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llvm-project/llvm/lib/Transforms/Scalar/SCCP.cpp

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//===- SCCP.cpp - Sparse Conditional Constant Propagation -----------------===//
//
// The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This file implements sparse conditional constant propagation and merging:
//
// Specifically, this:
// * Assumes values are constant unless proven otherwise
// * Assumes BasicBlocks are dead unless proven otherwise
// * Proves values to be constant, and replaces them with constants
// * Proves conditional branches to be unconditional
//
//===----------------------------------------------------------------------===//
#include "llvm/Transforms/Scalar/SCCP.h"
#include "llvm/ADT/ArrayRef.h"
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/DenseSet.h"
#include "llvm/ADT/PointerIntPair.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Analysis/ConstantFolding.h"
#include "llvm/Analysis/GlobalsModRef.h"
#include "llvm/Analysis/TargetLibraryInfo.h"
#include "llvm/Transforms/Utils/Local.h"
#include "llvm/Analysis/ValueLattice.h"
#include "llvm/Analysis/ValueLatticeUtils.h"
#include "llvm/IR/BasicBlock.h"
#include "llvm/IR/CallSite.h"
#include "llvm/IR/Constant.h"
#include "llvm/IR/Constants.h"
#include "llvm/IR/DataLayout.h"
#include "llvm/IR/DerivedTypes.h"
#include "llvm/IR/Function.h"
#include "llvm/IR/GlobalVariable.h"
#include "llvm/IR/InstVisitor.h"
#include "llvm/IR/InstrTypes.h"
#include "llvm/IR/Instruction.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/Module.h"
#include "llvm/IR/PassManager.h"
#include "llvm/IR/Type.h"
#include "llvm/IR/User.h"
#include "llvm/IR/Value.h"
#include "llvm/Pass.h"
#include "llvm/Support/Casting.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/ErrorHandling.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/Transforms/Scalar.h"
#include <cassert>
#include <utility>
#include <vector>
using namespace llvm;
#define DEBUG_TYPE "sccp"
STATISTIC(NumInstRemoved, "Number of instructions removed");
STATISTIC(NumDeadBlocks , "Number of basic blocks unreachable");
STATISTIC(IPNumInstRemoved, "Number of instructions removed by IPSCCP");
STATISTIC(IPNumArgsElimed ,"Number of arguments constant propagated by IPSCCP");
STATISTIC(IPNumGlobalConst, "Number of globals found to be constant by IPSCCP");
namespace {
/// LatticeVal class - This class represents the different lattice values that
/// an LLVM value may occupy. It is a simple class with value semantics.
///
class LatticeVal {
enum LatticeValueTy {
/// unknown - This LLVM Value has no known value yet.
unknown,
/// constant - This LLVM Value has a specific constant value.
constant,
/// forcedconstant - This LLVM Value was thought to be undef until
/// ResolvedUndefsIn. This is treated just like 'constant', but if merged
/// with another (different) constant, it goes to overdefined, instead of
/// asserting.
forcedconstant,
/// overdefined - This instruction is not known to be constant, and we know
/// it has a value.
overdefined
};
/// Val: This stores the current lattice value along with the Constant* for
/// the constant if this is a 'constant' or 'forcedconstant' value.
PointerIntPair<Constant *, 2, LatticeValueTy> Val;
LatticeValueTy getLatticeValue() const {
return Val.getInt();
}
public:
LatticeVal() : Val(nullptr, unknown) {}
bool isUnknown() const { return getLatticeValue() == unknown; }
bool isConstant() const {
return getLatticeValue() == constant || getLatticeValue() == forcedconstant;
}
bool isOverdefined() const { return getLatticeValue() == overdefined; }
Constant *getConstant() const {
assert(isConstant() && "Cannot get the constant of a non-constant!");
return Val.getPointer();
}
/// markOverdefined - Return true if this is a change in status.
bool markOverdefined() {
if (isOverdefined())
return false;
Val.setInt(overdefined);
return true;
}
/// markConstant - Return true if this is a change in status.
bool markConstant(Constant *V) {
if (getLatticeValue() == constant) { // Constant but not forcedconstant.
assert(getConstant() == V && "Marking constant with different value");
return false;
}
if (isUnknown()) {
Val.setInt(constant);
assert(V && "Marking constant with NULL");
Val.setPointer(V);
} else {
assert(getLatticeValue() == forcedconstant &&
"Cannot move from overdefined to constant!");
// Stay at forcedconstant if the constant is the same.
if (V == getConstant()) return false;
// Otherwise, we go to overdefined. Assumptions made based on the
// forced value are possibly wrong. Assuming this is another constant
// could expose a contradiction.
Val.setInt(overdefined);
}
return true;
}
/// getConstantInt - If this is a constant with a ConstantInt value, return it
/// otherwise return null.
ConstantInt *getConstantInt() const {
if (isConstant())
return dyn_cast<ConstantInt>(getConstant());
return nullptr;
}
/// getBlockAddress - If this is a constant with a BlockAddress value, return
/// it, otherwise return null.
BlockAddress *getBlockAddress() const {
if (isConstant())
return dyn_cast<BlockAddress>(getConstant());
return nullptr;
}
void markForcedConstant(Constant *V) {
assert(isUnknown() && "Can't force a defined value!");
Val.setInt(forcedconstant);
Val.setPointer(V);
}
ValueLatticeElement toValueLattice() const {
if (isOverdefined())
return ValueLatticeElement::getOverdefined();
if (isConstant())
return ValueLatticeElement::get(getConstant());
return ValueLatticeElement();
}
};
//===----------------------------------------------------------------------===//
//
/// SCCPSolver - This class is a general purpose solver for Sparse Conditional
/// Constant Propagation.
///
class SCCPSolver : public InstVisitor<SCCPSolver> {
const DataLayout &DL;
const TargetLibraryInfo *TLI;
SmallPtrSet<BasicBlock *, 8> BBExecutable; // The BBs that are executable.
DenseMap<Value *, LatticeVal> ValueState; // The state each value is in.
// The state each parameter is in.
DenseMap<Value *, ValueLatticeElement> ParamState;
/// StructValueState - This maintains ValueState for values that have
/// StructType, for example for formal arguments, calls, insertelement, etc.
DenseMap<std::pair<Value *, unsigned>, LatticeVal> StructValueState;
/// GlobalValue - If we are tracking any values for the contents of a global
/// variable, we keep a mapping from the constant accessor to the element of
/// the global, to the currently known value. If the value becomes
/// overdefined, it's entry is simply removed from this map.
DenseMap<GlobalVariable *, LatticeVal> TrackedGlobals;
/// TrackedRetVals - If we are tracking arguments into and the return
/// value out of a function, it will have an entry in this map, indicating
/// what the known return value for the function is.
DenseMap<Function *, LatticeVal> TrackedRetVals;
/// TrackedMultipleRetVals - Same as TrackedRetVals, but used for functions
/// that return multiple values.
DenseMap<std::pair<Function *, unsigned>, LatticeVal> TrackedMultipleRetVals;
/// MRVFunctionsTracked - Each function in TrackedMultipleRetVals is
/// represented here for efficient lookup.
SmallPtrSet<Function *, 16> MRVFunctionsTracked;
/// MustTailFunctions - Each function here is a callee of non-removable
/// musttail call site.
SmallPtrSet<Function *, 16> MustTailCallees;
/// TrackingIncomingArguments - This is the set of functions for whose
/// arguments we make optimistic assumptions about and try to prove as
/// constants.
SmallPtrSet<Function *, 16> TrackingIncomingArguments;
/// The reason for two worklists is that overdefined is the lowest state
/// on the lattice, and moving things to overdefined as fast as possible
/// makes SCCP converge much faster.
///
/// By having a separate worklist, we accomplish this because everything
/// possibly overdefined will become overdefined at the soonest possible
/// point.
SmallVector<Value *, 64> OverdefinedInstWorkList;
SmallVector<Value *, 64> InstWorkList;
// The BasicBlock work list
SmallVector<BasicBlock *, 64> BBWorkList;
/// KnownFeasibleEdges - Entries in this set are edges which have already had
/// PHI nodes retriggered.
using Edge = std::pair<BasicBlock *, BasicBlock *>;
DenseSet<Edge> KnownFeasibleEdges;
public:
SCCPSolver(const DataLayout &DL, const TargetLibraryInfo *tli)
: DL(DL), TLI(tli) {}
/// 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.
