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//==- llvm/CodeGen/GlobalISel/RegBankSelect.cpp - RegBankSelect --*- 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 implements the RegBankSelect class.
#include "llvm/CodeGen/GlobalISel/RegBankSelect.h"
#include "llvm/ADT/PostOrderIterator.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/CodeGen/GlobalISel/LegalizerInfo.h"
#include "llvm/CodeGen/GlobalISel/RegisterBank.h"
#include "llvm/CodeGen/GlobalISel/RegisterBankInfo.h"
#include "llvm/CodeGen/GlobalISel/Utils.h"
#include "llvm/CodeGen/MachineBasicBlock.h"
#include "llvm/CodeGen/MachineBlockFrequencyInfo.h"
#include "llvm/CodeGen/MachineBranchProbabilityInfo.h"
#include "llvm/CodeGen/MachineFunction.h"
#include "llvm/CodeGen/MachineInstr.h"
#include "llvm/CodeGen/MachineOperand.h"
#include "llvm/CodeGen/MachineOptimizationRemarkEmitter.h"
#include "llvm/CodeGen/MachineRegisterInfo.h"
#include "llvm/CodeGen/TargetOpcodes.h"
#include "llvm/CodeGen/TargetPassConfig.h"
#include "llvm/CodeGen/TargetRegisterInfo.h"
#include "llvm/CodeGen/TargetSubtargetInfo.h"
#include "llvm/Config/llvm-config.h"
#include "llvm/IR/Attributes.h"
#include "llvm/IR/Function.h"
#include "llvm/InitializePasses.h"
#include "llvm/Pass.h"
#include "llvm/Support/BlockFrequency.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Compiler.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/ErrorHandling.h"
#include "llvm/Support/raw_ostream.h"
#include <algorithm>
#include <cassert>
#include <cstdint>
#include <limits>
#include <memory>
#include <utility>
#define DEBUG_TYPE "regbankselect"
using namespace llvm;
static cl::opt<RegBankSelect::Mode> RegBankSelectMode(
cl::desc("Mode of the RegBankSelect pass"), cl::Hidden, cl::Optional,
cl::values(clEnumValN(RegBankSelect::Mode::Fast, "regbankselect-fast",
"Run the Fast mode (default mapping)"),
clEnumValN(RegBankSelect::Mode::Greedy, "regbankselect-greedy",
"Use the Greedy mode (best local mapping)")));
char RegBankSelect::ID = 0;
"Assign register bank of generic virtual registers",
false, false);
"Assign register bank of generic virtual registers", false,
RegBankSelect::RegBankSelect(Mode RunningMode)
: MachineFunctionPass(ID), OptMode(RunningMode) {
if (RegBankSelectMode.getNumOccurrences() != 0) {
OptMode = RegBankSelectMode;
if (RegBankSelectMode != RunningMode)
LLVM_DEBUG(dbgs() << "RegBankSelect mode overrided by command line\n");
void RegBankSelect::init(MachineFunction &MF) {
RBI = MF.getSubtarget().getRegBankInfo();
assert(RBI && "Cannot work without RegisterBankInfo");
MRI = &MF.getRegInfo();
TRI = MF.getSubtarget().getRegisterInfo();
TPC = &getAnalysis<TargetPassConfig>();
if (OptMode != Mode::Fast) {
MBFI = &getAnalysis<MachineBlockFrequencyInfo>();
MBPI = &getAnalysis<MachineBranchProbabilityInfo>();
} else {
MBFI = nullptr;
MBPI = nullptr;
MORE = std::make_unique<MachineOptimizationRemarkEmitter>(MF, MBFI);
void RegBankSelect::getAnalysisUsage(AnalysisUsage &AU) const {
if (OptMode != Mode::Fast) {
// We could preserve the information from these two analysis but
// the APIs do not allow to do so yet.
bool RegBankSelect::assignmentMatch(
Register Reg, const RegisterBankInfo::ValueMapping &ValMapping,
bool &OnlyAssign) const {
// By default we assume we will have to repair something.
OnlyAssign = false;
// Each part of a break down needs to end up in a different register.
// In other word, Reg assignment does not match.
if (ValMapping.NumBreakDowns != 1)
return false;
const RegisterBank *CurRegBank = RBI->getRegBank(Reg, *MRI, *TRI);
const RegisterBank *DesiredRegBank = ValMapping.BreakDown[0].RegBank;
// Reg is free of assignment, a simple assignment will make the
// register bank to match.
OnlyAssign = CurRegBank == nullptr;
LLVM_DEBUG(dbgs() << "Does assignment already match: ";
if (CurRegBank) dbgs() << *CurRegBank; else dbgs() << "none";
dbgs() << " against ";
assert(DesiredRegBank && "The mapping must be valid");
dbgs() << *DesiredRegBank << '\n';);
return CurRegBank == DesiredRegBank;
bool RegBankSelect::repairReg(
MachineOperand &MO, const RegisterBankInfo::ValueMapping &ValMapping,
RegBankSelect::RepairingPlacement &RepairPt,
const iterator_range<SmallVectorImpl<Register>::const_iterator> &NewVRegs) {
assert(ValMapping.NumBreakDowns == (unsigned)size(NewVRegs) &&
"need new vreg for each breakdown");
// An empty range of new register means no repairing.
assert(!NewVRegs.empty() && "We should not have to repair");
MachineInstr *MI;
if (ValMapping.NumBreakDowns == 1) {
// Assume we are repairing a use and thus, the original reg will be
// the source of the repairing.
