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llvm-project/llvm/lib/CodeGen/VirtRegMap.cpp

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//===- llvm/CodeGen/VirtRegMap.cpp - Virtual Register Map -----------------===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
//
// This file implements the VirtRegMap class.
//
// It also contains implementations of the Spiller interface, which, given a
// virtual register map and a machine function, eliminates all virtual
// references by replacing them with physical register references - adding spill
// code as necessary.
//
//===----------------------------------------------------------------------===//
#include "llvm/CodeGen/VirtRegMap.h"
#include "LiveDebugVariables.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/CodeGen/LiveInterval.h"
#include "llvm/CodeGen/LiveIntervals.h"
#include "llvm/CodeGen/LiveStacks.h"
#include "llvm/CodeGen/MachineBasicBlock.h"
#include "llvm/CodeGen/MachineFrameInfo.h"
#include "llvm/CodeGen/MachineFunction.h"
#include "llvm/CodeGen/MachineFunctionPass.h"
#include "llvm/CodeGen/MachineInstr.h"
#include "llvm/CodeGen/MachineOperand.h"
#include "llvm/CodeGen/MachineRegisterInfo.h"
#include "llvm/CodeGen/SlotIndexes.h"
#include "llvm/CodeGen/TargetInstrInfo.h"
#include "llvm/CodeGen/TargetOpcodes.h"
#include "llvm/CodeGen/TargetRegisterInfo.h"
#include "llvm/CodeGen/TargetSubtargetInfo.h"
#include "llvm/Config/llvm-config.h"
#include "llvm/MC/LaneBitmask.h"
#include "llvm/Pass.h"
#include "llvm/Support/Compiler.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/raw_ostream.h"
#include <cassert>
#include <iterator>
#include <utility>
using namespace llvm;
#define DEBUG_TYPE "regalloc"
STATISTIC(NumSpillSlots, "Number of spill slots allocated");
STATISTIC(NumIdCopies, "Number of identity moves eliminated after rewriting");
//===----------------------------------------------------------------------===//
// VirtRegMap implementation
//===----------------------------------------------------------------------===//
char VirtRegMap::ID = 0;
INITIALIZE_PASS(VirtRegMap, "virtregmap", "Virtual Register Map", false, false)
bool VirtRegMap::runOnMachineFunction(MachineFunction &mf) {
MRI = &mf.getRegInfo();
TII = mf.getSubtarget().getInstrInfo();
TRI = mf.getSubtarget().getRegisterInfo();
MF = &mf;
Virt2PhysMap.clear();
Virt2StackSlotMap.clear();
Virt2SplitMap.clear();
grow();
return false;
}
void VirtRegMap::grow() {
unsigned NumRegs = MF->getRegInfo().getNumVirtRegs();
Virt2PhysMap.resize(NumRegs);
Virt2StackSlotMap.resize(NumRegs);
Virt2SplitMap.resize(NumRegs);
}
void VirtRegMap::assignVirt2Phys(Register virtReg, MCPhysReg physReg) {
assert(virtReg.isVirtual() && Register::isPhysicalRegister(physReg));
assert(Virt2PhysMap[virtReg.id()] == NO_PHYS_REG &&
"attempt to assign physical register to already mapped "
"virtual register");
