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      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/ADT/SmallSet.h"
     20 #include "llvm/ADT/SmallVector.h"
     21 #include "llvm/Analysis/AliasAnalysis.h"
     22 #include "llvm/Analysis/CFG.h"
     23 #include "llvm/Analysis/CaptureTracking.h"
     24 #include "llvm/Analysis/OrderedBasicBlock.h"
     25 #include "llvm/IR/CallSite.h"
     26 #include "llvm/IR/Constants.h"
     27 #include "llvm/IR/Dominators.h"
     28 #include "llvm/IR/Instructions.h"
     29 
     30 using namespace llvm;
     31 
     32 CaptureTracker::~CaptureTracker() {}
     33 
     34 bool CaptureTracker::shouldExplore(const Use *U) { return true; }
     35 
     36 namespace {
     37   struct SimpleCaptureTracker : public CaptureTracker {
     38     explicit SimpleCaptureTracker(bool ReturnCaptures)
     39       : ReturnCaptures(ReturnCaptures), Captured(false) {}
     40 
     41     void tooManyUses() override { Captured = true; }
     42 
     43     bool captured(const Use *U) override {
     44       if (isa<ReturnInst>(U->getUser()) && !ReturnCaptures)
     45         return false;
     46 
     47       Captured = true;
     48       return true;
     49     }
     50 
     51     bool ReturnCaptures;
     52 
     53     bool Captured;
     54   };
     55 
     56   /// Only find pointer captures which happen before the given instruction. Uses
     57   /// the dominator tree to determine whether one instruction is before another.
     58   /// Only support the case where the Value is defined in the same basic block
     59   /// as the given instruction and the use.
     60   struct CapturesBefore : public CaptureTracker {
     61 
     62     CapturesBefore(bool ReturnCaptures, const Instruction *I, DominatorTree *DT,
     63                    bool IncludeI, OrderedBasicBlock *IC)
     64       : OrderedBB(IC), BeforeHere(I), DT(DT),
     65         ReturnCaptures(ReturnCaptures), IncludeI(IncludeI), Captured(false) {}
     66 
     67     void tooManyUses() override { Captured = true; }
     68 
     69     bool isSafeToPrune(Instruction *I) {
     70       BasicBlock *BB = I->getParent();
     71       // We explore this usage only if the usage can reach "BeforeHere".
     72       // If use is not reachable from entry, there is no need to explore.
     73       if (BeforeHere != I && !DT->isReachableFromEntry(BB))
     74         return true;
     75 
     76       // Compute the case where both instructions are inside the same basic
     77       // block. Since instructions in the same BB as BeforeHere are numbered in
     78       // 'OrderedBB', avoid using 'dominates' and 'isPotentiallyReachable'
     79       // which are very expensive for large basic blocks.
     80       if (BB == BeforeHere->getParent()) {
     81         // 'I' dominates 'BeforeHere' => not safe to prune.
     82         //
     83         // The value defined by an invoke dominates an instruction only
     84         // if it dominates every instruction in UseBB. A PHI is dominated only
     85         // if the instruction dominates every possible use in the UseBB. Since
     86         // UseBB == BB, avoid pruning.
     87         if (isa<InvokeInst>(BeforeHere) || isa<PHINode>(I) || I == BeforeHere)
     88           return false;
     89         if (!OrderedBB->dominates(BeforeHere, I))
     90           return false;
     91 
     92         // 'BeforeHere' comes before 'I', it's safe to prune if we also
     93         // guarantee that 'I' never reaches 'BeforeHere' through a back-edge or
     94         // by its successors, i.e, prune if:
     95         //
     96         //  (1) BB is an entry block or have no sucessors.
     97         //  (2) There's no path coming back through BB sucessors.
     98         if (BB == &BB->getParent()->getEntryBlock() ||
     99             !BB->getTerminator()->getNumSuccessors())
    100           return true;
    101 
    102         SmallVector<BasicBlock*, 32> Worklist;
    103         Worklist.append(succ_begin(BB), succ_end(BB));
    104         return !isPotentiallyReachableFromMany(Worklist, BB, DT);
    105       }
    106 
    107       // If the value is defined in the same basic block as use and BeforeHere,
    108       // there is no need to explore the use if BeforeHere dominates use.
    109       // Check whether there is a path from I to BeforeHere.
