LCOV - code coverage report
Current view: top level - lib/Analysis - CaptureTracking.cpp (source / functions) Hit Total Coverage
Test: llvm-toolchain.info Lines: 93 95 97.9 %
Date: 2018-07-13 00:08:38 Functions: 12 17 70.6 %
Legend: Lines: hit not hit

          Line data    Source code
       1             : //===--- CaptureTracking.cpp - Determine whether a pointer is captured ----===//
       2             : //
       3             : //                     The LLVM Compiler Infrastructure
       4             : //
       5             : // This file is distributed under the University of Illinois Open Source
       6             : // License. See LICENSE.TXT for details.
       7             : //
       8             : //===----------------------------------------------------------------------===//
       9             : //
      10             : // This file contains routines that help determine which pointers are captured.
      11             : // A pointer value is captured if the function makes a copy of any part of the
      12             : // pointer that outlives the call.  Not being captured means, more or less, that
      13             : // the pointer is only dereferenced and not stored in a global.  Returning part
      14             : // of the pointer as the function return value may or may not count as capturing
      15             : // the pointer, depending on the context.
      16             : //
      17             : //===----------------------------------------------------------------------===//
      18             : 
      19             : #include "llvm/Analysis/CaptureTracking.h"
      20             : #include "llvm/ADT/SmallSet.h"
      21             : #include "llvm/ADT/SmallVector.h"
      22             : #include "llvm/Analysis/AliasAnalysis.h"
      23             : #include "llvm/Analysis/CFG.h"
      24             : #include "llvm/Analysis/OrderedBasicBlock.h"
      25             : #include "llvm/Analysis/ValueTracking.h"
      26             : #include "llvm/IR/CallSite.h"
      27             : #include "llvm/IR/Constants.h"
      28             : #include "llvm/IR/Dominators.h"
      29             : #include "llvm/IR/Instructions.h"
      30             : #include "llvm/IR/IntrinsicInst.h"
      31             : 
      32             : using namespace llvm;
      33             : 
      34      919031 : CaptureTracker::~CaptureTracker() {}
      35             : 
      36     7209771 : bool CaptureTracker::shouldExplore(const Use *U) { return true; }
      37             : 
      38             : namespace {
      39      807847 :   struct SimpleCaptureTracker : public CaptureTracker {
      40             :     explicit SimpleCaptureTracker(bool ReturnCaptures)
      41      807847 :       : ReturnCaptures(ReturnCaptures), Captured(false) {}
      42             : 
      43       20618 :     void tooManyUses() override { Captured = true; }
      44             : 
      45      754295 :     bool captured(const Use *U) override {
      46      754862 :       if (isa<ReturnInst>(U->getUser()) && !ReturnCaptures)
      47             :         return false;
      48             : 
      49      753769 :       Captured = true;
      50      753769 :       return true;
      51             :     }
      52             : 
      53             :     bool ReturnCaptures;
      54             : 
      55             :     bool Captured;
      56             :   };
      57             : 
      58             :   /// Only find pointer captures which happen before the given instruction. Uses
      59             :   /// the dominator tree to determine whether one instruction is before another.
      60             :   /// Only support the case where the Value is defined in the same basic block
      61             :   /// as the given instruction and the use.
      62      106317 :   struct CapturesBefore : public CaptureTracker {
      63             : 
      64             :     CapturesBefore(bool ReturnCaptures, const Instruction *I, const DominatorTree *DT,
      65             :                    bool IncludeI, OrderedBasicBlock *IC)
      66      106317 :       : OrderedBB(IC), BeforeHere(I), DT(DT),
      67      106317 :         ReturnCaptures(ReturnCaptures), IncludeI(IncludeI), Captured(false) {}
      68             : 
      69        1698 :     void tooManyUses() override { Captured = true; }
      70             : 
      71      661797 :     bool isSafeToPrune(Instruction *I) {
      72      661797 :       BasicBlock *BB = I->getParent();
      73             :       // We explore this usage only if the usage can reach "BeforeHere".
      74             :       // If use is not reachable from entry, there is no need to explore.
      75      661797 :       if (BeforeHere != I && !DT->isReachableFromEntry(BB))
      76             :         return true;
      77             : 
      78             :       // Compute the case where both instructions are inside the same basic
      79             :       // block. Since instructions in the same BB as BeforeHere are numbered in
      80             :       // 'OrderedBB', avoid using 'dominates' and 'isPotentiallyReachable'
      81             :       // which are very expensive for large basic blocks.
      82      661797 :       if (BB == BeforeHere->getParent()) {
      83             :         // 'I' dominates 'BeforeHere' => not safe to prune.