assert(!getRegInfo().isReserved(physReg) &&
"Attempt to map virtReg to a reserved physReg");
Virt2PhysMap[virtReg.id()] = physReg;
}
unsigned VirtRegMap::createSpillSlot(const TargetRegisterClass *RC) {
unsigned Size = TRI->getSpillSize(*RC);
Align Alignment = TRI->getSpillAlign(*RC);
int SS = MF->getFrameInfo().CreateSpillStackObject(Size, Alignment);
++NumSpillSlots;
return SS;
}
bool VirtRegMap::hasPreferredPhys(Register VirtReg) {
Register Hint = MRI->getSimpleHint(VirtReg);
if (!Hint.isValid())
return false;
if (Hint.isVirtual())
Hint = getPhys(Hint);
return Register(getPhys(VirtReg)) == Hint;
}
bool VirtRegMap::hasKnownPreference(Register VirtReg) {
std::pair<unsigned, unsigned> Hint = MRI->getRegAllocationHint(VirtReg);
if (Register::isPhysicalRegister(Hint.second))
return true;
if (Register::isVirtualRegister(Hint.second))
return hasPhys(Hint.second);
return false;
}
int VirtRegMap::assignVirt2StackSlot(Register virtReg) {
assert(virtReg.isVirtual());
assert(Virt2StackSlotMap[virtReg.id()] == NO_STACK_SLOT &&
"attempt to assign stack slot to already spilled register");
const TargetRegisterClass* RC = MF->getRegInfo().getRegClass(virtReg);
return Virt2StackSlotMap[virtReg.id()] = createSpillSlot(RC);
}
void VirtRegMap::assignVirt2StackSlot(Register virtReg, int SS) {
assert(virtReg.isVirtual());
assert(Virt2StackSlotMap[virtReg.id()] == NO_STACK_SLOT &&
"attempt to assign stack slot to already spilled register");
assert((SS >= 0 ||
(SS >= MF->getFrameInfo().getObjectIndexBegin())) &&
"illegal fixed frame index");
Virt2StackSlotMap[virtReg.id()] = SS;
}
void VirtRegMap::print(raw_ostream &OS, const Module*) const {
OS << "********** REGISTER MAP **********\n";
for (unsigned i = 0, e = MRI->getNumVirtRegs(); i != e; ++i) {
unsigned Reg = Register::index2VirtReg(i);
if (Virt2PhysMap[Reg] != (unsigned)VirtRegMap::NO_PHYS_REG) {
OS << '[' << printReg(Reg, TRI) << " -> "
<< printReg(Virt2PhysMap[Reg], TRI) << "] "
<< TRI->getRegClassName(MRI->getRegClass(Reg)) << "\n";
}
}
for (unsigned i = 0, e = MRI->getNumVirtRegs(); i != e; ++i) {
unsigned Reg = Register::index2VirtReg(i);
if (Virt2StackSlotMap[Reg] != VirtRegMap::NO_STACK_SLOT) {
OS << '[' << printReg(Reg, TRI) << " -> fi#" << Virt2StackSlotMap[Reg]
<< "] " << TRI->getRegClassName(MRI->getRegClass(Reg)) << "\n";
}
}
OS << '\n';
}
#if !defined(NDEBUG) || defined(LLVM_ENABLE_DUMP)
LLVM_DUMP_METHOD void VirtRegMap::dump() const {
print(dbgs());
}
#endif
//===----------------------------------------------------------------------===//
// VirtRegRewriter
//===----------------------------------------------------------------------===//
//
// The VirtRegRewriter is the last of the register allocator passes.
// It rewrites virtual registers to physical registers as specified in the
// VirtRegMap analysis. It also updates live-in information on basic blocks
// according to LiveIntervals.