Register Src = MO.getReg();
Register Dst = *NewVRegs.begin();
// If we repair a definition, swap the source and destination for
// the repairing.
if (MO.isDef())
std::swap(Src, Dst);
assert((RepairPt.getNumInsertPoints() == 1 ||
Register::isPhysicalRegister(Dst)) &&
"We are about to create several defs for Dst");
// Build the instruction used to repair, then clone it at the right
// places. Avoiding buildCopy bypasses the check that Src and Dst have the
// same types because the type is a placeholder when this function is called.
MI = MIRBuilder.buildInstrNoInsert(TargetOpcode::COPY)
LLVM_DEBUG(dbgs() << "Copy: " << printReg(Src) << " to: " << printReg(Dst)
<< '\n');
} else {
// TODO: Support with G_IMPLICIT_DEF + G_INSERT sequence or G_EXTRACT
// sequence.
assert(ValMapping.partsAllUniform() && "irregular breakdowns not supported");
LLT RegTy = MRI->getType(MO.getReg());
if (MO.isDef()) {
unsigned MergeOp;
if (RegTy.isVector()) {
if (ValMapping.NumBreakDowns == RegTy.getNumElements())
MergeOp = TargetOpcode::G_BUILD_VECTOR;
else {
(ValMapping.BreakDown[0].Length * ValMapping.NumBreakDowns ==
RegTy.getSizeInBits()) &&
(ValMapping.BreakDown[0].Length % RegTy.getScalarSizeInBits() ==
0) &&
"don't understand this value breakdown");
MergeOp = TargetOpcode::G_CONCAT_VECTORS;
} else
MergeOp = TargetOpcode::G_MERGE_VALUES;
auto MergeBuilder =
for (Register SrcReg : NewVRegs)
MI = MergeBuilder;
} else {
MachineInstrBuilder UnMergeBuilder =
for (Register DefReg : NewVRegs)
MI = UnMergeBuilder;
if (RepairPt.getNumInsertPoints() != 1)
report_fatal_error("need testcase to support multiple insertion points");
// TODO:
// Check if MI is legal. if not, we need to legalize all the
// instructions we are going to insert.
std::unique_ptr<MachineInstr *[]> NewInstrs(
new MachineInstr *[RepairPt.getNumInsertPoints()]);
bool IsFirst = true;
unsigned Idx = 0;
for (const std::unique_ptr<InsertPoint> &InsertPt : RepairPt) {
MachineInstr *CurMI;
if (IsFirst)
CurMI = MI;
CurMI = MIRBuilder.getMF().CloneMachineInstr(MI);
NewInstrs[Idx++] = CurMI;
IsFirst = false;
// TODO:
// Legalize NewInstrs if need be.
return true;
uint64_t RegBankSelect::getRepairCost(
const MachineOperand &MO,
const RegisterBankInfo::ValueMapping &ValMapping) const {
assert(MO.isReg() && "We should only repair register operand");
assert(ValMapping.NumBreakDowns && "Nothing to map??");
bool IsSameNumOfValues = ValMapping.NumBreakDowns == 1;
const RegisterBank *CurRegBank = RBI->getRegBank(MO.getReg(), *MRI, *TRI);
// If MO does not have a register bank, we should have just been
// able to set one unless we have to break the value down.
assert(CurRegBank || MO.isDef());
// Def: Val <- NewDefs
// Same number of values: copy
// Different number: Val = build_sequence Defs1, Defs2, ...
// Use: NewSources <- Val.
// Same number of values: copy.
// Different number: Src1, Src2, ... =
// extract_value Val, Src1Begin, Src1Len, Src2Begin, Src2Len, ...
// We should remember that this value is available somewhere else to
// coalesce the value.
if (ValMapping.NumBreakDowns != 1)
return RBI->getBreakDownCost(ValMapping, CurRegBank);
if (IsSameNumOfValues) {
const RegisterBank *DesiredRegBank = ValMapping.BreakDown[0].RegBank;
// If we repair a definition, swap the source and destination for
// the repairing.
if (MO.isDef())
std::swap(CurRegBank, DesiredRegBank);
// TODO: It may be possible to actually avoid the copy.
// If we repair something where the source is defined by a copy
// and the source of that copy is on the right bank, we can reuse
// it for free.
// E.g.,
// RegToRepair<BankA> = copy AlternativeSrc<BankB>
// = op RegToRepair<BankA>
// We can simply propagate AlternativeSrc instead of copying RegToRepair
// into a new virtual register.
