//===- SparsePropagation.h - Sparse Conditional Property Propagation ------===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
//
// This file implements an abstract sparse conditional propagation algorithm,
// modeled after SCCP, but with a customizable lattice function.
//
//===----------------------------------------------------------------------===//
#ifndef LLVM_ANALYSIS_SPARSEPROPAGATION_H
#define LLVM_ANALYSIS_SPARSEPROPAGATION_H
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/IR/Constants.h"
#include "llvm/IR/Instructions.h"
#include "llvm/Support/Debug.h"
#include <set>
#define DEBUG_TYPE "sparseprop"
namespace llvm {
/// A template for translating between LLVM Values and LatticeKeys. Clients must
/// provide a specialization of LatticeKeyInfo for their LatticeKey type.
template <class LatticeKey> struct LatticeKeyInfo {
// static inline Value *getValueFromLatticeKey(LatticeKey Key);
// static inline LatticeKey getLatticeKeyFromValue(Value *V);
};
template <class LatticeKey, class LatticeVal,
class KeyInfo = LatticeKeyInfo<LatticeKey>>
class SparseSolver;
/// AbstractLatticeFunction - This class is implemented by the dataflow instance
/// to specify what the lattice values are and how they handle merges etc. This
/// gives the client the power to compute lattice values from instructions,
/// constants, etc. The current requirement is that lattice values must be
/// copyable. At the moment, nothing tries to avoid copying. Additionally,
/// lattice keys must be able to be used as keys of a mapping data structure.
/// Internally, the generic solver currently uses a DenseMap to map lattice keys
/// to lattice values. If the lattice key is a non-standard type, a
/// specialization of DenseMapInfo must be provided.
template <class LatticeKey, class LatticeVal> class AbstractLatticeFunction {
private:
LatticeVal UndefVal, OverdefinedVal, UntrackedVal;
public:
AbstractLatticeFunction(LatticeVal undefVal, LatticeVal overdefinedVal,
LatticeVal untrackedVal) {
UndefVal = undefVal;
OverdefinedVal = overdefinedVal;
UntrackedVal = untrackedVal;
}
virtual ~AbstractLatticeFunction() = default;
LatticeVal getUndefVal() const { return UndefVal; }
LatticeVal getOverdefinedVal() const { return OverdefinedVal; }
LatticeVal getUntrackedVal() const { return UntrackedVal; }
/// IsUntrackedValue - If the specified LatticeKey is obviously uninteresting
/// to the analysis (i.e., it would always return UntrackedVal), this
/// function can return true to avoid pointless work.
virtual bool IsUntrackedValue(LatticeKey Key) { return false; }
/// ComputeLatticeVal - Compute and return a LatticeVal corresponding to the
/// given LatticeKey.
virtual LatticeVal ComputeLatticeVal(LatticeKey Key) {
return getOverdefinedVal();
}
/// IsSpecialCasedPHI - Given a PHI node, determine whether this PHI node is
/// one that the we want to handle through ComputeInstructionState.
virtual bool IsSpecialCasedPHI(PHINode *PN) { return false; }
/// MergeValues - Compute and return the merge of the two specified lattice
/// values. Merging should only move one direction down the lattice to
/// guarantee convergence (toward overdefined).
virtual LatticeVal MergeValues(LatticeVal X, LatticeVal Y) {
return getOverdefinedVal(); // always safe, never useful.
}
/// ComputeInstructionState - Compute the LatticeKeys that change as a result
/// of executing instruction \p I. Their associated LatticeVals are store in
/// \p ChangedValues.
virtual void
ComputeInstructionState(Instruction &I,
DenseMap<LatticeKey, LatticeVal> &ChangedValues,
SparseSolver<LatticeKey, LatticeVal> &SS) = 0;
/// PrintLatticeVal - Render the given LatticeVal to the specified stream.
virtual void PrintLatticeVal(LatticeVal LV, raw_ostream &OS);
/// PrintLatticeKey - Render the given LatticeKey to the specified stream.
virtual void PrintLatticeKey(LatticeKey Key, raw_ostream &OS);
/// GetValueFromLatticeVal - If the given LatticeVal is representable as an
/// LLVM value, return it; otherwise, return nullptr. If a type is given, the
/// returned value must have the same type. This function is used by the
/// generic solver in attempting to resolve branch and switch conditions.
virtual Value *GetValueFromLatticeVal(LatticeVal LV, Type *Ty = nullptr) {
return nullptr;
}
};
/// SparseSolver - This class is a general purpose solver for Sparse Conditional
/// Propagation with a programmable lattice function.
template <class LatticeKey, class LatticeVal, class KeyInfo>
class SparseSolver {
/// LatticeFunc - This is the object that knows the lattice and how to
/// compute transfer functions.
