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//===-- SafepointIRVerifier.cpp - Verify gc.statepoint invariants ---------===//
// The LLVM Compiler Infrastructure
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
// Run a sanity check on the IR to ensure that Safepoints - if they've been
// inserted - were inserted correctly. In particular, look for use of
// non-relocated values after a safepoint. It's primary use is to check the
// correctness of safepoint insertion immediately after insertion, but it can
// also be used to verify that later transforms have not found a way to break
// safepoint semenatics.
// In its current form, this verify checks a property which is sufficient, but
// not neccessary for correctness. There are some cases where an unrelocated
// pointer can be used after the safepoint. Consider this example:
// a = ...
// b = ...
// (a',b') = safepoint(a,b)
// c = cmp eq a b
// br c, ..., ....
// Because it is valid to reorder 'c' above the safepoint, this is legal. In
// practice, this is a somewhat uncommon transform, but CodeGenPrep does create
// idioms like this. The verifier knows about these cases and avoids reporting
// false positives.
#include "llvm/ADT/DenseSet.h"
#include "llvm/ADT/PostOrderIterator.h"
#include "llvm/ADT/SetOperations.h"
#include "llvm/ADT/SetVector.h"
#include "llvm/IR/BasicBlock.h"
#include "llvm/IR/Dominators.h"
#include "llvm/IR/Function.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/Intrinsics.h"
#include "llvm/IR/IntrinsicInst.h"
#include "llvm/IR/Module.h"
#include "llvm/IR/Value.h"
#include "llvm/IR/SafepointIRVerifier.h"
#include "llvm/IR/Statepoint.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/raw_ostream.h"
#define DEBUG_TYPE "safepoint-ir-verifier"
using namespace llvm;
/// This option is used for writing test cases. Instead of crashing the program
/// when verification fails, report a message to the console (for FileCheck
/// usage) and continue execution as if nothing happened.
static cl::opt<bool> PrintOnly("safepoint-ir-verifier-print-only",
namespace {
/// This CFG Deadness finds dead blocks and edges. Algorithm starts with a set
/// of blocks unreachable from entry then propagates deadness using foldable
/// conditional branches without modifying CFG. So GVN does but it changes CFG
/// by splitting critical edges. In most cases passes rely on SimplifyCFG to
/// clean up dead blocks, but in some cases, like verification or loop passes
/// it's not possible.
class CFGDeadness {
const DominatorTree *DT = nullptr;
SetVector<const BasicBlock *> DeadBlocks;
SetVector<const Use *> DeadEdges; // Contains all dead edges from live blocks.
/// Return the edge that coresponds to the predecessor.
static const Use& getEdge(const_pred_iterator &PredIt) {
auto &PU = PredIt.getUse();
return PU.getUser()->getOperandUse(PU.getOperandNo());
/// Return true if there is at least one live edge that corresponds to the
/// basic block InBB listed in the phi node.
bool hasLiveIncomingEdge(const PHINode *PN, const BasicBlock *InBB) const {
assert(!isDeadBlock(InBB) && "block must be live");
const BasicBlock* BB = PN->getParent();
bool Listed = false;
for (const_pred_iterator PredIt(BB), End(BB, true); PredIt != End; ++PredIt) {
if (InBB == *PredIt) {
if (!isDeadEdge(&getEdge(PredIt)))
return true;
Listed = true;
assert(Listed && "basic block is not found among incoming blocks");
return false;
bool isDeadBlock(const BasicBlock *BB) const {
return DeadBlocks.count(BB);
bool isDeadEdge(const Use *U) const {
assert(dyn_cast<Instruction>(U->getUser())->isTerminator() &&
"edge must be operand of terminator");
assert(cast_or_null<BasicBlock>(U->get()) &&
"edge must refer to basic block");
assert(!isDeadBlock(dyn_cast<Instruction>(U->getUser())->getParent()) &&
"isDeadEdge() must be applied to edge from live block");
return DeadEdges.count(U);
bool hasLiveIncomingEdges(const BasicBlock *BB) const {
// Check if all incoming edges are dead.
