llvm/lib/CodeGen/GlobalISel/CombinerHelper.cpp

//===-- lib/CodeGen/GlobalISel/GICombinerHelper.cpp -----------------------===//
//
// 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
//
//===----------------------------------------------------------------------===//
#include "llvm/CodeGen/GlobalISel/CombinerHelper.h"
#include "llvm/CodeGen/GlobalISel/Combiner.h"
#include "llvm/CodeGen/GlobalISel/GISelChangeObserver.h"
#include "llvm/CodeGen/GlobalISel/MachineIRBuilder.h"
#include "llvm/CodeGen/GlobalISel/Utils.h"
#include "llvm/CodeGen/MachineInstr.h"
#include "llvm/CodeGen/MachineRegisterInfo.h"
#include "llvm/CodeGen/TargetInstrInfo.h"

#define DEBUG_TYPE "gi-combiner"

using namespace llvm;

CombinerHelper::CombinerHelper(GISelChangeObserver &Observer,
                               MachineIRBuilder &B)
    : Builder(B), MRI(Builder.getMF().getRegInfo()), Observer(Observer) {}

void CombinerHelper::replaceRegWith(MachineRegisterInfo &MRI, Register FromReg,
                                    Register ToReg) const {
  Observer.changingAllUsesOfReg(MRI, FromReg);

  if (MRI.constrainRegAttrs(ToReg, FromReg))
    MRI.replaceRegWith(FromReg, ToReg);
  else
    Builder.buildCopy(ToReg, FromReg);

  Observer.finishedChangingAllUsesOfReg();
}

void CombinerHelper::replaceRegOpWith(MachineRegisterInfo &MRI,
                                      MachineOperand &FromRegOp,
                                      Register ToReg) const {
  assert(FromRegOp.getParent() && "Expected an operand in an MI");
  Observer.changingInstr(*FromRegOp.getParent());

  FromRegOp.setReg(ToReg);

  Observer.changedInstr(*FromRegOp.getParent());
}

bool CombinerHelper::tryCombineCopy(MachineInstr &MI) {
  if (matchCombineCopy(MI)) {
    applyCombineCopy(MI);
    return true;
  }
  return false;
}
bool CombinerHelper::matchCombineCopy(MachineInstr &MI) {
  if (MI.getOpcode() != TargetOpcode::COPY)
    return false;
  unsigned DstReg = MI.getOperand(0).getReg();
  unsigned SrcReg = MI.getOperand(1).getReg();
  LLT DstTy = MRI.getType(DstReg);
  LLT SrcTy = MRI.getType(SrcReg);
  // Simple Copy Propagation.
  // a(sx) = COPY b(sx) -> Replace all uses of a with b.
  if (DstTy.isValid() && SrcTy.isValid() && DstTy == SrcTy)
    return true;
  return false;
}
void CombinerHelper::applyCombineCopy(MachineInstr &MI) {
  unsigned DstReg = MI.getOperand(0).getReg();
  unsigned SrcReg = MI.getOperand(1).getReg();
  MI.eraseFromParent();
  replaceRegWith(MRI, DstReg, SrcReg);
}

namespace {

/// Select a preference between two uses. CurrentUse is the current preference
/// while *ForCandidate is attributes of the candidate under consideration.
PreferredTuple ChoosePreferredUse(PreferredTuple &CurrentUse,
                                  const LLT &TyForCandidate,
                                  unsigned OpcodeForCandidate,
                                  MachineInstr *MIForCandidate) {
  if (!CurrentUse.Ty.isValid()) {
    if (CurrentUse.ExtendOpcode == OpcodeForCandidate ||
        CurrentUse.ExtendOpcode == TargetOpcode::G_ANYEXT)
      return {TyForCandidate, OpcodeForCandidate, MIForCandidate};
    return CurrentUse;
  }

  // We permit the extend to hoist through basic blocks but this is only
  // sensible if the target has extending loads. If you end up lowering back
  // into a load and extend during the legalizer then the end result is
  // hoisting the extend up to the load.

