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[LVI] Normalize pointer behavior 

Related to D69686. As noted there, LVI currently behaves differently
for integer and pointer values: For integers, the block value is always
valid inside the basic block, while for pointers it is only valid at
the end of the basic block. I believe the integer behavior is the
correct one, and CVP relies on it via its getConstantRange() uses.

The reason for the special pointer behavior is that LVI checks whether
a pointer is dereferenced in a given basic block and marks it as
non-null in that case. Of course, this information is valid only after
the dereferencing instruction, or in conservative approximation,
at the end of the block.

This patch changes the treatment of dereferencability: Instead of
including it inside the block value, we instead treat it as something
similar to an assume (it essentially is a non-nullness assume) and
incorporate this information in intersectAssumeOrGuardBlockValueConstantRange()
if the context instruction is the terminator of the basic block.
This happens either when determining an edge-value internally in LVI,
or when a terminator was explicitly passed to getValueAt(). The latter
case makes this change not fully NFC, because we can now fold
terminator icmps based on the dereferencability information in the
same block. This is the reason why I changed one JumpThreading test
(it would optimize the condition away without the change).

Of course, we do not want to recompute dereferencability on each
intersectAssume call, so we need a new cache for this. The
dereferencability analysis requires walking the entire basic block
and computing underlying objects of all memory operands. This was
previously done separately for each queried pointer value. In the
new implementation (both because this makes the caching simpler,
and because it is faster), I instead only walk the full BB once and
cache all the dereferenced pointers. So the traversal is now performed
only once per BB, instead of once per queried pointer value.

I think the overall model now makes more sense than before, and there
will be no more pitfalls due to differing integer/pointer behavior.

Differential Revision: https://reviews.llvm.org/D69914
This commit is contained in:
Nikita Popov 2019-11-05 23:14:22 +01:00
parent 1471a7ddee
commit 15bc4dc9a8
2 changed files with 97 additions and 91 deletions
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185
llvm/lib/Analysis/LazyValueInfo.cpp
View File

