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//=- llvm/CodeGen/GlobalISel/RegBankSelect.h - Reg Bank Selector --*- C++ -*-=//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See 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,
/// = 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,
/// = 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
#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 {
static char ID;
/// List of the modes supported by the RegBankSelect pass.
enum Mode {
/// Assign the register banks as fast as possible (default).
/// Greedily minimize the cost of assigning register banks.
/// This should produce code of greater quality, but will
/// require more compile time.
/// Abstract class used to represent an insertion point in a CFG.
/// This class records an insertion point and materializes it on
/// demand.
/// It allows to reason about the frequency of this insertion point,
/// without having to logically materialize it (e.g., on an edge),
/// before we actually need to insert something.
class InsertPoint {
/// Tell if the insert point has already been materialized.
bool WasMaterialized = false;
/// Materialize the insertion point.
/// If isSplit() is true, this involves actually splitting
/// the block or edge.
/// \post getPointImpl() returns a valid iterator.
/// \post getInsertMBBImpl() returns a valid basic block.
/// \post isSplit() == false ; no more splitting should be required.
virtual void materialize() = 0;
/// Return the materialized insertion basic block.
/// Code will be inserted into that basic block.
/// \pre ::materialize has been called.
virtual MachineBasicBlock &getInsertMBBImpl() = 0;
/// Return the materialized insertion point.
/// Code will be inserted before that point.
/// \pre ::materialize has been called.
virtual MachineBasicBlock::iterator getPointImpl() = 0;
virtual ~InsertPoint() = default;
/// The first call to this method will cause the splitting to
/// happen if need be, then sub sequent calls just return
/// the iterator to that point. I.e., no more splitting will
/// occur.
/// \return The iterator that should be used with
/// MachineBasicBlock::insert. I.e., additional code happens
/// before that point.
MachineBasicBlock::iterator getPoint() {
if (!WasMaterialized) {
WasMaterialized = true;
assert(canMaterialize() && "Impossible to materialize this point");
// When we materialized the point we should have done the splitting.
assert(!isSplit() && "Wrong pre-condition");
return getPointImpl();
/// The first call to this method will cause the splitting to
/// happen if need be, then sub sequent calls just return
/// the basic block that contains the insertion point.
/// I.e., no more splitting will occur.
/// \return The basic block should be used with
/// MachineBasicBlock::insert and ::getPoint. The new code should
/// happen before that point.
MachineBasicBlock &getInsertMBB() {
if (!WasMaterialized) {
WasMaterialized = true;
assert(canMaterialize() && "Impossible to materialize this point");
// When we materialized the point we should have done the splitting.
assert(!isSplit() && "Wrong pre-condition");
return getInsertMBBImpl();
/// Insert \p MI in the just before ::getPoint()
MachineBasicBlock::iterator insert(MachineInstr &MI) {
return getInsertMBB().insert(getPoint(), &MI);
/// Does this point involve splitting an edge or block?
/// As soon as ::getPoint is called and thus, the point
/// materialized, the point will not require splitting anymore,
/// i.e., this will return false.
virtual bool isSplit() const { return false; }
/// Frequency of the insertion point.
/// \p P is used to access the various analysis that will help to
/// get that information, like MachineBlockFrequencyInfo. If \p P
/// does not contain enough enough to return the actual frequency,
/// this returns 1.
virtual uint64_t frequency(const Pass &P) const { return 1; }
/// Check whether this insertion point can be materialized.
/// As soon as ::getPoint is called and thus, the point materialized
/// calling this method does not make sense.
virtual bool canMaterialize() const { return false; }
/// Insertion point before or after an instruction.
class InstrInsertPoint : public InsertPoint {
/// Insertion point.
MachineInstr &Instr;
/// Does the insertion point is before or after Instr.
bool Before;
void materialize() override;
MachineBasicBlock::iterator getPointImpl() override {
if (Before)
return Instr;
return Instr.getNextNode() ? *Instr.getNextNode()
: Instr.getParent()->end();
MachineBasicBlock &getInsertMBBImpl() override {
return *Instr.getParent();
/// Create an insertion point before (\p Before=true) or after \p Instr.
