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Diffstat (limited to 'llvm/lib/Transforms/Scalar/ConstantHoisting.cpp')
| -rw-r--r-- | llvm/lib/Transforms/Scalar/ConstantHoisting.cpp | 979 |
1 files changed, 979 insertions, 0 deletions
diff --git a/llvm/lib/Transforms/Scalar/ConstantHoisting.cpp b/llvm/lib/Transforms/Scalar/ConstantHoisting.cpp new file mode 100644 index 0000000000000..9f340afbf7c28 --- /dev/null +++ b/llvm/lib/Transforms/Scalar/ConstantHoisting.cpp @@ -0,0 +1,979 @@ +//===- ConstantHoisting.cpp - Prepare code for expensive constants --------===// +// +// 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 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/ConstantHoisting.h" +#include "llvm/ADT/APInt.h" +#include "llvm/ADT/DenseMap.h" +#include "llvm/ADT/None.h" +#include "llvm/ADT/Optional.h" +#include "llvm/ADT/SmallPtrSet.h" +#include "llvm/ADT/SmallVector.h" +#include "llvm/ADT/Statistic.h" +#include "llvm/Analysis/BlockFrequencyInfo.h" +#include "llvm/Analysis/ProfileSummaryInfo.h" +#include "llvm/Analysis/TargetTransformInfo.h" +#include "llvm/Transforms/Utils/Local.h" +#include "llvm/IR/BasicBlock.h" +#include "llvm/IR/Constants.h" +#include "llvm/IR/DebugInfoMetadata.h" +#include "llvm/IR/Dominators.h" +#include "llvm/IR/Function.h" +#include "llvm/IR/InstrTypes.h" +#include "llvm/IR/Instruction.h" +#include "llvm/IR/Instructions.h" +#include "llvm/IR/IntrinsicInst.h" +#include "llvm/IR/Value.h" +#include "llvm/Pass.h" +#include "llvm/Support/BlockFrequency.h" +#include "llvm/Support/Casting.h" +#include "llvm/Support/CommandLine.h" +#include "llvm/Support/Debug.h" +#include "llvm/Support/raw_ostream.h" +#include "llvm/Transforms/Scalar.h" +#include "llvm/Transforms/Utils/SizeOpts.h" +#include <algorithm> +#include <cassert> +#include <cstdint> +#include <iterator> +#include <tuple> +#include <utility> + +using namespace llvm; +using namespace consthoist; + +#define DEBUG_TYPE "consthoist" + +STATISTIC(NumConstantsHoisted, "Number of constants hoisted"); +STATISTIC(NumConstantsRebased, "Number of constants rebased"); + +static cl::opt<bool> ConstHoistWithBlockFrequency( + "consthoist-with-block-frequency", cl::init(true), cl::Hidden, + cl::desc("Enable the use of the block frequency analysis to reduce the " + "chance to execute const materialization more frequently than " + "without hoisting.")); + +static cl::opt<bool> ConstHoistGEP( + "consthoist-gep", cl::init(false), cl::Hidden, + cl::desc("Try hoisting constant gep expressions")); + +static cl::opt<unsigned> +MinNumOfDependentToRebase("consthoist-min-num-to-rebase", + cl::desc("Do not rebase if number of dependent constants of a Base is less " + "than this number."), + cl::init(0), cl::Hidden); + +namespace { + +/// The constant hoisting pass. +class ConstantHoistingLegacyPass : public FunctionPass { +public: + static char ID; // Pass identification, replacement for typeid + + ConstantHoistingLegacyPass() : FunctionPass(ID) { + initializeConstantHoistingLegacyPassPass(*PassRegistry::getPassRegistry()); + } + + bool runOnFunction(Function &Fn) override; + + StringRef getPassName() const override { return "Constant Hoisting"; } + + void getAnalysisUsage(AnalysisUsage &AU) const override { + AU.setPreservesCFG(); + if (ConstHoistWithBlockFrequency) + AU.addRequired<BlockFrequencyInfoWrapperPass>(); + AU.addRequired<DominatorTreeWrapperPass>(); + AU.addRequired<ProfileSummaryInfoWrapperPass>(); + AU.addRequired<TargetTransformInfoWrapperPass>(); + } + +private: + ConstantHoistingPass Impl; +}; + +} // end anonymous namespace + +char ConstantHoistingLegacyPass::ID = 0; + +INITIALIZE_PASS_BEGIN(ConstantHoistingLegacyPass, "consthoist", + "Constant