for (const_pred_iterator PredIt(BB), End(BB, true); PredIt != End; ++PredIt) {
auto &PU = PredIt.getUse();
const Use &U = PU.getUser()->getOperandUse(PU.getOperandNo());
if (!isDeadBlock(*PredIt) && !isDeadEdge(&U))
return true; // Found a live edge.
return false;
void processFunction(const Function &F, const DominatorTree &DT) {
this->DT = &DT;
// Start with all blocks unreachable from entry.
for (const BasicBlock &BB : F)
if (!DT.isReachableFromEntry(&BB))
// Top-down walk of the dominator tree
ReversePostOrderTraversal<const Function *> RPOT(&F);
for (const BasicBlock *BB : RPOT) {
const TerminatorInst *TI = BB->getTerminator();
assert(TI && "blocks must be well formed");
// For conditional branches, we can perform simple conditional propagation on
// the condition value itself.
const BranchInst *BI = dyn_cast<BranchInst>(TI);
if (!BI || !BI->isConditional() || !isa<Constant>(BI->getCondition()))
// If a branch has two identical successors, we cannot declare either dead.
if (BI->getSuccessor(0) == BI->getSuccessor(1))
ConstantInt *Cond = dyn_cast<ConstantInt>(BI->getCondition());
if (!Cond)
addDeadEdge(BI->getOperandUse(Cond->getZExtValue() ? 1 : 2));
void addDeadBlock(const BasicBlock *BB) {
SmallVector<const BasicBlock *, 4> NewDead;
SmallSetVector<const BasicBlock *, 4> DF;
while (!NewDead.empty()) {
const BasicBlock *D = NewDead.pop_back_val();
if (isDeadBlock(D))
// All blocks dominated by D are dead.
SmallVector<BasicBlock *, 8> Dom;
DT->getDescendants(const_cast<BasicBlock*>(D), Dom);
// Do not need to mark all in and out edges dead
// because BB is marked dead and this is enough
// to run further.
DeadBlocks.insert(Dom.begin(), Dom.end());
// Figure out the dominance-frontier(D).
for (BasicBlock *B : Dom)
for (BasicBlock *S : successors(B))
if (!isDeadBlock(S) && !hasLiveIncomingEdges(S))
void addDeadEdge(const Use &DeadEdge) {
if (!DeadEdges.insert(&DeadEdge))
BasicBlock *BB = cast_or_null<BasicBlock>(DeadEdge.get());
if (hasLiveIncomingEdges(BB))
} // namespace
static void Verify(const Function &F, const DominatorTree &DT,
const CFGDeadness &CD);
namespace {
struct SafepointIRVerifier : public FunctionPass {
static char ID; // Pass identification, replacement for typeid
SafepointIRVerifier() : FunctionPass(ID) {
bool runOnFunction(Function &F) override {
auto &DT = getAnalysis<DominatorTreeWrapperPass>().getDomTree();
CFGDeadness CD;
CD.processFunction(F, DT);
Verify(F, DT, CD);
return false; // no modifications
void getAnalysisUsage(AnalysisUsage &AU) const override {
StringRef getPassName() const override { return "safepoint verifier"; }
} // namespace
void llvm::verifySafepointIR(Function &F) {
SafepointIRVerifier pass;
char SafepointIRVerifier::ID = 0;
FunctionPass *llvm::createSafepointIRVerifierPass() {
return new SafepointIRVerifier();
INITIALIZE_PASS_BEGIN(SafepointIRVerifier, "verify-safepoint-ir",
"Safepoint IR Verifier", false, false)
INITIALIZE_PASS_END(SafepointIRVerifier, "verify-safepoint-ir",
"Safepoint IR Verifier", false, false)
static bool isGCPointerType(Type *T) {
if (auto *PT = dyn_cast<PointerType>(T))
// For the sake of this example GC, we arbitrarily pick addrspace(1) as our
// GC managed heap. We know that a pointer into this heap needs to be
// updated and that no other pointer does.