InstrInsertPoint(MachineInstr &Instr, bool Before = true);
bool isSplit() const override;
uint64_t frequency(const Pass &P) const override;
// Worst case, we need to slice the basic block, but that is still doable.
bool canMaterialize() const override { return true; }
/// Insertion point at the beginning or end of a basic block.
class MBBInsertPoint : public InsertPoint {
/// Insertion point.
MachineBasicBlock &MBB;
/// Does the insertion point is at the beginning or end of MBB.
bool Beginning;
void materialize() override { /*Nothing to do to materialize*/
MachineBasicBlock::iterator getPointImpl() override {
return Beginning ? MBB.begin() : MBB.end();
MachineBasicBlock &getInsertMBBImpl() override { return MBB; }
MBBInsertPoint(MachineBasicBlock &MBB, bool Beginning = true)
: MBB(MBB), Beginning(Beginning) {
// If we try to insert before phis, we should use the insertion
// points on the incoming edges.
assert((!Beginning || MBB.getFirstNonPHI() == MBB.begin()) &&
"Invalid beginning point");
// If we try to insert after the terminators, we should use the
// points on the outcoming edges.
assert((Beginning || MBB.getFirstTerminator() == MBB.end()) &&
"Invalid end point");
bool isSplit() const override { return false; }
uint64_t frequency(const Pass &P) const override;
bool canMaterialize() const override { return true; };
/// Insertion point on an edge.
class EdgeInsertPoint : public InsertPoint {
/// Source of the edge.
MachineBasicBlock &Src;
/// Destination of the edge.
/// After the materialization is done, this hold the basic block
/// that resulted from the splitting.
MachineBasicBlock *DstOrSplit;
/// P is used to update the analysis passes as applicable.
Pass &P;
void materialize() override;
MachineBasicBlock::iterator getPointImpl() override {
// DstOrSplit should be the Split block at this point.
// I.e., it should have one predecessor, Src, and one successor,
// the original Dst.
assert(DstOrSplit && DstOrSplit->isPredecessor(&Src) &&
DstOrSplit->pred_size() == 1 && DstOrSplit->succ_size() == 1 &&
"Did not split?!");
return DstOrSplit->begin();
MachineBasicBlock &getInsertMBBImpl() override { return *DstOrSplit; }
EdgeInsertPoint(MachineBasicBlock &Src, MachineBasicBlock &Dst, Pass &P)
: Src(Src), DstOrSplit(&Dst), P(P) {}
bool isSplit() const override {
return Src.succ_size() > 1 && DstOrSplit->pred_size() > 1;
uint64_t frequency(const Pass &P) const override;
bool canMaterialize() const override;
/// Struct used to represent the placement of a repairing point for
/// a given operand.
class RepairingPlacement {
/// Define the kind of action this repairing needs.
enum RepairingKind {
/// Nothing to repair, just drop this action.
/// Reparing code needs to happen before InsertPoints.
/// (Re)assign the register bank of the operand.
/// Mark this repairing placement as impossible.
/// \name Convenient types for a list of insertion points.
/// @{
using InsertionPoints = SmallVector<std::unique_ptr<InsertPoint>, 2>;
using insertpt_iterator = InsertionPoints::iterator;
using const_insertpt_iterator = InsertionPoints::const_iterator;
/// @}
/// Kind of repairing.
RepairingKind Kind;
/// Index of the operand that will be repaired.
unsigned OpIdx;
/// Are all the insert points materializeable?
bool CanMaterialize;
/// Is there any of the insert points needing splitting?
bool HasSplit = false;
/// Insertion point for the repair code.
/// The repairing code needs to happen just before these points.
InsertionPoints InsertPoints;
/// Some insertion points may need to update the liveness and such.