// We would also need to propagate this information in the
// repairing placement.
unsigned Cost = RBI->copyCost(*DesiredRegBank, *CurRegBank,
RBI->getSizeInBits(MO.getReg(), *MRI, *TRI));
// TODO: use a dedicated constant for ImpossibleCost.
if (Cost != std::numeric_limits<unsigned>::max())
return Cost;
// Return the legalization cost of that repairing.
return std::numeric_limits<unsigned>::max();
const RegisterBankInfo::InstructionMapping &RegBankSelect::findBestMapping(
MachineInstr &MI, RegisterBankInfo::InstructionMappings &PossibleMappings,
SmallVectorImpl<RepairingPlacement> &RepairPts) {
assert(!PossibleMappings.empty() &&
"Do not know how to map this instruction");
const RegisterBankInfo::InstructionMapping *BestMapping = nullptr;
MappingCost Cost = MappingCost::ImpossibleCost();
SmallVector<RepairingPlacement, 4> LocalRepairPts;
for (const RegisterBankInfo::InstructionMapping *CurMapping :
PossibleMappings) {
MappingCost CurCost =
computeMapping(MI, *CurMapping, LocalRepairPts, &Cost);
if (CurCost < Cost) {
LLVM_DEBUG(dbgs() << "New best: " << CurCost << '\n');
Cost = CurCost;
BestMapping = CurMapping;
for (RepairingPlacement &RepairPt : LocalRepairPts)
if (!BestMapping && !TPC->isGlobalISelAbortEnabled()) {
// If none of the mapping worked that means they are all impossible.
// Thus, pick the first one and set an impossible repairing point.
// It will trigger the failed isel mode.
BestMapping = *PossibleMappings.begin();
RepairingPlacement(MI, 0, *TRI, *this, RepairingPlacement::Impossible));
} else
assert(BestMapping && "No suitable mapping for instruction");
return *BestMapping;
void RegBankSelect::tryAvoidingSplit(
RegBankSelect::RepairingPlacement &RepairPt, const MachineOperand &MO,
const RegisterBankInfo::ValueMapping &ValMapping) const {
const MachineInstr &MI = *MO.getParent();
assert(RepairPt.hasSplit() && "We should not have to adjust for split");
// Splitting should only occur for PHIs or between terminators,
// because we only do local repairing.
assert((MI.isPHI() || MI.isTerminator()) && "Why do we split?");
assert(&MI.getOperand(RepairPt.getOpIdx()) == &MO &&
"Repairing placement does not match operand");
// If we need splitting for phis, that means it is because we
// could not find an insertion point before the terminators of
// the predecessor block for this argument. In other words,
// the input value is defined by one of the terminators.
assert((!MI.isPHI() || !MO.isDef()) && "Need split for phi def?");
// We split to repair the use of a phi or a terminator.
if (!MO.isDef()) {
if (MI.isTerminator()) {
assert(&MI != &(*MI.getParent()->getFirstTerminator()) &&
"Need to split for the first terminator?!");
} else {
// For the PHI case, the split may not be actually required.
// In the copy case, a phi is already a copy on the incoming edge,
// therefore there is no need to split.
if (ValMapping.NumBreakDowns == 1)
// This is a already a copy, there is nothing to do.
// At this point, we need to repair a defintion of a terminator.
// Technically we need to fix the def of MI on all outgoing
// edges of MI to keep the repairing local. In other words, we
// will create several definitions of the same register. This
// does not work for SSA unless that definition is a physical
// register.
// However, there are other cases where we can get away with
// that while still keeping the repairing local.
assert(MI.isTerminator() && MO.isDef() &&
"This code is for the def of a terminator");
// Since we use RPO traversal, if we need to repair a definition
// this means this definition could be:
// 1. Used by PHIs (i.e., this VReg has been visited as part of the
// uses of a phi.), or
// 2. Part of a target specific instruction (i.e., the target applied
// some register class constraints when creating the instruction.)
// If the constraints come for #2, the target said that another mapping
// is supported so we may just drop them. Indeed, if we do not change
// the number of registers holding that value, the uses will get fixed
// when we get to them.
// Uses in PHIs may have already been proceeded though.
// If the constraints come for #1, then, those are weak constraints and
// no actual uses may rely on them. However, the problem remains mainly
// the same as for #2. If the value stays in one register, we could
// just switch the register bank of the definition, but we would need to
// account for a repairing cost for each phi we silently change.
// In any case, if the value needs to be broken down into several
// registers, the repairing is not local anymore as we need to patch
// every uses to rebuild the value in just one register.
// To summarize:
// - If the value is in a physical register, we can do the split and
// fix locally.
// Otherwise if the value is in a virtual register:
// - If the value remains in one register, we do not have to split
// just switching the register bank would do, but we need to account
// in the repairing cost all the phi we changed.
// - If the value spans several registers, then we cannot do a local
// repairing.
// Check if this is a physical or virtual register.
Register Reg = MO.getReg();
if (Register::isPhysicalRegister(Reg)) {
// We are going to split every outgoing edges.
// Check that this is possible.
// FIXME: The machine representation is currently broken
// since it also several terminators in one basic block.
// Because of that we would technically need a way to get
// the targets of just one terminator to know which edges
// we have to split.