return (1 == PT->getAddressSpace());
return false;
static bool containsGCPtrType(Type *Ty) {
if (isGCPointerType(Ty))
return true;
if (VectorType *VT = dyn_cast<VectorType>(Ty))
return isGCPointerType(VT->getScalarType());
if (ArrayType *AT = dyn_cast<ArrayType>(Ty))
return containsGCPtrType(AT->getElementType());
if (StructType *ST = dyn_cast<StructType>(Ty))
return std::any_of(ST->subtypes().begin(), ST->subtypes().end(),
return false;
// Debugging aid -- prints a [Begin, End) range of values.
template<typename IteratorTy>
static void PrintValueSet(raw_ostream &OS, IteratorTy Begin, IteratorTy End) {
OS << "[ ";
while (Begin != End) {
OS << **Begin << " ";
OS << "]";
/// The verifier algorithm is phrased in terms of availability. The set of
/// values "available" at a given point in the control flow graph is the set of
/// correctly relocated value at that point, and is a subset of the set of
/// definitions dominating that point.
using AvailableValueSet = DenseSet<const Value *>;
/// State we compute and track per basic block.
struct BasicBlockState {
// Set of values available coming in, before the phi nodes
AvailableValueSet AvailableIn;
// Set of values available going out
AvailableValueSet AvailableOut;
// AvailableOut minus AvailableIn.
// All elements are Instructions
AvailableValueSet Contribution;
// True if this block contains a safepoint and thus AvailableIn does not
// contribute to AvailableOut.
bool Cleared = false;
/// A given derived pointer can have multiple base pointers through phi/selects.
/// This type indicates when the base pointer is exclusively constant
/// (ExclusivelySomeConstant), and if that constant is proven to be exclusively
/// null, we record that as ExclusivelyNull. In all other cases, the BaseType is
/// NonConstant.
enum BaseType {
NonConstant = 1, // Base pointers is not exclusively constant.
ExclusivelySomeConstant // Base pointers for a given derived pointer is from a
// set of constants, but they are not exclusively
// null.
/// Return the baseType for Val which states whether Val is exclusively
/// derived from constant/null, or not exclusively derived from constant.
/// Val is exclusively derived off a constant base when all operands of phi and
/// selects are derived off a constant base.
static enum BaseType getBaseType(const Value *Val) {
SmallVector<const Value *, 32> Worklist;
DenseSet<const Value *> Visited;
bool isExclusivelyDerivedFromNull = true;
// Strip through all the bitcasts and geps to get base pointer. Also check for
// the exclusive value when there can be multiple base pointers (through phis
// or selects).
while(!Worklist.empty()) {
const Value *V = Worklist.pop_back_val();
if (!Visited.insert(V).second)
if (const auto *CI = dyn_cast<CastInst>(V)) {
if (const auto *GEP = dyn_cast<GetElementPtrInst>(V)) {
// Push all the incoming values of phi node into the worklist for
// processing.
if (const auto *PN = dyn_cast<PHINode>(V)) {
for (Value *InV: PN->incoming_values())
if (const auto *SI = dyn_cast<SelectInst>(V)) {
// Push in the true and false values
if (isa<Constant>(V)) {
// We found at least one base pointer which is non-null, so this derived
// pointer is not exclusively derived from null.
if (V != Constant::getNullValue(V->getType()))
isExclusivelyDerivedFromNull = false;
// Continue processing the remaining values to make sure it's exclusively
// constant.
// At this point, we know that the base pointer is not exclusively
// constant.
return BaseType::NonConstant;
// Now, we know that the base pointer is exclusively constant, but we need to
// differentiate between exclusive null constant and non-null constant.
return isExclusivelyDerivedFromNull ? BaseType::ExclusivelyNull
: BaseType::ExclusivelySomeConstant;
static bool isNotExclusivelyConstantDerived(const Value *V) {
return getBaseType(V) == BaseType::NonConstant;
namespace {
class InstructionVerifier;
/// Builds BasicBlockState for each BB of the function.
/// It can traverse function for verification and provides all required
/// information.
