llvm-project/llvm/lib/Target/AMDGPU/SILoadStoreOptimizer.cpp

//===-- SILoadStoreOptimizer.cpp ------------------------------------------===//
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This pass tries to fuse DS instructions with close by immediate offsets.
// This will fuse operations such as
//  ds_read_b32 v0, v2 offset:16
//  ds_read_b32 v1, v2 offset:32
// ==>
//   ds_read2_b32 v[0:1], v2, offset0:4 offset1:8
//
//
// Future improvements:
//
// - This currently relies on the scheduler to place loads and stores next to
//   each other, and then only merges adjacent pairs of instructions. It would
//   be good to be more flexible with interleaved instructions, and possibly run
//   before scheduling. It currently missing stores of constants because loading
//   the constant into the data register is placed between the stores, although
//   this is arguably a scheduling problem.
//
// - Live interval recomputing seems inefficient. This currently only matches
//   one pair, and recomputes live intervals and moves on to the next pair. It
//   would be better to compute a list of all merges that need to occur.
//
// - With a list of instructions to process, we can also merge more. If a
//   cluster of loads have offsets that are too large to fit in the 8-bit
//   offsets, but are close enough to fit in the 8 bits, we can add to the base
//   pointer and use the new reduced offsets.
//
//===----------------------------------------------------------------------===//

#include "AMDGPU.h"
#include "AMDGPUSubtarget.h"
#include "SIInstrInfo.h"
#include "SIRegisterInfo.h"
#include "Utils/AMDGPUBaseInfo.h"
#include "llvm/ADT/ArrayRef.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/StringRef.h"
#include "llvm/Analysis/AliasAnalysis.h"
#include "llvm/CodeGen/MachineBasicBlock.h"
#include "llvm/CodeGen/MachineFunction.h"
#include "llvm/CodeGen/MachineFunctionPass.h"
#include "llvm/CodeGen/MachineInstr.h"
#include "llvm/CodeGen/MachineInstrBuilder.h"
#include "llvm/CodeGen/MachineOperand.h"
#include "llvm/CodeGen/MachineRegisterInfo.h"
#include "llvm/IR/DebugLoc.h"
#include "llvm/Pass.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/MathExtras.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/Target/TargetMachine.h"
#include <cassert>
#include <iterator>
#include <utility>

using namespace llvm;

#define DEBUG_TYPE "si-load-store-opt"

namespace {

class SILoadStoreOptimizer : public MachineFunctionPass {
private:
  const SIInstrInfo *TII = nullptr;
  const SIRegisterInfo *TRI = nullptr;
  MachineRegisterInfo *MRI = nullptr;
  AliasAnalysis *AA = nullptr;

  static bool offsetsCanBeCombined(unsigned Offset0,
                                   unsigned Offset1,
                                   unsigned EltSize);

  MachineBasicBlock::iterator findMatchingDSInst(
    MachineBasicBlock::iterator I,
    unsigned EltSize,
    SmallVectorImpl<MachineInstr*> &InstsToMove);

  MachineBasicBlock::iterator mergeRead2Pair(
    MachineBasicBlock::iterator I,
    MachineBasicBlock::iterator Paired,
    unsigned EltSize,
    ArrayRef<MachineInstr*> InstsToMove);

  MachineBasicBlock::iterator mergeWrite2Pair(
    MachineBasicBlock::iterator I,
    MachineBasicBlock::iterator Paired,
    unsigned EltSize,
    ArrayRef<MachineInstr*> InstsToMove);

public:
  static char ID;

  SILoadStoreOptimizer() : MachineFunctionPass(ID) {}

  SILoadStoreOptimizer(const TargetMachine &TM_) : MachineFunctionPass(ID) {
    initializeSILoadStoreOptimizerPass(*PassRegistry::getPassRegistry());
  }

  bool optimizeBlock(MachineBasicBlock &MBB);

  bool runOnMachineFunction(MachineFunction &MF) override;

  StringRef getPassName() const override { return "SI Load / Store Optimizer"; }

  void getAnalysisUsage(AnalysisUsage &AU) const override {
    AU.setPreservesCFG();
    AU.addRequired<AAResultsWrapperPass>();

    MachineFunctionPass::getAnalysisUsage(AU);
  }
};

} // end anonymous namespace.