  // Prefer defined extensions to undefined extensions as these are more
  // likely to reduce the number of instructions.
  if (OpcodeForCandidate == TargetOpcode::G_ANYEXT &&
      CurrentUse.ExtendOpcode != TargetOpcode::G_ANYEXT)
    return CurrentUse;
  else if (CurrentUse.ExtendOpcode == TargetOpcode::G_ANYEXT &&
           OpcodeForCandidate != TargetOpcode::G_ANYEXT)
    return {TyForCandidate, OpcodeForCandidate, MIForCandidate};

  // Prefer sign extensions to zero extensions as sign-extensions tend to be
  // more expensive.
  if (CurrentUse.Ty == TyForCandidate) {
    if (CurrentUse.ExtendOpcode == TargetOpcode::G_SEXT &&
        OpcodeForCandidate == TargetOpcode::G_ZEXT)
      return CurrentUse;
    else if (CurrentUse.ExtendOpcode == TargetOpcode::G_ZEXT &&
             OpcodeForCandidate == TargetOpcode::G_SEXT)
      return {TyForCandidate, OpcodeForCandidate, MIForCandidate};
  }

  // This is potentially target specific. We've chosen the largest type
  // because G_TRUNC is usually free. One potential catch with this is that
  // some targets have a reduced number of larger registers than smaller
  // registers and this choice potentially increases the live-range for the
  // larger value.
  if (TyForCandidate.getSizeInBits() > CurrentUse.Ty.getSizeInBits()) {
    return {TyForCandidate, OpcodeForCandidate, MIForCandidate};
  }
  return CurrentUse;
}

/// Find a suitable place to insert some instructions and insert them. This
/// function accounts for special cases like inserting before a PHI node.
/// The current strategy for inserting before PHI's is to duplicate the
/// instructions for each predecessor. However, while that's ok for G_TRUNC
/// on most targets since it generally requires no code, other targets/cases may
/// want to try harder to find a dominating block.
static void InsertInsnsWithoutSideEffectsBeforeUse(
    MachineIRBuilder &Builder, MachineInstr &DefMI, MachineOperand &UseMO,
    std::function<void(MachineBasicBlock *, MachineBasicBlock::iterator,
                       MachineOperand &UseMO)>
        Inserter) {
  MachineInstr &UseMI = *UseMO.getParent();

  MachineBasicBlock *InsertBB = UseMI.getParent();

  // If the use is a PHI then we want the predecessor block instead.
  if (UseMI.isPHI()) {
    MachineOperand *PredBB = std::next(&UseMO);
    InsertBB = PredBB->getMBB();
  }

  // If the block is the same block as the def then we want to insert just after
  // the def instead of at the start of the block.
  if (InsertBB == DefMI.getParent()) {
    MachineBasicBlock::iterator InsertPt = &DefMI;
    Inserter(InsertBB, std::next(InsertPt), UseMO);
    return;
  }

  // Otherwise we want the start of the BB
  Inserter(InsertBB, InsertBB->getFirstNonPHI(), UseMO);
}
} // end anonymous namespace

bool CombinerHelper::tryCombineExtendingLoads(MachineInstr &MI) {
  PreferredTuple Preferred;
  if (matchCombineExtendingLoads(MI, Preferred)) {
    applyCombineExtendingLoads(MI, Preferred);
    return true;
  }
  return false;
}

bool CombinerHelper::matchCombineExtendingLoads(MachineInstr &MI,
                                                PreferredTuple &Preferred) {
  // We match the loads and follow the uses to the extend instead of matching
  // the extends and following the def to the load. This is because the load
  // must remain in the same position for correctness (unless we also add code
  // to find a safe place to sink it) whereas the extend is freely movable.
  // It also prevents us from duplicating the load for the volatile case or just
  // for performance.

  if (MI.getOpcode() != TargetOpcode::G_LOAD &&
      MI.getOpcode() != TargetOpcode::G_SEXTLOAD &&
      MI.getOpcode() != TargetOpcode::G_ZEXTLOAD)
    return false;

  auto &LoadValue = MI.getOperand(0);
  assert(LoadValue.isReg() && "Result wasn't a register?");