// Assert that we do not hit the ill-formed representation.
// If there are other terminators before that one, some of
// the outgoing edges may not be dominated by this definition.
assert(&MI == &(*MI.getParent()->getFirstTerminator()) &&
"Do not know which outgoing edges are relevant");
const MachineInstr *Next = MI.getNextNode();
assert((!Next || Next->isUnconditionalBranch()) &&
"Do not know where each terminator ends up");
if (Next)
// If the next terminator uses Reg, this means we have
// to split right after MI and thus we need a way to ask
// which outgoing edges are affected.
assert(!Next->readsRegister(Reg) && "Need to split between terminators");
// We will split all the edges and repair there.
} else {
// This is a virtual register defined by a terminator.
if (ValMapping.NumBreakDowns == 1) {
// There is nothing to repair, but we may actually lie on
// the repairing cost because of the PHIs already proceeded
// as already stated.
// Though the code will be correct.
assert(false && "Repairing cost may not be accurate");
} else {
// We need to do non-local repairing. Basically, patch all
// the uses (i.e., phis) that we already proceeded.
// For now, just say this mapping is not possible.
RegBankSelect::MappingCost RegBankSelect::computeMapping(
MachineInstr &MI, const RegisterBankInfo::InstructionMapping &InstrMapping,
SmallVectorImpl<RepairingPlacement> &RepairPts,
const RegBankSelect::MappingCost *BestCost) {
assert((MBFI || !BestCost) && "Costs comparison require MBFI");
if (!InstrMapping.isValid())
return MappingCost::ImpossibleCost();
// If mapped with InstrMapping, MI will have the recorded cost.
MappingCost Cost(MBFI ? MBFI->getBlockFreq(MI.getParent()) : 1);
bool Saturated = Cost.addLocalCost(InstrMapping.getCost());
assert(!Saturated && "Possible mapping saturated the cost");
LLVM_DEBUG(dbgs() << "Evaluating mapping cost for: " << MI);
LLVM_DEBUG(dbgs() << "With: " << InstrMapping << '\n');
if (BestCost && Cost > *BestCost) {
LLVM_DEBUG(dbgs() << "Mapping is too expensive from the start\n");
return Cost;
// Moreover, to realize this mapping, the register bank of each operand must
// match this mapping. In other words, we may need to locally reassign the
// register banks. Account for that repairing cost as well.
// In this context, local means in the surrounding of MI.
for (unsigned OpIdx = 0, EndOpIdx = InstrMapping.getNumOperands();
OpIdx != EndOpIdx; ++OpIdx) {
const MachineOperand &MO = MI.getOperand(OpIdx);
if (!MO.isReg())
Register Reg = MO.getReg();
if (!Reg)
LLVM_DEBUG(dbgs() << "Opd" << OpIdx << '\n');
const RegisterBankInfo::ValueMapping &ValMapping =
// If Reg is already properly mapped, this is free.
bool Assign;
if (assignmentMatch(Reg, ValMapping, Assign)) {
LLVM_DEBUG(dbgs() << "=> is free (match).\n");
if (Assign) {
LLVM_DEBUG(dbgs() << "=> is free (simple assignment).\n");
RepairPts.emplace_back(RepairingPlacement(MI, OpIdx, *TRI, *this,
// Find the insertion point for the repairing code.
RepairingPlacement(MI, OpIdx, *TRI, *this, RepairingPlacement::Insert));
RepairingPlacement &RepairPt = RepairPts.back();
// If we need to split a basic block to materialize this insertion point,
// we may give a higher cost to this mapping.
// Nevertheless, we may get away with the split, so try that first.
if (RepairPt.hasSplit())
tryAvoidingSplit(RepairPt, MO, ValMapping);
// Check that the materialization of the repairing is possible.
if (!RepairPt.canMaterialize()) {
LLVM_DEBUG(dbgs() << "Mapping involves impossible repairing\n");
return MappingCost::ImpossibleCost();
// Account for the split cost and repair cost.
// Unless the cost is already saturated or we do not care about the cost.
if (!BestCost || Saturated)
// To get accurate information we need MBFI and MBPI.
// Thus, if we end up here this information should be here.
assert(MBFI && MBPI && "Cost computation requires MBFI and MBPI");
// FIXME: We will have to rework the repairing cost model.
// The repairing cost depends on the register bank that MO has.
// However, when we break down the value into different values,
// MO may not have a register bank while still needing repairing.
// For the fast mode, we don't compute the cost so that is fine,
// but still for the repairing code, we will have to make a choice.
// For the greedy mode, we should choose greedily what is the best
// choice based on the next use of MO.