///
/// This returns true if the block was not considered live before.
bool MarkBlockExecutable(BasicBlock *BB) {
if (!BBExecutable.insert(BB).second)
return false;
LLVM_DEBUG(dbgs() << "Marking Block Executable: " << BB->getName() << '\n');
BBWorkList.push_back(BB); // Add the block to the work list!
return true;
}
/// TrackValueOfGlobalVariable - Clients can use this method to
/// inform the SCCPSolver that it should track loads and stores to the
/// specified global variable if it can. This is only legal to call if
/// performing Interprocedural SCCP.
void TrackValueOfGlobalVariable(GlobalVariable *GV) {
// We only track the contents of scalar globals.
if (GV->getValueType()->isSingleValueType()) {
LatticeVal &IV = TrackedGlobals[GV];
if (!isa<UndefValue>(GV->getInitializer()))
IV.markConstant(GV->getInitializer());
}
}
/// AddTrackedFunction - If the SCCP solver is supposed to track calls into
/// and out of the specified function (which cannot have its address taken),
/// this method must be called.
void AddTrackedFunction(Function *F) {
// Add an entry, F -> undef.
if (auto *STy = dyn_cast<StructType>(F->getReturnType())) {
MRVFunctionsTracked.insert(F);
for (unsigned i = 0, e = STy->getNumElements(); i != e; ++i)
TrackedMultipleRetVals.insert(std::make_pair(std::make_pair(F, i),
LatticeVal()));
} else
TrackedRetVals.insert(std::make_pair(F, LatticeVal()));
}
/// AddMustTailCallee - If the SCCP solver finds that this function is called
/// from non-removable musttail call site.
void AddMustTailCallee(Function *F) {
MustTailCallees.insert(F);
}
/// Returns true if the given function is called from non-removable musttail
/// call site.
bool isMustTailCallee(Function *F) {
return MustTailCallees.count(F);
}
void AddArgumentTrackedFunction(Function *F) {
TrackingIncomingArguments.insert(F);
}
/// Returns true if the given function is in the solver's set of
/// argument-tracked functions.
bool isArgumentTrackedFunction(Function *F) {
return TrackingIncomingArguments.count(F);
}
/// Solve - Solve for constants and executable blocks.
void Solve();
/// ResolvedUndefsIn - While solving the dataflow for a function, we assume
/// that branches on undef values cannot reach any of their successors.
/// However, this is not a safe assumption. After we solve dataflow, this
/// method should be use to handle this. If this returns true, the solver
/// should be rerun.
bool ResolvedUndefsIn(Function &F);
bool isBlockExecutable(BasicBlock *BB) const {
return BBExecutable.count(BB);
}
// isEdgeFeasible - Return true if the control flow edge from the 'From' basic
// block to the 'To' basic block is currently feasible.
bool isEdgeFeasible(BasicBlock *From, BasicBlock *To);
std::vector<LatticeVal> getStructLatticeValueFor(Value *V) const {
std::vector<LatticeVal> StructValues;
auto *STy = dyn_cast<StructType>(V->getType());
assert(STy && "getStructLatticeValueFor() can be called only on structs");
for (unsigned i = 0, e = STy->getNumElements(); i != e; ++i) {
auto I = StructValueState.find(std::make_pair(V, i));
assert(I != StructValueState.end() && "Value not in valuemap!");
StructValues.push_back(I->second);
}
return StructValues;
}
const LatticeVal &getLatticeValueFor(Value *V) const {
assert(!V->getType()->isStructTy() &&
"Should use getStructLatticeValueFor");
DenseMap<Value *, LatticeVal>::const_iterator I = ValueState.find(V);
assert(I != ValueState.end() &&
"V not found in ValueState nor Paramstate map!");
return I->second;
}
/// getTrackedRetVals - Get the inferred return value map.
const DenseMap<Function*, LatticeVal> &getTrackedRetVals() {
return TrackedRetVals;
}
/// getTrackedGlobals - Get and return the set of inferred initializers for
/// global variables.
const DenseMap<GlobalVariable*, LatticeVal> &getTrackedGlobals() {
return TrackedGlobals;
}
/// getMRVFunctionsTracked - Get the set of functions which return multiple
/// values tracked by the pass.
const SmallPtrSet<Function *, 16> getMRVFunctionsTracked() {
return MRVFunctionsTracked;
}
/// getMustTailCallees - Get the set of functions which are called
/// from non-removable musttail call sites.
const SmallPtrSet<Function *, 16> getMustTailCallees() {
return MustTailCallees;
}
/// markOverdefined - Mark the specified value overdefined. This
/// works with both scalars and structs.
void markOverdefined(Value *V) {
if (auto *STy = dyn_cast<StructType>(V->getType()))
for (unsigned i = 0, e = STy->getNumElements(); i != e; ++i)
markOverdefined(getStructValueState(V, i), V);
else
markOverdefined(ValueState[V], V);
}
// isStructLatticeConstant - Return true if all the lattice values
// corresponding to elements of the structure are not overdefined,
// false otherwise.
bool isStructLatticeConstant(Function *F, StructType *STy) {
for (unsigned i = 0, e = STy->getNumElements(); i != e; ++i) {
const auto &It = TrackedMultipleRetVals.find(std::make_pair(F, i));
assert(It != TrackedMultipleRetVals.end());
LatticeVal LV = It->second;
if (LV.isOverdefined())
return false;
}
return true;
}
private:
// pushToWorkList - Helper for markConstant/markForcedConstant/markOverdefined
void pushToWorkList(LatticeVal &IV, Value *V) {
if (IV.isOverdefined())
return OverdefinedInstWorkList.push_back(V);
InstWorkList.push_back(V);
}
// markConstant - Make a value be marked as "constant". If the value
// is not already a constant, add it to the instruction work list so that
// the users of the instruction are updated later.
bool markConstant(LatticeVal &IV, Value *V, Constant *C) {
if (!IV.markConstant(C)) return false;
LLVM_DEBUG(dbgs() << "markConstant: " << *C << ": " << *V << '\n');
pushToWorkList(IV, V);
return true;
}
bool markConstant(Value *V, Constant *C) {
assert(!V->getType()->isStructTy() && "structs should use mergeInValue");
return markConstant(ValueState[V], V, C);
}
void markForcedConstant(Value *V, Constant *C) {
assert(!V->getType()->isStructTy() && "structs should use mergeInValue");
LatticeVal &IV = ValueState[V];
IV.markForcedConstant(C);
LLVM_DEBUG(dbgs() << "markForcedConstant: " << *C << ": " << *V << '\n');
pushToWorkList(IV, V);
}
// markOverdefined - Make a value be marked as "overdefined". If the
// value is not already overdefined, add it to the overdefined instruction
// work list so that the users of the instruction are updated later.
bool markOverdefined(LatticeVal &IV, Value *V) {
if (!IV.markOverdefined()) return false;
LLVM_DEBUG(dbgs() << "markOverdefined: ";
if (auto *F = dyn_cast<Function>(V)) dbgs()
<< "Function '" << F->getName() << "'\n";
else dbgs() << *V << '\n');
// Only instructions go on the work list
pushToWorkList(IV, V);
return true;
}
bool mergeInValue(LatticeVal &IV, Value *V, LatticeVal MergeWithV) {
if (IV.isOverdefined() || MergeWithV.isUnknown())
return false; // Noop.
if (MergeWithV.isOverdefined())
return markOverdefined(IV, V);
if (IV.isUnknown())
return markConstant(IV, V, MergeWithV.getConstant());
if (IV.getConstant() != MergeWithV.getConstant())
return markOverdefined(IV, V);
return false;
}
bool mergeInValue(Value *V, LatticeVal MergeWithV) {
assert(!V->getType()->isStructTy() &&
"non-structs should use markConstant");
return mergeInValue(ValueState[V], V, MergeWithV);
}
/// getValueState - Return the LatticeVal object that corresponds to the
/// value. This function handles the case when the value hasn't been seen yet
/// by properly seeding constants etc.
LatticeVal &getValueState(Value *V) {
assert(!V->getType()->isStructTy() && "Should use getStructValueState");
std::pair<DenseMap<Value*, LatticeVal>::iterator, bool> I =
ValueState.insert(std::make_pair(V, LatticeVal()));
LatticeVal &LV = I.first->second;
if (!I.second)
return LV; // Common case, already in the map.
if (auto *C = dyn_cast<Constant>(V)) {
// Undef values remain unknown.
if (!isa<UndefValue>(V))
LV.markConstant(C); // Constants are constant
}
// All others are underdefined by default.
return LV;
}
ValueLatticeElement &getParamState(Value *V) {
assert(!V->getType()->isStructTy() && "Should use getStructValueState");
std::pair<DenseMap<Value*, ValueLatticeElement>::iterator, bool>
PI = ParamState.insert(std::make_pair(V, ValueLatticeElement()));
ValueLatticeElement &LV = PI.first->second;
if (PI.second)
LV = getValueState(V).toValueLattice();
return LV;
}
/// getStructValueState - Return the LatticeVal object that corresponds to the
/// value/field pair. This function handles the case when the value hasn't
/// been seen yet by properly seeding constants etc.
