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llvm-project/llvm/lib/Transforms/Utils/CodeMoverUtils.cpp

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//===- CodeMoverUtils.cpp - CodeMover Utilities ----------------------------==//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
//
// This family of functions perform movements on basic blocks, and instructions
// contained within a function.
//
//===----------------------------------------------------------------------===//
#include "llvm/Transforms/Utils/CodeMoverUtils.h"
#include "llvm/ADT/Optional.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Analysis/DependenceAnalysis.h"
#include "llvm/Analysis/OrderedInstructions.h"
#include "llvm/Analysis/PostDominators.h"
#include "llvm/Analysis/ValueTracking.h"
#include "llvm/IR/Dominators.h"
using namespace llvm;
#define DEBUG_TYPE "codemover-utils"
STATISTIC(HasDependences,
"Cannot move across instructions that has memory dependences");
STATISTIC(MayThrowException, "Cannot move across instructions that may throw");
STATISTIC(NotControlFlowEquivalent,
"Instructions are not control flow equivalent");
STATISTIC(NotMovedPHINode, "Movement of PHINodes are not supported");
STATISTIC(NotMovedTerminator, "Movement of Terminator are not supported");
namespace {
/// Represent a control condition. A control condition is a condition of a
/// terminator to decide which successors to execute. The pointer field
/// represents the address of the condition of the terminator. The integer field
/// is a bool, it is true when the basic block is executed when V is true. For
/// example, `br %cond, bb0, bb1` %cond is a control condition of bb0 with the
/// integer field equals to true, while %cond is a control condition of bb1 with
/// the integer field equals to false.
using ControlCondition = PointerIntPair<Value *, 1, bool>;
#ifndef NDEBUG
raw_ostream &operator<<(raw_ostream &OS, const ControlCondition &C) {
OS << "[" << *C.getPointer() << ", " << (C.getInt() ? "true" : "false")
<< "]";
return OS;
}
#endif
/// Represent a set of control conditions required to execute ToBB from FromBB.
class ControlConditions {
using ConditionVectorTy = SmallVector<ControlCondition, 6>;
/// A SmallVector of control conditions.
ConditionVectorTy Conditions;
public:
/// Return a ControlConditions which stores all conditions required to execute
/// \p BB from \p Dominator. If \p MaxLookup is non-zero, it limits the
/// number of conditions to collect. Return None if not all conditions are
/// collected successfully, or we hit the limit.
static const Optional<ControlConditions>
collectControlConditions(const BasicBlock &BB, const BasicBlock &Dominator,
const DominatorTree &DT,
const PostDominatorTree &PDT,
unsigned MaxLookup = 6);
/// Return true if there exists no control conditions required to execute ToBB
/// from FromBB.
bool isUnconditional() const { return Conditions.empty(); }
/// Return a constant reference of Conditions.
const ConditionVectorTy &getControlConditions() const { return Conditions; }
/// Add \p V as one of the ControlCondition in Condition with IsTrueCondition
/// equals to \p True. Return true if inserted successfully.
bool addControlCondition(ControlCondition C);
/// Return true if for all control conditions in Conditions, there exists an
/// equivalent control condition in \p Other.Conditions.
bool isEquivalent(const ControlConditions &Other) const;
/// Return true if \p C1 and \p C2 are equivalent.
static bool isEquivalent(const ControlCondition &C1,
const ControlCondition &C2);
private:
ControlConditions() = default;
static bool isEquivalent(const Value &V1, const Value &V2);
static bool isInverse(const Value &V1, const Value &V2);
};
} // namespace
const Optional<ControlConditions> ControlConditions::collectControlConditions(
const BasicBlock &BB, const BasicBlock &Dominator, const DominatorTree &DT,
const PostDominatorTree &PDT, unsigned MaxLookup) {
assert(DT.dominates(&Dominator, &BB) && "Expecting Dominator to dominate BB");
ControlConditions Conditions;
unsigned NumConditions = 0;
// BB is executed unconditional from itself.
