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Diffstat (limited to 'contrib/llvm-project/llvm/lib/CodeGen/MachineLICM.cpp')
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diff --git a/contrib/llvm-project/llvm/lib/CodeGen/MachineLICM.cpp b/contrib/llvm-project/llvm/lib/CodeGen/MachineLICM.cpp new file mode 100644 index 000000000000..6a898ff6ef88 --- /dev/null +++ b/contrib/llvm-project/llvm/lib/CodeGen/MachineLICM.cpp @@ -0,0 +1,1528 @@ +//===- MachineLICM.cpp - Machine Loop Invariant Code Motion Pass ----------===// +// +// 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 performs loop invariant code motion on machine instructions. We +// attempt to remove as much code from the body of a loop as possible. +// +// This pass is not intended to be a replacement or a complete alternative +// for the LLVM-IR-level LICM pass. It is only designed to hoist simple +// constructs that are not exposed before lowering and instruction selection. +// +//===----------------------------------------------------------------------===// + +#include "llvm/ADT/BitVector.h" +#include "llvm/ADT/DenseMap.h" +#include "llvm/ADT/STLExtras.h" +#include "llvm/ADT/SmallSet.h" +#include "llvm/ADT/SmallVector.h" +#include "llvm/ADT/Statistic.h" +#include "llvm/Analysis/AliasAnalysis.h" +#include "llvm/CodeGen/MachineBasicBlock.h" +#include "llvm/CodeGen/MachineDominators.h" +#include "llvm/CodeGen/MachineFrameInfo.h" +#include "llvm/CodeGen/MachineFunction.h" +#include "llvm/CodeGen/MachineFunctionPass.h" +#include "llvm/CodeGen/MachineInstr.h" +#include "llvm/CodeGen/MachineLoopInfo.h" +#include "llvm/CodeGen/MachineMemOperand.h" +#include "llvm/CodeGen/MachineOperand.h" +#include "llvm/CodeGen/MachineRegisterInfo.h" +#include "llvm/CodeGen/PseudoSourceValue.h" +#include "llvm/CodeGen/TargetInstrInfo.h" +#include "llvm/CodeGen/TargetLowering.h" +#include "llvm/CodeGen/TargetRegisterInfo.h" +#include "llvm/CodeGen/TargetSchedule.h" +#include "llvm/CodeGen/TargetSubtargetInfo.h" +#include "llvm/IR/DebugLoc.h" +#include "llvm/MC/MCInstrDesc.h" +#include "llvm/MC/MCRegisterInfo.h" +#include "llvm/Pass.h" +#include "llvm/Support/Casting.h" +#include "llvm/Support/CommandLine.h" +#include "llvm/Support/Debug.h" +#include "llvm/Support/raw_ostream.h" +#include <algorithm> +#include <cassert> +#include <limits> +#include <vector> + +using namespace llvm; + +#define DEBUG_TYPE "machinelicm" + +static cl::opt<bool> +AvoidSpeculation("avoid-speculation", + cl::desc("MachineLICM should avoid speculation"), + cl::init(true), cl::Hidden); + +static cl::opt<bool> +HoistCheapInsts("hoist-cheap-insts", + cl::desc("MachineLICM should hoist even cheap instructions"), + cl::init(false), cl::Hidden); + +static cl::opt<bool> +SinkInstsToAvoidSpills("sink-insts-to-avoid-spills", + cl::desc("MachineLICM should sink instructions into " + "loops to avoid register spills"), + cl::init(false), cl::Hidden); +static cl::opt<bool> +HoistConstStores("hoist-const-stores", + cl::desc("Hoist invariant stores"), + cl::init(true), cl::Hidden); + +STATISTIC(NumHoisted, + "Number of machine instructions hoisted out of loops"); +STATISTIC(NumLowRP, + "Number of instructions hoisted in low reg pressure situation"); +STATISTIC(NumHighLatency, + "Number of high latency instructions hoisted"); +STATISTIC(NumCSEed, + "Number of hoisted machine instructions CSEed"); +STATISTIC(NumPostRAHoisted, + "Number of machine instructions hoisted out of loops post regalloc"); +STATISTIC(NumStoreConst, + "Number of stores of const phys reg hoisted out of loops"); + +namespace { + + class MachineLICMBase : public MachineFunctionPass { + const TargetInstrInfo *TII; + const TargetLoweringBase *TLI; + const TargetRegisterInfo *TRI; + const MachineFrameInfo *MFI; + MachineRegisterInfo *MRI; + TargetSchedModel SchedModel; + bool PreRegAlloc; + + // Various analyses that we use... + AliasAnalysis *AA; // Alias analysis info. + MachineLoopInfo *MLI; // Current MachineLoopInfo + MachineDominatorTree *DT; // Machine dominator tree for the cur loop + + // State that is updated as we process loops + bool Changed; // True if a loop is changed. + bool FirstInLoop; // True if it's the first LICM in the loop. + MachineLoop *CurLoop; // The current loop we are working on. + MachineBasicBlock *CurPreheader; // The preheader for CurLoop. + + // Exit blocks for CurLoop. + SmallVector<MachineBasicBlock *, 8> ExitBlocks; + + bool isExitBlock(const MachineBasicBlock *MBB) const { + return is_contained(ExitBlocks, MBB); + } + + // Track 'estimated' register pressure. + SmallSet<unsigned, 32> RegSeen; + SmallVector<unsigned, 8> RegPressure; + + // Register pressure "limit" per register pressure set. If the pressure + // is higher than the limit, then it's considered high. + SmallVector<unsigned, 8> RegLimit; + + // Register pressure on path leading from loop preheader to current BB. + SmallVector<SmallVector<unsigned, 8>, 16> BackTrace; + + // For each opcode, keep a list of potential CSE instructions. + DenseMap<unsigned, std::vector<const MachineInstr *>> CSEMap; + + enum { + SpeculateFalse = 0, + SpeculateTrue = 1, + SpeculateUnknown = 2 + }; + + // If a MBB does not dominate loop exiting blocks then it may not safe + // to hoist loads from this block. + // Tri-state: 0 - false, 1 - true, 2 - unknown + unsigned SpeculationState; + + public: + MachineLICMBase(char &PassID, bool PreRegAlloc) + : MachineFunctionPass(PassID), PreRegAlloc(PreRegAlloc) {} + + bool runOnMachineFunction(MachineFunction &MF) override; + + void getAnalysisUsage(AnalysisUsage &AU) const override { + AU.addRequired<MachineLoopInfo>(); + AU.addRequired<MachineDominatorTree>(); + AU.addRequired<AAResultsWrapperPass>(); + AU.addPreserved<MachineLoopInfo>(); + MachineFunctionPass::getAnalysisUsage(AU); + } + + void releaseMemory() override { + RegSeen.clear(); + RegPressure.clear(); + RegLimit.clear(); + BackTrace.clear(); + CSEMap.clear(); + } + + private: + /// Keep track of information about hoisting candidates. + struct CandidateInfo { + MachineInstr *MI; + unsigned Def; + int FI; + + CandidateInfo(MachineInstr *mi, unsigned def, int fi) + : MI(mi), Def(def), FI(fi) {} + }; + + void HoistRegionPostRA(); + + void HoistPostRA(MachineInstr *MI, unsigned Def); + + void ProcessMI(MachineInstr *MI, BitVector &PhysRegDefs, + BitVector &PhysRegClobbers, SmallSet<int, 32> &StoredFIs, + SmallVectorImpl<CandidateInfo> &Candidates); + + void AddToLiveIns(unsigned