// Sums up the repairing cost of MO at each insertion point.
uint64_t RepairCost = getRepairCost(MO, ValMapping);
// This is an impossible to repair cost.
if (RepairCost == std::numeric_limits<unsigned>::max())
return MappingCost::ImpossibleCost();
// Bias used for splitting: 5%.
const uint64_t PercentageForBias = 5;
uint64_t Bias = (RepairCost * PercentageForBias + 99) / 100;
// We should not need more than a couple of instructions to repair
// an assignment. In other words, the computation should not
// overflow because the repairing cost is free of basic block
// frequency.
assert(((RepairCost < RepairCost * PercentageForBias) &&
(RepairCost * PercentageForBias <
RepairCost * PercentageForBias + 99)) &&
"Repairing involves more than a billion of instructions?!");
for (const std::unique_ptr<InsertPoint> &InsertPt : RepairPt) {
assert(InsertPt->canMaterialize() && "We should not have made it here");
// We will applied some basic block frequency and those uses uint64_t.
if (!InsertPt->isSplit())
Saturated = Cost.addLocalCost(RepairCost);
else {
uint64_t CostForInsertPt = RepairCost;
// Again we shouldn't overflow here givent that
// CostForInsertPt is frequency free at this point.
assert(CostForInsertPt + Bias > CostForInsertPt &&
"Repairing + split bias overflows");
CostForInsertPt += Bias;
uint64_t PtCost = InsertPt->frequency(*this) * CostForInsertPt;
// Check if we just overflowed.
if ((Saturated = PtCost < CostForInsertPt))
Saturated = Cost.addNonLocalCost(PtCost);
// Stop looking into what it takes to repair, this is already
// too expensive.
if (BestCost && Cost > *BestCost) {
LLVM_DEBUG(dbgs() << "Mapping is too expensive, stop processing\n");
return Cost;
// No need to accumulate more cost information.
// We need to still gather the repairing information though.
if (Saturated)
LLVM_DEBUG(dbgs() << "Total cost is: " << Cost << "\n");
return Cost;
bool RegBankSelect::applyMapping(
MachineInstr &MI, const RegisterBankInfo::InstructionMapping &InstrMapping,
SmallVectorImpl<RegBankSelect::RepairingPlacement> &RepairPts) {
// OpdMapper will hold all the information needed for the rewriting.
RegisterBankInfo::OperandsMapper OpdMapper(MI, InstrMapping, *MRI);
// First, place the repairing code.
for (RepairingPlacement &RepairPt : RepairPts) {
if (!RepairPt.canMaterialize() ||
RepairPt.getKind() == RepairingPlacement::Impossible)
return false;
assert(RepairPt.getKind() != RepairingPlacement::None &&
"This should not make its way in the list");
unsigned OpIdx = RepairPt.getOpIdx();
MachineOperand &MO = MI.getOperand(OpIdx);
const RegisterBankInfo::ValueMapping &ValMapping =
Register Reg = MO.getReg();
switch (RepairPt.getKind()) {
case RepairingPlacement::Reassign:
assert(ValMapping.NumBreakDowns == 1 &&
"Reassignment should only be for simple mapping");
MRI->setRegBank(Reg, *ValMapping.BreakDown[0].RegBank);
case RepairingPlacement::Insert:
if (!repairReg(MO, ValMapping, RepairPt, OpdMapper.getVRegs(OpIdx)))
return false;
llvm_unreachable("Other kind should not happen");
// Second, rewrite the instruction.
LLVM_DEBUG(dbgs() << "Actual mapping of the operands: " << OpdMapper << '\n');
return true;
bool RegBankSelect::assignInstr(MachineInstr &MI) {
LLVM_DEBUG(dbgs() << "Assign: " << MI);
unsigned Opc = MI.getOpcode();
if (isPreISelGenericOptimizationHint(Opc)) {
assert((Opc == TargetOpcode::G_ASSERT_ZEXT ||
Opc == TargetOpcode::G_ASSERT_SEXT) &&
"Unexpected hint opcode!");
// The only correct mapping for these is to always use the source register
// bank.
const RegisterBank *RB = MRI->getRegBankOrNull(MI.getOperand(1).getReg());
// We can assume every instruction above this one has a selected register
// bank.
assert(RB && "Expected source register to have a register bank?");
LLVM_DEBUG(dbgs() << "... Hint always uses source's register bank.\n");
MRI->setRegBank(MI.getOperand(0).getReg(), *RB);
return true;
// Remember the repairing placement for all the operands.
SmallVector<RepairingPlacement, 4> RepairPts;
const RegisterBankInfo::InstructionMapping *BestMapping;
if (OptMode == RegBankSelect::Mode::Fast) {
BestMapping = &RBI->getInstrMapping(MI);
MappingCost DefaultCost = computeMapping(MI, *BestMapping, RepairPts);
if (DefaultCost == MappingCost::ImpossibleCost())
return false;
} else {
RegisterBankInfo::InstructionMappings PossibleMappings =
if (PossibleMappings.empty())
return false;
BestMapping = &findBestMapping(MI, PossibleMappings, RepairPts);
// Make sure the mapping is valid for MI.
assert(BestMapping->verify(MI) && "Invalid instruction mapping");
LLVM_DEBUG(dbgs() << "Best Mapping: " << *BestMapping << '\n');
// After this call, MI may not be valid anymore.
