1 //===--- CaptureTracking.cpp - Determine whether a pointer is captured ----===//
3 // The LLVM Compiler Infrastructure
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
8 //===----------------------------------------------------------------------===//
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.
17 //===----------------------------------------------------------------------===//
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"
32 CaptureTracker::~CaptureTracker() {}
34 bool CaptureTracker::shouldExplore(const Use *U) { return true; }
37 struct SimpleCaptureTracker : public CaptureTracker {
38 explicit SimpleCaptureTracker(bool ReturnCaptures)
39 : ReturnCaptures(ReturnCaptures), Captured(false) {}
41 void tooManyUses() override { Captured = true; }
43 bool captured(const Use *U) override {
44 if (isa<ReturnInst>(U->getUser()) && !ReturnCaptures)
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 {
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) {}
67 void tooManyUses() override { Captured = true; }
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))
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.
83 // The value defined by an invoke/catchpad 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<CatchPadInst>(BeforeHere) ||
88 isa<PHINode>(I) || I == BeforeHere)
90 if (!OrderedBB->dominates(BeforeHere, I))
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:
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())
103 SmallVector<BasicBlock*, 32> Worklist;
104 Worklist.append(succ_begin(BB), succ_end(BB));
105 if (!isPotentiallyReachableFromMany(Worklist, BB, DT))
111 // If the value is defined in the same basic block as use and BeforeHere,
112 // there is no need to explore the use if BeforeHere dominates use.
113 // Check whether there is a path from I to BeforeHere.
114 if (BeforeHere != I && DT->dominates(BeforeHere, I) &&
115 !isPotentiallyReachable(I, BeforeHere, DT))
121 bool shouldExplore(const Use *U) override {
122 Instruction *I = cast<Instruction>(U->getUser());
124 if (BeforeHere == I && !IncludeI)
127 if (isSafeToPrune(I))
133 bool captured(const Use *U) override {
134 if (isa<ReturnInst>(U->getUser()) && !ReturnCaptures)
137 if (!shouldExplore(U))
144 OrderedBasicBlock *OrderedBB;
145 const Instruction *BeforeHere;
155 /// PointerMayBeCaptured - Return true if this pointer value may be captured
156 /// by the enclosing function (which is required to exist). This routine can
157 /// be expensive, so consider caching the results. The boolean ReturnCaptures
158 /// specifies whether returning the value (or part of it) from the function
159 /// counts as capturing it or not. The boolean StoreCaptures specified whether
160 /// storing the value (or part of it) into memory anywhere automatically
161 /// counts as capturing it or not.
162 bool llvm::PointerMayBeCaptured(const Value *V,
163 bool ReturnCaptures, bool StoreCaptures) {
164 assert(!isa<GlobalValue>(V) &&
165 "It doesn't make sense to ask whether a global is captured.");
167 // TODO: If StoreCaptures is not true, we could do Fancy analysis
168 // to determine whether this store is not actually an escape point.
169 // In that case, BasicAliasAnalysis should be updated as well to
170 // take advantage of this.
173 SimpleCaptureTracker SCT(ReturnCaptures);
174 PointerMayBeCaptured(V, &SCT);
178 /// PointerMayBeCapturedBefore - Return true if this pointer value may be
179 /// captured by the enclosing function (which is required to exist). If a
180 /// DominatorTree is provided, only captures which happen before the given
181 /// instruction are considered. This routine can be expensive, so consider
182 /// caching the results. The boolean ReturnCaptures specifies whether
183 /// returning the value (or part of it) from the function counts as capturing
184 /// it or not. The boolean StoreCaptures specified whether storing the value
185 /// (or part of it) into memory anywhere automatically counts as capturing it
186 /// or not. A ordered basic block \p OBB can be used in order to speed up
187 /// queries about relative order among instructions in the same basic block.
188 bool llvm::PointerMayBeCapturedBefore(const Value *V, bool ReturnCaptures,
189 bool StoreCaptures, const Instruction *I,
190 DominatorTree *DT, bool IncludeI,
191 OrderedBasicBlock *OBB) {
192 assert(!isa<GlobalValue>(V) &&
193 "It doesn't make sense to ask whether a global is captured.");
194 bool UseNewOBB = OBB == nullptr;
197 return PointerMayBeCaptured(V, ReturnCaptures, StoreCaptures);
199 OBB = new OrderedBasicBlock(I->getParent());
201 // TODO: See comment in PointerMayBeCaptured regarding what could be done
202 // with StoreCaptures.