/// GC pointer may be in one of three states: relocated, unrelocated and
/// poisoned.
/// Relocated pointer may be used without any restrictions.
/// Unrelocated pointer cannot be dereferenced, passed as argument to any call
/// or returned. Unrelocated pointer may be safely compared against another
/// unrelocated pointer or against a pointer exclusively derived from null.
/// Poisoned pointers are produced when we somehow derive pointer from relocated
/// and unrelocated pointers (e.g. phi, select). This pointers may be safely
/// used in a very limited number of situations. Currently the only way to use
/// it is comparison against constant exclusively derived from null. All
/// limitations arise due to their undefined state: this pointers should be
/// treated as relocated and unrelocated simultaneously.
/// Rules of deriving:
/// R + U = P - that's where the poisoned pointers come from
/// P + X = P
/// U + U = U
/// R + R = R
/// X + C = X
/// Where "+" - any operation that somehow derive pointer, U - unrelocated,
/// R - relocated and P - poisoned, C - constant, X - U or R or P or C or
/// nothing (in case when "+" is unary operation).
/// Deriving of pointers by itself is always safe.
/// NOTE: when we are making decision on the status of instruction's result:
/// a) for phi we need to check status of each input *at the end of
/// corresponding predecessor BB*.
/// b) for other instructions we need to check status of each input *at the
/// current point*.
/// FIXME: This works fairly well except one case
/// bb1:
/// p = *some GC-ptr def*
/// p1 = gep p, offset
/// / |
/// / |
/// bb2: |
/// safepoint |
/// \ |
/// \ |
/// bb3:
/// p2 = phi [p, bb2] [p1, bb1]
/// p3 = phi [p, bb2] [p, bb1]
/// here p and p1 is unrelocated
/// p2 and p3 is poisoned (though they shouldn't be)
/// This leads to some weird results:
/// cmp eq p, p2 - illegal instruction (false-positive)
/// cmp eq p1, p2 - illegal instruction (false-positive)
/// cmp eq p, p3 - illegal instruction (false-positive)
/// cmp eq p, p1 - ok
/// To fix this we need to introduce conception of generations and be able to
/// check if two values belong to one generation or not. This way p2 will be
/// considered to be unrelocated and no false alarm will happen.
class GCPtrTracker {
const Function &F;
const CFGDeadness &CD;
SpecificBumpPtrAllocator<BasicBlockState> BSAllocator;
DenseMap<const BasicBlock *, BasicBlockState *> BlockMap;
// This set contains defs of unrelocated pointers that are proved to be legal
// and don't need verification.
DenseSet<const Instruction *> ValidUnrelocatedDefs;
// This set contains poisoned defs. They can be safely ignored during
// verification too.
DenseSet<const Value *> PoisonedDefs;
GCPtrTracker(const Function &F, const DominatorTree &DT,
const CFGDeadness &CD);
bool hasLiveIncomingEdge(const PHINode *PN, const BasicBlock *InBB) const {
return CD.hasLiveIncomingEdge(PN, InBB);
BasicBlockState *getBasicBlockState(const BasicBlock *BB);
const BasicBlockState *getBasicBlockState(const BasicBlock *BB) const;
bool isValuePoisoned(const Value *V) const { return PoisonedDefs.count(V); }
/// Traverse each BB of the function and call
/// InstructionVerifier::verifyInstruction for each possibly invalid
/// instruction.
/// It destructively modifies GCPtrTracker so it's passed via rvalue reference
/// in order to prohibit further usages of GCPtrTracker as it'll be in
/// inconsistent state.
static void verifyFunction(GCPtrTracker &&Tracker,
InstructionVerifier &Verifier);
/// Returns true for reachable and live blocks.
bool isMapped(const BasicBlock *BB) const {
return BlockMap.find(BB) != BlockMap.end();
/// Returns true if the instruction may be safely skipped during verification.
bool instructionMayBeSkipped(const Instruction *I) const;
/// Iterates over all BBs from BlockMap and recalculates AvailableIn/Out for
/// each of them until it converges.
void recalculateBBsStates();
/// Remove from Contribution all defs that legally produce unrelocated
/// pointers and saves them to ValidUnrelocatedDefs.
