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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 #include "llvm/IR/IntrinsicInst.h"
30 
31 using namespace llvm;
32 
~CaptureTracker()33 CaptureTracker::~CaptureTracker() {}
34 
shouldExplore(const Use * U)35 bool CaptureTracker::shouldExplore(const Use *U) { return true; }
36 
37 namespace {
38   struct SimpleCaptureTracker : public CaptureTracker {
SimpleCaptureTracker__anon90316b0b0111::SimpleCaptureTracker39     explicit SimpleCaptureTracker(bool ReturnCaptures)
40       : ReturnCaptures(ReturnCaptures), Captured(false) {}
41 
tooManyUses__anon90316b0b0111::SimpleCaptureTracker42     void tooManyUses() override { Captured = true; }
43 
captured__anon90316b0b0111::SimpleCaptureTracker44     bool captured(const Use *U) override {
45       if (isa<ReturnInst>(U->getUser()) && !ReturnCaptures)
46         return false;
47 
48       Captured = true;
49       return true;
50     }
51 
52     bool ReturnCaptures;
53 
54     bool Captured;
55   };
56 
57   /// Only find pointer captures which happen before the given instruction. Uses
58   /// the dominator tree to determine whether one instruction is before another.
59   /// Only support the case where the Value is defined in the same basic block
60   /// as the given instruction and the use.
61   struct CapturesBefore : public CaptureTracker {
62 
CapturesBefore__anon90316b0b0111::CapturesBefore63     CapturesBefore(bool ReturnCaptures, const Instruction *I, DominatorTree *DT,
64                    bool IncludeI, OrderedBasicBlock *IC)
65       : OrderedBB(IC), BeforeHere(I), DT(DT),
66         ReturnCaptures(ReturnCaptures), IncludeI(IncludeI), Captured(false) {}
67 
tooManyUses__anon90316b0b0111::CapturesBefore68     void tooManyUses() override { Captured = true; }
69 
isSafeToPrune__anon90316b0b0111::CapturesBefore70     bool isSafeToPrune(Instruction *I) {
71       BasicBlock *BB = I->getParent();
72       // We explore this usage only if the usage can reach "BeforeHere".
73       // If use is not reachable from entry, there is no need to explore.
74       if (BeforeHere != I && !DT->isReachableFromEntry(BB))
75         return true;
76 
77       // Compute the case where both instructions are inside the same basic
78       // block. Since instructions in the same BB as BeforeHere are numbered in
79       // 'OrderedBB', avoid using 'dominates' and 'isPotentiallyReachable'
80       // which are very expensive for large basic blocks.
81       if (BB == BeforeHere->getParent()) {
82         // 'I' dominates 'BeforeHere' => not safe to prune.
83         //
84         // The value defined by an invoke dominates an instruction only
85         // if it dominates every instruction in UseBB. A PHI is dominated only
86         // if the instruction dominates every possible use in the UseBB. Since
87         // UseBB == BB, avoid pruning.
88         if (isa<InvokeInst>(BeforeHere) || isa<PHINode>(I) || I == BeforeHere)
89           return false;
90         if (!OrderedBB->dominates(BeforeHere, I))
91           return false;
92 
93         // 'BeforeHere' comes before 'I', it's safe to prune if we also
94         // guarantee that 'I' never reaches 'BeforeHere' through a back-edge or
95         // by its successors, i.e, prune if:
96         //
97         //  (1) BB is an entry block or have no sucessors.
98         //  (2) There's no path coming back through BB sucessors.
99         if (BB == &BB->getParent()->getEntryBlock() ||
100             !BB->getTerminator()->getNumSuccessors())
101           return true;
102 
103         SmallVector<BasicBlock*, 32> Worklist;
104         Worklist.append(succ_begin(BB), succ_end(BB));
105         return !isPotentiallyReachableFromMany(Worklist, BB, DT);
106       }
107 
108       // If the value is defined in the same basic block as use and BeforeHere,
109       // there is no need to explore the use if BeforeHere dominates use.
110       // Check whether there is a path from I to BeforeHere.
