OSchip
/
llvm-project
mirror of https://github.com/THU-DSP-LAB/llvm-project.git
13
0
Fork 2
Code Issues Packages Projects Releases Wiki Activity
llvm-project/llvm/lib/Transforms/Scalar/ConstraintElimination.cpp

1073 lines
37 KiB
C++
Raw Blame History

//===-- ConstraintElimination.cpp - Eliminate conds using constraints. ----===//
//
// 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
//
//===----------------------------------------------------------------------===//
//
// Eliminate conditions based on constraints collected from dominating
// conditions.
//
//===----------------------------------------------------------------------===//
#include "llvm/Transforms/Scalar/ConstraintElimination.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/ADT/ScopeExit.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Analysis/ConstraintSystem.h"
#include "llvm/Analysis/GlobalsModRef.h"
#include "llvm/Analysis/ValueTracking.h"
#include "llvm/IR/DataLayout.h"
#include "llvm/IR/Dominators.h"
#include "llvm/IR/Function.h"
#include "llvm/IR/GetElementPtrTypeIterator.h"
#include "llvm/IR/IRBuilder.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/PatternMatch.h"
#include "llvm/InitializePasses.h"
#include "llvm/Pass.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/DebugCounter.h"
#include "llvm/Support/MathExtras.h"
#include "llvm/Transforms/Scalar.h"
#include <cmath>
#include <string>
using namespace llvm;
using namespace PatternMatch;
#define DEBUG_TYPE "constraint-elimination"
STATISTIC(NumCondsRemoved, "Number of instructions removed");
DEBUG_COUNTER(EliminatedCounter, "conds-eliminated",
"Controls which conditions are eliminated");
static int64_t MaxConstraintValue = std::numeric_limits<int64_t>::max();
static int64_t MinSignedConstraintValue = std::numeric_limits<int64_t>::min();
// A helper to multiply 2 signed integers where overflowing is allowed.
static int64_t multiplyWithOverflow(int64_t A, int64_t B) {
int64_t Result;
MulOverflow(A, B, Result);
return Result;
}
// A helper to add 2 signed integers where overflowing is allowed.
static int64_t addWithOverflow(int64_t A, int64_t B) {
int64_t Result;
AddOverflow(A, B, Result);
return Result;
}
namespace {
class ConstraintInfo;
struct StackEntry {
unsigned NumIn;
unsigned NumOut;
bool IsSigned = false;
/// Variables that can be removed from the system once the stack entry gets
/// removed.
SmallVector<Value *, 2> ValuesToRelease;
StackEntry(unsigned NumIn, unsigned NumOut, bool IsSigned,
SmallVector<Value *, 2> ValuesToRelease)
: NumIn(NumIn), NumOut(NumOut), IsSigned(IsSigned),
ValuesToRelease(ValuesToRelease) {}
};
/// Struct to express a pre-condition of the form %Op0 Pred %Op1.
struct PreconditionTy {
CmpInst::Predicate Pred;
Value *Op0;
Value *Op1;
PreconditionTy(CmpInst::Predicate Pred, Value *Op0, Value *Op1)
: Pred(Pred), Op0(Op0), Op1(Op1) {}
};
struct ConstraintTy {
SmallVector<int64_t, 8> Coefficients;
SmallVector<PreconditionTy, 2> Preconditions;
SmallVector<SmallVector<int64_t, 8>> ExtraInfo;
bool IsSigned = false;
bool IsEq = false;
ConstraintTy() = default;
ConstraintTy(SmallVector<int64_t, 8> Coefficients, bool IsSigned)
: Coefficients(Coefficients), IsSigned(IsSigned) {}
unsigned size() const { return Coefficients.size(); }
unsigned empty() const { return Coefficients.empty(); }
/// Returns true if all preconditions for this list of constraints are
/// satisfied given \p CS and the corresponding \p Value2Index mapping.
bool isValid(const ConstraintInfo &Info) const;
};
/// Wrapper encapsulating separate constraint systems and corresponding value
/// mappings for both unsigned and signed information. Facts are added to and
/// conditions are checked against the corresponding system depending on the
/// signed-ness of their predicates. While the information is kept separate
/// based on signed-ness, certain conditions can be transferred between the two
/// systems.
class ConstraintInfo {
DenseMap<Value *, unsigned> UnsignedValue2Index;
DenseMap<Value *, unsigned> SignedValue2Index;
ConstraintSystem UnsignedCS;
ConstraintSystem SignedCS;
const DataLayout &DL;
public:
ConstraintInfo(const DataLayout &DL) : DL(DL) {}
DenseMap<Value *, unsigned> &getValue2Index(bool Signed) {
return Signed ? SignedValue2Index : UnsignedValue2Index;
}
const DenseMap<Value *, unsigned> &getValue2Index(bool Signed) const {
return Signed ? SignedValue2Index : UnsignedValue2Index;
}
ConstraintSystem &getCS(bool Signed) {
return Signed ? SignedCS : UnsignedCS;
}
const ConstraintSystem &getCS(bool Signed) const {
return Signed ? SignedCS : UnsignedCS;
}
void popLastConstraint(bool Signed) { getCS(Signed).popLastConstraint(); }
void popLastNVariables(bool Signed, unsigned N) {
getCS(Signed).popLastNVariables(N);
}
bool doesHold(CmpInst::Predicate Pred, Value *A, Value *B) const;
void addFact(CmpInst::Predicate Pred, Value *A, Value *B, unsigned NumIn,
unsigned NumOut, SmallVectorImpl<StackEntry> &DFSInStack);
/// Turn a comparison of the form \p Op0 \p Pred \p Op1 into a vector of
/// constraints, using indices from the corresponding constraint system.