Reg); + + bool IsLICMCandidate(MachineInstr &I); + + bool IsLoopInvariantInst(MachineInstr &I); + + bool HasLoopPHIUse(const MachineInstr *MI) const; + + bool HasHighOperandLatency(MachineInstr &MI, unsigned DefIdx, + unsigned Reg) const; + + bool IsCheapInstruction(MachineInstr &MI) const; + + bool CanCauseHighRegPressure(const DenseMap<unsigned, int> &Cost, + bool Cheap); + + void UpdateBackTraceRegPressure(const MachineInstr *MI); + + bool IsProfitableToHoist(MachineInstr &MI); + + bool IsGuaranteedToExecute(MachineBasicBlock *BB); + + void EnterScope(MachineBasicBlock *MBB); + + void ExitScope(MachineBasicBlock *MBB); + + void ExitScopeIfDone( + MachineDomTreeNode *Node, + DenseMap<MachineDomTreeNode *, unsigned> &OpenChildren, + DenseMap<MachineDomTreeNode *, MachineDomTreeNode *> &ParentMap); + + void HoistOutOfLoop(MachineDomTreeNode *HeaderN); + + void HoistRegion(MachineDomTreeNode *N, bool IsHeader); + + void SinkIntoLoop(); + + void InitRegPressure(MachineBasicBlock *BB); + + DenseMap<unsigned, int> calcRegisterCost(const MachineInstr *MI, + bool ConsiderSeen, + bool ConsiderUnseenAsDef); + + void UpdateRegPressure(const MachineInstr *MI, + bool ConsiderUnseenAsDef = false); + + MachineInstr *ExtractHoistableLoad(MachineInstr *MI); + + const MachineInstr * + LookForDuplicate(const MachineInstr *MI, + std::vector<const MachineInstr *> &PrevMIs); + + bool EliminateCSE( + MachineInstr *MI, + DenseMap<unsigned, std::vector<const MachineInstr *>>::iterator &CI); + + bool MayCSE(MachineInstr *MI); + + bool Hoist(MachineInstr *MI, MachineBasicBlock *Preheader); + + void InitCSEMap(MachineBasicBlock *BB); + + MachineBasicBlock *getCurPreheader(); + }; + + class MachineLICM : public MachineLICMBase { + public: + static char ID; + MachineLICM() : MachineLICMBase(ID, false) { + initializeMachineLICMPass(*PassRegistry::getPassRegistry()); + } + }; + + class EarlyMachineLICM : public MachineLICMBase { + public: + static char ID; + EarlyMachineLICM() : MachineLICMBase(ID, true) { + initializeEarlyMachineLICMPass(*PassRegistry::getPassRegistry()); + } + }; + +} // end anonymous namespace + +char MachineLICM::ID; +char EarlyMachineLICM::ID; + +char &llvm::MachineLICMID = MachineLICM::ID; +char &llvm::EarlyMachineLICMID = EarlyMachineLICM::ID; + +INITIALIZE_PASS_BEGIN(MachineLICM, DEBUG_TYPE, + "Machine Loop Invariant Code Motion", false, false) +INITIALIZE_PASS_DEPENDENCY(MachineLoopInfo) +INITIALIZE_PASS_DEPENDENCY(MachineDominatorTree) +INITIALIZE_PASS_DEPENDENCY(AAResultsWrapperPass) +INITIALIZE_PASS_END(MachineLICM, DEBUG_TYPE, + "Machine Loop Invariant Code Motion", false, false) + +INITIALIZE_PASS_BEGIN(EarlyMachineLICM, "early-machinelicm", + "Early Machine Loop Invariant Code Motion", false, false) +INITIALIZE_PASS_DEPENDENCY(MachineLoopInfo) +INITIALIZE_PASS_DEPENDENCY(MachineDominatorTree) +INITIALIZE_PASS_DEPENDENCY(AAResultsWrapperPass) +INITIALIZE_PASS_END(EarlyMachineLICM, "early-machinelicm", + "Early Machine Loop Invariant Code Motion", false, false) + +/// Test if the given loop is the outer-most loop that has a unique predecessor. +static bool LoopIsOuterMostWithPredecessor(MachineLoop *CurLoop) { + // Check whether this loop even has a unique predecessor. + if (!CurLoop->getLoopPredecessor()) + return false; + // Ok, now check to see if any of its outer loops do. + for (MachineLoop *L = CurLoop->getParentLoop(); L; L = L->getParentLoop()) + if (L->getLoopPredecessor()) + return false; + // None of them did, so this is the outermost with a unique predecessor. + return true; +} + +bool MachineLICMBase::runOnMachineFunction(MachineFunction &MF) { + if (skipFunction(MF.getFunction())) + return false; + + Changed = FirstInLoop = false; + const TargetSubtargetInfo &ST = MF.getSubtarget(); + TII = ST.getInstrInfo(); + TLI = ST.getTargetLowering(); + TRI = ST.getRegisterInfo(); + MFI = &MF.getFrameInfo(); + MRI = &MF.getRegInfo(); + SchedModel.init(&ST); + + PreRegAlloc = MRI->isSSA(); + + if (PreRegAlloc) + LLVM_DEBUG(dbgs() << "******** Pre-regalloc Machine LICM: "); + else + LLVM_DEBUG(dbgs() << "******** Post-regalloc Machine LICM: "); + LLVM_DEBUG(dbgs() << MF.getName() << " ********\n"); + + if (PreRegAlloc) { + // Estimate register pressure during pre-regalloc pass. + unsigned NumRPS = TRI->getNumRegPressureSets(); + RegPressure.resize(NumRPS); + std::fill(RegPressure.begin(), RegPressure.end(), 0); + RegLimit.resize(NumRPS); + for (unsigned i = 0, e = NumRPS; i != e; ++i) + RegLimit[i] = TRI->getRegPressureSetLimit(MF, i); + } + + // Get our Loop information... + MLI = &getAnalysis<MachineLoopInfo>(); + DT = &getAnalysis<MachineDominatorTree>(); + AA = &getAnalysis<AAResultsWrapperPass>().getAAResults(); + + SmallVector<MachineLoop *, 8> Worklist(MLI->begin(), MLI->end()); + while (!Worklist.empty()) { + CurLoop = Worklist.pop_back_val(); + CurPreheader = nullptr; + ExitBlocks.clear(); + + // If this is done before regalloc, only visit outer-most preheader-sporting + // loops. + if (PreRegAlloc && !LoopIsOuterMostWithPredecessor(CurLoop)) { + Worklist.append(CurLoop->begin(), CurLoop->end()); + continue; + } + + CurLoop->getExitBlocks(ExitBlocks); + + if (!PreRegAlloc) + HoistRegionPostRA(); + else { + // CSEMap is initialized for loop header when the first instruction is + // being hoisted. + MachineDomTreeNode *N = DT->getNode(CurLoop->getHeader()); + FirstInLoop = true; + HoistOutOfLoop(N); + CSEMap.clear(); + + if (SinkInstsToAvoidSpills) + SinkIntoLoop(); + } + } + + return Changed; +} + +/// Return true if instruction stores to the specified frame. +static bool InstructionStoresToFI(const MachineInstr *MI, int FI) { + // Check mayStore before memory operands so that e.g. DBG_VALUEs will return + // true since they have no memory operands. + if (!MI->mayStore()) + return false; + // If we lost memory operands, conservatively assume that the instruction + // writes to all slots. + if (MI->memoperands_empty()) + return true; + for (const MachineMemOperand *MemOp : MI->memoperands()) { + if (!MemOp->isStore() || !MemOp->getPseudoValue()) + continue; + if (const FixedStackPseudoSourceValue *Value = + dyn_cast<FixedStackPseudoSourceValue>(MemOp->getPseudoValue())) { + if (Value->getFrameIndex() == FI) + return true; + } + } + return false; +} + +/// Examine the instruction for potentai LICM candidate. Also +/// gather register def and frame object update information. +void MachineLICMBase::ProcessMI(MachineInstr *MI, + BitVector &PhysRegDefs, + BitVector &PhysRegClobbers, + SmallSet<int, 32> &StoredFIs, + SmallVectorImpl<CandidateInfo> &Candidates) { + bool RuledOut = false; + bool HasNonInvariantUse = false; + unsigned Def = 0; + for (const MachineOperand &MO : MI->operands()) { + if (MO.isFI()) { + // Remember if the instruction stores to the frame index. + int FI = MO.getIndex(); + if (!StoredFIs.count(FI) && + MFI->isSpillSlotObjectIndex(FI) && + InstructionStoresToFI(MI, FI)) + StoredFIs.insert(FI); + HasNonInvariantUse = true; + continue; + } + + // We can't hoist an instruction defining a physreg that is clobbered in + // the loop. + if (MO.isRegMask()) { + PhysRegClobbers.setBitsNotInMask(MO.getRegMask()); + continue; + } + + if (!MO.isReg()) + continue; + Register Reg = MO.getReg(); + if (!Reg) + continue; + assert(Register::isPhysicalRegister(Reg) && + "Not expecting virtual register!"); + + if (!MO.isDef()) { + if (Reg && (PhysRegDefs.test(Reg) || PhysRegClobbers.test(Reg))) + // If it's using a non-loop-invariant register, then it's obviously not + // safe to hoist. + HasNonInvariantUse = true; + continue; + } + + if (MO.isImplicit()) { + for (MCRegAliasIterator AI(Reg, TRI, true); AI.isValid(); ++AI) + PhysRegClobbers.set(*AI); + if (!MO.isDead()) + // Non-dead implicit def? This cannot be hoisted. + RuledOut = true; + // No need to check if a dead implicit def is also defined by + // another instruction. + continue; + } + + // FIXME: For now, avoid instructions with multiple defs, unless + // it's a dead implicit def. + if (Def) + RuledOut = true; + else + Def = Reg; + + // If we have already seen another instruction that defines the same + // register, then this is not safe. Two defs is indicated by setting a + // PhysRegClobbers bit. + for (MCRegAliasIterator AS(Reg, TRI, true); AS.isValid(); ++AS) { + if (PhysRegDefs.test(*AS)) + PhysRegClobbers.set(*AS); + } + // Need a second loop because MCRegAliasIterator can visit the same + // register twice. + for (MCRegAliasIterator AS(Reg, TRI, true); AS.isValid(); ++AS) + PhysRegDefs.set(*AS); + + if (PhysRegClobbers.test(Reg)) + // MI defined register is seen defined by another instruction in + // the loop, it cannot be a LICM candidate. + RuledOut = true; + } + + // Only consider reloads for now and remats which do not have register + // operands. FIXME: Consider unfold load folding instructions. + if (Def && !RuledOut) { + int FI = std::numeric_limits<int>::min(); + if ((!HasNonInvariantUse && IsLICMCandidate(*MI)) || + (TII->isLoadFromStackSlot(*MI, FI) && MFI->isSpillSlotObjectIndex(FI))) + Candidates.push_back(CandidateInfo(MI, Def, FI)); + } +} + +/// Walk the specified region of the CFG and hoist loop invariants out to the +/// preheader. +void MachineLICMBase::HoistRegionPostRA() { + MachineBasicBlock *Preheader = getCurPreheader(); + if (!Preheader) + return; + + unsigned NumRegs = TRI->getNumRegs(); + BitVector PhysRegDefs(NumRegs); // Regs defined once in the loop. + BitVector PhysRegClobbers(NumRegs); // Regs defined more than once. + + SmallVector<CandidateInfo, 32> Candidates; + SmallSet<int, 32> StoredFIs; + + // Walk the entire region, count number of defs for each register, and + // collect potential LICM candidates. + for (MachineBasicBlock *BB : CurLoop->getBlocks()) { + // If the header of the loop containing this basic block is a landing pad, + // then don't try to hoist instructions out of this loop. + const MachineLoop *ML = MLI->getLoopFor(BB); + if (ML && ML->getHeader()->isEHPad()) continue; + + // Conservatively treat live-in's as an external def. + // FIXME: That means a reload that're reused in successor block(s) will not + // be LICM'ed. + for (const auto &LI : BB->liveins()) { + for (MCRegAliasIterator AI(LI.PhysReg, TRI, true); AI.isValid(); ++AI) + PhysRegDefs.set(*AI); + } + + SpeculationState = SpeculateUnknown; + for (MachineInstr &MI : *BB) + ProcessMI(&MI, PhysRegDefs, PhysRegClobbers, StoredFIs, Candidates); + } + + // Gather the registers read / clobbered by the terminator. + BitVector TermRegs(NumRegs); + MachineBasicBlock::iterator TI = Preheader->getFirstTerminator(); + if (TI != Preheader->end()) { + for (const MachineOperand &MO : TI->operands()) { + if (!MO.isReg()) + continue; + Register Reg = MO.getReg(); + if (!Reg) + continue; + for (MCRegAliasIterator AI(Reg, TRI, true); AI.isValid(); ++AI) + TermRegs.set(*AI); + } + } + + // Now evaluate whether the potential candidates qualify. + // 1. Check if the candidate defined register is defined by another + // instruction in the loop. + // 2. If the candidate is a load from stack slot (always true for now), + // check if the slot is stored anywhere in the loop. + // 3. Make sure candidate def should not clobber + // registers read by the terminator. Similarly its def should not be + // clobbered by the terminator. + for (CandidateInfo &Candidate : Candidates) { + if (Candidate.FI != std::numeric_limits<int>::min() && + StoredFIs.count(Candidate.FI)) + continue; + + unsigned Def = Candidate.Def; + if (!PhysRegClobbers.test(Def) && !TermRegs.test(Def)) { + bool Safe = true; + MachineInstr *MI = Candidate.MI; + for (const MachineOperand &MO : MI->operands()) { + if (!MO.isReg() || MO.isDef() || !MO.getReg()) + continue; + Register Reg = MO.getReg(); + if (PhysRegDefs.test(Reg) || + PhysRegClobbers.test(Reg)) { + // If it's using a non-loop-invariant register, then it's obviously + // not safe to hoist. + Safe = false; + break; + } + } + if (Safe) + HoistPostRA(MI, Candidate.Def); + } + } +} + +/// Add register 'Reg' to the livein sets of BBs in the current loop, and make +/// sure it is not killed by any instructions in the loop. +void MachineLICMBase::AddToLiveIns(unsigned Reg) { + for (MachineBasicBlock *BB : CurLoop->getBlocks()) { + if (!BB->isLiveIn(Reg)) + BB->addLiveIn(Reg); + for (MachineInstr &MI : *BB) { + for (MachineOperand &MO : MI.operands()) { + if (!MO.isReg() || !MO.getReg() || MO.isDef()) continue; + if (MO.getReg() == Reg || TRI->isSuperRegister(Reg, MO.getReg())) + MO.setIsKill(false); + } + } + } +} + +/// When an instruction is found to only use loop invariant operands that is +/// safe to hoist, this instruction is called to do the dirty work. +void MachineLICMBase::HoistPostRA(MachineInstr *MI, unsigned Def) { + MachineBasicBlock *Preheader = getCurPreheader(); + + // Now move the instructions to the predecessor, inserting it before any + // terminator instructions. + LLVM_DEBUG(dbgs() << "Hoisting