    110       if (BeforeHere != I && DT->dominates(BeforeHere, I) &&
    111           !isPotentiallyReachable(I, BeforeHere, DT))
    112         return true;
    113 
    114       return false;
    115     }
    116 
    117     bool shouldExplore(const Use *U) override {
    118       Instruction *I = cast<Instruction>(U->getUser());
    119 
    120       if (BeforeHere == I && !IncludeI)
    121         return false;
    122 
    123       if (isSafeToPrune(I))
    124         return false;
    125 
    126       return true;
    127     }
    128 
    129     bool captured(const Use *U) override {
    130       if (isa<ReturnInst>(U->getUser()) && !ReturnCaptures)
    131         return false;
    132 
    133       if (!shouldExplore(U))
    134         return false;
    135 
    136       Captured = true;
    137       return true;
    138     }
    139 
    140     OrderedBasicBlock *OrderedBB;
    141     const Instruction *BeforeHere;
    142     DominatorTree *DT;
    143 
    144     bool ReturnCaptures;
    145     bool IncludeI;
    146 
    147     bool Captured;
    148   };
    149 }
    150 
    151 /// PointerMayBeCaptured - Return true if this pointer value may be captured
    152 /// by the enclosing function (which is required to exist).  This routine can
    153 /// be expensive, so consider caching the results.  The boolean ReturnCaptures
    154 /// specifies whether returning the value (or part of it) from the function
    155 /// counts as capturing it or not.  The boolean StoreCaptures specified whether
    156 /// storing the value (or part of it) into memory anywhere automatically
    157 /// counts as capturing it or not.
    158 bool llvm::PointerMayBeCaptured(const Value *V,
    159                                 bool ReturnCaptures, bool StoreCaptures) {
    160   assert(!isa<GlobalValue>(V) &&
    161          "It doesn't make sense to ask whether a global is captured.");
    162 
    163   // TODO: If StoreCaptures is not true, we could do Fancy analysis
    164   // to determine whether this store is not actually an escape point.
    165   // In that case, BasicAliasAnalysis should be updated as well to
    166   // take advantage of this.
    167   (void)StoreCaptures;
    168 
    169   SimpleCaptureTracker SCT(ReturnCaptures);
    170   PointerMayBeCaptured(V, &SCT);
    171   return SCT.Captured;
    172 }
    173 
    174 /// PointerMayBeCapturedBefore - Return true if this pointer value may be
    175 /// captured by the enclosing function (which is required to exist). If a
    176 /// DominatorTree is provided, only captures which happen before the given
    177 /// instruction are considered. This routine can be expensive, so consider
    178 /// caching the results.  The boolean ReturnCaptures specifies whether
    179 /// returning the value (or part of it) from the function counts as capturing
    180 /// it or not.  The boolean StoreCaptures specified whether storing the value
    181 /// (or part of it) into memory anywhere automatically counts as capturing it
    182 /// or not. A ordered basic block \p OBB can be used in order to speed up
    183 /// queries about relative order among instructions in the same basic block.
    184 bool llvm::PointerMayBeCapturedBefore(const Value *V, bool ReturnCaptures,
    185                                       bool StoreCaptures, const Instruction *I,
    186                                       DominatorTree *DT, bool IncludeI,
    187                                       OrderedBasicBlock *OBB) {
    188   assert(!isa<GlobalValue>(V) &&
    189          "It doesn't make sense to ask whether a global is captured.");
    190   bool UseNewOBB = OBB == nullptr;
    191 
    192   if (!DT)
    193     return PointerMayBeCaptured(V, ReturnCaptures, StoreCaptures);
    194   if (UseNewOBB)
    195     OBB = new OrderedBasicBlock(I->getParent());
    196 
    197   // TODO: See comment in PointerMayBeCaptured regarding what could be done
    198   // with StoreCaptures.
    199 
    200   CapturesBefore CB(ReturnCaptures, I, DT, IncludeI, OBB);
    201   PointerMayBeCaptured(V, &CB);
    202 
    203   if (UseNewOBB)
    204     delete OBB;
    205   return CB.Captured;
    206 }
    207 
    208 /// TODO: Write a new FunctionPass AliasAnalysis so that it can keep
    209 /// a cache. Then we can move the code from BasicAliasAnalysis into
    210 /// that path, and remove this threshold.