111       if (BeforeHere != I && DT->dominates(BeforeHere, I) &&
112           !isPotentiallyReachable(I, BeforeHere, DT))
113         return true;
114 
115       return false;
116     }
117 
shouldExplore__anon90316b0b0111::CapturesBefore118     bool shouldExplore(const Use *U) override {
119       Instruction *I = cast<Instruction>(U->getUser());
120 
121       if (BeforeHere == I && !IncludeI)
122         return false;
123 
124       if (isSafeToPrune(I))
125         return false;
126 
127       return true;
128     }
129 
captured__anon90316b0b0111::CapturesBefore130     bool captured(const Use *U) override {
131       if (isa<ReturnInst>(U->getUser()) && !ReturnCaptures)
132         return false;
133 
134       if (!shouldExplore(U))
135         return false;
136 
137       Captured = true;
138       return true;
139     }
140 
141     OrderedBasicBlock *OrderedBB;
142     const Instruction *BeforeHere;
143     DominatorTree *DT;
144 
145     bool ReturnCaptures;
146     bool IncludeI;
147 
148     bool Captured;
149   };
150 }
151 
152 /// PointerMayBeCaptured - Return true if this pointer value may be captured
153 /// by the enclosing function (which is required to exist).  This routine can
154 /// be expensive, so consider caching the results.  The boolean ReturnCaptures
155 /// specifies whether returning the value (or part of it) from the function
156 /// counts as capturing it or not.  The boolean StoreCaptures specified whether
157 /// storing the value (or part of it) into memory anywhere automatically
158 /// counts as capturing it or not.
PointerMayBeCaptured(const Value * V,bool ReturnCaptures,bool StoreCaptures)159 bool llvm::PointerMayBeCaptured(const Value *V,
160                                 bool ReturnCaptures, bool StoreCaptures) {
161   assert(!isa<GlobalValue>(V) &&
162          "It doesn't make sense to ask whether a global is captured.");
163 
164   // TODO: If StoreCaptures is not true, we could do Fancy analysis
165   // to determine whether this store is not actually an escape point.
166   // In that case, BasicAliasAnalysis should be updated as well to
167   // take advantage of this.
168   (void)StoreCaptures;
169 
170   SimpleCaptureTracker SCT(ReturnCaptures);
171   PointerMayBeCaptured(V, &SCT);
172   return SCT.Captured;
173 }
174 
175 /// PointerMayBeCapturedBefore - Return true if this pointer value may be
176 /// captured by the enclosing function (which is required to exist). If a
177 /// DominatorTree is provided, only captures which happen before the given
178 /// instruction are considered. This routine can be expensive, so consider
179 /// caching the results.  The boolean ReturnCaptures specifies whether
180 /// returning the value (or part of it) from the function counts as capturing
181 /// it or not.  The boolean StoreCaptures specified whether storing the value
182 /// (or part of it) into memory anywhere automatically counts as capturing it
183 /// or not. A ordered basic block \p OBB can be used in order to speed up
184 /// queries about relative order among instructions in the same basic block.
PointerMayBeCapturedBefore(const Value * V,bool ReturnCaptures,bool StoreCaptures,const Instruction * I,DominatorTree * DT,bool IncludeI,OrderedBasicBlock * OBB)185 bool llvm::PointerMayBeCapturedBefore(const Value *V, bool ReturnCaptures,
186                                       bool StoreCaptures, const Instruction *I,
187                                       DominatorTree *DT, bool IncludeI,
188                                       OrderedBasicBlock *OBB) {
189   assert(!isa<GlobalValue>(V) &&
190          "It doesn't make sense to ask whether a global is captured.");
191   bool UseNewOBB = OBB == nullptr;
192 
193   if (!DT)
194     return PointerMayBeCaptured(V, ReturnCaptures, StoreCaptures);
195   if (UseNewOBB)
196     OBB = new OrderedBasicBlock(I->getParent());
197 
198   // TODO: See comment in PointerMayBeCaptured regarding what could be done
199   // with StoreCaptures.
200 
201   CapturesBefore CB(ReturnCaptures, I, DT, IncludeI, OBB);
202   PointerMayBeCaptured(V, &CB);
203 
204   if (UseNewOBB)
205     delete OBB;
206   return CB.Captured;
207 }
208 
209 /// TODO: Write a new FunctionPass AliasAnalysis so that it can keep
210 /// a cache. Then we can move the code from BasicAliasAnalysis into
211 /// that path, and remove this threshold.
212 static int const Threshold = 20;
213 
PointerMayBeCaptured(const Value * V,CaptureTracker * Tracker)214 void llvm::PointerMayBeCaptured(const Value *V, CaptureTracker *Tracker) {
215   assert(V->getType()->isPointerTy() && "Capture is for pointers only!");
216   SmallVector<const Use *, Threshold> Worklist;
217   SmallSet<const Use *, Threshold> Visited;
218   int Count = 0;
219 
220   for (const Use &U : V->uses()) {
221     // If there are lots of uses, conservatively say that the value
222     // is captured to avoid taking too much compile time.
223     if (Count++ >= Threshold)
224       return Tracker->tooManyUses();
225 
226     if (!Tracker->shouldExplore(&U)) continue;
227     Visited.insert(&U);
228     Worklist.push_back(&U);
229   }
230 
231   while (!Worklist.empty()) {
232     const Use *U = Worklist.pop_back_val();
233     Instruction *I = cast<Instruction>(U->getUser());
234     V = U->get();
235 
236     switch (I->getOpcode()) {
237     case Instruction::Call:
238     case Instruction::Invoke: {
239       CallSite CS(I);
240       // Not captured if the callee is readonly, doesn't return a copy through
241       // its return value and doesn't unwind (a readonly function can leak bits
242       // by throwing an exception or not depending on the input value).