204 CapturesBefore CB(ReturnCaptures, I, DT, IncludeI, OBB);
205 PointerMayBeCaptured(V, &CB);
212 /// TODO: Write a new FunctionPass AliasAnalysis so that it can keep
213 /// a cache. Then we can move the code from BasicAliasAnalysis into
214 /// that path, and remove this threshold.
215 static int const Threshold = 20;
217 void llvm::PointerMayBeCaptured(const Value *V, CaptureTracker *Tracker) {
218 assert(V->getType()->isPointerTy() && "Capture is for pointers only!");
219 SmallVector<const Use *, Threshold> Worklist;
220 SmallSet<const Use *, Threshold> Visited;
223 for (const Use &U : V->uses()) {
224 // If there are lots of uses, conservatively say that the value
225 // is captured to avoid taking too much compile time.
226 if (Count++ >= Threshold)
227 return Tracker->tooManyUses();
229 if (!Tracker->shouldExplore(&U)) continue;
231 Worklist.push_back(&U);
234 while (!Worklist.empty()) {
235 const Use *U = Worklist.pop_back_val();
236 Instruction *I = cast<Instruction>(U->getUser());
239 switch (I->getOpcode()) {
240 case Instruction::Call:
241 case Instruction::Invoke: {
243 // Not captured if the callee is readonly, doesn't return a copy through
244 // its return value and doesn't unwind (a readonly function can leak bits
245 // by throwing an exception or not depending on the input value).
246 if (CS.onlyReadsMemory() && CS.doesNotThrow() && I->getType()->isVoidTy())
249 // Not captured if only passed via 'nocapture' arguments. Note that
250 // calling a function pointer does not in itself cause the pointer to
251 // be captured. This is a subtle point considering that (for example)
252 // the callee might return its own address. It is analogous to saying
253 // that loading a value from a pointer does not cause the pointer to be
254 // captured, even though the loaded value might be the pointer itself
255 // (think of self-referential objects).
256 CallSite::arg_iterator B = CS.arg_begin(), E = CS.arg_end();
257 for (CallSite::arg_iterator A = B; A != E; ++A)
258 if (A->get() == V && !CS.doesNotCapture(A - B))
259 // The parameter is not marked 'nocapture' - captured.
260 if (Tracker->captured(U))
264 case Instruction::Load:
265 // Loading from a pointer does not cause it to be captured.
267 case Instruction::VAArg:
268 // "va-arg" from a pointer does not cause it to be captured.
270 case Instruction::Store:
271 if (V == I->getOperand(0))
272 // Stored the pointer - conservatively assume it may be captured.
273 if (Tracker->captured(U))
275 // Storing to the pointee does not cause the pointer to be captured.
277 case Instruction::BitCast:
278 case Instruction::GetElementPtr:
279 case Instruction::PHI:
280 case Instruction::Select:
281 case Instruction::AddrSpaceCast:
282 // The original value is not captured via this if the new value isn't.
284 for (Use &UU : I->uses()) {
285 // If there are lots of uses, conservatively say that the value
286 // is captured to avoid taking too much compile time.
287 if (Count++ >= Threshold)
288 return Tracker->tooManyUses();
290 if (Visited.insert(&UU).second)
291 if (Tracker->shouldExplore(&UU))
292 Worklist.push_back(&UU);
295 case Instruction::ICmp:
296 // Don't count comparisons of a no-alias return value against null as
297 // captures. This allows us to ignore comparisons of malloc results
298 // with null, for example.
299 if (ConstantPointerNull *CPN =
300 dyn_cast<ConstantPointerNull>(I->getOperand(1)))
301 if (CPN->getType()->getAddressSpace() == 0)
302 if (isNoAliasCall(V->stripPointerCasts()))
304 // Otherwise, be conservative. There are crazy ways to capture pointers
305 // using comparisons.
306 if (Tracker->captured(U))
310 // Something else - be conservative and say it is captured.
311 if (Tracker->captured(U))
317 // All uses examined.