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

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//===-- LiveVariables.cpp - Live Variable Analysis for Machine Code -------===//
//
// 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 LiveVariable analysis pass. For each machine
// instruction in the function, this pass calculates the set of registers that
// are immediately dead after the instruction (i.e., the instruction calculates
// the value, but it is never used) and the set of registers that are used by
// the instruction, but are never used after the instruction (i.e., they are
// killed).
//
// This class computes live variables using a sparse implementation based on
// the machine code SSA form. This class computes live variable information for
// each virtual and _register allocatable_ physical register in a function. It
// uses the dominance properties of SSA form to efficiently compute live
// variables for virtual registers, and assumes that physical registers are only
// live within a single basic block (allowing it to do a single local analysis
// to resolve physical register lifetimes in each basic block). If a physical
// register is not register allocatable, it is not tracked. This is useful for
// things like the stack pointer and condition codes.
//
//===----------------------------------------------------------------------===//
#include "llvm/CodeGen/LiveVariables.h"
#include "llvm/ADT/DenseSet.h"
#include "llvm/ADT/DepthFirstIterator.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/SmallSet.h"
#include "llvm/CodeGen/MachineInstr.h"
#include "llvm/CodeGen/MachineRegisterInfo.h"
#include "llvm/CodeGen/Passes.h"
#include "llvm/Config/llvm-config.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/ErrorHandling.h"
#include "llvm/Support/raw_ostream.h"
#include <algorithm>
using namespace llvm;
char LiveVariables::ID = 0;
char &llvm::LiveVariablesID = LiveVariables::ID;
INITIALIZE_PASS_BEGIN(LiveVariables, "livevars",
"Live Variable Analysis", false, false)
INITIALIZE_PASS_DEPENDENCY(UnreachableMachineBlockElim)
INITIALIZE_PASS_END(LiveVariables, "livevars",
"Live Variable Analysis", false, false)
void LiveVariables::getAnalysisUsage(AnalysisUsage &AU) const {
AU.addRequiredID(UnreachableMachineBlockElimID);
AU.setPreservesAll();
MachineFunctionPass::getAnalysisUsage(AU);
}
MachineInstr *
LiveVariables::VarInfo::findKill(const MachineBasicBlock *MBB) const {
for (MachineInstr *MI : Kills)
if (MI->getParent() == MBB)
return MI;
return nullptr;
}
#if !defined(NDEBUG) || defined(LLVM_ENABLE_DUMP)
LLVM_DUMP_METHOD void LiveVariables::VarInfo::dump() const {
dbgs() << " Alive in blocks: ";
for (unsigned AB : AliveBlocks)
dbgs() << AB << ", ";
dbgs() << "\n Killed by:";
if (Kills.empty())
dbgs() << " No instructions.\n";
else {
for (unsigned i = 0, e = Kills.size(); i != e; ++i)
dbgs() << "\n #" << i << ": " << *Kills[i];
dbgs() << "\n";
}
}
#endif
/// getVarInfo - Get (possibly creating) a VarInfo object for the given vreg.
LiveVariables::VarInfo &LiveVariables::getVarInfo(Register Reg) {
assert(Reg.isVirtual() && "getVarInfo: not a virtual register!");
VirtRegInfo.grow(Reg);
return VirtRegInfo[Reg];
}
void LiveVariables::MarkVirtRegAliveInBlock(
VarInfo &VRInfo, MachineBasicBlock *DefBlock, MachineBasicBlock *MBB,
SmallVectorImpl<MachineBasicBlock *> &WorkList) {
unsigned BBNum = MBB->getNumber();
// Check to see if this basic block is one of the killing blocks. If so,
// remove it.
for (unsigned i = 0, e = VRInfo.Kills.size(); i != e; ++i)
if (VRInfo.Kills[i]->getParent() == MBB) {
VRInfo.Kills.erase(VRInfo.Kills.begin()+i); // Erase entry
break;
}
if (MBB == DefBlock) return; // Terminate recursion
if (VRInfo.AliveBlocks.test(BBNum))
return; // We already know the block is live
// Mark the variable known alive in this bb
VRInfo.AliveBlocks.set(BBNum);
assert(MBB != &MF->front() && "Can't find reaching def for virtreg");
WorkList.insert(WorkList.end(), MBB->pred_rbegin(), MBB->pred_rend());
}
void LiveVariables::MarkVirtRegAliveInBlock(VarInfo &VRInfo,
MachineBasicBlock *DefBlock,
MachineBasicBlock *MBB) {
SmallVector<MachineBasicBlock *, 16> WorkList;
MarkVirtRegAliveInBlock(VRInfo, DefBlock, MBB, WorkList);
while (!WorkList.empty()) {
MachineBasicBlock *Pred = WorkList.pop_back_val();
MarkVirtRegAliveInBlock(VRInfo, DefBlock, Pred, WorkList);
}
}
void LiveVariables::HandleVirtRegUse(Register Reg, MachineBasicBlock *MBB,
MachineInstr &MI) {
assert(MRI->getVRegDef(Reg) && "Register use before def!");
unsigned BBNum = MBB->getNumber();
VarInfo &VRInfo = getVarInfo(Reg);
// Check to see if this basic block is already a kill block.