/// New variables that need to be added to the system are collected in
/// \p NewVariables.
ConstraintTy getConstraint(CmpInst::Predicate Pred, Value *Op0, Value *Op1,
SmallVectorImpl<Value *> &NewVariables) const;
/// Turns a comparison of the form \p Op0 \p Pred \p Op1 into a vector of
/// constraints using getConstraint. Returns an empty constraint if the result
/// cannot be used to query the existing constraint system, e.g. because it
/// would require adding new variables. Also tries to convert signed
/// predicates to unsigned ones if possible to allow using the unsigned system
/// which increases the effectiveness of the signed <-> unsigned transfer
/// logic.
ConstraintTy getConstraintForSolving(CmpInst::Predicate Pred, Value *Op0,
Value *Op1) const;
/// Try to add information from \p A \p Pred \p B to the unsigned/signed
/// system if \p Pred is signed/unsigned.
void transferToOtherSystem(CmpInst::Predicate Pred, Value *A, Value *B,
unsigned NumIn, unsigned NumOut,
SmallVectorImpl<StackEntry> &DFSInStack);
};
/// Represents a (Coefficient * Variable) entry after IR decomposition.
struct DecompEntry {
int64_t Coefficient;
Value *Variable;
/// True if the variable is known positive in the current constraint.
bool IsKnownPositive;
DecompEntry(int64_t Coefficient, Value *Variable,
bool IsKnownPositive = false)
: Coefficient(Coefficient), Variable(Variable),
IsKnownPositive(IsKnownPositive) {}
};
/// Represents an Offset + Coefficient1 * Variable1 + ... decomposition.
struct Decomposition {
int64_t Offset = 0;
SmallVector<DecompEntry, 3> Vars;
Decomposition(int64_t Offset) : Offset(Offset) {}
Decomposition(Value *V, bool IsKnownPositive = false) {
Vars.emplace_back(1, V, IsKnownPositive);
}
Decomposition(int64_t Offset, ArrayRef<DecompEntry> Vars)
: Offset(Offset), Vars(Vars) {}
void add(int64_t OtherOffset) {
Offset = addWithOverflow(Offset, OtherOffset);
}
void add(const Decomposition &Other) {
add(Other.Offset);
append_range(Vars, Other.Vars);
}
void mul(int64_t Factor) {
Offset = multiplyWithOverflow(Offset, Factor);
for (auto &Var : Vars)
Var.Coefficient = multiplyWithOverflow(Var.Coefficient, Factor);
}
};
} // namespace
static Decomposition decompose(Value *V,
SmallVectorImpl<PreconditionTy> &Preconditions,
bool IsSigned, const DataLayout &DL);
static bool canUseSExt(ConstantInt *CI) {
const APInt &Val = CI->getValue();
return Val.sgt(MinSignedConstraintValue) && Val.slt(MaxConstraintValue);
}
static Decomposition
decomposeGEP(GetElementPtrInst &GEP,
SmallVectorImpl<PreconditionTy> &Preconditions, bool IsSigned,
const DataLayout &DL) {
// Do not reason about pointers where the index size is larger than 64 bits,
// as the coefficients used to encode constraints are 64 bit integers.
if (DL.getIndexTypeSizeInBits(GEP.getPointerOperand()->getType()) > 64)
return &GEP;
if (!GEP.isInBounds())
return &GEP;
Type *PtrTy = GEP.getType()->getScalarType();
unsigned BitWidth = DL.getIndexTypeSizeInBits(PtrTy);
MapVector<Value *, APInt> VariableOffsets;
APInt ConstantOffset(BitWidth, 0);
if (!GEP.collectOffset(DL, BitWidth, VariableOffsets, ConstantOffset))
return &GEP;
// Handle the (gep (gep ....), C) case by incrementing the constant
// coefficient of the inner GEP, if C is a constant.
auto *InnerGEP = dyn_cast<GetElementPtrInst>(GEP.getPointerOperand());
if (VariableOffsets.empty() && InnerGEP && InnerGEP->getNumOperands() == 2) {
auto Result = decompose(InnerGEP, Preconditions, IsSigned, DL);
Result.add(ConstantOffset.getSExtValue());
if (ConstantOffset.isNegative()) {
unsigned Scale = DL.getTypeAllocSize(InnerGEP->getResultElementType());
int64_t ConstantOffsetI = ConstantOffset.getSExtValue();
if (ConstantOffsetI % Scale != 0)
return &GEP;
// Add pre-condition ensuring the GEP is increasing monotonically and
// can be de-composed.
// Both sides are normalized by being divided by Scale.