/// Though Contribution should belong to BBS it is passed separately with
/// different const-modifier in order to emphasize (and guarantee) that only
/// Contribution will be changed.
/// Returns true if Contribution was changed otherwise false.
bool removeValidUnrelocatedDefs(const BasicBlock *BB,
const BasicBlockState *BBS,
AvailableValueSet &Contribution);
/// Gather all the definitions dominating the start of BB into Result. This is
/// simply the defs introduced by every dominating basic block and the
/// function arguments.
void gatherDominatingDefs(const BasicBlock *BB, AvailableValueSet &Result,
const DominatorTree &DT);
/// Compute the AvailableOut set for BB, based on the BasicBlockState BBS,
/// which is the BasicBlockState for BB.
/// ContributionChanged is set when the verifier runs for the first time
/// (in this case Contribution was changed from 'empty' to its initial state)
/// or when Contribution of this BB was changed since last computation.
static void transferBlock(const BasicBlock *BB, BasicBlockState &BBS,
bool ContributionChanged);
/// Model the effect of an instruction on the set of available values.
static void transferInstruction(const Instruction &I, bool &Cleared,
AvailableValueSet &Available);
/// It is a visitor for GCPtrTracker::verifyFunction. It decides if the
/// instruction (which uses heap reference) is legal or not, given our safepoint
/// semantics.
class InstructionVerifier {
bool AnyInvalidUses = false;
void verifyInstruction(const GCPtrTracker *Tracker, const Instruction &I,
const AvailableValueSet &AvailableSet);
bool hasAnyInvalidUses() const { return AnyInvalidUses; }
void reportInvalidUse(const Value &V, const Instruction &I);
} // end anonymous namespace
GCPtrTracker::GCPtrTracker(const Function &F, const DominatorTree &DT,
const CFGDeadness &CD) : F(F), CD(CD) {
// Calculate Contribution of each live BB.
// Allocate BB states for live blocks.
for (const BasicBlock &BB : F)
if (!CD.isDeadBlock(&BB)) {
BasicBlockState *BBS = new (BSAllocator.Allocate()) BasicBlockState;
for (const auto &I : BB)
transferInstruction(I, BBS->Cleared, BBS->Contribution);
BlockMap[&BB] = BBS;
// Initialize AvailableIn/Out sets of each BB using only information about
// dominating BBs.
for (auto &BBI : BlockMap) {
gatherDominatingDefs(BBI.first, BBI.second->AvailableIn, DT);
transferBlock(BBI.first, *BBI.second, true);
// Simulate the flow of defs through the CFG and recalculate AvailableIn/Out
// sets of each BB until it converges. If any def is proved to be an
// unrelocated pointer, it will be removed from all BBSs.
BasicBlockState *GCPtrTracker::getBasicBlockState(const BasicBlock *BB) {
auto it = BlockMap.find(BB);
return it != BlockMap.end() ? it->second : nullptr;
const BasicBlockState *GCPtrTracker::getBasicBlockState(
const BasicBlock *BB) const {
return const_cast<GCPtrTracker *>(this)->getBasicBlockState(BB);
bool GCPtrTracker::instructionMayBeSkipped(const Instruction *I) const {
// Poisoned defs are skipped since they are always safe by itself by
// definition (for details see comment to this class).
return ValidUnrelocatedDefs.count(I) || PoisonedDefs.count(I);
void GCPtrTracker::verifyFunction(GCPtrTracker &&Tracker,
InstructionVerifier &Verifier) {
// We need RPO here to a) report always the first error b) report errors in
// same order from run to run.
ReversePostOrderTraversal<const Function *> RPOT(&Tracker.F);
for (const BasicBlock *BB : RPOT) {
BasicBlockState *BBS = Tracker.getBasicBlockState(BB);
if (!BBS)
// We destructively modify AvailableIn as we traverse the block instruction
// by instruction.
