llvm-project/llvm/lib/CodeGen/MachineCopyPropagation.cpp

//===- MachineCopyPropagation.cpp - Machine Copy Propagation Pass ---------===//
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This is an extremely simple MachineInstr-level copy propagation pass.
//
//===----------------------------------------------------------------------===//

#define DEBUG_TYPE "codegen-cp"
#include "llvm/CodeGen/Passes.h"
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/SetVector.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/CodeGen/MachineFunction.h"
#include "llvm/CodeGen/MachineFunctionPass.h"
#include "llvm/CodeGen/MachineRegisterInfo.h"
#include "llvm/Pass.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/ErrorHandling.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/Target/TargetInstrInfo.h"
#include "llvm/Target/TargetRegisterInfo.h"
using namespace llvm;

STATISTIC(NumDeletes, "Number of dead copies deleted");

namespace {
  class MachineCopyPropagation : public MachineFunctionPass {
    const TargetRegisterInfo *TRI;
    const TargetInstrInfo *TII;
    MachineRegisterInfo *MRI;

  public:
    static char ID; // Pass identification, replacement for typeid
    MachineCopyPropagation() : MachineFunctionPass(ID) {
     initializeMachineCopyPropagationPass(*PassRegistry::getPassRegistry());
    }

    bool runOnMachineFunction(MachineFunction &MF) override;

  private:
    typedef SmallVector<unsigned, 4> DestList;
    typedef DenseMap<unsigned, DestList> SourceMap;

    void SourceNoLongerAvailable(unsigned Reg,
                                 SourceMap &SrcMap,
                                 DenseMap<unsigned, MachineInstr*> &AvailCopyMap);
    bool CopyPropagateBlock(MachineBasicBlock &MBB);
    void removeCopy(MachineInstr *MI);
  };
}
char MachineCopyPropagation::ID = 0;
char &llvm::MachineCopyPropagationID = MachineCopyPropagation::ID;

INITIALIZE_PASS(MachineCopyPropagation, "machine-cp",
                "Machine Copy Propagation Pass", false, false)

void
MachineCopyPropagation::SourceNoLongerAvailable(unsigned Reg,
                              SourceMap &SrcMap,
                              DenseMap<unsigned, MachineInstr*> &AvailCopyMap) {
  for (MCRegAliasIterator AI(Reg, TRI, true); AI.isValid(); ++AI) {
    SourceMap::iterator SI = SrcMap.find(*AI);
    if (SI != SrcMap.end()) {
      const DestList& Defs = SI->second;
      for (DestList::const_iterator I = Defs.begin(), E = Defs.end();
           I != E; ++I) {
        unsigned MappedDef = *I;
        // Source of copy is no longer available for propagation.
        if (AvailCopyMap.erase(MappedDef)) {
          for (MCSubRegIterator SR(MappedDef, TRI); SR.isValid(); ++SR)
            AvailCopyMap.erase(*SR);
        }
      }
    }
  }
}

static bool NoInterveningSideEffect(const MachineInstr *CopyMI,
                                    const MachineInstr *MI) {
  const MachineBasicBlock *MBB = CopyMI->getParent();
  if (MI->getParent() != MBB)
    return false;
  MachineBasicBlock::const_iterator I = CopyMI;
  MachineBasicBlock::const_iterator E = MBB->end();
  MachineBasicBlock::const_iterator E2 = MI;

  ++I;
  while (I != E && I != E2) {
    if (I->hasUnmodeledSideEffects() || I->isCall() ||
        I->isTerminator())
      return false;
    ++I;
  }
  return true;
}

/// isNopCopy - Return true if the specified copy is really a nop. That is
/// if the source of the copy is the same of the definition of the copy that
/// supplied the source. If the source of the copy is a sub-register than it
/// must check the sub-indices match. e.g.
/// ecx = mov eax
/// al  = mov cl
/// But not
/// ecx = mov eax
/// al  = mov ch
static bool isNopCopy(MachineInstr *CopyMI, unsigned Def, unsigned Src,
                      const TargetRegisterInfo *TRI) {
  unsigned SrcSrc = CopyMI->getOperand(1).getReg();
  if (Def == SrcSrc)
    return true;
  if (TRI->isSubRegister(SrcSrc, Def)) {
    unsigned SrcDef = CopyMI->getOperand(0).getReg();
    unsigned SubIdx = TRI->getSubRegIndex(SrcSrc, Def);
    if (!SubIdx)
      return false;
    return SubIdx == TRI->getSubRegIndex(SrcDef, Src);
  }

