llvm/lib/CodeGen/RegAllocBasic.h

//===-- RegAllocBasic.h - Basic Register Allocator Header -----------------===//
//
// 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 file declares the RABasic class, which provides a minimal
/// implementation of the basic register allocator.
///
//===----------------------------------------------------------------------===//

#ifndef LLVM_CODEGEN_REGALLOCBASIC_H
#define LLVM_CODEGEN_REGALLOCBASIC_H

#include "RegAllocBase.h"
#include "llvm/CodeGen/LiveRangeEdit.h"
#include "llvm/CodeGen/MachineFunctionPass.h"
#include "llvm/CodeGen/Spiller.h"
#include <queue>
#include <tuple>

namespace llvm {

struct CompSpillWeight {
  bool operator()(const LiveInterval *A, const LiveInterval *B) const {
    // Compare by weight first, then use register number as a stable tie-breaker
    // to ensure deterministic ordering when the weights are equal.
    return std::tuple(A->weight(), A->reg()) <
           std::tuple(B->weight(), B->reg());
  }
};

/// RABasic provides a minimal implementation of the basic register allocation
/// algorithm. It prioritizes live virtual registers by spill weight and spills
/// whenever a register is unavailable. This is not practical in production but
/// provides a useful baseline both for measuring other allocators and comparing
/// the speed of the basic algorithm against other styles of allocators.
class LLVM_LIBRARY_VISIBILITY RABasic : public MachineFunctionPass,
                                        public RegAllocBase,
                                        private LiveRangeEdit::Delegate {
  // context
  MachineFunction *MF = nullptr;

  // state
  std::unique_ptr<Spiller> SpillerInstance;
  std::priority_queue<const LiveInterval *, std::vector<const LiveInterval *>,
                      CompSpillWeight>
      Queue;

  bool LRE_CanEraseVirtReg(Register) override;
  void LRE_WillShrinkVirtReg(Register) override;

public:
  RABasic(const RegAllocFilterFunc F = nullptr);

  /// Return the pass name.
  StringRef getPassName() const override { return "Basic Register Allocator"; }

  /// RABasic analysis usage.
  void getAnalysisUsage(AnalysisUsage &AU) const override;

  void releaseMemory() override;

  Spiller &spiller() override { return *SpillerInstance; }

  void enqueueImpl(const LiveInterval *LI) override { Queue.push(LI); }

  const LiveInterval *dequeue() override {
    if (Queue.empty())
      return nullptr;
    const LiveInterval *LI = Queue.top();
    Queue.pop();
    return LI;
  }

  MCRegister selectOrSplit(const LiveInterval &VirtReg,
                           SmallVectorImpl<Register> &SplitVRegs) override;

  /// Perform register allocation.
  bool runOnMachineFunction(MachineFunction &mf) override;

  MachineFunctionProperties getRequiredProperties() const override {
    return MachineFunctionProperties().set(
        MachineFunctionProperties::Property::NoPHIs);
  }

  MachineFunctionProperties getClearedProperties() const override {
    return MachineFunctionProperties().set(
        MachineFunctionProperties::Property::IsSSA);
  }

  // Helper for spilling all live virtual registers currently unified under preg
  // that interfere with the most recently queried lvr.  Return true if spilling
  // was successful, and append any new spilled/split intervals to splitLVRs.
  bool spillInterferences(const LiveInterval &VirtReg, MCRegister PhysReg,
                          SmallVectorImpl<Register> &SplitVRegs);

  static char ID;
};
} // namespace llvm
#endif