AbstractLatticeFunction<LatticeKey, LatticeVal> *LatticeFunc;
/// ValueState - Holds the LatticeVals associated with LatticeKeys.
DenseMap<LatticeKey, LatticeVal> ValueState;
/// BBExecutable - Holds the basic blocks that are executable.
SmallPtrSet<BasicBlock *, 16> BBExecutable;
/// ValueWorkList - Holds values that should be processed.
SmallVector<Value *, 64> ValueWorkList;
/// BBWorkList - Holds basic blocks that should be processed.
SmallVector<BasicBlock *, 64> BBWorkList;
using Edge = std::pair<BasicBlock *, BasicBlock *>;
/// KnownFeasibleEdges - Entries in this set are edges which have already had
/// PHI nodes retriggered.
std::set<Edge> KnownFeasibleEdges;
public:
explicit SparseSolver(
AbstractLatticeFunction<LatticeKey, LatticeVal> *Lattice)
: LatticeFunc(Lattice) {}
SparseSolver(const SparseSolver &) = delete;
SparseSolver &operator=(const SparseSolver &) = delete;
/// Solve - Solve for constants and executable blocks.
void Solve();
void Print(raw_ostream &OS) const;
/// getExistingValueState - Return the LatticeVal object corresponding to the
/// given value from the ValueState map. If the value is not in the map,
/// UntrackedVal is returned, unlike the getValueState method.
LatticeVal getExistingValueState(LatticeKey Key) const {
auto I = ValueState.find(Key);
return I != ValueState.end() ? I->second : LatticeFunc->getUntrackedVal();
}
/// getValueState - Return the LatticeVal object corresponding to the given
/// value from the ValueState map. If the value is not in the map, its state
/// is initialized.
LatticeVal getValueState(LatticeKey Key);
/// isEdgeFeasible - Return true if the control flow edge from the 'From'
/// basic block to the 'To' basic block is currently feasible. If
/// AggressiveUndef is true, then this treats values with unknown lattice
/// values as undefined. This is generally only useful when solving the
/// lattice, not when querying it.
bool isEdgeFeasible(BasicBlock *From, BasicBlock *To,
bool AggressiveUndef = false);
/// isBlockExecutable - Return true if there are any known feasible
/// edges into the basic block. This is generally only useful when
/// querying the lattice.
bool isBlockExecutable(BasicBlock *BB) const {
return BBExecutable.count(BB);
}
/// MarkBlockExecutable - This method can be used by clients to mark all of
/// the blocks that are known to be intrinsically live in the processed unit.
void MarkBlockExecutable(BasicBlock *BB);
private:
/// UpdateState - When the state of some LatticeKey is potentially updated to
/// the given LatticeVal, this function notices and adds the LLVM value
/// corresponding the key to the work list, if needed.
void UpdateState(LatticeKey Key, LatticeVal LV);
/// markEdgeExecutable - Mark a basic block as executable, adding it to the BB
/// work list if it is not already executable.
void markEdgeExecutable(BasicBlock *Source, BasicBlock *Dest);
/// getFeasibleSuccessors - Return a vector of booleans to indicate which
/// successors are reachable from a given terminator instruction.
