RISCVMergeBaseOffset.cpp

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//===----- RISCVMergeBaseOffset.cpp - Optimise address calculations  ------===//
//
// 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
//
//===----------------------------------------------------------------------===//
//
// Merge the offset of address calculation into the offset field
// of instructions in a global address lowering sequence. This pass transforms:
//   lui  vreg1, %hi(s)
//   addi vreg2, vreg1, %lo(s)
//   addi vreg3, verg2, Offset
//
//   Into:
//   lui  vreg1, %hi(s+Offset)
//   addi vreg2, vreg1, %lo(s+Offset)
//
// The transformation is carried out under certain conditions:
// 1) The offset field in the base of global address lowering sequence is zero.
// 2) The lowered global address has only one use.
//
// The offset field can be in a different form. This pass handles all of them.
//===----------------------------------------------------------------------===//
 
#include "RISCV.h"
#include "RISCVTargetMachine.h"
#include "llvm/CodeGen/MachineFunctionPass.h"
#include "llvm/CodeGen/Passes.h"
#include "llvm/MC/TargetRegistry.h"
#include "llvm/Support/Debug.h"
#include "llvm/Target/TargetOptions.h"
#include <set>
using namespace llvm;
 
#define DEBUG_TYPE "riscv-merge-base-offset"
#define RISCV_MERGE_BASE_OFFSET_NAME "RISCV Merge Base Offset"
namespace {
 
struct RISCVMergeBaseOffsetOpt : public MachineFunctionPass {
private:
  const RISCVSubtarget *ST = nullptr;
 
public:
  static char ID;
  bool runOnMachineFunction(MachineFunction &Fn) override;
  bool detectLuiAddiGlobal(MachineInstr &LUI, MachineInstr *&ADDI);
 
  bool detectAndFoldOffset(MachineInstr &HiLUI, MachineInstr &LoADDI);
  void foldOffset(MachineInstr &HiLUI, MachineInstr &LoADDI, MachineInstr &Tail,
                  int64_t Offset);
  bool matchLargeOffset(MachineInstr &TailAdd, Register GSReg, int64_t &Offset);
  bool matchShiftedOffset(MachineInstr &TailShXAdd, Register GSReg,
                          int64_t &Offset);
 
  RISCVMergeBaseOffsetOpt() : MachineFunctionPass(ID) {}
 
  MachineFunctionProperties getRequiredProperties() const override {
    return MachineFunctionProperties().set(
        MachineFunctionProperties::Property::IsSSA);
  }
 
  void getAnalysisUsage(AnalysisUsage &AU) const override {
    AU.setPreservesCFG();
    MachineFunctionPass::getAnalysisUsage(AU);
  }
 
  StringRef getPassName() const override {
    return RISCV_MERGE_BASE_OFFSET_NAME;
  }
 
private:
  MachineRegisterInfo *MRI;
  std::set<MachineInstr *> DeadInstrs;
};
} // end anonymous namespace
 
char RISCVMergeBaseOffsetOpt::ID = 0;
INITIALIZE_PASS(RISCVMergeBaseOffsetOpt, DEBUG_TYPE,
                RISCV_MERGE_BASE_OFFSET_NAME, false, false)
 
// Detect the pattern:
//   lui   vreg1, %hi(s)
//   addi  vreg2, vreg1, %lo(s)
//
//   Pattern only accepted if:
//     1) ADDI has only one use.
//     2) LUI has only one use; which is the ADDI.
//     3) Both ADDI and LUI have GlobalAddress type which indicates that these
//        are generated from global address lowering.
//     4) Offset value in the Global Address is 0.
bool RISCVMergeBaseOffsetOpt::detectLuiAddiGlobal(MachineInstr &HiLUI,
                                                  MachineInstr *&LoADDI) {
  if (HiLUI.getOpcode() != RISCV::LUI ||
      HiLUI.getOperand(1).getTargetFlags() != RISCVII::MO_HI ||
      !HiLUI.getOperand(1).isGlobal() ||
      HiLUI.getOperand(1).getOffset() != 0 ||
      !MRI->hasOneUse(HiLUI.getOperand(0).getReg()))
    return false;
  Register HiLuiDestReg = HiLUI.getOperand(0).getReg();
  LoADDI = &*MRI->use_instr_begin(HiLuiDestReg);
  if (LoADDI->getOpcode() != RISCV::ADDI ||
      LoADDI->getOperand(2).getTargetFlags() != RISCVII::MO_LO ||
      !LoADDI->getOperand(2).isGlobal() ||
      LoADDI->getOperand(2).getOffset() != 0)
    return false;
  return true;
}
 