if (!VRInfo.Kills.empty() && VRInfo.Kills.back()->getParent() == MBB) {
// Yes, this register is killed in this basic block already. Increase the
// live range by updating the kill instruction.
VRInfo.Kills.back() = &MI;
return;
}
#ifndef NDEBUG
for (MachineInstr *Kill : VRInfo.Kills)
assert(Kill->getParent() != MBB && "entry should be at end!");
#endif
// This situation can occur:
//
// ,------.
// | |
// | v
// | t2 = phi ... t1 ...
// | |
// | v
// | t1 = ...
// | ... = ... t1 ...
// | |
// `------'
//
// where there is a use in a PHI node that's a predecessor to the defining
// block. We don't want to mark all predecessors as having the value "alive"
// in this case.
if (MBB == MRI->getVRegDef(Reg)->getParent())
return;
// Add a new kill entry for this basic block. If this virtual register is
// already marked as alive in this basic block, that means it is alive in at
// least one of the successor blocks, it's not a kill.
if (!VRInfo.AliveBlocks.test(BBNum))
VRInfo.Kills.push_back(&MI);
// Update all dominating blocks to mark them as "known live".
for (MachineBasicBlock *Pred : MBB->predecessors())
MarkVirtRegAliveInBlock(VRInfo, MRI->getVRegDef(Reg)->getParent(), Pred);
}
void LiveVariables::HandleVirtRegDef(Register Reg, MachineInstr &MI) {
VarInfo &VRInfo = getVarInfo(Reg);
if (VRInfo.AliveBlocks.empty())
// If vr is not alive in any block, then defaults to dead.
VRInfo.Kills.push_back(&MI);
}
/// FindLastPartialDef - Return the last partial def of the specified register.
/// Also returns the sub-registers that're defined by the instruction.
MachineInstr *
LiveVariables::FindLastPartialDef(Register Reg,
SmallSet<unsigned, 4> &PartDefRegs) {
unsigned LastDefReg = 0;
unsigned LastDefDist = 0;
MachineInstr *LastDef = nullptr;
for (MCSubRegIterator SubRegs(Reg, TRI); SubRegs.isValid(); ++SubRegs) {
unsigned SubReg = *SubRegs;
MachineInstr *Def = PhysRegDef[SubReg];
if (!Def)
continue;
unsigned Dist = DistanceMap[Def];
if (Dist > LastDefDist) {
LastDefReg = SubReg;
LastDef = Def;
LastDefDist = Dist;
}
}
if (!LastDef)
return nullptr;
PartDefRegs.insert(LastDefReg);
for (unsigned i = 0, e = LastDef->getNumOperands(); i != e; ++i) {
MachineOperand &MO = LastDef->getOperand(i);
if (!MO.isReg() || !MO.isDef() || MO.getReg() == 0)
continue;
Register DefReg = MO.getReg();
if (TRI->isSubRegister(Reg, DefReg)) {
for (MCSubRegIterator SubRegs(DefReg, TRI, /*IncludeSelf=*/true);
SubRegs.isValid(); ++SubRegs)
PartDefRegs.insert(*SubRegs);
}
}
return LastDef;
}
/// HandlePhysRegUse - Turn previous partial def's into read/mod/writes. Add
/// implicit defs to a machine instruction if there was an earlier def of its
/// super-register.
void LiveVariables::HandlePhysRegUse(Register Reg, MachineInstr &MI) {
MachineInstr *LastDef = PhysRegDef[Reg];
// If there was a previous use or a "full" def all is well.
if (!LastDef && !PhysRegUse[Reg]) {
// Otherwise, the last sub-register def implicitly defines this register.
// e.g.
// AH =
// AL = ... implicit-def EAX, implicit killed AH
// = AH
// ...
// = EAX
// All of the sub-registers must have been defined before the use of Reg!