  LLT LoadValueTy = MRI.getType(LoadValue.getReg());
  if (!LoadValueTy.isScalar())
    return false;

  // Most architectures are going to legalize <s8 loads into at least a 1 byte
  // load, and the MMOs can only describe memory accesses in multiples of bytes.
  // If we try to perform extload combining on those, we can end up with
  // %a(s8) = extload %ptr (load 1 byte from %ptr)
  // ... which is an illegal extload instruction.
  if (LoadValueTy.getSizeInBits() < 8)
    return false;

  // For non power-of-2 types, they will very likely be legalized into multiple
  // loads. Don't bother trying to match them into extending loads.
  if (!isPowerOf2_32(LoadValueTy.getSizeInBits()))
    return false;

  // Find the preferred type aside from the any-extends (unless it's the only
  // one) and non-extending ops. We'll emit an extending load to that type and
  // and emit a variant of (extend (trunc X)) for the others according to the
  // relative type sizes. At the same time, pick an extend to use based on the
  // extend involved in the chosen type.
  unsigned PreferredOpcode = MI.getOpcode() == TargetOpcode::G_LOAD
                                 ? TargetOpcode::G_ANYEXT
                                 : MI.getOpcode() == TargetOpcode::G_SEXTLOAD
                                       ? TargetOpcode::G_SEXT
                                       : TargetOpcode::G_ZEXT;
  Preferred = {LLT(), PreferredOpcode, nullptr};
  for (auto &UseMI : MRI.use_instructions(LoadValue.getReg())) {
    if (UseMI.getOpcode() == TargetOpcode::G_SEXT ||
        UseMI.getOpcode() == TargetOpcode::G_ZEXT ||
        UseMI.getOpcode() == TargetOpcode::G_ANYEXT) {
      Preferred = ChoosePreferredUse(Preferred,
                                     MRI.getType(UseMI.getOperand(0).getReg()),
                                     UseMI.getOpcode(), &UseMI);
    }
  }

  // There were no extends
  if (!Preferred.MI)
    return false;
  // It should be impossible to chose an extend without selecting a different
  // type since by definition the result of an extend is larger.
  assert(Preferred.Ty != LoadValueTy && "Extending to same type?");

  LLVM_DEBUG(dbgs() << "Preferred use is: " << *Preferred.MI);
  return true;
}

void CombinerHelper::applyCombineExtendingLoads(MachineInstr &MI,
                                                PreferredTuple &Preferred) {
  // Rewrite the load to the chosen extending load.
  Register ChosenDstReg = Preferred.MI->getOperand(0).getReg();

  // Inserter to insert a truncate back to the original type at a given point
  // with some basic CSE to limit truncate duplication to one per BB.
  DenseMap<MachineBasicBlock *, MachineInstr *> EmittedInsns;
  auto InsertTruncAt = [&](MachineBasicBlock *InsertIntoBB,
                           MachineBasicBlock::iterator InsertBefore,
                           MachineOperand &UseMO) {
    MachineInstr *PreviouslyEmitted = EmittedInsns.lookup(InsertIntoBB);
    if (PreviouslyEmitted) {
      Observer.changingInstr(*UseMO.getParent());
      UseMO.setReg(PreviouslyEmitted->getOperand(0).getReg());
      Observer.changedInstr(*UseMO.getParent());
      return;
    }

    Builder.setInsertPt(*InsertIntoBB, InsertBefore);
    Register NewDstReg = MRI.cloneVirtualRegister(MI.getOperand(0).getReg());
    MachineInstr *NewMI = Builder.buildTrunc(NewDstReg, ChosenDstReg);
    EmittedInsns[InsertIntoBB] = NewMI;
    replaceRegOpWith(MRI, UseMO, NewDstReg);
  };

