RISCVZilsdOptimizer.cpp

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//===-- RISCVZilsdOptimizer.cpp - RISC-V Zilsd Load/Store Optimizer ------===//
//
// 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 file contains a pass that performs load/store optimizations for the
// RISC-V Zilsd extension. It combines pairs of 32-bit load/store instructions
// into single 64-bit LD/SD instructions when possible.
//
// The pass runs in two phases:
// 1. Pre-allocation: Reschedules loads/stores to bring consecutive memory
//    accesses closer together and forms LD/SD pairs with register hints.
// 2. Post-allocation: Fixes invalid LD/SD instructions if register allocation
//    didn't provide suitable consecutive registers.
//
// Note: second phase is integrated into RISCVLoadStoreOptimizer
//
//===----------------------------------------------------------------------===//
 
#include "RISCV.h"
#include "RISCVInstrInfo.h"
#include "RISCVRegisterInfo.h"
#include "RISCVSubtarget.h"
#include "llvm/ADT/DenseMap.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/MachineFunction.h"
#include "llvm/CodeGen/MachineFunctionPass.h"
#include "llvm/CodeGen/MachineInstr.h"
#include "llvm/CodeGen/MachineInstrBuilder.h"
#include "llvm/CodeGen/MachineRegisterInfo.h"
#include "llvm/InitializePasses.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Debug.h"
#include <algorithm>
 
using namespace llvm;
 
#define DEBUG_TYPE "riscv-zilsd-opt"
 
STATISTIC(NumLDFormed, "Number of LD instructions formed");
STATISTIC(NumSDFormed, "Number of SD instructions formed");
 
static cl::opt<bool>
    DisableZilsdOpt("disable-riscv-zilsd-opt", cl::Hidden, cl::init(false),
                    cl::desc("Disable Zilsd load/store optimization"));
 
static cl::opt<unsigned> MaxRescheduleDistance(
    "riscv-zilsd-max-reschedule-distance", cl::Hidden, cl::init(10),
    cl::desc("Maximum distance for rescheduling load/store instructions"));
 
namespace {
 
//===----------------------------------------------------------------------===//
// Pre-allocation Zilsd optimization pass
//===----------------------------------------------------------------------===//
class RISCVPreAllocZilsdOpt : public MachineFunctionPass {
public:
  static char ID;
 
  RISCVPreAllocZilsdOpt() : MachineFunctionPass(ID) {}
 
  bool runOnMachineFunction(MachineFunction &MF) override;
 
  StringRef getPassName() const override {
    return "RISC-V pre-allocation Zilsd load/store optimization";
  }
 
  MachineFunctionProperties getRequiredProperties() const override {
    return MachineFunctionProperties().setIsSSA();
  }
 
  void getAnalysisUsage(AnalysisUsage &AU) const override {
    AU.addRequired<AAResultsWrapperPass>();
    AU.addRequired<MachineDominatorTreeWrapperPass>();
    AU.setPreservesCFG();
    MachineFunctionPass::getAnalysisUsage(AU);
  }
  enum class MemoryOffsetKind {
    Imm = 0,
    Global = 1,
    CPI = 2,
    BlockAddr = 3,
    FrameIdx = 4,
    Unknown = 5,
  };
  using MemOffset = std::pair<MemoryOffsetKind, int>;
  using BaseRegInfo = std::pair<unsigned, MemoryOffsetKind>;
 
private:
  bool isMemoryOp(const MachineInstr &MI);
  bool rescheduleLoadStoreInstrs(MachineBasicBlock *MBB);
  bool canFormLdSdPair(MachineInstr *MI0, MachineInstr *MI1);
  bool rescheduleOps(MachineBasicBlock *MBB,
                     SmallVectorImpl<MachineInstr *> &MIs, BaseRegInfo Base,
                     bool IsLoad,
                     DenseMap<MachineInstr *, unsigned> &MI2LocMap);
  bool isSafeToMove(MachineInstr *MI, MachineInstr *Target, bool MoveForward);
  MemOffset getMemoryOpOffset(const MachineInstr &MI);
 
  const RISCVSubtarget *STI;
  const RISCVInstrInfo *TII;
  const RISCVRegisterInfo *TRI;
  MachineRegisterInfo *MRI;
  AliasAnalysis *AA;
  MachineDominatorTree *DT;
  Align RequiredAlign;
};
 