AvailableValueSet &AvailableSet = BBS->AvailableIn;
for (const Instruction &I : *BB) {
if (Tracker.instructionMayBeSkipped(&I))
continue; // This instruction shouldn't be added to AvailableSet.
Verifier.verifyInstruction(&Tracker, I, AvailableSet);
// Model the effect of current instruction on AvailableSet to keep the set
// relevant at each point of BB.
bool Cleared = false;
transferInstruction(I, Cleared, AvailableSet);
void GCPtrTracker::recalculateBBsStates() {
SetVector<const BasicBlock *> Worklist;
// TODO: This order is suboptimal, it's better to replace it with priority
// queue where priority is RPO number of BB.
for (auto &BBI : BlockMap)
// This loop iterates the AvailableIn/Out sets until it converges.
// The AvailableIn and AvailableOut sets decrease as we iterate.
while (!Worklist.empty()) {
const BasicBlock *BB = Worklist.pop_back_val();
BasicBlockState *BBS = getBasicBlockState(BB);
if (!BBS)
continue; // Ignore dead successors.
size_t OldInCount = BBS->AvailableIn.size();
for (const_pred_iterator PredIt(BB), End(BB, true); PredIt != End; ++PredIt) {
const BasicBlock *PBB = *PredIt;
BasicBlockState *PBBS = getBasicBlockState(PBB);
if (PBBS && !CD.isDeadEdge(&CFGDeadness::getEdge(PredIt)))
set_intersect(BBS->AvailableIn, PBBS->AvailableOut);
assert(OldInCount >= BBS->AvailableIn.size() && "invariant!");
bool InputsChanged = OldInCount != BBS->AvailableIn.size();
bool ContributionChanged =
removeValidUnrelocatedDefs(BB, BBS, BBS->Contribution);
if (!InputsChanged && !ContributionChanged)
size_t OldOutCount = BBS->AvailableOut.size();
transferBlock(BB, *BBS, ContributionChanged);
if (OldOutCount != BBS->AvailableOut.size()) {
assert(OldOutCount > BBS->AvailableOut.size() && "invariant!");
Worklist.insert(succ_begin(BB), succ_end(BB));
bool GCPtrTracker::removeValidUnrelocatedDefs(const BasicBlock *BB,
const BasicBlockState *BBS,
AvailableValueSet &Contribution) {
assert(&BBS->Contribution == &Contribution &&
"Passed Contribution should be from the passed BasicBlockState!");
AvailableValueSet AvailableSet = BBS->AvailableIn;
bool ContributionChanged = false;
// For explanation why instructions are processed this way see
// "Rules of deriving" in the comment to this class.
for (const Instruction &I : *BB) {
bool ValidUnrelocatedPointerDef = false;
bool PoisonedPointerDef = false;
// TODO: `select` instructions should be handled here too.
if (const PHINode *PN = dyn_cast<PHINode>(&I)) {
if (containsGCPtrType(PN->getType())) {
// If both is true, output is poisoned.
bool HasRelocatedInputs = false;
bool HasUnrelocatedInputs = false;
for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
const BasicBlock *InBB = PN->getIncomingBlock(i);
if (!isMapped(InBB) ||
!CD.hasLiveIncomingEdge(PN, InBB))
continue; // Skip dead block or dead edge.
const Value *InValue = PN->getIncomingValue(i);
if (isNotExclusivelyConstantDerived(InValue)) {
if (isValuePoisoned(InValue)) {
// If any of inputs is poisoned, output is always poisoned too.