to " << printMBBReference(*Preheader) + << " from " << printMBBReference(*MI->getParent()) << ": " + << *MI); + + // Splice the instruction to the preheader. + MachineBasicBlock *MBB = MI->getParent(); + Preheader->splice(Preheader->getFirstTerminator(), MBB, MI); + + // Add register to livein list to all the BBs in the current loop since a + // loop invariant must be kept live throughout the whole loop. This is + // important to ensure later passes do not scavenge the def register. + AddToLiveIns(Def); + + ++NumPostRAHoisted; + Changed = true; +} + +/// Check if this mbb is guaranteed to execute. If not then a load from this mbb +/// may not be safe to hoist. +bool MachineLICMBase::IsGuaranteedToExecute(MachineBasicBlock *BB) { + if (SpeculationState != SpeculateUnknown) + return SpeculationState == SpeculateFalse; + + if (BB != CurLoop->getHeader()) { + // Check loop exiting blocks. + SmallVector<MachineBasicBlock*, 8> CurrentLoopExitingBlocks; + CurLoop->getExitingBlocks(CurrentLoopExitingBlocks); + for (MachineBasicBlock *CurrentLoopExitingBlock : CurrentLoopExitingBlocks) + if (!DT->dominates(BB, CurrentLoopExitingBlock)) { + SpeculationState = SpeculateTrue; + return false; + } + } + + SpeculationState = SpeculateFalse; + return true; +} + +void MachineLICMBase::EnterScope(MachineBasicBlock *MBB) { + LLVM_DEBUG(dbgs() << "Entering " << printMBBReference(*MBB) << '\n'); + + // Remember livein register pressure. + BackTrace.push_back(RegPressure); +} + +void MachineLICMBase::ExitScope(MachineBasicBlock *MBB) { + LLVM_DEBUG(dbgs() << "Exiting " << printMBBReference(*MBB) << '\n'); + BackTrace.pop_back(); +} + +/// Destroy scope for the MBB that corresponds to the given dominator tree node +/// if its a leaf or all of its children are done. Walk up the dominator tree to +/// destroy ancestors which are now done. +void MachineLICMBase::ExitScopeIfDone(MachineDomTreeNode *Node, + DenseMap<MachineDomTreeNode*, unsigned> &OpenChildren, + DenseMap<MachineDomTreeNode*, MachineDomTreeNode*> &ParentMap) { + if (OpenChildren[Node]) + return; + + // Pop scope. + ExitScope(Node->getBlock()); + + // Now traverse upwards to pop ancestors whose offsprings are all done. + while (MachineDomTreeNode *Parent = ParentMap[Node]) { + unsigned Left = --OpenChildren[Parent]; + if (Left != 0) + break; + ExitScope(Parent->getBlock()); + Node = Parent; + } +} + +/// Walk the specified loop in the CFG (defined by all blocks dominated by the +/// specified header block, and that are in the current loop) in depth first +/// order w.r.t the DominatorTree. This allows us to visit definitions before +/// uses, allowing us to hoist a loop body in one pass without iteration. +void MachineLICMBase::HoistOutOfLoop(MachineDomTreeNode *HeaderN) { + MachineBasicBlock *Preheader = getCurPreheader(); + if (!Preheader) + return; + + SmallVector<MachineDomTreeNode*, 32> Scopes; + SmallVector<MachineDomTreeNode*, 8> WorkList; + DenseMap<MachineDomTreeNode*, MachineDomTreeNode*> ParentMap; + DenseMap<MachineDomTreeNode*, unsigned> OpenChildren; + + // Perform a DFS walk to determine the order of visit. + WorkList.push_back(HeaderN); + while (!WorkList.empty()) { + MachineDomTreeNode *Node = WorkList.pop_back_val(); + assert(Node && "Null dominator tree node?"); + MachineBasicBlock *BB = Node->getBlock(); + + // If the header of the loop containing this basic block is a landing pad, + // then don't try to hoist instructions out of this loop. + const MachineLoop *ML = MLI->getLoopFor(BB); + if (ML && ML->getHeader()->isEHPad()) + continue; + + // If this subregion is not in the top level loop at all, exit. + if (!CurLoop->contains(BB)) + continue; + + Scopes.push_back(Node); + const std::vector<MachineDomTreeNode*> &Children = Node->getChildren(); + unsigned NumChildren = Children.size(); + + // Don't hoist things out of a large switch statement. This often causes + // code to be hoisted that wasn't going to be executed, and increases + // register pressure in a situation where it's likely to matter. + if (BB->succ_size() >= 25) + NumChildren = 0; + + OpenChildren[Node] = NumChildren; + // Add children in reverse order as then the next popped worklist node is + // the first child of this node. This means we ultimately traverse the + // DOM tree in exactly the same order as if we'd recursed. + for (int i = (int)NumChildren-1; i >= 0; --i) { + MachineDomTreeNode *Child = Children[i]; + ParentMap[Child] = Node; + WorkList.push_back(Child); + } + } + + if (Scopes.size() == 0) + return; + + // Compute registers which are livein into the loop headers. + RegSeen.clear(); + BackTrace.clear(); + InitRegPressure(Preheader); + + // Now perform LICM. + for (MachineDomTreeNode *Node : Scopes) { + MachineBasicBlock *MBB = Node->getBlock(); + + EnterScope(MBB); + + // Process the block + SpeculationState = SpeculateUnknown; + for (MachineBasicBlock::iterator + MII = MBB->begin(), E = MBB->end(); MII != E; ) { + MachineBasicBlock::iterator NextMII = MII; ++NextMII; + MachineInstr *MI = &*MII; + if (!Hoist(MI, Preheader)) + UpdateRegPressure(MI); + // If we have hoisted an instruction that may store, it can only be a + // constant store. + MII = NextMII; + } + + // If it's a leaf node, it's done. Traverse upwards to pop ancestors. + ExitScopeIfDone(Node, OpenChildren, ParentMap); + } +} + +/// Sink instructions into loops if profitable. This especially tries to prevent +/// register spills caused by register pressure if there is little to no +/// overhead moving instructions into loops. +void MachineLICMBase::SinkIntoLoop() { + MachineBasicBlock *Preheader = getCurPreheader(); + if (!Preheader) + return; + + SmallVector<MachineInstr *, 8> Candidates; + for (MachineBasicBlock::instr_iterator I = Preheader->instr_begin(); + I != Preheader->instr_end(); ++I) { + // We need to ensure that we can safely move this instruction into the loop. + // As such, it must not have side-effects, e.g. such as a call has. + if (IsLoopInvariantInst(*I) && !HasLoopPHIUse(&*I)) + Candidates.push_back(&*I); + } + + for (MachineInstr *I : Candidates) { + const MachineOperand &MO = I->getOperand(0); + if (!MO.isDef() || !MO.isReg() || !MO.getReg()) + continue; + if (!MRI->hasOneDef(MO.getReg())) + continue; + bool CanSink = true; + MachineBasicBlock *B = nullptr; + for (MachineInstr &MI : MRI->use_instructions(MO.getReg())) { + // FIXME: Come up with a proper cost model that estimates whether sinking + // the