      84             :         //
      85             :         // The value defined by an invoke dominates an instruction only
      86             :         // if it dominates every instruction in UseBB. A PHI is dominated only
      87             :         // if the instruction dominates every possible use in the UseBB. Since
      88             :         // UseBB == BB, avoid pruning.
      89      207062 :         if (isa<InvokeInst>(BeforeHere) || isa<PHINode>(I) || I == BeforeHere)
      90             :           return false;
      91       86678 :         if (!OrderedBB->dominates(BeforeHere, I))
      92             :           return false;
      93             : 
      94             :         // 'BeforeHere' comes before 'I', it's safe to prune if we also
      95             :         // guarantee that 'I' never reaches 'BeforeHere' through a back-edge or
      96             :         // by its successors, i.e, prune if:
      97             :         //
      98             :         //  (1) BB is an entry block or have no successors.
      99             :         //  (2) There's no path coming back through BB successors.
     100      164544 :         if (BB == &BB->getParent()->getEntryBlock() ||
     101       39024 :             !BB->getTerminator()->getNumSuccessors())
     102             :           return true;
     103             : 
     104             :         SmallVector<BasicBlock*, 32> Worklist;
     105       69620 :         Worklist.append(succ_begin(BB), succ_end(BB));
     106       34810 :         return !isPotentiallyReachableFromMany(Worklist, BB, DT);
     107             :       }
     108             : 
     109             :       // If the value is defined in the same basic block as use and BeforeHere,
     110             :       // there is no need to explore the use if BeforeHere dominates use.
     111             :       // Check whether there is a path from I to BeforeHere.
     112      637524 :       if (BeforeHere != I && DT->dominates(BeforeHere, I) &&
     113       80610 :           !isPotentiallyReachable(I, BeforeHere, DT))
     114             :         return true;
     115             : 
     116             :       return false;
     117             :     }
     118             : 
     119      661797 :     bool shouldExplore(const Use *U) override {
     120      661797 :       Instruction *I = cast<Instruction>(U->getUser());
     121             : 
     122      661797 :       if (BeforeHere == I && !IncludeI)
     123             :         return false;
     124             : 
     125      661797 :       if (isSafeToPrune(I))
     126             :         return false;
     127             : 
     128      570442 :       return true;
     129             :     }
     130             : 
     131       71366 :     bool captured(const Use *U) override {
     132       71376 :       if (isa<ReturnInst>(U->getUser()) && !ReturnCaptures)
     133             :         return false;
     134             : 
     135       71366 :       if (!shouldExplore(U))
     136             :         return false;
     137             : 
     138       71366 :       Captured = true;
     139       71366 :       return true;
     140             :     }
     141             : 
     142             :     OrderedBasicBlock *OrderedBB;
     143             :     const Instruction *BeforeHere;
     144             :     const DominatorTree *DT;
     145             : 
     146             :     bool ReturnCaptures;
     147             :     bool IncludeI;
     148             : 
     149             :     bool Captured;
     150             :   };
     151             : }
     152             : 
     153             : /// PointerMayBeCaptured - Return true if this pointer value may be captured
     154             : /// by the enclosing function (which is required to exist).  This routine can
     155             : /// be expensive, so consider caching the results.  The boolean ReturnCaptures
     156             : /// specifies whether returning the value (or part of it) from the function
     157             : /// counts as capturing it or not.  The boolean StoreCaptures specified whether
     158             : /// storing the value (or part of it) into memory anywhere automatically
     159             : /// counts as capturing it or not.
     160      807847 : bool llvm::PointerMayBeCaptured(const Value *V,
     161             :                                 bool ReturnCaptures, bool StoreCaptures) {
     162             :   assert(!isa<GlobalValue>(V) &&
     163             :          "It doesn't make sense to ask whether a global is captured.");
     164             : 
     165             :   // TODO: If StoreCaptures is not true, we could do Fancy analysis
     166             :   // to determine whether this store is not actually an escape point.
     167             :   // In that case, BasicAliasAnalysis should be updated as well to
     168             :   // take advantage of this.
     169             :   (void)StoreCaptures;
     170             : 
     171             :   SimpleCaptureTracker SCT(ReturnCaptures);
     172      807847 :   PointerMayBeCaptured(V, &SCT);
     173     1615694 :   return SCT.Captured;
     174             : }
     175             : 
     176             : /// PointerMayBeCapturedBefore - Return true if this pointer value may be
     177             : /// captured by the enclosing function (which is required to exist). If a
     178             : /// DominatorTree is provided, only captures which happen before the given
     179             : /// instruction are considered. This routine can be expensive, so consider
     180             : /// caching the results.  The boolean ReturnCaptures specifies whether
     181             : /// returning the value (or part of it) from the function counts as capturing
     182             : /// it or not.  The boolean StoreCaptures specified whether storing the value
     183             : /// (or part of it) into memory anywhere automatically counts as capturing it
     184             : /// or not. A ordered basic block \p OBB can be used in order to speed up
     185             : /// queries about relative order among instructions in the same basic block.