243       if (CS.onlyReadsMemory() && CS.doesNotThrow() && I->getType()->isVoidTy())
244         break;
245 
246       // Volatile operations effectively capture the memory location that they
247       // load and store to.
248       if (auto *MI = dyn_cast<MemIntrinsic>(I))
249         if (MI->isVolatile())
250           if (Tracker->captured(U))
251             return;
252 
253       // Not captured if only passed via 'nocapture' arguments.  Note that
254       // calling a function pointer does not in itself cause the pointer to
255       // be captured.  This is a subtle point considering that (for example)
256       // the callee might return its own address.  It is analogous to saying
257       // that loading a value from a pointer does not cause the pointer to be
258       // captured, even though the loaded value might be the pointer itself
259       // (think of self-referential objects).
260       CallSite::data_operand_iterator B =
261         CS.data_operands_begin(), E = CS.data_operands_end();
262       for (CallSite::data_operand_iterator A = B; A != E; ++A)
263         if (A->get() == V && !CS.doesNotCapture(A - B))
264           // The parameter is not marked 'nocapture' - captured.
265           if (Tracker->captured(U))
266             return;
267       break;
268     }
269     case Instruction::Load:
270       // Volatile loads make the address observable.
271       if (cast<LoadInst>(I)->isVolatile())
272         if (Tracker->captured(U))
273           return;
274       break;
275     case Instruction::VAArg:
276       // "va-arg" from a pointer does not cause it to be captured.
277       break;
278     case Instruction::Store:
279         // Stored the pointer - conservatively assume it may be captured.
280         // Volatile stores make the address observable.
281       if (V == I->getOperand(0) || cast<StoreInst>(I)->isVolatile())
282         if (Tracker->captured(U))
283           return;
284       break;
285     case Instruction::AtomicRMW: {
286       // atomicrmw conceptually includes both a load and store from
287       // the same location.
288       // As with a store, the location being accessed is not captured,
289       // but the value being stored is.
290       // Volatile stores make the address observable.
291       auto *ARMWI = cast<AtomicRMWInst>(I);
292       if (ARMWI->getValOperand() == V || ARMWI->isVolatile())
293         if (Tracker->captured(U))
294           return;
295       break;
296     }
297     case Instruction::AtomicCmpXchg: {
298       // cmpxchg conceptually includes both a load and store from
299       // the same location.
300       // As with a store, the location being accessed is not captured,
301       // but the value being stored is.
302       // Volatile stores make the address observable.
303       auto *ACXI = cast<AtomicCmpXchgInst>(I);
304       if (ACXI->getCompareOperand() == V || ACXI->getNewValOperand() == V ||
305           ACXI->isVolatile())
306         if (Tracker->captured(U))
307           return;
308       break;
309     }
310     case Instruction::BitCast:
311     case Instruction::GetElementPtr:
312     case Instruction::PHI:
313     case Instruction::Select:
314     case Instruction::AddrSpaceCast:
315       // The original value is not captured via this if the new value isn't.
316       Count = 0;
317       for (Use &UU : I->uses()) {
318         // If there are lots of uses, conservatively say that the value
319         // is captured to avoid taking too much compile time.
320         if (Count++ >= Threshold)
321           return Tracker->tooManyUses();
322 
323         if (Visited.insert(&UU).second)
324           if (Tracker->shouldExplore(&UU))
325             Worklist.push_back(&UU);
326       }
327       break;
328     case Instruction::ICmp: {
329       // Don't count comparisons of a no-alias return value against null as
330       // captures. This allows us to ignore comparisons of malloc results
331       // with null, for example.
332       if (ConstantPointerNull *CPN =
333           dyn_cast<ConstantPointerNull>(I->getOperand(1)))
334         if (CPN->getType()->getAddressSpace() == 0)
335           if (isNoAliasCall(V->stripPointerCasts()))
336             break;
337       // Comparison against value stored in global variable. Given the pointer
338       // does not escape, its value cannot be guessed and stored separately in a
339       // global variable.
340       unsigned OtherIndex = (I->getOperand(0) == V) ? 1 : 0;
341       auto *LI = dyn_cast<LoadInst>(I->getOperand(OtherIndex));
342       if (LI && isa<GlobalVariable>(LI->getPointerOperand()))
343         break;
344       // Otherwise, be conservative. There are crazy ways to capture pointers
345       // using comparisons.
346       if (Tracker->captured(U))
347         return;
348       break;
349     }
350     default:
351       // Something else - be conservative and say it is captured.
352       if (Tracker->captured(U))
353         return;
354       break;
355     }
356   }
357 
358   // All uses examined.
359 }
360