if (&Dominator == &BB)
return Conditions;
const BasicBlock *CurBlock = &BB;
// Walk up the dominator tree from the associated DT node for BB to the
// associated DT node for Dominator.
do {
assert(DT.getNode(CurBlock) && "Expecting a valid DT node for CurBlock");
BasicBlock *IDom = DT.getNode(CurBlock)->getIDom()->getBlock();
assert(DT.dominates(&Dominator, IDom) &&
"Expecting Dominator to dominate IDom");
// Limitation: can only handle branch instruction currently.
const BranchInst *BI = dyn_cast<BranchInst>(IDom->getTerminator());
if (!BI)
return None;
bool Inserted = false;
if (PDT.dominates(CurBlock, IDom)) {
LLVM_DEBUG(dbgs() << CurBlock->getName()
<< " is executed unconditionally from "
<< IDom->getName() << "\n");
} else if (PDT.dominates(CurBlock, BI->getSuccessor(0))) {
LLVM_DEBUG(dbgs() << CurBlock->getName() << " is executed when \""
<< *BI->getCondition() << "\" is true from "
<< IDom->getName() << "\n");
Inserted = Conditions.addControlCondition(
ControlCondition(BI->getCondition(), true));
} else if (PDT.dominates(CurBlock, BI->getSuccessor(1))) {
LLVM_DEBUG(dbgs() << CurBlock->getName() << " is executed when \""
<< *BI->getCondition() << "\" is false from "
<< IDom->getName() << "\n");
Inserted = Conditions.addControlCondition(
ControlCondition(BI->getCondition(), false));
} else
return None;
if (Inserted)
++NumConditions;
if (MaxLookup != 0 && NumConditions > MaxLookup)
return None;
CurBlock = IDom;
} while (CurBlock != &Dominator);
return Conditions;
}
bool ControlConditions::addControlCondition(ControlCondition C) {
bool Inserted = false;
if (none_of(Conditions, [&](ControlCondition &Exists) {
return ControlConditions::isEquivalent(C, Exists);
})) {
Conditions.push_back(C);
Inserted = true;
}
LLVM_DEBUG(dbgs() << (Inserted ? "Inserted " : "Not inserted ") << C << "\n");
return Inserted;
}
bool ControlConditions::isEquivalent(const ControlConditions &Other) const {
if (Conditions.empty() && Other.Conditions.empty())
return true;
if (Conditions.size() != Other.Conditions.size())
return false;
return all_of(Conditions, [&](const ControlCondition &C) {
return any_of(Other.Conditions, [&](const ControlCondition &OtherC) {
return ControlConditions::isEquivalent(C, OtherC);
});
});
}
bool ControlConditions::isEquivalent(const ControlCondition &C1,
const ControlCondition &C2) {
if (C1.getInt() == C2.getInt()) {
if (isEquivalent(*C1.getPointer(), *C2.getPointer()))
return true;
} else if (isInverse(*C1.getPointer(), *C2.getPointer()))
return true;
return false;
}
// FIXME: Use SCEV and reuse GVN/CSE logic to check for equivalence between
// Values.
// Currently, isEquivalent rely on other passes to ensure equivalent conditions
// have the same value, e.g. GVN.
