auto *GEP = dyn_cast<GetElementPtrInst>(O)) 70 if (GEP->isInBounds()) 71 return true; 72 bool CanBeNull, CanBeFreed; 73 return O->getPointerDereferenceableBytes(DL, CanBeNull, CanBeFreed); 74 } 75 76 namespace { 77 struct SimpleCaptureTracker : public CaptureTracker { 78 explicit SimpleCaptureTracker( 79 80 const SmallPtrSetImpl<const Value *> &EphValues, bool ReturnCaptures) 81 : EphValues(EphValues), ReturnCaptures(ReturnCaptures) {} 82 83 void tooManyUses() override { Captured = true; } 84 85 bool captured(const Use *U) override { 86 if (isa<ReturnInst>(U->getUser()) && !ReturnCaptures) 87 return false; 88 89 if (EphValues.contains(U->getUser())) 90 return false; 91 92 Captured = true; 93 return true; 94 } 95 96 const SmallPtrSetImpl<const Value *> &EphValues; 97 98 bool ReturnCaptures; 99 100 bool Captured = false; 101 }; 102 103 /// Only find pointer captures which happen before the given instruction. Uses 104 /// the dominator tree to determine whether one instruction is before another. 105 /// Only support the case where the Value is defined in the same basic block 106 /// as the given instruction and the use. 107 struct CapturesBefore : public CaptureTracker { 108 109 CapturesBefore(bool ReturnCaptures, const Instruction *I, 110 const DominatorTree *DT, bool IncludeI, const LoopInfo *LI) 111 : BeforeHere(I), DT(DT), ReturnCaptures(ReturnCaptures), 112 IncludeI(IncludeI), LI(LI) {} 113 114 void tooManyUses() override { Captured = true; } 115 116 bool isSafeToPrune(Instruction *I) { 117 if (BeforeHere == I) 118 return !IncludeI; 119 120 // We explore this usage only if the usage can reach "BeforeHere". 121 // If use is not reachable from entry, there is no need to explore. 122 if (!DT->isReachableFromEntry(I->getParent())) 123 return true; 124 125 // Check whether there is a path from I to BeforeHere. 126 return !isPotentiallyReachable(I, BeforeHere, nullptr, DT, LI); 127 } 128 129 bool captured(const Use *U) override { 130 Instruction *I = cast<Instruction>(U->getUser()); 131 if (isa<ReturnInst>(I) && !ReturnCaptures) 132 return false; 133 134 // Check isSafeToPrune() here rather than in shouldExplore() to avoid 135 // an expensive reachability query for every instruction we look at. 136 // Instead we only do one for actual capturing candidates. 137 if (isSafeToPrune(I)) 138 return false; 139 140 Captured = true; 141 return true; 142 } 143 144 const Instruction *BeforeHere; 145 const DominatorTree *DT; 146 147 bool ReturnCaptures; 148 bool IncludeI; 149 150 bool Captured = false; 151 152 const LoopInfo *LI; 153 }; 154 155 /// Find the 'earliest' instruction before which the pointer is known not to 156 /// be captured. Here an instruction A is considered earlier than instruction 157 /// B, if A dominates B. If 2 escapes do not dominate each other, the 158 /// terminator of the common dominator is chosen. If not all uses cannot be 159 /// analyzed, the earliest escape is set to the first instruction in the 160 /// function entry block. 161 // NOTE: Users have to make sure instructions compared against the earliest 162 // escape are not in a cycle. 163 struct EarliestCaptures : public CaptureTracker { 164 165 EarliestCaptures(bool ReturnCaptures, Function &F, const DominatorTree &DT, 166 const SmallPtrSetImpl<const Value *> &EphValues) 167 : EphValues(EphValues), DT(DT), ReturnCaptures(ReturnCaptures), F(F) {} 168 169 void tooManyUses() override { 170 Captured = true; 171 EarliestCapture = &*F.getEntryBlock().begin(); 172 } 173 174 bool captured(const Use *U) override { 175 Instruction *I = cast<Instruction>(U->getUser()); 176 if (isa<ReturnInst>(I) && !ReturnCaptures) 177 return false; 178 179 if (EphValues.contains(I)) 180 return false; 181 182 if (!EarliestCapture) 183 EarliestCapture = I; 184 else 185 EarliestCapture = DT.findNearestCommonDominator(EarliestCapture, I); 186 Captured = true; 187 188 // Return false to continue analysis; we need to see all potential 189 // captures. 