} // end anonymous namespace
 
char RISCVPreAllocZilsdOpt::ID = 0;
 
INITIALIZE_PASS_BEGIN(RISCVPreAllocZilsdOpt, "riscv-prera-zilsd-opt",
                      "RISC-V pre-allocation Zilsd optimization", false, false)
INITIALIZE_PASS_DEPENDENCY(AAResultsWrapperPass)
INITIALIZE_PASS_DEPENDENCY(MachineDominatorTreeWrapperPass)
INITIALIZE_PASS_END(RISCVPreAllocZilsdOpt, "riscv-prera-zilsd-opt",
                    "RISC-V pre-allocation Zilsd optimization", false, false)
 
//===----------------------------------------------------------------------===//
// Pre-allocation pass implementation
//===----------------------------------------------------------------------===//
 
bool RISCVPreAllocZilsdOpt::runOnMachineFunction(MachineFunction &MF) {
 
  if (DisableZilsdOpt || skipFunction(MF.getFunction()))
    return false;
 
  STI = &MF.getSubtarget<RISCVSubtarget>();
 
  // Only run on RV32 with Zilsd extension
  if (STI->is64Bit() || !STI->hasStdExtZilsd())
    return false;
 
  TII = STI->getInstrInfo();
  TRI = STI->getRegisterInfo();
  MRI = &MF.getRegInfo();
  AA = &getAnalysis<AAResultsWrapperPass>().getAAResults();
  DT = &getAnalysis<MachineDominatorTreeWrapperPass>().getDomTree();
 
  // Check alignment: default is 8-byte, but allow 4-byte with tune feature
  // If unaligned scalar memory is enabled, allow any alignment
  RequiredAlign = STI->getZilsdAlign();
  bool Modified = false;
  for (auto &MBB : MF) {
    Modified |= rescheduleLoadStoreInstrs(&MBB);
  }
 
  return Modified;
}
 
RISCVPreAllocZilsdOpt::MemOffset
RISCVPreAllocZilsdOpt::getMemoryOpOffset(const MachineInstr &MI) {
  switch (MI.getOpcode()) {
  case RISCV::LW:
  case RISCV::SW: {
    // For LW/SW, the base is in operand 1 and offset is in operand 2
    const MachineOperand &BaseOp = MI.getOperand(1);
    const MachineOperand &OffsetOp = MI.getOperand(2);
 
    // Handle immediate offset
    if (OffsetOp.isImm()) {
      if (BaseOp.isFI())
        return std::make_pair(MemoryOffsetKind::FrameIdx, OffsetOp.getImm());
      return std::make_pair(MemoryOffsetKind::Imm, OffsetOp.getImm());
    }
 
    // Handle symbolic operands with MO_LO flag (from MergeBaseOffset)
    if (OffsetOp.getTargetFlags() & RISCVII::MO_LO) {
      if (OffsetOp.isGlobal())
        return std::make_pair(MemoryOffsetKind::Global, OffsetOp.getOffset());
      if (OffsetOp.isCPI())
        return std::make_pair(MemoryOffsetKind::CPI, OffsetOp.getOffset());
      if (OffsetOp.isBlockAddress())
        return std::make_pair(MemoryOffsetKind::BlockAddr,
                              OffsetOp.getOffset());
    }
 
    break;
  }
  default:
    break;
  }
 
  return std::make_pair(MemoryOffsetKind::Unknown, 0);
}
 
bool RISCVPreAllocZilsdOpt::canFormLdSdPair(MachineInstr *MI0,
                                            MachineInstr *MI1) {
  if (!MI0->hasOneMemOperand() || !MI1->hasOneMemOperand())
    return false;
 
  // Get offsets and check they are consecutive
  int Offset0 = getMemoryOpOffset(*MI0).second;
  int Offset1 = getMemoryOpOffset(*MI1).second;
 
  // Offsets must be 4 bytes apart
  if (Offset1 - Offset0 != 4)
    return false;
 
  // We need to guarantee the alignment(base + offset) is legal.
  const MachineMemOperand *MMO = *MI0->memoperands_begin();
  if (MMO->getAlign() < RequiredAlign)
    return false;
 