  return false;
}

// Remove MI from the function because it has been determined it is dead.
// Turn it into a noop KILL instruction as opposed to removing it to
// maintain imp-use/imp-def chains.
void MachineCopyPropagation::removeCopy(MachineInstr *MI) {
  MI->setDesc(TII->get(TargetOpcode::KILL));
}

bool MachineCopyPropagation::CopyPropagateBlock(MachineBasicBlock &MBB) {
  SmallSetVector<MachineInstr*, 8> MaybeDeadCopies;  // Candidates for deletion
  DenseMap<unsigned, MachineInstr*> AvailCopyMap;    // Def -> available copies map
  DenseMap<unsigned, MachineInstr*> CopyMap;         // Def -> copies map
  SourceMap SrcMap; // Src -> Def map

  DEBUG(dbgs() << "MCP: CopyPropagateBlock " << MBB.getName() << "\n");

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

    if (MI->isCopy()) {
      unsigned Def = MI->getOperand(0).getReg();
      unsigned Src = MI->getOperand(1).getReg();

      if (TargetRegisterInfo::isVirtualRegister(Def) ||
          TargetRegisterInfo::isVirtualRegister(Src))
        report_fatal_error("MachineCopyPropagation should be run after"
                           " register allocation!");

      DenseMap<unsigned, MachineInstr*>::iterator CI = AvailCopyMap.find(Src);
      if (CI != AvailCopyMap.end()) {
        MachineInstr *CopyMI = CI->second;
        if (!MRI->isReserved(Def) &&
            (!MRI->isReserved(Src) || NoInterveningSideEffect(CopyMI, MI)) &&
            isNopCopy(CopyMI, Def, Src, TRI)) {
          // The two copies cancel out and the source of the first copy
          // hasn't been overridden, eliminate the second one. e.g.
          //  %ECX<def> = COPY %EAX<kill>
          //  ... nothing clobbered EAX.
          //  %EAX<def> = COPY %ECX
          // =>
          //  %ECX<def> = COPY %EAX
          //
          // Also avoid eliminating a copy from reserved registers unless the
          // definition is proven not clobbered. e.g.
          // %RSP<def> = COPY %RAX
          // CALL
          // %RAX<def> = COPY %RSP

          DEBUG(dbgs() << "MCP: copy is a NOP, removing: "; MI->dump());

          // Clear any kills of Def between CopyMI and MI. This extends the
          // live range.
          for (MachineBasicBlock::iterator I = CopyMI, E = MI; I != E; ++I)
            I->clearRegisterKills(Def, TRI);

          removeCopy(MI);
          Changed = true;
          ++NumDeletes;
          continue;
        }
      }

      // If Src is defined by a previous copy, it cannot be eliminated.
      for (MCRegAliasIterator AI(Src, TRI, true); AI.isValid(); ++AI) {
        CI = CopyMap.find(*AI);
        if (CI != CopyMap.end()) {
          DEBUG(dbgs() << "MCP: Copy is no longer dead: "; CI->second->dump());
          MaybeDeadCopies.remove(CI->second);
        }
      }

      DEBUG(dbgs() << "MCP: Copy is a deletion candidate: "; MI->dump());

      // Copy is now a candidate for deletion.
      MaybeDeadCopies.insert(MI);

      // If 'Src' is previously source of another copy, then this earlier copy's
      // source is no longer available. e.g.
      // %xmm9<def> = copy %xmm2
      // ...
      // %xmm2<def> = copy %xmm0
      // ...
      // %xmm2<def> = copy %xmm9
      SourceNoLongerAvailable(Def, SrcMap, AvailCopyMap);

      // Remember Def is defined by the copy.
      // ... Make sure to clear the def maps of aliases first.
      for (MCRegAliasIterator AI(Def, TRI, false); AI.isValid(); ++AI) {
        CopyMap.erase(*AI);
        AvailCopyMap.erase(*AI);
      }
      for (MCSubRegIterator SR(Def, TRI, /*IncludeSelf=*/true); SR.isValid();
           ++SR) {
        CopyMap[*SR] = MI;
        AvailCopyMap[*SR] = MI;
      }

