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Diffstat (limited to 'contrib/llvm/lib/Transforms/Scalar/ConstantHoisting.cpp')
| -rw-r--r-- | contrib/llvm/lib/Transforms/Scalar/ConstantHoisting.cpp | 606 |
1 files changed, 606 insertions, 0 deletions
diff --git a/contrib/llvm/lib/Transforms/Scalar/ConstantHoisting.cpp b/contrib/llvm/lib/Transforms/Scalar/ConstantHoisting.cpp new file mode 100644 index 0000000..4288742 --- /dev/null +++ b/contrib/llvm/lib/Transforms/Scalar/ConstantHoisting.cpp @@ -0,0 +1,606 @@ +//===- ConstantHoisting.cpp - Prepare code for expensive constants --------===// +// +// The LLVM Compiler Infrastructure +// +// This file is distributed under the University of Illinois Open Source +// License. See LICENSE.TXT for details. +// +//===----------------------------------------------------------------------===// +// +// This pass identifies expensive constants to hoist and coalesces them to +// better prepare it for SelectionDAG-based code generation. This works around +// the limitations of the basic-block-at-a-time approach. +// +// First it scans all instructions for integer constants and calculates its +// cost. If the constant can be folded into the instruction (the cost is +// TCC_Free) or the cost is just a simple operation (TCC_BASIC), then we don't +// consider it expensive and leave it alone. This is the default behavior and +// the default implementation of getIntImmCost will always return TCC_Free. +// +// If the cost is more than TCC_BASIC, then the integer constant can't be folded +// into the instruction and it might be beneficial to hoist the constant. +// Similar constants are coalesced to reduce register pressure and +// materialization code. +// +// When a constant is hoisted, it is also hidden behind a bitcast to force it to +// be live-out of the basic block. Otherwise the constant would be just +// duplicated and each basic block would have its own copy in the SelectionDAG. +// The SelectionDAG recognizes such constants as opaque and doesn't perform +// certain transformations on them, which would create a new expensive constant. +// +// This optimization is only applied to integer constants in instructions and +// simple (this means not nested) constant cast expressions. For example: +// %0 = load i64* inttoptr (i64 big_constant to i64*) +//===----------------------------------------------------------------------===// + +#include "llvm/Transforms/Scalar.h" +#include "llvm/ADT/SmallSet.h" +#include "llvm/ADT/SmallVector.h" +#include "llvm/ADT/Statistic.h" +#include "llvm/Analysis/TargetTransformInfo.h" +#include "llvm/IR/Constants.h" +#include "llvm/IR/Dominators.h" +#include "llvm/IR/IntrinsicInst.h" +#include "llvm/Pass.h" +#include "llvm/Support/Debug.h" +#include "llvm/Support/raw_ostream.h" +#include <tuple> + +using namespace llvm; + +#define DEBUG_TYPE "consthoist" + +STATISTIC(NumConstantsHoisted, "Number of constants hoisted"); +STATISTIC(NumConstantsRebased, "Number of constants rebased"); + +namespace { +struct ConstantUser; +struct RebasedConstantInfo; + +typedef SmallVector<ConstantUser, 8> ConstantUseListType; +typedef SmallVector<RebasedConstantInfo, 4> RebasedConstantListType; + +/// \brief Keeps track of the user of a constant and the operand index where the +/// constant is used. +struct ConstantUser { + Instruction *Inst; + unsigned OpndIdx; + + ConstantUser(Instruction *Inst, unsigned Idx) : Inst(Inst), OpndIdx(Idx) { } +}; + +/// \brief Keeps track of a constant candidate and its uses. +struct ConstantCandidate { + ConstantUseListType Uses; + ConstantInt *ConstInt; + unsigned CumulativeCost; + + ConstantCandidate(ConstantInt *ConstInt) + : ConstInt(ConstInt), CumulativeCost(0) { } + + /// \brief