//
namespace {
class VirtRegRewriter : public MachineFunctionPass {
MachineFunction *MF;
const TargetRegisterInfo *TRI;
const TargetInstrInfo *TII;
MachineRegisterInfo *MRI;
SlotIndexes *Indexes;
LiveIntervals *LIS;
VirtRegMap *VRM;
void rewrite();
void addMBBLiveIns();
bool readsUndefSubreg(const MachineOperand &MO) const;
void addLiveInsForSubRanges(const LiveInterval &LI, Register PhysReg) const;
void handleIdentityCopy(MachineInstr &MI) const;
void expandCopyBundle(MachineInstr &MI) const;
bool subRegLiveThrough(const MachineInstr &MI, Register SuperPhysReg) const;
public:
static char ID;
VirtRegRewriter() : MachineFunctionPass(ID) {}
void getAnalysisUsage(AnalysisUsage &AU) const override;
bool runOnMachineFunction(MachineFunction&) override;
MachineFunctionProperties getSetProperties() const override {
return MachineFunctionProperties().set(
MachineFunctionProperties::Property::NoVRegs);
}
};
} // end anonymous namespace
char VirtRegRewriter::ID = 0;
char &llvm::VirtRegRewriterID = VirtRegRewriter::ID;
INITIALIZE_PASS_BEGIN(VirtRegRewriter, "virtregrewriter",
"Virtual Register Rewriter", false, false)
INITIALIZE_PASS_DEPENDENCY(SlotIndexes)
INITIALIZE_PASS_DEPENDENCY(LiveIntervals)
INITIALIZE_PASS_DEPENDENCY(LiveDebugVariables)
INITIALIZE_PASS_DEPENDENCY(LiveStacks)
INITIALIZE_PASS_DEPENDENCY(VirtRegMap)
INITIALIZE_PASS_END(VirtRegRewriter, "virtregrewriter",
"Virtual Register Rewriter", false, false)
void VirtRegRewriter::getAnalysisUsage(AnalysisUsage &AU) const {
AU.setPreservesCFG();
AU.addRequired<LiveIntervals>();
AU.addRequired<SlotIndexes>();
AU.addPreserved<SlotIndexes>();
AU.addRequired<LiveDebugVariables>();
AU.addRequired<LiveStacks>();
AU.addPreserved<LiveStacks>();
AU.addRequired<VirtRegMap>();
MachineFunctionPass::getAnalysisUsage(AU);
}
bool VirtRegRewriter::runOnMachineFunction(MachineFunction &fn) {
MF = &fn;
TRI = MF->getSubtarget().getRegisterInfo();
TII = MF->getSubtarget().getInstrInfo();
MRI = &MF->getRegInfo();
Indexes = &getAnalysis<SlotIndexes>();
LIS = &getAnalysis<LiveIntervals>();
VRM = &getAnalysis<VirtRegMap>();
LLVM_DEBUG(dbgs() << "********** REWRITE VIRTUAL REGISTERS **********\n"
<< "********** Function: " << MF->getName() << '\n');
LLVM_DEBUG(VRM->dump());
// Add kill flags while we still have virtual registers.
LIS->addKillFlags(VRM);
// Live-in lists on basic blocks are required for physregs.
addMBBLiveIns();
// Rewrite virtual registers.
rewrite();
// Write out new DBG_VALUE instructions.
getAnalysis<LiveDebugVariables>().emitDebugValues(VRM);
// All machine operands and other references to virtual registers have been
// replaced. Remove the virtual registers and release all the transient data.
VRM->clearAllVirt();
MRI->clearVirtRegs();
return true;
}
void VirtRegRewriter::addLiveInsForSubRanges(const LiveInterval &LI,
Register PhysReg) const {
assert(!LI.empty());
assert(LI.hasSubRanges());
using SubRangeIteratorPair =
std::pair<const LiveInterval::SubRange *, LiveInterval::const_iterator>;
SmallVector<SubRangeIteratorPair, 4> SubRanges;
SlotIndex First;
SlotIndex Last;
for (const LiveInterval::SubRange &SR : LI.subranges()) {
SubRanges.push_back(std::make_pair(&SR, SR.begin()));
if (!First.isValid() || SR.segments.front().start < First)
First = SR.segments.front().start;
if (!Last.isValid() || SR.segments.back().end > Last)
Last = SR.segments.back().end;
}
// Check all mbb start positions between First and Last while
// simulatenously advancing an iterator for each subrange.
for (SlotIndexes::MBBIndexIterator MBBI = Indexes->findMBBIndex(First);
MBBI != Indexes->MBBIndexEnd() && MBBI->first <= Last; ++MBBI) {
SlotIndex MBBBegin = MBBI->first;
// Advance all subrange iterators so that their end position is just
// behind MBBBegin (or the iterator is at the end).