INITIALIZE_PASS_BEGIN(SILoadStoreOptimizer, DEBUG_TYPE,
                      "SI Load / Store Optimizer", false, false)
INITIALIZE_PASS_DEPENDENCY(AAResultsWrapperPass)
INITIALIZE_PASS_END(SILoadStoreOptimizer, DEBUG_TYPE,
                    "SI Load / Store Optimizer", false, false)

char SILoadStoreOptimizer::ID = 0;

char &llvm::SILoadStoreOptimizerID = SILoadStoreOptimizer::ID;

FunctionPass *llvm::createSILoadStoreOptimizerPass(TargetMachine &TM) {
  return new SILoadStoreOptimizer(TM);
}

static void moveInstsAfter(MachineBasicBlock::iterator I,
                           ArrayRef<MachineInstr*> InstsToMove) {
  MachineBasicBlock *MBB = I->getParent();
  ++I;
  for (MachineInstr *MI : InstsToMove) {
    MI->removeFromParent();
    MBB->insert(I, MI);
  }
}

static void addDefsToList(const MachineInstr &MI,
                          SmallVectorImpl<const MachineOperand *> &Defs) {
  for (const MachineOperand &Def : MI.defs()) {
    Defs.push_back(&Def);
  }
}

static bool memAccessesCanBeReordered(MachineBasicBlock::iterator A,
                                      MachineBasicBlock::iterator B,
                                      const SIInstrInfo *TII,
                                      AliasAnalysis * AA) {
  return (TII->areMemAccessesTriviallyDisjoint(*A, *B, AA) ||
    // RAW or WAR - cannot reorder
    // WAW - cannot reorder
    // RAR - safe to reorder
    !(A->mayStore() || B->mayStore()));
}

// Add MI and its defs to the lists if MI reads one of the defs that are
// already in the list. Returns true in that case.
static bool
addToListsIfDependent(MachineInstr &MI,
                      SmallVectorImpl<const MachineOperand *> &Defs,
                      SmallVectorImpl<MachineInstr*> &Insts) {
  for (const MachineOperand *Def : Defs) {
    bool ReadDef = MI.readsVirtualRegister(Def->getReg());
    // If ReadDef is true, then there is a use of Def between I
    // and the instruction that I will potentially be merged with. We
    // will need to move this instruction after the merged instructions.
    if (ReadDef) {
      Insts.push_back(&MI);
      addDefsToList(MI, Defs);
      return true;
    }
  }

  return false;
}

static bool
canMoveInstsAcrossMemOp(MachineInstr &MemOp,
                        ArrayRef<MachineInstr*> InstsToMove,
                        const SIInstrInfo *TII,
                        AliasAnalysis *AA) {
  assert(MemOp.mayLoadOrStore());

  for (MachineInstr *InstToMove : InstsToMove) {
    if (!InstToMove->mayLoadOrStore())
      continue;
    if (!memAccessesCanBeReordered(MemOp, *InstToMove, TII, AA))
        return false;
  }
  return true;
}

bool SILoadStoreOptimizer::offsetsCanBeCombined(unsigned Offset0,
                                                unsigned Offset1,
                                                unsigned Size) {
  // XXX - Would the same offset be OK? Is there any reason this would happen or
  // be useful?
  if (Offset0 == Offset1)
    return false;

  // This won't be valid if the offset isn't aligned.
  if ((Offset0 % Size != 0) || (Offset1 % Size != 0))
    return false;

  unsigned EltOffset0 = Offset0 / Size;
  unsigned EltOffset1 = Offset1 / Size;

  // Check if the new offsets fit in the reduced 8-bit range.
  if (isUInt<8>(EltOffset0) && isUInt<8>(EltOffset1))
    return true;

  // If the offset in elements doesn't fit in 8-bits, we might be able to use
  // the stride 64 versions.
  if ((EltOffset0 % 64 != 0) || (EltOffset1 % 64) != 0)
    return false;

  return isUInt<8>(EltOffset0 / 64) && isUInt<8>(EltOffset1 / 64);
}

MachineBasicBlock::iterator
SILoadStoreOptimizer::findMatchingDSInst(MachineBasicBlock::iterator I,
                                  unsigned EltSize,
                                  SmallVectorImpl<MachineInstr*> &InstsToMove) {
  MachineBasicBlock::iterator E = I->getParent()->end();
  MachineBasicBlock::iterator MBBI = I;
  ++MBBI;

