//
//===----------------------------------------------------------------------===//
-#include "llvm/Analysis/CaptureTracking.h"
-#include "llvm/Instructions.h"
-#include "llvm/Value.h"
#include "llvm/ADT/SmallSet.h"
#include "llvm/ADT/SmallVector.h"
-#include "llvm/Support/CallSite.h"
+#include "llvm/Analysis/AliasAnalysis.h"
+#include "llvm/Analysis/CFG.h"
+#include "llvm/Analysis/CaptureTracking.h"
+#include "llvm/IR/CallSite.h"
+#include "llvm/IR/Constants.h"
+#include "llvm/IR/Dominators.h"
+#include "llvm/IR/Instructions.h"
+
using namespace llvm;
+CaptureTracker::~CaptureTracker() {}
+
+bool CaptureTracker::shouldExplore(const Use *U) { return true; }
+
+namespace {
+ struct SimpleCaptureTracker : public CaptureTracker {
+ explicit SimpleCaptureTracker(bool ReturnCaptures)
+ : ReturnCaptures(ReturnCaptures), Captured(false) {}
+
+ void tooManyUses() override { Captured = true; }
+
+ bool captured(const Use *U) override {
+ if (isa<ReturnInst>(U->getUser()) && !ReturnCaptures)
+ return false;
+
+ Captured = true;
+ return true;
+ }
+
+ bool ReturnCaptures;
+
+ bool Captured;
+ };
+
+ struct NumberedInstCache {
+ SmallDenseMap<const Instruction *, unsigned, 32> NumberedInsts;
+ BasicBlock::const_iterator LastInstFound;
+ unsigned LastInstPos;
+ const BasicBlock *BB;
+
+ NumberedInstCache(const BasicBlock *BasicB) : LastInstPos(0), BB(BasicB) {
+ LastInstFound = BB->end();
+ }
+
+ /// \brief Find the first instruction 'A' or 'B' in 'BB'. Number out
+ /// instruction while walking 'BB'.
+ const Instruction *find(const Instruction *A, const Instruction *B) {
+ const Instruction *Inst = nullptr;
+ assert(!(LastInstFound == BB->end() && LastInstPos != 0) &&
+ "Instruction supposed to be in NumberedInsts");
+
+ // Start the search with the instruction found in the last lookup round.
+ auto II = BB->begin();
+ auto IE = BB->end();
+ if (LastInstFound != IE)
+ II = std::next(LastInstFound);
+
+ // Number all instructions up to the point where we find 'A' or 'B'.
+ for (++LastInstPos; II != IE; ++II, ++LastInstPos) {
+ Inst = cast<Instruction>(II);
+ NumberedInsts[Inst] = LastInstPos;
+ if (Inst == A || Inst == B)
+ break;
+ }
+
+ assert(II != IE && "Instruction not found?");
+ LastInstFound = II;
+ return Inst;
+ }
+
+ /// \brief Find out whether 'A' dominates 'B', meaning whether 'A'
+ /// comes before 'B' in 'BB'. This is a simplification that considers
+ /// cached instruction positions and ignores other basic blocks, being
+ /// only relevant to compare relative instructions positions inside 'BB'.
+ bool dominates(const Instruction *A, const Instruction *B) {
+ assert(A->getParent() == B->getParent() &&
+ "Instructions must be in the same basic block!");
+
+ unsigned NA = NumberedInsts.lookup(A);
+ unsigned NB = NumberedInsts.lookup(B);
+ if (NA && NB)
+ return NA < NB;
+ if (NA)
+ return true;
+ if (NB)
+ return false;
+
+ return A == find(A, B);
+ }
+ };
+
+ /// Only find pointer captures which happen before the given instruction. Uses
+ /// the dominator tree to determine whether one instruction is before another.
+ /// Only support the case where the Value is defined in the same basic block
+ /// as the given instruction and the use.
+ struct CapturesBefore : public CaptureTracker {
+
+ CapturesBefore(bool ReturnCaptures, const Instruction *I, DominatorTree *DT,
+ bool IncludeI)
+ : LocalInstCache(I->getParent()), BeforeHere(I), DT(DT),
+ ReturnCaptures(ReturnCaptures), IncludeI(IncludeI), Captured(false) {}
+
+ void tooManyUses() override { Captured = true; }
+
+ bool isSafeToPrune(Instruction *I) {
+ BasicBlock *BB = I->getParent();
+ // We explore this usage only if the usage can reach "BeforeHere".
+ // If use is not reachable from entry, there is no need to explore.
+ if (BeforeHere != I && !DT->isReachableFromEntry(BB))
+ return true;
+
+ // Compute the case where both instructions are inside the same basic
+ // block. Since instructions in the same BB as BeforeHere are numbered in
+ // 'LocalInstCache', avoid using 'dominates' and 'isPotentiallyReachable'
+ // which are very expensive for large basic blocks.
+ if (BB == BeforeHere->getParent()) {
+ // 'I' dominates 'BeforeHere' => not safe to prune.