Preconditions.emplace_back(
CmpInst::ICMP_SGE, InnerGEP->getOperand(1),
ConstantInt::get(InnerGEP->getOperand(1)->getType(),
-1 * (ConstantOffsetI / Scale)));
}
return Result;
}
Decomposition Result(ConstantOffset.getSExtValue(),
DecompEntry(1, GEP.getPointerOperand()));
for (auto [Index, Scale] : VariableOffsets) {
auto IdxResult = decompose(Index, Preconditions, IsSigned, DL);
IdxResult.mul(Scale.getSExtValue());
Result.add(IdxResult);
// If Op0 is signed non-negative, the GEP is increasing monotonically and
// can be de-composed.
if (!isKnownNonNegative(Index, DL, /*Depth=*/MaxAnalysisRecursionDepth - 1))
Preconditions.emplace_back(CmpInst::ICMP_SGE, Index,
ConstantInt::get(Index->getType(), 0));
}
return Result;
}
// Decomposes \p V into a vector of entries of the form { Coefficient, Variable
// } where Coefficient * Variable. The sum of the pairs equals \p V. The first
// pair is the constant-factor and X must be nullptr. If the expression cannot
// be decomposed, returns an empty vector.
static Decomposition decompose(Value *V,
SmallVectorImpl<PreconditionTy> &Preconditions,
bool IsSigned, const DataLayout &DL) {
auto MergeResults = [&Preconditions, IsSigned, &DL](Value *A, Value *B,
bool IsSignedB) {
auto ResA = decompose(A, Preconditions, IsSigned, DL);
auto ResB = decompose(B, Preconditions, IsSignedB, DL);
ResA.add(ResB);
return ResA;
};
// Decompose \p V used with a signed predicate.
if (IsSigned) {
if (auto *CI = dyn_cast<ConstantInt>(V)) {
if (canUseSExt(CI))
return CI->getSExtValue();
}
Value *Op0;
Value *Op1;
if (match(V, m_NSWAdd(m_Value(Op0), m_Value(Op1))))
return MergeResults(Op0, Op1, IsSigned);
return V;
}
if (auto *CI = dyn_cast<ConstantInt>(V)) {
if (CI->uge(MaxConstraintValue))
return V;
return int64_t(CI->getZExtValue());
}
if (auto *GEP = dyn_cast<GetElementPtrInst>(V))
return decomposeGEP(*GEP, Preconditions, IsSigned, DL);
Value *Op0;
bool IsKnownPositive = false;
if (match(V, m_ZExt(m_Value(Op0)))) {
IsKnownPositive = true;
V = Op0;
}
Value *Op1;
ConstantInt *CI;
if (match(V, m_NUWAdd(m_Value(Op0), m_Value(Op1)))) {
return MergeResults(Op0, Op1, IsSigned);
}
if (match(V, m_NSWAdd(m_Value(Op0), m_Value(Op1)))) {
if (!isKnownNonNegative(Op0, DL, /*Depth=*/MaxAnalysisRecursionDepth - 1))
Preconditions.emplace_back(CmpInst::ICMP_SGE, Op0,
ConstantInt::get(Op0->getType(), 0));
if (!isKnownNonNegative(Op1, DL, /*Depth=*/MaxAnalysisRecursionDepth - 1))
Preconditions.emplace_back(CmpInst::ICMP_SGE, Op1,
ConstantInt::get(Op1->getType(), 0));
return MergeResults(Op0, Op1, IsSigned);
}
if (match(V, m_Add(m_Value(Op0), m_ConstantInt(CI))) && CI->isNegative() &&
canUseSExt(CI)) {
Preconditions.emplace_back(
CmpInst::ICMP_UGE, Op0,
ConstantInt::get(Op0->getType(), CI->getSExtValue() * -1));
return MergeResults(Op0, CI, true);
}
if (match(V, m_NUWShl(m_Value(Op1), m_ConstantInt(CI))) && canUseSExt(CI)) {
int64_t Mult = int64_t(std::pow(int64_t(2), CI->getSExtValue()));
auto Result = decompose(Op1, Preconditions, IsSigned, DL);
Result.mul(Mult);
return Result;
}
if (match(V, m_NUWMul(m_Value(Op1), m_ConstantInt(CI))) && canUseSExt(CI) &&
(!CI->isNegative())) {
auto Result = decompose(Op1, Preconditions, IsSigned, DL);
Result.mul(CI->getSExtValue());
return Result;
}
if (match(V, m_NUWSub(m_Value(Op0), m_ConstantInt(CI))) && canUseSExt(CI))
return {-1 * CI->getSExtValue(), {{1, Op0}}};
if (match(V, m_NUWSub(m_Value(Op0), m_Value(Op1))))
return {0, {{1, Op0}, {-1, Op1}}};
return {V, IsKnownPositive};
}
ConstraintTy
ConstraintInfo::getConstraint(CmpInst::Predicate Pred, Value *Op0, Value *Op1,
SmallVectorImpl<Value *> &NewVariables) const {
assert(NewVariables.empty() && "NewVariables must be empty when passed in");
bool IsEq = false;
// Try to convert Pred to one of ULE/SLT/SLE/SLT.