bool ControlConditions::isEquivalent(const Value &V1, const Value &V2) {
return &V1 == &V2;
}
bool ControlConditions::isInverse(const Value &V1, const Value &V2) {
if (const CmpInst *Cmp1 = dyn_cast<CmpInst>(&V1))
if (const CmpInst *Cmp2 = dyn_cast<CmpInst>(&V2)) {
if (Cmp1->getPredicate() == Cmp2->getInversePredicate() &&
Cmp1->getOperand(0) == Cmp2->getOperand(0) &&
Cmp1->getOperand(1) == Cmp2->getOperand(1))
return true;
if (Cmp1->getPredicate() ==
CmpInst::getSwappedPredicate(Cmp2->getInversePredicate()) &&
Cmp1->getOperand(0) == Cmp2->getOperand(1) &&
Cmp1->getOperand(1) == Cmp2->getOperand(0))
return true;
}
return false;
}
bool llvm::isControlFlowEquivalent(const Instruction &I0, const Instruction &I1,
const DominatorTree &DT,
const PostDominatorTree &PDT) {
return isControlFlowEquivalent(*I0.getParent(), *I1.getParent(), DT, PDT);
}
bool llvm::isControlFlowEquivalent(const BasicBlock &BB0, const BasicBlock &BB1,
const DominatorTree &DT,
const PostDominatorTree &PDT) {
if (&BB0 == &BB1)
return true;
if ((DT.dominates(&BB0, &BB1) && PDT.dominates(&BB1, &BB0)) ||
(PDT.dominates(&BB0, &BB1) && DT.dominates(&BB1, &BB0)))
return true;
// If the set of conditions required to execute BB0 and BB1 from their common
// dominator are the same, then BB0 and BB1 are control flow equivalent.
const BasicBlock *CommonDominator = DT.findNearestCommonDominator(&BB0, &BB1);
LLVM_DEBUG(dbgs() << "The nearest common dominator of " << BB0.getName()
<< " and " << BB1.getName() << " is "
<< CommonDominator->getName() << "\n");
const Optional<ControlConditions> BB0Conditions =
ControlConditions::collectControlConditions(BB0, *CommonDominator, DT,
PDT);
if (BB0Conditions == None)
return false;
const Optional<ControlConditions> BB1Conditions =
ControlConditions::collectControlConditions(BB1, *CommonDominator, DT,
PDT);
if (BB1Conditions == None)
return false;
return BB0Conditions->isEquivalent(*BB1Conditions);
}
static bool reportInvalidCandidate(const Instruction &I,
llvm::Statistic &Stat) {
++Stat;
LLVM_DEBUG(dbgs() << "Unable to move instruction: " << I << ". "
<< Stat.getDesc());
return false;
}
/// Collect all instructions in between \p StartInst and \p EndInst, and store
/// them in \p InBetweenInsts.
static void
collectInstructionsInBetween(Instruction &StartInst, const Instruction &EndInst,
SmallPtrSetImpl<Instruction *> &InBetweenInsts) {
assert(InBetweenInsts.empty() && "Expecting InBetweenInsts to be empty");
/// Get the next instructions of \p I, and push them to \p WorkList.
auto getNextInsts = [](Instruction &I,
SmallPtrSetImpl<Instruction *> &WorkList) {
if (Instruction *NextInst = I.getNextNode())
WorkList.insert(NextInst);
else {
assert(I.isTerminator() && "Expecting a terminator instruction");
for (BasicBlock *Succ : successors(&I))
WorkList.insert(&Succ->front());
}
};
SmallPtrSet<Instruction *, 10> WorkList;
getNextInsts(StartInst, WorkList);
while (!WorkList.empty()) {
Instruction *CurInst = *WorkList.begin();
WorkList.erase(CurInst);
if (CurInst == &EndInst)
continue;
if (!InBetweenInsts.insert(CurInst).second)
continue;
getNextInsts(*CurInst, WorkList);
}
}
bool llvm::isSafeToMoveBefore(Instruction &I, Instruction &InsertPoint,
DominatorTree &DT, const PostDominatorTree &PDT,
DependenceInfo &DI) {
// Cannot move itself before itself.
if (&I == &InsertPoint)
return false;
// Not moved.
if (I.getNextNode() == &InsertPoint)
return true;
if (isa<PHINode>(I) || isa<PHINode>(InsertPoint))
return reportInvalidCandidate(I, NotMovedPHINode);
if (I.isTerminator())
return reportInvalidCandidate(I, NotMovedTerminator);
// TODO remove this limitation.
