llvm/llvm/include/llvm/CodeGen/GlobalISel/RegBankSelect.h

//=- llvm/CodeGen/GlobalISel/RegBankSelect.h - Reg Bank Selector --*- C++ -*-=//
//
// 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 describes the interface of the MachineFunctionPass
/// responsible for assigning the generic virtual registers to register bank.
///
/// By default, the reg bank selector relies on local decisions to
/// assign the register bank. In other words, it looks at one instruction
/// at a time to decide where the operand of that instruction should live.
///
/// At higher optimization level, we could imagine that the reg bank selector
/// would use more global analysis and do crazier thing like duplicating
/// instructions and so on. This is future work.
///
/// For now, the pass uses a greedy algorithm to decide where the operand
/// of an instruction should live. It asks the target which banks may be
/// used for each operand of the instruction and what is the cost. Then,
/// it chooses the solution which minimize the cost of the instruction plus
/// the cost of any move that may be needed to the values into the right
/// register bank.
/// In other words, the cost for an instruction on a register bank RegBank
/// is: Cost of I on RegBank plus the sum of the cost for bringing the
/// input operands from their current register bank to RegBank.
/// Thus, the following formula:
/// cost(I, RegBank) = cost(I.Opcode, RegBank) +
///    sum(for each arg in I.arguments: costCrossCopy(arg.RegBank, RegBank))
///
/// E.g., Let say we are assigning the register bank for the instruction
/// defining v2.
/// v0(A_REGBANK) = ...
/// v1(A_REGBANK) = ...
/// v2 = G_ADD i32 v0, v1 <-- MI
///
/// The target may say it can generate G_ADD i32 on register bank A and B
/// with a cost of respectively 5 and 1.
/// Then, let say the cost of a cross register bank copies from A to B is 1.
/// The reg bank selector would compare the following two costs:
/// cost(MI, A_REGBANK) = cost(G_ADD, A_REGBANK) + cost(v0.RegBank, A_REGBANK) +
///    cost(v1.RegBank, A_REGBANK)
///                     = 5 + cost(A_REGBANK, A_REGBANK) + cost(A_REGBANK,
///                                                             A_REGBANK)
///                     = 5 + 0 + 0 = 5
/// cost(MI, B_REGBANK) = cost(G_ADD, B_REGBANK) + cost(v0.RegBank, B_REGBANK) +
///    cost(v1.RegBank, B_REGBANK)
///                     = 1 + cost(A_REGBANK, B_REGBANK) + cost(A_REGBANK,
///                                                             B_REGBANK)
///                     = 1 + 1 + 1 = 3
/// Therefore, in this specific example, the reg bank selector would choose
/// bank B for MI.
/// v0(A_REGBANK) = ...
/// v1(A_REGBANK) = ...
/// tmp0(B_REGBANK) = COPY v0
/// tmp1(B_REGBANK) = COPY v1
/// v2(B_REGBANK) = G_ADD i32 tmp0, tmp1
//
//===----------------------------------------------------------------------===//

#ifndef LLVM_CODEGEN_GLOBALISEL_REGBANKSELECT_H
#define LLVM_CODEGEN_GLOBALISEL_REGBANKSELECT_H

#include "llvm/ADT/SmallVector.h"
#include "llvm/CodeGen/GlobalISel/MachineIRBuilder.h"
#include "llvm/CodeGen/MachineBasicBlock.h"
#include "llvm/CodeGen/MachineFunctionPass.h"
#include "llvm/CodeGen/MachineOptimizationRemarkEmitter.h"
#include "llvm/CodeGen/RegisterBankInfo.h"
#include <cassert>
#include <cstdint>
#include <memory>

namespace llvm {

class BlockFrequency;
class MachineBlockFrequencyInfo;
class MachineBranchProbabilityInfo;
class MachineOperand;
class MachineRegisterInfo;
class Pass;
class raw_ostream;
class TargetPassConfig;
class TargetRegisterInfo;

/// This pass implements the reg bank selector pass used in the GlobalISel
/// pipeline. At the end of this pass, all register operands have been assigned
class RegBankSelect : public MachineFunctionPass { … };

} // end namespace llvm

#endif // LLVM_CODEGEN_GLOBALISEL_REGBANKSELECT_H