LaneBitmask LaneMask;
for (auto &RangeIterPair : SubRanges) {
const LiveInterval::SubRange *SR = RangeIterPair.first;
LiveInterval::const_iterator &SRI = RangeIterPair.second;
while (SRI != SR->end() && SRI->end <= MBBBegin)
++SRI;
if (SRI == SR->end())
continue;
if (SRI->start <= MBBBegin)
LaneMask |= SR->LaneMask;
}
if (LaneMask.none())
continue;
MachineBasicBlock *MBB = MBBI->second;
MBB->addLiveIn(PhysReg, LaneMask);
}
}
// Compute MBB live-in lists from virtual register live ranges and their
// assignments.
void VirtRegRewriter::addMBBLiveIns() {
for (unsigned Idx = 0, IdxE = MRI->getNumVirtRegs(); Idx != IdxE; ++Idx) {
Register VirtReg = Register::index2VirtReg(Idx);
if (MRI->reg_nodbg_empty(VirtReg))
continue;
LiveInterval &LI = LIS->getInterval(VirtReg);
if (LI.empty() || LIS->intervalIsInOneMBB(LI))
continue;
// This is a virtual register that is live across basic blocks. Its
// assigned PhysReg must be marked as live-in to those blocks.
Register PhysReg = VRM->getPhys(VirtReg);
assert(PhysReg != VirtRegMap::NO_PHYS_REG && "Unmapped virtual register.");
if (LI.hasSubRanges()) {
addLiveInsForSubRanges(LI, PhysReg);
} else {
// Go over MBB begin positions and see if we have segments covering them.
// The following works because segments and the MBBIndex list are both
// sorted by slot indexes.
SlotIndexes::MBBIndexIterator I = Indexes->MBBIndexBegin();
for (const auto &Seg : LI) {
I = Indexes->advanceMBBIndex(I, Seg.start);
for (; I != Indexes->MBBIndexEnd() && I->first < Seg.end; ++I) {
MachineBasicBlock *MBB = I->second;
MBB->addLiveIn(PhysReg);
}
}
}
}
// Sort and unique MBB LiveIns as we've not checked if SubReg/PhysReg were in
// each MBB's LiveIns set before calling addLiveIn on them.
for (MachineBasicBlock &MBB : *MF)
MBB.sortUniqueLiveIns();
}
/// Returns true if the given machine operand \p MO only reads undefined lanes.
/// The function only works for use operands with a subregister set.
bool VirtRegRewriter::readsUndefSubreg(const MachineOperand &MO) const {
// Shortcut if the operand is already marked undef.
if (MO.isUndef())
return true;
Register Reg = MO.getReg();
const LiveInterval &LI = LIS->getInterval(Reg);
const MachineInstr &MI = *MO.getParent();
SlotIndex BaseIndex = LIS->getInstructionIndex(MI);
// This code is only meant to handle reading undefined subregisters which
// we couldn't properly detect before.
assert(LI.liveAt(BaseIndex) &&
"Reads of completely dead register should be marked undef already");
unsigned SubRegIdx = MO.getSubReg();
assert(SubRegIdx != 0 && LI.hasSubRanges());
LaneBitmask UseMask = TRI->getSubRegIndexLaneMask(SubRegIdx);
// See if any of the relevant subregister liveranges is defined at this point.
for (const LiveInterval::SubRange &SR : LI.subranges()) {
if ((SR.LaneMask & UseMask).any() && SR.liveAt(BaseIndex))
return false;
}
return true;
}
void VirtRegRewriter::handleIdentityCopy(MachineInstr &MI) const {
if (!MI.isIdentityCopy())
return;
LLVM_DEBUG(dbgs() << "Identity copy: " << MI);
++NumIdCopies;
// Copies like:
// %r0 = COPY undef %r0
// %al = COPY %al, implicit-def %eax
// give us additional liveness information: The target (super-)register
// must not be valid before this point. Replace the COPY with a KILL
// instruction to maintain this information.
