| //===- 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 getIntImmCostInst 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/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/InitializePasses.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/Local.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)->children()) { |
| 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->getIntImmCostIntrin(IntrInst->getIntrinsicID(), Idx, |
| ConstInt->getValue(), ConstInt->getType(), |
| TargetTransformInfo::TCK_SizeAndLatency); |
| else |
| Cost = TTI->getIntImmCostInst(Inst->getOpcode(), Idx, ConstInt->getValue(), |
| ConstInt->getType(), |
| TargetTransformInfo::TCK_SizeAndLatency); |
| |
| // 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->getIntImmCostInst(Instruction::Add, 1, Offset, PtrIntTy, |
| TargetTransformInfo::TCK_SizeAndLatency); |
| 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::getIntImmCostInst`) 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)) { |
| 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) { |
| // Ignore unreachable basic blocks. |
| if (!DT->isReachableFromEntry(&BB)) |
| continue; |
| 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, |
| PGSOQueryType::IRPass); |
| 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->getIntImmCostInst(Opcode, OpndIdx, Value, Ty, |
| TargetTransformInfo::TCK_SizeAndLatency); |
| 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 &MAMProxy = AM.getResult<ModuleAnalysisManagerFunctionProxy>(F); |
| auto *PSI = MAMProxy.getCachedResult<ProfileSummaryAnalysis>(*F.getParent()); |
| if (!runImpl(F, TTI, DT, BFI, F.getEntryBlock(), PSI)) |
| return PreservedAnalyses::all(); |
| |
| PreservedAnalyses PA; |
| PA.preserveSet<CFGAnalyses>(); |
| return PA; |
| } |
