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//===-- LiveRangeEdit.cpp - Basic tools for editing a register live range -===//
// 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
// The LiveRangeEdit class represents changes done to a virtual register when it
// is spilled or split.
#include "llvm/CodeGen/LiveRangeEdit.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/CodeGen/CalcSpillWeights.h"
#include "llvm/CodeGen/LiveIntervals.h"
#include "llvm/CodeGen/MachineRegisterInfo.h"
#include "llvm/CodeGen/TargetInstrInfo.h"
#include "llvm/CodeGen/VirtRegMap.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/raw_ostream.h"
using namespace llvm;
#define DEBUG_TYPE "regalloc"
STATISTIC(NumDCEDeleted, "Number of instructions deleted by DCE");
STATISTIC(NumDCEFoldedLoads, "Number of single use loads folded after DCE");
STATISTIC(NumFracRanges, "Number of live ranges fractured by DCE");
STATISTIC(NumReMaterialization, "Number of instructions rematerialized");
void LiveRangeEdit::Delegate::anchor() { }
LiveInterval &LiveRangeEdit::createEmptyIntervalFrom(Register OldReg,
bool createSubRanges) {
Register VReg = MRI.cloneVirtualRegister(OldReg);
if (VRM)
VRM->setIsSplitFromReg(VReg, VRM->getOriginal(OldReg));
LiveInterval &LI = LIS.createEmptyInterval(VReg);
if (Parent && !Parent->isSpillable())
if (createSubRanges) {
// Create empty subranges if the OldReg's interval has them. Do not create
// the main range here---it will be constructed later after the subranges
// have been finalized.
LiveInterval &OldLI = LIS.getInterval(OldReg);
VNInfo::Allocator &Alloc = LIS.getVNInfoAllocator();
for (LiveInterval::SubRange &S : OldLI.subranges())
LI.createSubRange(Alloc, S.LaneMask);
return LI;
Register LiveRangeEdit::createFrom(Register OldReg) {
Register VReg = MRI.cloneVirtualRegister(OldReg);
if (VRM) {
VRM->setIsSplitFromReg(VReg, VRM->getOriginal(OldReg));
// FIXME: Getting the interval here actually computes it.
// In theory, this may not be what we want, but in practice
// the createEmptyIntervalFrom API is used when this is not
// the case. Generally speaking we just want to annotate the
// LiveInterval when it gets created but we cannot do that at
// the moment.
if (Parent && !Parent->isSpillable())
return VReg;
bool LiveRangeEdit::checkRematerializable(VNInfo *VNI,
const MachineInstr *DefMI) {
assert(DefMI && "Missing instruction");
ScannedRemattable = true;
if (!TII.isTriviallyReMaterializable(*DefMI))
return false;
return true;
void LiveRangeEdit::scanRemattable() {
for (VNInfo *VNI : getParent().valnos) {
if (VNI->isUnused())
Register Original = VRM->getOriginal(getReg());
LiveInterval &OrigLI = LIS.getInterval(Original);
VNInfo *OrigVNI = OrigLI.getVNInfoAt(VNI->def);
if (!OrigVNI)
MachineInstr *DefMI = LIS.getInstructionFromIndex(OrigVNI->def);
if (!DefMI)
checkRematerializable(OrigVNI, DefMI);
ScannedRemattable = true;
bool LiveRangeEdit::anyRematerializable() {
if (!ScannedRemattable)
return !Remattable.empty();
/// allUsesAvailableAt - Return true if all registers used by OrigMI at
/// OrigIdx are also available with the same value at UseIdx.
bool LiveRangeEdit::allUsesAvailableAt(const MachineInstr *OrigMI,
SlotIndex OrigIdx,
SlotIndex UseIdx) const {
OrigIdx = OrigIdx.getRegSlot(true);
UseIdx = std::max(UseIdx, UseIdx.getRegSlot(true));
for (const MachineOperand &MO : OrigMI->operands()) {
if (!MO.isReg() || !MO.getReg() || !MO.readsReg())
// We can't remat physreg uses, unless it is a constant or target wants
// to ignore this use.
if (MO.getReg().isPhysical()) {
if (MRI.isConstantPhysReg(MO.getReg()) || TII.isIgnorableUse(MO))
return false;
LiveInterval &li = LIS.getInterval(MO.getReg());
const VNInfo *OVNI = li.getVNInfoAt(OrigIdx);
if (!OVNI)
// Don't allow rematerialization immediately after the original def.
// It would be incorrect if OrigMI redefines the register.