instruction (and thus possibly executing it on every loop + // iteration) is more expensive than a register. + // For now assumes that copies are cheap and thus almost always worth it. + if (!MI.isCopy()) { + CanSink = false; + break; + } + if (!B) { + B = MI.getParent(); + continue; + } + B = DT->findNearestCommonDominator(B, MI.getParent()); + if (!B) { + CanSink = false; + break; + } + } + if (!CanSink || !B || B == Preheader) + continue; + B->splice(B->getFirstNonPHI(), Preheader, I); + } +} + +static bool isOperandKill(const MachineOperand &MO, MachineRegisterInfo *MRI) { + return MO.isKill() || MRI->hasOneNonDBGUse(MO.getReg()); +} + +/// Find all virtual register references that are liveout of the preheader to +/// initialize the starting "register pressure". Note this does not count live +/// through (livein but not used) registers. +void MachineLICMBase::InitRegPressure(MachineBasicBlock *BB) { + std::fill(RegPressure.begin(), RegPressure.end(), 0); + + // If the preheader has only a single predecessor and it ends with a + // fallthrough or an unconditional branch, then scan its predecessor for live + // defs as well. This happens whenever the preheader is created by splitting + // the critical edge from the loop predecessor to the loop header. + if (BB->pred_size() == 1) { + MachineBasicBlock *TBB = nullptr, *FBB = nullptr; + SmallVector<MachineOperand, 4> Cond; + if (!TII->analyzeBranch(*BB, TBB, FBB, Cond, false) && Cond.empty()) + InitRegPressure(*BB->pred_begin()); + } + + for (const MachineInstr &MI : *BB) + UpdateRegPressure(&MI, /*ConsiderUnseenAsDef=*/true); +} + +/// Update estimate of register pressure after the specified instruction. +void MachineLICMBase::UpdateRegPressure(const MachineInstr *MI, + bool ConsiderUnseenAsDef) { + auto Cost = calcRegisterCost(MI, /*ConsiderSeen=*/true, ConsiderUnseenAsDef); + for (const auto &RPIdAndCost : Cost) { + unsigned Class = RPIdAndCost.first; + if (static_cast<int>(RegPressure[Class]) < -RPIdAndCost.second) + RegPressure[Class] = 0; + else + RegPressure[Class] += RPIdAndCost.second; + } +} + +/// Calculate the additional register pressure that the registers used in MI +/// cause. +/// +/// If 'ConsiderSeen' is true, updates 'RegSeen' and uses the information to +/// figure out which usages are live-ins. +/// FIXME: Figure out a way to consider 'RegSeen' from all code paths. +DenseMap<unsigned, int> +MachineLICMBase::calcRegisterCost(const MachineInstr *MI, bool ConsiderSeen, + bool ConsiderUnseenAsDef) { + DenseMap<unsigned, int> Cost; + if (MI->isImplicitDef()) + return Cost; + for (unsigned i = 0, e = MI->getDesc().getNumOperands(); i != e; ++i) { + const MachineOperand &MO = MI->getOperand(i); + if (!MO.isReg() || MO.isImplicit()) + continue; + Register Reg = MO.getReg(); + if (!Register::isVirtualRegister(Reg)) + continue; + + // FIXME: It seems bad to use RegSeen only for some of these calculations. + bool isNew = ConsiderSeen ? RegSeen.insert(Reg).second : false; + const TargetRegisterClass *RC = MRI->getRegClass(Reg); + + RegClassWeight W = TRI->getRegClassWeight(RC); + int RCCost = 0; + if (MO.isDef()) + RCCost = W.RegWeight; + else { + bool isKill = isOperandKill(MO, MRI); + if (isNew && !isKill && ConsiderUnseenAsDef) + // Haven't seen this, it must be a livein. + RCCost = W.RegWeight; + else if (!isNew && isKill) + RCCost = -W.RegWeight; + } + if (RCCost == 0) + continue; + const int *PS = TRI->getRegClassPressureSets(RC); + for (; *PS != -1; ++PS) { + if (Cost.find(*PS) == Cost.end()) + Cost[*PS] = RCCost; + else + Cost[*PS] += RCCost; + } + } + return Cost; +} + +/// Return true if this machine instruction loads from global offset table or +/// constant pool. +static bool mayLoadFromGOTOrConstantPool(MachineInstr &MI) { + assert(MI.mayLoad() && "Expected MI that loads!"); + + // If we lost memory operands, conservatively assume that the instruction + // reads from everything.. + if (MI.memoperands_empty()) + return true; + + for (MachineMemOperand *MemOp : MI.memoperands()) + if (const PseudoSourceValue *PSV = MemOp->getPseudoValue()) + if (PSV->isGOT() || PSV->isConstantPool()) + return true; + + return false; +} + +// This function iterates through all the operands of the input store MI and +// checks that each register operand statisfies isCallerPreservedPhysReg. +// This means, the value being stored and the address where it is being stored +// is constant throughout the body of the function (not including prologue and +// epilogue). When called with an MI that isn't a store, it returns false. +// A future improvement can be to check if the store registers are constant +// throughout the loop rather than throughout the funtion. +static bool isInvariantStore(const MachineInstr &MI, + const TargetRegisterInfo *TRI, + const MachineRegisterInfo *MRI) { + + bool FoundCallerPresReg = false; + if (!MI.mayStore() || MI.hasUnmodeledSideEffects() || + (MI.getNumOperands() == 0)) + return false; + + // Check that all register operands are caller-preserved physical registers. + for (const MachineOperand &MO : MI.operands()) { + if (MO.isReg()) { + Register Reg = MO.getReg(); + // If operand is a virtual register, check if it comes from a copy of a + // physical register. + if (Register::isVirtualRegister(Reg)) + Reg = TRI->lookThruCopyLike(MO.getReg(), MRI); + if (Register::isVirtualRegister(Reg)) + return false; + if (!TRI->isCallerPreservedPhysReg(Reg, *MI.getMF())) + return false; + else + FoundCallerPresReg = true; + } else if (!MO.isImm()) { + return false; + } + } + return FoundCallerPresReg; +} + +// Return true if the input MI is a copy instruction that feeds an invariant +// store instruction. This means that the src of the copy has to satisfy +// isCallerPreservedPhysReg and atleast one of it's users should satisfy +// isInvariantStore. +static bool isCopyFeedingInvariantStore(const MachineInstr &MI, + const MachineRegisterInfo *MRI, + const TargetRegisterInfo *TRI) { + + // FIXME: If targets would like to look through instructions that aren't + // pure copies, this can be updated to a query. + if (!MI.isCopy()) + return false; + + const MachineFunction *MF = MI.getMF(); + // Check that we are copying a constant physical register. + Register CopySrcReg = MI.getOperand(1).getReg(); + if (Register::isVirtualRegister(CopySrcReg)) + return false; + + if (!TRI->isCallerPreservedPhysReg(CopySrcReg, *MF)) + return false; + + Register CopyDstReg = MI.getOperand(0).getReg(); + // Check if any of the uses of the copy are invariant stores. + assert(Register::isVirtualRegister(CopyDstReg) && + "copy dst is not a virtual reg"); + + for (MachineInstr &UseMI : MRI->use_instructions(CopyDstReg)) { + if (UseMI.mayStore() && isInvariantStore(UseMI, TRI, MRI)) + return true; + } + return false; +} + +/// Returns true if the instruction may be a suitable candidate for LICM. +/// e.g. If the instruction is a call, then it's obviously not safe to hoist it. +bool MachineLICMBase::IsLICMCandidate(MachineInstr &I) { + // Check if it's safe to move the instruction. + bool DontMoveAcrossStore = true; + if ((!I.isSafeToMove(AA, DontMoveAcrossStore)) && + !