    211 static int const Threshold = 20;
    212 
    213 void llvm::PointerMayBeCaptured(const Value *V, CaptureTracker *Tracker) {
    214   assert(V->getType()->isPointerTy() && "Capture is for pointers only!");
    215   SmallVector<const Use *, Threshold> Worklist;
    216   SmallSet<const Use *, Threshold> Visited;
    217   int Count = 0;
    218 
    219   for (const Use &U : V->uses()) {
    220     // If there are lots of uses, conservatively say that the value
    221     // is captured to avoid taking too much compile time.
    222     if (Count++ >= Threshold)
    223       return Tracker->tooManyUses();
    224 
    225     if (!Tracker->shouldExplore(&U)) continue;
    226     Visited.insert(&U);
    227     Worklist.push_back(&U);
    228   }
    229 
    230   while (!Worklist.empty()) {
    231     const Use *U = Worklist.pop_back_val();
    232     Instruction *I = cast<Instruction>(U->getUser());
    233     V = U->get();
    234 
    235     switch (I->getOpcode()) {
    236     case Instruction::Call:
    237     case Instruction::Invoke: {
    238       CallSite CS(I);
    239       // Not captured if the callee is readonly, doesn't return a copy through
    240       // its return value and doesn't unwind (a readonly function can leak bits
    241       // by throwing an exception or not depending on the input value).
    242       if (CS.onlyReadsMemory() && CS.doesNotThrow() && I->getType()->isVoidTy())
    243         break;
    244 
    245       // Not captured if only passed via 'nocapture' arguments.  Note that
    246       // calling a function pointer does not in itself cause the pointer to
    247       // be captured.  This is a subtle point considering that (for example)
    248       // the callee might return its own address.  It is analogous to saying
    249       // that loading a value from a pointer does not cause the pointer to be
    250       // captured, even though the loaded value might be the pointer itself
    251       // (think of self-referential objects).
    252       CallSite::data_operand_iterator B =
    253         CS.data_operands_begin(), E = CS.data_operands_end();
    254       for (CallSite::data_operand_iterator A = B; A != E; ++A)
    255         if (A->get() == V && !CS.doesNotCapture(A - B))
    256           // The parameter is not marked 'nocapture' - captured.
    257           if (Tracker->captured(U))
    258             return;
    259       break;
    260     }
    261     case Instruction::Load:
    262       // Loading from a pointer does not cause it to be captured.
    263       break;
    264     case Instruction::VAArg:
    265       // "va-arg" from a pointer does not cause it to be captured.
    266       break;
    267     case Instruction::Store:
    268       if (V == I->getOperand(0))
    269         // Stored the pointer - conservatively assume it may be captured.
    270         if (Tracker->captured(U))
    271           return;
    272       // Storing to the pointee does not cause the pointer to be captured.
    273       break;
    274     case Instruction::BitCast:
    275     case Instruction::GetElementPtr:
    276     case Instruction::PHI:
    277     case Instruction::Select:
    278     case Instruction::AddrSpaceCast:
    279       // The original value is not captured via this if the new value isn't.
    280       Count = 0;
    281       for (Use &UU : I->uses()) {
    282         // If there are lots of uses, conservatively say that the value
    283         // is captured to avoid taking too much compile time.
    284         if (Count++ >= Threshold)
    285           return Tracker->tooManyUses();
    286 
    287         if (Visited.insert(&UU).second)
    288           if (Tracker->shouldExplore(&UU))
    289             Worklist.push_back(&UU);
    290       }
    291       break;
    292     case Instruction::ICmp:
    293       // Don't count comparisons of a no-alias return value against null as
    294       // captures. This allows us to ignore comparisons of malloc results
    295       // with null, for example.
    296       if (ConstantPointerNull *CPN =
    297           dyn_cast<ConstantPointerNull>(I->getOperand(1)))
    298         if (CPN->getType()->getAddressSpace() == 0)
    299           if (isNoAliasCall(V->stripPointerCasts()))
    300             break;
    301       // Otherwise, be conservative. There are crazy ways to capture pointers
    302       // using comparisons.
    303       if (Tracker->captured(U))
    304         return;
    305       break;
    306     default:
    307       // Something else - be conservative and say it is captured.
    308       if (Tracker->captured(U))
    309         return;
    310       break;
    311     }
    312   }
    313 
    314   // All uses examined.
    315 }
    316