// Do not use it.
return applyMapping(MI, *BestMapping, RepairPts);
bool RegBankSelect::runOnMachineFunction(MachineFunction &MF) {
// If the ISel pipeline failed, do not bother running that pass.
if (MF.getProperties().hasProperty(
return false;
LLVM_DEBUG(dbgs() << "Assign register banks for: " << MF.getName() << '\n');
const Function &F = MF.getFunction();
Mode SaveOptMode = OptMode;
if (F.hasOptNone())
OptMode = Mode::Fast;
#ifndef NDEBUG
// 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.
if (!DisableGISelLegalityCheck)
if (const MachineInstr *MI = machineFunctionIsIllegal(MF)) {
reportGISelFailure(MF, *TPC, *MORE, "gisel-regbankselect",
"instruction is not legal", *MI);
return false;
// Walk the function and assign register banks to all operands.
// Use a RPOT to make sure all registers are assigned before we choose
// the best mapping of the current instruction.
ReversePostOrderTraversal<MachineFunction*> RPOT(&MF);
for (MachineBasicBlock *MBB : RPOT) {
// Set a sensible insertion point so that subsequent calls to
// MIRBuilder.
for (MachineBasicBlock::iterator MII = MBB->begin(), End = MBB->end();
MII != End;) {
// MI might be invalidated by the assignment, so move the
// iterator before hand.
MachineInstr &MI = *MII++;
// Ignore target-specific post-isel instructions: they should use proper
// regclasses.
if (isTargetSpecificOpcode(MI.getOpcode()) && !MI.isPreISelOpcode())
// Ignore inline asm instructions: they should use physical
// registers/regclasses
if (MI.isInlineAsm())
// Ignore debug info.
if (MI.isDebugInstr())
// Ignore IMPLICIT_DEF which must have a regclass.
if (MI.isImplicitDef())
if (!assignInstr(MI)) {
reportGISelFailure(MF, *TPC, *MORE, "gisel-regbankselect",
"unable to map instruction", MI);
return false;
// It's possible the mapping changed control flow, and moved the following
// instruction to a new block, so figure out the new parent.
if (MII != End) {
MachineBasicBlock *NextInstBB = MII->getParent();
if (NextInstBB != MBB) {
LLVM_DEBUG(dbgs() << "Instruction mapping changed control flow\n");
MBB = NextInstBB;
End = MBB->end();
OptMode = SaveOptMode;
return false;
// Helper Classes Implementation
MachineInstr &MI, unsigned OpIdx, const TargetRegisterInfo &TRI, Pass &P,
RepairingPlacement::RepairingKind Kind)
// Default is, we are going to insert code to repair OpIdx.
: Kind(Kind), OpIdx(OpIdx),
CanMaterialize(Kind != RepairingKind::Impossible), P(P) {
const MachineOperand &MO = MI.getOperand(OpIdx);
assert(MO.isReg() && "Trying to repair a non-reg operand");
if (Kind != RepairingKind::Insert)
// Repairings for definitions happen after MI, uses happen before.
bool Before = !MO.isDef();
// Check if we are done with MI.
if (!MI.isPHI() && !MI.isTerminator()) {
addInsertPoint(MI, Before);
// We are done with the initialization.
// Now, look for the special cases.
if (MI.isPHI()) {
// - PHI must be the first instructions:
// * Before, we have to split the related incoming edge.
// * After, move the insertion point past the last phi.
if (!Before) {
MachineBasicBlock::iterator It = MI.getParent()->getFirstNonPHI();
if (It != MI.getParent()->end())
addInsertPoint(*It, /*Before*/ true);
addInsertPoint(*(--It), /*Before*/ false);
// We repair a use of a phi, we may need to split the related edge.
MachineBasicBlock &Pred = *MI.getOperand(OpIdx + 1).getMBB();
// Check if we can move the insertion point prior to the
// terminators of the predecessor.
Register Reg = MO.getReg();
MachineBasicBlock::iterator It = Pred.getLastNonDebugInstr();
for (auto Begin = Pred.begin(); It != Begin && It->isTerminator(); --It)
if (It->modifiesRegister(Reg, &TRI)) {
// We cannot hoist the repairing code in the predecessor.
// Split the edge.
addInsertPoint(Pred, *MI.getParent());
// At this point, we can insert in Pred.
// - If It is invalid, Pred is empty and we can insert in Pred
// wherever we want.
// - If It is valid, It is the first non-terminator, insert after It.
if (It == Pred.end())
addInsertPoint(Pred, /*Beginning*/ false);
addInsertPoint(*It, /*Before*/ false);
} else {
// - Terminators must be the last instructions:
// * Before, move the insert point before the first terminator.
// * After, we have to split the outcoming edges.
if (Before) {
// Check whether Reg is defined by any terminator.
MachineBasicBlock::reverse_iterator It = MI;
auto REnd = MI.getParent()->rend();
for (; It != REnd && It->isTerminator(); ++It) {
assert(!It->modifiesRegister(MO.getReg(), &TRI) &&
"copy insertion in middle of terminators not handled");
if (It == REnd) {
addInsertPoint(*MI.getParent()->begin(), true);
// We are sure to be right before the first terminator.
addInsertPoint(*It, /*Before*/ false);
// Make sure Reg is not redefined by other terminators, otherwise
// we do not know how to split.
for (MachineBasicBlock::iterator It = MI, End = MI.getParent()->end();
++It != End;)
// The machine verifier should reject this kind of code.
assert(It->modifiesRegister(MO.getReg(), &TRI) &&
"Do not know where to split");
// Split each outcoming edges.