void getFeasibleSuccessors(Instruction &TI, SmallVectorImpl<bool> &Succs,
bool AggressiveUndef);
void visitInst(Instruction &I);
void visitPHINode(PHINode &I);
void visitTerminator(Instruction &TI);
};
//===----------------------------------------------------------------------===//
// AbstractLatticeFunction Implementation
//===----------------------------------------------------------------------===//
template <class LatticeKey, class LatticeVal>
void AbstractLatticeFunction<LatticeKey, LatticeVal>::PrintLatticeVal(
LatticeVal V, raw_ostream &OS) {
if (V == UndefVal)
OS << "undefined";
else if (V == OverdefinedVal)
OS << "overdefined";
else if (V == UntrackedVal)
OS << "untracked";
else
OS << "unknown lattice value";
}
template <class LatticeKey, class LatticeVal>
void AbstractLatticeFunction<LatticeKey, LatticeVal>::PrintLatticeKey(
LatticeKey Key, raw_ostream &OS) {
OS << "unknown lattice key";
}
//===----------------------------------------------------------------------===//
// SparseSolver Implementation
//===----------------------------------------------------------------------===//
template <class LatticeKey, class LatticeVal, class KeyInfo>
LatticeVal
SparseSolver<LatticeKey, LatticeVal, KeyInfo>::getValueState(LatticeKey Key) {
auto I = ValueState.find(Key);
if (I != ValueState.end())
return I->second; // Common case, in the map
if (LatticeFunc->IsUntrackedValue(Key))
return LatticeFunc->getUntrackedVal();
LatticeVal LV = LatticeFunc->ComputeLatticeVal(Key);
// If this value is untracked, don't add it to the map.
if (LV == LatticeFunc->getUntrackedVal())
return LV;
return ValueState[Key] = std::move(LV);
}
template <class LatticeKey, class LatticeVal, class KeyInfo>
void SparseSolver<LatticeKey, LatticeVal, KeyInfo>::UpdateState(LatticeKey Key,
LatticeVal LV) {
auto I = ValueState.find(Key);
if (I != ValueState.end() && I->second == LV)
return; // No change.
// Update the state of the given LatticeKey and add its corresponding LLVM
// value to the work list.
ValueState[Key] = std::move(LV);
if (Value *V = KeyInfo::getValueFromLatticeKey(Key))
ValueWorkList.push_back(V);
}
template <class LatticeKey, class LatticeVal, class KeyInfo>
void SparseSolver<LatticeKey, LatticeVal, KeyInfo>::MarkBlockExecutable(
BasicBlock *BB) {
if (!BBExecutable.insert(BB).second)
return;
LLVM_DEBUG(dbgs() << "Marking Block Executable: " << BB->getName() << "\n");
BBWorkList.push_back(BB); // Add the block to the work list!
}
template <class LatticeKey, class LatticeVal, class KeyInfo>
void SparseSolver<LatticeKey, LatticeVal, KeyInfo>::markEdgeExecutable(
BasicBlock *Source, BasicBlock *Dest) {
if (!KnownFeasibleEdges.insert(Edge(Source, Dest)).second)
return; // This edge is already known to be executable!
LLVM_DEBUG(dbgs() << "Marking Edge Executable: " << Source->getName()
<< " -> " << Dest->getName() << "\n");
if (BBExecutable.count(Dest)) {
// The destination is already executable, but we just made an edge
// feasible that wasn't before. Revisit the PHI nodes in the block
// because they have potentially new operands.
for (BasicBlock::iterator I = Dest->begin(); isa<PHINode>(I); ++I)
visitPHINode(*cast<PHINode>(I));
} else {
MarkBlockExecutable(Dest);
}
}
template <class LatticeKey, class LatticeVal, class KeyInfo>
void SparseSolver<LatticeKey, LatticeVal, KeyInfo>::getFeasibleSuccessors(
Instruction &TI, SmallVectorImpl<bool> &Succs, bool AggressiveUndef) {
Succs.resize(TI.getNumSuccessors());
if (TI.getNumSuccessors() == 0)
return;
if (BranchInst *BI = dyn_cast<BranchInst>(&TI)) {
if (BI->isUnconditional()) {
Succs[0] = true;
return;
}
LatticeVal BCValue;
if (AggressiveUndef)
BCValue =
getValueState(KeyInfo::getLatticeKeyFromValue(BI->getCondition()));
else
BCValue = getExistingValueState(
KeyInfo::getLatticeKeyFromValue(BI->getCondition()));
if (BCValue == LatticeFunc->getOverdefinedVal() ||
BCValue == LatticeFunc->getUntrackedVal()) {
// Overdefined condition variables can branch either way.