Pass &P;
/// Create a repairing placement for the \p OpIdx-th operand of
/// \p MI. \p TRI is used to make some checks on the register aliases
/// if the machine operand is a physical register. \p P is used to
/// to update liveness information and such when materializing the
/// points.
RepairingPlacement(MachineInstr &MI, unsigned OpIdx,
const TargetRegisterInfo &TRI, Pass &P,
RepairingKind Kind = RepairingKind::Insert);
/// \name Getters.
/// @{
RepairingKind getKind() const { return Kind; }
unsigned getOpIdx() const { return OpIdx; }
bool canMaterialize() const { return CanMaterialize; }
bool hasSplit() { return HasSplit; }
/// @}
/// \name Overloaded methods to add an insertion point.
/// @{
/// Add a MBBInsertionPoint to the list of InsertPoints.
void addInsertPoint(MachineBasicBlock &MBB, bool Beginning);
/// Add a InstrInsertionPoint to the list of InsertPoints.
void addInsertPoint(MachineInstr &MI, bool Before);
/// Add an EdgeInsertionPoint (\p Src, \p Dst) to the list of InsertPoints.
void addInsertPoint(MachineBasicBlock &Src, MachineBasicBlock &Dst);
/// Add an InsertPoint to the list of insert points.
/// This method takes the ownership of &\p Point.
void addInsertPoint(InsertPoint &Point);
/// @}
/// \name Accessors related to the insertion points.
/// @{
insertpt_iterator begin() { return InsertPoints.begin(); }
insertpt_iterator end() { return InsertPoints.end(); }
const_insertpt_iterator begin() const { return InsertPoints.begin(); }
const_insertpt_iterator end() const { return InsertPoints.end(); }
unsigned getNumInsertPoints() const { return InsertPoints.size(); }
/// @}
/// Change the type of this repairing placement to \p NewKind.
/// It is not possible to switch a repairing placement to the
/// RepairingKind::Insert. There is no fundamental problem with
/// that, but no uses as well, so do not support it for now.
/// \pre NewKind != RepairingKind::Insert
/// \post getKind() == NewKind
void switchTo(RepairingKind NewKind) {
assert(NewKind != Kind && "Already of the right Kind");
Kind = NewKind;
CanMaterialize = NewKind != RepairingKind::Impossible;
HasSplit = false;
assert(NewKind != RepairingKind::Insert &&
"We would need more MI to switch to Insert");
/// Helper class used to represent the cost for mapping an instruction.
/// When mapping an instruction, we may introduce some repairing code.
/// In most cases, the repairing code is local to the instruction,
/// thus, we can omit the basic block frequency from the cost.
/// However, some alternatives may produce non-local cost, e.g., when
/// repairing a phi, and thus we then need to scale the local cost
/// to the non-local cost. This class does this for us.
/// \note: We could simply always scale the cost. The problem is that
/// there are higher chances that we saturate the cost easier and end
/// up having the same cost for actually different alternatives.
/// Another option would be to use APInt everywhere.
class MappingCost {
/// Cost of the local instructions.
/// This cost is free of basic block frequency.
uint64_t LocalCost = 0;
/// Cost of the non-local instructions.
/// This cost should include the frequency of the related blocks.
uint64_t NonLocalCost = 0;
/// Frequency of the block where the local instructions live.
uint64_t LocalFreq;
MappingCost(uint64_t LocalCost, uint64_t NonLocalCost, uint64_t LocalFreq)
: LocalCost(LocalCost), NonLocalCost(NonLocalCost),
LocalFreq(LocalFreq) {}
/// Check if this cost is saturated.
bool isSaturated() const;
/// Create a MappingCost assuming that most of the instructions
/// will occur in a basic block with \p LocalFreq frequency.
MappingCost(const BlockFrequency &LocalFreq);
/// Add \p Cost to the local cost.
/// \return true if this cost is saturated, false otherwise.
bool addLocalCost(uint64_t Cost);
/// Add \p Cost to the non-local cost.
/// Non-local cost should reflect the frequency of their placement.