@ -151,6 +151,11 @@ namespace {
/// This is the cache kept by LazyValueInfo which
/// maintains information about queries across the clients' queries.
class LazyValueInfoCache {
public:
typedef DenseMap<PoisoningVH<BasicBlock>, SmallPtrSet<Value *, 4>>
PerBlockValueCacheTy;
private:
/// This is all of the cached block information for exactly one Value*.
/// The entries are sorted by the BasicBlock* of the
/// entries, allowing us to do a lookup with a binary search.
@ -162,10 +167,6 @@ namespace {
SmallDenseMap<PoisoningVH<BasicBlock>, ValueLatticeElement, 4> BlockVals;
};
/// This tracks, on a per-block basis, the set of values that are
/// over-defined at the end of that block.
typedef DenseMap<PoisoningVH<BasicBlock>, SmallPtrSet<Value *, 4>>
OverDefinedCacheTy;
/// Keep track of all blocks that we have ever seen, so we
/// don't spend time removing unused blocks from our caches.
DenseSet<PoisoningVH<BasicBlock> > SeenBlocks;
@ -173,7 +174,12 @@ namespace {
/// This is all of the cached information for all values,
/// mapped from Value* to key information.
DenseMap<Value *, std::unique_ptr<ValueCacheEntryTy>> ValueCache;
OverDefinedCacheTy OverDefinedCache;
/// This tracks, on a per-block basis, the set of values that are
/// over-defined at the end of that block.
PerBlockValueCacheTy OverDefinedCache;
/// This tracks, on a per-block basis, the set of pointers that are
/// dereferenced in the block (and thus non-null at the end of the block).
PerBlockValueCacheTy DereferencedPointerCache;
public:
@ -229,11 +235,17 @@ namespace {
return BBI->second;
}
std::pair<PerBlockValueCacheTy::iterator, bool>
getOrInitDereferencedPointers(BasicBlock *BB) {
return DereferencedPointerCache.try_emplace(BB);
}
/// clear - Empty the cache.
void clear() {
SeenBlocks.clear();
ValueCache.clear();
OverDefinedCache.clear();
DereferencedPointerCache.clear();
}
/// Inform the cache that a given value has been deleted.
@ -252,17 +264,22 @@ namespace {
};
}
void LazyValueInfoCache::eraseValue(Value *V) {
for (auto I = OverDefinedCache.begin(), E = OverDefinedCache.end(); I != E;) {
static void eraseValueFromPerBlockValueCache(
Value *V, LazyValueInfoCache::PerBlockValueCacheTy &Cache) {
for (auto I = Cache.begin(), E = Cache.end(); I != E;) {
// Copy and increment the iterator immediately so we can erase behind
// ourselves.
auto Iter = I++;
SmallPtrSetImpl<Value *> &ValueSet = Iter->second;
ValueSet.erase(V);
if (ValueSet.empty())
OverDefinedCache.erase(Iter);
Cache.erase(Iter);
}
}
void LazyValueInfoCache::eraseValue(Value *V) {
eraseValueFromPerBlockValueCache(V, OverDefinedCache);
eraseValueFromPerBlockValueCache(V, DereferencedPointerCache);
ValueCache.erase(V);
}
@ -279,9 +296,8 @@ void LazyValueInfoCache::eraseBlock(BasicBlock *BB) {
return;
SeenBlocks.erase(I);
auto ODI = OverDefinedCache.find(BB);
if (ODI != OverDefinedCache.end())
OverDefinedCache.erase(ODI);
OverDefinedCache.erase(BB);
DereferencedPointerCache.erase(BB);
for (auto &I : ValueCache)
I.second->BlockVals.erase(BB);
@ -438,6 +454,7 @@ namespace {
BasicBlock *BB);
bool solveBlockValueExtractValue(ValueLatticeElement &BBLV,
ExtractValueInst *EVI, BasicBlock *BB);
bool isNonNullDueToDereferenceInBlock(Value *Val, BasicBlock *BB);
void intersectAssumeOrGuardBlockValueConstantRange(Value *Val,
ValueLatticeElement &BBLV,
Instruction *BBI);
@ -619,6 +636,20 @@ bool LazyValueInfoImpl::solveBlockValue(Value *Val, BasicBlock *BB) {
bool LazyValueInfoImpl::solveBlockValueImpl(ValueLatticeElement &Res,
Value *Val, BasicBlock *BB) {
// If this value is a nonnull pointer, record it's range and bailout. Note
// that for all other pointer typed values, we terminate the search at the
// definition. We could easily extend this to look through geps, bitcasts,
// and the like to prove non-nullness, but it's not clear that's worth it
// compile time wise. The context-insensitive value walk done inside
// isKnownNonZero gets most of the profitable cases at much less expense.