LatticeVal &getStructValueState(Value *V, unsigned i) {
assert(V->getType()->isStructTy() && "Should use getValueState");
assert(i < cast<StructType>(V->getType())->getNumElements() &&
"Invalid element #");
std::pair<DenseMap<std::pair<Value*, unsigned>, LatticeVal>::iterator,
bool> I = StructValueState.insert(
std::make_pair(std::make_pair(V, i), LatticeVal()));
LatticeVal &LV = I.first->second;
if (!I.second)
return LV; // Common case, already in the map.
if (auto *C = dyn_cast<Constant>(V)) {
Constant *Elt = C->getAggregateElement(i);
if (!Elt)
LV.markOverdefined(); // Unknown sort of constant.
else if (isa<UndefValue>(Elt))
; // Undef values remain unknown.
else
LV.markConstant(Elt); // Constants are constant.
}
// All others are underdefined by default.
return LV;
}
/// markEdgeExecutable - Mark a basic block as executable, adding it to the BB
/// work list if it is not already executable.
bool markEdgeExecutable(BasicBlock *Source, BasicBlock *Dest) {
if (!KnownFeasibleEdges.insert(Edge(Source, Dest)).second)
return false; // This edge is already known to be executable!
if (!MarkBlockExecutable(Dest)) {
// If the destination is already executable, we just made an *edge*
// feasible that wasn't before. Revisit the PHI nodes in the block
// because they have potentially new operands.
LLVM_DEBUG(dbgs() << "Marking Edge Executable: " << Source->getName()
<< " -> " << Dest->getName() << '\n');
for (PHINode &PN : Dest->phis())
visitPHINode(PN);
}
return true;
}
// getFeasibleSuccessors - Return a vector of booleans to indicate which
// successors are reachable from a given terminator instruction.
void getFeasibleSuccessors(TerminatorInst &TI, SmallVectorImpl<bool> &Succs);
// OperandChangedState - This method is invoked on all of the users of an
// instruction that was just changed state somehow. Based on this
// information, we need to update the specified user of this instruction.
void OperandChangedState(Instruction *I) {
if (BBExecutable.count(I->getParent())) // Inst is executable?
visit(*I);
}
private:
friend class InstVisitor<SCCPSolver>;
// visit implementations - Something changed in this instruction. Either an
// operand made a transition, or the instruction is newly executable. Change
// the value type of I to reflect these changes if appropriate.
void visitPHINode(PHINode &I);
// Terminators
void visitReturnInst(ReturnInst &I);
void visitTerminatorInst(TerminatorInst &TI);
void visitCastInst(CastInst &I);
void visitSelectInst(SelectInst &I);
void visitBinaryOperator(Instruction &I);
void visitCmpInst(CmpInst &I);
void visitExtractValueInst(ExtractValueInst &EVI);
void visitInsertValueInst(InsertValueInst &IVI);
void visitCatchSwitchInst(CatchSwitchInst &CPI) {
markOverdefined(&CPI);
visitTerminatorInst(CPI);
}
// Instructions that cannot be folded away.
void visitStoreInst (StoreInst &I);
void visitLoadInst (LoadInst &I);
void visitGetElementPtrInst(GetElementPtrInst &I);
void visitCallInst (CallInst &I) {
visitCallSite(&I);
}
void visitInvokeInst (InvokeInst &II) {
visitCallSite(&II);
visitTerminatorInst(II);
}
void visitCallSite (CallSite CS);
void visitResumeInst (TerminatorInst &I) { /*returns void*/ }
void visitUnreachableInst(TerminatorInst &I) { /*returns void*/ }
void visitFenceInst (FenceInst &I) { /*returns void*/ }
void visitInstruction(Instruction &I) {
// All the instructions we don't do any special handling for just
// go to overdefined.
LLVM_DEBUG(dbgs() << "SCCP: Don't know how to handle: " << I << '\n');
markOverdefined(&I);
}
};
} // end anonymous namespace
// getFeasibleSuccessors - Return a vector of booleans to indicate which
// successors are reachable from a given terminator instruction.
void SCCPSolver::getFeasibleSuccessors(TerminatorInst &TI,
SmallVectorImpl<bool> &Succs) {
Succs.resize(TI.getNumSuccessors());
if (auto *BI = dyn_cast<BranchInst>(&TI)) {
if (BI->isUnconditional()) {
Succs[0] = true;
return;
}
LatticeVal BCValue = getValueState(BI->getCondition());
ConstantInt *CI = BCValue.getConstantInt();
if (!CI) {
// Overdefined condition variables, and branches on unfoldable constant
// conditions, mean the branch could go either way.
if (!BCValue.isUnknown())
Succs[0] = Succs[1] = true;
return;
}
// Constant condition variables mean the branch can only go a single way.
Succs[CI->isZero()] = true;
return;
}
// Unwinding instructions successors are always executable.
if (TI.isExceptional()) {
Succs.assign(TI.getNumSuccessors(), true);
return;
}
if (auto *SI = dyn_cast<SwitchInst>(&TI)) {
if (!SI->getNumCases()) {
Succs[0] = true;
return;
}
LatticeVal SCValue = getValueState(SI->getCondition());
ConstantInt *CI = SCValue.getConstantInt();
if (!CI) { // Overdefined or unknown condition?
// All destinations are executable!
if (!SCValue.isUnknown())
Succs.assign(TI.getNumSuccessors(), true);
return;
}
Succs[SI->findCaseValue(CI)->getSuccessorIndex()] = true;
return;
}
// In case of indirect branch and its address is a blockaddress, we mark
// the target as executable.
if (auto *IBR = dyn_cast<IndirectBrInst>(&TI)) {
// Casts are folded by visitCastInst.
LatticeVal IBRValue = getValueState(IBR->getAddress());
BlockAddress *Addr = IBRValue.getBlockAddress();
if (!Addr) { // Overdefined or unknown condition?
// All destinations are executable!
if (!IBRValue.isUnknown())
Succs.assign(TI.getNumSuccessors(), true);
return;
}
BasicBlock* T = Addr->getBasicBlock();
assert(Addr->getFunction() == T->getParent() &&
"Block address of a different function ?");
for (unsigned i = 0; i < IBR->getNumSuccessors(); ++i) {
// This is the target.
if (IBR->getDestination(i) == T) {
Succs[i] = true;
return;
}
}
// If we didn't find our destination in the IBR successor list, then we
// have undefined behavior. Its ok to assume no successor is executable.
return;
}
LLVM_DEBUG(dbgs() << "Unknown terminator instruction: " << TI << '\n');
llvm_unreachable("SCCP: Don't know how to handle this terminator!");
}
// isEdgeFeasible - Return true if the control flow edge from the 'From' basic
// block to the 'To' basic block is currently feasible.
bool SCCPSolver::isEdgeFeasible(BasicBlock *From, BasicBlock *To) {
// Check if we've called markEdgeExecutable on the edge yet. (We could
// be more aggressive and try to consider edges which haven't been marked
// yet, but there isn't any need.)
return KnownFeasibleEdges.count(Edge(From, To));
}
// visit Implementations - Something changed in this instruction, either an
// operand made a transition, or the instruction is newly executable. Change
// the value type of I to reflect these changes if appropriate. This method
// makes sure to do the following actions:
//
// 1. If a phi node merges two constants in, and has conflicting value coming
// from different branches, or if the PHI node merges in an overdefined
// value, then the PHI node becomes overdefined.
// 2. If a phi node merges only constants in, and they all agree on value, the
// PHI node becomes a constant value equal to that.
// 3. If V <- x (op) y && isConstant(x) && isConstant(y) V = Constant
// 4. If V <- x (op) y && (isOverdefined(x) || isOverdefined(y)) V = Overdefined
// 5. If V <- MEM or V <- CALL or V <- (unknown) then V = Overdefined
// 6. If a conditional branch has a value that is constant, make the selected
// destination executable
// 7. If a conditional branch has a value that is overdefined, make all
// successors executable.
void SCCPSolver::visitPHINode(PHINode &PN) {
// If this PN returns a struct, just mark the result overdefined.
// TODO: We could do a lot better than this if code actually uses this.
if (PN.getType()->isStructTy())
return (void)markOverdefined(&PN);
if (getValueState(&PN).isOverdefined())
return; // Quick exit
// Super-extra-high-degree PHI nodes are unlikely to ever be marked constant,
// and slow us down a lot. Just mark them overdefined.
if (PN.getNumIncomingValues() > 64)
return (void)markOverdefined(&PN);
// Look at all of the executable operands of the PHI node. If any of them
// are overdefined, the PHI becomes overdefined as well. If they are all
// constant, and they agree with each other, the PHI becomes the identical
// constant. If they are constant and don't agree, the PHI is overdefined.
// If there are no executable operands, the PHI remains unknown.
Constant *OperandVal = nullptr;
for (unsigned i = 0, e = PN.getNumIncomingValues(); i != e; ++i) {
LatticeVal IV = getValueState(PN.getIncomingValue(i));
if (IV.isUnknown()) continue; // Doesn't influence PHI node.
if (!isEdgeFeasible(PN.getIncomingBlock(i), PN.getParent()))
continue;
if (IV.isOverdefined()) // PHI node becomes overdefined!
return (void)markOverdefined(&PN);
if (!OperandVal) { // Grab the first value.