SmallSet<unsigned, 4> PartDefRegs;
MachineInstr *LastPartialDef = FindLastPartialDef(Reg, PartDefRegs);
// If LastPartialDef is NULL, it must be using a livein register.
if (LastPartialDef) {
LastPartialDef->addOperand(MachineOperand::CreateReg(Reg, true/*IsDef*/,
true/*IsImp*/));
PhysRegDef[Reg] = LastPartialDef;
SmallSet<unsigned, 8> Processed;
for (MCSubRegIterator SubRegs(Reg, TRI); SubRegs.isValid(); ++SubRegs) {
unsigned SubReg = *SubRegs;
if (Processed.count(SubReg))
continue;
if (PartDefRegs.count(SubReg))
continue;
// This part of Reg was defined before the last partial def. It's killed
// here.
LastPartialDef->addOperand(MachineOperand::CreateReg(SubReg,
false/*IsDef*/,
true/*IsImp*/));
PhysRegDef[SubReg] = LastPartialDef;
for (MCSubRegIterator SS(SubReg, TRI); SS.isValid(); ++SS)
Processed.insert(*SS);
}
}
} else if (LastDef && !PhysRegUse[Reg] &&
!LastDef->findRegisterDefOperand(Reg))
// Last def defines the super register, add an implicit def of reg.
LastDef->addOperand(MachineOperand::CreateReg(Reg, true/*IsDef*/,
true/*IsImp*/));
// Remember this use.
for (MCSubRegIterator SubRegs(Reg, TRI, /*IncludeSelf=*/true);
SubRegs.isValid(); ++SubRegs)
PhysRegUse[*SubRegs] = &MI;
}
/// FindLastRefOrPartRef - Return the last reference or partial reference of
/// the specified register.
MachineInstr *LiveVariables::FindLastRefOrPartRef(Register Reg) {
MachineInstr *LastDef = PhysRegDef[Reg];
MachineInstr *LastUse = PhysRegUse[Reg];
if (!LastDef && !LastUse)
return nullptr;
MachineInstr *LastRefOrPartRef = LastUse ? LastUse : LastDef;
unsigned LastRefOrPartRefDist = DistanceMap[LastRefOrPartRef];
unsigned LastPartDefDist = 0;
for (MCSubRegIterator SubRegs(Reg, TRI); SubRegs.isValid(); ++SubRegs) {
unsigned SubReg = *SubRegs;
MachineInstr *Def = PhysRegDef[SubReg];
if (Def && Def != LastDef) {
// There was a def of this sub-register in between. This is a partial
// def, keep track of the last one.
unsigned Dist = DistanceMap[Def];
if (Dist > LastPartDefDist)
LastPartDefDist = Dist;
} else if (MachineInstr *Use = PhysRegUse[SubReg]) {
unsigned Dist = DistanceMap[Use];
if (Dist > LastRefOrPartRefDist) {
LastRefOrPartRefDist = Dist;
LastRefOrPartRef = Use;
}
}
}
return LastRefOrPartRef;
}
bool LiveVariables::HandlePhysRegKill(Register Reg, MachineInstr *MI) {
MachineInstr *LastDef = PhysRegDef[Reg];
MachineInstr *LastUse = PhysRegUse[Reg];
if (!LastDef && !LastUse)
return false;
MachineInstr *LastRefOrPartRef = LastUse ? LastUse : LastDef;
unsigned LastRefOrPartRefDist = DistanceMap[LastRefOrPartRef];
// The whole register is used.
// AL =
// AH =
//
// = AX
// = AL, implicit killed AX
// AX =
//
// Or whole register is defined, but not used at all.
// dead AX =
// ...
// AX =
//
// Or whole register is defined, but only partly used.
// dead AX = implicit-def AL
// = killed AL
// AX =
MachineInstr *LastPartDef = nullptr;
unsigned LastPartDefDist = 0;
SmallSet<unsigned, 8> PartUses;
for (MCSubRegIterator SubRegs(Reg, TRI); SubRegs.isValid(); ++SubRegs) {
unsigned SubReg = *SubRegs;
MachineInstr *Def = PhysRegDef[SubReg];
if (Def && Def != LastDef) {
// There was a def of this sub-register in between. This is a partial
// def, keep track of the last one.
unsigned Dist = DistanceMap[Def];
if (Dist > LastPartDefDist) {
LastPartDefDist = Dist;
LastPartDef = Def;
}
continue;
}
if (MachineInstr *Use = PhysRegUse[SubReg]) {
for (MCSubRegIterator SS(SubReg, TRI, /*IncludeSelf=*/true); SS.isValid();
++SS)
PartUses.insert(*SS);
unsigned Dist = DistanceMap[Use];
if (Dist > LastRefOrPartRefDist) {
LastRefOrPartRefDist = Dist;
LastRefOrPartRef = Use;
}
}
}
if (!PhysRegUse[Reg]) {
// Partial uses. Mark register def dead and add implicit def of
// sub-registers which are used.