(HoistConstStores && isInvariantStore(I, TRI, MRI))) { + return false; + } + + // If it is load then check if it is guaranteed to execute by making sure that + // it dominates all exiting blocks. If it doesn't, then there is a path out of + // the loop which does not execute this load, so we can't hoist it. Loads + // from constant memory are not safe to speculate all the time, for example + // indexed load from a jump table. + // Stores and side effects are already checked by isSafeToMove. + if (I.mayLoad() && !mayLoadFromGOTOrConstantPool(I) && + !IsGuaranteedToExecute(I.getParent())) + return false; + + return true; +} + +/// Returns true if the instruction is loop invariant. +/// I.e., all virtual register operands are defined outside of the loop, +/// physical registers aren't accessed explicitly, and there are no side +/// effects that aren't captured by the operands or other flags. +bool MachineLICMBase::IsLoopInvariantInst(MachineInstr &I) { + if (!IsLICMCandidate(I)) + return false; + + // The instruction is loop invariant if all of its operands are. + for (const MachineOperand &MO : I.operands()) { + if (!MO.isReg()) + continue; + + Register Reg = MO.getReg(); + if (Reg == 0) continue; + + // Don't hoist an instruction that uses or defines a physical register. + if (Register::isPhysicalRegister(Reg)) { + if (MO.isUse()) { + // If the physreg has no defs anywhere, it's just an ambient register + // and we can freely move its uses. Alternatively, if it's allocatable, + // it could get allocated to something with a def during allocation. + // However, if the physreg is known to always be caller saved/restored + // then this use is safe to hoist. + if (!MRI->isConstantPhysReg(Reg) && + !(TRI->isCallerPreservedPhysReg(Reg, *I.getMF()))) + return false; + // Otherwise it's safe to move. + continue; + } else if (!MO.isDead()) { + // A def that isn't dead. We can't move it. + return false; + } else if (CurLoop->getHeader()->isLiveIn(Reg)) { + // If the reg is live into the loop, we can't hoist an instruction + // which would clobber it. + return false; + } + } + + if (!MO.isUse()) + continue; + + assert(MRI->getVRegDef(Reg) && + "Machine instr not mapped for this vreg?!"); + + // If the loop contains the definition of an operand, then the instruction + // isn't loop invariant. + if (CurLoop->contains(MRI->getVRegDef(Reg))) + return false; + } + + // If we got this far, the instruction is loop invariant! + return true; +} + +/// Return true if the specified instruction is used by a phi node and hoisting +/// it could cause a copy to be inserted. +bool MachineLICMBase::HasLoopPHIUse(const MachineInstr *MI) const { + SmallVector<const MachineInstr*, 8> Work(1, MI); + do { + MI = Work.pop_back_val(); + for (const MachineOperand &MO : MI->operands()) { + if (!MO.isReg() || !MO.isDef()) + continue; + Register Reg = MO.getReg(); + if (!Register::isVirtualRegister(Reg)) + continue; + for (MachineInstr &UseMI : MRI->use_instructions(Reg)) { + // A PHI may cause a copy to be inserted. + if (UseMI.isPHI()) { + // A PHI inside the loop causes a copy because the live range of Reg is + // extended across the PHI. + if (CurLoop->contains(&UseMI)) + return true; + // A PHI in an exit block can cause a copy to be inserted if the PHI + // has multiple predecessors in the loop with different values. + // For now, approximate by rejecting all exit blocks. + if (isExitBlock(UseMI.getParent())) + return true; + continue; + } + // Look past copies as well. + if (UseMI.isCopy() && CurLoop->contains(&UseMI)) + Work.push_back(&UseMI); + } + } + } while (!Work.empty()); + return false; +} + +/// Compute operand latency between a def of 'Reg' and an use in the current +/// loop, return true if the target considered it high. +bool MachineLICMBase::HasHighOperandLatency(MachineInstr &MI, + unsigned DefIdx, + unsigned Reg) const { + if (MRI->use_nodbg_empty(Reg)) + return false; + + for (MachineInstr &UseMI : MRI->use_nodbg_instructions(Reg)) { + if (UseMI.isCopyLike()) + continue; + if (!CurLoop->contains(UseMI.getParent())) + continue; + for (unsigned i = 0, e = UseMI.getNumOperands(); i != e; ++i) { + const MachineOperand &MO = UseMI.getOperand(i); + if (!MO.isReg() || !MO.isUse()) + continue; + Register MOReg = MO.getReg(); + if (MOReg != Reg) + continue; + + if (TII->hasHighOperandLatency(SchedModel, MRI, MI, DefIdx, UseMI, i)) + return true; + } + + // Only look at the first in loop use. + break; + } + + return false; +} + +/// Return true if the instruction is marked "cheap" or the operand latency +/// between its def and a use is one or less. +bool MachineLICMBase::IsCheapInstruction(MachineInstr &MI) const { + if (TII->isAsCheapAsAMove(MI) || MI.isCopyLike()) + return true; + + bool isCheap = false; + unsigned NumDefs = MI.getDesc().getNumDefs(); + for (unsigned i = 0, e = MI.getNumOperands(); NumDefs && i != e; ++i) { + MachineOperand &DefMO = MI.getOperand(i); + if (!DefMO.isReg() || !DefMO.isDef()) + continue; + --NumDefs; + Register Reg = DefMO.getReg(); + if (Register::isPhysicalRegister(Reg)) + continue; + + if (!TII->hasLowDefLatency(SchedModel, MI, i)) + return false; + isCheap = true; + } + + return isCheap; +} + +/// Visit BBs from header to current BB, check if hoisting an instruction of the +/// given cost matrix can cause high register pressure. +bool +MachineLICMBase::CanCauseHighRegPressure(const DenseMap<unsigned, int>& Cost, + bool CheapInstr) { + for (const auto &RPIdAndCost : Cost) { + if (RPIdAndCost.second <= 0) + continue; + + unsigned Class = RPIdAndCost.first; + int Limit = RegLimit[Class]; + + // Don't hoist cheap instructions if they would increase register pressure, + // even if we're under the limit. + if (CheapInstr && !HoistCheapInsts) + return true; + + for (const