// See PR14098.
if (SlotIndex::isSameInstr(OrigIdx, UseIdx))
return false;
if (OVNI != li.getVNInfoAt(UseIdx))
return false;
// Check that subrange is live at UseIdx.
if (li.hasSubRanges()) {
const TargetRegisterInfo *TRI = MRI.getTargetRegisterInfo();
unsigned SubReg = MO.getSubReg();
LaneBitmask LM = SubReg ? TRI->getSubRegIndexLaneMask(SubReg)
: MRI.getMaxLaneMaskForVReg(MO.getReg());
for (LiveInterval::SubRange &SR : li.subranges()) {
if ((SR.LaneMask & LM).none())
if (!SR.liveAt(UseIdx))
return false;
// Early exit if all used lanes are checked. No need to continue.
LM &= ~SR.LaneMask;
if (LM.none())
return true;
bool LiveRangeEdit::canRematerializeAt(Remat &RM, VNInfo *OrigVNI,
SlotIndex UseIdx, bool cheapAsAMove) {
assert(ScannedRemattable && "Call anyRematerializable first");
// Use scanRemattable info.
if (!Remattable.count(OrigVNI))
return false;
// No defining instruction provided.
SlotIndex DefIdx;
assert(RM.OrigMI && "No defining instruction for remattable value");
DefIdx = LIS.getInstructionIndex(*RM.OrigMI);
// If only cheap remats were requested, bail out early.
if (cheapAsAMove && !TII.isAsCheapAsAMove(*RM.OrigMI))
return false;
// Verify that all used registers are available with the same values.
if (!allUsesAvailableAt(RM.OrigMI, DefIdx, UseIdx))
return false;
return true;
SlotIndex LiveRangeEdit::rematerializeAt(MachineBasicBlock &MBB,
MachineBasicBlock::iterator MI,
Register DestReg, const Remat &RM,
const TargetRegisterInfo &tri,
bool Late, unsigned SubIdx,
MachineInstr *ReplaceIndexMI) {
assert(RM.OrigMI && "Invalid remat");
TII.reMaterialize(MBB, MI, DestReg, SubIdx, *RM.OrigMI, tri);
// DestReg of the cloned instruction cannot be Dead. Set isDead of DestReg
// to false anyway in case the isDead flag of RM.OrigMI's dest register
// is true.
if (ReplaceIndexMI)
return LIS.ReplaceMachineInstrInMaps(*ReplaceIndexMI, *MI).getRegSlot();
return LIS.getSlotIndexes()->insertMachineInstrInMaps(*MI, Late).getRegSlot();
void LiveRangeEdit::eraseVirtReg(Register Reg) {
if (TheDelegate && TheDelegate->LRE_CanEraseVirtReg(Reg))
bool LiveRangeEdit::foldAsLoad(LiveInterval *LI,
SmallVectorImpl<MachineInstr*> &Dead) {
MachineInstr *DefMI = nullptr, *UseMI = nullptr;
// Check that there is a single def and a single use.
for (MachineOperand &MO : MRI.reg_nodbg_operands(LI->reg())) {
MachineInstr *MI = MO.getParent();
if (MO.isDef()) {
if (DefMI && DefMI != MI)
return false;
if (!MI->canFoldAsLoad())
return false;
DefMI = MI;
} else if (!MO.isUndef()) {
if (UseMI && UseMI != MI)
return false;
// FIXME: Targets don't know how to fold subreg uses.
if (MO.getSubReg())
return false;
UseMI = MI;
if (!DefMI || !UseMI)
return false;
// Since we're moving the DefMI load, make sure we're not extending any live
// ranges.
if (!allUsesAvailableAt(DefMI, LIS.getInstructionIndex(*DefMI),
return false;
// We also need to make sure it is safe to move the load.
// Assume there are stores between DefMI and UseMI.
bool SawStore = true;
if (!DefMI->isSafeToMove(nullptr, SawStore))
return false;
LLVM_DEBUG(dbgs() << "Try to fold single def: " << *DefMI
<< " into single use: " << *UseMI);
SmallVector<unsigned, 8> Ops;
if (UseMI->readsWritesVirtualRegister(LI->reg(), &Ops).second)
return false;
MachineInstr *FoldMI = TII.foldMemoryOperand(*UseMI, Ops, *DefMI, &LIS);
if (!FoldMI)
return false;
LLVM_DEBUG(dbgs() << " folded: " << *FoldMI);
LIS.ReplaceMachineInstrInMaps(*UseMI, *FoldMI);
// Update the call site info.
