//===--- CaptureTracking.cpp - Determine whether a pointer is captured ----===//
//
// The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This file contains routines that help determine which pointers are captured.
// A pointer value is captured if the function makes a copy of any part of the
// pointer that outlives the call. Not being captured means, more or less, that
// the pointer is only dereferenced and not stored in a global. Returning part
// of the pointer as the function return value may or may not count as capturing
// the pointer, depending on the context.
//
//===----------------------------------------------------------------------===//
#include "llvm/Analysis/CaptureTracking.h"
#include "llvm/ADT/SmallSet.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/Analysis/AliasAnalysis.h"
#include "llvm/Analysis/CFG.h"
#include "llvm/Analysis/OrderedBasicBlock.h"
#include "llvm/Analysis/ValueTracking.h"
#include "llvm/IR/CallSite.h"
#include "llvm/IR/Constants.h"
#include "llvm/IR/Dominators.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/IntrinsicInst.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;
};
/// 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, const DominatorTree *DT,
bool IncludeI, OrderedBasicBlock *IC)
: OrderedBB(IC), 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
// 'OrderedBB', 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 (!OrderedBB->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 successors.
// (2) There's no path coming back through BB successors.
if (BB == &BB->getParent()->getEntryBlock() ||
!BB->getTerminator()->getNumSuccessors())
return true;
SmallVector<BasicBlock*, 32> Worklist;
Worklist.append(succ_begin(BB), succ_end(BB));
return !isPotentiallyReachableFromMany(Worklist, BB, DT);
}
// 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;
}
OrderedBasicBlock *OrderedBB;
const Instruction *BeforeHere;
const 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, 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. A ordered basic block \p OBB can be used in order to speed up
/// queries about relative order among instructions in the same basic block.
bool llvm::PointerMayBeCapturedBefore(const Value *V, bool ReturnCaptures,
bool StoreCaptures, const Instruction *I,
const DominatorTree *DT, bool IncludeI,
OrderedBasicBlock *OBB) {
assert(!isa<GlobalValue>(V) &&
"It doesn't make sense to ask whether a global is captured.");
bool UseNewOBB = OBB == nullptr;
if (!DT)
return PointerMayBeCaptured(V, ReturnCaptures, StoreCaptures);
if (UseNewOBB)
OBB = new OrderedBasicBlock(I->getParent());
// TODO: See comment in PointerMayBeCaptured regarding what could be done
// with StoreCaptures.
CapturesBefore CB(ReturnCaptures, I, DT, IncludeI, OBB);
PointerMayBeCaptured(V, &CB);
if (UseNewOBB)
delete OBB;
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;
auto AddUses = [&](const Value *V) {
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 (!Visited.insert(&U).second)
continue;
if (!Tracker->shouldExplore(&U))
continue;
Worklist.push_back(&U);
}
};
AddUses(V);
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;
// The pointer is not captured if returned pointer is not captured.
// NOTE: CaptureTracking users should not assume that only functions
// marked with nocapture do not capture. This means that places like
// GetUnderlyingObject in ValueTracking or DecomposeGEPExpression
// in BasicAA also need to know about this property.
if (isIntrinsicReturningPointerAliasingArgumentWithoutCapturing(CS)) {
AddUses(I);
break;
}
// Volatile operations effectively capture the memory location that they
// load and store to.
if (auto *MI = dyn_cast<MemIntrinsic>(I))
if (MI->isVolatile())
if (Tracker->captured(U))
return;
// 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::data_operand_iterator B =
CS.data_operands_begin(), E = CS.data_operands_end();
for (CallSite::data_operand_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:
// Volatile loads make the address observable.
if (cast<LoadInst>(I)->isVolatile())
if (Tracker->captured(U))
return;
break;
case Instruction::VAArg:
// "va-arg" from a pointer does not cause it to be captured.
break;
case Instruction::Store:
// Stored the pointer - conservatively assume it may be captured.
// Volatile stores make the address observable.
if (V == I->getOperand(0) || cast<StoreInst>(I)->isVolatile())
if (Tracker->captured(U))
return;
break;
case Instruction::AtomicRMW: {
// atomicrmw conceptually includes both a load and store from
// the same location.
// As with a store, the location being accessed is not captured,
// but the value being stored is.
// Volatile stores make the address observable.
auto *ARMWI = cast<AtomicRMWInst>(I);
if (ARMWI->getValOperand() == V || ARMWI->isVolatile())
if (Tracker->captured(U))
return;
break;
}
case Instruction::AtomicCmpXchg: {
// cmpxchg conceptually includes both a load and store from
// the same location.
// As with a store, the location being accessed is not captured,
// but the value being stored is.
// Volatile stores make the address observable.
auto *ACXI = cast<AtomicCmpXchgInst>(I);
if (ACXI->getCompareOperand() == V || ACXI->getNewValOperand() == V ||
ACXI->isVolatile())
if (Tracker->captured(U))
return;
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.
AddUses(I);
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;
// Comparison against value stored in global variable. Given the pointer
// does not escape, its value cannot be guessed and stored separately in a
// global variable.
unsigned OtherIndex = (I->getOperand(0) == V) ? 1 : 0;
auto *LI = dyn_cast<LoadInst>(I->getOperand(OtherIndex));
if (LI && isa<GlobalVariable>(LI->getPointerOperand()))
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.
}