  // Check that the two destination/source registers are different for
  // load/store respectively.
  // The only case two destinations/sources can be same is (x0, x0). This pass
  // is run before register coalescer so it will be the form of:
  //   %0 = COPY $x0
  //   SW %0, %ptr
  // instead of:
  //   SW $x0, %ptr
  Register FirstReg = MI0->getOperand(0).getReg();
  Register SecondReg = MI1->getOperand(0).getReg();
  if (FirstReg == SecondReg) {
    const MachineInstr *FirstOpDefInst = MRI->getUniqueVRegDef(FirstReg);
    if (FirstOpDefInst->isCopy() &&
        FirstOpDefInst->getOperand(1).getReg() == RISCV::X0)
      return true;
    return false;
  }
 
  return true;
}
 
bool RISCVPreAllocZilsdOpt::isSafeToMove(MachineInstr *MI, MachineInstr *Target,
                                         bool MoveForward) {
  MachineBasicBlock *MBB = MI->getParent();
  MachineBasicBlock::iterator Start = MI->getIterator();
  MachineBasicBlock::iterator End = Target->getIterator();
 
  if (!MoveForward)
    std::swap(Start, End);
 
  // Increment Start to skip the current instruction
  if (Start != MBB->end())
    ++Start;
 
  Register DefReg = MI->getOperand(0).getReg();
  const MachineOperand &BaseOp = MI->getOperand(1);
 
  unsigned ScanCount = 0;
  for (auto It = Start; It != End; ++It, ++ScanCount) {
    // Don't move across calls or terminators
    if (It->isCall() || It->isTerminator()) {
      LLVM_DEBUG(dbgs() << "Cannot move across call/terminator: " << *It);
      return false;
    }
 
    // Don't move across instructions that modify memory barrier
    if (It->hasUnmodeledSideEffects()) {
      LLVM_DEBUG(dbgs() << "Cannot move across instruction with side effects: "
                        << *It);
      return false;
    }
 
    // Check if the base register is modified
    if (BaseOp.isReg() && It->modifiesRegister(BaseOp.getReg(), TRI)) {
      LLVM_DEBUG(dbgs() << "Base register " << BaseOp.getReg()
                        << " modified by: " << *It);
      return false;
    }
 
    // For loads, check if the loaded value is used
    if (MI->mayLoad() &&
        (It->readsRegister(DefReg, TRI) || It->modifiesRegister(DefReg, TRI))) {
      LLVM_DEBUG(dbgs() << "Destination register " << DefReg
                        << " used by: " << *It);
      return false;
    }
 
    // For stores, check if the stored register is modified
    if (MI->mayStore() && It->modifiesRegister(DefReg, TRI)) {
      LLVM_DEBUG(dbgs() << "Source register " << DefReg
                        << " modified by: " << *It);
      return false;
    }
 
    // Check for memory operation interference
    if (It->mayLoadOrStore() && It->mayAlias(AA, *MI, /*UseTBAA*/ false)) {
      LLVM_DEBUG(dbgs() << "Memory operation interference detected\n");
      return false;
    }
  }
 
  return true;
}
 
bool RISCVPreAllocZilsdOpt::rescheduleOps(
    MachineBasicBlock *MBB, SmallVectorImpl<MachineInstr *> &MIs,
    BaseRegInfo Base, bool IsLoad,
    DenseMap<MachineInstr *, unsigned> &MI2LocMap) {
  // Sort by offset, at this point it ensure base reg and MemoryOffsetKind are
  // same, so we just need to simply sort by offset value.
  llvm::sort(MIs.begin(), MIs.end(), [this](MachineInstr *A, MachineInstr *B) {
    return getMemoryOpOffset(*A).second < getMemoryOpOffset(*B).second;
  });
 
  bool Modified = false;
 
  // Try to pair consecutive operations
  for (size_t i = 0; i + 1 < MIs.size(); i++) {
    MachineInstr *MI0 = MIs[i];
    MachineInstr *MI1 = MIs[i + 1];
 
    Register FirstReg = MI0->getOperand(0).getReg();
    Register SecondReg = MI1->getOperand(0).getReg();
    const MachineOperand &BaseOp = MI0->getOperand(1);
    const MachineOperand &OffsetOp = MI0->getOperand(2);
    assert((BaseOp.isReg() || BaseOp.isFI()) &&
           "Base register should be register or frame index");
 