// dead EAX = op implicit-def AL
// That is, EAX def is dead but AL def extends pass it.
PhysRegDef[Reg]->addRegisterDead(Reg, TRI, true);
for (MCSubRegIterator SubRegs(Reg, TRI); SubRegs.isValid(); ++SubRegs) {
unsigned SubReg = *SubRegs;
if (!PartUses.count(SubReg))
continue;
bool NeedDef = true;
if (PhysRegDef[Reg] == PhysRegDef[SubReg]) {
MachineOperand *MO = PhysRegDef[Reg]->findRegisterDefOperand(SubReg);
if (MO) {
NeedDef = false;
assert(!MO->isDead());
}
}
if (NeedDef)
PhysRegDef[Reg]->addOperand(MachineOperand::CreateReg(SubReg,
true/*IsDef*/, true/*IsImp*/));
MachineInstr *LastSubRef = FindLastRefOrPartRef(SubReg);
if (LastSubRef)
LastSubRef->addRegisterKilled(SubReg, TRI, true);
else {
LastRefOrPartRef->addRegisterKilled(SubReg, TRI, true);
for (MCSubRegIterator SS(SubReg, TRI, /*IncludeSelf=*/true);
SS.isValid(); ++SS)
PhysRegUse[*SS] = LastRefOrPartRef;
}
for (MCSubRegIterator SS(SubReg, TRI); SS.isValid(); ++SS)
PartUses.erase(*SS);
}
} else if (LastRefOrPartRef == PhysRegDef[Reg] && LastRefOrPartRef != MI) {
if (LastPartDef)
// The last partial def kills the register.
LastPartDef->addOperand(MachineOperand::CreateReg(Reg, false/*IsDef*/,
true/*IsImp*/, true/*IsKill*/));
else {
MachineOperand *MO =
LastRefOrPartRef->findRegisterDefOperand(Reg, false, false, TRI);
bool NeedEC = MO->isEarlyClobber() && MO->getReg() != Reg;
// If the last reference is the last def, then it's not used at all.
// That is, unless we are currently processing the last reference itself.
LastRefOrPartRef->addRegisterDead(Reg, TRI, true);
if (NeedEC) {
// If we are adding a subreg def and the superreg def is marked early
// clobber, add an early clobber marker to the subreg def.
MO = LastRefOrPartRef->findRegisterDefOperand(Reg);
if (MO)
MO->setIsEarlyClobber();
}
}
} else
LastRefOrPartRef->addRegisterKilled(Reg, TRI, true);
return true;
}
void LiveVariables::HandleRegMask(const MachineOperand &MO) {
// Call HandlePhysRegKill() for all live registers clobbered by Mask.
// Clobbered registers are always dead, sp there is no need to use
// HandlePhysRegDef().
for (unsigned Reg = 1, NumRegs = TRI->getNumRegs(); Reg != NumRegs; ++Reg) {
// Skip dead regs.
if (!PhysRegDef[Reg] && !PhysRegUse[Reg])
continue;
// Skip mask-preserved regs.
if (!MO.clobbersPhysReg(Reg))
continue;
// Kill the largest clobbered super-register.
// This avoids needless implicit operands.
unsigned Super = Reg;
for (MCSuperRegIterator SR(Reg, TRI); SR.isValid(); ++SR)
if ((PhysRegDef[*SR] || PhysRegUse[*SR]) && MO.clobbersPhysReg(*SR))
Super = *SR;
HandlePhysRegKill(Super, nullptr);
}
}
void LiveVariables::HandlePhysRegDef(Register Reg, MachineInstr *MI,
SmallVectorImpl<unsigned> &Defs) {
// What parts of the register are previously defined?
SmallSet<unsigned, 32> Live;
if (PhysRegDef[Reg] || PhysRegUse[Reg]) {
for (MCSubRegIterator SubRegs(Reg, TRI, /*IncludeSelf=*/true);
SubRegs.isValid(); ++SubRegs)
Live.insert(*SubRegs);
} else {
for (MCSubRegIterator SubRegs(Reg, TRI); SubRegs.isValid(); ++SubRegs) {
unsigned SubReg = *SubRegs;
// If a register isn't itself defined, but all parts that make up of it
// are defined, then consider it also defined.