switch (Pred) {
case CmpInst::ICMP_UGT:
case CmpInst::ICMP_UGE:
case CmpInst::ICMP_SGT:
case CmpInst::ICMP_SGE: {
Pred = CmpInst::getSwappedPredicate(Pred);
std::swap(Op0, Op1);
break;
}
case CmpInst::ICMP_EQ:
if (match(Op1, m_Zero())) {
Pred = CmpInst::ICMP_ULE;
} else {
IsEq = true;
Pred = CmpInst::ICMP_ULE;
}
break;
case CmpInst::ICMP_NE:
if (!match(Op1, m_Zero()))
return {};
Pred = CmpInst::getSwappedPredicate(CmpInst::ICMP_UGT);
std::swap(Op0, Op1);
break;
default:
break;
}
// Only ULE and ULT predicates are supported at the moment.
if (Pred != CmpInst::ICMP_ULE && Pred != CmpInst::ICMP_ULT &&
Pred != CmpInst::ICMP_SLE && Pred != CmpInst::ICMP_SLT)
return {};
SmallVector<PreconditionTy, 4> Preconditions;
bool IsSigned = CmpInst::isSigned(Pred);
auto &Value2Index = getValue2Index(IsSigned);
auto ADec = decompose(Op0->stripPointerCastsSameRepresentation(),
Preconditions, IsSigned, DL);
auto BDec = decompose(Op1->stripPointerCastsSameRepresentation(),
Preconditions, IsSigned, DL);
int64_t Offset1 = ADec.Offset;
int64_t Offset2 = BDec.Offset;
Offset1 *= -1;
auto &VariablesA = ADec.Vars;
auto &VariablesB = BDec.Vars;
// First try to look up \p V in Value2Index and NewVariables. Otherwise add a
// new entry to NewVariables.
DenseMap<Value *, unsigned> NewIndexMap;
auto GetOrAddIndex = [&Value2Index, &NewVariables,
&NewIndexMap](Value *V) -> unsigned {
auto V2I = Value2Index.find(V);
if (V2I != Value2Index.end())
return V2I->second;
auto Insert =
NewIndexMap.insert({V, Value2Index.size() + NewVariables.size() + 1});
if (Insert.second)
NewVariables.push_back(V);
return Insert.first->second;
};
// Make sure all variables have entries in Value2Index or NewVariables.
for (const auto &KV : concat<DecompEntry>(VariablesA, VariablesB))
GetOrAddIndex(KV.Variable);
// Build result constraint, by first adding all coefficients from A and then
// subtracting all coefficients from B.
ConstraintTy Res(
SmallVector<int64_t, 8>(Value2Index.size() + NewVariables.size() + 1, 0),
IsSigned);
// Collect variables that are known to be positive in all uses in the
// constraint.
DenseMap<Value *, bool> KnownPositiveVariables;
Res.IsEq = IsEq;
auto &R = Res.Coefficients;
for (const auto &KV : VariablesA) {
R[GetOrAddIndex(KV.Variable)] += KV.Coefficient;
auto I = KnownPositiveVariables.insert({KV.Variable, KV.IsKnownPositive});
I.first->second &= KV.IsKnownPositive;
}
for (const auto &KV : VariablesB) {
R[GetOrAddIndex(KV.Variable)] -= KV.Coefficient;
auto I = KnownPositiveVariables.insert({KV.Variable, KV.IsKnownPositive});
I.first->second &= KV.IsKnownPositive;
}
int64_t OffsetSum;
if (AddOverflow(Offset1, Offset2, OffsetSum))
return {};
if (Pred == (IsSigned ? CmpInst::ICMP_SLT : CmpInst::ICMP_ULT))
if (AddOverflow(OffsetSum, int64_t(-1), OffsetSum))
return {};
R[0] = OffsetSum;
Res.Preconditions = std::move(Preconditions);
// Remove any (Coefficient, Variable) entry where the Coefficient is 0 for new
// variables.
while (!NewVariables.empty()) {
int64_t Last = R.back();
if (Last != 0)
break;
R.pop_back();
Value *RemovedV = NewVariables.pop_back_val();
NewIndexMap.erase(RemovedV);
}
// Add extra constraints for variables that are known positive.
for (auto &KV : KnownPositiveVariables) {
if (!KV.second || (Value2Index.find(KV.first) == Value2Index.end() &&
NewIndexMap.find(KV.first) == NewIndexMap.end()))
continue;
SmallVector<int64_t, 8> C(Value2Index.size() + NewVariables.size() + 1, 0);
C[GetOrAddIndex(KV.first)] = -1;
Res.ExtraInfo.push_back(C);
}
return Res;
}
ConstraintTy ConstraintInfo::getConstraintForSolving(CmpInst::Predicate Pred,
Value *Op0,
Value *Op1) const {
// If both operands are known to be non-negative, change signed predicates to
// unsigned ones. This increases the reasoning effectiveness in combination
// with the signed <-> unsigned transfer logic.