+ //
+ // The value defined by an invoke dominates an instruction only if it
+ // dominates every instruction in UseBB. A PHI is dominated only if
+ // the instruction dominates every possible use in the UseBB. Since
+ // UseBB == BB, avoid pruning.
+ if (isa<InvokeInst>(BeforeHere) || isa<PHINode>(I) || I == BeforeHere)
+ return false;
+ if (!LocalInstCache.dominates(BeforeHere, I))
+ return false;
+
+ // 'BeforeHere' comes before 'I', it's safe to prune if we also
+ // guarantee that 'I' never reaches 'BeforeHere' through a back-edge or
+ // by its successors, i.e, prune if:
+ //
+ // (1) BB is an entry block or have no sucessors.
+ // (2) There's no path coming back through BB sucessors.
+ if (BB == &BB->getParent()->getEntryBlock() ||
+ !BB->getTerminator()->getNumSuccessors())
+ return true;
+
+ SmallVector<BasicBlock*, 32> Worklist;
+ Worklist.append(succ_begin(BB), succ_end(BB));
+ if (!isPotentiallyReachableFromMany(Worklist, BB, DT))
+ return true;
+
+ return false;
+ }
+
+ // If the value is defined in the same basic block as use and BeforeHere,
+ // there is no need to explore the use if BeforeHere dominates use.
+ // Check whether there is a path from I to BeforeHere.
+ if (BeforeHere != I && DT->dominates(BeforeHere, I) &&
+ !isPotentiallyReachable(I, BeforeHere, DT))
+ return true;
+
+ return false;
+ }
+
+ bool shouldExplore(const Use *U) override {
+ Instruction *I = cast<Instruction>(U->getUser());
+
+ if (BeforeHere == I && !IncludeI)
+ return false;
+
+ if (isSafeToPrune(I))
+ return false;
+
+ return true;
+ }
+
+ bool captured(const Use *U) override {
+ if (isa<ReturnInst>(U->getUser()) && !ReturnCaptures)
+ return false;
+
+ if (!shouldExplore(U))
+ return false;
+
+ Captured = true;
+ return true;
+ }
+
+ NumberedInstCache LocalInstCache;
+ const Instruction *BeforeHere;
+ DominatorTree *DT;
+
+ bool ReturnCaptures;
+ bool IncludeI;
+
+ bool Captured;
+ };
+}
+
/// PointerMayBeCaptured - Return true if this pointer value may be captured
/// by the enclosing function (which is required to exist). This routine can
/// be expensive, so consider caching the results. The boolean ReturnCaptures
/// specifies whether returning the value (or part of it) from the function
+/// counts as capturing it or not. The boolean StoreCaptures specified whether
+/// storing the value (or part of it) into memory anywhere automatically
/// counts as capturing it or not.
-bool llvm::PointerMayBeCaptured(const Value *V, bool ReturnCaptures) {
- assert(isa<PointerType>(V->getType()) && "Capture is for pointers only!");
- SmallVector<Use*, 16> Worklist;
- SmallSet<Use*, 16> Visited;
-
- for (Value::use_const_iterator UI = V->use_begin(), UE = V->use_end();
- UI != UE; ++UI) {
- Use *U = &UI.getUse();
- Visited.insert(U);
- Worklist.push_back(U);
+bool llvm::PointerMayBeCaptured(const Value *V,
+ bool ReturnCaptures, bool StoreCaptures) {
+ assert(!isa<GlobalValue>(V) &&
+ "It doesn't make sense to ask whether a global is captured.");
+
+ // TODO: If StoreCaptures is not true, we could do Fancy analysis
+ // to determine whether this store is not actually an escape point.
+ // In that case, BasicAliasAnalysis should be updated as well to
+ // take advantage of this.
+ (void)StoreCaptures;
+
+ SimpleCaptureTracker SCT(ReturnCaptures);
+ PointerMayBeCaptured(V, &SCT);
+ return SCT.Captured;
+}
+
+/// PointerMayBeCapturedBefore - Return true if this pointer value may be
+/// captured by the enclosing function (which is required to exist). If a
+/// DominatorTree is provided, only captures which happen before the given
+/// instruction are considered. This routine can be expensive, so consider
+/// caching the results. The boolean ReturnCaptures specifies whether
+/// returning the value (or part of it) from the function counts as capturing
+/// it or not. The boolean StoreCaptures specified whether storing the value
+/// (or part of it) into memory anywhere automatically counts as capturing it
+/// or not.
+bool llvm::PointerMayBeCapturedBefore(const Value *V, bool ReturnCaptures,
+ bool StoreCaptures, const Instruction *I,
+ DominatorTree *DT, bool IncludeI) {
+ assert(!isa<GlobalValue>(V) &&
+ "It doesn't make sense to ask whether a global is captured.");
+
+ if (!DT)
+ return PointerMayBeCaptured(V, ReturnCaptures, StoreCaptures);
+
+ // TODO: See comment in PointerMayBeCaptured regarding what could be done
+ // with StoreCaptures.