190 return false; 191 } 192 193 const SmallPtrSetImpl<const Value *> &EphValues; 194 195 Instruction *EarliestCapture = nullptr; 196 197 const DominatorTree &DT; 198 199 bool ReturnCaptures; 200 201 bool Captured = false; 202 203 Function &F; 204 }; 205 } 206 207 /// PointerMayBeCaptured - Return true if this pointer value may be captured 208 /// by the enclosing function (which is required to exist). This routine can 209 /// be expensive, so consider caching the results. The boolean ReturnCaptures 210 /// specifies whether returning the value (or part of it) from the function 211 /// counts as capturing it or not. The boolean StoreCaptures specified whether 212 /// storing the value (or part of it) into memory anywhere automatically 213 /// counts as capturing it or not. 214 bool llvm::PointerMayBeCaptured(const Value *V, bool ReturnCaptures, 215 bool StoreCaptures, unsigned MaxUsesToExplore) { 216 SmallPtrSet<const Value *, 1> Empty; 217 return PointerMayBeCaptured(V, ReturnCaptures, StoreCaptures, Empty, 218 MaxUsesToExplore); 219 } 220 221 /// Variant of the above function which accepts a set of Values that are 222 /// ephemeral and cannot cause pointers to escape. 223 bool llvm::PointerMayBeCaptured(const Value *V, bool ReturnCaptures, 224 bool StoreCaptures, 225 const SmallPtrSetImpl<const Value *> &EphValues, 226 unsigned MaxUsesToExplore) { 227 assert(!isa<GlobalValue>(V) && 228 "It doesn't make sense to ask whether a global is captured."); 229 230 // TODO: If StoreCaptures is not true, we could do Fancy analysis 231 // to determine whether this store is not actually an escape point. 232 // In that case, BasicAliasAnalysis should be updated as well to 233 // take advantage of this. 234 (void)StoreCaptures; 235 236 SimpleCaptureTracker SCT(EphValues, ReturnCaptures); 237 PointerMayBeCaptured(V, &SCT, MaxUsesToExplore); 238 if (SCT.Captured) 239 ++NumCaptured; 240 else 241 ++NumNotCaptured; 242 return SCT.Captured; 243 } 244 245 /// PointerMayBeCapturedBefore - Return true if this pointer value may be 246 /// captured by the enclosing function (which is required to exist). If a 247 /// DominatorTree is provided, only captures which happen before the given 248 /// instruction are considered. This routine can be expensive, so consider 249 /// caching the results. The boolean ReturnCaptures specifies whether 250 /// returning the value (or part of it) from the function counts as capturing 251 /// it or not. The boolean StoreCaptures specified whether storing the value 252 /// (or part of it) into memory anywhere automatically counts as capturing it 253 /// or not. 254 bool llvm::PointerMayBeCapturedBefore(const Value *V, bool ReturnCaptures, 255 bool StoreCaptures, const Instruction *I, 256 const DominatorTree *DT, bool IncludeI, 257 unsigned MaxUsesToExplore, 258 const LoopInfo *LI) { 259 assert(!isa<GlobalValue>(V) && 260 "It doesn't make sense to ask whether a global is captured."); 261 262 if (!DT) 263 return PointerMayBeCaptured(V, ReturnCaptures, StoreCaptures, 264 MaxUsesToExplore); 265 266 // TODO: See comment in PointerMayBeCaptured regarding what could be done 267 // with StoreCaptures. 268 269 CapturesBefore CB(ReturnCaptures, I, DT, IncludeI, LI); 270 PointerMayBeCaptured(V, &CB, MaxUsesToExplore); 271 if (CB.Captured) 272 ++NumCapturedBefore; 273 else 274 ++NumNotCapturedBefore; 275 return CB.Captured; 276 } 277 278 Instruction * 279 llvm::FindEarliestCapture(const Value *V, Function &F, bool ReturnCaptures, 280 bool StoreCaptures, const DominatorTree &DT, 281 282 const SmallPtrSetImpl<const Value *> &EphValues, 283 unsigned MaxUsesToExplore) { 284 assert(!isa<GlobalValue>(V) && 285 "It doesn't make sense to ask whether a global is captured."); 286 287 EarliestCaptures CB(ReturnCaptures, F, DT, EphValues); 288 PointerMayBeCaptured(V, &CB, MaxUsesToExplore); 289 if (CB.Captured) 290 ++NumCapturedBefore; 291 else 292 ++NumNotCapturedBefore; 293 return CB.EarliestCapture; 294 } 295 296 UseCaptureKind llvm::DetermineUseCaptureKind( 297 const Use &U, 298 function_ref<bool(Value *, const DataLayout &)> IsDereferenceableOrNull) { 299 Instruction *I = cast<Instruction>(U.getUser()); 300 301 switch (I->getOpcode()) { 302 case Instruction::Call: 303 case Instruction::Invoke: { 304 auto *Call = cast<CallBase>(I); 305 // Not captured if the callee is readonly, doesn't return a copy through 306 // its return value and doesn't unwind (a readonly function can leak bits 307 // by throwing an exception or not depending on the input value). 