OperandVal = IV.getConstant();
continue;
}
// There is already a reachable operand. If we conflict with it,
// then the PHI node becomes overdefined. If we agree with it, we
// can continue on.
// Check to see if there are two different constants merging, if so, the PHI
// node is overdefined.
if (IV.getConstant() != OperandVal)
return (void)markOverdefined(&PN);
}
// If we exited the loop, this means that the PHI node only has constant
// arguments that agree with each other(and OperandVal is the constant) or
// OperandVal is null because there are no defined incoming arguments. If
// this is the case, the PHI remains unknown.
if (OperandVal)
markConstant(&PN, OperandVal); // Acquire operand value
}
void SCCPSolver::visitReturnInst(ReturnInst &I) {
if (I.getNumOperands() == 0) return; // ret void
Function *F = I.getParent()->getParent();
Value *ResultOp = I.getOperand(0);
// If we are tracking the return value of this function, merge it in.
if (!TrackedRetVals.empty() && !ResultOp->getType()->isStructTy()) {
DenseMap<Function*, LatticeVal>::iterator TFRVI =
TrackedRetVals.find(F);
if (TFRVI != TrackedRetVals.end()) {
mergeInValue(TFRVI->second, F, getValueState(ResultOp));
return;
}
}
// Handle functions that return multiple values.
if (!TrackedMultipleRetVals.empty()) {
if (auto *STy = dyn_cast<StructType>(ResultOp->getType()))
if (MRVFunctionsTracked.count(F))
for (unsigned i = 0, e = STy->getNumElements(); i != e; ++i)
mergeInValue(TrackedMultipleRetVals[std::make_pair(F, i)], F,
getStructValueState(ResultOp, i));
}
}
void SCCPSolver::visitTerminatorInst(TerminatorInst &TI) {
SmallVector<bool, 16> SuccFeasible;
getFeasibleSuccessors(TI, SuccFeasible);
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));
}
void SCCPSolver::visitCastInst(CastInst &I) {
LatticeVal OpSt = getValueState(I.getOperand(0));
if (OpSt.isOverdefined()) // Inherit overdefinedness of operand
markOverdefined(&I);
else if (OpSt.isConstant()) {
// Fold the constant as we build.
Constant *C = ConstantFoldCastOperand(I.getOpcode(), OpSt.getConstant(),
I.getType(), DL);
if (isa<UndefValue>(C))
return;
// Propagate constant value
markConstant(&I, C);
}
}
void SCCPSolver::visitExtractValueInst(ExtractValueInst &EVI) {
// If this returns a struct, mark all elements over defined, we don't track
// structs in structs.
if (EVI.getType()->isStructTy())
return (void)markOverdefined(&EVI);
// If this is extracting from more than one level of struct, we don't know.
if (EVI.getNumIndices() != 1)
return (void)markOverdefined(&EVI);
Value *AggVal = EVI.getAggregateOperand();
if (AggVal->getType()->isStructTy()) {
unsigned i = *EVI.idx_begin();
LatticeVal EltVal = getStructValueState(AggVal, i);
mergeInValue(getValueState(&EVI), &EVI, EltVal);
} else {
// Otherwise, must be extracting from an array.
return (void)markOverdefined(&EVI);
}
}
void SCCPSolver::visitInsertValueInst(InsertValueInst &IVI) {
auto *STy = dyn_cast<StructType>(IVI.getType());
if (!STy)
return (void)markOverdefined(&IVI);
// If this has more than one index, we can't handle it, drive all results to
// undef.
if (IVI.getNumIndices() != 1)
return (void)markOverdefined(&IVI);
Value *Aggr = IVI.getAggregateOperand();
unsigned Idx = *IVI.idx_begin();
// Compute the result based on what we're inserting.
for (unsigned i = 0, e = STy->getNumElements(); i != e; ++i) {
// This passes through all values that aren't the inserted element.
if (i != Idx) {
LatticeVal EltVal = getStructValueState(Aggr, i);
mergeInValue(getStructValueState(&IVI, i), &IVI, EltVal);
continue;
}
Value *Val = IVI.getInsertedValueOperand();
if (Val->getType()->isStructTy())
// We don't track structs in structs.
markOverdefined(getStructValueState(&IVI, i), &IVI);
else {
LatticeVal InVal = getValueState(Val);
mergeInValue(getStructValueState(&IVI, i), &IVI, InVal);
}
}
}
void SCCPSolver::visitSelectInst(SelectInst &I) {
// If this select returns a struct, just mark the result overdefined.
// TODO: We could do a lot better than this if code actually uses this.
if (I.getType()->isStructTy())
return (void)markOverdefined(&I);
LatticeVal CondValue = getValueState(I.getCondition());
if (CondValue.isUnknown())
return;
if (ConstantInt *CondCB = CondValue.getConstantInt()) {
Value *OpVal = CondCB->isZero() ? I.getFalseValue() : I.getTrueValue();
mergeInValue(&I, getValueState(OpVal));
return;
}
// Otherwise, the condition is overdefined or a constant we can't evaluate.
// See if we can produce something better than overdefined based on the T/F
// value.
LatticeVal TVal = getValueState(I.getTrueValue());
LatticeVal FVal = getValueState(I.getFalseValue());
// select ?, C, C -> C.
if (TVal.isConstant() && FVal.isConstant() &&
TVal.getConstant() == FVal.getConstant())
return (void)markConstant(&I, FVal.getConstant());
if (TVal.isUnknown()) // select ?, undef, X -> X.
return (void)mergeInValue(&I, FVal);
if (FVal.isUnknown()) // select ?, X, undef -> X.
return (void)mergeInValue(&I, TVal);
markOverdefined(&I);
}
// Handle Binary Operators.
void SCCPSolver::visitBinaryOperator(Instruction &I) {
LatticeVal V1State = getValueState(I.getOperand(0));
LatticeVal V2State = getValueState(I.getOperand(1));
LatticeVal &IV = ValueState[&I];
if (IV.isOverdefined()) return;
if (V1State.isConstant() && V2State.isConstant()) {
Constant *C = ConstantExpr::get(I.getOpcode(), V1State.getConstant(),
V2State.getConstant());
// X op Y -> undef.
if (isa<UndefValue>(C))
return;
return (void)markConstant(IV, &I, C);
}
// If something is undef, wait for it to resolve.
if (!V1State.isOverdefined() && !V2State.isOverdefined())
return;
// Otherwise, one of our operands is overdefined. Try to produce something
// better than overdefined with some tricks.
// If this is 0 / Y, it doesn't matter that the second operand is
// overdefined, and we can replace it with zero.
if (I.getOpcode() == Instruction::UDiv || I.getOpcode() == Instruction::SDiv)
if (V1State.isConstant() && V1State.getConstant()->isNullValue())
return (void)markConstant(IV, &I, V1State.getConstant());
// If this is:
// -> AND/MUL with 0
// -> OR with -1
// it doesn't matter that the other operand is overdefined.
if (I.getOpcode() == Instruction::And || I.getOpcode() == Instruction::Mul ||
I.getOpcode() == Instruction::Or) {
LatticeVal *NonOverdefVal = nullptr;
if (!V1State.isOverdefined())
NonOverdefVal = &V1State;
else if (!V2State.isOverdefined())
NonOverdefVal = &V2State;
if (NonOverdefVal) {
if (NonOverdefVal->isUnknown())
return;
if (I.getOpcode() == Instruction::And ||
I.getOpcode() == Instruction::Mul) {
// X and 0 = 0
// X * 0 = 0
if (NonOverdefVal->getConstant()->isNullValue())
return (void)markConstant(IV, &I, NonOverdefVal->getConstant());
} else {
// X or -1 = -1
if (ConstantInt *CI = NonOverdefVal->getConstantInt())
if (CI->isMinusOne())
return (void)markConstant(IV, &I, NonOverdefVal->getConstant());
}
}
}
markOverdefined(&I);
}
// Handle ICmpInst instruction.
void SCCPSolver::visitCmpInst(CmpInst &I) {
LatticeVal &IV = ValueState[&I];
if (IV.isOverdefined()) return;
Value *Op1 = I.getOperand(0);
Value *Op2 = I.getOperand(1);
// For parameters, use ParamState which includes constant range info if
// available.
auto V1Param = ParamState.find(Op1);
ValueLatticeElement V1State = (V1Param != ParamState.end())
? V1Param->second
: getValueState(Op1).toValueLattice();
auto V2Param = ParamState.find(Op2);
ValueLatticeElement V2State = V2Param != ParamState.end()
? V2Param->second
: getValueState(Op2).toValueLattice();
Constant *C = V1State.getCompare(I.getPredicate(), I.getType(), V2State);
if (C) {
if (isa<UndefValue>(C))
return;
LatticeVal CV;
CV.markConstant(C);
mergeInValue(&I, CV);
return;
}
// If operands are still unknown, wait for it to resolve.