// e.g.
// AL =
// AH =
// = AX
if (Live.count(SubReg))
continue;
if (PhysRegDef[SubReg] || PhysRegUse[SubReg]) {
for (MCSubRegIterator SS(SubReg, TRI, /*IncludeSelf=*/true);
SS.isValid(); ++SS)
Live.insert(*SS);
}
}
}
// Start from the largest piece, find the last time any part of the register
// is referenced.
HandlePhysRegKill(Reg, MI);
// Only some of the sub-registers are used.
for (MCSubRegIterator SubRegs(Reg, TRI); SubRegs.isValid(); ++SubRegs) {
unsigned SubReg = *SubRegs;
if (!Live.count(SubReg))
// Skip if this sub-register isn't defined.
continue;
HandlePhysRegKill(SubReg, MI);
}
if (MI)
Defs.push_back(Reg); // Remember this def.
}
void LiveVariables::UpdatePhysRegDefs(MachineInstr &MI,
SmallVectorImpl<unsigned> &Defs) {
while (!Defs.empty()) {
Register Reg = Defs.pop_back_val();
for (MCSubRegIterator SubRegs(Reg, TRI, /*IncludeSelf=*/true);
SubRegs.isValid(); ++SubRegs) {
unsigned SubReg = *SubRegs;
PhysRegDef[SubReg] = &MI;
PhysRegUse[SubReg] = nullptr;
}
}
}
void LiveVariables::runOnInstr(MachineInstr &MI,
SmallVectorImpl<unsigned> &Defs) {
assert(!MI.isDebugOrPseudoInstr());
// Process all of the operands of the instruction...
unsigned NumOperandsToProcess = MI.getNumOperands();
// Unless it is a PHI node. In this case, ONLY process the DEF, not any
// of the uses. They will be handled in other basic blocks.
if (MI.isPHI())
NumOperandsToProcess = 1;
// Clear kill and dead markers. LV will recompute them.
SmallVector<unsigned, 4> UseRegs;
SmallVector<unsigned, 4> DefRegs;
SmallVector<unsigned, 1> RegMasks;
for (unsigned i = 0; i != NumOperandsToProcess; ++i) {
MachineOperand &MO = MI.getOperand(i);
if (MO.isRegMask()) {
RegMasks.push_back(i);
continue;
}
if (!MO.isReg() || MO.getReg() == 0)
continue;
Register MOReg = MO.getReg();
if (MO.isUse()) {
if (!(Register::isPhysicalRegister(MOReg) && MRI->isReserved(MOReg)))
MO.setIsKill(false);
if (MO.readsReg())
UseRegs.push_back(MOReg);
} else {
assert(MO.isDef());
// FIXME: We should not remove any dead flags. However the MIPS RDDSP
// instruction needs it at the moment: http://llvm.org/PR27116.
if (Register::isPhysicalRegister(MOReg) && !MRI->isReserved(MOReg))
MO.setIsDead(false);
DefRegs.push_back(MOReg);
}
}
MachineBasicBlock *MBB = MI.getParent();
// Process all uses.
for (unsigned MOReg : UseRegs) {
if (Register::isVirtualRegister(MOReg))
HandleVirtRegUse(MOReg, MBB, MI);
else if (!MRI->isReserved(MOReg))
HandlePhysRegUse(MOReg, MI);
}
// Process all masked registers. (Call clobbers).
for (unsigned Mask : RegMasks)
HandleRegMask(MI.getOperand(Mask));
// Process all defs.
for (unsigned MOReg : DefRegs) {
if (Register::isVirtualRegister(MOReg))
HandleVirtRegDef(MOReg, MI);
else if (!MRI->isReserved(MOReg))
HandlePhysRegDef(MOReg, &MI, Defs);
}
UpdatePhysRegDefs(MI, Defs);
}
void LiveVariables::runOnBlock(MachineBasicBlock *MBB, const unsigned NumRegs) {
// Mark live-in registers as live-in.
SmallVector<unsigned, 4> Defs;
for (const auto &LI : MBB->liveins()) {
assert(Register::isPhysicalRegister(LI.PhysReg) &&
"Cannot have a live-in virtual register!");
HandlePhysRegDef(LI.PhysReg, nullptr, Defs);
}
// Loop over all of the instructions, processing them.