308 if (Call->onlyReadsMemory() && Call->doesNotThrow() && 309 Call->getType()->isVoidTy()) 310 return UseCaptureKind::NO_CAPTURE; 311 312 // The pointer is not captured if returned pointer is not captured. 313 // NOTE: CaptureTracking users should not assume that only functions 314 // marked with nocapture do not capture. This means that places like 315 // getUnderlyingObject in ValueTracking or DecomposeGEPExpression 316 // in BasicAA also need to know about this property. 317 if (isIntrinsicReturningPointerAliasingArgumentWithoutCapturing(Call, true)) 318 return UseCaptureKind::PASSTHROUGH; 319 320 // Volatile operations effectively capture the memory location that they 321 // load and store to. 322 if (auto *MI = dyn_cast<MemIntrinsic>(Call)) 323 if (MI->isVolatile()) 324 return UseCaptureKind::MAY_CAPTURE; 325 326 // Calling a function pointer does not in itself cause the pointer to 327 // be captured. This is a subtle point considering that (for example) 328 // the callee might return its own address. It is analogous to saying 329 // that loading a value from a pointer does not cause the pointer to be 330 // captured, even though the loaded value might be the pointer itself 331 // (think of self-referential objects). 332 if (Call->isCallee(&U)) 333 return UseCaptureKind::NO_CAPTURE; 334 335 // Not captured if only passed via 'nocapture' arguments. 336 if (Call->isDataOperand(&U) && 337 !Call->doesNotCapture(Call->getDataOperandNo(&U))) { 338 // The parameter is not marked 'nocapture' - captured. 339 return UseCaptureKind::MAY_CAPTURE; 340 } 341 return UseCaptureKind::NO_CAPTURE; 342 } 343 case Instruction::Load: 344 // Volatile loads make the address observable. 345 if (cast<LoadInst>(I)->isVolatile()) 346 return UseCaptureKind::MAY_CAPTURE; 347 return UseCaptureKind::NO_CAPTURE; 348 case Instruction::VAArg: 349 // "va-arg" from a pointer does not cause it to be captured. 350 return UseCaptureKind::NO_CAPTURE; 351 case Instruction::Store: 352 // Stored the pointer - conservatively assume it may be captured. 353 // Volatile stores make the address observable. 354 if (U.getOperandNo() == 0 || cast<StoreInst>(I)->isVolatile()) 355 return UseCaptureKind::MAY_CAPTURE; 356 return UseCaptureKind::NO_CAPTURE; 357 case Instruction::AtomicRMW: { 358 // atomicrmw conceptually includes both a load and store from 359 // the same location. 360 // As with a store, the location being accessed is not captured, 361 // but the value being stored is. 362 // Volatile stores make the address observable. 363 auto *ARMWI = cast<AtomicRMWInst>(I); 364 if (U.getOperandNo() == 1 || ARMWI->isVolatile()) 365 return UseCaptureKind::MAY_CAPTURE; 366 return UseCaptureKind::NO_CAPTURE; 367 } 368 case Instruction::AtomicCmpXchg: { 369 // cmpxchg conceptually includes both a load and store from 370 // the same location. 371 // As with a store, the location being accessed is not captured, 372 // but the value being stored is. 373 // Volatile stores make the address observable. 374 auto *ACXI = cast<AtomicCmpXchgInst>(I); 375 if (U.getOperandNo() == 1 || U.getOperandNo() == 2 || ACXI->isVolatile()) 376 return UseCaptureKind::MAY_CAPTURE; 377 return UseCaptureKind::NO_CAPTURE; 378 } 379 case Instruction::BitCast: 380 case Instruction::GetElementPtr: 381 case Instruction::PHI: 382 case Instruction::Select: 383 case Instruction::AddrSpaceCast: 384 // The original value is not captured via this if the new value isn't. 385 return UseCaptureKind::PASSTHROUGH; 386 case Instruction::ICmp: { 387 unsigned Idx = U.getOperandNo(); 388 unsigned OtherIdx = 1 - Idx; 389 if (auto *CPN = dyn_cast<ConstantPointerNull>(I->getOperand(OtherIdx))) { 390 // Don't count comparisons of a no-alias return value against null as 391 // captures. This allows us to ignore comparisons of malloc results 392 // with null, for example. 