if (!V1State.isOverdefined() && !V2State.isOverdefined() && !IV.isConstant())
return;
markOverdefined(&I);
}
// Handle getelementptr instructions. If all operands are constants then we
// can turn this into a getelementptr ConstantExpr.
void SCCPSolver::visitGetElementPtrInst(GetElementPtrInst &I) {
if (ValueState[&I].isOverdefined()) return;
SmallVector<Constant*, 8> Operands;
Operands.reserve(I.getNumOperands());
for (unsigned i = 0, e = I.getNumOperands(); i != e; ++i) {
LatticeVal State = getValueState(I.getOperand(i));
if (State.isUnknown())
return; // Operands are not resolved yet.
if (State.isOverdefined())
return (void)markOverdefined(&I);
assert(State.isConstant() && "Unknown state!");
Operands.push_back(State.getConstant());
}
Constant *Ptr = Operands[0];
auto Indices = makeArrayRef(Operands.begin() + 1, Operands.end());
Constant *C =
ConstantExpr::getGetElementPtr(I.getSourceElementType(), Ptr, Indices);
if (isa<UndefValue>(C))
return;
markConstant(&I, C);
}
void SCCPSolver::visitStoreInst(StoreInst &SI) {
// If this store is of a struct, ignore it.
if (SI.getOperand(0)->getType()->isStructTy())
return;
if (TrackedGlobals.empty() || !isa<GlobalVariable>(SI.getOperand(1)))
return;
GlobalVariable *GV = cast<GlobalVariable>(SI.getOperand(1));
DenseMap<GlobalVariable*, LatticeVal>::iterator I = TrackedGlobals.find(GV);
if (I == TrackedGlobals.end() || I->second.isOverdefined()) return;
// Get the value we are storing into the global, then merge it.
mergeInValue(I->second, GV, getValueState(SI.getOperand(0)));
if (I->second.isOverdefined())
TrackedGlobals.erase(I); // No need to keep tracking this!
}
// Handle load instructions. If the operand is a constant pointer to a constant
// global, we can replace the load with the loaded constant value!
void SCCPSolver::visitLoadInst(LoadInst &I) {
// If this load is of a struct, just mark the result overdefined.
if (I.getType()->isStructTy())
return (void)markOverdefined(&I);
LatticeVal PtrVal = getValueState(I.getOperand(0));
if (PtrVal.isUnknown()) return; // The pointer is not resolved yet!
LatticeVal &IV = ValueState[&I];
if (IV.isOverdefined()) return;
if (!PtrVal.isConstant() || I.isVolatile())
return (void)markOverdefined(IV, &I);
Constant *Ptr = PtrVal.getConstant();
// load null is undefined.
if (isa<ConstantPointerNull>(Ptr)) {
if (NullPointerIsDefined(I.getFunction(), I.getPointerAddressSpace()))
return (void)markOverdefined(IV, &I);
else
return;
}
// Transform load (constant global) into the value loaded.
if (auto *GV = dyn_cast<GlobalVariable>(Ptr)) {
if (!TrackedGlobals.empty()) {
// If we are tracking this global, merge in the known value for it.
DenseMap<GlobalVariable*, LatticeVal>::iterator It =
TrackedGlobals.find(GV);
if (It != TrackedGlobals.end()) {
mergeInValue(IV, &I, It->second);
return;
}
}
}
// Transform load from a constant into a constant if possible.
if (Constant *C = ConstantFoldLoadFromConstPtr(Ptr, I.getType(), DL)) {
if (isa<UndefValue>(C))
return;
return (void)markConstant(IV, &I, C);
}
// Otherwise we cannot say for certain what value this load will produce.
// Bail out.
markOverdefined(IV, &I);
}
void SCCPSolver::visitCallSite(CallSite CS) {
Function *F = CS.getCalledFunction();
Instruction *I = CS.getInstruction();
// The common case is that we aren't tracking the callee, either because we
// are not doing interprocedural analysis or the callee is indirect, or is
// external. Handle these cases first.
if (!F || F->isDeclaration()) {
CallOverdefined:
// Void return and not tracking callee, just bail.
if (I->getType()->isVoidTy()) return;
// Otherwise, if we have a single return value case, and if the function is
// a declaration, maybe we can constant fold it.
if (F && F->isDeclaration() && !I->getType()->isStructTy() &&
canConstantFoldCallTo(CS, F)) {
SmallVector<Constant*, 8> Operands;
for (CallSite::arg_iterator AI = CS.arg_begin(), E = CS.arg_end();
AI != E; ++AI) {
LatticeVal State = getValueState(*AI);
if (State.isUnknown())
return; // Operands are not resolved yet.
if (State.isOverdefined())
return (void)markOverdefined(I);
assert(State.isConstant() && "Unknown state!");
Operands.push_back(State.getConstant());
}
if (getValueState(I).isOverdefined())
return;
// If we can constant fold this, mark the result of the call as a
// constant.
if (Constant *C = ConstantFoldCall(CS, F, Operands, TLI)) {
// call -> undef.
if (isa<UndefValue>(C))
return;
return (void)markConstant(I, C);
}
}
// Otherwise, we don't know anything about this call, mark it overdefined.
return (void)markOverdefined(I);
}
// If this is a local function that doesn't have its address taken, mark its
// entry block executable and merge in the actual arguments to the call into
// the formal arguments of the function.
if (!TrackingIncomingArguments.empty() && TrackingIncomingArguments.count(F)){
MarkBlockExecutable(&F->front());
// Propagate information from this call site into the callee.
CallSite::arg_iterator CAI = CS.arg_begin();
for (Function::arg_iterator AI = F->arg_begin(), E = F->arg_end();
AI != E; ++AI, ++CAI) {
// If this argument is byval, and if the function is not readonly, there
// will be an implicit copy formed of the input aggregate.
if (AI->hasByValAttr() && !F->onlyReadsMemory()) {
markOverdefined(&*AI);
continue;
}
if (auto *STy = dyn_cast<StructType>(AI->getType())) {
for (unsigned i = 0, e = STy->getNumElements(); i != e; ++i) {
LatticeVal CallArg = getStructValueState(*CAI, i);
mergeInValue(getStructValueState(&*AI, i), &*AI, CallArg);
}
} else {
// Most other parts of the Solver still only use the simpler value
// lattice, so we propagate changes for parameters to both lattices.
LatticeVal ConcreteArgument = getValueState(*CAI);
bool ParamChanged =
getParamState(&*AI).mergeIn(ConcreteArgument.toValueLattice(), DL);
bool ValueChanged = mergeInValue(&*AI, ConcreteArgument);
// Add argument to work list, if the state of a parameter changes but
// ValueState does not change (because it is already overdefined there),
// We have to take changes in ParamState into account, as it is used
// when evaluating Cmp instructions.
if (!ValueChanged && ParamChanged)
pushToWorkList(ValueState[&*AI], &*AI);
}
}
}
// If this is a single/zero retval case, see if we're tracking the function.
if (auto *STy = dyn_cast<StructType>(F->getReturnType())) {
if (!MRVFunctionsTracked.count(F))
goto CallOverdefined; // Not tracking this callee.
// If we are tracking this callee, propagate the result of the function
// into this call site.
for (unsigned i = 0, e = STy->getNumElements(); i != e; ++i)
mergeInValue(getStructValueState(I, i), I,
TrackedMultipleRetVals[std::make_pair(F, i)]);
} else {
DenseMap<Function*, LatticeVal>::iterator TFRVI = TrackedRetVals.find(F);
if (TFRVI == TrackedRetVals.end())
goto CallOverdefined; // Not tracking this callee.
// If so, propagate the return value of the callee into this call result.
mergeInValue(I, TFRVI->second);
}
}
void SCCPSolver::Solve() {
// Process the work lists until they are empty!
while (!BBWorkList.empty() || !InstWorkList.empty() ||
!OverdefinedInstWorkList.empty()) {
// Process the overdefined instruction's work list first, which drives other
// things to overdefined more quickly.
while (!OverdefinedInstWorkList.empty()) {
Value *I = OverdefinedInstWorkList.pop_back_val();
LLVM_DEBUG(dbgs() << "\nPopped off OI-WL: " << *I << '\n');
// "I" got into the work list because it either made the transition from
// bottom to constant, or to overdefined.
//
// Anything on this worklist that is overdefined need not be visited
// since all of its users will have already been marked as overdefined
// Update all of the users of this instruction's value.
//
for (User *U : I->users())
if (auto *UI = dyn_cast<Instruction>(U))
OperandChangedState(UI);
}
// Process the instruction work list.
while (!InstWorkList.empty()) {
Value *I = InstWorkList.pop_back_val();
LLVM_DEBUG(dbgs() << "\nPopped off I-WL: " << *I << '\n');
// "I" got into the work list because it made the transition from undef to
// constant.
//
// Anything on this worklist that is overdefined need not be visited
// since all of its users will have already been marked as overdefined.