Hoisting", false, false) +INITIALIZE_PASS_DEPENDENCY(BlockFrequencyInfoWrapperPass) +INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass) +INITIALIZE_PASS_DEPENDENCY(ProfileSummaryInfoWrapperPass) +INITIALIZE_PASS_DEPENDENCY(TargetTransformInfoWrapperPass) +INITIALIZE_PASS_END(ConstantHoistingLegacyPass, "consthoist", + "Constant Hoisting", false, false) + +FunctionPass *llvm::createConstantHoistingPass() { + return new ConstantHoistingLegacyPass(); +} + +/// Perform the constant hoisting optimization for the given function. +bool ConstantHoistingLegacyPass::runOnFunction(Function &Fn) { + if (skipFunction(Fn)) + return false; + + LLVM_DEBUG(dbgs() << "********** Begin Constant Hoisting **********\n"); + LLVM_DEBUG(dbgs() << "********** Function: " << Fn.getName() << '\n'); + + bool MadeChange = + Impl.runImpl(Fn, getAnalysis<TargetTransformInfoWrapperPass>().getTTI(Fn), + getAnalysis<DominatorTreeWrapperPass>().getDomTree(), + ConstHoistWithBlockFrequency + ? &getAnalysis<BlockFrequencyInfoWrapperPass>().getBFI() + : nullptr, + Fn.getEntryBlock(), + &getAnalysis<ProfileSummaryInfoWrapperPass>().getPSI()); + + if (MadeChange) { + LLVM_DEBUG(dbgs() << "********** Function after Constant Hoisting: " + << Fn.getName() << '\n'); + LLVM_DEBUG(dbgs() << Fn); + } + LLVM_DEBUG(dbgs() << "********** End Constant Hoisting **********\n"); + + return MadeChange; +} + +/// Find the constant materialization insertion point. +Instruction *ConstantHoistingPass::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) && !Inst->isEHPad()) + return Inst; + + // We can't insert directly before a phi node or an eh 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(); + + // This must be an EH pad. Iterate over immediate dominators until we find a + // non-EH pad. We need to skip over catchswitch blocks, which are both EH pads + // and terminators. + auto IDom = DT->getNode(Inst->getParent())->getIDom(); + while (IDom->getBlock()->isEHPad()) { + assert(Entry != IDom->getBlock() && "eh pad in entry block"); + IDom = IDom->getIDom(); + } + + return IDom->getBlock()->getTerminator(); +} + +/// Given \p BBs as input, find another set of BBs which collectively +/// dominates \p BBs and have the minimal sum of frequencies. Return the BB +/// set found in \p BBs. +static void findBestInsertionSet(DominatorTree &DT, BlockFrequencyInfo &BFI, + BasicBlock *Entry, + SetVector<BasicBlock *> &BBs) { + assert(!BBs.count(Entry) && "Assume Entry is not in BBs"); + // Nodes on the current path to the root. + SmallPtrSet<BasicBlock *, 8> Path; + // Candidates includes any block 'BB' in set 'BBs' that is not strictly + // dominated by any other blocks in set 'BBs', and all nodes in the path + // in the dominator tree from Entry to 'BB'. + SmallPtrSet<BasicBlock *, 16> Candidates; + for (auto BB : BBs) { + // Ignore unreachable basic blocks. + if (!DT.isReachableFromEntry(BB)) + continue; + Path.clear(); + // Walk up the dominator tree until Entry or another BB in BBs + // is reached. Insert the nodes on the way to the Path. + BasicBlock *Node = BB; + // The "Path" is a candidate path to be added into Candidates set. + bool isCandidate = false; + do { + Path.insert(Node); + if (Node == Entry || Candidates.count(Node)) { + isCandidate = true; + break; + } + assert(DT.getNode(Node)->getIDom() && + "Entry doens't dominate current Node"); + Node = DT.getNode(Node)->getIDom()->getBlock(); + } while (!BBs.count(Node)); + + // If isCandidate is false, Node is another Block in BBs dominating + // current 'BB'. Drop the nodes on the Path. + if (!isCandidate) + continue; + + // Add nodes on the Path into Candidates. + Candidates.insert(Path.begin(), Path.end()); + } + + // Sort the nodes in Candidates in top-down order and save the nodes + // in Orders. + unsigned Idx = 0; + SmallVector<BasicBlock *, 16> Orders; + Orders.push_back(Entry); + while (Idx != Orders.size()) { + BasicBlock *Node = Orders[Idx++]; + for (auto ChildDomNode : DT.getNode(Node)->getChildren()) { + if (Candidates.count(ChildDomNode->getBlock())) + Orders.push_back(ChildDomNode->getBlock()); + } + } + + // Visit Orders in bottom-up order. + using InsertPtsCostPair = + std::pair<SetVector<BasicBlock *>, BlockFrequency>; + + // InsertPtsMap is a map from a BB to the best insertion points for the + // subtree of BB (subtree not including the BB itself). + DenseMap<BasicBlock *, InsertPtsCostPair> InsertPtsMap; + InsertPtsMap.reserve(Orders.size() + 1); + for (auto RIt = Orders.rbegin(); RIt != Orders.rend(); RIt++) { + BasicBlock *Node = *RIt; + bool NodeInBBs = BBs.count(Node); + auto &InsertPts = InsertPtsMap[Node].first; + BlockFrequency &InsertPtsFreq = InsertPtsMap[Node].second; + + // Return the optimal insert points in BBs. + if (Node == Entry) { + BBs.clear(); + if (InsertPtsFreq > BFI.getBlockFreq(Node) || + (InsertPtsFreq == BFI.getBlockFreq(Node) && InsertPts.size() > 1)) + BBs.insert(Entry); + else + BBs.insert(InsertPts.begin(), InsertPts.end()); + break; + } + + BasicBlock *Parent = DT.getNode(Node)->getIDom()->getBlock(); + // Initially, ParentInsertPts is empty and ParentPtsFreq is 0. Every child + // will update its parent's ParentInsertPts and ParentPtsFreq. + auto &ParentInsertPts = InsertPtsMap[Parent].first; + BlockFrequency &ParentPtsFreq = InsertPtsMap[Parent].second; + // Choose to insert in Node or in subtree of Node. + // Don't hoist to EHPad because we may not find a proper place to insert + // in EHPad. + // If the total frequency of InsertPts is the same as the frequency of the + // target Node, and InsertPts contains more than one nodes, choose hoisting + // to reduce code size. + if (NodeInBBs || + (!Node->isEHPad() && + (InsertPtsFreq > BFI.getBlockFreq(Node) || + (InsertPtsFreq == BFI.getBlockFreq(Node) && InsertPts.size() > 1)))) { + ParentInsertPts.insert(Node); + ParentPtsFreq += BFI.getBlockFreq(Node); + } else { + ParentInsertPts.insert(InsertPts.begin(), InsertPts.end()); + ParentPtsFreq += InsertPtsFreq; + } + } +} + +/// Find an insertion point that dominates all uses. +SetVector<Instruction *> ConstantHoistingPass::findConstantInsertionPoint( + const ConstantInfo &ConstInfo) const { + assert(!ConstInfo.RebasedConstants.empty() && "Invalid constant info entry."); + // Collect all basic blocks. + SetVector<BasicBlock *> BBs; + SetVector<Instruction *> InsertPts; + for (auto const &RCI : ConstInfo.RebasedConstants) + for (auto const &U : RCI.Uses) + BBs.insert(findMatInsertPt(U.Inst, U.OpndIdx)->getParent()); + + if (BBs.count(Entry)) { + InsertPts.insert(&Entry->front()); + return InsertPts; + } + + if (BFI) { + findBestInsertionSet(*DT, *BFI, Entry, BBs); + for (auto BB : BBs) { + BasicBlock::iterator InsertPt = BB->begin(); + for (; isa<PHINode>(InsertPt) || InsertPt->isEHPad(); ++InsertPt) + ; + InsertPts.insert(&*InsertPt); + } + return InsertPts; + } + + while (BBs.size() >= 2) { + BasicBlock *BB, *BB1, *BB2; + BB1 = BBs.pop_back_val(); + BB2 = BBs.pop_back_val(); + BB = DT->findNearestCommonDominator(BB1, BB2); + if (BB == Entry) { + InsertPts.insert(&Entry->front()); + return InsertPts; + } + BBs.insert(BB); + } + assert((BBs.size() == 1) && "Expected only one element."); + Instruction &FirstInst = (*BBs.begin())->front(); + InsertPts.insert(findMatInsertPt(&FirstInst)); + return InsertPts; +} + +/// 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 ConstantHoistingPass::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; + ConstPtrUnionType Cand = ConstInt; + std::tie(Itr, Inserted) = ConstCandMap.insert(std::make_pair(Cand, 0)); + if (Inserted) { + ConstIntCandVec.push_back(ConstantCandidate(ConstInt)); + Itr->second = ConstIntCandVec.size() - 