// This does mean that we have a sensitivity to where the defining
// instruction is placed, even if it could legally be hoisted much higher.
// That is unfortunate.
PointerType *PT = dyn_cast<PointerType>(Val->getType());
if (PT && isKnownNonZero(Val, DL)) {
Res = ValueLatticeElement::getNot(ConstantPointerNull::get(PT));
return true;
}
Instruction *BBI = dyn_cast<Instruction>(Val);
if (!BBI || BBI->getParent() != BB)
@ -630,20 +661,6 @@ bool LazyValueInfoImpl::solveBlockValueImpl(ValueLatticeElement &Res,
if (auto *SI = dyn_cast<SelectInst>(BBI))
return solveBlockValueSelect(Res, SI, BB);
// If this value is a nonnull pointer, record it's range and bailout. Note
// that for all other pointer typed values, we terminate the search at the
// definition. We could easily extend this to look through geps, bitcasts,
// and the like to prove non-nullness, but it's not clear that's worth it
// compile time wise. The context-insensitive value walk done inside
// isKnownNonZero gets most of the profitable cases at much less expense.
// This does mean that we have a sensitivity to where the defining
// instruction is placed, even if it could legally be hoisted much higher.
// That is unfortunate.
PointerType *PT = dyn_cast<PointerType>(BBI->getType());
if (PT && isKnownNonZero(BBI, DL)) {
Res = ValueLatticeElement::getNot(ConstantPointerNull::get(PT));
return true;
}
if (BBI->getType()->isIntegerTy()) {
if (auto *CI = dyn_cast<CastInst>(BBI))
return solveBlockValueCast(Res, CI, BB);
@ -664,75 +681,63 @@ bool LazyValueInfoImpl::solveBlockValueImpl(ValueLatticeElement &Res,
return true;
}
static bool InstructionDereferencesPointer(Instruction *I, Value *Ptr) {
static void AddDereferencedPointer(
Value *Ptr, SmallPtrSet<Value *, 4> &PtrSet, const DataLayout &DL) {
// TODO: Use NullPointerIsDefined instead.
if (Ptr->getType()->getPointerAddressSpace() == 0) {
Ptr = GetUnderlyingObject(Ptr, DL);
PtrSet.insert(Ptr);
}
}
static void AddPointersDereferencedByInstruction(
Instruction *I, SmallPtrSet<Value *, 4> &PtrSet, const DataLayout &DL) {
if (LoadInst *L = dyn_cast<LoadInst>(I)) {
return L->getPointerAddressSpace() == 0 &&
GetUnderlyingObject(L->getPointerOperand(),
L->getModule()->getDataLayout()) == Ptr;
}
if (StoreInst *S = dyn_cast<StoreInst>(I)) {
return S->getPointerAddressSpace() == 0 &&
GetUnderlyingObject(S->getPointerOperand(),
S->getModule()->getDataLayout()) == Ptr;
}
if (MemIntrinsic *MI = dyn_cast<MemIntrinsic>(I)) {
if (MI->isVolatile()) return false;
AddDereferencedPointer(L->getPointerOperand(), PtrSet, DL);
} else if (StoreInst *S = dyn_cast<StoreInst>(I)) {
AddDereferencedPointer(S->getPointerOperand(), PtrSet, DL);
} else if (MemIntrinsic *MI = dyn_cast<MemIntrinsic>(I)) {
if (MI->isVolatile()) return;
// FIXME: check whether it has a valuerange that excludes zero?
ConstantInt *Len = dyn_cast<ConstantInt>(MI->getLength());
if (!Len || Len->isZero()) return false;
if (!Len || Len->isZero()) return;
if (MI->getDestAddressSpace() == 0)
if (GetUnderlyingObject(MI->getRawDest(),
MI->getModule()->getDataLayout()) == Ptr)
return true;
AddDereferencedPointer(MI->getRawDest(), PtrSet, DL);
if (MemTransferInst *MTI = dyn_cast<MemTransferInst>(MI))
if (MTI->getSourceAddressSpace() == 0)
if (GetUnderlyingObject(MTI->getRawSource(),
MTI->getModule()->getDataLayout()) == Ptr)
return true;
AddDereferencedPointer(MTI->getRawSource(), PtrSet, DL);
}
return false;
}
/// Return true if the allocation associated with Val is ever dereferenced
/// within the given basic block. This establishes the fact Val is not null,
/// but does not imply that the memory at Val is dereferenceable. (Val may
/// point off the end of the dereferenceable part of the object.)
static bool isObjectDereferencedInBlock(Value *Val, BasicBlock *BB) {
assert(Val->getType()->isPointerTy());
bool LazyValueInfoImpl::isNonNullDueToDereferenceInBlock(
Value *Val, BasicBlock *BB) {
if (NullPointerIsDefined(BB->getParent(),