if (MI.getOperand(1).isUndef() || MI.getNumOperands() > 2) {
MI.setDesc(TII->get(TargetOpcode::KILL));
LLVM_DEBUG(dbgs() << " replace by: " << MI);
return;
}
if (Indexes)
Indexes->removeSingleMachineInstrFromMaps(MI);
MI.eraseFromBundle();
LLVM_DEBUG(dbgs() << " deleted.\n");
}
/// The liverange splitting logic sometimes produces bundles of copies when
/// subregisters are involved. Expand these into a sequence of copy instructions
/// after processing the last in the bundle. Does not update LiveIntervals
/// which we shouldn't need for this instruction anymore.
void VirtRegRewriter::expandCopyBundle(MachineInstr &MI) const {
if (!MI.isCopy() && !MI.isKill())
return;
if (MI.isBundledWithPred() && !MI.isBundledWithSucc()) {
SmallVector<MachineInstr *, 2> MIs({&MI});
// Only do this when the complete bundle is made out of COPYs and KILLs.
MachineBasicBlock &MBB = *MI.getParent();
for (MachineBasicBlock::reverse_instr_iterator I =
std::next(MI.getReverseIterator()), E = MBB.instr_rend();
I != E && I->isBundledWithSucc(); ++I) {
if (!I->isCopy() && !I->isKill())
return;
MIs.push_back(&*I);
}
MachineInstr *FirstMI = MIs.back();
auto anyRegsAlias = [](const MachineInstr *Dst,
ArrayRef<MachineInstr *> Srcs,
const TargetRegisterInfo *TRI) {
for (const MachineInstr *Src : Srcs)
if (Src != Dst)
if (TRI->regsOverlap(Dst->getOperand(0).getReg(),
Src->getOperand(1).getReg()))
return true;
return false;
};
// If any of the destination registers in the bundle of copies alias any of
// the source registers, try to schedule the instructions to avoid any
// clobbering.
for (int E = MIs.size(), PrevE = E; E > 1; PrevE = E) {
for (int I = E; I--; )
if (!anyRegsAlias(MIs[I], makeArrayRef(MIs).take_front(E), TRI)) {
if (I + 1 != E)
std::swap(MIs[I], MIs[E - 1]);
--E;
}
if (PrevE == E) {
MF->getFunction().getContext().emitError(
"register rewriting failed: cycle in copy bundle");
break;
}
}
MachineInstr *BundleStart = FirstMI;
for (MachineInstr *BundledMI : llvm::reverse(MIs)) {
// If instruction is in the middle of the bundle, move it before the
// bundle starts, otherwise, just unbundle it. When we get to the last
// instruction, the bundle will have been completely undone.
if (BundledMI != BundleStart) {
BundledMI->removeFromBundle();
MBB.insert(BundleStart, BundledMI);
} else if (BundledMI->isBundledWithSucc()) {
BundledMI->unbundleFromSucc();
BundleStart = &*std::next(BundledMI->getIterator());
}
if (Indexes && BundledMI != FirstMI)
Indexes->insertMachineInstrInMaps(*BundledMI);
}
}
}
/// Check whether (part of) \p SuperPhysReg is live through \p MI.
/// \pre \p MI defines a subregister of a virtual register that
/// has been assigned to \p SuperPhysReg.
bool VirtRegRewriter::subRegLiveThrough(const MachineInstr &MI,
Register SuperPhysReg) const {
SlotIndex MIIndex = LIS->getInstructionIndex(MI);
SlotIndex BeforeMIUses = MIIndex.getBaseIndex();
SlotIndex AfterMIDefs = MIIndex.getBoundaryIndex();
for (MCRegUnitIterator Unit(SuperPhysReg, TRI); Unit.isValid(); ++Unit) {
const LiveRange &UnitRange = LIS->getRegUnit(*Unit);
// If the regunit is live both before and after MI,
// we assume it is live through.
// Generally speaking, this is not true, because something like
// "RU = op RU" would match that description.
// However, we know that we are trying to assess whether
// a def of a virtual reg, vreg, is live at the same time of RU.
// If we are in the "RU = op RU" situation, that means that vreg
// is defined at the same time as RU (i.e., "vreg, RU = op RU").