if (!isControlFlowEquivalent(I, InsertPoint, DT, PDT))
return reportInvalidCandidate(I, NotControlFlowEquivalent);
OrderedInstructions OI(&DT);
DT.updateDFSNumbers();
const bool MoveForward = OI.dfsBefore(&I, &InsertPoint);
if (MoveForward) {
// When I is being moved forward, we need to make sure the InsertPoint
// dominates every users. Or else, a user may be using an undefined I.
for (const Use &U : I.uses())
if (auto *UserInst = dyn_cast<Instruction>(U.getUser()))
if (UserInst != &InsertPoint && !DT.dominates(&InsertPoint, U))
return false;
} else {
// When I is being moved backward, we need to make sure all its opernads
// dominates the InsertPoint. Or else, an operand may be undefined for I.
for (const Value *Op : I.operands())
if (auto *OpInst = dyn_cast<Instruction>(Op))
if (&InsertPoint == OpInst || !OI.dominates(OpInst, &InsertPoint))
return false;
}
Instruction &StartInst = (MoveForward ? I : InsertPoint);
Instruction &EndInst = (MoveForward ? InsertPoint : I);
SmallPtrSet<Instruction *, 10> InstsToCheck;
collectInstructionsInBetween(StartInst, EndInst, InstsToCheck);
if (!MoveForward)
InstsToCheck.insert(&InsertPoint);
// Check if there exists instructions which may throw, may synchonize, or may
// never return, from I to InsertPoint.
if (!isSafeToSpeculativelyExecute(&I))
if (std::any_of(InstsToCheck.begin(), InstsToCheck.end(),
[](Instruction *I) {
if (I->mayThrow())
return true;
const CallBase *CB = dyn_cast<CallBase>(I);
if (!CB)
return false;
if (!CB->hasFnAttr(Attribute::WillReturn))
return true;
if (!CB->hasFnAttr(Attribute::NoSync))
return true;
return false;
})) {
return reportInvalidCandidate(I, MayThrowException);
}
// Check if I has any output/flow/anti dependences with instructions from \p
// StartInst to \p EndInst.
if (std::any_of(InstsToCheck.begin(), InstsToCheck.end(),
[&DI, &I](Instruction *CurInst) {
auto DepResult = DI.depends(&I, CurInst, true);
if (DepResult &&
(DepResult->isOutput() || DepResult->isFlow() ||
DepResult->isAnti()))
return true;
return false;
}))
return reportInvalidCandidate(I, HasDependences);
return true;
}
bool llvm::isSafeToMoveBefore(BasicBlock &BB, Instruction &InsertPoint,
DominatorTree &DT, const PostDominatorTree &PDT,
DependenceInfo &DI) {
return llvm::all_of(BB, [&](Instruction &I) {
if (BB.getTerminator() == &I)
return true;
return isSafeToMoveBefore(I, InsertPoint, DT, PDT, DI);
});
}
void llvm::moveInstructionsToTheBeginning(BasicBlock &FromBB, BasicBlock &ToBB,
DominatorTree &DT,
const PostDominatorTree &PDT,
DependenceInfo &DI) {
for (auto It = ++FromBB.rbegin(); It != FromBB.rend();) {
Instruction *MovePos = ToBB.getFirstNonPHIOrDbg();
Instruction &I = *It;
// Increment the iterator before modifying FromBB.
++It;
if (isSafeToMoveBefore(I, *MovePos, DT, PDT, DI))
I.moveBefore(MovePos);
}
}
void llvm::moveInstructionsToTheEnd(BasicBlock &FromBB, BasicBlock &ToBB,
DominatorTree &DT,
const PostDominatorTree &PDT,
DependenceInfo &DI) {
Instruction *MovePos = ToBB.getTerminator();
while (FromBB.size() > 1) {
Instruction &I = FromBB.front();
if (isSafeToMoveBefore(I, *MovePos, DT, PDT, DI))
I.moveBefore(MovePos);
}
}
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