llvm/lib/Target/AMDGPU/SIPreEmitPeephole.cpp

//===-- SIPreEmitPeephole.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
//
//===----------------------------------------------------------------------===//
//
/// \file
/// This pass performs the peephole optimizations before code emission.
///
//===----------------------------------------------------------------------===//

#include "AMDGPU.h"
#include "AMDGPUSubtarget.h"
#include "MCTargetDesc/AMDGPUMCTargetDesc.h"
#include "SIInstrInfo.h"
#include "llvm/CodeGen/MachineFunctionPass.h"
#include "llvm/Support/CommandLine.h"

using namespace llvm;

#define DEBUG_TYPE "si-pre-emit-peephole"

namespace {

class SIPreEmitPeephole : public MachineFunctionPass {
private:
  const SIInstrInfo *TII = nullptr;
  const SIRegisterInfo *TRI = nullptr;

  bool optimizeVccBranch(MachineInstr &MI) const;

public:
  static char ID;

  SIPreEmitPeephole() : MachineFunctionPass(ID) {
    initializeSIPreEmitPeepholePass(*PassRegistry::getPassRegistry());
  }

  bool runOnMachineFunction(MachineFunction &MF) override;
};

} // End anonymous namespace.

INITIALIZE_PASS(SIPreEmitPeephole, DEBUG_TYPE,
                "SI peephole optimizations", false, false)

char SIPreEmitPeephole::ID = 0;

char &llvm::SIPreEmitPeepholeID = SIPreEmitPeephole::ID;

bool SIPreEmitPeephole::optimizeVccBranch(MachineInstr &MI) const {
  // Match:
  // sreg = -1
  // vcc = S_AND_B64 exec, sreg
  // S_CBRANCH_VCC[N]Z
  // =>
  // S_CBRANCH_EXEC[N]Z
  // We end up with this pattern sometimes after basic block placement.
  // It happens while combining a block which assigns -1 to a saved mask and
  // another block which consumes that saved mask and then a branch.
  bool Changed = false;
  MachineBasicBlock &MBB = *MI.getParent();
  const GCNSubtarget &ST = MBB.getParent()->getSubtarget<GCNSubtarget>();
  const bool IsWave32 = ST.isWave32();
  const unsigned CondReg = TRI->getVCC();
  const unsigned ExecReg = IsWave32 ? AMDGPU::EXEC_LO : AMDGPU::EXEC;
  const unsigned And = IsWave32 ? AMDGPU::S_AND_B32 : AMDGPU::S_AND_B64;

  MachineBasicBlock::reverse_iterator A = MI.getReverseIterator(),
                                      E = MBB.rend();
  bool ReadsCond = false;
  unsigned Threshold = 5;
  for (++A; A != E; ++A) {
    if (!--Threshold)
      return false;
    if (A->modifiesRegister(ExecReg, TRI))
      return false;
    if (A->modifiesRegister(CondReg, TRI)) {
      if (!A->definesRegister(CondReg, TRI) || A->getOpcode() != And)
        return false;
      break;
    }
    ReadsCond |= A->readsRegister(CondReg, TRI);
  }
  if (A == E)
    return false;

  MachineOperand &Op1 = A->getOperand(1);
  MachineOperand &Op2 = A->getOperand(2);
  if (Op1.getReg() != ExecReg && Op2.isReg() && Op2.getReg() == ExecReg) {
    TII->commuteInstruction(*A);
    Changed = true;
  }
  if (Op1.getReg() != ExecReg)
    return Changed;
  if (Op2.isImm() && Op2.getImm() != -1)
    return Changed;

  Register SReg;
  if (Op2.isReg()) {
    SReg = Op2.getReg();
    auto M = std::next(A);
    bool ReadsSreg = false;
    for (; M != E; ++M) {
      if (M->definesRegister(SReg, TRI))
        break;
      if (M->modifiesRegister(SReg, TRI))
        return Changed;
      ReadsSreg |= M->readsRegister(SReg, TRI);
    }
    if (M == E || !M->isMoveImmediate() || !M->getOperand(1).isImm() ||
        M->getOperand(1).getImm() != -1)
      return Changed;
    // First if sreg is only used in and instruction fold the immediate
    // into that and.
    if (!ReadsSreg && Op2.isKill()) {
      A->getOperand(2).ChangeToImmediate(-1);
      M->eraseFromParent();
    }
  }

  if (!ReadsCond && A->registerDefIsDead(AMDGPU::SCC) &&
      MI.killsRegister(CondReg, TRI))
    A->eraseFromParent();

  bool IsVCCZ = MI.getOpcode() == AMDGPU::S_CBRANCH_VCCZ;
  if (SReg == ExecReg) {
    if (IsVCCZ) {
      MI.eraseFromParent();
      return true;
    }
    MI.setDesc(TII->get(AMDGPU::S_BRANCH));
  } else {
    MI.setDesc(
        TII->get(IsVCCZ ? AMDGPU::S_CBRANCH_EXECZ : AMDGPU::S_CBRANCH_EXECNZ));
  }

  MI.RemoveOperand(MI.findRegisterUseOperandIdx(CondReg, false /*Kill*/, TRI));
  MI.addImplicitDefUseOperands(*MBB.getParent());

  return true;
}

bool SIPreEmitPeephole::runOnMachineFunction(MachineFunction &MF) {
  const GCNSubtarget &ST = MF.getSubtarget<GCNSubtarget>();
  TII = ST.getInstrInfo();
  TRI = &TII->getRegisterInfo();
  bool Changed = false;

  for (MachineBasicBlock &MBB : MF) {
    MachineBasicBlock::iterator MBBI = MBB.getFirstTerminator();
    if (MBBI == MBB.end())
      continue;

    MachineInstr &MI = *MBBI;
    switch (MI.getOpcode()) {
    case AMDGPU::S_CBRANCH_VCCZ:
    case AMDGPU::S_CBRANCH_VCCNZ:
      Changed |= optimizeVccBranch(MI);
      break;
    default:
      break;
    }
  }

  return Changed;
}