    // At this point, MI0 and MI1 are:
    //    1. both either LW or SW.
    //    2. guaranteed to have same memory kind.
    //    3. guaranteed to have same base register.
    //    4. already be sorted by offset value.
    // so we don't have to check these in canFormLdSdPair.
    if (!canFormLdSdPair(MI0, MI1))
      continue;
 
    // Use MI2LocMap to determine which instruction appears later in program
    // order
    bool MI1IsLater = MI2LocMap[MI1] > MI2LocMap[MI0];
 
    // For loads: move later instruction up (backwards) to earlier instruction
    // For stores: move earlier instruction down (forwards) to later instruction
    MachineInstr *MoveInstr, *TargetInstr;
    if (IsLoad) {
      // For loads: move the later instruction to the earlier one
      MoveInstr = MI1IsLater ? MI1 : MI0;
      TargetInstr = MI1IsLater ? MI0 : MI1;
    } else {
      // For stores: move the earlier instruction to the later one
      MoveInstr = MI1IsLater ? MI0 : MI1;
      TargetInstr = MI1IsLater ? MI1 : MI0;
    }
 
    unsigned Distance = MI1IsLater ? MI2LocMap[MI1] - MI2LocMap[MI0]
                                   : MI2LocMap[MI0] - MI2LocMap[MI1];
    if (!isSafeToMove(MoveInstr, TargetInstr, !IsLoad) ||
        Distance > MaxRescheduleDistance)
      continue;
 
    // Move the instruction to the target position
    MachineBasicBlock::iterator InsertPos = TargetInstr->getIterator();
    ++InsertPos;
 
    // If we need to move an instruction, do it now
    if (MoveInstr != TargetInstr)
      MBB->splice(InsertPos, MBB, MoveInstr->getIterator());
 
    // Create the paired instruction
    MachineInstrBuilder MIB;
    DebugLoc DL = MI0->getDebugLoc();
 
    if (IsLoad) {
      MIB = BuildMI(*MBB, InsertPos, DL, TII->get(RISCV::PseudoLD_RV32_OPT))
                .addReg(FirstReg, RegState::Define)
                .addReg(SecondReg, RegState::Define);
      ++NumLDFormed;
      LLVM_DEBUG(dbgs() << "Formed LD: " << *MIB << "\n");
    } else {
      MIB = BuildMI(*MBB, InsertPos, DL, TII->get(RISCV::PseudoSD_RV32_OPT))
                .addReg(FirstReg)
                .addReg(SecondReg);
      ++NumSDFormed;
      LLVM_DEBUG(dbgs() << "Formed SD: " << *MIB << "\n");
    }
 
    if (BaseOp.isReg())
      MIB = MIB.addReg(BaseOp.getReg());
    else
      MIB = MIB.addFrameIndex(BaseOp.getIndex());
    MIB = MIB.add(OffsetOp);
 
    // Copy memory operands
    MIB.cloneMergedMemRefs({MI0, MI1});
 
    // Add register allocation hints for consecutive registers
    // RISC-V Zilsd requires even/odd register pairs
    // Only set hints for virtual registers (physical registers already have
    // encoding)
    if (FirstReg.isVirtual() && SecondReg.isVirtual()) {
      // For virtual registers, we can't determine even/odd yet, but we can hint
      // that they should be allocated as a consecutive pair
      MRI->setRegAllocationHint(FirstReg, RISCVRI::RegPairEven, SecondReg);
      MRI->setRegAllocationHint(SecondReg, RISCVRI::RegPairOdd, FirstReg);
    }
 
    // Remove the original instructions
    MI0->eraseFromParent();
    MI1->eraseFromParent();
 
    Modified = true;
 
    // Skip the next instruction since we've already processed it
    i++;
  }
 
  return Modified;
}
 
bool RISCVPreAllocZilsdOpt::isMemoryOp(const MachineInstr &MI) {
  unsigned Opcode = MI.getOpcode();
  if (Opcode != RISCV::LW && Opcode != RISCV::SW)
    return false;
 
  if (!MI.getOperand(1).isReg() && !MI.getOperand(1).isFI())
    return false;
 
  // When no memory operands are present, conservatively assume unaligned,
  // volatile, unfoldable.
  if (!MI.hasOneMemOperand())
    return false;
 
  const MachineMemOperand *MMO = *MI.memoperands_begin();
 
  if (MMO->isVolatile() || MMO->isAtomic())
    return false;
 