      // Remember source that's copied to Def. Once it's clobbered, then
      // it's no longer available for copy propagation.
      if (std::find(SrcMap[Src].begin(), SrcMap[Src].end(), Def) ==
          SrcMap[Src].end()) {
        SrcMap[Src].push_back(Def);
      }

      continue;
    }

    // Not a copy.
    SmallVector<unsigned, 2> Defs;
    int RegMaskOpNum = -1;
    for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) {
      MachineOperand &MO = MI->getOperand(i);
      if (MO.isRegMask())
        RegMaskOpNum = i;
      if (!MO.isReg())
        continue;
      unsigned Reg = MO.getReg();
      if (!Reg)
        continue;

      if (TargetRegisterInfo::isVirtualRegister(Reg))
        report_fatal_error("MachineCopyPropagation should be run after"
                           " register allocation!");

      if (MO.isDef()) {
        Defs.push_back(Reg);
        continue;
      }

      // If 'Reg' is defined by a copy, the copy is no longer a candidate
      // for elimination.
      for (MCRegAliasIterator AI(Reg, TRI, true); AI.isValid(); ++AI) {
        DenseMap<unsigned, MachineInstr*>::iterator CI = CopyMap.find(*AI);
        if (CI != CopyMap.end()) {
          DEBUG(dbgs() << "MCP: Copy is used - not dead: "; CI->second->dump());
          MaybeDeadCopies.remove(CI->second);
        }
      }
    }

    // The instruction has a register mask operand which means that it clobbers
    // a large set of registers.  It is possible to use the register mask to
    // prune the available copies, but treat it like a basic block boundary for
    // now.
    if (RegMaskOpNum >= 0) {
      // Erase any MaybeDeadCopies whose destination register is clobbered.
      const MachineOperand &MaskMO = MI->getOperand(RegMaskOpNum);
      for (SmallSetVector<MachineInstr*, 8>::iterator
           DI = MaybeDeadCopies.begin(), DE = MaybeDeadCopies.end();
           DI != DE; ++DI) {
        unsigned Reg = (*DI)->getOperand(0).getReg();
        if (MRI->isReserved(Reg) || !MaskMO.clobbersPhysReg(Reg))
          continue;
        DEBUG(dbgs() << "MCP: Removing copy due to regmask clobbering: ";
              (*DI)->dump());
        removeCopy(*DI);
        Changed = true;
        ++NumDeletes;
      }

      // Clear all data structures as if we were beginning a new basic block.
      MaybeDeadCopies.clear();
      AvailCopyMap.clear();
      CopyMap.clear();
      SrcMap.clear();
      continue;
    }

    for (unsigned i = 0, e = Defs.size(); i != e; ++i) {
      unsigned Reg = Defs[i];

      // No longer defined by a copy.
      for (MCRegAliasIterator AI(Reg, TRI, true); AI.isValid(); ++AI) {
        CopyMap.erase(*AI);
        AvailCopyMap.erase(*AI);
      }

      // If 'Reg' is previously source of a copy, it is no longer available for
      // copy propagation.
      SourceNoLongerAvailable(Reg, SrcMap, AvailCopyMap);
    }
  }

  // If MBB doesn't have successors, delete the copies whose defs are not used.
  // If MBB does have successors, then conservative assume the defs are live-out
  // since we don't want to trust live-in lists.
  if (MBB.succ_empty()) {
    for (SmallSetVector<MachineInstr*, 8>::iterator
           DI = MaybeDeadCopies.begin(), DE = MaybeDeadCopies.end();
         DI != DE; ++DI) {
      if (!MRI->isReserved((*DI)->getOperand(0).getReg())) {
        removeCopy(*DI);
        Changed = true;
        ++NumDeletes;
      }
    }
  }

  return Changed;
}

bool MachineCopyPropagation::runOnMachineFunction(MachineFunction &MF) {
  bool Changed = false;

  TRI = MF.getTarget().getRegisterInfo();
  TII = MF.getTarget().getInstrInfo();
  MRI = &MF.getRegInfo();

  for (MachineFunction::iterator I = MF.begin(), E = MF.end(); I != E; ++I)
    Changed |= CopyPropagateBlock(*I);

  return Changed;
}