// Update all of the users of this instruction's value.
//
if (I->getType()->isStructTy() || !getValueState(I).isOverdefined())
for (User *U : I->users())
if (auto *UI = dyn_cast<Instruction>(U))
OperandChangedState(UI);
}
// Process the basic block work list.
while (!BBWorkList.empty()) {
BasicBlock *BB = BBWorkList.back();
BBWorkList.pop_back();
LLVM_DEBUG(dbgs() << "\nPopped off BBWL: " << *BB << '\n');
// Notify all instructions in this basic block that they are newly
// executable.
visit(BB);
}
}
}
/// ResolvedUndefsIn - While solving the dataflow for a function, we assume
/// that branches on undef values cannot reach any of their successors.
/// However, this is not a safe assumption. After we solve dataflow, this
/// method should be use to handle this. If this returns true, the solver
/// should be rerun.
///
/// This method handles this by finding an unresolved branch and marking it one
/// of the edges from the block as being feasible, even though the condition
/// doesn't say it would otherwise be. This allows SCCP to find the rest of the
/// CFG and only slightly pessimizes the analysis results (by marking one,
/// potentially infeasible, edge feasible). This cannot usefully modify the
/// constraints on the condition of the branch, as that would impact other users
/// of the value.
///
/// This scan also checks for values that use undefs, whose results are actually
/// defined. For example, 'zext i8 undef to i32' should produce all zeros
/// conservatively, as "(zext i8 X -> i32) & 0xFF00" must always return zero,
/// even if X isn't defined.
bool SCCPSolver::ResolvedUndefsIn(Function &F) {
for (BasicBlock &BB : F) {
if (!BBExecutable.count(&BB))
continue;
for (Instruction &I : BB) {
// Look for instructions which produce undef values.
if (I.getType()->isVoidTy()) continue;
if (auto *STy = dyn_cast<StructType>(I.getType())) {
// Only a few things that can be structs matter for undef.
// Tracked calls must never be marked overdefined in ResolvedUndefsIn.
if (CallSite CS = CallSite(&I))
if (Function *F = CS.getCalledFunction())
if (MRVFunctionsTracked.count(F))
continue;
// extractvalue and insertvalue don't need to be marked; they are
// tracked as precisely as their operands.
if (isa<ExtractValueInst>(I) || isa<InsertValueInst>(I))
continue;
// Send the results of everything else to overdefined. We could be
// more precise than this but it isn't worth bothering.
for (unsigned i = 0, e = STy->getNumElements(); i != e; ++i) {
LatticeVal &LV = getStructValueState(&I, i);
if (LV.isUnknown())
markOverdefined(LV, &I);
}
continue;
}
LatticeVal &LV = getValueState(&I);
if (!LV.isUnknown()) continue;
// extractvalue is safe; check here because the argument is a struct.
if (isa<ExtractValueInst>(I))
continue;
// Compute the operand LatticeVals, for convenience below.
// Anything taking a struct is conservatively assumed to require
// overdefined markings.
if (I.getOperand(0)->getType()->isStructTy()) {
markOverdefined(&I);
return true;
}
LatticeVal Op0LV = getValueState(I.getOperand(0));
LatticeVal Op1LV;
if (I.getNumOperands() == 2) {
if (I.getOperand(1)->getType()->isStructTy()) {
markOverdefined(&I);
return true;
}
Op1LV = getValueState(I.getOperand(1));
}
// If this is an instructions whose result is defined even if the input is
// not fully defined, propagate the information.
Type *ITy = I.getType();
switch (I.getOpcode()) {
case Instruction::Add:
case Instruction::Sub:
case Instruction::Trunc:
case Instruction::FPTrunc:
case Instruction::BitCast:
break; // Any undef -> undef
case Instruction::FSub:
case Instruction::FAdd:
case Instruction::FMul:
case Instruction::FDiv:
case Instruction::FRem:
// Floating-point binary operation: be conservative.
if (Op0LV.isUnknown() && Op1LV.isUnknown())
markForcedConstant(&I, Constant::getNullValue(ITy));
else
markOverdefined(&I);
return true;
case Instruction::ZExt:
case Instruction::SExt:
case Instruction::FPToUI:
case Instruction::FPToSI:
case Instruction::FPExt:
case Instruction::PtrToInt:
case Instruction::IntToPtr:
case Instruction::SIToFP:
case Instruction::UIToFP:
// undef -> 0; some outputs are impossible
markForcedConstant(&I, Constant::getNullValue(ITy));
return true;
case Instruction::Mul:
case Instruction::And:
// Both operands undef -> undef
if (Op0LV.isUnknown() && Op1LV.isUnknown())
break;
// undef * X -> 0. X could be zero.
// undef & X -> 0. X could be zero.
markForcedConstant(&I, Constant::getNullValue(ITy));
return true;
case Instruction::Or:
// Both operands undef -> undef
if (Op0LV.isUnknown() && Op1LV.isUnknown())
break;
// undef | X -> -1. X could be -1.
markForcedConstant(&I, Constant::getAllOnesValue(ITy));
return true;
case Instruction::Xor:
// undef ^ undef -> 0; strictly speaking, this is not strictly
// necessary, but we try to be nice to people who expect this
// behavior in simple cases
if (Op0LV.isUnknown() && Op1LV.isUnknown()) {
markForcedConstant(&I, Constant::getNullValue(ITy));
return true;
}
// undef ^ X -> undef
break;
case Instruction::SDiv:
case Instruction::UDiv:
case Instruction::SRem:
case Instruction::URem:
// X / undef -> undef. No change.
// X % undef -> undef. No change.
if (Op1LV.isUnknown()) break;
// X / 0 -> undef. No change.
// X % 0 -> undef. No change.
if (Op1LV.isConstant() && Op1LV.getConstant()->isZeroValue())
break;
// undef / X -> 0. X could be maxint.
// undef % X -> 0. X could be 1.
markForcedConstant(&I, Constant::getNullValue(ITy));
return true;
case Instruction::AShr:
// X >>a undef -> undef.
if (Op1LV.isUnknown()) break;
// Shifting by the bitwidth or more is undefined.
if (Op1LV.isConstant()) {
if (auto *ShiftAmt = Op1LV.getConstantInt())
if (ShiftAmt->getLimitedValue() >=
ShiftAmt->getType()->getScalarSizeInBits())
break;
}
// undef >>a X -> 0
markForcedConstant(&I, Constant::getNullValue(ITy));
return true;
case Instruction::LShr:
case Instruction::Shl:
// X << undef -> undef.
// X >> undef -> undef.
if (Op1LV.isUnknown()) break;
// Shifting by the bitwidth or more is undefined.
if (Op1LV.isConstant()) {
if (auto *ShiftAmt = Op1LV.getConstantInt())
if (ShiftAmt->getLimitedValue() >=
ShiftAmt->getType()->getScalarSizeInBits())
break;
}
// undef << X -> 0
// undef >> X -> 0
markForcedConstant(&I, Constant::getNullValue(ITy));
return true;
case Instruction::Select:
Op1LV = getValueState(I.getOperand(1));
// undef ? X : Y -> X or Y. There could be commonality between X/Y.
if (Op0LV.isUnknown()) {
if (!Op1LV.isConstant()) // Pick the constant one if there is any.
Op1LV = getValueState(I.getOperand(2));
} else if (Op1LV.isUnknown()) {
// c ? undef : undef -> undef. No change.
Op1LV = getValueState(I.getOperand(2));
if (Op1LV.isUnknown())
break;
// Otherwise, c ? undef : x -> x.
} else {
// Leave Op1LV as Operand(1)'s LatticeValue.
}
if (Op1LV.isConstant())
markForcedConstant(&I, Op1LV.getConstant());
else
markOverdefined(&I);
return true;
case Instruction::Load:
// A load here means one of two things: a load of undef from a global,
// a load from an unknown pointer. Either way, having it return undef
// is okay.
break;
case Instruction::ICmp:
// X == undef -> undef. Other comparisons get more complicated.
Op0LV = getValueState(I.getOperand(0));
Op1LV = getValueState(I.getOperand(1));
if ((Op0LV.isUnknown() || Op1LV.isUnknown()) &&
cast<ICmpInst>(&I)->isEquality())
break;
markOverdefined(&I);
return true;
case Instruction::Call:
case Instruction::Invoke:
// There are two reasons a call can have an undef result
// 1. It could be tracked.
// 2. It could be constant-foldable.
// Because of the way we solve return values, tracked calls must
// never be marked overdefined in ResolvedUndefsIn.
if (Function *F = CallSite(&I).getCalledFunction())
if (TrackedRetVals.count(F))
break;
// If the call is constant-foldable, we mark it overdefined because
// we do not know what return values are valid.
markOverdefined(&I);
return true;
default:
// If we don't know what should happen here, conservatively mark it
// overdefined.
markOverdefined(&I);
return true;
}
}
// Check to see if we have a branch or switch on an undefined value. If so
// we force the branch to go one way or the other to make the successor
// values live. It doesn't really matter which way we force it.