DistanceMap.clear();
unsigned Dist = 0;
for (MachineInstr &MI : *MBB) {
if (MI.isDebugOrPseudoInstr())
continue;
DistanceMap.insert(std::make_pair(&MI, Dist++));
runOnInstr(MI, Defs);
}
// Handle any virtual assignments from PHI nodes which might be at the
// bottom of this basic block. We check all of our successor blocks to see
// if they have PHI nodes, and if so, we simulate an assignment at the end
// of the current block.
if (!PHIVarInfo[MBB->getNumber()].empty()) {
SmallVectorImpl<unsigned> &VarInfoVec = PHIVarInfo[MBB->getNumber()];
for (unsigned I : VarInfoVec)
// Mark it alive only in the block we are representing.
MarkVirtRegAliveInBlock(getVarInfo(I), MRI->getVRegDef(I)->getParent(),
MBB);
}
// MachineCSE may CSE instructions which write to non-allocatable physical
// registers across MBBs. Remember if any reserved register is liveout.
SmallSet<unsigned, 4> LiveOuts;
for (const MachineBasicBlock *SuccMBB : MBB->successors()) {
if (SuccMBB->isEHPad())
continue;
for (const auto &LI : SuccMBB->liveins()) {
if (!TRI->isInAllocatableClass(LI.PhysReg))
// Ignore other live-ins, e.g. those that are live into landing pads.
LiveOuts.insert(LI.PhysReg);
}
}
// Loop over PhysRegDef / PhysRegUse, killing any registers that are
// available at the end of the basic block.
for (unsigned i = 0; i != NumRegs; ++i)
if ((PhysRegDef[i] || PhysRegUse[i]) && !LiveOuts.count(i))
HandlePhysRegDef(i, nullptr, Defs);
}
bool LiveVariables::runOnMachineFunction(MachineFunction &mf) {
MF = &mf;
MRI = &mf.getRegInfo();
TRI = MF->getSubtarget().getRegisterInfo();
const unsigned NumRegs = TRI->getNumRegs();
PhysRegDef.assign(NumRegs, nullptr);
PhysRegUse.assign(NumRegs, nullptr);
PHIVarInfo.resize(MF->getNumBlockIDs());
PHIJoins.clear();
// FIXME: LiveIntervals will be updated to remove its dependence on
// LiveVariables to improve compilation time and eliminate bizarre pass
// dependencies. Until then, we can't change much in -O0.
if (!MRI->isSSA())
report_fatal_error("regalloc=... not currently supported with -O0");
analyzePHINodes(mf);
// Calculate live variable information in depth first order on the CFG of the
// function. This guarantees that we will see the definition of a virtual
// register before its uses due to dominance properties of SSA (except for PHI
// nodes, which are treated as a special case).
MachineBasicBlock *Entry = &MF->front();
df_iterator_default_set<MachineBasicBlock*,16> Visited;
for (MachineBasicBlock *MBB : depth_first_ext(Entry, Visited)) {
runOnBlock(MBB, NumRegs);
PhysRegDef.assign(NumRegs, nullptr);
PhysRegUse.assign(NumRegs, nullptr);
}
// Convert and transfer the dead / killed information we have gathered into
// VirtRegInfo onto MI's.
for (unsigned i = 0, e1 = VirtRegInfo.size(); i != e1; ++i) {
const Register Reg = Register::index2VirtReg(i);
for (unsigned j = 0, e2 = VirtRegInfo[Reg].Kills.size(); j != e2; ++j)
if (VirtRegInfo[Reg].Kills[j] == MRI->getVRegDef(Reg))
VirtRegInfo[Reg].Kills[j]->addRegisterDead(Reg, TRI);
else
VirtRegInfo[Reg].Kills[j]->addRegisterKilled(Reg, TRI);
}
// Check to make sure there are no unreachable blocks in the MC CFG for the
// function. If so, it is due to a bug in the instruction selector or some
// other part of the code generator if this happens.
#ifndef NDEBUG
for (const MachineBasicBlock &MBB : *MF)
assert(Visited.contains(&MBB) && "unreachable basic block found");
#endif
PhysRegDef.clear();
PhysRegUse.clear();
PHIVarInfo.clear();
return false;
}
void LiveVariables::recomputeForSingleDefVirtReg(Register Reg) {
assert(Reg.isVirtual());
VarInfo &VI = getVarInfo(Reg);
VI.AliveBlocks.clear();
VI.Kills.clear();
MachineInstr &DefMI = *MRI->getUniqueVRegDef(Reg);
MachineBasicBlock &DefBB = *DefMI.getParent();
// Handle the case where all uses have been removed.
if (MRI->use_nodbg_empty(Reg)) {
VI.Kills.push_back(&DefMI);
DefMI.addRegisterDead(Reg, nullptr);
return;
}
DefMI.clearRegisterDeads(Reg);
// Initialize a worklist of BBs that Reg is live-to-end of. (Here
// "live-to-end" means Reg is live at the end of a block even if it is only
// live because of phi uses in a successor. This is different from isLiveOut()
// which does not consider phi uses.)