if (UseMI->shouldUpdateCallSiteInfo())
UseMI->getMF()->moveCallSiteInfo(UseMI, FoldMI);
DefMI->addRegisterDead(LI->reg(), nullptr);
return true;
bool LiveRangeEdit::useIsKill(const LiveInterval &LI,
const MachineOperand &MO) const {
const MachineInstr &MI = *MO.getParent();
SlotIndex Idx = LIS.getInstructionIndex(MI).getRegSlot();
if (LI.Query(Idx).isKill())
return true;
const TargetRegisterInfo &TRI = *MRI.getTargetRegisterInfo();
unsigned SubReg = MO.getSubReg();
LaneBitmask LaneMask = TRI.getSubRegIndexLaneMask(SubReg);
for (const LiveInterval::SubRange &S : LI.subranges()) {
if ((S.LaneMask & LaneMask).any() && S.Query(Idx).isKill())
return true;
return false;
/// Find all live intervals that need to shrink, then remove the instruction.
void LiveRangeEdit::eliminateDeadDef(MachineInstr *MI, ToShrinkSet &ToShrink) {
assert(MI->allDefsAreDead() && "Def isn't really dead");
SlotIndex Idx = LIS.getInstructionIndex(*MI).getRegSlot();
// Never delete a bundled instruction.
if (MI->isBundled()) {
// Never delete inline asm.
if (MI->isInlineAsm()) {
LLVM_DEBUG(dbgs() << "Won't delete: " << Idx << '\t' << *MI);
// Use the same criteria as DeadMachineInstructionElim.
bool SawStore = false;
if (!MI->isSafeToMove(nullptr, SawStore)) {
LLVM_DEBUG(dbgs() << "Can't delete: " << Idx << '\t' << *MI);
LLVM_DEBUG(dbgs() << "Deleting dead def " << Idx << '\t' << *MI);
// Collect virtual registers to be erased after MI is gone.
SmallVector<Register, 8> RegsToErase;
bool ReadsPhysRegs = false;
bool isOrigDef = false;
Register Dest;
unsigned DestSubReg;
// Only optimize rematerialize case when the instruction has one def, since
// otherwise we could leave some dead defs in the code. This case is
// extremely rare.
if (VRM && MI->getOperand(0).isReg() && MI->getOperand(0).isDef() &&
MI->getDesc().getNumDefs() == 1) {
Dest = MI->getOperand(0).getReg();
DestSubReg = MI->getOperand(0).getSubReg();
Register Original = VRM->getOriginal(Dest);
LiveInterval &OrigLI = LIS.getInterval(Original);
VNInfo *OrigVNI = OrigLI.getVNInfoAt(Idx);
// The original live-range may have been shrunk to
// an empty live-range. It happens when it is dead, but
// we still keep it around to be able to rematerialize
// other values that depend on it.
if (OrigVNI)
isOrigDef = SlotIndex::isSameInstr(OrigVNI->def, Idx);
bool HasLiveVRegUses = false;
// Check for live intervals that may shrink
for (const MachineOperand &MO : MI->operands()) {
if (!MO.isReg())
Register Reg = MO.getReg();
if (!Reg.isVirtual()) {
// Check if MI reads any unreserved physregs.
if (Reg && MO.readsReg() && !MRI.isReserved(Reg))
ReadsPhysRegs = true;
else if (MO.isDef())
LIS.removePhysRegDefAt(Reg.asMCReg(), Idx);
LiveInterval &LI = LIS.getInterval(Reg);
// Shrink read registers, unless it is likely to be expensive and
// unlikely to change anything. We typically don't want to shrink the
// PIC base register that has lots of uses everywhere.
// Always shrink COPY uses that probably come from live range splitting.
if ((MI->readsVirtualRegister(Reg) && (MI->isCopy() || MO.isDef())) ||
(MO.readsReg() && (MRI.hasOneNonDBGUse(Reg) || useIsKill(LI, MO))))
else if (MO.readsReg())
HasLiveVRegUses = true;
// Remove defined value.
if (MO.isDef()) {
if (TheDelegate && LI.getVNInfoAt(Idx) != nullptr)
LIS.removeVRegDefAt(LI, Idx);
if (LI.empty())
// Currently, we don't support DCE of physreg live ranges. If MI reads
// any unreserved physregs, don't erase the instruction, but turn it into
// a KILL instead. This way, the physreg live ranges don't end up
// dangling.