  Observer.changingInstr(MI);
  MI.setDesc(
      Builder.getTII().get(Preferred.ExtendOpcode == TargetOpcode::G_SEXT
                               ? TargetOpcode::G_SEXTLOAD
                               : Preferred.ExtendOpcode == TargetOpcode::G_ZEXT
                                     ? TargetOpcode::G_ZEXTLOAD
                                     : TargetOpcode::G_LOAD));

  // Rewrite all the uses to fix up the types.
  auto &LoadValue = MI.getOperand(0);
  SmallVector<MachineOperand *, 4> Uses;
  for (auto &UseMO : MRI.use_operands(LoadValue.getReg()))
    Uses.push_back(&UseMO);

  for (auto *UseMO : Uses) {
    MachineInstr *UseMI = UseMO->getParent();

    // If the extend is compatible with the preferred extend then we should fix
    // up the type and extend so that it uses the preferred use.
    if (UseMI->getOpcode() == Preferred.ExtendOpcode ||
        UseMI->getOpcode() == TargetOpcode::G_ANYEXT) {
      unsigned UseDstReg = UseMI->getOperand(0).getReg();
      MachineOperand &UseSrcMO = UseMI->getOperand(1);
      const LLT &UseDstTy = MRI.getType(UseDstReg);
      if (UseDstReg != ChosenDstReg) {
        if (Preferred.Ty == UseDstTy) {
          // If the use has the same type as the preferred use, then merge
          // the vregs and erase the extend. For example:
          //    %1:_(s8) = G_LOAD ...
          //    %2:_(s32) = G_SEXT %1(s8)
          //    %3:_(s32) = G_ANYEXT %1(s8)
          //    ... = ... %3(s32)
          // rewrites to:
          //    %2:_(s32) = G_SEXTLOAD ...
          //    ... = ... %2(s32)
          replaceRegWith(MRI, UseDstReg, ChosenDstReg);
          Observer.erasingInstr(*UseMO->getParent());
          UseMO->getParent()->eraseFromParent();
        } else if (Preferred.Ty.getSizeInBits() < UseDstTy.getSizeInBits()) {
          // If the preferred size is smaller, then keep the extend but extend
          // from the result of the extending load. For example:
          //    %1:_(s8) = G_LOAD ...
          //    %2:_(s32) = G_SEXT %1(s8)
          //    %3:_(s64) = G_ANYEXT %1(s8)
          //    ... = ... %3(s64)
          /// rewrites to:
          //    %2:_(s32) = G_SEXTLOAD ...
          //    %3:_(s64) = G_ANYEXT %2:_(s32)
          //    ... = ... %3(s64)
          replaceRegOpWith(MRI, UseSrcMO, ChosenDstReg);
        } else {
          // If the preferred size is large, then insert a truncate. For
          // example:
          //    %1:_(s8) = G_LOAD ...
          //    %2:_(s64) = G_SEXT %1(s8)
          //    %3:_(s32) = G_ZEXT %1(s8)
          //    ... = ... %3(s32)
          /// rewrites to:
          //    %2:_(s64) = G_SEXTLOAD ...
          //    %4:_(s8) = G_TRUNC %2:_(s32)
          //    %3:_(s64) = G_ZEXT %2:_(s8)
          //    ... = ... %3(s64)
          InsertInsnsWithoutSideEffectsBeforeUse(Builder, MI, *UseMO,
                                                 InsertTruncAt);
        }
        continue;
      }
      // The use is (one of) the uses of the preferred use we chose earlier.
      // We're going to update the load to def this value later so just erase
      // the old extend.
      Observer.erasingInstr(*UseMO->getParent());
      UseMO->getParent()->eraseFromParent();
      continue;
    }

    // The use isn't an extend. Truncate back to the type we originally loaded.
    // This is free on many targets.
    InsertInsnsWithoutSideEffectsBeforeUse(Builder, MI, *UseMO, InsertTruncAt);
  }

  MI.getOperand(0).setReg(ChosenDstReg);
  Observer.changedInstr(MI);
}

bool CombinerHelper::tryCombine(MachineInstr &MI) {
  if (tryCombineCopy(MI))
    return true;
  return tryCombineExtendingLoads(MI);
}