// Update the offset in HiLUI and LoADDI instructions.
// Delete the tail instruction and update all the uses to use the
// output from LoADDI.
void RISCVMergeBaseOffsetOpt::foldOffset(MachineInstr &HiLUI,
                                         MachineInstr &LoADDI,
                                         MachineInstr &Tail, int64_t Offset) {
  assert(isInt<32>(Offset) && "Unexpected offset");
  // Put the offset back in HiLUI and the LoADDI
  HiLUI.getOperand(1).setOffset(Offset);
  LoADDI.getOperand(2).setOffset(Offset);
  // Delete the tail instruction.
  DeadInstrs.insert(&Tail);
  MRI->replaceRegWith(Tail.getOperand(0).getReg(),
                      LoADDI.getOperand(0).getReg());
  LLVM_DEBUG(dbgs() << "  Merged offset " << Offset << " into base.\n"
                    << "     " << HiLUI << "     " << LoADDI;);
}
 
// Detect patterns for large offsets that are passed into an ADD instruction.
//
//                     Base address lowering is of the form:
//                        HiLUI:  lui   vreg1, %hi(s)
//                       LoADDI:  addi  vreg2, vreg1, %lo(s)
//                       /                                  \
//                      /                                    \
//                     /                                      \
//                    /  The large offset can be of two forms: \
//  1) Offset that has non zero bits in lower      2) Offset that has non zero
//     12 bits and upper 20 bits                      bits in upper 20 bits only
//   OffseLUI: lui   vreg3, 4
// OffsetTail: addi  voff, vreg3, 188                OffsetTail: lui  voff, 128
//                    \                                        /
//                     \                                      /
//                      \                                    /
//                       \                                  /
//                         TailAdd: add  vreg4, vreg2, voff
bool RISCVMergeBaseOffsetOpt::matchLargeOffset(MachineInstr &TailAdd,
                                               Register GAReg,
                                               int64_t &Offset) {
  assert((TailAdd.getOpcode() == RISCV::ADD) && "Expected ADD instruction!");
  Register Rs = TailAdd.getOperand(1).getReg();
  Register Rt = TailAdd.getOperand(2).getReg();
  Register Reg = Rs == GAReg ? Rt : Rs;
 
  // Can't fold if the register has more than one use.
  if (!MRI->hasOneUse(Reg))
    return false;
  // This can point to an ADDI or a LUI:
  MachineInstr &OffsetTail = *MRI->getVRegDef(Reg);
  if (OffsetTail.getOpcode() == RISCV::ADDI ||
      OffsetTail.getOpcode() == RISCV::ADDIW) {
    // The offset value has non zero bits in both %hi and %lo parts.
    // Detect an ADDI that feeds from a LUI instruction.
    MachineOperand &AddiImmOp = OffsetTail.getOperand(2);
    if (AddiImmOp.getTargetFlags() != RISCVII::MO_None)
      return false;
    int64_t OffLo = AddiImmOp.getImm();
    MachineInstr &OffsetLui =
        *MRI->getVRegDef(OffsetTail.getOperand(1).getReg());
    MachineOperand &LuiImmOp = OffsetLui.getOperand(1);
    if (OffsetLui.getOpcode() != RISCV::LUI ||
        LuiImmOp.getTargetFlags() != RISCVII::MO_None ||
        !MRI->hasOneUse(OffsetLui.getOperand(0).getReg()))
      return false;
    Offset = SignExtend64<32>(LuiImmOp.getImm() << 12);
    Offset += OffLo;
    // RV32 ignores the upper 32 bits. ADDIW sign extends the result.
    if (!ST->is64Bit() || OffsetTail.getOpcode() == RISCV::ADDIW)
       Offset = SignExtend64<32>(Offset);
    // We can only fold simm32 offsets.
    if (!isInt<32>(Offset))
      return false;
    LLVM_DEBUG(dbgs() << "  Offset Instrs: " << OffsetTail
                      << "                 " << OffsetLui);
    DeadInstrs.insert(&OffsetTail);
    DeadInstrs.insert(&OffsetLui);
    return true;
  } else if (OffsetTail.getOpcode() == RISCV::LUI) {
    // The offset value has all zero bits in the lower 12 bits. Only LUI
    // exists.
    LLVM_DEBUG(dbgs() << "  Offset Instr: " << OffsetTail);
    Offset = SignExtend64<32>(OffsetTail.getOperand(1).getImm() << 12);
    DeadInstrs.insert(&OffsetTail);
    return true;
  }
  return false;
}
 
