//
//===----------------------------------------------------------------------===//
+#include "llvm/ADT/SmallSet.h"
+#include "llvm/ADT/SmallVector.h"
+#include "llvm/Analysis/AliasAnalysis.h"
#include "llvm/Analysis/CaptureTracking.h"
+#include "llvm/Analysis/CFG.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 {
+ struct SimpleCaptureTracker : public CaptureTracker {
explicit SimpleCaptureTracker(bool ReturnCaptures)
: ReturnCaptures(ReturnCaptures), Captured(false) {}
- void tooManyUses() { Captured = true; }
+ 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 shouldExplore(Use *U) { return true; }
+ bool Captured;
+ };
- bool captured(Instruction *I) {
- if (isa<ReturnInst>(I) && !ReturnCaptures)
- return false;
+ /// 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)
+ : BeforeHere(I), DT(DT), ReturnCaptures(ReturnCaptures),
+ Captured(false) {}
+ void tooManyUses() override { Captured = true; }
+
+ bool shouldExplore(const Use *U) override {
+ Instruction *I = cast<Instruction>(U->getUser());
+ 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 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 false;
+ return true;
+ }
+
+ bool captured(const Use *U) override {
+ if (isa<ReturnInst>(U->getUser()) && !ReturnCaptures)
+ return false;
+
+ Instruction *I = cast<Instruction>(U->getUser());
+ BasicBlock *BB = I->getParent();
+ // Same logic as in shouldExplore.
+ if (BeforeHere != I && !DT->isReachableFromEntry(BB))
+ return false;
+ if (BeforeHere != I && DT->dominates(BeforeHere, I) &&
+ !isPotentiallyReachable(I, BeforeHere, DT))
+ return false;
Captured = true;
return true;
}
+ const Instruction *BeforeHere;
+ DominatorTree *DT;
+
bool ReturnCaptures;
bool Captured;
/// counts as capturing it or not.
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
(void)StoreCaptures;
SimpleCaptureTracker SCT(ReturnCaptures);
- PointerMayBeCaptured(V, SCT);
+ 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) {
+ 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);
+ 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()) {
+ 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(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()->isVoidTy())
+ break;
+
+ // Not captured if only passed via 'nocapture' arguments. Note that
+ // calling a function pointer does not in itself cause the pointer to
+ // be captured. This is a subtle point considering that (for example)
+ // the callee might return its own address. It is analogous to saying
+ // that loading a value from a pointer does not cause the pointer to be
+ // captured, even though the loaded value might be the pointer itself
+ // (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.doesNotCapture(A - B))
+ // The parameter is not marked 'nocapture' - captured.
+ if (Tracker->captured(U))
+ return;
+ break;
+ }
+ case Instruction::Load:
+ // Loading from a pointer does not cause it to be captured.
+ break;
+ 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 - 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.
+ 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))
+ 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.
+ if (Tracker->captured(U))
+ return;
+ break;
+ }
+ }
+
+ // All uses examined.
+}
projects / oota-llvm.git / blobdiff