// FIXME: It would be better to have something like shrinkToUses() for
// physregs. That could potentially enable more DCE and it would free up
// the physreg. It would not happen often, though.
if (ReadsPhysRegs) {
// Remove all operands that aren't physregs.
for (unsigned i = MI->getNumOperands(); i; --i) {
const MachineOperand &MO = MI->getOperand(i-1);
if (MO.isReg() && MO.getReg().isPhysical())
LLVM_DEBUG(dbgs() << "Converted physregs to:\t" << *MI);
} else {
// If the dest of MI is an original reg and MI is reMaterializable,
// don't delete the inst. Replace the dest with a new reg, and keep
// the inst for remat of other siblings. The inst is saved in
// LiveRangeEdit::DeadRemats and will be deleted after all the
// allocations of the func are done.
// However, immediately delete instructions which have unshrunk virtual
// register uses. That may provoke RA to split an interval at the KILL
// and later result in an invalid live segment end.
if (isOrigDef && DeadRemats && !HasLiveVRegUses &&
TII.isTriviallyReMaterializable(*MI)) {
LiveInterval &NewLI = createEmptyIntervalFrom(Dest, false);
VNInfo::Allocator &Alloc = LIS.getVNInfoAllocator();
VNInfo *VNI = NewLI.getNextValue(Idx, Alloc);
NewLI.addSegment(LiveInterval::Segment(Idx, Idx.getDeadSlot(), VNI));
if (DestSubReg) {
const TargetRegisterInfo *TRI = MRI.getTargetRegisterInfo();
auto *SR = NewLI.createSubRange(
Alloc, TRI->getSubRegIndexLaneMask(DestSubReg));
SR->addSegment(LiveInterval::Segment(Idx, Idx.getDeadSlot(),
SR->getNextValue(Idx, Alloc)));
const TargetRegisterInfo &TRI = *MRI.getTargetRegisterInfo();
MI->substituteRegister(Dest, NewLI.reg(), 0, TRI);
} else {
if (TheDelegate)
// Erase any virtregs that are now empty and unused. There may be <undef>
// uses around. Keep the empty live range in that case.
for (unsigned i = 0, e = RegsToErase.size(); i != e; ++i) {
Register Reg = RegsToErase[i];
if (LIS.hasInterval(Reg) && MRI.reg_nodbg_empty(Reg)) {
void LiveRangeEdit::eliminateDeadDefs(SmallVectorImpl<MachineInstr *> &Dead,
ArrayRef<Register> RegsBeingSpilled) {
ToShrinkSet ToShrink;
for (;;) {
// Erase all dead defs.
while (!Dead.empty())
eliminateDeadDef(Dead.pop_back_val(), ToShrink);
if (ToShrink.empty())
// Shrink just one live interval. Then delete new dead defs.
LiveInterval *LI = ToShrink.pop_back_val();
if (foldAsLoad(LI, Dead))
Register VReg = LI->reg();
if (TheDelegate)
if (!LIS.shrinkToUses(LI, &Dead))
// Don't create new intervals for a register being spilled.
// The new intervals would have to be spilled anyway so its not worth it.
// Also they currently aren't spilled so creating them and not spilling
// them results in incorrect code.
if (llvm::is_contained(RegsBeingSpilled, VReg))
// LI may have been separated, create new intervals.
SmallVector<LiveInterval*, 8> SplitLIs;
LIS.splitSeparateComponents(*LI, SplitLIs);
if (!SplitLIs.empty())
Register Original = VRM ? VRM->getOriginal(VReg) : Register();
for (const LiveInterval *SplitLI : SplitLIs) {
// If LI is an original interval that hasn't been split yet, make the new
// intervals their own originals instead of referring to LI. The original
// interval must contain all the split products, and LI doesn't.
if (Original != VReg && Original != 0)
VRM->setIsSplitFromReg(SplitLI->reg(), Original);
if (TheDelegate)
TheDelegate->LRE_DidCloneVirtReg(SplitLI->reg(), VReg);
// Keep track of new virtual registers created via
// MachineRegisterInfo::createVirtualRegister.
LiveRangeEdit::MRI_NoteNewVirtualRegister(Register VReg) {
if (VRM)
void LiveRangeEdit::calculateRegClassAndHint(MachineFunction &MF,
VirtRegAuxInfo &VRAI) {
for (unsigned I = 0, Size = size(); I < Size; ++I) {
LiveInterval &LI = LIS.getInterval(get(I));
if (MRI.recomputeRegClass(LI.reg()))
const TargetRegisterInfo *TRI = MF.getSubtarget().getRegisterInfo();
dbgs() << "Inflated " << printReg(LI.reg()) << " to "
<< TRI->getRegClassName(MRI.getRegClass(LI.reg())) << '\n';