// Detect patterns for offsets that are passed into a SHXADD instruction.
// The offset has 1,2, or 3 trailing zeros and fits in simm13, simm14, simm15.
// The constant is created with addi    voff, x0, C, and shXadd is used to
// fill insert the trailing zeros and do the addition.
//
// HiLUI:      lui     vreg1, %hi(s)
// LoADDI:     addi    vreg2, vreg1, %lo(s)
// OffsetTail: addi    voff, x0, C
// TailAdd:    shXadd  vreg4, voff, vreg2
bool RISCVMergeBaseOffsetOpt::matchShiftedOffset(MachineInstr &TailShXAdd,
                                                 Register GAReg,
                                                 int64_t &Offset) {
  assert((TailShXAdd.getOpcode() == RISCV::SH1ADD ||
          TailShXAdd.getOpcode() == RISCV::SH2ADD ||
          TailShXAdd.getOpcode() == RISCV::SH3ADD) &&
         "Expected SHXADD instruction!");
 
  // The first source is the shifted operand.
  Register Rs1 = TailShXAdd.getOperand(1).getReg();
 
  if (GAReg != TailShXAdd.getOperand(2).getReg())
    return false;
 
  // Can't fold if the register has more than one use.
  if (!MRI->hasOneUse(Rs1))
    return false;
  // This can point to an ADDI X0, C.
  MachineInstr &OffsetTail = *MRI->getVRegDef(Rs1);
  if (OffsetTail.getOpcode() != RISCV::ADDI)
    return false;
  if (!OffsetTail.getOperand(1).isReg() ||
      OffsetTail.getOperand(1).getReg() != RISCV::X0 ||
      !OffsetTail.getOperand(2).isImm())
    return false;
 
  Offset = OffsetTail.getOperand(2).getImm();
  assert(isInt<12>(Offset) && "Unexpected offset");
 
  unsigned ShAmt;
  switch (TailShXAdd.getOpcode()) {
  default: llvm_unreachable("Unexpected opcode");
  case RISCV::SH1ADD: ShAmt = 1; break;
  case RISCV::SH2ADD: ShAmt = 2; break;
  case RISCV::SH3ADD: ShAmt = 3; break;
  }
 
  Offset = (uint64_t)Offset << ShAmt;
 
  LLVM_DEBUG(dbgs() << "  Offset Instr: " << OffsetTail);
  DeadInstrs.insert(&OffsetTail);
  return true;
}
 
bool RISCVMergeBaseOffsetOpt::detectAndFoldOffset(MachineInstr &HiLUI,
                                                  MachineInstr &LoADDI) {
  Register DestReg = LoADDI.getOperand(0).getReg();
 
  // First, look for arithmetic instructions we can get an offset from.
  // We might be able to remove the arithmetic instructions by folding the
  // offset into the LUI+ADDI.
  if (MRI->hasOneUse(DestReg)) {
    // LoADDI has only one use.
    MachineInstr &Tail = *MRI->use_instr_begin(DestReg);
    switch (Tail.getOpcode()) {
    default:
      LLVM_DEBUG(dbgs() << "Don't know how to get offset from this instr:"
                        << Tail);
      break;
    case RISCV::ADDI: {
      // Offset is simply an immediate operand.
      int64_t Offset = Tail.getOperand(2).getImm();
 
      // We might have two ADDIs in a row.
      Register TailDestReg = Tail.getOperand(0).getReg();
      if (MRI->hasOneUse(TailDestReg)) {
        MachineInstr &TailTail = *MRI->use_instr_begin(TailDestReg);
        if (TailTail.getOpcode() == RISCV::ADDI) {
          Offset += TailTail.getOperand(2).getImm();
          LLVM_DEBUG(dbgs() << "  Offset Instrs: " << Tail << TailTail);
          DeadInstrs.insert(&Tail);
          foldOffset(HiLUI, LoADDI, TailTail, Offset);
          return true;
        }
      }
 