1; + } + ConstIntCandVec[Itr->second].addUser(Inst, Idx, Cost); + LLVM_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';); + } +} + +/// Record constant GEP expression for instruction Inst at operand index Idx. +void ConstantHoistingPass::collectConstantCandidates( + ConstCandMapType &ConstCandMap, Instruction *Inst, unsigned Idx, + ConstantExpr *ConstExpr) { + // TODO: Handle vector GEPs + if (ConstExpr->getType()->isVectorTy()) + return; + + GlobalVariable *BaseGV = dyn_cast<GlobalVariable>(ConstExpr->getOperand(0)); + if (!BaseGV) + return; + + // Get offset from the base GV. + PointerType *GVPtrTy = cast<PointerType>(BaseGV->getType()); + IntegerType *PtrIntTy = DL->getIntPtrType(*Ctx, GVPtrTy->getAddressSpace()); + APInt Offset(DL->getTypeSizeInBits(PtrIntTy), /*val*/0, /*isSigned*/true); + auto *GEPO = cast<GEPOperator>(ConstExpr); + if (!GEPO->accumulateConstantOffset(*DL, Offset)) + return; + + if (!Offset.isIntN(32)) + return; + + // A constant GEP expression that has a GlobalVariable as base pointer is + // usually lowered to a load from constant pool. Such operation is unlikely + // to be cheaper than compute it by <Base + Offset>, which can be lowered to + // an ADD instruction or folded into Load/Store instruction. + int Cost = TTI->getIntImmCost(Instruction::Add, 1, Offset, PtrIntTy); + ConstCandVecType &ExprCandVec = ConstGEPCandMap[BaseGV]; + ConstCandMapType::iterator Itr; + bool Inserted; + ConstPtrUnionType Cand = ConstExpr; + std::tie(Itr, Inserted) = ConstCandMap.insert(std::make_pair(Cand, 0)); + if (Inserted) { + ExprCandVec.push_back(ConstantCandidate( + ConstantInt::get(Type::getInt32Ty(*Ctx), Offset.getLimitedValue()), + ConstExpr)); + Itr->second = ExprCandVec.size() - 1; + } + ExprCandVec[Itr->second].addUser(Inst, Idx, Cost); +} + +/// Check the operand for instruction Inst at index Idx. +void ConstantHoistingPass::collectConstantCandidates( + ConstCandMapType &ConstCandMap, Instruction *Inst, unsigned Idx) { + Value *Opnd = Inst->getOperand(Idx); + + // Visit constant integers. + if (auto ConstInt = dyn_cast<ConstantInt>(Opnd)) { + collectConstantCandidates(ConstCandMap, Inst, Idx, ConstInt); + return; + } + + // 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()) + return; + + 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); + return; + } + } + + // Visit constant expressions that have constant integers. + if (auto ConstExpr = dyn_cast<ConstantExpr>(Opnd)) { + // Handle constant gep expressions. + if (ConstHoistGEP && ConstExpr->isGEPWithNoNotionalOverIndexing()) + collectConstantCandidates(ConstCandMap, Inst, Idx, ConstExpr); + + // Only visit constant cast expressions. + if (!ConstExpr->isCast()) + return; + + 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); + return; + } + } +} + +/// Scan the instruction for expensive integer constants and record them +/// in the constant candidate vector. +void ConstantHoistingPass::collectConstantCandidates( + ConstCandMapType &ConstCandMap, Instruction *Inst) { + // Skip all cast instructions. They are visited indirectly later on. + if (Inst->isCast()) + return; + + // Scan all operands. + for (unsigned Idx = 0, E = Inst->getNumOperands(); Idx != E; ++Idx) { + // The cost of materializing the constants (defined in + // `TargetTransformInfo::getIntImmCost`) for instructions which only take + // constant variables is lower than `TargetTransformInfo::TCC_Basic`. So + // it's safe for us to collect constant candidates from all IntrinsicInsts. + if (canReplaceOperandWithVariable(Inst, Idx) || isa<IntrinsicInst>(Inst)) { + collectConstantCandidates(ConstCandMap, Inst, Idx); + } + } // end of for all operands +} + +/// Collect all integer constants in the function that cannot be folded +/// into an instruction itself. +void