if (CmpInst::isSigned(Pred) &&
isKnownNonNegative(Op0, DL, /*Depth=*/MaxAnalysisRecursionDepth - 1) &&
isKnownNonNegative(Op1, DL, /*Depth=*/MaxAnalysisRecursionDepth - 1))
Pred = CmpInst::getUnsignedPredicate(Pred);
SmallVector<Value *> NewVariables;
ConstraintTy R = getConstraint(Pred, Op0, Op1, NewVariables);
if (R.IsEq || !NewVariables.empty())
return {};
return R;
}
bool ConstraintTy::isValid(const ConstraintInfo &Info) const {
return Coefficients.size() > 0 &&
all_of(Preconditions, [&Info](const PreconditionTy &C) {
return Info.doesHold(C.Pred, C.Op0, C.Op1);
});
}
bool ConstraintInfo::doesHold(CmpInst::Predicate Pred, Value *A,
Value *B) const {
auto R = getConstraintForSolving(Pred, A, B);
return R.Preconditions.empty() && !R.empty() &&
getCS(R.IsSigned).isConditionImplied(R.Coefficients);
}
void ConstraintInfo::transferToOtherSystem(
CmpInst::Predicate Pred, Value *A, Value *B, unsigned NumIn,
unsigned NumOut, SmallVectorImpl<StackEntry> &DFSInStack) {
// Check if we can combine facts from the signed and unsigned systems to
// derive additional facts.
if (!A->getType()->isIntegerTy())
return;
// FIXME: This currently depends on the order we add facts. Ideally we
// would first add all known facts and only then try to add additional
// facts.
switch (Pred) {
default:
break;
case CmpInst::ICMP_ULT:
// If B is a signed positive constant, A >=s 0 and A <s B.
if (doesHold(CmpInst::ICMP_SGE, B, ConstantInt::get(B->getType(), 0))) {
addFact(CmpInst::ICMP_SGE, A, ConstantInt::get(B->getType(), 0), NumIn,
NumOut, DFSInStack);
addFact(CmpInst::ICMP_SLT, A, B, NumIn, NumOut, DFSInStack);
}
break;
case CmpInst::ICMP_SLT:
if (doesHold(CmpInst::ICMP_SGE, A, ConstantInt::get(B->getType(), 0)))
addFact(CmpInst::ICMP_ULT, A, B, NumIn, NumOut, DFSInStack);
break;
case CmpInst::ICMP_SGT:
if (doesHold(CmpInst::ICMP_SGE, B, ConstantInt::get(B->getType(), -1)))
addFact(CmpInst::ICMP_UGE, A, ConstantInt::get(B->getType(), 0), NumIn,
NumOut, DFSInStack);
break;
case CmpInst::ICMP_SGE:
if (doesHold(CmpInst::ICMP_SGE, B, ConstantInt::get(B->getType(), 0))) {
addFact(CmpInst::ICMP_UGE, A, B, NumIn, NumOut, DFSInStack);
}
break;
}
}
namespace {
/// Represents either a condition that holds on entry to a block or a basic
/// block, with their respective Dominator DFS in and out numbers.
struct ConstraintOrBlock {
unsigned NumIn;
unsigned NumOut;
bool IsBlock;
bool Not;
union {
BasicBlock *BB;
CmpInst *Condition;
};
ConstraintOrBlock(DomTreeNode *DTN)
: NumIn(DTN->getDFSNumIn()), NumOut(DTN->getDFSNumOut()), IsBlock(true),
BB(DTN->getBlock()) {}
ConstraintOrBlock(DomTreeNode *DTN, CmpInst *Condition, bool Not)
: NumIn(DTN->getDFSNumIn()), NumOut(DTN->getDFSNumOut()), IsBlock(false),
Not(Not), Condition(Condition) {}
};
/// Keep state required to build worklist.
struct State {
DominatorTree &DT;
SmallVector<ConstraintOrBlock, 64> WorkList;
State(DominatorTree &DT) : DT(DT) {}
/// Process block \p BB and add known facts to work-list.
void addInfoFor(BasicBlock &BB);
/// Returns true if we can add a known condition from BB to its successor
/// block Succ. Each predecessor of Succ can either be BB or be dominated
/// by Succ (e.g. the case when adding a condition from a pre-header to a
/// loop header).
bool canAddSuccessor(BasicBlock &BB, BasicBlock *Succ) const {
if (BB.getSingleSuccessor()) {
assert(BB.getSingleSuccessor() == Succ);
return DT.properlyDominates(&BB, Succ);
}
return any_of(successors(&BB),
[Succ](const BasicBlock *S) { return S != Succ; }) &&
all_of(predecessors(Succ), [&BB, Succ, this](BasicBlock *Pred) {
return Pred == &BB || DT.dominates(Succ, Pred);
});
}
};
} // namespace
#ifndef NDEBUG
static void dumpWithNames(const ConstraintSystem &CS,
DenseMap<Value *, unsigned> &Value2Index) {
SmallVector<std::string> Names(Value2Index.size(), "");
for (auto &KV : Value2Index) {
Names[KV.second - 1] = std::string("%") + KV.first->getName().str();
}
CS.dump(Names);
}
static void dumpWithNames(ArrayRef<int64_t> C,
DenseMap<Value *, unsigned> &Value2Index) {
ConstraintSystem CS;
CS.addVariableRowFill(C);
dumpWithNames(CS, Value2Index);
}
#endif
void State::addInfoFor(BasicBlock &BB) {
WorkList.emplace_back(DT.getNode(&BB));
// True as long as long as the current instruction is guaranteed to execute.
bool GuaranteedToExecute = true;
// Scan BB for assume calls.
// TODO: also use this scan to queue conditions to simplify, so we can
// interleave facts from assumes and conditions to simplify in a single
// basic block. And to skip another traversal of each basic block when
// simplifying.
for (Instruction &I : BB) {
Value *Cond;
// For now, just handle assumes with a single compare as condition.
if (match(&I, m_Intrinsic<Intrinsic::assume>(m_Value(Cond))) &&
isa<ICmpInst>(Cond)) {
if (GuaranteedToExecute) {
// The assume is guaranteed to execute when BB is entered, hence Cond
// holds on entry to BB.