Add the user to the use list and update the cost. + void addUser(Instruction *Inst, unsigned Idx, unsigned Cost) { + CumulativeCost += Cost; + Uses.push_back(ConstantUser(Inst, Idx)); + } +}; + +/// \brief This represents a constant that has been rebased with respect to a +/// base constant. The difference to the base constant is recorded in Offset. +struct RebasedConstantInfo { + ConstantUseListType Uses; + Constant *Offset; + + RebasedConstantInfo(ConstantUseListType &&Uses, Constant *Offset) + : Uses(std::move(Uses)), Offset(Offset) { } +}; + +/// \brief A base constant and all its rebased constants. +struct ConstantInfo { + ConstantInt *BaseConstant; + RebasedConstantListType RebasedConstants; +}; + +/// \brief The constant hoisting pass. +class ConstantHoisting : public FunctionPass { + typedef DenseMap<ConstantInt *, unsigned> ConstCandMapType; + typedef std::vector<ConstantCandidate> ConstCandVecType; + + const TargetTransformInfo *TTI; + DominatorTree *DT; + BasicBlock *Entry; + + /// Keeps track of constant candidates found in the function. + ConstCandVecType ConstCandVec; + + /// Keep track of cast instructions we already cloned. + SmallDenseMap<Instruction *, Instruction *> ClonedCastMap; + + /// These are the final constants we decided to hoist. + SmallVector<ConstantInfo, 8> ConstantVec; +public: + static char ID; // Pass identification, replacement for typeid + ConstantHoisting() : FunctionPass(ID), TTI(nullptr), DT(nullptr), + Entry(nullptr) { + initializeConstantHoistingPass(*PassRegistry::getPassRegistry()); + } + + bool runOnFunction(Function &Fn) override; + + const char *getPassName() const override { return "Constant Hoisting"; } + + void getAnalysisUsage(AnalysisUsage &AU) const override { + AU.setPreservesCFG(); + AU.addRequired<DominatorTreeWrapperPass>(); + AU.addRequired<TargetTransformInfoWrapperPass>(); + } + +private: + /// \brief Initialize the pass. + void setup(Function &Fn) { + DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree(); + TTI = &getAnalysis<TargetTransformInfoWrapperPass>().getTTI(Fn); + Entry = &Fn.getEntryBlock(); + } + + /// \brief Cleanup. + void cleanup() { + ConstantVec.clear(); + ClonedCastMap.clear(); + ConstCandVec.clear(); + + TTI = nullptr; + DT = nullptr; + Entry = nullptr; + } + + Instruction *findMatInsertPt(Instruction *Inst, unsigned Idx = ~0U) const; + Instruction *findConstantInsertionPoint(const ConstantInfo &ConstInfo) const; + void collectConstantCandidates(ConstCandMapType &ConstCandMap, + Instruction *Inst, unsigned Idx, + ConstantInt *ConstInt); + void collectConstantCandidates(ConstCandMapType &ConstCandMap, + Instruction *Inst); + void collectConstantCandidates(Function &Fn); + void findAndMakeBaseConstant(ConstCandVecType::iterator S, + ConstCandVecType::iterator E); + void findBaseConstants(); + void emitBaseConstants(Instruction *Base, Constant *Offset, + const ConstantUser &ConstUser); + bool emitBaseConstants(); + void deleteDeadCastInst() const; + bool optimizeConstants(Function &Fn); +}; +} + +char ConstantHoisting::ID = 0; +INITIALIZE_PASS_BEGIN(ConstantHoisting, "consthoist", "Constant Hoisting", + false, false) +INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass) +INITIALIZE_PASS_DEPENDENCY(TargetTransformInfoWrapperPass) +INITIALIZE_PASS_END(ConstantHoisting, "consthoist", "Constant Hoisting", + false, false) + +FunctionPass *llvm::createConstantHoistingPass() { + return new ConstantHoisting(); +} + +/// \brief Perform the constant hoisting optimization for the given function. +bool ConstantHoisting::runOnFunction(Function &Fn) { + if (skipOptnoneFunction(Fn)) + return false; + + DEBUG(dbgs() << "********** Begin Constant Hoisting **********\n"); + DEBUG(dbgs() << "********** Function: " << Fn.getName() << '\n'); + + setup(Fn); + + bool MadeChange = optimizeConstants(Fn); + + if (MadeChange) { + DEBUG(dbgs() << "********** Function after Constant Hoisting: " + << Fn.getName() << '\n'); + DEBUG(dbgs() << Fn); + } + DEBUG(dbgs() << "********** End Constant Hoisting **********\n"); + + cleanup(); + + return MadeChange; +} + + +/// \brief Find the constant materialization insertion point. +Instruction *ConstantHoisting::findMatInsertPt(Instruction *Inst, + unsigned Idx) const { + // If the operand is a cast instruction, then we have to materialize the + // constant before the cast instruction. + if (Idx != ~0U) { + Value *Opnd = Inst->getOperand(Idx); + if (auto CastInst = dyn_cast<Instruction>(Opnd)) + if (CastInst->isCast()) + return CastInst; + } + + // The simple and common case. This also includes constant expressions. + if (!isa<PHINode>(Inst) && !isa<LandingPadInst>(Inst)) + return Inst; + + // We can't insert directly before a phi node or landing pad. Insert before + // the terminator of the incoming or dominating block. + assert(Entry != Inst->getParent() && "PHI or landing pad in entry block!"); + if (Idx != ~0U && isa<PHINode>(Inst)) + return cast<PHINode>(Inst)->getIncomingBlock(Idx)->getTerminator(); + + BasicBlock *IDom = DT->getNode(Inst->getParent())->getIDom()->getBlock(); + return IDom->getTerminator(); +} + +/// \brief Find an insertion point that dominates all uses. +Instruction *ConstantHoisting:: +findConstantInsertionPoint(const ConstantInfo &ConstInfo) const { + assert(!ConstInfo.RebasedConstants.empty() && "Invalid constant info entry."); + // Collect all basic blocks. + SmallPtrSet<BasicBlock *, 8> BBs; + for (auto const &RCI : ConstInfo.RebasedConstants) + for (auto const &U : RCI.Uses) + BBs.insert(findMatInsertPt(U.Inst, U.OpndIdx)->getParent()); + + if (BBs.count(Entry)) + return &Entry->front(); + + while (BBs.size() >= 2) { + BasicBlock *BB, *BB1, *BB2; + BB1 = *BBs.begin(); + BB2 = *std::next(BBs.begin()); + BB = DT->findNearestCommonDominator(BB1, BB2); + if (BB == Entry) + return &Entry->front(); + BBs.erase(BB1); + BBs.erase(BB2); + BBs.insert(BB); + } + assert((BBs.size() == 1) && "Expected only one element."); + Instruction &FirstInst = (*BBs.begin())->front(); + return findMatInsertPt(&FirstInst); +} + + +/// \brief Record constant integer ConstInt for instruction Inst at operand +/// index Idx. +/// +/// The operand at index Idx is not necessarily the constant integer itself. It +/// could also be a cast instruction or a constant expression that uses the +// constant integer. +void ConstantHoisting::collectConstantCandidates(ConstCandMapType &ConstCandMap, + Instruction *Inst, + unsigned Idx, + ConstantInt *ConstInt) { + unsigned Cost; + // Ask the target about the cost of materializing the constant for the given + // instruction and operand index. + if (auto IntrInst = dyn_cast<IntrinsicInst>(Inst)) + Cost = TTI->getIntImmCost(IntrInst->getIntrinsicID(), Idx, + ConstInt->getValue(), ConstInt->getType()); + else + Cost = TTI->getIntImmCost(Inst->getOpcode(), Idx, ConstInt->getValue(), + ConstInt->getType()); + + // Ignore cheap integer constants. + if (Cost > TargetTransformInfo::TCC_Basic) { + ConstCandMapType::iterator Itr; + bool Inserted; + std::tie(Itr, Inserted) = ConstCandMap.insert(std::make_pair(ConstInt, 0)); + if (Inserted) { + ConstCandVec.push_back(ConstantCandidate(ConstInt)); + Itr->second = ConstCandVec.size() - 1; + } + ConstCandVec[Itr->second].addUser(Inst, Idx, Cost); + DEBUG(if (isa<ConstantInt>(Inst->getOperand(Idx))) + dbgs() << "Collect constant " << *ConstInt << " from " << *Inst + << " with cost " << Cost << '\n'; + else + dbgs() << "Collect constant " << *ConstInt << " indirectly from " + << *Inst << " via " << *Inst->getOperand(Idx) << " with cost " + << Cost << '\n'; + ); + } +} + +/// \brief Scan the instruction for expensive integer constants and record them +/// in the constant candidate vector. +void ConstantHoisting::collectConstantCandidates(ConstCandMapType &ConstCandMap, + Instruction *Inst) { + // Skip all cast instructions. They are visited indirectly later on. + if (Inst->isCast()) + return; + + // Can't handle inline asm. Skip it. + if (auto Call = dyn_cast<CallInst>(Inst)) + if (isa<InlineAsm>(Call->getCalledValue())) + return; + + // Scan all operands. + for (unsigned Idx = 0, E = Inst->getNumOperands(); Idx != E; ++Idx) { + Value *Opnd = Inst->getOperand(Idx); + + // Visit constant integers. + if (auto ConstInt = dyn_cast<ConstantInt>(Opnd)) { + collectConstantCandidates(ConstCandMap, Inst, Idx, ConstInt); + continue; + } + + // Visit cast instructions that have constant integers. + if (auto CastInst = dyn_cast<Instruction>(Opnd)) { + // Only visit cast instructions, which have been skipped. All other + // instructions should have already been visited. + if (!CastInst->isCast()) + continue; + + if (auto *ConstInt = dyn_cast<ConstantInt>(CastInst->getOperand(0))) { + // Pretend the constant is directly used by the instruction and ignore + // the cast instruction. + collectConstantCandidates(ConstCandMap, Inst, Idx, ConstInt); + continue; + } + } + + // Visit constant expressions that have constant integers. + if (auto ConstExpr = dyn_cast<ConstantExpr>(Opnd)) { + // Only visit constant cast expressions. + if (!ConstExpr->isCast()) + continue; + + if (auto ConstInt = dyn_cast<ConstantInt>(ConstExpr->getOperand(0))) { + // Pretend the constant is directly used by the instruction and ignore + // the constant expression. + collectConstantCandidates(ConstCandMap, Inst, Idx, ConstInt); + continue; + } + } + } // end of for all operands +} + +/// \brief Collect all integer constants in the function that cannot be folded +/// into an instruction itself. +void ConstantHoisting::collectConstantCandidates(Function &Fn) { + ConstCandMapType ConstCandMap; + for (Function::iterator BB : Fn) + for (BasicBlock::iterator Inst : *BB) + collectConstantCandidates(ConstCandMap, Inst); +} + +/// \brief Find the base constant within the given range and rebase all other +/// constants with respect to the base constant. +void ConstantHoisting::findAndMakeBaseConstant(ConstCandVecType::iterator S, + ConstCandVecType::iterator E) { + auto MaxCostItr = S; + unsigned NumUses = 0; + // Use the constant that has the maximum cost as base constant. + for (auto ConstCand = S; ConstCand != E; ++ConstCand) { + NumUses += ConstCand->Uses.size(); + if (ConstCand->CumulativeCost > MaxCostItr->CumulativeCost) + MaxCostItr = ConstCand; + } + + // Don't hoist constants that have only one use. + if (NumUses <= 1) + return; + + ConstantInfo ConstInfo; + ConstInfo.BaseConstant = MaxCostItr->ConstInt; + Type *Ty = ConstInfo.BaseConstant->getType(); + + // Rebase the constants with respect to the base constant. + for (auto ConstCand = S; ConstCand != E; ++ConstCand) { + APInt Diff = ConstCand->ConstInt->getValue() - + ConstInfo.BaseConstant->getValue(); + Constant *Offset = Diff == 0 ? nullptr : ConstantInt::get(Ty, Diff); + ConstInfo.RebasedConstants.push_back( + RebasedConstantInfo(std::move(ConstCand->Uses), Offset)); + } + ConstantVec.push_back(std::move(ConstInfo)); +} + +/// \brief Finds and combines constant candidates that can be easily +/// rematerialized with an add from a common base constant. +void ConstantHoisting::findBaseConstants() { + // Sort the constants by value and type. This