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//===- LiveRangeCalc.cpp - Calculate live ranges -------------------------===//
// 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 LiveRangeCalc class.
#include "llvm/CodeGen/LiveRangeCalc.h"
#include "llvm/ADT/BitVector.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/ADT/SetVector.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/CodeGen/LiveInterval.h"
#include "llvm/CodeGen/MachineBasicBlock.h"
#include "llvm/CodeGen/MachineDominators.h"
#include "llvm/CodeGen/MachineFunction.h"
#include "llvm/CodeGen/MachineInstr.h"
#include "llvm/CodeGen/MachineRegisterInfo.h"
#include "llvm/CodeGen/SlotIndexes.h"
#include "llvm/CodeGen/TargetRegisterInfo.h"
#include "llvm/Support/ErrorHandling.h"
#include "llvm/Support/raw_ostream.h"
#include <algorithm>
#include <cassert>
#include <iterator>
#include <tuple>
#include <utility>
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());
void LiveRangeCalc::resetLiveOutMap() {
unsigned NumBlocks = MF->getNumBlockIDs();
void LiveRangeCalc::reset(const MachineFunction *mf,
SlotIndexes *SI,
MachineDominatorTree *MDT,
VNInfo::Allocator *VNIA) {
MF = mf;
MRI = &MF->getRegInfo();
Indexes = SI;
DomTree = MDT;
Alloc = VNIA;
void LiveRangeCalc::updateFromLiveIns() {
LiveRangeUpdater Updater;
for (const LiveInBlock &I : LiveIn) {
if (!I.DomNode)
MachineBasicBlock *MBB = I.DomNode->getBlock();
assert(I.Value && "No live-in value found");
SlotIndex Start, End;
std::tie(Start, End) = Indexes->getMBBRange(MBB);
if (I.Kill.isValid())
// Value is killed inside this block.
End = I.Kill;
else {
// The value is live-through, update LiveOut as well.
// Defer the Domtree lookup until it is needed.
Map[MBB] = LiveOutPair(I.Value, nullptr);
Updater.add(Start, End, I.Value);
void LiveRangeCalc::extend(LiveRange &LR, SlotIndex Use, unsigned PhysReg,
ArrayRef<SlotIndex> Undefs) {
assert(Use.isValid() && "Invalid SlotIndex");
assert(Indexes && "Missing SlotIndexes");
assert(DomTree && "Missing dominator tree");
MachineBasicBlock *UseMBB = Indexes->getMBBFromIndex(Use.getPrevSlot());
assert(UseMBB && "No MBB at Use");
// Is there a def in the same MBB we can extend?
auto EP = LR.extendInBlock(Undefs, Indexes->getMBBStartIdx(UseMBB), Use);
if (EP.first != nullptr || EP.second)
// Find the single reaching def, or determine if Use is jointly dominated by
// multiple values, and we may need to create even more phi-defs to preserve
// VNInfo SSA form. Perform a search for all predecessor blocks where we
// know the dominating VNInfo.
if (findReachingDefs(LR, *UseMBB, Use, PhysReg, Undefs))
// When there were multiple different values, we may need new PHIs.
// This function is called by a client after using the low-level API to add
// live-out and live-in blocks. The unique value optimization is not
// available, SplitEditor::transferValues handles that case directly anyway.
void LiveRangeCalc::calculateValues() {
assert(Indexes && "Missing SlotIndexes");
assert(DomTree && "Missing dominator tree");
bool LiveRangeCalc::isDefOnEntry(LiveRange &LR, ArrayRef<SlotIndex> Undefs,
MachineBasicBlock &MBB, BitVector &DefOnEntry,
BitVector &UndefOnEntry) {
unsigned BN = MBB.getNumber();
if (DefOnEntry[BN])
return true;
if (UndefOnEntry[BN])
return false;
auto MarkDefined = [BN, &DefOnEntry](MachineBasicBlock &B) -> bool {
for (MachineBasicBlock *S : B.successors())
DefOnEntry[S->getNumber()] = true;
DefOnEntry[BN] = true;
return true;
SetVector<unsigned> WorkList;
// Checking if the entry of MBB is reached by some def: add all predecessors
// that are potentially defined-on-exit to the work list.
for (MachineBasicBlock *P : MBB.predecessors())
for (unsigned i = 0; i != WorkList.size(); ++i) {
// Determine if the exit from the block is reached by some def.
unsigned N = WorkList[i];
MachineBasicBlock &B = *MF->getBlockNumbered(N);
if (Seen[N]) {
const LiveOutPair &LOB = Map[&B];
if (LOB.first != nullptr && LOB.first != &UndefVNI)
return MarkDefined(B);
SlotIndex Begin, End;
std::tie(Begin, End) = Indexes->getMBBRange(&B);
// Treat End as not belonging to B.