WorkList.emplace_back(DT.getNode(&BB), cast<ICmpInst>(Cond), false);
} else {
// Otherwise the condition only holds in the successors.
for (BasicBlock *Succ : successors(&BB)) {
if (!canAddSuccessor(BB, Succ))
continue;
WorkList.emplace_back(DT.getNode(Succ), cast<ICmpInst>(Cond), false);
}
}
}
GuaranteedToExecute &= isGuaranteedToTransferExecutionToSuccessor(&I);
}
auto *Br = dyn_cast<BranchInst>(BB.getTerminator());
if (!Br || !Br->isConditional())
return;
Value *Cond = Br->getCondition();
// If the condition is a chain of ORs/AND and the successor only has the
// current block as predecessor, queue conditions for the successor.
Value *Op0, *Op1;
if (match(Cond, m_LogicalOr(m_Value(Op0), m_Value(Op1))) ||
match(Cond, m_LogicalAnd(m_Value(Op0), m_Value(Op1)))) {
bool IsOr = match(Cond, m_LogicalOr());
bool IsAnd = match(Cond, m_LogicalAnd());
// If there's a select that matches both AND and OR, we need to commit to
// one of the options. Arbitrarily pick OR.
if (IsOr && IsAnd)
IsAnd = false;
BasicBlock *Successor = Br->getSuccessor(IsOr ? 1 : 0);
if (canAddSuccessor(BB, Successor)) {
SmallVector<Value *> CondWorkList;
SmallPtrSet<Value *, 8> SeenCond;
auto QueueValue = [&CondWorkList, &SeenCond](Value *V) {
if (SeenCond.insert(V).second)
CondWorkList.push_back(V);
};
QueueValue(Op1);
QueueValue(Op0);
while (!CondWorkList.empty()) {
Value *Cur = CondWorkList.pop_back_val();
if (auto *Cmp = dyn_cast<ICmpInst>(Cur)) {
WorkList.emplace_back(DT.getNode(Successor), Cmp, IsOr);
continue;
}
if (IsOr && match(Cur, m_LogicalOr(m_Value(Op0), m_Value(Op1)))) {
QueueValue(Op1);
QueueValue(Op0);
continue;
}
if (IsAnd && match(Cur, m_LogicalAnd(m_Value(Op0), m_Value(Op1)))) {
QueueValue(Op1);
QueueValue(Op0);
continue;
}
}
}
return;
}
auto *CmpI = dyn_cast<ICmpInst>(Br->getCondition());
if (!CmpI)
return;
if (canAddSuccessor(BB, Br->getSuccessor(0)))
WorkList.emplace_back(DT.getNode(Br->getSuccessor(0)), CmpI, false);
if (canAddSuccessor(BB, Br->getSuccessor(1)))
WorkList.emplace_back(DT.getNode(Br->getSuccessor(1)), CmpI, true);
}
static bool checkAndReplaceCondition(CmpInst *Cmp, ConstraintInfo &Info) {
LLVM_DEBUG(dbgs() << "Checking " << *Cmp << "\n");
CmpInst::Predicate Pred = Cmp->getPredicate();
Value *A = Cmp->getOperand(0);
Value *B = Cmp->getOperand(1);
auto R = Info.getConstraintForSolving(Pred, A, B);
if (R.empty() || !R.isValid(Info)){
LLVM_DEBUG(dbgs() << " failed to decompose condition\n");
return false;
}
auto &CSToUse = Info.getCS(R.IsSigned);
// If there was extra information collected during decomposition, apply
// it now and remove it immediately once we are done with reasoning
// about the constraint.
for (auto &Row : R.ExtraInfo)
CSToUse.addVariableRow(Row);
auto InfoRestorer = make_scope_exit([&]() {
for (unsigned I = 0; I < R.ExtraInfo.size(); ++I)
CSToUse.popLastConstraint();
});
bool Changed = false;
if (CSToUse.isConditionImplied(R.Coefficients)) {
if (!DebugCounter::shouldExecute(EliminatedCounter))
return false;
LLVM_DEBUG({
dbgs() << "Condition " << *Cmp << " implied by dominating constraints\n";
dumpWithNames(CSToUse, Info.getValue2Index(R.IsSigned));
});
Constant *TrueC =
ConstantInt::getTrue(CmpInst::makeCmpResultType(Cmp->getType()));
Cmp->replaceUsesWithIf(TrueC, [](Use &U) {
// Conditions in an assume trivially simplify to true. Skip uses
// in assume calls to not destroy the available information.
auto *II = dyn_cast<IntrinsicInst>(U.getUser());
return !II || II->getIntrinsicID() != Intrinsic::assume;
});
NumCondsRemoved++;
Changed = true;
}
if (CSToUse.isConditionImplied(ConstraintSystem::negate(R.Coefficients))) {
if (!DebugCounter::shouldExecute(EliminatedCounter))
return false;
LLVM_DEBUG({
dbgs() << "Condition !" << *Cmp << " implied by dominating constraints\n";
dumpWithNames(CSToUse, Info.getValue2Index(R.IsSigned));
});
Constant *FalseC =
ConstantInt::getFalse(CmpInst::makeCmpResultType(Cmp->getType()));
Cmp->replaceAllUsesWith(FalseC);
NumCondsRemoved++;
Changed = true;
}
return Changed;
}
void ConstraintInfo::addFact(CmpInst::Predicate Pred, Value *A, Value *B,
unsigned NumIn, unsigned NumOut,
SmallVectorImpl<StackEntry> &DFSInStack) {
// If the constraint has a pre-condition, skip the constraint if it does not
// hold.