/// \return true if this cost is saturated, false otherwise.
bool addNonLocalCost(uint64_t Cost);
/// Saturate the cost to the maximal representable value.
void saturate();
/// Return an instance of MappingCost that represents an
/// impossible mapping.
static MappingCost ImpossibleCost();
/// Check if this is less than \p Cost.
bool operator<(const MappingCost &Cost) const;
/// Check if this is equal to \p Cost.
bool operator==(const MappingCost &Cost) const;
/// Check if this is not equal to \p Cost.
bool operator!=(const MappingCost &Cost) const { return !(*this == Cost); }
/// Check if this is greater than \p Cost.
bool operator>(const MappingCost &Cost) const {
return *this != Cost && Cost < *this;
/// Print this on dbgs() stream.
void dump() const;
/// Print this on \p OS;
void print(raw_ostream &OS) const;
/// Overload the stream operator for easy debug printing.
friend raw_ostream &operator<<(raw_ostream &OS, const MappingCost &Cost) {
return OS;
/// Interface to the target lowering info related
/// to register banks.
const RegisterBankInfo *RBI = nullptr;
/// MRI contains all the register class/bank information that this
/// pass uses and updates.
MachineRegisterInfo *MRI = nullptr;
/// Information on the register classes for the current function.
const TargetRegisterInfo *TRI = nullptr;
/// Get the frequency of blocks.
/// This is required for non-fast mode.
MachineBlockFrequencyInfo *MBFI = nullptr;
/// Get the frequency of the edges.
/// This is required for non-fast mode.
MachineBranchProbabilityInfo *MBPI = nullptr;
/// Current optimization remark emitter. Used to report failures.
std::unique_ptr<MachineOptimizationRemarkEmitter> MORE;
/// Helper class used for every code morphing.
MachineIRBuilder MIRBuilder;
/// Optimization mode of the pass.
Mode OptMode;
/// Current target configuration. Controls how the pass handles errors.
const TargetPassConfig *TPC;
/// Assign the register bank of each operand of \p MI.
/// \return True on success, false otherwise.
bool assignInstr(MachineInstr &MI);
/// Initialize the field members using \p MF.
void init(MachineFunction &MF);
/// Check if \p Reg is already assigned what is described by \p ValMapping.
/// \p OnlyAssign == true means that \p Reg just needs to be assigned a
/// register bank. I.e., no repairing is necessary to have the
/// assignment match.
bool assignmentMatch(Register Reg,
const RegisterBankInfo::ValueMapping &ValMapping,
bool &OnlyAssign) const;
/// Insert repairing code for \p Reg as specified by \p ValMapping.
/// The repairing placement is specified by \p RepairPt.
/// \p NewVRegs contains all the registers required to remap \p Reg.
/// In other words, the number of registers in NewVRegs must be equal
/// to ValMapping.BreakDown.size().
/// The transformation could be sketched as:
/// \code
/// ... = op Reg
/// \endcode
/// Becomes
/// \code
/// <NewRegs> = COPY or extract Reg
/// ... = op Reg
/// \endcode
/// and
/// \code
/// Reg = op ...
/// \endcode
/// Becomes
/// \code
/// Reg = op ...
/// Reg = COPY or build_sequence <NewRegs>
/// \endcode
/// \pre NewVRegs.size() == ValMapping.BreakDown.size()
/// \note The caller is supposed to do the rewriting of op if need be.
/// I.e., Reg = op ... => <NewRegs> = NewOp ...
/// \return True if the repairing worked, false otherwise.
bool repairReg(MachineOperand &MO,
const RegisterBankInfo::ValueMapping &ValMapping,
RegBankSelect::RepairingPlacement &RepairPt,
const iterator_range<SmallVectorImpl<Register>::const_iterator>
/// Return the cost of the instruction needed to map \p MO to \p ValMapping.
/// The cost is free of basic block frequencies.
/// \pre MO.isReg()
/// \pre MO is assigned to a register bank.