     186      106317 : bool llvm::PointerMayBeCapturedBefore(const Value *V, bool ReturnCaptures,
     187             :                                       bool StoreCaptures, const Instruction *I,
     188             :                                       const DominatorTree *DT, bool IncludeI,
     189             :                                       OrderedBasicBlock *OBB) {
     190             :   assert(!isa<GlobalValue>(V) &&
     191             :          "It doesn't make sense to ask whether a global is captured.");
     192             :   bool UseNewOBB = OBB == nullptr;
     193             : 
     194      106317 :   if (!DT)
     195           0 :     return PointerMayBeCaptured(V, ReturnCaptures, StoreCaptures);
     196      106317 :   if (UseNewOBB)
     197       53821 :     OBB = new OrderedBasicBlock(I->getParent());
     198             : 
     199             :   // TODO: See comment in PointerMayBeCaptured regarding what could be done
     200             :   // with StoreCaptures.
     201             : 
     202             :   CapturesBefore CB(ReturnCaptures, I, DT, IncludeI, OBB);
     203      106317 :   PointerMayBeCaptured(V, &CB);
     204             : 
     205      106317 :   if (UseNewOBB)
     206      107642 :     delete OBB;
     207      106317 :   return CB.Captured;
     208             : }
     209             : 
     210             : /// TODO: Write a new FunctionPass AliasAnalysis so that it can keep
     211             : /// a cache. Then we can move the code from BasicAliasAnalysis into
     212             : /// that path, and remove this threshold.
     213             : static int const Threshold = 20;
     214             : 
     215      919031 : void llvm::PointerMayBeCaptured(const Value *V, CaptureTracker *Tracker) {
     216             :   assert(V->getType()->isPointerTy() && "Capture is for pointers only!");
     217             :   SmallVector<const Use *, Threshold> Worklist;
     218             :   SmallSet<const Use *, Threshold> Visited;
     219             : 
     220     2336129 :   auto AddUses = [&](const Value *V) {
     221             :     int Count = 0;
     222    10281875 :     for (const Use &U : V->uses()) {
     223             :       // If there are lots of uses, conservatively say that the value
     224             :       // is captured to avoid taking too much compile time.
     225     7968074 :       if (Count++ >= Threshold)
     226     7822530 :         return Tracker->tooManyUses();
     227     7945746 :       if (!Visited.insert(&U).second)
     228      145544 :         continue;
     229     7800202 :       if (!Tracker->shouldExplore(&U))
     230       91355 :         continue;
     231     7708847 :       Worklist.push_back(&U);
     232             :     }
     233      919031 :   };
     234      919031 :   AddUses(V);
     235             : 
     236     5336273 :   while (!Worklist.empty()) {
     237             :     const Use *U = Worklist.pop_back_val();
     238     5244565 :     Instruction *I = cast<Instruction>(U->getUser());
     239     5244565 :     V = U->get();
     240             : 
     241     5244565 :     switch (I->getOpcode()) {
     242             :     case Instruction::Call:
     243             :     case Instruction::Invoke: {
     244             :       CallSite CS(I);
     245             :       // Not captured if the callee is readonly, doesn't return a copy through
     246             :       // its return value and doesn't unwind (a readonly function can leak bits
     247             :       // by throwing an exception or not depending on the input value).
     248     2556222 :       if (CS.onlyReadsMemory() && CS.doesNotThrow() && I->getType()->isVoidTy())
     249             :         break;
     250             : 
     251             :       // The pointer is not captured if returned pointer is not captured.
     252             :       // NOTE: CaptureTracking users should not assume that only functions
     253             :       // marked with nocapture do not capture. This means that places like
     254             :       // GetUnderlyingObject in ValueTracking or DecomposeGEPExpression
     255             :       // in BasicAA also need to know about this property.
     256     2555593 :       if (isIntrinsicReturningPointerAliasingArgumentWithoutCapturing(CS)) {
     257          68 :         AddUses(I);
     258          68 :         break;
     259             :       }
     260             : 
     261             :       // Volatile operations effectively capture the memory location that they
     262             :       // load and store to.