// If LR has a segment S that starts at the next block, i.e. [End, ...),
// std::upper_bound will return the segment following S. Instead,
// S should be treated as the first segment that does not overlap B.
LiveRange::iterator UB = upper_bound(LR, End.getPrevSlot());
if (UB != LR.begin()) {
LiveRange::Segment &Seg = *std::prev(UB);
if (Seg.end > Begin) {
// There is a segment that overlaps B. If the range is not explicitly
// undefined between the end of the segment and the end of the block,
// treat the block as defined on exit. If it is, go to the next block
// on the work list.
if (LR.isUndefIn(Undefs, Seg.end, End))
return MarkDefined(B);
// No segment overlaps with this block. If this block is not defined on
// entry, or it undefines the range, do not process its predecessors.
if (UndefOnEntry[N] || LR.isUndefIn(Undefs, Begin, End)) {
UndefOnEntry[N] = true;
if (DefOnEntry[N])
return MarkDefined(B);
// Still don't know: add all predecessors to the work list.
for (MachineBasicBlock *P : B.predecessors())
UndefOnEntry[BN] = true;
return false;
bool LiveRangeCalc::findReachingDefs(LiveRange &LR, MachineBasicBlock &UseMBB,
SlotIndex Use, unsigned PhysReg,
ArrayRef<SlotIndex> Undefs) {
unsigned UseMBBNum = UseMBB.getNumber();
// Block numbers where LR should be live-in.
SmallVector<unsigned, 16> WorkList(1, UseMBBNum);
// Remember if we have seen more than one value.
bool UniqueVNI = true;
VNInfo *TheVNI = nullptr;
bool FoundUndef = false;
// Using Seen as a visited set, perform a BFS for all reaching defs.
for (unsigned i = 0; i != WorkList.size(); ++i) {
MachineBasicBlock *MBB = MF->getBlockNumbered(WorkList[i]);
#ifndef NDEBUG
if (MBB->pred_empty()) {
errs() << "Use of " << printReg(PhysReg, MRI->getTargetRegisterInfo())
<< " does not have a corresponding definition on every path:\n";
const MachineInstr *MI = Indexes->getInstructionFromIndex(Use);
if (MI != nullptr)
errs() << Use << " " << *MI;
report_fatal_error("Use not jointly dominated by defs.");
if (Register::isPhysicalRegister(PhysReg) && !MBB->isLiveIn(PhysReg)) {
const TargetRegisterInfo *TRI = MRI->getTargetRegisterInfo();
errs() << "The register " << printReg(PhysReg, TRI)
<< " needs to be live in to " << printMBBReference(*MBB)
<< ", but is missing from the live-in list.\n";
report_fatal_error("Invalid global physical register");
FoundUndef |= MBB->pred_empty();
for (MachineBasicBlock *Pred : MBB->predecessors()) {
// Is this a known live-out block?
if (Seen.test(Pred->getNumber())) {
if (VNInfo *VNI = Map[Pred].first) {
if (TheVNI && TheVNI != VNI)
UniqueVNI = false;
SlotIndex Start, End;
std::tie(Start, End) = Indexes->getMBBRange(Pred);
// First time we see Pred. Try to determine the live-out value, but set
// it as null if Pred is live-through with an unknown value.
auto EP = LR.extendInBlock(Undefs, Start, End);
VNInfo *VNI = EP.first;
FoundUndef |= EP.second;
setLiveOutValue(Pred, EP.second ? &UndefVNI : VNI);
if (VNI) {
if (TheVNI && TheVNI != VNI)
UniqueVNI = false;
if (VNI || EP.second)
// No, we need a live-in value for Pred as well
if (Pred != &UseMBB)
// Loopback to UseMBB, so value is really live through.
Use = SlotIndex();
FoundUndef |= (TheVNI == nullptr || TheVNI == &UndefVNI);
if (!Undefs.empty() && FoundUndef)
UniqueVNI = false;
// Both updateSSA() and LiveRangeUpdater benefit from ordered blocks, but
// neither require it. Skip the sorting overhead for small updates.
if (WorkList.size() > 4)
array_pod_sort(WorkList.begin(), WorkList.end());
// If a unique reaching def was found, blit in the live ranges immediately.
if (UniqueVNI) {
assert(TheVNI != nullptr && TheVNI != &UndefVNI);
LiveRangeUpdater Updater(&LR);
for (unsigned BN : WorkList) {
SlotIndex Start, End;
std::tie(Start, End) = Indexes->getMBBRange(BN);
// Trim the live range in UseMBB.
if (BN == UseMBBNum && Use.isValid())
End = Use;
Map[MF->getBlockNumbered(BN)] = LiveOutPair(TheVNI, nullptr);
Updater.add(Start, End, TheVNI);
return true;
// Prepare the defined/undefined bit vectors.