HasRelocatedInputs = true;
HasUnrelocatedInputs = true;
if (BlockMap[InBB]->AvailableOut.count(InValue))
HasRelocatedInputs = true;
HasUnrelocatedInputs = true;
if (HasUnrelocatedInputs) {
if (HasRelocatedInputs)
PoisonedPointerDef = true;
ValidUnrelocatedPointerDef = true;
} else if ((isa<GetElementPtrInst>(I) || isa<BitCastInst>(I)) &&
containsGCPtrType(I.getType())) {
// GEP/bitcast of unrelocated pointer is legal by itself but this def
// shouldn't appear in any AvailableSet.
for (const Value *V : I.operands())
if (containsGCPtrType(V->getType()) &&
isNotExclusivelyConstantDerived(V) && !AvailableSet.count(V)) {
if (isValuePoisoned(V))
PoisonedPointerDef = true;
ValidUnrelocatedPointerDef = true;
assert(!(ValidUnrelocatedPointerDef && PoisonedPointerDef) &&
"Value cannot be both unrelocated and poisoned!");
if (ValidUnrelocatedPointerDef) {
// Remove def of unrelocated pointer from Contribution of this BB and
// trigger update of all its successors.
LLVM_DEBUG(dbgs() << "Removing urelocated " << I
<< " from Contribution of " << BB->getName() << "\n");
ContributionChanged = true;
} else if (PoisonedPointerDef) {
// Mark pointer as poisoned, remove its def from Contribution and trigger
// update of all successors.
LLVM_DEBUG(dbgs() << "Removing poisoned " << I << " from Contribution of "
<< BB->getName() << "\n");
ContributionChanged = true;
} else {
bool Cleared = false;
transferInstruction(I, Cleared, AvailableSet);
return ContributionChanged;
void GCPtrTracker::gatherDominatingDefs(const BasicBlock *BB,
AvailableValueSet &Result,
const DominatorTree &DT) {
DomTreeNode *DTN = DT[const_cast<BasicBlock *>(BB)];
assert(DTN && "Unreachable blocks are ignored");
while (DTN->getIDom()) {
DTN = DTN->getIDom();
auto BBS = getBasicBlockState(DTN->getBlock());
assert(BBS && "immediate dominator cannot be dead for a live block");
const auto &Defs = BBS->Contribution;
Result.insert(Defs.begin(), Defs.end());
// If this block is 'Cleared', then nothing LiveIn to this block can be
// available after this block completes. Note: This turns out to be
// really important for reducing memory consuption of the initial available
// sets and thus peak memory usage by this verifier.
if (BBS->Cleared)
for (const Argument &A : BB->getParent()->args())
if (containsGCPtrType(A.getType()))
void GCPtrTracker::transferBlock(const BasicBlock *BB, BasicBlockState &BBS,
bool ContributionChanged) {
const AvailableValueSet &AvailableIn = BBS.AvailableIn;
AvailableValueSet &AvailableOut = BBS.AvailableOut;
if (BBS.Cleared) {
// AvailableOut will change only when Contribution changed.
if (ContributionChanged)
AvailableOut = BBS.Contribution;
} else {
// Otherwise, we need to reduce the AvailableOut set by things which are no
// longer in our AvailableIn
AvailableValueSet Temp = BBS.Contribution;
set_union(Temp, AvailableIn);
AvailableOut = std::move(Temp);
LLVM_DEBUG(dbgs() << "Transfered block " << BB->getName() << " from ";
PrintValueSet(dbgs(), AvailableIn.begin(), AvailableIn.end());
dbgs() << " to ";
PrintValueSet(dbgs(), AvailableOut.begin(), AvailableOut.end());
dbgs() << "\n";);
void GCPtrTracker::transferInstruction(const Instruction &I, bool &Cleared,
AvailableValueSet &Available) {
if (isStatepoint(I)) {
Cleared = true;
} else if (containsGCPtrType(I.getType()))
void InstructionVerifier::verifyInstruction(
const GCPtrTracker *Tracker, const Instruction &I,
const AvailableValueSet &AvailableSet) {
if (const PHINode *PN = dyn_cast<PHINode>(&I)) {
if (containsGCPtrType(PN->getType()))
for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
const BasicBlock *InBB = PN->getIncomingBlock(i);
const BasicBlockState *InBBS = Tracker->getBasicBlockState(InBB);
if (!InBBS ||
!Tracker->hasLiveIncomingEdge(PN, InBB))
continue; // Skip dead block or dead edge.