Succs[0] = Succs[1] = true;
return;
}
// If undefined, neither is feasible yet.
if (BCValue == LatticeFunc->getUndefVal())
return;
Constant *C =
dyn_cast_or_null<Constant>(LatticeFunc->GetValueFromLatticeVal(
std::move(BCValue), BI->getCondition()->getType()));
if (!C || !isa<ConstantInt>(C)) {
// Non-constant values can go either way.
Succs[0] = Succs[1] = true;
return;
}
// Constant condition variables mean the branch can only go a single way
Succs[C->isNullValue()] = true;
return;
}
if (!isa<SwitchInst>(TI)) {
// Unknown termintor, assume all successors are feasible.
Succs.assign(Succs.size(), true);
return;
}
SwitchInst &SI = cast<SwitchInst>(TI);
LatticeVal SCValue;
if (AggressiveUndef)
SCValue = getValueState(KeyInfo::getLatticeKeyFromValue(SI.getCondition()));
else
SCValue = getExistingValueState(
KeyInfo::getLatticeKeyFromValue(SI.getCondition()));
if (SCValue == LatticeFunc->getOverdefinedVal() ||
SCValue == LatticeFunc->getUntrackedVal()) {
// All destinations are executable!
Succs.assign(TI.getNumSuccessors(), true);
return;
}
// If undefined, neither is feasible yet.
if (SCValue == LatticeFunc->getUndefVal())
return;
Constant *C = dyn_cast_or_null<Constant>(LatticeFunc->GetValueFromLatticeVal(
std::move(SCValue), SI.getCondition()->getType()));
if (!C || !isa<ConstantInt>(C)) {
// All destinations are executable!
Succs.assign(TI.getNumSuccessors(), true);
return;
}
SwitchInst::CaseHandle Case = *SI.findCaseValue(cast<ConstantInt>(C));
Succs[Case.getSuccessorIndex()] = true;
}
template <class LatticeKey, class LatticeVal, class KeyInfo>
bool SparseSolver<LatticeKey, LatticeVal, KeyInfo>::isEdgeFeasible(
BasicBlock *From, BasicBlock *To, bool AggressiveUndef) {
SmallVector<bool, 16> SuccFeasible;
Instruction *TI = From->getTerminator();
getFeasibleSuccessors(*TI, SuccFeasible, AggressiveUndef);
for (unsigned i = 0, e = TI->getNumSuccessors(); i != e; ++i)
if (TI->getSuccessor(i) == To && SuccFeasible[i])
return true;
return false;
}
template <class LatticeKey, class LatticeVal, class KeyInfo>
void SparseSolver<LatticeKey, LatticeVal, KeyInfo>::visitTerminator(
Instruction &TI) {
SmallVector<bool, 16> SuccFeasible;
getFeasibleSuccessors(TI, SuccFeasible, true);
BasicBlock *BB = TI.getParent();
// Mark all feasible successors executable...
for (unsigned i = 0, e = SuccFeasible.size(); i != e; ++i)
if (SuccFeasible[i])
markEdgeExecutable(BB, TI.getSuccessor(i));
}
template <class LatticeKey, class LatticeVal, class KeyInfo>
void SparseSolver<LatticeKey, LatticeVal, KeyInfo>::visitPHINode(PHINode &PN) {
// The lattice function may store more information on a PHINode than could be
// computed from its incoming values. For example, SSI form stores its sigma
// functions as PHINodes with a single incoming value.
if (LatticeFunc->IsSpecialCasedPHI(&PN)) {
DenseMap<LatticeKey, LatticeVal> ChangedValues;
LatticeFunc->ComputeInstructionState(PN, ChangedValues, *this);
for (auto &ChangedValue : ChangedValues)
if (ChangedValue.second != LatticeFunc->getUntrackedVal())
UpdateState(std::move(ChangedValue.first),
std::move(ChangedValue.second));
return;
}
LatticeKey Key = KeyInfo::getLatticeKeyFromValue(&PN);
LatticeVal PNIV = getValueState(Key);
LatticeVal Overdefined = LatticeFunc->getOverdefinedVal();
// If this value is already overdefined (common) just return.