EntryInfoMap::iterator Entry;
bool DidInsert;
std::tie(Entry, DidInsert) = EntryInfos.insert(
std::make_pair(&LR, std::make_pair(BitVector(), BitVector())));
if (DidInsert) {
// Initialize newly inserted entries.
unsigned N = MF->getNumBlockIDs();
BitVector &DefOnEntry = Entry->second.first;
BitVector &UndefOnEntry = Entry->second.second;
// Multiple values were found, so transfer the work list to the LiveIn array
// where UpdateSSA will use it as a work list.
for (unsigned BN : WorkList) {
MachineBasicBlock *MBB = MF->getBlockNumbered(BN);
if (!Undefs.empty() &&
!isDefOnEntry(LR, Undefs, *MBB, DefOnEntry, UndefOnEntry))
addLiveInBlock(LR, DomTree->getNode(MBB));
if (MBB == &UseMBB)
LiveIn.back().Kill = Use;
return false;
// This is essentially the same iterative algorithm that SSAUpdater uses,
// except we already have a dominator tree, so we don't have to recompute it.
void LiveRangeCalc::updateSSA() {
assert(Indexes && "Missing SlotIndexes");
assert(DomTree && "Missing dominator tree");
// Interate until convergence.
bool Changed;
do {
Changed = false;
// Propagate live-out values down the dominator tree, inserting phi-defs
// when necessary.
for (LiveInBlock &I : LiveIn) {
MachineDomTreeNode *Node = I.DomNode;
// Skip block if the live-in value has already been determined.
if (!Node)
MachineBasicBlock *MBB = Node->getBlock();
MachineDomTreeNode *IDom = Node->getIDom();
LiveOutPair IDomValue;
// We need a live-in value to a block with no immediate dominator?
// This is probably an unreachable block that has survived somehow.
bool needPHI = !IDom || !Seen.test(IDom->getBlock()->getNumber());
// IDom dominates all of our predecessors, but it may not be their
// immediate dominator. Check if any of them have live-out values that are
// properly dominated by IDom. If so, we need a phi-def here.
if (!needPHI) {
IDomValue = Map[IDom->getBlock()];
// Cache the DomTree node that defined the value.
if (IDomValue.first && IDomValue.first != &UndefVNI &&
!IDomValue.second) {
Map[IDom->getBlock()].second = IDomValue.second =
for (MachineBasicBlock *Pred : MBB->predecessors()) {
LiveOutPair &Value = Map[Pred];
if (!Value.first || Value.first == IDomValue.first)
if (Value.first == &UndefVNI) {
needPHI = true;
// Cache the DomTree node that defined the value.
if (!Value.second)
Value.second =
// This predecessor is carrying something other than IDomValue.
// It could be because IDomValue hasn't propagated yet, or it could be
// because MBB is in the dominance frontier of that value.
if (DomTree->dominates(IDom, Value.second)) {
needPHI = true;
// The value may be live-through even if Kill is set, as can happen when
// we are called from extendRange. In that case LiveOutSeen is true, and
// LiveOut indicates a foreign or missing value.
LiveOutPair &LOP = Map[MBB];
// Create a phi-def if required.
if (needPHI) {
Changed = true;
assert(Alloc && "Need VNInfo allocator to create PHI-defs");
SlotIndex Start, End;
std::tie(Start, End) = Indexes->getMBBRange(MBB);
LiveRange &LR = I.LR;
VNInfo *VNI = LR.getNextValue(Start, *Alloc);
I.Value = VNI;
// This block is done, we know the final value.
I.DomNode = nullptr;
// Add liveness since updateFromLiveIns now skips this node.
if (I.Kill.isValid()) {
if (VNI)
LR.addSegment(LiveInterval::Segment(Start, I.Kill, VNI));
} else {
if (VNI)
LR.addSegment(LiveInterval::Segment(Start, End, VNI));
LOP = LiveOutPair(VNI, Node);
} else if (IDomValue.first && IDomValue.first != &UndefVNI) {
// No phi-def here. Remember incoming value.
I.Value = IDomValue.first;
// If the IDomValue is killed in the block, don't propagate through.
if (I.Kill.isValid())
// Propagate IDomValue if it isn't killed:
// MBB is live-out and doesn't define its own value.
if (LOP.first == IDomValue.first)
Changed = true;
LOP = IDomValue;
} while (Changed);
bool LiveRangeCalc::isJointlyDominated(const MachineBasicBlock *MBB,
ArrayRef<SlotIndex> Defs,
const SlotIndexes &Indexes) {
const MachineFunction &MF = *MBB->getParent();
BitVector DefBlocks(MF.getNumBlockIDs());
for (SlotIndex I : Defs)
SetVector<unsigned> PredQueue;
for (unsigned i = 0; i != PredQueue.size(); ++i) {
unsigned BN = PredQueue[i];
if (DefBlocks[BN])
return true;
const MachineBasicBlock *B = MF.getBlockNumbered(BN);
for (const MachineBasicBlock *P : B->predecessors())
return false;