SmallVector<MachineBasicBlock *> LiveToEndBlocks;
SparseBitVector<> UseBlocks;
for (auto &UseMO : MRI->use_nodbg_operands(Reg)) {
UseMO.setIsKill(false);
MachineInstr &UseMI = *UseMO.getParent();
MachineBasicBlock &UseBB = *UseMI.getParent();
UseBlocks.set(UseBB.getNumber());
if (UseMI.isPHI()) {
// If Reg is used in a phi then it is live-to-end of the corresponding
// predecessor.
unsigned Idx = UseMI.getOperandNo(&UseMO);
LiveToEndBlocks.push_back(UseMI.getOperand(Idx + 1).getMBB());
} else if (&UseBB == &DefBB) {
// A non-phi use in the same BB as the single def must come after the def.
} else {
// Otherwise Reg must be live-to-end of all predecessors.
LiveToEndBlocks.append(UseBB.pred_begin(), UseBB.pred_end());
}
}
// Iterate over the worklist adding blocks to AliveBlocks.
bool LiveToEndOfDefBB = false;
while (!LiveToEndBlocks.empty()) {
MachineBasicBlock &BB = *LiveToEndBlocks.pop_back_val();
if (&BB == &DefBB) {
LiveToEndOfDefBB = true;
continue;
}
if (VI.AliveBlocks.test(BB.getNumber()))
continue;
VI.AliveBlocks.set(BB.getNumber());
LiveToEndBlocks.append(BB.pred_begin(), BB.pred_end());
}
// Recompute kill flags. For each block in which Reg is used but is not
// live-through, find the last instruction that uses Reg. Ignore phi nodes
// because they should not be included in Kills.
for (unsigned UseBBNum : UseBlocks) {
if (VI.AliveBlocks.test(UseBBNum))
continue;
MachineBasicBlock &UseBB = *MF->getBlockNumbered(UseBBNum);
if (&UseBB == &DefBB && LiveToEndOfDefBB)
continue;
for (auto &MI : reverse(UseBB)) {
if (MI.isDebugOrPseudoInstr())
continue;
if (MI.isPHI())
break;
if (MI.readsRegister(Reg)) {
assert(!MI.killsRegister(Reg));
MI.addRegisterKilled(Reg, nullptr);
VI.Kills.push_back(&MI);
break;
}
}
}
}
/// replaceKillInstruction - Update register kill info by replacing a kill
/// instruction with a new one.
void LiveVariables::replaceKillInstruction(Register Reg, MachineInstr &OldMI,
MachineInstr &NewMI) {
VarInfo &VI = getVarInfo(Reg);
std::replace(VI.Kills.begin(), VI.Kills.end(), &OldMI, &NewMI);
}
/// removeVirtualRegistersKilled - Remove all killed info for the specified
/// instruction.
void LiveVariables::removeVirtualRegistersKilled(MachineInstr &MI) {
for (unsigned i = 0, e = MI.getNumOperands(); i != e; ++i) {
MachineOperand &MO = MI.getOperand(i);
if (MO.isReg() && MO.isKill()) {
MO.setIsKill(false);
Register Reg = MO.getReg();
if (Register::isVirtualRegister(Reg)) {
bool removed = getVarInfo(Reg).removeKill(MI);
assert(removed && "kill not in register's VarInfo?");
(void)removed;
}
}
}
}
/// analyzePHINodes - Gather information about the PHI nodes in here. In
/// particular, we want to map the variable information of a virtual register
/// which is used in a PHI node. We map that to the BB the vreg is coming from.