     263             :       if (auto *MI = dyn_cast<MemIntrinsic>(I))
     264       21502 :         if (MI->isVolatile())
     265          52 :           if (Tracker->captured(U))
     266      794167 :             return;
     267             : 
     268             :       // Not captured if only passed via 'nocapture' arguments.  Note that
     269             :       // calling a function pointer does not in itself cause the pointer to
     270             :       // be captured.  This is a subtle point considering that (for example)
     271             :       // the callee might return its own address.  It is analogous to saying
     272             :       // that loading a value from a pointer does not cause the pointer to be
     273             :       // captured, even though the loaded value might be the pointer itself
     274             :       // (think of self-referential objects).
     275             :       CallSite::data_operand_iterator B =
     276     2555473 :         CS.data_operands_begin(), E = CS.data_operands_end();
     277    11166055 :       for (CallSite::data_operand_iterator A = B; A != E; ++A)
     278     7654945 :         if (A->get() == V && !CS.doesNotCapture(A - B))
     279             :           // The parameter is not marked 'nocapture' - captured.
     280      794159 :           if (Tracker->captured(U))
     281             :             return;
     282             :       break;
     283             :     }
     284             :     case Instruction::Load:
     285             :       // Volatile loads make the address observable.
     286      822703 :       if (cast<LoadInst>(I)->isVolatile())
     287          32 :         if (Tracker->captured(U))
     288             :           return;
     289             :       break;
     290             :     case Instruction::VAArg:
     291             :       // "va-arg" from a pointer does not cause it to be captured.
     292             :       break;
     293      443668 :     case Instruction::Store:
     294             :         // Stored the pointer - conservatively assume it may be captured.
     295             :         // Volatile stores make the address observable.
     296     1301562 :       if (V == I->getOperand(0) || cast<StoreInst>(I)->isVolatile())
     297       29592 :         if (Tracker->captured(U))
     298             :           return;
     299             :       break;
     300             :     case Instruction::AtomicRMW: {
     301             :       // atomicrmw conceptually includes both a load and store from
     302             :       // the same location.
     303             :       // As with a store, the location being accessed is not captured,
     304             :       // but the value being stored is.
     305             :       // Volatile stores make the address observable.
     306             :       auto *ARMWI = cast<AtomicRMWInst>(I);
     307         210 :       if (ARMWI->getValOperand() == V || ARMWI->isVolatile())
     308           0 :         if (Tracker->captured(U))
     309             :           return;
     310             :       break;
     311             :     }
     312             :     case Instruction::AtomicCmpXchg: {
     313             :       // cmpxchg conceptually includes both a load and store from
     314             :       // the same location.
     315             :       // As with a store, the location being accessed is not captured,
     316             :       // but the value being stored is.
     317             :       // Volatile stores make the address observable.
     318             :       auto *ACXI = cast<AtomicCmpXchgInst>(I);
     319        3670 :       if (ACXI->getCompareOperand() == V || ACXI->getNewValOperand() == V ||
     320             :           ACXI->isVolatile())
     321          20 :         if (Tracker->captured(U))
     322             :           return;
     323             :       break;
     324             :     }
     325     1417030 :     case Instruction::BitCast:
     326             :     case Instruction::GetElementPtr:
     327             :     case Instruction::PHI:
     328             :     case Instruction::Select:
     329             :     case Instruction::AddrSpaceCast:
     330             :       // The original value is not captured via this if the new value isn't.
     331     1417030 :       AddUses(I);
     332     1417030 :       break;
     333        3002 :     case Instruction::ICmp: {
     334             :       // Don't count comparisons of a no-alias return value against null as
     335             :       // captures. This allows us to ignore comparisons of malloc results
     336             :       // with null, for example.
     337             :       if (ConstantPointerNull *CPN =
     338        3002 :           dyn_cast<ConstantPointerNull>(I->getOperand(1)))
     339         300 :         if (CPN->getType()->getAddressSpace() == 0)
     340         278 :           if (isNoAliasCall(V->stripPointerCasts()))
     341             :             break;
     342             :       // Comparison against value stored in global variable. Given the pointer
     343             :       // does not escape, its value cannot be guessed and stored separately in a
     344             :       // global variable.
     345        2813 :       unsigned OtherIndex = (I->getOperand(0) == V) ? 1 : 0;
     346             :       auto *LI = dyn_cast<LoadInst>(I->getOperand(OtherIndex));
     347        2029 :       if (LI && isa<GlobalVariable>(LI->getPointerOperand()))
     348             :         break;
     349             :       // Otherwise, be conservative. There are crazy ways to capture pointers
     350             :       // using comparisons.
     351        2808 :       if (Tracker->captured(U))
     352             :         return;
     353             :       break;
     354             :     }
     355        1230 :     default:
     356             :       // Something else - be conservative and say it is captured.
     357        1230 :       if (Tracker->captured(U))
     358             :         return;
     359             :       break;
     360             :     }
     361             :   }
     362             : 
     363             :   // All uses examined.
     364             : }

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