ConstantHoistingPass::collectConstantCandidates(Function &Fn) { + ConstCandMapType ConstCandMap; + for (BasicBlock &BB : Fn) + for (Instruction &Inst : BB) + collectConstantCandidates(ConstCandMap, &Inst); +} + +// This helper function is necessary to deal with values that have different +// bit widths (APInt Operator- does not like that). If the value cannot be +// represented in uint64 we return an "empty" APInt. This is then interpreted +// as the value is not in range. +static Optional<APInt> calculateOffsetDiff(const APInt &V1, const APInt &V2) { + Optional<APInt> Res = None; + unsigned BW = V1.getBitWidth() > V2.getBitWidth() ? + V1.getBitWidth() : V2.getBitWidth(); + uint64_t LimVal1 = V1.getLimitedValue(); + uint64_t LimVal2 = V2.getLimitedValue(); + + if (LimVal1 == ~0ULL || LimVal2 == ~0ULL) + return Res; + + uint64_t Diff = LimVal1 - LimVal2; + return APInt(BW, Diff, true); +} + +// From a list of constants, one needs to picked as the base and the other +// constants will be transformed into an offset from that base constant. The +// question is which we can pick best? For example, consider these constants +// and their number of uses: +// +// Constants| 2 | 4 | 12 | 42 | +// NumUses | 3 | 2 | 8 | 7 | +// +// Selecting constant 12 because it has the most uses will generate negative +// offsets for constants 2 and 4 (i.e. -10 and -8 respectively). If negative +// offsets lead to less optimal code generation, then there might be better +// solutions. Suppose immediates in the range of 0..35 are most optimally +// supported by the architecture, then selecting constant 2 is most optimal +// because this will generate offsets: 0, 2, 10, 40. Offsets 0, 2 and 10 are in +// range 0..35, and thus 3 + 2 + 8 = 13 uses are in range. Selecting 12 would +// have only 8 uses in range, so choosing 2 as a base is more optimal. Thus, in +// selecting the base constant the range of the offsets is a very important +// factor too that we take into account here. This algorithm calculates a total +// costs for selecting a constant as the base and substract the costs if +// immediates are out of range. It has quadratic complexity, so we call this +// function only when we're optimising for size and there are less than 100 +// constants, we fall back to the straightforward algorithm otherwise +// which does not do all the offset calculations. +unsigned +ConstantHoistingPass::maximizeConstantsInRange(ConstCandVecType::iterator S, + ConstCandVecType::iterator E, + ConstCandVecType::iterator &MaxCostItr) { + unsigned NumUses = 0; + + bool OptForSize = Entry->getParent()->hasOptSize() || + llvm::shouldOptimizeForSize(Entry->getParent(), PSI, BFI); + if (!OptForSize || std::distance(S,E) > 100) { + for (auto ConstCand = S; ConstCand != E; ++ConstCand) { + NumUses += ConstCand->Uses.size(); + if (ConstCand->CumulativeCost > MaxCostItr->CumulativeCost) + MaxCostItr = ConstCand; + } + return NumUses; + } + + LLVM_DEBUG(dbgs() << "== Maximize constants in range ==\n"); + int MaxCost = -1; + for (auto ConstCand = S; ConstCand != E; ++ConstCand) { + auto Value = ConstCand->ConstInt->getValue(); + Type *Ty = ConstCand->ConstInt->getType(); + int Cost = 0; + NumUses += ConstCand->Uses.size(); + LLVM_DEBUG(dbgs() << "= Constant: " << ConstCand->ConstInt->getValue() + << "\n"); + + for (auto User : ConstCand->Uses) { + unsigned Opcode = User.Inst->getOpcode(); + unsigned OpndIdx = User.OpndIdx; + Cost += TTI->getIntImmCost(Opcode, OpndIdx, Value, Ty); + LLVM_DEBUG(dbgs() << "Cost: " << Cost << "\n"); + + for (auto C2 = S; C2 != E; ++C2) { + Optional<APInt> Diff = calculateOffsetDiff( + C2->ConstInt->getValue(), + ConstCand->ConstInt->getValue()); + if (Diff) { + const int ImmCosts = + TTI->getIntImmCodeSizeCost(Opcode, OpndIdx, Diff.getValue(), Ty); + Cost -= ImmCosts; + LLVM_DEBUG(dbgs() << "Offset " << Diff.getValue() << " " + << "has penalty: " << ImmCosts << "\n" + << "Adjusted cost: " << Cost << "\n"); + } + } + } + LLVM_DEBUG(dbgs() << "Cumulative cost: " << Cost << "\n"); + if (Cost > MaxCost) { + MaxCost = Cost; + MaxCostItr = ConstCand; + LLVM_DEBUG(dbgs() << "New candidate: " << MaxCostItr->ConstInt->getValue() + << "\n"); + } + } + return NumUses; +} + +/// Find the base constant within the given range and rebase all other +/// constants with respect to the base constant. +void ConstantHoistingPass::findAndMakeBaseConstant( + ConstCandVecType::iterator S, ConstCandVecType::iterator E, + SmallVectorImpl<consthoist::ConstantInfo> &ConstInfoVec) { + auto MaxCostItr = S; + unsigned NumUses = maximizeConstantsInRange(S, E, MaxCostItr); + + // Don't hoist constants that have only one use. + if (NumUses <= 1) + return; + + ConstantInt *ConstInt = MaxCostItr->ConstInt; + ConstantExpr *ConstExpr = MaxCostItr->ConstExpr; + ConstantInfo ConstInfo; + ConstInfo.BaseInt = ConstInt; + ConstInfo.BaseExpr = ConstExpr; + Type *Ty = ConstInt->getType(); + + // Rebase the constants with respect to the base constant. + for (auto ConstCand = S; ConstCand != E; ++ConstCand) { + APInt Diff = ConstCand->ConstInt->getValue() - ConstInt->getValue(); + Constant *Offset = Diff == 0 ? nullptr : ConstantInt::get(Ty, Diff); + Type *ConstTy = + ConstCand->ConstExpr ? ConstCand->ConstExpr->getType() : nullptr; + ConstInfo.RebasedConstants.push_back( + RebasedConstantInfo(std::move(ConstCand->Uses), Offset, ConstTy)); + } + ConstInfoVec.push_back(std::move(ConstInfo)); +} + +/// Finds and combines constant candidates that can be easily +/// rematerialized with an add from a common base constant. +void ConstantHoistingPass::findBaseConstants(GlobalVariable *BaseGV) { + // If BaseGV is nullptr, find base among candidate constant integers; + // Otherwise find base among constant GEPs that share the same BaseGV. + ConstCandVecType &ConstCandVec = BaseGV ? + ConstGEPCandMap[BaseGV] : ConstIntCandVec; + ConstInfoVecType &ConstInfoVec = BaseGV ? + ConstGEPInfoMap[BaseGV] : ConstIntInfoVec; + + // Sort the constants by value and type. This invalidates the mapping! + llvm::stable_sort(ConstCandVec, [](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()) { + Type *MemUseValTy = nullptr; + for (auto &U : CC->Uses) { + auto *UI = U.Inst; + if (LoadInst *LI = dyn_cast<LoadInst>(UI)) { + MemUseValTy = LI->getType(); + break; + } else if (StoreInst *SI = dyn_cast<StoreInst>(UI)) { + // Make sure the constant is used as pointer operand of the StoreInst. + if (SI->getPointerOperand() == SI->getOperand(U.OpndIdx)) { + MemUseValTy = SI->getValueOperand()->getType(); + break; + } + } + } + + // 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()) && + // Check if Diff can be used as offset in addressing mode of the user + // memory instruction. + (!MemUseValTy || TTI->isLegalAddressingMode(MemUseValTy, + /*BaseGV*/nullptr, /*BaseOffset*/Diff.getSExtValue(), + /*HasBaseReg*/true, /*Scale*/0))) + 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, ConstInfoVec); + // Start a new base constant search. + MinValItr = CC; + } + // Finalize the last base constant search. + findAndMakeBaseConstant(MinValItr, ConstCandVec.end(), ConstInfoVec); +} + +/// 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 incoming 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; +} + +/// Emit materialization code for all rebased constants and update their +/// users. +void ConstantHoistingPass::emitBaseConstants(Instruction *Base, + Constant *Offset, + Type *Ty, + const ConstantUser &ConstUser) { + Instruction *Mat = Base; + + // The same offset can be dereferenced to different types in nested struct. + if (!Offset && Ty && Ty != Base->getType()) + Offset = ConstantInt::get(Type::getInt32Ty(*Ctx), 0); + + if (Offset) { + Instruction *InsertionPt = findMatInsertPt(ConstUser.Inst, + ConstUser.OpndIdx); + if (Ty) { + // Constant being rebased is a ConstantExpr. + PointerType *Int8PtrTy = Type::getInt8PtrTy(*Ctx, + cast<PointerType>(Ty)->getAddressSpace()); + Base = new BitCastInst(Base, Int8PtrTy, "base_bitcast", InsertionPt); + Mat = GetElementPtrInst::Create(Int8PtrTy->getElementType(), Base, + Offset, "mat_gep", InsertionPt); + Mat = new BitCastInst(Mat, Ty, "mat_bitcast", InsertionPt); + } else + // Constant being rebased is a ConstantInt. + Mat = BinaryOperator::Create(Instruction::Add, Base, Offset, + "const_mat", InsertionPt); + + LLVM_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)) { + LLVM_DEBUG(dbgs() << "Update: " << *ConstUser.Inst << '\n'); + if (!updateOperand(ConstUser.Inst, ConstUser.OpndIdx, Mat) && Offset) + Mat->eraseFromParent(); + LLVM_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()); + LLVM_DEBUG(dbgs() << "Clone instruction: " << *CastInst << '\n' + << "To : " << *ClonedCastInst << '\n'); + } + + LLVM_DEBUG(dbgs() << "Update: " << *ConstUser.Inst << '\n'); + updateOperand(ConstUser.Inst, ConstUser.OpndIdx, ClonedCastInst); + LLVM_DEBUG(dbgs() << "To : " << *ConstUser.Inst << '\n'); + return; + } + + // Visit constant expression. + if (auto ConstExpr = dyn_cast<ConstantExpr>(Opnd)) { + if (ConstExpr->isGEPWithNoNotionalOverIndexing()) { + // Operand is a ConstantGEP, replace it. + updateOperand(ConstUser.Inst, ConstUser.OpndIdx, Mat); + return; + } + + // Aside from constant GEPs, only constant cast expressions are collected. + assert(ConstExpr->isCast() && "ConstExpr should be a cast"); + 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()); + + LLVM_DEBUG(dbgs() << "Create instruction: " << *ConstExprInst << '\n' + << "From : " << *ConstExpr << '\n'); + LLVM_DEBUG(dbgs() << "Update: " << *ConstUser.Inst << '\n'); + if (!updateOperand(ConstUser.Inst, ConstUser.OpndIdx, ConstExprInst)) { + ConstExprInst->eraseFromParent(); + if (Offset) + Mat->eraseFromParent(); + } + LLVM_DEBUG(dbgs() << "To : " << *ConstUser.Inst << '\n'); + return; + } +} + +/// Hoist and hide the base constant behind a bitcast and emit +/// materialization code for derived constants. +bool ConstantHoistingPass::emitBaseConstants(GlobalVariable *BaseGV) { + bool MadeChange = false; + SmallVectorImpl<consthoist::ConstantInfo> &ConstInfoVec = + BaseGV ? ConstGEPInfoMap[BaseGV] : ConstIntInfoVec; + for (auto const &ConstInfo : ConstInfoVec) { + SetVector<Instruction *> IPSet = findConstantInsertionPoint(ConstInfo); + // We can have an empty set if the function contains unreachable blocks. + if (IPSet.empty()) + continue; + + unsigned UsesNum = 0; + unsigned ReBasesNum = 0; + unsigned NotRebasedNum = 0; + for (Instruction *IP : IPSet) { + // First, collect constants depending on this IP of the base. + unsigned Uses = 0; + using RebasedUse = std::tuple<Constant *, Type *, ConstantUser>; + SmallVector<RebasedUse, 4> ToBeRebased; + for (auto const &RCI : ConstInfo.RebasedConstants) { + for (auto const &U : RCI.Uses) { + Uses++; + BasicBlock *OrigMatInsertBB = + findMatInsertPt(U.Inst, U.OpndIdx)->getParent(); + // If Base constant is to be inserted in multiple places, + // generate rebase for U using the Base dominating U. + if (IPSet.size() == 1 || + DT->dominates(IP->getParent(), OrigMatInsertBB)) + ToBeRebased.push_back(RebasedUse(RCI.Offset, RCI.Ty, U)); + } + } + UsesNum = Uses; + + // If only few constants depend on this IP of base, skip rebasing, + // assuming the base and the rebased have the same materialization cost. + if (ToBeRebased.size() < MinNumOfDependentToRebase) { + NotRebasedNum += ToBeRebased.size(); + continue; + } + + // Emit an instance of the base at this IP. + Instruction *Base = nullptr; + // Hoist and hide the base constant behind a bitcast. + if (ConstInfo.BaseExpr) { + assert(BaseGV && "A base constant expression must have an base GV"); + Type *Ty = ConstInfo.BaseExpr->getType(); + Base = new BitCastInst(ConstInfo.BaseExpr, Ty, "const", IP); + } else { + IntegerType *Ty = ConstInfo.BaseInt->getType(); + Base = new BitCastInst(ConstInfo.BaseInt, Ty, "const", IP); + } + + Base->setDebugLoc(IP->getDebugLoc()); + + LLVM_DEBUG(dbgs() << "Hoist constant (" << *ConstInfo.BaseInt + << ") to BB " << IP->getParent()->getName() << '\n' + << *Base << '\n'); + + // Emit materialization code for rebased constants depending on this IP. + for (auto const &R : ToBeRebased) { + Constant *Off = std::get<0>(R); + Type *Ty = std::get<1>(R); + ConstantUser U = std::get<2>(R); + emitBaseConstants(Base, Off, Ty, U); + ReBasesNum++; + // Use the same debug location as the last user of the constant. + Base->setDebugLoc(DILocation::getMergedLocation( + Base->getDebugLoc(), U.Inst->getDebugLoc())); + } + assert(!Base->use_empty() && "The use list is empty!?"); + assert(isa<Instruction>(Base->user_back()) && + "All uses should be instructions."); + } + (void)UsesNum; + (void)ReBasesNum; + (void)NotRebasedNum; + // Expect all uses are rebased after rebase is done. + assert(UsesNum == (ReBasesNum + NotRebasedNum) && + "Not all uses are rebased"); + + NumConstantsHoisted++; + + // Base constant is also included in ConstInfo.RebasedConstants, so + // deduct 1 from ConstInfo.RebasedConstants.size(). + NumConstantsRebased += ConstInfo.RebasedConstants.size() - 1; + + MadeChange = true; + } + return MadeChange; +} + +/// Check all cast instructions we made a copy of and remove them if they +/// have no more users. +void ConstantHoistingPass::deleteDeadCastInst() const { + for (auto const &I : ClonedCastMap) + if (I.first->use_empty()) + I.first->eraseFromParent(); +} + +/// Optimize expensive integer constants in the given function. +bool ConstantHoistingPass::runImpl(Function &Fn, TargetTransformInfo &TTI, + DominatorTree &DT, BlockFrequencyInfo *BFI, + BasicBlock &Entry, ProfileSummaryInfo *PSI) { + this->TTI = &TTI; + this->DT = &DT; + this->BFI = BFI; + this->DL = &Fn.getParent()->getDataLayout(); + this->Ctx = &Fn.getContext(); + this->Entry = &Entry; + this->PSI = PSI; + // Collect all constant candidates. + collectConstantCandidates(Fn); + + // Combine constants that can be easily materialized with an add from a common + // base constant. + if (!ConstIntCandVec.empty()) + findBaseConstants(nullptr); + for (auto &MapEntry : ConstGEPCandMap) + if (!MapEntry.second.empty()) + findBaseConstants(MapEntry.first); + + // Finally hoist the base constant and emit materialization code for dependent + // constants. + bool MadeChange = false; + if (!ConstIntInfoVec.empty()) + MadeChange = emitBaseConstants(nullptr); + for (auto MapEntry : ConstGEPInfoMap) + if (!MapEntry.second.empty()) + MadeChange |= emitBaseConstants(MapEntry.first); + + + // Cleanup dead instructions. + deleteDeadCastInst(); + + cleanup(); + + return MadeChange; +} + +PreservedAnalyses ConstantHoistingPass::run(Function &F, + FunctionAnalysisManager &AM) { + auto &DT = AM.getResult<DominatorTreeAnalysis>(F); + auto &TTI = AM.getResult<TargetIRAnalysis>(F); + auto BFI = ConstHoistWithBlockFrequency + ? &AM.getResult<BlockFrequencyAnalysis>(F) + : nullptr; + auto &MAM = AM.getResult<ModuleAnalysisManagerFunctionProxy>(F).getManager(); + auto *PSI = MAM.getCachedResult<ProfileSummaryAnalysis>(*F.getParent()); + if (!runImpl(F, TTI, DT, BFI, F.getEntryBlock(), PSI)) + return PreservedAnalyses::all(); + + PreservedAnalyses PA; + PA.preserveSet<CFGAnalyses>(); + return PA; +} |