MachineBasicBlock &Src = *MI.getParent();
for (auto &Succ : Src.successors())
addInsertPoint(Src, Succ);
void RegBankSelect::RepairingPlacement::addInsertPoint(MachineInstr &MI,
bool Before) {
addInsertPoint(*new InstrInsertPoint(MI, Before));
void RegBankSelect::RepairingPlacement::addInsertPoint(MachineBasicBlock &MBB,
bool Beginning) {
addInsertPoint(*new MBBInsertPoint(MBB, Beginning));
void RegBankSelect::RepairingPlacement::addInsertPoint(MachineBasicBlock &Src,
MachineBasicBlock &Dst) {
addInsertPoint(*new EdgeInsertPoint(Src, Dst, P));
void RegBankSelect::RepairingPlacement::addInsertPoint(
RegBankSelect::InsertPoint &Point) {
CanMaterialize &= Point.canMaterialize();
HasSplit |= Point.isSplit();
RegBankSelect::InstrInsertPoint::InstrInsertPoint(MachineInstr &Instr,
bool Before)
: InsertPoint(), Instr(Instr), Before(Before) {
// Since we do not support splitting, we do not need to update
// liveness and such, so do not do anything with P.
assert((!Before || !Instr.isPHI()) &&
"Splitting before phis requires more points");
assert((!Before || !Instr.getNextNode() || !Instr.getNextNode()->isPHI()) &&
"Splitting between phis does not make sense");
void RegBankSelect::InstrInsertPoint::materialize() {
if (isSplit()) {
// Slice and return the beginning of the new block.
// If we need to split between the terminators, we theoritically
// need to know where the first and second set of terminators end
// to update the successors properly.
// Now, in pratice, we should have a maximum of 2 branch
// instructions; one conditional and one unconditional. Therefore
// we know how to update the successor by looking at the target of
// the unconditional branch.
// If we end up splitting at some point, then, we should update
// the liveness information and such. I.e., we would need to
// access P here.
// The machine verifier should actually make sure such cases
// cannot happen.
llvm_unreachable("Not yet implemented");
// Otherwise the insertion point is just the current or next
// instruction depending on Before. I.e., there is nothing to do
// here.
bool RegBankSelect::InstrInsertPoint::isSplit() const {
// If the insertion point is after a terminator, we need to split.
if (!Before)
return Instr.isTerminator();
// If we insert before an instruction that is after a terminator,
// we are still after a terminator.
return Instr.getPrevNode() && Instr.getPrevNode()->isTerminator();
uint64_t RegBankSelect::InstrInsertPoint::frequency(const Pass &P) const {
// Even if we need to split, because we insert between terminators,
// this split has actually the same frequency as the instruction.
const MachineBlockFrequencyInfo *MBFI =
if (!MBFI)
return 1;
return MBFI->getBlockFreq(Instr.getParent()).getFrequency();
uint64_t RegBankSelect::MBBInsertPoint::frequency(const Pass &P) const {
const MachineBlockFrequencyInfo *MBFI =
if (!MBFI)
return 1;
return MBFI->getBlockFreq(&MBB).getFrequency();
void RegBankSelect::EdgeInsertPoint::materialize() {
// If we end up repairing twice at the same place before materializing the
// insertion point, we may think we have to split an edge twice.
// We should have a factory for the insert point such that identical points
// are the same instance.
assert(Src.isSuccessor(DstOrSplit) && DstOrSplit->isPredecessor(&Src) &&
"This point has already been split");
MachineBasicBlock *NewBB = Src.SplitCriticalEdge(DstOrSplit, P);
assert(NewBB && "Invalid call to materialize");
// We reuse the destination block to hold the information of the new block.
DstOrSplit = NewBB;
uint64_t RegBankSelect::EdgeInsertPoint::frequency(const Pass &P) const {
const MachineBlockFrequencyInfo *MBFI =
if (!MBFI)
return 1;
if (WasMaterialized)
return MBFI->getBlockFreq(DstOrSplit).getFrequency();
const MachineBranchProbabilityInfo *MBPI =
if (!MBPI)
return 1;
// The basic block will be on the edge.
return (MBFI->getBlockFreq(&Src) * MBPI->getEdgeProbability(&Src, DstOrSplit))
bool RegBankSelect::EdgeInsertPoint::canMaterialize() const {
// If this is not a critical edge, we should not have used this insert
// point. Indeed, either the successor or the predecessor should
// have do.
assert(Src.succ_size() > 1 && DstOrSplit->pred_size() > 1 &&
"Edge is not critical");
return Src.canSplitCriticalEdge(DstOrSplit);
RegBankSelect::MappingCost::MappingCost(const BlockFrequency &LocalFreq)
: LocalFreq(LocalFreq.getFrequency()) {}
bool RegBankSelect::MappingCost::addLocalCost(uint64_t Cost) {
// Check if this overflows.