invalidates the mapping! + std::sort(ConstCandVec.begin(), ConstCandVec.end(), + [](const ConstantCandidate &LHS, const ConstantCandidate &RHS) { + if (LHS.ConstInt->getType() != RHS.ConstInt->getType()) + return LHS.ConstInt->getType()->getBitWidth() < + RHS.ConstInt->getType()->getBitWidth(); + return LHS.ConstInt->getValue().ult(RHS.ConstInt->getValue()); + }); + + // Simple linear scan through the sorted constant candidate vector for viable + // merge candidates. + auto MinValItr = ConstCandVec.begin(); + for (auto CC = std::next(ConstCandVec.begin()), E = ConstCandVec.end(); + CC != E; ++CC) { + if (MinValItr->ConstInt->getType() == CC->ConstInt->getType()) { + // Check if the constant is in range of an add with immediate. + APInt Diff = CC->ConstInt->getValue() - MinValItr->ConstInt->getValue(); + if ((Diff.getBitWidth() <= 64) && + TTI->isLegalAddImmediate(Diff.getSExtValue())) + continue; + } + // We either have now a different constant type or the constant is not in + // range of an add with immediate anymore. + findAndMakeBaseConstant(MinValItr, CC); + // Start a new base constant search. + MinValItr = CC; + } + // Finalize the last base constant search. + findAndMakeBaseConstant(MinValItr, ConstCandVec.end()); +} + +/// \brief Updates the operand at Idx in instruction Inst with the result of +/// instruction Mat. If the instruction is a PHI node then special +/// handling for duplicate values form the same incomming basic block is +/// required. +/// \return The update will always succeed, but the return value indicated if +/// Mat was used for the update or not. +static bool updateOperand(Instruction *Inst, unsigned Idx, Instruction *Mat) { + if (auto PHI = dyn_cast<PHINode>(Inst)) { + // Check if any previous operand of the PHI node has the same incoming basic + // block. This is a very odd case that happens when the incoming basic block + // has a switch statement. In this case use the same value as the previous + // operand(s), otherwise we will fail verification due to different values. + // The values are actually the same, but the variable names are different + // and the verifier doesn't like that. + BasicBlock *IncomingBB = PHI->getIncomingBlock(Idx); + for (unsigned i = 0; i < Idx; ++i) { + if (PHI->getIncomingBlock(i) == IncomingBB) { + Value *IncomingVal = PHI->getIncomingValue(i); + Inst->setOperand(Idx, IncomingVal); + return false; + } + } + } + + Inst->setOperand(Idx, Mat); + return true; +} + +/// \brief Emit materialization code for all rebased constants and update their +/// users. +void ConstantHoisting::emitBaseConstants(Instruction *Base, Constant *Offset, + const ConstantUser &ConstUser) { + Instruction *Mat = Base; + if (Offset) { + Instruction *InsertionPt = findMatInsertPt(ConstUser.Inst, + ConstUser.OpndIdx); + Mat = BinaryOperator::Create(Instruction::Add, Base, Offset, + "const_mat", InsertionPt); + + DEBUG(dbgs() << "Materialize constant (" << *Base->getOperand(0) + << " + " << *Offset << ") in BB " + << Mat->getParent()->getName() << '\n' << *Mat << '\n'); + Mat->setDebugLoc(ConstUser.Inst->getDebugLoc()); + } + Value *Opnd = ConstUser.Inst->getOperand(ConstUser.OpndIdx); + + // Visit constant integer. + if (isa<ConstantInt>(Opnd)) { + DEBUG(dbgs() << "Update: " << *ConstUser.Inst << '\n'); + if (!updateOperand(ConstUser.Inst, ConstUser.OpndIdx, Mat) && Offset) + Mat->eraseFromParent(); + DEBUG(dbgs() << "To : " << *ConstUser.Inst << '\n'); + return; + } + + // Visit cast instruction. + if (auto CastInst = dyn_cast<Instruction>(Opnd)) { + assert(CastInst->isCast() && "Expected an cast instruction!"); + // Check if we already have visited this cast instruction before to avoid + // unnecessary cloning. + Instruction *&ClonedCastInst = ClonedCastMap[CastInst]; + if (!ClonedCastInst) { + ClonedCastInst = CastInst->clone(); + ClonedCastInst->setOperand(0, Mat); + ClonedCastInst->insertAfter(CastInst); + // Use the same debug location as the original cast instruction. + ClonedCastInst->setDebugLoc(CastInst->getDebugLoc()); + DEBUG(dbgs() << "Clone instruction: " << *CastInst << '\n' + << "To : " << *ClonedCastInst << '\n'); + } + + DEBUG(dbgs() << "Update: " << *ConstUser.Inst << '\n'); + updateOperand(ConstUser.Inst, ConstUser.OpndIdx, ClonedCastInst); + DEBUG(dbgs() << "To : " << *ConstUser.Inst << '\n'); + return; + } + + // Visit constant expression. + if (auto ConstExpr = dyn_cast<ConstantExpr>(Opnd)) { + Instruction *ConstExprInst = ConstExpr->getAsInstruction(); + ConstExprInst->setOperand(0, Mat); + ConstExprInst->insertBefore(findMatInsertPt(ConstUser.Inst, + ConstUser.OpndIdx)); + + // Use the same debug location as the instruction we are about to update. + ConstExprInst->setDebugLoc(ConstUser.Inst->getDebugLoc()); + + DEBUG(dbgs() << "Create instruction: " << *ConstExprInst << '\n' + << "From : " << *ConstExpr << '\n'); + DEBUG(dbgs() << "Update: " << *ConstUser.Inst << '\n'); + if (!updateOperand(ConstUser.Inst, ConstUser.OpndIdx, ConstExprInst)) { + ConstExprInst->eraseFromParent(); + if (Offset) + Mat->eraseFromParent(); + } + DEBUG(dbgs() << "To : " << *ConstUser.Inst << '\n'); + return; + } +} + +/// \brief Hoist and hide the base constant behind a bitcast and emit +/// materialization code for derived constants. +bool ConstantHoisting::emitBaseConstants() { + bool MadeChange = false; + for (auto const &ConstInfo : ConstantVec) { + // Hoist and hide the base constant behind a bitcast. + Instruction *IP = findConstantInsertionPoint(ConstInfo); + IntegerType *Ty = ConstInfo.BaseConstant->getType(); + Instruction *Base = + new BitCastInst(ConstInfo.BaseConstant, Ty, "const", IP); + DEBUG(dbgs() << "Hoist constant (" << *ConstInfo.BaseConstant << ") to BB " + << IP->getParent()->getName() << '\n' << *Base << '\n'); + NumConstantsHoisted++; + + // Emit materialization code for all rebased constants. + for (auto const &RCI : ConstInfo.RebasedConstants) { + NumConstantsRebased++; + for (auto const &U : RCI.Uses) + emitBaseConstants(Base, RCI.Offset, U); + } + + // Use the same debug location as the last user of the constant. + assert(!Base->use_empty() && "The use list is empty!?"); + assert(isa<Instruction>(Base->user_back()) && + "All uses should be instructions."); + Base->setDebugLoc(cast<Instruction>(Base->user_back())->getDebugLoc()); + + // Correct for base constant, which we counted above too. + NumConstantsRebased--; + MadeChange = true; + } + return MadeChange; +} + +/// \brief Check all cast instructions we made a copy of and remove them if they +/// have no more users. +void ConstantHoisting::deleteDeadCastInst() const { + for (auto const &I : ClonedCastMap) + if (I.first->use_empty()) + I.first->eraseFromParent(); +} + +/// \brief Optimize expensive integer constants in the given function. +bool ConstantHoisting::optimizeConstants(Function &Fn) { + // Collect all constant candidates. + collectConstantCandidates(Fn); + + // There are no constant candidates to worry about. + if (ConstCandVec.empty()) + return false; + + // Combine constants that can be easily materialized with an add from a common + // base constant. + findBaseConstants(); + + // There are no constants to emit. + if (ConstantVec.empty()) + return false; + + // Finally hoist the base constant and emit materialization code for dependent + // constants. + bool MadeChange = emitBaseConstants(); + + // Cleanup dead instructions. + deleteDeadCastInst(); + + return MadeChange; +} |