auto &RP : BackTrace) + if (static_cast<int>(RP[Class]) + RPIdAndCost.second >= Limit) + return true; + } + + return false; +} + +/// Traverse the back trace from header to the current block and update their +/// register pressures to reflect the effect of hoisting MI from the current +/// block to the preheader. +void MachineLICMBase::UpdateBackTraceRegPressure(const MachineInstr *MI) { + // First compute the 'cost' of the instruction, i.e. its contribution + // to register pressure. + auto Cost = calcRegisterCost(MI, /*ConsiderSeen=*/false, + /*ConsiderUnseenAsDef=*/false); + + // Update register pressure of blocks from loop header to current block. + for (auto &RP : BackTrace) + for (const auto &RPIdAndCost : Cost) + RP[RPIdAndCost.first] += RPIdAndCost.second; +} + +/// Return true if it is potentially profitable to hoist the given loop +/// invariant. +bool MachineLICMBase::IsProfitableToHoist(MachineInstr &MI) { + if (MI.isImplicitDef()) + return true; + + // Besides removing computation from the loop, hoisting an instruction has + // these effects: + // + // - The value defined by the instruction becomes live across the entire + // loop. This increases register pressure in the loop. + // + // - If the value is used by a PHI in the loop, a copy will be required for + // lowering the PHI after extending the live range. + // + // - When hoisting the last use of a value in the loop, that value no longer + // needs to be live in the loop. This lowers register pressure in the loop. + + if (HoistConstStores && isCopyFeedingInvariantStore(MI, MRI, TRI)) + return true; + + bool CheapInstr = IsCheapInstruction(MI); + bool CreatesCopy = HasLoopPHIUse(&MI); + + // Don't hoist a cheap instruction if it would create a copy in the loop. + if (CheapInstr && CreatesCopy) { + LLVM_DEBUG(dbgs() << "Won't hoist cheap instr with loop PHI use: " << MI); + return false; + } + + // Rematerializable instructions should always be hoisted since the register + // allocator can just pull them down again when needed. + if (TII->isTriviallyReMaterializable(MI, AA)) + return true; + + // FIXME: If there are long latency loop-invariant instructions inside the + // loop at this point, why didn't the optimizer's LICM hoist them? + for (unsigned i = 0, e = MI.getDesc().getNumOperands(); i != e; ++i) { + const MachineOperand &MO = MI.getOperand(i); + if (!MO.isReg() || MO.isImplicit()) + continue; + Register Reg = MO.getReg(); + if (!Register::isVirtualRegister(Reg)) + continue; + if (MO.isDef() && HasHighOperandLatency(MI, i, Reg)) { + LLVM_DEBUG(dbgs() << "Hoist High Latency: " << MI); + ++NumHighLatency; + return true; + } + } + + // Estimate register pressure to determine whether to LICM the instruction. + // In low register pressure situation, we can be more aggressive about + // hoisting. Also, favors hoisting long latency instructions even in + // moderately high pressure situation. + // Cheap instructions will only be hoisted if they don't increase register + // pressure at all. + auto Cost = calcRegisterCost(&MI, /*ConsiderSeen=*/false, + /*ConsiderUnseenAsDef=*/false); + + // Visit BBs from header to current BB, if hoisting this doesn't cause + // high register pressure, then it's safe to proceed. + if (!CanCauseHighRegPressure(Cost, CheapInstr)) { + LLVM_DEBUG(dbgs() << "Hoist non-reg-pressure: " << MI); + ++NumLowRP; + return true; + } + + // Don't risk increasing register pressure if it would create copies. + if (CreatesCopy) { + LLVM_DEBUG(dbgs() << "Won't hoist instr with loop PHI use: " << MI); + return false; + } + + // Do not "speculate" in high register pressure situation. If an + // instruction is not guaranteed to be executed in the loop, it's best to be + // conservative. + if (AvoidSpeculation && + (!IsGuaranteedToExecute(MI.getParent()) && !MayCSE(&MI))) { + LLVM_DEBUG(dbgs() << "Won't speculate: " << MI); + return false; + } + + // High register pressure situation, only hoist if the instruction is going + // to be remat'ed. + if (!TII->isTriviallyReMaterializable(MI, AA) && + !MI.isDereferenceableInvariantLoad(AA)) { + LLVM_DEBUG(dbgs() << "Can't remat / high reg-pressure: " << MI); + return false; + } + + return true; +} + +/// Unfold a load from the given machineinstr if the load itself could be +/// hoisted. Return the unfolded and hoistable load, or null if the load +/// couldn't be unfolded or if it wouldn't be hoistable. +MachineInstr *MachineLICMBase::ExtractHoistableLoad(MachineInstr *MI) { + // Don't unfold simple loads. + if (MI->canFoldAsLoad()) + return nullptr; + + // If not, we may be able to unfold a load and hoist that. + // First test whether the instruction is loading from an amenable + // memory location. + if (!MI->isDereferenceableInvariantLoad(AA)) + return nullptr; + + // Next determine the register class for a temporary register. + unsigned LoadRegIndex; + unsigned NewOpc = + TII->getOpcodeAfterMemoryUnfold(MI->getOpcode(), + /*UnfoldLoad=*/true, + /*UnfoldStore=*/false, + &LoadRegIndex); + if (NewOpc == 0) return nullptr; + const MCInstrDesc &MID = TII->get(NewOpc); + MachineFunction &MF = *MI->getMF(); + const TargetRegisterClass *RC = TII->getRegClass(MID, LoadRegIndex, TRI, MF); + // Ok, we're unfolding. Create a temporary register and do the unfold. + Register Reg = MRI->createVirtualRegister(RC); + + SmallVector<MachineInstr *, 2> NewMIs; + bool Success = TII->unfoldMemoryOperand(MF, *MI, Reg, + /*UnfoldLoad=*/true, + /*UnfoldStore=*/false, NewMIs); + (void)Success; + assert(Success && + "unfoldMemoryOperand failed when getOpcodeAfterMemoryUnfold " + "succeeded!"); + assert(NewMIs.size() == 2 && + "Unfolded a load into multiple instructions!"); + MachineBasicBlock *MBB = MI->getParent(); + MachineBasicBlock::iterator Pos = MI; + MBB->insert(Pos, NewMIs[0]); + MBB->insert(Pos, NewMIs[1]); + // If unfolding produced a load that wasn't loop-invariant or profitable to + // hoist, discard the new instructions and bail. + if (!IsLoopInvariantInst(*NewMIs[0]) || !IsProfitableToHoist(*NewMIs[0])) { + NewMIs[0]->eraseFromParent(); + NewMIs[1]->eraseFromParent(); + return nullptr; + } + + // Update register pressure for the unfolded instruction. + UpdateRegPressure(NewMIs[1]); + + // Otherwise we successfully unfolded a load that we can hoist. + MI->eraseFromParent(); + return NewMIs[0]; +} + +/// Initialize the CSE map with instructions that are in the current loop +/// preheader that may become duplicates of instructions