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//===- LiveIntervalCalc.cpp - Calculate live interval --------------------===//
// 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
// Implementation of the LiveIntervalCalc class.
#include "llvm/CodeGen/LiveIntervalCalc.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/CodeGen/LiveInterval.h"
#include "llvm/CodeGen/MachineInstr.h"
#include "llvm/CodeGen/MachineOperand.h"
#include "llvm/CodeGen/MachineRegisterInfo.h"
#include "llvm/CodeGen/SlotIndexes.h"
#include "llvm/CodeGen/TargetRegisterInfo.h"
#include "llvm/MC/LaneBitmask.h"
#include "llvm/Support/ErrorHandling.h"
#include <cassert>
using namespace llvm;
#define DEBUG_TYPE "regalloc"
// Reserve an address that indicates a value that is known to be "undef".
static VNInfo UndefVNI(0xbad, SlotIndex());
static void createDeadDef(SlotIndexes &Indexes, VNInfo::Allocator &Alloc,
LiveRange &LR, const MachineOperand &MO) {
const MachineInstr &MI = *MO.getParent();
SlotIndex DefIdx =
// Create the def in LR. This may find an existing def.
LR.createDeadDef(DefIdx, Alloc);
void LiveIntervalCalc::calculate(LiveInterval &LI, bool TrackSubRegs) {
const MachineRegisterInfo *MRI = getRegInfo();
SlotIndexes *Indexes = getIndexes();
VNInfo::Allocator *Alloc = getVNAlloc();
assert(MRI && Indexes && "call reset() first");
// Step 1: Create minimal live segments for every definition of Reg.
// Visit all def operands. If the same instruction has multiple defs of Reg,
// createDeadDef() will deduplicate.
const TargetRegisterInfo &TRI = *MRI->getTargetRegisterInfo();
Register Reg = LI.reg();
for (const MachineOperand &MO : MRI->reg_nodbg_operands(Reg)) {
if (!MO.isDef() && !MO.readsReg())
unsigned SubReg = MO.getSubReg();
if (LI.hasSubRanges() || (SubReg != 0 && TrackSubRegs)) {
LaneBitmask SubMask = SubReg != 0 ? TRI.getSubRegIndexLaneMask(SubReg)
: MRI->getMaxLaneMaskForVReg(Reg);
// If this is the first time we see a subregister def, initialize
// subranges by creating a copy of the main range.
if (!LI.hasSubRanges() && !LI.empty()) {
LaneBitmask ClassMask = MRI->getMaxLaneMaskForVReg(Reg);
LI.createSubRangeFrom(*Alloc, ClassMask, LI);
*Alloc, SubMask,
[&MO, Indexes, Alloc](LiveInterval::SubRange &SR) {
if (MO.isDef())
createDeadDef(*Indexes, *Alloc, SR, MO);
*Indexes, TRI);
// Create the def in the main liverange. We do not have to do this if
// subranges are tracked as we recreate the main range later in this case.
if (MO.isDef() && !LI.hasSubRanges())
createDeadDef(*Indexes, *Alloc, LI, MO);
// We may have created empty live ranges for partially undefined uses, we
// can't keep them because we won't find defs in them later.
const MachineFunction *MF = getMachineFunction();
MachineDominatorTree *DomTree = getDomTree();
// Step 2: Extend live segments to all uses, constructing SSA form as
// necessary.
if (LI.hasSubRanges()) {
for (LiveInterval::SubRange &S : LI.subranges()) {
LiveIntervalCalc SubLIC;
SubLIC.reset(MF, Indexes, DomTree, Alloc);
SubLIC.extendToUses(S, Reg, S.LaneMask, &LI);
} else {
extendToUses(LI, Reg, LaneBitmask::getAll());
void LiveIntervalCalc::constructMainRangeFromSubranges(LiveInterval &LI) {
// First create dead defs at all defs found in subranges.
LiveRange &MainRange = LI;
assert(MainRange.segments.empty() && MainRange.valnos.empty() &&
"Expect empty main liverange");
VNInfo::Allocator *Alloc = getVNAlloc();
for (const LiveInterval::SubRange &SR : LI.subranges()) {
for (const VNInfo *VNI : SR.valnos) {
if (!VNI->isUnused() && !VNI->isPHIDef())
MainRange.createDeadDef(VNI->def, *Alloc);
extendToUses(MainRange, LI.reg(), LaneBitmask::getAll(), &LI);
void LiveIntervalCalc::createDeadDefs(LiveRange &LR, Register Reg) {
const MachineRegisterInfo *MRI = getRegInfo();
SlotIndexes *Indexes = getIndexes();
VNInfo::Allocator *Alloc = getVNAlloc();
assert(MRI && Indexes && "call reset() first");
// Visit all def operands. If the same instruction has multiple defs of Reg,
// LR.createDeadDef() will deduplicate.
for (MachineOperand &MO : MRI->def_operands(Reg))
createDeadDef(*Indexes, *Alloc, LR, MO);
void LiveIntervalCalc::extendToUses(LiveRange &LR, Register Reg,
LaneBitmask Mask, LiveInterval *LI) {
const MachineRegisterInfo *MRI = getRegInfo();
SlotIndexes *Indexes = getIndexes();
SmallVector<SlotIndex, 4> Undefs;
if (LI != nullptr)
LI->computeSubRangeUndefs(Undefs, Mask, *MRI, *Indexes);
// Visit all operands that read Reg. This may include partial defs.
bool IsSubRange = !Mask.all();
const TargetRegisterInfo &TRI = *MRI->getTargetRegisterInfo();
for (MachineOperand &MO : MRI->reg_nodbg_operands(Reg)) {
// Clear all kill flags. They will be reinserted after register allocation
// by LiveIntervals::addKillFlags().
if (MO.isUse())
// MO::readsReg returns "true" for subregister defs. This is for keeping
// liveness of the entire register (i.e. for the main range of the live
// interval). For subranges, definitions of non-overlapping subregisters
// do not count as uses.
if (!MO.readsReg() || (IsSubRange && MO.isDef()))
unsigned SubReg = MO.getSubReg();
if (SubReg != 0) {
LaneBitmask SLM = TRI.getSubRegIndexLaneMask(SubReg);
if (MO.isDef())
// Ignore uses not reading the current (sub)range.
if ((SLM & Mask).none())
// Determine the actual place of the use.
const MachineInstr *MI = MO.getParent();
unsigned OpNo = (&MO - &MI->getOperand(0));
SlotIndex UseIdx;
if (MI->isPHI()) {
assert(!MO.isDef() && "Cannot handle PHI def of partial register.");
// The actual place where a phi operand is used is the end of the pred
// MBB. PHI operands are paired: (Reg, PredMBB).
UseIdx = Indexes->getMBBEndIdx(MI->getOperand(OpNo + 1).getMBB());
} else {
// Check for early-clobber redefs.
bool isEarlyClobber = false;
unsigned DefIdx;
if (MO.isDef())
isEarlyClobber = MO.isEarlyClobber();
else if (MI->isRegTiedToDefOperand(OpNo, &DefIdx)) {
// FIXME: This would be a lot easier if tied early-clobber uses also
// had an early-clobber flag.
isEarlyClobber = MI->getOperand(DefIdx).isEarlyClobber();
UseIdx = Indexes->getInstructionIndex(*MI).getRegSlot(isEarlyClobber);
// MI is reading Reg. We may have visited MI before if it happens to be
// reading Reg multiple times. That is OK, extend() is idempotent.
extend(LR, UseIdx, Reg, Undefs);