TerminatorInst *TI = BB.getTerminator();
if (auto *BI = dyn_cast<BranchInst>(TI)) {
if (!BI->isConditional()) continue;
if (!getValueState(BI->getCondition()).isUnknown())
continue;
// If the input to SCCP is actually branch on undef, fix the undef to
// false.
if (isa<UndefValue>(BI->getCondition())) {
BI->setCondition(ConstantInt::getFalse(BI->getContext()));
markEdgeExecutable(&BB, TI->getSuccessor(1));
return true;
}
// Otherwise, it is a branch on a symbolic value which is currently
// considered to be undef. Make sure some edge is executable, so a
// branch on "undef" always flows somewhere.
// FIXME: Distinguish between dead code and an LLVM "undef" value.
BasicBlock *DefaultSuccessor = TI->getSuccessor(1);
if (markEdgeExecutable(&BB, DefaultSuccessor))
return true;
continue;
}
if (auto *IBR = dyn_cast<IndirectBrInst>(TI)) {
// Indirect branch with no successor ?. Its ok to assume it branches
// to no target.
if (IBR->getNumSuccessors() < 1)
continue;
if (!getValueState(IBR->getAddress()).isUnknown())
continue;
// If the input to SCCP is actually branch on undef, fix the undef to
// the first successor of the indirect branch.
if (isa<UndefValue>(IBR->getAddress())) {
IBR->setAddress(BlockAddress::get(IBR->getSuccessor(0)));
markEdgeExecutable(&BB, IBR->getSuccessor(0));
return true;
}
// Otherwise, it is a branch on a symbolic value which is currently
// considered to be undef. Make sure some edge is executable, so a
// branch on "undef" always flows somewhere.
// FIXME: IndirectBr on "undef" doesn't actually need to go anywhere:
// we can assume the branch has undefined behavior instead.
BasicBlock *DefaultSuccessor = IBR->getSuccessor(0);
if (markEdgeExecutable(&BB, DefaultSuccessor))
return true;
continue;
}
if (auto *SI = dyn_cast<SwitchInst>(TI)) {
if (!SI->getNumCases() || !getValueState(SI->getCondition()).isUnknown())
continue;
// If the input to SCCP is actually switch on undef, fix the undef to
// the first constant.
if (isa<UndefValue>(SI->getCondition())) {
SI->setCondition(SI->case_begin()->getCaseValue());
markEdgeExecutable(&BB, SI->case_begin()->getCaseSuccessor());
return true;
}
// Otherwise, it is a branch on a symbolic value which is currently
// considered to be undef. Make sure some edge is executable, so a
// branch on "undef" always flows somewhere.
// FIXME: Distinguish between dead code and an LLVM "undef" value.
BasicBlock *DefaultSuccessor = SI->case_begin()->getCaseSuccessor();
if (markEdgeExecutable(&BB, DefaultSuccessor))
return true;
continue;
}
}
return false;
}
static bool tryToReplaceWithConstant(SCCPSolver &Solver, Value *V) {
Constant *Const = nullptr;
if (V->getType()->isStructTy()) {
std::vector<LatticeVal> IVs = Solver.getStructLatticeValueFor(V);
if (llvm::any_of(IVs,
[](const LatticeVal &LV) { return LV.isOverdefined(); }))
return false;
std::vector<Constant *> ConstVals;
auto *ST = dyn_cast<StructType>(V->getType());
for (unsigned i = 0, e = ST->getNumElements(); i != e; ++i) {
LatticeVal V = IVs[i];
ConstVals.push_back(V.isConstant()
? V.getConstant()
: UndefValue::get(ST->getElementType(i)));
}
Const = ConstantStruct::get(ST, ConstVals);
} else {
const LatticeVal &IV = Solver.getLatticeValueFor(V);
if (IV.isOverdefined())
return false;
Const = IV.isConstant() ? IV.getConstant() : UndefValue::get(V->getType());
}
assert(Const && "Constant is nullptr here!");
// Replacing `musttail` instructions with constant breaks `musttail` invariant
// unless the call itself can be removed
CallInst *CI = dyn_cast<CallInst>(V);
if (CI && CI->isMustTailCall() && !CI->isSafeToRemove()) {
CallSite CS(CI);
Function *F = CS.getCalledFunction();
// Don't zap returns of the callee
if (F)
Solver.AddMustTailCallee(F);
LLVM_DEBUG(dbgs() << " Can\'t treat the result of musttail call : " << *CI
<< " as a constant\n");
return false;
}
LLVM_DEBUG(dbgs() << " Constant: " << *Const << " = " << *V << '\n');
// Replaces all of the uses of a variable with uses of the constant.
V->replaceAllUsesWith(Const);
return true;
}
// runSCCP() - Run the Sparse Conditional Constant Propagation algorithm,
// and return true if the function was modified.
static bool runSCCP(Function &F, const DataLayout &DL,
const TargetLibraryInfo *TLI) {
LLVM_DEBUG(dbgs() << "SCCP on function '" << F.getName() << "'\n");
SCCPSolver Solver(DL, TLI);
// Mark the first block of the function as being executable.
Solver.MarkBlockExecutable(&F.front());
// Mark all arguments to the function as being overdefined.
for (Argument &AI : F.args())
Solver.markOverdefined(&AI);
// Solve for constants.
bool ResolvedUndefs = true;
while (ResolvedUndefs) {
Solver.Solve();
LLVM_DEBUG(dbgs() << "RESOLVING UNDEFs\n");
ResolvedUndefs = Solver.ResolvedUndefsIn(F);
}
bool MadeChanges = false;
// If we decided that there are basic blocks that are dead in this function,
// delete their contents now. Note that we cannot actually delete the blocks,
// as we cannot modify the CFG of the function.
for (BasicBlock &BB : F) {
if (!Solver.isBlockExecutable(&BB)) {
LLVM_DEBUG(dbgs() << " BasicBlock Dead:" << BB);
++NumDeadBlocks;
NumInstRemoved += removeAllNonTerminatorAndEHPadInstructions(&BB);
MadeChanges = true;
continue;
}
// Iterate over all of the instructions in a function, replacing them with
// constants if we have found them to be of constant values.
for (BasicBlock::iterator BI = BB.begin(), E = BB.end(); BI != E;) {
Instruction *Inst = &*BI++;
if (Inst->getType()->isVoidTy() || isa<TerminatorInst>(Inst))
continue;
if (tryToReplaceWithConstant(Solver, Inst)) {
if (isInstructionTriviallyDead(Inst))
Inst->eraseFromParent();
// Hey, we just changed something!
MadeChanges = true;
++NumInstRemoved;
}
}
}
return MadeChanges;
}
PreservedAnalyses SCCPPass::run(Function &F, FunctionAnalysisManager &AM) {
const DataLayout &DL = F.getParent()->getDataLayout();
auto &TLI = AM.getResult<TargetLibraryAnalysis>(F);
if (!runSCCP(F, DL, &TLI))
return PreservedAnalyses::all();
auto PA = PreservedAnalyses();
PA.preserve<GlobalsAA>();
PA.preserveSet<CFGAnalyses>();
return PA;
}
namespace {
//===--------------------------------------------------------------------===//
//
/// SCCP Class - This class uses the SCCPSolver to implement a per-function
/// Sparse Conditional Constant Propagator.
///
class SCCPLegacyPass : public FunctionPass {
public:
// Pass identification, replacement for typeid
static char ID;
SCCPLegacyPass() : FunctionPass(ID) {
initializeSCCPLegacyPassPass(*PassRegistry::getPassRegistry());
}
void getAnalysisUsage(AnalysisUsage &AU) const override {
AU.addRequired<TargetLibraryInfoWrapperPass>();
AU.addPreserved<GlobalsAAWrapperPass>();
AU.setPreservesCFG();
}
// runOnFunction - Run the Sparse Conditional Constant Propagation
// algorithm, and return true if the function was modified.
bool runOnFunction(Function &F) override {
if (skipFunction(F))
return false;
const DataLayout &DL = F.getParent()->getDataLayout();
const TargetLibraryInfo *TLI =
&getAnalysis<TargetLibraryInfoWrapperPass>().getTLI();
return runSCCP(F, DL, TLI);
}
};
} // end anonymous namespace
char SCCPLegacyPass::ID = 0;
INITIALIZE_PASS_BEGIN(SCCPLegacyPass, "sccp",
"Sparse Conditional Constant Propagation", false, false)
INITIALIZE_PASS_DEPENDENCY(TargetLibraryInfoWrapperPass)
INITIALIZE_PASS_END(SCCPLegacyPass, "sccp",
"Sparse Conditional Constant Propagation", false, false)
// createSCCPPass - This is the public interface to this file.