  SmallVector<const MachineOperand *, 8> DefsToMove;
  addDefsToList(*I, DefsToMove);

  for ( ; MBBI != E; ++MBBI) {
    if (MBBI->getOpcode() != I->getOpcode()) {

      // This is not a matching DS instruction, but we can keep looking as
      // long as one of these conditions are met:
      // 1. It is safe to move I down past MBBI.
      // 2. It is safe to move MBBI down past the instruction that I will
      //    be merged into.

      if (MBBI->hasUnmodeledSideEffects())
        // We can't re-order this instruction with respect to other memory
        // opeations, so we fail both conditions mentioned above.
        return E;

      if (MBBI->mayLoadOrStore() &&
        !memAccessesCanBeReordered(*I, *MBBI, TII, AA)) {
        // We fail condition #1, but we may still be able to satisfy condition
        // #2.  Add this instruction to the move list and then we will check
        // if condition #2 holds once we have selected the matching instruction.
        InstsToMove.push_back(&*MBBI);
        addDefsToList(*MBBI, DefsToMove);
        continue;
      }

      // When we match I with another DS instruction we will be moving I down
      // to the location of the matched instruction any uses of I will need to
      // be moved down as well.
      addToListsIfDependent(*MBBI, DefsToMove, InstsToMove);
      continue;
    }

    // Don't merge volatiles.
    if (MBBI->hasOrderedMemoryRef())
      return E;

    // Handle a case like
    //   DS_WRITE_B32 addr, v, idx0
    //   w = DS_READ_B32 addr, idx0
    //   DS_WRITE_B32 addr, f(w), idx1
    // where the DS_READ_B32 ends up in InstsToMove and therefore prevents
    // merging of the two writes.
    if (addToListsIfDependent(*MBBI, DefsToMove, InstsToMove))
      continue;

    int AddrIdx = AMDGPU::getNamedOperandIdx(I->getOpcode(), AMDGPU::OpName::addr);
    const MachineOperand &AddrReg0 = I->getOperand(AddrIdx);
    const MachineOperand &AddrReg1 = MBBI->getOperand(AddrIdx);

    // Check same base pointer. Be careful of subregisters, which can occur with
    // vectors of pointers.
    if (AddrReg0.getReg() == AddrReg1.getReg() &&
        AddrReg0.getSubReg() == AddrReg1.getSubReg()) {
      int OffsetIdx = AMDGPU::getNamedOperandIdx(I->getOpcode(),
                                                 AMDGPU::OpName::offset);
      unsigned Offset0 = I->getOperand(OffsetIdx).getImm() & 0xffff;
      unsigned Offset1 = MBBI->getOperand(OffsetIdx).getImm() & 0xffff;

      // Check both offsets fit in the reduced range.
      // We also need to go through the list of instructions that we plan to
      // move and make sure they are all safe to move down past the merged
      // instruction.
      if (offsetsCanBeCombined(Offset0, Offset1, EltSize) &&
          canMoveInstsAcrossMemOp(*MBBI, InstsToMove, TII, AA))
        return MBBI;
    }

    // We've found a load/store that we couldn't merge for some reason.
    // We could potentially keep looking, but we'd need to make sure that
    // it was safe to move I and also all the instruction in InstsToMove
    // down past this instruction.
    if (!memAccessesCanBeReordered(*I, *MBBI, TII, AA) ||   // check if we can move I across MBBI
      !canMoveInstsAcrossMemOp(*MBBI, InstsToMove, TII, AA) // check if we can move all I's users
     )
      break;
  }
  return E;
}

MachineBasicBlock::iterator  SILoadStoreOptimizer::mergeRead2Pair(
  MachineBasicBlock::iterator I,
  MachineBasicBlock::iterator Paired,
  unsigned EltSize,
  ArrayRef<MachineInstr*> InstsToMove) {
  MachineBasicBlock *MBB = I->getParent();

  // Be careful, since the addresses could be subregisters themselves in weird
  // cases, like vectors of pointers.
  const MachineOperand *AddrReg = TII->getNamedOperand(*I, AMDGPU::OpName::addr);

  const MachineOperand *Dest0 = TII->getNamedOperand(*I, AMDGPU::OpName::vdst);
  const MachineOperand *Dest1 = TII->getNamedOperand(*Paired, AMDGPU::OpName::vdst);

  unsigned Offset0
    = TII->getNamedOperand(*I, AMDGPU::OpName::offset)->getImm() & 0xffff;
  unsigned Offset1
    = TII->getNamedOperand(*Paired, AMDGPU::OpName::offset)->getImm() & 0xffff;

  unsigned NewOffset0 = Offset0 / EltSize;
  unsigned NewOffset1 = Offset1 / EltSize;
  unsigned Opc = (EltSize == 4) ? AMDGPU::DS_READ2_B32 : AMDGPU::DS_READ2_B64;