SmallVector<Value *> NewVariables;
auto R = getConstraint(Pred, A, B, NewVariables);
if (!R.isValid(*this))
return;
LLVM_DEBUG(dbgs() << "Adding '" << CmpInst::getPredicateName(Pred) << " ";
A->printAsOperand(dbgs(), false); dbgs() << ", ";
B->printAsOperand(dbgs(), false); dbgs() << "'\n");
bool Added = false;
auto &CSToUse = getCS(R.IsSigned);
if (R.Coefficients.empty())
return;
Added |= CSToUse.addVariableRowFill(R.Coefficients);
// If R has been added to the system, add the new variables and queue it for
// removal once it goes out-of-scope.
if (Added) {
SmallVector<Value *, 2> ValuesToRelease;
auto &Value2Index = getValue2Index(R.IsSigned);
for (Value *V : NewVariables) {
Value2Index.insert({V, Value2Index.size() + 1});
ValuesToRelease.push_back(V);
}
LLVM_DEBUG({
dbgs() << " constraint: ";
dumpWithNames(R.Coefficients, getValue2Index(R.IsSigned));
dbgs() << "\n";
});
DFSInStack.emplace_back(NumIn, NumOut, R.IsSigned, ValuesToRelease);
if (R.IsEq) {
// Also add the inverted constraint for equality constraints.
for (auto &Coeff : R.Coefficients)
Coeff *= -1;
CSToUse.addVariableRowFill(R.Coefficients);
DFSInStack.emplace_back(NumIn, NumOut, R.IsSigned,
SmallVector<Value *, 2>());
}
}
}
static bool replaceSubOverflowUses(IntrinsicInst *II, Value *A, Value *B,
SmallVectorImpl<Instruction *> &ToRemove) {
bool Changed = false;
IRBuilder<> Builder(II->getParent(), II->getIterator());
Value *Sub = nullptr;
for (User *U : make_early_inc_range(II->users())) {
if (match(U, m_ExtractValue<0>(m_Value()))) {
if (!Sub)
Sub = Builder.CreateSub(A, B);
U->replaceAllUsesWith(Sub);
Changed = true;
} else if (match(U, m_ExtractValue<1>(m_Value()))) {
U->replaceAllUsesWith(Builder.getFalse());
Changed = true;
} else
continue;
if (U->use_empty()) {
auto *I = cast<Instruction>(U);
ToRemove.push_back(I);
I->setOperand(0, PoisonValue::get(II->getType()));
Changed = true;
}
}
if (II->use_empty()) {
II->eraseFromParent();
Changed = true;
}
return Changed;
}
static bool
tryToSimplifyOverflowMath(IntrinsicInst *II, ConstraintInfo &Info,
SmallVectorImpl<Instruction *> &ToRemove) {
auto DoesConditionHold = [](CmpInst::Predicate Pred, Value *A, Value *B,
ConstraintInfo &Info) {
auto R = Info.getConstraintForSolving(Pred, A, B);
if (R.size() < 2 || !R.isValid(Info))
return false;
auto &CSToUse = Info.getCS(R.IsSigned);
return CSToUse.isConditionImplied(R.Coefficients);
};
bool Changed = false;
if (II->getIntrinsicID() == Intrinsic::ssub_with_overflow) {
// If A s>= B && B s>= 0, ssub.with.overflow(a, b) should not overflow and
// can be simplified to a regular sub.
Value *A = II->getArgOperand(0);
Value *B = II->getArgOperand(1);
if (!DoesConditionHold(CmpInst::ICMP_SGE, A, B, Info) ||
!DoesConditionHold(CmpInst::ICMP_SGE, B,
ConstantInt::get(A->getType(), 0), Info))
return false;
Changed = replaceSubOverflowUses(II, A, B, ToRemove);
}
return Changed;
}
static bool eliminateConstraints(Function &F, DominatorTree &DT) {
bool Changed = false;
DT.updateDFSNumbers();
ConstraintInfo Info(F.getParent()->getDataLayout());
State S(DT);
// First, collect conditions implied by branches and blocks with their
// Dominator DFS in and out numbers.
for (BasicBlock &BB : F) {
if (!DT.getNode(&BB))
continue;
S.addInfoFor(BB);
}
// Next, sort worklist by dominance, so that dominating blocks and conditions
// come before blocks and conditions dominated by them. If a block and a
// condition have the same numbers, the condition comes before the block, as
// it holds on entry to the block. Also make sure conditions with constant
// operands come before conditions without constant operands. This increases
// the effectiveness of the current signed <-> unsigned fact transfer logic.
stable_sort(
S.WorkList, [](const ConstraintOrBlock &A, const ConstraintOrBlock &B) {
auto HasNoConstOp = [](const ConstraintOrBlock &B) {
return !B.IsBlock && !isa<ConstantInt>(B.Condition->getOperand(0)) &&
!isa<ConstantInt>(B.Condition->getOperand(1));
};
bool NoConstOpA = HasNoConstOp(A);
bool NoConstOpB = HasNoConstOp(B);
return std::tie(A.NumIn, A.IsBlock, NoConstOpA) <
std::tie(B.NumIn, B.IsBlock, NoConstOpB);
});
SmallVector<Instruction *> ToRemove;
// Finally, process ordered worklist and eliminate implied conditions.