FunctionPass *llvm::createSCCPPass() { return new SCCPLegacyPass(); }
static void findReturnsToZap(Function &F,
SmallVector<ReturnInst *, 8> &ReturnsToZap,
SCCPSolver &Solver) {
// We can only do this if we know that nothing else can call the function.
if (!Solver.isArgumentTrackedFunction(&F))
return;
// There is a non-removable musttail call site of this function. Zapping
// returns is not allowed.
if (Solver.isMustTailCallee(&F)) {
LLVM_DEBUG(dbgs() << "Can't zap returns of the function : " << F.getName()
<< " due to present musttail call of it\n");
return;
}
for (BasicBlock &BB : F) {
if (CallInst *CI = BB.getTerminatingMustTailCall()) {
LLVM_DEBUG(dbgs() << "Can't zap return of the block due to present "
<< "musttail call : " << *CI << "\n");
(void)CI;
return;
}
if (auto *RI = dyn_cast<ReturnInst>(BB.getTerminator()))
if (!isa<UndefValue>(RI->getOperand(0)))
ReturnsToZap.push_back(RI);
}
}
bool llvm::runIPSCCP(Module &M, const DataLayout &DL,
const TargetLibraryInfo *TLI) {
SCCPSolver Solver(DL, TLI);
// Loop over all functions, marking arguments to those with their addresses
// taken or that are external as overdefined.
for (Function &F : M) {
if (F.isDeclaration())
continue;
// Determine if we can track the function's return values. If so, add the
// function to the solver's set of return-tracked functions.
if (canTrackReturnsInterprocedurally(&F))
Solver.AddTrackedFunction(&F);
// Determine if we can track the function's arguments. If so, add the
// function to the solver's set of argument-tracked functions.
if (canTrackArgumentsInterprocedurally(&F)) {
Solver.AddArgumentTrackedFunction(&F);
continue;
}
// Assume the function is called.
Solver.MarkBlockExecutable(&F.front());
// Assume nothing about the incoming arguments.
for (Argument &AI : F.args())
Solver.markOverdefined(&AI);
}
// Determine if we can track any of the module's global variables. If so, add
// the global variables we can track to the solver's set of tracked global
// variables.
for (GlobalVariable &G : M.globals()) {
G.removeDeadConstantUsers();
if (canTrackGlobalVariableInterprocedurally(&G))
Solver.TrackValueOfGlobalVariable(&G);
}
// Solve for constants.
bool ResolvedUndefs = true;
Solver.Solve();
while (ResolvedUndefs) {
LLVM_DEBUG(dbgs() << "RESOLVING UNDEFS\n");
ResolvedUndefs = false;
for (Function &F : M)
if (Solver.ResolvedUndefsIn(F)) {
// We run Solve() after we resolved an undef in a function, because
// we might deduce a fact that eliminates an undef in another function.
Solver.Solve();
ResolvedUndefs = true;
}
}
bool MadeChanges = false;
// Iterate over all of the instructions in the module, replacing them with
// constants if we have found them to be of constant values.
SmallVector<BasicBlock*, 512> BlocksToErase;
for (Function &F : M) {
if (F.isDeclaration())
continue;
if (Solver.isBlockExecutable(&F.front()))
for (Function::arg_iterator AI = F.arg_begin(), E = F.arg_end(); AI != E;
++AI) {
if (!AI->use_empty() && tryToReplaceWithConstant(Solver, &*AI)) {
++IPNumArgsElimed;
continue;
}
}
for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB) {
if (!Solver.isBlockExecutable(&*BB)) {
LLVM_DEBUG(dbgs() << " BasicBlock Dead:" << *BB);
++NumDeadBlocks;
MadeChanges = true;
if (&*BB != &F.front())
BlocksToErase.push_back(&*BB);
continue;
}
for (BasicBlock::iterator BI = BB->begin(), E = BB->end(); BI != E; ) {
Instruction *Inst = &*BI++;
if (Inst->getType()->isVoidTy())
continue;
if (tryToReplaceWithConstant(Solver, Inst)) {
if (Inst->isSafeToRemove())
Inst->eraseFromParent();
// Hey, we just changed something!
MadeChanges = true;
++IPNumInstRemoved;
}
}
}
// Change dead blocks to unreachable. We do it after replacing constants in
// all executable blocks, because changeToUnreachable may remove PHI nodes
// in executable blocks we found values for. The function's entry block is
// not part of BlocksToErase, so we have to handle it separately.
for (BasicBlock *BB : BlocksToErase)
NumInstRemoved +=
changeToUnreachable(BB->getFirstNonPHI(), /*UseLLVMTrap=*/false);
if (!Solver.isBlockExecutable(&F.front()))
NumInstRemoved += changeToUnreachable(F.front().getFirstNonPHI(),
/*UseLLVMTrap=*/false);
// Now that all instructions in the function are constant folded, erase dead
// blocks, because we can now use ConstantFoldTerminator to get rid of
// in-edges.
for (unsigned i = 0, e = BlocksToErase.size(); i != e; ++i) {
// If there are any PHI nodes in this successor, drop entries for BB now.
BasicBlock *DeadBB = BlocksToErase[i];
for (Value::user_iterator UI = DeadBB->user_begin(),
UE = DeadBB->user_end();
UI != UE;) {
// Grab the user and then increment the iterator early, as the user
// will be deleted. Step past all adjacent uses from the same user.
auto *I = dyn_cast<Instruction>(*UI);
do { ++UI; } while (UI != UE && *UI == I);
// Ignore blockaddress users; BasicBlock's dtor will handle them.
if (!I) continue;
bool Folded = ConstantFoldTerminator(I->getParent());
if (!Folded) {
// If the branch can't be folded, we must have forced an edge
// for an indeterminate value. Force the terminator to fold
// to that edge.
Constant *C;
BasicBlock *Dest;
if (SwitchInst *SI = dyn_cast<SwitchInst>(I)) {
Dest = SI->case_begin()->getCaseSuccessor();
C = SI->case_begin()->getCaseValue();
} else if (BranchInst *BI = dyn_cast<BranchInst>(I)) {
Dest = BI->getSuccessor(1);
C = ConstantInt::getFalse(BI->getContext());
} else if (IndirectBrInst *IBR = dyn_cast<IndirectBrInst>(I)) {
Dest = IBR->getSuccessor(0);
C = BlockAddress::get(IBR->getSuccessor(0));
} else {
llvm_unreachable("Unexpected terminator instruction");
}
assert(Solver.isEdgeFeasible(I->getParent(), Dest) &&
"Didn't find feasible edge?");
(void)Dest;
I->setOperand(0, C);
Folded = ConstantFoldTerminator(I->getParent());
}
assert(Folded &&
"Expect TermInst on constantint or blockaddress to be folded");
(void) Folded;
}
// Finally, delete the basic block.
F.getBasicBlockList().erase(DeadBB);
}
BlocksToErase.clear();
}
// If we inferred constant or undef return values for a function, we replaced
// all call uses with the inferred value. This means we don't need to bother
// actually returning anything from the function. Replace all return
// instructions with return undef.
//
// Do this in two stages: first identify the functions we should process, then
// actually zap their returns. This is important because we can only do this
// if the address of the function isn't taken. In cases where a return is the
// last use of a function, the order of processing functions would affect
// whether other functions are optimizable.
SmallVector<ReturnInst*, 8> ReturnsToZap;
const DenseMap<Function*, LatticeVal> &RV = Solver.getTrackedRetVals();
for (const auto &I : RV) {
Function *F = I.first;
if (I.second.isOverdefined() || F->getReturnType()->isVoidTy())
continue;
findReturnsToZap(*F, ReturnsToZap, Solver);
}
for (const auto &F : Solver.getMRVFunctionsTracked()) {
assert(F->getReturnType()->isStructTy() &&
"The return type should be a struct");
StructType *STy = cast<StructType>(F->getReturnType());
if (Solver.isStructLatticeConstant(F, STy))
findReturnsToZap(*F, ReturnsToZap, Solver);
}
// Zap all returns which we've identified as zap to change.
for (unsigned i = 0, e = ReturnsToZap.size(); i != e; ++i) {
Function *F = ReturnsToZap[i]->getParent()->getParent();
ReturnsToZap[i]->setOperand(0, UndefValue::get(F->getReturnType()));
}
// If we inferred constant or undef values for globals variables, we can
// delete the global and any stores that remain to it.
const DenseMap<GlobalVariable*, LatticeVal> &TG = Solver.getTrackedGlobals();
for (DenseMap<GlobalVariable*, LatticeVal>::const_iterator I = TG.begin(),
E = TG.end(); I != E; ++I) {
GlobalVariable *GV = I->first;
assert(!I->second.isOverdefined() &&
"Overdefined values should have been taken out of the map!");
LLVM_DEBUG(dbgs() << "Found that GV '" << GV->getName()
<< "' is constant!\n");
while (!GV->use_empty()) {
StoreInst *SI = cast<StoreInst>(GV->user_back());
SI->eraseFromParent();
}
M.getGlobalList().erase(GV);
++IPNumGlobalConst;
}
return MadeChanges;
}
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