393 if (CPN->getType()->getAddressSpace() == 0) 394 if (isNoAliasCall(U.get()->stripPointerCasts())) 395 return UseCaptureKind::NO_CAPTURE; 396 if (!I->getFunction()->nullPointerIsDefined()) { 397 auto *O = I->getOperand(Idx)->stripPointerCastsSameRepresentation(); 398 // Comparing a dereferenceable_or_null pointer against null cannot 399 // lead to pointer escapes, because if it is not null it must be a 400 // valid (in-bounds) pointer. 401 const DataLayout &DL = I->getModule()->getDataLayout(); 402 if (IsDereferenceableOrNull && IsDereferenceableOrNull(O, DL)) 403 return UseCaptureKind::NO_CAPTURE; 404 } 405 } 406 // Comparison against value stored in global variable. Given the pointer 407 // does not escape, its value cannot be guessed and stored separately in a 408 // global variable. 409 auto *LI = dyn_cast<LoadInst>(I->getOperand(OtherIdx)); 410 if (LI && isa<GlobalVariable>(LI->getPointerOperand())) 411 return UseCaptureKind::NO_CAPTURE; 412 // Otherwise, be conservative. There are crazy ways to capture pointers 413 // using comparisons. 414 return UseCaptureKind::MAY_CAPTURE; 415 } 416 default: 417 // Something else - be conservative and say it is captured. 418 return UseCaptureKind::MAY_CAPTURE; 419 } 420 } 421 422 void llvm::PointerMayBeCaptured(const Value *V, CaptureTracker *Tracker, 423 unsigned MaxUsesToExplore) { 424 assert(V->getType()->isPointerTy() && "Capture is for pointers only!"); 425 if (MaxUsesToExplore == 0) 426 MaxUsesToExplore = DefaultMaxUsesToExplore; 427 428 SmallVector<const Use *, 20> Worklist; 429 Worklist.reserve(getDefaultMaxUsesToExploreForCaptureTracking()); 430 SmallSet<const Use *, 20> Visited; 431 432 auto AddUses = [&](const Value *V) { 433 for (const Use &U : V->uses()) { 434 // If there are lots of uses, conservatively say that the value 435 // is captured to avoid taking too much compile time. 436 if (Visited.size() >= MaxUsesToExplore) { 437 Tracker->tooManyUses(); 438 return false; 439 } 440 if (!Visited.insert(&U).second) 441 continue; 442 if (!Tracker->shouldExplore(&U)) 443 continue; 444 Worklist.push_back(&U); 445 } 446 return true; 447 }; 448 if (!AddUses(V)) 449 return; 450 451 auto IsDereferenceableOrNull = [Tracker](Value *V, const DataLayout &DL) { 452 return Tracker->isDereferenceableOrNull(V, DL); 453 }; 454 while (!Worklist.empty()) { 455 const Use *U = Worklist.pop_back_val(); 456 switch (DetermineUseCaptureKind(*U, IsDereferenceableOrNull)) { 457 case UseCaptureKind::NO_CAPTURE: 458 continue; 459 case UseCaptureKind::MAY_CAPTURE: 460 if (Tracker->captured(U)) 461 return; 462 continue; 463 case UseCaptureKind::PASSTHROUGH: 464 if (!AddUses(U->getUser())) 465 return; 466 continue; 467 } 468 } 469 470 // All uses examined. 471 } 472 473 bool llvm::isNonEscapingLocalObject( 474 const Value *V, SmallDenseMap<const Value *, bool, 8> *IsCapturedCache) { 475 SmallDenseMap<const Value *, bool, 8>::iterator CacheIt; 476 if (IsCapturedCache) { 477 bool Inserted; 478 std::tie(CacheIt, Inserted) = IsCapturedCache->insert({V, false}); 479 if (!Inserted) 480 // Found cached result, return it! 481 return CacheIt->second; 482 } 483 484 // If this is an identified function-local object, check to see if it escapes. 485 if (isIdentifiedFunctionLocal(V)) { 486 // Set StoreCaptures to True so that we can assume in our callers that the 487 // pointer is not the result of a load instruction. Currently 488 // PointerMayBeCaptured doesn't have any special analysis for the 489 // StoreCaptures=false case; if it did, our callers could be refined to be 490 // more precise. 491 auto Ret = !PointerMayBeCaptured(V, false, /*StoreCaptures=*/true); 492 if (IsCapturedCache) 493 CacheIt->second = Ret; 494 return Ret; 495 } 496 497 return false; 498 } 499