      LLVM_DEBUG(dbgs() << "  Offset Instr: " << Tail);
      foldOffset(HiLUI, LoADDI, Tail, Offset);
      return true;
    }
    case RISCV::ADD: {
      // The offset is too large to fit in the immediate field of ADDI.
      // This can be in two forms:
      // 1) LUI hi_Offset followed by:
      //    ADDI lo_offset
      //    This happens in case the offset has non zero bits in
      //    both hi 20 and lo 12 bits.
      // 2) LUI (offset20)
      //    This happens in case the lower 12 bits of the offset are zeros.
      int64_t Offset;
      if (!matchLargeOffset(Tail, DestReg, Offset))
        return false;
      foldOffset(HiLUI, LoADDI, Tail, Offset);
      return true;
    }
    case RISCV::SH1ADD:
    case RISCV::SH2ADD:
    case RISCV::SH3ADD: {
      // The offset is too large to fit in the immediate field of ADDI.
      // It may be encoded as (SH2ADD (ADDI X0, C), DestReg) or
      // (SH3ADD (ADDI X0, C), DestReg).
      int64_t Offset;
      if (!matchShiftedOffset(Tail, DestReg, Offset))
        return false;
      foldOffset(HiLUI, LoADDI, Tail, Offset);
      return true;
    }
    }
  }
 
  // We didn't find an arithmetic instruction. If all the uses are memory ops
  // with the same offset, we can transform
  // HiLUI:  lui vreg1, %hi(foo)          --->  lui vreg1, %hi(foo+8)
  // LoADDI: addi vreg2, vreg1, %lo(foo)  --->  lw vreg3, lo(foo+8)(vreg1)
  // Tail:   lw vreg3, 8(vreg2)
 
  Optional<int64_t> CommonOffset;
  for (const MachineInstr &UseMI : MRI->use_instructions(DestReg)) {
    switch (UseMI.getOpcode()) {
    default:
      LLVM_DEBUG(dbgs() << "Not a load or store instruction: " << UseMI);
      return false;
    case RISCV::LB:
    case RISCV::LH:
    case RISCV::LW:
    case RISCV::LBU:
    case RISCV::LHU:
    case RISCV::LWU:
    case RISCV::LD:
    case RISCV::FLH:
    case RISCV::FLW:
    case RISCV::FLD:
    case RISCV::SB:
    case RISCV::SH:
    case RISCV::SW:
    case RISCV::SD:
    case RISCV::FSH:
    case RISCV::FSW:
    case RISCV::FSD: {
      if (UseMI.getOperand(1).isFI())
        return false;
      // Register defined by LoADDI should not be the value register.
      if (DestReg == UseMI.getOperand(0).getReg())
        return false;
      assert(DestReg == UseMI.getOperand(1).getReg() &&
             "Expected base address use");
      // All load/store instructions must use the same offset.
      int64_t Offset = UseMI.getOperand(2).getImm();
      if (CommonOffset && Offset != CommonOffset)
        return false;
      CommonOffset = Offset;
    }
    }
  }
 
  // We found a common offset.
  // Update the offsets in global address lowering.
  HiLUI.getOperand(1).setOffset(*CommonOffset);
  MachineOperand &ImmOp = LoADDI.getOperand(2);
  ImmOp.setOffset(*CommonOffset);
 
  // Update the immediate in the load/store instructions to add the offset.
  for (MachineInstr &UseMI :
       llvm::make_early_inc_range(MRI->use_instructions(DestReg))) {
    UseMI.removeOperand(2);
    UseMI.addOperand(ImmOp);
    // Update the base reg in the Tail instruction to feed from LUI.
    // Output of HiLUI is only used in LoADDI, no need to use
    // MRI->replaceRegWith().
    UseMI.getOperand(1).setReg(HiLUI.getOperand(0).getReg());
  }
 
  DeadInstrs.insert(&LoADDI);
  return true;
}
 
bool RISCVMergeBaseOffsetOpt::runOnMachineFunction(MachineFunction &Fn) {
  if (skipFunction(Fn.getFunction()))
    return false;
 
  ST = &Fn.getSubtarget<RISCVSubtarget>();
 
  bool MadeChange = false;
  DeadInstrs.clear();
  MRI = &Fn.getRegInfo();
  for (MachineBasicBlock &MBB : Fn) {
    LLVM_DEBUG(dbgs() << "MBB: " << MBB.getName() << "\n");
    for (MachineInstr &HiLUI : MBB) {
      MachineInstr *LoADDI = nullptr;
      if (!detectLuiAddiGlobal(HiLUI, LoADDI))
        continue;
      LLVM_DEBUG(dbgs() << "  Found lowered global address: "
                        << *LoADDI->getOperand(2).getGlobal() << "\n");
      MadeChange |= detectAndFoldOffset(HiLUI, *LoADDI);
    }
  }
  // Delete dead instructions.
  for (auto *MI : DeadInstrs)
    MI->eraseFromParent();
  return MadeChange;
}
 
/// Returns an instance of the Merge Base Offset Optimization pass.
FunctionPass *llvm::createRISCVMergeBaseOffsetOptPass() {
  return new RISCVMergeBaseOffsetOpt();
}