  // Prefer the st64 form if we can use it, even if we can fit the offset in the
  // non st64 version. I'm not sure if there's any real reason to do this.
  bool UseST64 = (NewOffset0 % 64 == 0) && (NewOffset1 % 64 == 0);
  if (UseST64) {
    NewOffset0 /= 64;
    NewOffset1 /= 64;
    Opc = (EltSize == 4) ? AMDGPU::DS_READ2ST64_B32 : AMDGPU::DS_READ2ST64_B64;
  }

  unsigned SubRegIdx0 = (EltSize == 4) ? AMDGPU::sub0 : AMDGPU::sub0_sub1;
  unsigned SubRegIdx1 = (EltSize == 4) ? AMDGPU::sub1 : AMDGPU::sub2_sub3;

  if (NewOffset0 > NewOffset1) {
    // Canonicalize the merged instruction so the smaller offset comes first.
    std::swap(NewOffset0, NewOffset1);
    std::swap(SubRegIdx0, SubRegIdx1);
  }

  assert((isUInt<8>(NewOffset0) && isUInt<8>(NewOffset1)) &&
         (NewOffset0 != NewOffset1) &&
         "Computed offset doesn't fit");

  const MCInstrDesc &Read2Desc = TII->get(Opc);

  const TargetRegisterClass *SuperRC
    = (EltSize == 4) ? &AMDGPU::VReg_64RegClass : &AMDGPU::VReg_128RegClass;
  unsigned DestReg = MRI->createVirtualRegister(SuperRC);

  DebugLoc DL = I->getDebugLoc();
  MachineInstrBuilder Read2 = BuildMI(*MBB, Paired, DL, Read2Desc, DestReg)
                                  .add(*AddrReg)      // addr
                                  .addImm(NewOffset0) // offset0
                                  .addImm(NewOffset1) // offset1
                                  .addImm(0)          // gds
                                  .addMemOperand(*I->memoperands_begin())
                                  .addMemOperand(*Paired->memoperands_begin());
  (void)Read2;

  const MCInstrDesc &CopyDesc = TII->get(TargetOpcode::COPY);

  // Copy to the old destination registers.
  BuildMI(*MBB, Paired, DL, CopyDesc)
      .add(*Dest0) // Copy to same destination including flags and sub reg.
      .addReg(DestReg, 0, SubRegIdx0);
  MachineInstr *Copy1 = BuildMI(*MBB, Paired, DL, CopyDesc)
                            .add(*Dest1)
                            .addReg(DestReg, RegState::Kill, SubRegIdx1);

  moveInstsAfter(Copy1, InstsToMove);

  MachineBasicBlock::iterator Next = std::next(I);
  I->eraseFromParent();
  Paired->eraseFromParent();

  DEBUG(dbgs() << "Inserted read2: " << *Read2 << '\n');
  return Next;
}

MachineBasicBlock::iterator SILoadStoreOptimizer::mergeWrite2Pair(
  MachineBasicBlock::iterator I,
  MachineBasicBlock::iterator Paired,
  unsigned EltSize,
  ArrayRef<MachineInstr*> InstsToMove) {
  MachineBasicBlock *MBB = I->getParent();

  // Be sure to use .addOperand(), and not .addReg() with these. We want to be
  // sure we preserve the subregister index and any register flags set on them.
  const MachineOperand *Addr = TII->getNamedOperand(*I, AMDGPU::OpName::addr);
  const MachineOperand *Data0 = TII->getNamedOperand(*I, AMDGPU::OpName::data0);
  const MachineOperand *Data1
    = TII->getNamedOperand(*Paired, AMDGPU::OpName::data0);


  unsigned Offset0
    = TII->getNamedOperand(*I, AMDGPU::OpName::offset)->getImm() & 0xffff;
  unsigned Offset1
    = TII->getNamedOperand(*Paired, AMDGPU::OpName::offset)->getImm() & 0xffff;

  unsigned NewOffset0 = Offset0 / EltSize;
  unsigned NewOffset1 = Offset1 / EltSize;
  unsigned Opc = (EltSize == 4) ? AMDGPU::DS_WRITE2_B32 : AMDGPU::DS_WRITE2_B64;