/// \pre ValMapping is a valid mapping for MO.
getRepairCost(const MachineOperand &MO,
const RegisterBankInfo::ValueMapping &ValMapping) const;
/// Find the best mapping for \p MI from \p PossibleMappings.
/// \return a reference on the best mapping in \p PossibleMappings.
const RegisterBankInfo::InstructionMapping &
findBestMapping(MachineInstr &MI,
RegisterBankInfo::InstructionMappings &PossibleMappings,
SmallVectorImpl<RepairingPlacement> &RepairPts);
/// Compute the cost of mapping \p MI with \p InstrMapping and
/// compute the repairing placement for such mapping in \p
/// RepairPts.
/// \p BestCost is used to specify when the cost becomes too high
/// and thus it is not worth computing the RepairPts. Moreover if
/// \p BestCost == nullptr, the mapping cost is actually not
/// computed.
computeMapping(MachineInstr &MI,
const RegisterBankInfo::InstructionMapping &InstrMapping,
SmallVectorImpl<RepairingPlacement> &RepairPts,
const MappingCost *BestCost = nullptr);
/// When \p RepairPt involves splitting to repair \p MO for the
/// given \p ValMapping, try to change the way we repair such that
/// the splitting is not required anymore.
/// \pre \p RepairPt.hasSplit()
/// \pre \p MO == MO.getParent()->getOperand(\p RepairPt.getOpIdx())
/// \pre \p ValMapping is the mapping of \p MO for MO.getParent()
/// that implied \p RepairPt.
void tryAvoidingSplit(RegBankSelect::RepairingPlacement &RepairPt,
const MachineOperand &MO,
const RegisterBankInfo::ValueMapping &ValMapping) const;
/// Apply \p Mapping to \p MI. \p RepairPts represents the different
/// mapping action that need to happen for the mapping to be
/// applied.
/// \return True if the mapping was applied sucessfully, false otherwise.
bool applyMapping(MachineInstr &MI,
const RegisterBankInfo::InstructionMapping &InstrMapping,
SmallVectorImpl<RepairingPlacement> &RepairPts);
/// Create a RegBankSelect pass with the specified \p RunningMode.
RegBankSelect(char &PassID = ID, Mode RunningMode = Fast);
StringRef getPassName() const override { return "RegBankSelect"; }
void getAnalysisUsage(AnalysisUsage &AU) const override;
MachineFunctionProperties getRequiredProperties() const override {
return MachineFunctionProperties()
MachineFunctionProperties getSetProperties() const override {
return MachineFunctionProperties().set(
MachineFunctionProperties getClearedProperties() const override {
return MachineFunctionProperties()
/// Check that our input is fully legal: we require the function to have the
/// Legalized property, so it should be.
/// FIXME: This should be in the MachineVerifier.
bool checkFunctionIsLegal(MachineFunction &MF) const;
/// Walk through \p MF and assign a register bank to every virtual register
/// that are still mapped to nothing.
/// The target needs to provide a RegisterBankInfo and in particular
/// override RegisterBankInfo::getInstrMapping.
/// Simplified algo:
/// \code
/// RBI = MF.subtarget.getRegBankInfo()
/// MIRBuilder.setMF(MF)
/// for each bb in MF
/// for each inst in bb
/// MIRBuilder.setInstr(inst)
/// MappingCosts = RBI.getMapping(inst);
/// Idx = findIdxOfMinCost(MappingCosts)
/// CurRegBank = MappingCosts[Idx].RegBank
/// MRI.setRegBank(inst.getOperand(0).getReg(), CurRegBank)
/// for each argument in inst
/// if (CurRegBank != argument.RegBank)
/// ArgReg = argument.getReg()
/// Tmp = MRI.createNewVirtual(MRI.getSize(ArgReg), CurRegBank)
/// MIRBuilder.buildInstr(COPY, Tmp, ArgReg)
/// inst.getOperand(argument.getOperandNo()).setReg(Tmp)
/// \endcode
bool assignRegisterBanks(MachineFunction &MF);
bool runOnMachineFunction(MachineFunction &MF) override;
} // end namespace llvm