llvm/lib/Target/AMDGPU/GCNPreRAOptimizations.cpp

//===-- GCNPreRAOptimizations.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 combines split register tuple initialization into a single psuedo:
///
///   undef %0.sub1:sreg_64 = S_MOV_B32 1
///   %0.sub0:sreg_64 = S_MOV_B32 2
/// =>
///   %0:sreg_64 = S_MOV_B64_IMM_PSEUDO 0x200000001
///
/// This is to allow rematerialization of a value instead of spilling. It is
/// supposed to be done after register coalescer to allow it to do its job and
/// before actual register allocation to allow rematerialization.
///
/// Right now the pass only handles 64 bit SGPRs with immediate initializers,
/// although the same shall be possible with other register classes and
/// instructions if necessary.
///
//===----------------------------------------------------------------------===//

#include "AMDGPU.h"
#include "GCNSubtarget.h"
#include "MCTargetDesc/AMDGPUMCTargetDesc.h"
#include "llvm/CodeGen/LiveIntervals.h"
#include "llvm/CodeGen/MachineFunctionPass.h"
#include "llvm/InitializePasses.h"

using namespace llvm;

#define DEBUG_TYPE "amdgpu-pre-ra-optimizations"

namespace {

class GCNPreRAOptimizations : public MachineFunctionPass {
private:
  const SIInstrInfo *TII;
  MachineRegisterInfo *MRI;
  LiveIntervals *LIS;

  bool processReg(Register Reg);

public:
  static char ID;

  GCNPreRAOptimizations() : MachineFunctionPass(ID) {
    initializeGCNPreRAOptimizationsPass(*PassRegistry::getPassRegistry());
  }

  bool runOnMachineFunction(MachineFunction &MF) override;

  StringRef getPassName() const override {
    return "AMDGPU Pre-RA optimizations";
  }

  void getAnalysisUsage(AnalysisUsage &AU) const override {
    AU.addRequired<LiveIntervals>();
    AU.setPreservesAll();
    MachineFunctionPass::getAnalysisUsage(AU);
  }
};

} // End anonymous namespace.

INITIALIZE_PASS_BEGIN(GCNPreRAOptimizations, DEBUG_TYPE,
                      "AMDGPU Pre-RA optimizations", false, false)
INITIALIZE_PASS_DEPENDENCY(LiveIntervals)
INITIALIZE_PASS_END(GCNPreRAOptimizations, DEBUG_TYPE, "Pre-RA optimizations",
                    false, false)

char GCNPreRAOptimizations::ID = 0;

char &llvm::GCNPreRAOptimizationsID = GCNPreRAOptimizations::ID;

FunctionPass *llvm::createGCNPreRAOptimizationsPass() {
  return new GCNPreRAOptimizations();
}

bool GCNPreRAOptimizations::processReg(Register Reg) {
  MachineInstr *Def0 = nullptr;
  MachineInstr *Def1 = nullptr;
  uint64_t Init = 0;

  for (MachineInstr &I : MRI->def_instructions(Reg)) {
    if (I.getOpcode() != AMDGPU::S_MOV_B32 || I.getOperand(0).getReg() != Reg ||
        !I.getOperand(1).isImm() || I.getNumOperands() != 2)
      return false;

    switch (I.getOperand(0).getSubReg()) {
    default:
      return false;
    case AMDGPU::sub0:
      if (Def0)
        return false;
      Def0 = &I;
      Init |= I.getOperand(1).getImm() & 0xffffffff;
      break;
    case AMDGPU::sub1:
      if (Def1)
        return false;
      Def1 = &I;
      Init |= static_cast<uint64_t>(I.getOperand(1).getImm()) << 32;
      break;
    }
  }

  if (!Def0 || !Def1 || Def0->getParent() != Def1->getParent())
    return false;

  LLVM_DEBUG(dbgs() << "Combining:\n  " << *Def0 << "  " << *Def1
                    << "    =>\n");

  if (SlotIndex::isEarlierInstr(LIS->getInstructionIndex(*Def1),
                                LIS->getInstructionIndex(*Def0)))
    std::swap(Def0, Def1);

  LIS->RemoveMachineInstrFromMaps(*Def0);
  LIS->RemoveMachineInstrFromMaps(*Def1);
  auto NewI = BuildMI(*Def0->getParent(), *Def0, Def0->getDebugLoc(),
                      TII->get(AMDGPU::S_MOV_B64_IMM_PSEUDO), Reg)
                  .addImm(Init);

  Def0->eraseFromParent();
  Def1->eraseFromParent();
  LIS->InsertMachineInstrInMaps(*NewI);
  LIS->removeInterval(Reg);
  LIS->createAndComputeVirtRegInterval(Reg);

  LLVM_DEBUG(dbgs() << "  " << *NewI);

  return true;
}

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

  const GCNSubtarget &ST = MF.getSubtarget<GCNSubtarget>();
  TII = ST.getInstrInfo();
  MRI = &MF.getRegInfo();
  LIS = &getAnalysis<LiveIntervals>();
  const SIRegisterInfo *TRI = ST.getRegisterInfo();

  bool Changed = false;

  for (unsigned I = 0, E = MRI->getNumVirtRegs(); I != E; ++I) {
    Register Reg = Register::index2VirtReg(I);
    if (!LIS->hasInterval(Reg))
      continue;
    const TargetRegisterClass *RC = MRI->getRegClass(Reg);
    if (RC->MC->getSizeInBits() != 64 || !TRI->isSGPRClass(RC))
      continue;
    Changed |= processReg(Reg);
  }

  return Changed;
}