that are hoisted +/// out of the loop. +void MachineLICMBase::InitCSEMap(MachineBasicBlock *BB) { + for (MachineInstr &MI : *BB) + CSEMap[MI.getOpcode()].push_back(&MI); +} + +/// Find an instruction amount PrevMIs that is a duplicate of MI. +/// Return this instruction if it's found. +const MachineInstr* +MachineLICMBase::LookForDuplicate(const MachineInstr *MI, + std::vector<const MachineInstr*> &PrevMIs) { + for (const MachineInstr *PrevMI : PrevMIs) + if (TII->produceSameValue(*MI, *PrevMI, (PreRegAlloc ? MRI : nullptr))) + return PrevMI; + + return nullptr; +} + +/// Given a LICM'ed instruction, look for an instruction on the preheader that +/// computes the same value. If it's found, do a RAU on with the definition of +/// the existing instruction rather than hoisting the instruction to the +/// preheader. +bool MachineLICMBase::EliminateCSE(MachineInstr *MI, + DenseMap<unsigned, std::vector<const MachineInstr *>>::iterator &CI) { + // Do not CSE implicit_def so ProcessImplicitDefs can properly propagate + // the undef property onto uses. + if (CI == CSEMap.end() || MI->isImplicitDef()) + return false; + + if (const MachineInstr *Dup = LookForDuplicate(MI, CI->second)) { + LLVM_DEBUG(dbgs() << "CSEing " << *MI << " with " << *Dup); + + // Replace virtual registers defined by MI by their counterparts defined + // by Dup. + SmallVector<unsigned, 2> Defs; + for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) { + const MachineOperand &MO = MI->getOperand(i); + + // Physical registers may not differ here. + assert((!MO.isReg() || MO.getReg() == 0 || + !Register::isPhysicalRegister(MO.getReg()) || + MO.getReg() == Dup->getOperand(i).getReg()) && + "Instructions with different phys regs are not identical!"); + + if (MO.isReg() && MO.isDef() && + !Register::isPhysicalRegister(MO.getReg())) + Defs.push_back(i); + } + + SmallVector<const TargetRegisterClass*, 2> OrigRCs; + for (unsigned i = 0, e = Defs.size(); i != e; ++i) { + unsigned Idx = Defs[i]; + Register Reg = MI->getOperand(Idx).getReg(); + Register DupReg = Dup->getOperand(Idx).getReg(); + OrigRCs.push_back(MRI->getRegClass(DupReg)); + + if (!MRI->constrainRegClass(DupReg, MRI->getRegClass(Reg))) { + // Restore old RCs if more than one defs. + for (unsigned j = 0; j != i; ++j) + MRI->setRegClass(Dup->getOperand(Defs[j]).getReg(), OrigRCs[j]); + return false; + } + } + + for (unsigned Idx : Defs) { + Register Reg = MI->getOperand(Idx).getReg(); + Register DupReg = Dup->getOperand(Idx).getReg(); + MRI->replaceRegWith(Reg, DupReg); + MRI->clearKillFlags(DupReg); + } + + MI->eraseFromParent(); + ++NumCSEed; + return true; + } + return false; +} + +/// Return true if the given instruction will be CSE'd if it's hoisted out of +/// the loop. +bool MachineLICMBase::MayCSE(MachineInstr *MI) { + unsigned Opcode = MI->getOpcode(); + DenseMap<unsigned, std::vector<const MachineInstr *>>::iterator + CI = CSEMap.find(Opcode); + // Do not CSE implicit_def so ProcessImplicitDefs can properly propagate + // the undef property onto uses. + if (CI == CSEMap.end() || MI->isImplicitDef()) + return false; + + return LookForDuplicate(MI, CI->second) != nullptr; +} + +/// When an instruction is found to use only loop invariant operands +/// that are safe to hoist, this instruction is called to do the dirty work. +/// It returns true if the instruction is hoisted. +bool MachineLICMBase::Hoist(MachineInstr *MI, MachineBasicBlock *Preheader) { + // First check whether we should hoist this instruction. + if (!IsLoopInvariantInst(*MI) || !IsProfitableToHoist(*MI)) { + // If not, try unfolding a hoistable load. + MI = ExtractHoistableLoad(MI); + if (!MI) return false; + } + + // If we have hoisted an instruction that may store, it can only be a constant + // store. + if (MI->mayStore()) + NumStoreConst++; + + // Now move the instructions to the predecessor, inserting it before any + // terminator instructions. + LLVM_DEBUG({ + dbgs() << "Hoisting " << *MI; + if (MI->getParent()->getBasicBlock()) + dbgs() << " from " << printMBBReference(*MI->getParent()); + if (Preheader->getBasicBlock()) + dbgs() << " to " << printMBBReference(*Preheader); + dbgs() << "\n"; + }); + + // If this is the first instruction being hoisted to the preheader, + // initialize the CSE map with potential common expressions. + if (FirstInLoop) { + InitCSEMap(Preheader); + FirstInLoop = false; + } + + // Look for opportunity to CSE the hoisted instruction. + unsigned Opcode = MI->getOpcode(); + DenseMap<unsigned, std::vector<const MachineInstr *>>::iterator + CI = CSEMap.find(Opcode); + if (!EliminateCSE(MI, CI)) { + // Otherwise, splice the instruction to the preheader. + Preheader->splice(Preheader->getFirstTerminator(),MI->getParent(),MI); + + // Since we are moving the instruction out of its basic block, we do not + // retain its debug location. Doing so would degrade the debugging + // experience and adversely affect the accuracy of profiling information. + MI->setDebugLoc(DebugLoc()); + + // Update register pressure for BBs from header to this block. + UpdateBackTraceRegPressure(MI); + + // Clear the kill flags of any register this instruction defines, + // since they may need to be live throughout the entire loop + // rather than just live for part of it. + for (MachineOperand &MO : MI->operands()) + if (MO.isReg() && MO.isDef() && !MO.isDead()) + MRI->clearKillFlags(MO.getReg()); + + // Add to the CSE map. + if (CI != CSEMap.end()) + CI->second.push_back(MI); + else + CSEMap[Opcode].push_back(MI); + } + + ++NumHoisted; + Changed = true; + + return true; +} + +/// Get the preheader for the current loop, splitting a critical edge if needed. +MachineBasicBlock *MachineLICMBase::getCurPreheader() { + // Determine the block to which to hoist instructions. If we can't find a + // suitable loop predecessor, we can't do any hoisting. + + // If we've tried to get a preheader and failed, don't try again. + if (CurPreheader == reinterpret_cast<MachineBasicBlock *>(-1)) + return nullptr; + + if (!CurPreheader) { + CurPreheader = CurLoop->getLoopPreheader(); + if (!CurPreheader) { + MachineBasicBlock *Pred = CurLoop->getLoopPredecessor(); + if (!Pred) { + CurPreheader = reinterpret_cast<MachineBasicBlock *>(-1); + return nullptr; + } + + CurPreheader = Pred->SplitCriticalEdge(CurLoop->getHeader(), *this); + if (!CurPreheader) { + CurPreheader = reinterpret_cast<MachineBasicBlock *>(-1); + return nullptr; + } + } + } + return CurPreheader; +} |