SmallVector<StackEntry, 16> DFSInStack;
for (ConstraintOrBlock &CB : S.WorkList) {
// First, pop entries from the stack that are out-of-scope for CB. Remove
// the corresponding entry from the constraint system.
while (!DFSInStack.empty()) {
auto &E = DFSInStack.back();
LLVM_DEBUG(dbgs() << "Top of stack : " << E.NumIn << " " << E.NumOut
<< "\n");
LLVM_DEBUG(dbgs() << "CB: " << CB.NumIn << " " << CB.NumOut << "\n");
assert(E.NumIn <= CB.NumIn);
if (CB.NumOut <= E.NumOut)
break;
LLVM_DEBUG({
dbgs() << "Removing ";
dumpWithNames(Info.getCS(E.IsSigned).getLastConstraint(),
Info.getValue2Index(E.IsSigned));
dbgs() << "\n";
});
Info.popLastConstraint(E.IsSigned);
// Remove variables in the system that went out of scope.
auto &Mapping = Info.getValue2Index(E.IsSigned);
for (Value *V : E.ValuesToRelease)
Mapping.erase(V);
Info.popLastNVariables(E.IsSigned, E.ValuesToRelease.size());
DFSInStack.pop_back();
}
LLVM_DEBUG({
dbgs() << "Processing ";
if (CB.IsBlock)
dbgs() << *CB.BB;
else
dbgs() << *CB.Condition;
dbgs() << "\n";
});
// For a block, check if any CmpInsts become known based on the current set
// of constraints.
if (CB.IsBlock) {
for (Instruction &I : make_early_inc_range(*CB.BB)) {
if (auto *II = dyn_cast<WithOverflowInst>(&I)) {
Changed |= tryToSimplifyOverflowMath(II, Info, ToRemove);
continue;
}
auto *Cmp = dyn_cast<ICmpInst>(&I);
if (!Cmp)
continue;
Changed |= checkAndReplaceCondition(Cmp, Info);
}
continue;
}
ICmpInst::Predicate Pred;
Value *A, *B;
if (match(CB.Condition, m_ICmp(Pred, m_Value(A), m_Value(B)))) {
// Use the inverse predicate if required.
if (CB.Not)
Pred = CmpInst::getInversePredicate(Pred);
Info.addFact(Pred, A, B, CB.NumIn, CB.NumOut, DFSInStack);
Info.transferToOtherSystem(Pred, A, B, CB.NumIn, CB.NumOut, DFSInStack);
}
}
#ifndef NDEBUG
unsigned SignedEntries =
count_if(DFSInStack, [](const StackEntry &E) { return E.IsSigned; });
assert(Info.getCS(false).size() == DFSInStack.size() - SignedEntries &&
"updates to CS and DFSInStack are out of sync");
assert(Info.getCS(true).size() == SignedEntries &&
"updates to CS and DFSInStack are out of sync");
#endif
for (Instruction *I : ToRemove)
I->eraseFromParent();
return Changed;
}
PreservedAnalyses ConstraintEliminationPass::run(Function &F,
FunctionAnalysisManager &AM) {
auto &DT = AM.getResult<DominatorTreeAnalysis>(F);
if (!eliminateConstraints(F, DT))
return PreservedAnalyses::all();
PreservedAnalyses PA;
PA.preserve<DominatorTreeAnalysis>();
PA.preserveSet<CFGAnalyses>();
return PA;
}
namespace {
class ConstraintElimination : public FunctionPass {
public:
static char ID;
ConstraintElimination() : FunctionPass(ID) {
initializeConstraintEliminationPass(*PassRegistry::getPassRegistry());
}
bool runOnFunction(Function &F) override {
auto &DT = getAnalysis<DominatorTreeWrapperPass>().getDomTree();
return eliminateConstraints(F, DT);
}
void getAnalysisUsage(AnalysisUsage &AU) const override {
AU.setPreservesCFG();
AU.addRequired<DominatorTreeWrapperPass>();
AU.addPreserved<GlobalsAAWrapperPass>();
AU.addPreserved<DominatorTreeWrapperPass>();
}
};
} // end anonymous namespace
char ConstraintElimination::ID = 0;
INITIALIZE_PASS_BEGIN(ConstraintElimination, "constraint-elimination",
"Constraint Elimination", false, false)
INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
INITIALIZE_PASS_DEPENDENCY(LazyValueInfoWrapperPass)
INITIALIZE_PASS_END(ConstraintElimination, "constraint-elimination",
"Constraint Elimination", false, false)
FunctionPass *llvm::createConstraintEliminationPass() {
return new ConstraintElimination();
}
Reference in New Issue View Git Blame Copy Permalink