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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/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 1625181 : CaptureTracker::~CaptureTracker() {}
35 :
36 12854111 : bool CaptureTracker::shouldExplore(const Use *U) { return true; }
37 :
38 : namespace {
39 1445630 : struct SimpleCaptureTracker : public CaptureTracker {
40 : explicit SimpleCaptureTracker(bool ReturnCaptures)
41 1445630 : : ReturnCaptures(ReturnCaptures), Captured(false) {}
42 :
43 28613 : void tooManyUses() override { Captured = true; }
44 :
45 1397081 : bool captured(const Use *U) override {
46 1397703 : if (isa<ReturnInst>(U->getUser()) && !ReturnCaptures)
47 : return false;
48 :
49 1396543 : Captured = true;
50 1396543 : 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 171214 : struct CapturesBefore : public CaptureTracker {
63 :
64 : CapturesBefore(bool ReturnCaptures, const Instruction *I, const DominatorTree *DT,
65 : bool IncludeI, OrderedBasicBlock *IC)
66 171214 : : OrderedBB(IC), BeforeHere(I), DT(DT),
67 171214 : ReturnCaptures(ReturnCaptures), IncludeI(IncludeI), Captured(false) {}
68 :
69 2027 : void tooManyUses() override { Captured = true; }
70 :
71 938374 : bool isSafeToPrune(Instruction *I) {
72 938374 : 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 938374 : 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 938374 : 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 137932 : if (isa<InvokeInst>(BeforeHere) || isa<PHINode>(I) || I == BeforeHere)
90 : return false;
91 102389 : 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 129377 : if (BB == &BB->getParent()->getEntryBlock() ||
101 22191 : !BB->getTerminator()->getNumSuccessors())
102 33496 : return true;
103 :
104 : SmallVector<BasicBlock*, 32> Worklist;
105 20097 : Worklist.append(succ_begin(BB), succ_end(BB));
106 20097 : 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 915216 : if (BeforeHere != I && DT->dominates(BeforeHere, I) &&
113 114774 : !isPotentiallyReachable(I, BeforeHere, DT))
114 71262 : return true;
115 :
116 : return false;
117 : }
118 :
119 938374 : bool shouldExplore(const Use *U) override {
120 938374 : Instruction *I = cast<Instruction>(U->getUser());
121 :
122 938374 : if (BeforeHere == I && !IncludeI)
123 : return false;
124 :
125 938374 : if (isSafeToPrune(I))
126 119925 : return false;
127 :
128 : return true;
129 : }
130 :
131 125753 : bool captured(const Use *U) override {
132 125755 : if (isa<ReturnInst>(U->getUser()) && !ReturnCaptures)
133 : return false;
134 :
135 125753 : if (!shouldExplore(U))
136 : return false;
137 :
138 125753 : Captured = true;
139 125753 : 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 1445630 : 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 1445630 : PointerMayBeCaptured(V, &SCT);
173 1445630 : 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 171214 : 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 171214 : if (!DT)
195 0 : return PointerMayBeCaptured(V, ReturnCaptures, StoreCaptures);
196 171214 : if (UseNewOBB)
197 118206 : 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 171214 : PointerMayBeCaptured(V, &CB);
204 :
205 171214 : if (UseNewOBB)
206 236412 : delete OBB;
207 171214 : 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 1625181 : 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 : auto AddUses = [&](const Value *V) {
221 : int Count = 0;
222 : 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 : if (Count++ >= Threshold)
226 : return Tracker->tooManyUses();
227 : if (!Visited.insert(&U).second)
228 : continue;
229 : if (!Tracker->shouldExplore(&U))
230 : continue;
231 : Worklist.push_back(&U);
232 : }
233 1625181 : };
234 1625181 : AddUses(V);
235 :
236 8800977 : while (!Worklist.empty()) {
237 : const Use *U = Worklist.pop_back_val();
238 8702037 : Instruction *I = cast<Instruction>(U->getUser());
239 8702037 : V = U->get();
240 :
241 8702037 : 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 5069437 : 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 5069429 : if (isIntrinsicReturningPointerAliasingArgumentWithoutCapturing(CS)) {
257 73 : AddUses(I);
258 73 : 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 70464 : if (MI->isVolatile())
265 16 : if (Tracker->captured(U))
266 1444801 : 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 5069340 : CS.data_operands_begin(), E = CS.data_operands_end();
277 13731480 : for (CallSite::data_operand_iterator A = B; A != E; ++A)
278 10106925 : if (A->get() == V && !CS.doesNotCapture(A - B))
279 : // The parameter is not marked 'nocapture' - captured.
280 1444910 : if (Tracker->captured(U))
281 : return;
282 : break;
283 : }
284 : case Instruction::Load:
285 : // Volatile loads make the address observable.
286 1024769 : if (cast<LoadInst>(I)->isVolatile())
287 37 : 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 619607 : case Instruction::Store:
294 : // Stored the pointer - conservatively assume it may be captured.
295 : // Volatile stores make the address observable.
296 1239214 : if (V == I->getOperand(0) || cast<StoreInst>(I)->isVolatile())
297 70677 : 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);
307 147 : 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 3630 : if (ACXI->getCompareOperand() == V || ACXI->getNewValOperand() == V ||
320 : ACXI->isVolatile())
321 29 : if (Tracker->captured(U))
322 : return;
323 : break;
324 : }
325 1963191 : 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 1963191 : AddUses(I);
332 1963191 : break;
333 17397 : 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 17397 : dyn_cast<ConstantPointerNull>(I->getOperand(1)))
339 12229 : if (CPN->getType()->getAddressSpace() == 0)
340 12207 : 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 5568 : unsigned OtherIndex = (I->getOperand(0) == V) ? 1 : 0;
346 : auto *LI = dyn_cast<LoadInst>(I->getOperand(OtherIndex));
347 2683 : if (LI && isa<GlobalVariable>(LI->getPointerOperand()))
348 : break;
349 : // Otherwise, be conservative. There are crazy ways to capture pointers
350 : // using comparisons.
351 5563 : if (Tracker->captured(U))
352 : return;
353 : break;
354 : }
355 5672 : default:
356 : // Something else - be conservative and say it is captured.
357 5672 : if (Tracker->captured(U))
358 : return;
359 : break;
360 : }
361 : }
362 :
363 : // All uses examined.
364 : }
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