  // Prefer the st64 form if we can use it, even if we can fit the offset in the
  // non st64 version. I'm not sure if there's any real reason to do this.
  bool UseST64 = (NewOffset0 % 64 == 0) && (NewOffset1 % 64 == 0);
  if (UseST64) {
    NewOffset0 /= 64;
    NewOffset1 /= 64;
    Opc = (EltSize == 4) ? AMDGPU::DS_WRITE2ST64_B32 : AMDGPU::DS_WRITE2ST64_B64;
  }

  if (NewOffset0 > NewOffset1) {
    // Canonicalize the merged instruction so the smaller offset comes first.
    std::swap(NewOffset0, NewOffset1);
    std::swap(Data0, Data1);
  }

  assert((isUInt<8>(NewOffset0) && isUInt<8>(NewOffset1)) &&
         (NewOffset0 != NewOffset1) &&
         "Computed offset doesn't fit");

  const MCInstrDesc &Write2Desc = TII->get(Opc);
  DebugLoc DL = I->getDebugLoc();

  MachineInstrBuilder Write2 = BuildMI(*MBB, Paired, DL, Write2Desc)
                                   .add(*Addr)         // addr
                                   .add(*Data0)        // data0
                                   .add(*Data1)        // data1
                                   .addImm(NewOffset0) // offset0
                                   .addImm(NewOffset1) // offset1
                                   .addImm(0)          // gds
                                   .addMemOperand(*I->memoperands_begin())
                                   .addMemOperand(*Paired->memoperands_begin());

  moveInstsAfter(Write2, InstsToMove);

  MachineBasicBlock::iterator Next = std::next(I);
  I->eraseFromParent();
  Paired->eraseFromParent();

  DEBUG(dbgs() << "Inserted write2 inst: " << *Write2 << '\n');
  return Next;
}

// Scan through looking for adjacent LDS operations with constant offsets from
// the same base register. We rely on the scheduler to do the hard work of
// clustering nearby loads, and assume these are all adjacent.
bool SILoadStoreOptimizer::optimizeBlock(MachineBasicBlock &MBB) {
  bool Modified = false;

  for (MachineBasicBlock::iterator I = MBB.begin(), E = MBB.end(); I != E;) {
    MachineInstr &MI = *I;

    // Don't combine if volatile.
    if (MI.hasOrderedMemoryRef()) {
      ++I;
      continue;
    }

    SmallVector<MachineInstr*, 8> InstsToMove;
    unsigned Opc = MI.getOpcode();
    if (Opc == AMDGPU::DS_READ_B32 || Opc == AMDGPU::DS_READ_B64) {
      unsigned Size = (Opc == AMDGPU::DS_READ_B64) ? 8 : 4;
      MachineBasicBlock::iterator Match = findMatchingDSInst(I, Size,
                                                             InstsToMove);
      if (Match != E) {
        Modified = true;
        I = mergeRead2Pair(I, Match, Size, InstsToMove);
      } else {
        ++I;
      }

      continue;
    } else if (Opc == AMDGPU::DS_WRITE_B32 || Opc == AMDGPU::DS_WRITE_B64) {
      unsigned Size = (Opc == AMDGPU::DS_WRITE_B64) ? 8 : 4;
      MachineBasicBlock::iterator Match = findMatchingDSInst(I, Size,
                                                             InstsToMove);
      if (Match != E) {
        Modified = true;
        I = mergeWrite2Pair(I, Match, Size, InstsToMove);
      } else {
        ++I;
      }

      continue;
    }

    ++I;
  }

  return Modified;
}

bool SILoadStoreOptimizer::runOnMachineFunction(MachineFunction &MF) {
  if (skipFunction(*MF.getFunction()))
    return false;

  const SISubtarget &STM = MF.getSubtarget<SISubtarget>();
  if (!STM.loadStoreOptEnabled())
    return false;

  TII = STM.getInstrInfo();
  TRI = &TII->getRegisterInfo();

  MRI = &MF.getRegInfo();
  AA = &getAnalysis<AAResultsWrapperPass>().getAAResults();

  DEBUG(dbgs() << "Running SILoadStoreOptimizer\n");

  bool Modified = false;

  for (MachineBasicBlock &MBB : MF)
    Modified |= optimizeBlock(MBB);

  return Modified;
}