// Thus, vreg and RU interferes and vreg cannot be assigned to
// SuperPhysReg. Therefore, this situation cannot happen.
if (UnitRange.liveAt(AfterMIDefs) && UnitRange.liveAt(BeforeMIUses))
return true;
}
return false;
}
void VirtRegRewriter::rewrite() {
bool NoSubRegLiveness = !MRI->subRegLivenessEnabled();
SmallVector<Register, 8> SuperDeads;
SmallVector<Register, 8> SuperDefs;
SmallVector<Register, 8> SuperKills;
for (MachineFunction::iterator MBBI = MF->begin(), MBBE = MF->end();
MBBI != MBBE; ++MBBI) {
LLVM_DEBUG(MBBI->print(dbgs(), Indexes));
for (MachineBasicBlock::instr_iterator
MII = MBBI->instr_begin(), MIE = MBBI->instr_end(); MII != MIE;) {
MachineInstr *MI = &*MII;
++MII;
for (MachineInstr::mop_iterator MOI = MI->operands_begin(),
MOE = MI->operands_end(); MOI != MOE; ++MOI) {
MachineOperand &MO = *MOI;
// Make sure MRI knows about registers clobbered by regmasks.
if (MO.isRegMask())
MRI->addPhysRegsUsedFromRegMask(MO.getRegMask());
if (!MO.isReg() || !MO.getReg().isVirtual())
continue;
Register VirtReg = MO.getReg();
Register PhysReg = VRM->getPhys(VirtReg);
assert(PhysReg != VirtRegMap::NO_PHYS_REG &&
"Instruction uses unmapped VirtReg");
assert(!MRI->isReserved(PhysReg) && "Reserved register assignment");
// Preserve semantics of sub-register operands.
unsigned SubReg = MO.getSubReg();
if (SubReg != 0) {
if (NoSubRegLiveness || !MRI->shouldTrackSubRegLiveness(VirtReg)) {
// A virtual register kill refers to the whole register, so we may
// have to add implicit killed operands for the super-register. A
// partial redef always kills and redefines the super-register.
if ((MO.readsReg() && (MO.isDef() || MO.isKill())) ||
(MO.isDef() && subRegLiveThrough(*MI, PhysReg)))
SuperKills.push_back(PhysReg);
if (MO.isDef()) {
// Also add implicit defs for the super-register.
if (MO.isDead())
SuperDeads.push_back(PhysReg);
else
SuperDefs.push_back(PhysReg);
}
} else {
if (MO.isUse()) {
if (readsUndefSubreg(MO))
// We need to add an <undef> flag if the subregister is
// completely undefined (and we are not adding super-register
// defs).
MO.setIsUndef(true);
} else if (!MO.isDead()) {
assert(MO.isDef());
}
}
// The def undef and def internal flags only make sense for
// sub-register defs, and we are substituting a full physreg. An
// implicit killed operand from the SuperKills list will represent the
// partial read of the super-register.
if (MO.isDef()) {
MO.setIsUndef(false);
MO.setIsInternalRead(false);
}
// PhysReg operands cannot have subregister indexes.
PhysReg = TRI->getSubReg(PhysReg, SubReg);
assert(PhysReg.isValid() && "Invalid SubReg for physical register");
MO.setSubReg(0);
}
// Rewrite. Note we could have used MachineOperand::substPhysReg(), but
// we need the inlining here.
MO.setReg(PhysReg);
MO.setIsRenamable(true);
}
// Add any missing super-register kills after rewriting the whole
// instruction.
while (!SuperKills.empty())
MI->addRegisterKilled(SuperKills.pop_back_val(), TRI, true);
while (!SuperDeads.empty())
MI->addRegisterDead(SuperDeads.pop_back_val(), TRI, true);
while (!SuperDefs.empty())
MI->addRegisterDefined(SuperDefs.pop_back_val(), TRI);
LLVM_DEBUG(dbgs() << "> " << *MI);
expandCopyBundle(*MI);
// We can remove identity copies right now.
handleIdentityCopy(*MI);
}
}
}
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