if (PNIV == Overdefined || PNIV == LatticeFunc->getUntrackedVal())
return; // Quick exit
// Super-extra-high-degree PHI nodes are unlikely to ever be interesting,
// and slow us down a lot. Just mark them overdefined.
if (PN.getNumIncomingValues() > 64) {
UpdateState(Key, Overdefined);
return;
}
// Look at all of the executable operands of the PHI node. If any of them
// are overdefined, the PHI becomes overdefined as well. Otherwise, ask the
// transfer function to give us the merge of the incoming values.
for (unsigned i = 0, e = PN.getNumIncomingValues(); i != e; ++i) {
// If the edge is not yet known to be feasible, it doesn't impact the PHI.
if (!isEdgeFeasible(PN.getIncomingBlock(i), PN.getParent(), true))
continue;
// Merge in this value.
LatticeVal OpVal =
getValueState(KeyInfo::getLatticeKeyFromValue(PN.getIncomingValue(i)));
if (OpVal != PNIV)
PNIV = LatticeFunc->MergeValues(PNIV, OpVal);
if (PNIV == Overdefined)
break; // Rest of input values don't matter.
}
// Update the PHI with the compute value, which is the merge of the inputs.
UpdateState(Key, PNIV);
}
template <class LatticeKey, class LatticeVal, class KeyInfo>
void SparseSolver<LatticeKey, LatticeVal, KeyInfo>::visitInst(Instruction &I) {
// PHIs are handled by the propagation logic, they are never passed into the
// transfer functions.
if (PHINode *PN = dyn_cast<PHINode>(&I))
return visitPHINode(*PN);
// Otherwise, ask the transfer function what the result is. If this is
// something that we care about, remember it.
DenseMap<LatticeKey, LatticeVal> ChangedValues;
LatticeFunc->ComputeInstructionState(I, ChangedValues, *this);
for (auto &ChangedValue : ChangedValues)
if (ChangedValue.second != LatticeFunc->getUntrackedVal())
UpdateState(ChangedValue.first, ChangedValue.second);
if (I.isTerminator())
visitTerminator(I);
}
template <class LatticeKey, class LatticeVal, class KeyInfo>
void SparseSolver<LatticeKey, LatticeVal, KeyInfo>::Solve() {
// Process the work lists until they are empty!
while (!BBWorkList.empty() || !ValueWorkList.empty()) {
// Process the value work list.
while (!ValueWorkList.empty()) {
Value *V = ValueWorkList.pop_back_val();
LLVM_DEBUG(dbgs() << "\nPopped off V-WL: " << *V << "\n");
// "V" got into the work list because it made a transition. See if any
// users are both live and in need of updating.
for (User *U : V->users())
if (Instruction *Inst = dyn_cast<Instruction>(U))
if (BBExecutable.count(Inst->getParent())) // Inst is executable?
visitInst(*Inst);
}
// Process the basic block work list.
while (!BBWorkList.empty()) {
BasicBlock *BB = BBWorkList.pop_back_val();
LLVM_DEBUG(dbgs() << "\nPopped off BBWL: " << *BB);
// Notify all instructions in this basic block that they are newly
// executable.
for (Instruction &I : *BB)
visitInst(I);
}
}
}
template <class LatticeKey, class LatticeVal, class KeyInfo>
void SparseSolver<LatticeKey, LatticeVal, KeyInfo>::Print(
raw_ostream &OS) const {
if (ValueState.empty())
return;
LatticeKey Key;
LatticeVal LV;
OS << "ValueState:\n";
for (auto &Entry : ValueState) {
std::tie(Key, LV) = Entry;
if (LV == LatticeFunc->getUntrackedVal())
continue;
OS << "\t";
LatticeFunc->PrintLatticeVal(LV, OS);
OS << ": ";
LatticeFunc->PrintLatticeKey(Key, OS);
OS << "\n";
}
}
} // end namespace llvm
#undef DEBUG_TYPE
#endif // LLVM_ANALYSIS_SPARSEPROPAGATION_H