///
void LiveVariables::analyzePHINodes(const MachineFunction& Fn) {
for (const auto &MBB : Fn)
for (const auto &BBI : MBB) {
if (!BBI.isPHI())
break;
for (unsigned i = 1, e = BBI.getNumOperands(); i != e; i += 2)
if (BBI.getOperand(i).readsReg())
PHIVarInfo[BBI.getOperand(i + 1).getMBB()->getNumber()]
.push_back(BBI.getOperand(i).getReg());
}
}
bool LiveVariables::VarInfo::isLiveIn(const MachineBasicBlock &MBB,
Register Reg, MachineRegisterInfo &MRI) {
unsigned Num = MBB.getNumber();
// Reg is live-through.
if (AliveBlocks.test(Num))
return true;
// Registers defined in MBB cannot be live in.
const MachineInstr *Def = MRI.getVRegDef(Reg);
if (Def && Def->getParent() == &MBB)
return false;
// Reg was not defined in MBB, was it killed here?
return findKill(&MBB);
}
bool LiveVariables::isLiveOut(Register Reg, const MachineBasicBlock &MBB) {
LiveVariables::VarInfo &VI = getVarInfo(Reg);
SmallPtrSet<const MachineBasicBlock *, 8> Kills;
for (MachineInstr *MI : VI.Kills)
Kills.insert(MI->getParent());
// Loop over all of the successors of the basic block, checking to see if
// the value is either live in the block, or if it is killed in the block.
for (const MachineBasicBlock *SuccMBB : MBB.successors()) {
// Is it alive in this successor?
unsigned SuccIdx = SuccMBB->getNumber();
if (VI.AliveBlocks.test(SuccIdx))
return true;
// Or is it live because there is a use in a successor that kills it?
if (Kills.count(SuccMBB))
return true;
}
return false;
}
/// addNewBlock - Add a new basic block BB as an empty succcessor to DomBB. All
/// variables that are live out of DomBB will be marked as passing live through
/// BB.
void LiveVariables::addNewBlock(MachineBasicBlock *BB,
MachineBasicBlock *DomBB,
MachineBasicBlock *SuccBB) {
const unsigned NumNew = BB->getNumber();
DenseSet<unsigned> Defs, Kills;
MachineBasicBlock::iterator BBI = SuccBB->begin(), BBE = SuccBB->end();
for (; BBI != BBE && BBI->isPHI(); ++BBI) {
// Record the def of the PHI node.
Defs.insert(BBI->getOperand(0).getReg());
// All registers used by PHI nodes in SuccBB must be live through BB.
for (unsigned i = 1, e = BBI->getNumOperands(); i != e; i += 2)
if (BBI->getOperand(i+1).getMBB() == BB)
getVarInfo(BBI->getOperand(i).getReg()).AliveBlocks.set(NumNew);
}
// Record all vreg defs and kills of all instructions in SuccBB.
for (; BBI != BBE; ++BBI) {
for (const MachineOperand &Op : BBI->operands()) {
if (Op.isReg() && Register::isVirtualRegister(Op.getReg())) {
if (Op.isDef())
Defs.insert(Op.getReg());
else if (Op.isKill())
Kills.insert(Op.getReg());
}
}
}
// Update info for all live variables
for (unsigned i = 0, e = MRI->getNumVirtRegs(); i != e; ++i) {
Register Reg = Register::index2VirtReg(i);
// If the Defs is defined in the successor it can't be live in BB.
if (Defs.count(Reg))
continue;
// If the register is either killed in or live through SuccBB it's also live
// through BB.
VarInfo &VI = getVarInfo(Reg);
if (Kills.count(Reg) || VI.AliveBlocks.test(SuccBB->getNumber()))
VI.AliveBlocks.set(NumNew);
}
}
/// addNewBlock - Add a new basic block BB as an empty succcessor to DomBB. All
/// variables that are live out of DomBB will be marked as passing live through
/// BB. LiveInSets[BB] is *not* updated (because it is not needed during
/// PHIElimination).
void LiveVariables::addNewBlock(MachineBasicBlock *BB,
MachineBasicBlock *DomBB,
MachineBasicBlock *SuccBB,
std::vector<SparseBitVector<>> &LiveInSets) {
const unsigned NumNew = BB->getNumber();
SparseBitVector<> &BV = LiveInSets[SuccBB->getNumber()];
for (unsigned R : BV) {
Register VirtReg = Register::index2VirtReg(R);
LiveVariables::VarInfo &VI = getVarInfo(VirtReg);
VI.AliveBlocks.set(NumNew);
}
// All registers used by PHI nodes in SuccBB must be live through BB.
for (MachineBasicBlock::iterator BBI = SuccBB->begin(),
BBE = SuccBB->end();
BBI != BBE && BBI->isPHI(); ++BBI) {
for (unsigned i = 1, e = BBI->getNumOperands(); i != e; i += 2)
if (BBI->getOperand(i + 1).getMBB() == BB &&
BBI->getOperand(i).readsReg())
getVarInfo(BBI->getOperand(i).getReg())
.AliveBlocks.set(NumNew);
}
}
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