if (LocalCost + Cost < LocalCost) {
return true;
LocalCost += Cost;
return isSaturated();
bool RegBankSelect::MappingCost::addNonLocalCost(uint64_t Cost) {
// Check if this overflows.
if (NonLocalCost + Cost < NonLocalCost) {
return true;
NonLocalCost += Cost;
return isSaturated();
bool RegBankSelect::MappingCost::isSaturated() const {
return LocalCost == UINT64_MAX - 1 && NonLocalCost == UINT64_MAX &&
LocalFreq == UINT64_MAX;
void RegBankSelect::MappingCost::saturate() {
*this = ImpossibleCost();
RegBankSelect::MappingCost RegBankSelect::MappingCost::ImpossibleCost() {
return MappingCost(UINT64_MAX, UINT64_MAX, UINT64_MAX);
bool RegBankSelect::MappingCost::operator<(const MappingCost &Cost) const {
// Sort out the easy cases.
if (*this == Cost)
return false;
// If one is impossible to realize the other is cheaper unless it is
// impossible as well.
if ((*this == ImpossibleCost()) || (Cost == ImpossibleCost()))
return (*this == ImpossibleCost()) < (Cost == ImpossibleCost());
// If one is saturated the other is cheaper, unless it is saturated
// as well.
if (isSaturated() || Cost.isSaturated())
return isSaturated() < Cost.isSaturated();
// At this point we know both costs hold sensible values.
// If both values have a different base frequency, there is no much
// we can do but to scale everything.
// However, if they have the same base frequency we can avoid making
// complicated computation.
uint64_t ThisLocalAdjust;
uint64_t OtherLocalAdjust;
if (LLVM_LIKELY(LocalFreq == Cost.LocalFreq)) {
// At this point, we know the local costs are comparable.
// Do the case that do not involve potential overflow first.
if (NonLocalCost == Cost.NonLocalCost)
// Since the non-local costs do not discriminate on the result,
// just compare the local costs.
return LocalCost < Cost.LocalCost;
// The base costs are comparable so we may only keep the relative
// value to increase our chances of avoiding overflows.
ThisLocalAdjust = 0;
OtherLocalAdjust = 0;
if (LocalCost < Cost.LocalCost)
OtherLocalAdjust = Cost.LocalCost - LocalCost;
ThisLocalAdjust = LocalCost - Cost.LocalCost;
} else {
ThisLocalAdjust = LocalCost;
OtherLocalAdjust = Cost.LocalCost;
// The non-local costs are comparable, just keep the relative value.
uint64_t ThisNonLocalAdjust = 0;
uint64_t OtherNonLocalAdjust = 0;
if (NonLocalCost < Cost.NonLocalCost)
OtherNonLocalAdjust = Cost.NonLocalCost - NonLocalCost;
ThisNonLocalAdjust = NonLocalCost - Cost.NonLocalCost;
// Scale everything to make them comparable.
uint64_t ThisScaledCost = ThisLocalAdjust * LocalFreq;
// Check for overflow on that operation.
bool ThisOverflows = ThisLocalAdjust && (ThisScaledCost < ThisLocalAdjust ||
ThisScaledCost < LocalFreq);
uint64_t OtherScaledCost = OtherLocalAdjust * Cost.LocalFreq;
// Check for overflow on the last operation.
bool OtherOverflows =
OtherLocalAdjust &&
(OtherScaledCost < OtherLocalAdjust || OtherScaledCost < Cost.LocalFreq);
// Add the non-local costs.
ThisOverflows |= ThisNonLocalAdjust &&
ThisScaledCost + ThisNonLocalAdjust < ThisNonLocalAdjust;
ThisScaledCost += ThisNonLocalAdjust;
OtherOverflows |= OtherNonLocalAdjust &&
OtherScaledCost + OtherNonLocalAdjust < OtherNonLocalAdjust;
OtherScaledCost += OtherNonLocalAdjust;
// If both overflows, we cannot compare without additional
// precision, e.g., APInt. Just give up on that case.
if (ThisOverflows && OtherOverflows)
return false;
// If one overflows but not the other, we can still compare.
if (ThisOverflows || OtherOverflows)
return ThisOverflows < OtherOverflows;
// Otherwise, just compare the values.
return ThisScaledCost < OtherScaledCost;
bool RegBankSelect::MappingCost::operator==(const MappingCost &Cost) const {
return LocalCost == Cost.LocalCost && NonLocalCost == Cost.NonLocalCost &&
LocalFreq == Cost.LocalFreq;
#if !defined(NDEBUG) || defined(LLVM_ENABLE_DUMP)
LLVM_DUMP_METHOD void RegBankSelect::MappingCost::dump() const {
dbgs() << '\n';
void RegBankSelect::MappingCost::print(raw_ostream &OS) const {
if (*this == ImpossibleCost()) {
OS << "impossible";
if (isSaturated()) {
OS << "saturated";
OS << LocalFreq << " * " << LocalCost << " + " << NonLocalCost;