Val->getType()->getPointerAddressSpace()))
return false;
const DataLayout &DL = BB->getModule()->getDataLayout();
Value *UnderlyingVal = GetUnderlyingObject(Val, DL);
// If 'GetUnderlyingObject' didn't converge, skip it. It won't converge
// inside InstructionDereferencesPointer either.
if (UnderlyingVal == GetUnderlyingObject(UnderlyingVal, DL, 1))
Val = GetUnderlyingObject(Val, DL);
LazyValueInfoCache::PerBlockValueCacheTy::iterator It;
bool NeedsInit;
std::tie(It, NeedsInit) = TheCache.getOrInitDereferencedPointers(BB);
if (NeedsInit)
for (Instruction &I : *BB)
if (InstructionDereferencesPointer(&I, UnderlyingVal))
return true;
return false;
AddPointersDereferencedByInstruction(&I, It->second, DL);
return It->second.count(Val);
}
bool LazyValueInfoImpl::solveBlockValueNonLocal(ValueLatticeElement &BBLV,
Value *Val, BasicBlock *BB) {
Value *Val, BasicBlock *BB) {
ValueLatticeElement Result; // Start Undefined.
// If this is the entry block, we must be asking about an argument. The
// value is overdefined.
if (BB == &BB->getParent()->getEntryBlock()) {
assert(isa<Argument>(Val) && "Unknown live-in to the entry block");
// Before giving up, see if we can prove the pointer non-null local to
// this particular block.
PointerType *PTy = dyn_cast<PointerType>(Val->getType());
if (PTy &&
(isKnownNonZero(Val, DL) ||
(isObjectDereferencedInBlock(Val, BB) &&
!NullPointerIsDefined(BB->getParent(), PTy->getAddressSpace())))) {
Result = ValueLatticeElement::getNot(ConstantPointerNull::get(PTy));
} else {
Result = ValueLatticeElement::getOverdefined();
}
BBLV = Result;
BBLV = ValueLatticeElement::getOverdefined();
return true;
}
@ -758,14 +763,6 @@ bool LazyValueInfoImpl::solveBlockValueNonLocal(ValueLatticeElement &BBLV,
if (Result.isOverdefined()) {
LLVM_DEBUG(dbgs() << " compute BB '" << BB->getName()
<< "' - overdefined because of pred (non local).\n");
// Before giving up, see if we can prove the pointer non-null local to
// this particular block.
PointerType *PTy = dyn_cast<PointerType>(Val->getType());
if (PTy && isObjectDereferencedInBlock(Val, BB) &&
!NullPointerIsDefined(BB->getParent(), PTy->getAddressSpace())) {
Result = ValueLatticeElement::getNot(ConstantPointerNull::get(PTy));
}
BBLV = Result;
return true;
}
@ -838,16 +835,24 @@ void LazyValueInfoImpl::intersectAssumeOrGuardBlockValueConstantRange(
// If guards are not used in the module, don't spend time looking for them
auto *GuardDecl = BBI->getModule()->getFunction(
Intrinsic::getName(Intrinsic::experimental_guard));
if (!GuardDecl || GuardDecl->use_empty())
return;
if (GuardDecl && !GuardDecl->use_empty()) {
if (BBI->getIterator() == BBI->getParent()->begin())
return;
for (Instruction &I : make_range(std::next(BBI->getIterator().getReverse()),
BBI->getParent()->rend())) {
Value *Cond = nullptr;
if (match(&I, m_Intrinsic<Intrinsic::experimental_guard>(m_Value(Cond))))
BBLV = intersect(BBLV, getValueFromCondition(Val, Cond));
}
}
if (BBI->getIterator() == BBI->getParent()->begin())
return;
for (Instruction &I : make_range(std::next(BBI->getIterator().getReverse()),
BBI->getParent()->rend())) {
Value *Cond = nullptr;
if (match(&I, m_Intrinsic<Intrinsic::experimental_guard>(m_Value(Cond))))
BBLV = intersect(BBLV, getValueFromCondition(Val, Cond));
if (BBLV.isOverdefined()) {
// Check whether we're checking at the terminator, and the pointer has
// been dereferenced in this block.
PointerType *PTy = dyn_cast<PointerType>(Val->getType());
if (PTy && BBI->getParent()->getTerminator() == BBI &&
isNonNullDueToDereferenceInBlock(Val, BBI->getParent()))
BBLV = ValueLatticeElement::getNot(ConstantPointerNull::get(PTy));
}
}

3
llvm/test/Transforms/JumpThreading/combine-metadata.ll
View File

@ -108,7 +108,7 @@ d2:
d3:
%y = load i32*, i32** %ptr
store i32 1, i32* %y
%c2 = icmp eq i32* %y, null
%c2 = icmp eq i32* %y, @p
br i1 %c2, label %ret1, label %ret2
ret1:
@ -118,5 +118,6 @@ ret2:
ret void
}
@p = external global i32
!0 = !{}