  // sw <undef> could probably be eliminated entirely, but for now we just want
  // to avoid making a mess of it.
  if (MI.getOperand(0).isReg() && MI.getOperand(0).isUndef())
    return false;
 
  // Likewise don't mess with references to undefined addresses.
  if (MI.getOperand(1).isReg() && MI.getOperand(1).isUndef())
    return false;
 
  return true;
}
 
bool RISCVPreAllocZilsdOpt::rescheduleLoadStoreInstrs(MachineBasicBlock *MBB) {
  bool Modified = false;
 
  // Process the basic block in windows delimited by calls, terminators,
  // or instructions with duplicate base+offset pairs
  MachineBasicBlock::iterator MBBI = MBB->begin();
  MachineBasicBlock::iterator E = MBB->end();
 
  while (MBBI != E) {
    // Map from instruction to its location in the current window
    DenseMap<MachineInstr *, unsigned> MI2LocMap;
 
    // Map from base register to list of load/store instructions
    using Base2InstMap = DenseMap<BaseRegInfo, SmallVector<MachineInstr *, 4>>;
    using BaseVec = SmallVector<BaseRegInfo, 4>;
    Base2InstMap Base2LdsMap;
    Base2InstMap Base2StsMap;
    BaseVec LdBases;
    BaseVec StBases;
 
    unsigned Loc = 0;
 
    // Build the current window of instructions
    for (; MBBI != E; ++MBBI) {
      MachineInstr &MI = *MBBI;
 
      // Stop at barriers (calls and terminators)
      if (MI.isCall() || MI.isTerminator()) {
        // Move past the barrier for next iteration
        ++MBBI;
        break;
      }
 
      // Track instruction location in window
      if (!MI.isDebugInstr())
        MI2LocMap[&MI] = ++Loc;
 
      MemOffset Offset = getMemoryOpOffset(MI);
      // Skip non-memory operations or it's not a valid memory offset kind.
      if (!isMemoryOp(MI) || Offset.first == MemoryOffsetKind::Unknown)
        continue;
 
      bool IsLd = (MI.getOpcode() == RISCV::LW);
      const MachineOperand &BaseOp = MI.getOperand(1);
      unsigned Base;
      if (BaseOp.isReg())
        Base = BaseOp.getReg().id();
      else
        Base = BaseOp.getIndex();
      bool StopHere = false;
 
      // Lambda to find or add base register entries
      auto FindBases = [&](Base2InstMap &Base2Ops, BaseVec &Bases) {
        auto [BI, Inserted] = Base2Ops.try_emplace({Base, Offset.first});
        if (Inserted) {
          // First time seeing this base register
          BI->second.push_back(&MI);
          Bases.push_back({Base, Offset.first});
          return;
        }
        // Check if we've seen this exact base+offset before
        if (any_of(BI->second, [&](const MachineInstr *PrevMI) {
              return Offset == getMemoryOpOffset(*PrevMI);
            })) {
          // Found duplicate base+offset - stop here to process current window
          StopHere = true;
        } else {
          BI->second.push_back(&MI);
        }
      };
 
      if (IsLd)
        FindBases(Base2LdsMap, LdBases);
      else
        FindBases(Base2StsMap, StBases);
 
      if (StopHere) {
        // Found a duplicate (a base+offset combination that's seen earlier).
        // Backtrack to process the current window.
        --Loc;
        break;
      }
    }
 
    // Process the current window - reschedule loads
    for (auto Base : LdBases) {
      SmallVectorImpl<MachineInstr *> &Lds = Base2LdsMap[Base];
      if (Lds.size() > 1) {
        Modified |= rescheduleOps(MBB, Lds, Base, true, MI2LocMap);
      }
    }
 
    // Process the current window - reschedule stores
    for (auto Base : StBases) {
      SmallVectorImpl<MachineInstr *> &Sts = Base2StsMap[Base];
      if (Sts.size() > 1) {
        Modified |= rescheduleOps(MBB, Sts, Base, false, MI2LocMap);
      }
    }
  }
 
  return Modified;
}
 
//===----------------------------------------------------------------------===//
// Pass creation functions
//===----------------------------------------------------------------------===//
 
FunctionPass *llvm::createRISCVPreAllocZilsdOptPass() {
  return new RISCVPreAllocZilsdOpt();
}