const Value *InValue = PN->getIncomingValue(i);
if (isNotExclusivelyConstantDerived(InValue) &&
reportInvalidUse(*InValue, *PN);
} else if (isa<CmpInst>(I) &&
containsGCPtrType(I.getOperand(0)->getType())) {
Value *LHS = I.getOperand(0), *RHS = I.getOperand(1);
enum BaseType baseTyLHS = getBaseType(LHS),
baseTyRHS = getBaseType(RHS);
// Returns true if LHS and RHS are unrelocated pointers and they are
// valid unrelocated uses.
auto hasValidUnrelocatedUse = [&AvailableSet, Tracker, baseTyLHS, baseTyRHS,
&LHS, &RHS] () {
// A cmp instruction has valid unrelocated pointer operands only if
// both operands are unrelocated pointers.
// In the comparison between two pointers, if one is an unrelocated
// use, the other *should be* an unrelocated use, for this
// instruction to contain valid unrelocated uses. This unrelocated
// use can be a null constant as well, or another unrelocated
// pointer.
if (AvailableSet.count(LHS) || AvailableSet.count(RHS))
return false;
// Constant pointers (that are not exclusively null) may have
// meaning in different VMs, so we cannot reorder the compare
// against constant pointers before the safepoint. In other words,
// comparison of an unrelocated use against a non-null constant
// maybe invalid.
if ((baseTyLHS == BaseType::ExclusivelySomeConstant &&
baseTyRHS == BaseType::NonConstant) ||
(baseTyLHS == BaseType::NonConstant &&
baseTyRHS == BaseType::ExclusivelySomeConstant))
return false;
// If one of pointers is poisoned and other is not exclusively derived
// from null it is an invalid expression: it produces poisoned result
// and unless we want to track all defs (not only gc pointers) the only
// option is to prohibit such instructions.
if ((Tracker->isValuePoisoned(LHS) && baseTyRHS != ExclusivelyNull) ||
(Tracker->isValuePoisoned(RHS) && baseTyLHS != ExclusivelyNull))
return false;
// All other cases are valid cases enumerated below:
// 1. Comparison between an exclusively derived null pointer and a
// constant base pointer.
// 2. Comparison between an exclusively derived null pointer and a
// non-constant unrelocated base pointer.
// 3. Comparison between 2 unrelocated pointers.
// 4. Comparison between a pointer exclusively derived from null and a
// non-constant poisoned pointer.
return true;
if (!hasValidUnrelocatedUse()) {
// Print out all non-constant derived pointers that are unrelocated
// uses, which are invalid.
if (baseTyLHS == BaseType::NonConstant && !AvailableSet.count(LHS))
reportInvalidUse(*LHS, I);
if (baseTyRHS == BaseType::NonConstant && !AvailableSet.count(RHS))
reportInvalidUse(*RHS, I);
} else {
for (const Value *V : I.operands())
if (containsGCPtrType(V->getType()) &&
isNotExclusivelyConstantDerived(V) && !AvailableSet.count(V))
reportInvalidUse(*V, I);
void InstructionVerifier::reportInvalidUse(const Value &V,
const Instruction &I) {
errs() << "Illegal use of unrelocated value found!\n";
errs() << "Def: " << V << "\n";
errs() << "Use: " << I << "\n";
if (!PrintOnly)
AnyInvalidUses = true;
static void Verify(const Function &F, const DominatorTree &DT,
const CFGDeadness &CD) {
LLVM_DEBUG(dbgs() << "Verifying gc pointers in function: " << F.getName()
<< "\n");
if (PrintOnly)
dbgs() << "Verifying gc pointers in function: " << F.getName() << "\n";
GCPtrTracker Tracker(F, DT, CD);
// We now have all the information we need to decide if the use of a heap
// reference is legal or not, given our safepoint semantics.
InstructionVerifier Verifier;
GCPtrTracker::verifyFunction(std::move(Tracker), Verifier);
if (PrintOnly && !Verifier.hasAnyInvalidUses()) {
dbgs() << "No illegal uses found by SafepointIRVerifier in: " << F.getName()
<< "\n";