+
+ CapturesBefore CB(ReturnCaptures, I, DT, IncludeI);
+ PointerMayBeCaptured(V, &CB);
+ return CB.Captured;
+}
+
+/// TODO: Write a new FunctionPass AliasAnalysis so that it can keep
+/// a cache. Then we can move the code from BasicAliasAnalysis into
+/// that path, and remove this threshold.
+static int const Threshold = 20;
+
+void llvm::PointerMayBeCaptured(const Value *V, CaptureTracker *Tracker) {
+ assert(V->getType()->isPointerTy() && "Capture is for pointers only!");
+ SmallVector<const Use *, Threshold> Worklist;
+ SmallSet<const Use *, Threshold> Visited;
+ int Count = 0;
+
+ for (const Use &U : V->uses()) {
+ // If there are lots of uses, conservatively say that the value
+ // is captured to avoid taking too much compile time.
+ if (Count++ >= Threshold)
+ return Tracker->tooManyUses();
+
+ if (!Tracker->shouldExplore(&U)) continue;
+ Visited.insert(&U);
+ Worklist.push_back(&U);
}
while (!Worklist.empty()) {
- Use *U = Worklist.pop_back_val();
+ const Use *U = Worklist.pop_back_val();
Instruction *I = cast<Instruction>(U->getUser());
V = U->get();
switch (I->getOpcode()) {
case Instruction::Call:
case Instruction::Invoke: {
- CallSite CS = CallSite::get(I);
+ CallSite CS(I);
// Not captured if the callee is readonly, doesn't return a copy through
// its return value and doesn't unwind (a readonly function can leak bits
// by throwing an exception or not depending on the input value).
- if (CS.onlyReadsMemory() && CS.doesNotThrow() &&
- I->getType() == Type::VoidTy)
+ if (CS.onlyReadsMemory() && CS.doesNotThrow() && I->getType()->isVoidTy())
break;
// Not captured if only passed via 'nocapture' arguments. Note that
// (think of self-referential objects).
CallSite::arg_iterator B = CS.arg_begin(), E = CS.arg_end();
for (CallSite::arg_iterator A = B; A != E; ++A)
- if (A->get() == V && !CS.paramHasAttr(A - B + 1, Attribute::NoCapture))
+ if (A->get() == V && !CS.doesNotCapture(A - B))
// The parameter is not marked 'nocapture' - captured.
- return true;
- // Only passed via 'nocapture' arguments, or is the called function - not
- // captured.
+ if (Tracker->captured(U))
+ return;
break;
}
- case Instruction::Free:
- // Freeing a pointer does not cause it to be captured.
- break;
case Instruction::Load:
// Loading from a pointer does not cause it to be captured.
break;
- case Instruction::Ret:
- if (ReturnCaptures)
- return true;
+ case Instruction::VAArg:
+ // "va-arg" from a pointer does not cause it to be captured.
break;
case Instruction::Store:
if (V == I->getOperand(0))
- // Stored the pointer - it may be captured.
- return true;
+ // Stored the pointer - conservatively assume it may be captured.
+ if (Tracker->captured(U))
+ return;
// Storing to the pointee does not cause the pointer to be captured.
break;
case Instruction::BitCast:
case Instruction::GetElementPtr:
case Instruction::PHI:
case Instruction::Select:
+ case Instruction::AddrSpaceCast:
// The original value is not captured via this if the new value isn't.
- for (Instruction::use_iterator UI = I->use_begin(), UE = I->use_end();
- UI != UE; ++UI) {
- Use *U = &UI.getUse();
- if (Visited.insert(U))
- Worklist.push_back(U);
+ Count = 0;
+ for (Use &UU : I->uses()) {
+ // If there are lots of uses, conservatively say that the value
+ // is captured to avoid taking too much compile time.
+ if (Count++ >= Threshold)
+ return Tracker->tooManyUses();
+
+ if (Visited.insert(&UU).second)
+ if (Tracker->shouldExplore(&UU))
+ Worklist.push_back(&UU);
}
break;
+ case Instruction::ICmp:
+ // Don't count comparisons of a no-alias return value against null as
+ // captures. This allows us to ignore comparisons of malloc results
+ // with null, for example.
+ if (ConstantPointerNull *CPN =
+ dyn_cast<ConstantPointerNull>(I->getOperand(1)))
+ if (CPN->getType()->getAddressSpace() == 0)
+ if (isNoAliasCall(V->stripPointerCasts()))
+ break;
+ // Otherwise, be conservative. There are crazy ways to capture pointers
+ // using comparisons.
+ if (Tracker->captured(U))
+ return;
+ break;
default:
// Something else - be conservative and say it is captured.
- return true;
+ if (Tracker->captured(U))
+ return;
+ break;
}
}
- // All uses examined - not captured.
- return false;
+ // All uses examined.
}
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