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llvm-mirror/lib/CodeGen/MachineLICM.cpp
Dan Gohman d88f1d63ac Add a FIXME comment. 
llvm-svn: 118803
2010-11-11 18:08:43 +00:00

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//===-- MachineLICM.cpp - Machine Loop Invariant Code Motion Pass ---------===//
//
// The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This pass performs loop invariant code motion on machine instructions. We
// attempt to remove as much code from the body of a loop as possible.
//
// This pass does not attempt to throttle itself to limit register pressure.
// The register allocation phases are expected to perform rematerialization
// to recover when register pressure is high.
//
// This pass is not intended to be a replacement or a complete alternative
// for the LLVM-IR-level LICM pass. It is only designed to hoist simple
// constructs that are not exposed before lowering and instruction selection.
//
//===----------------------------------------------------------------------===//
#define DEBUG_TYPE "machine-licm"
#include "llvm/CodeGen/Passes.h"
#include "llvm/CodeGen/MachineDominators.h"
#include "llvm/CodeGen/MachineFrameInfo.h"
#include "llvm/CodeGen/MachineLoopInfo.h"
#include "llvm/CodeGen/MachineMemOperand.h"
#include "llvm/CodeGen/MachineRegisterInfo.h"
#include "llvm/CodeGen/PseudoSourceValue.h"
#include "llvm/Target/TargetLowering.h"
#include "llvm/Target/TargetRegisterInfo.h"
#include "llvm/Target/TargetInstrInfo.h"
#include "llvm/Target/TargetInstrItineraries.h"
#include "llvm/Target/TargetMachine.h"
#include "llvm/Analysis/AliasAnalysis.h"
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/SmallSet.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/raw_ostream.h"
using namespace llvm;
STATISTIC(NumHoisted,
"Number of machine instructions hoisted out of loops");
STATISTIC(NumLowRP,
"Number of instructions hoisted in low reg pressure situation");
STATISTIC(NumHighLatency,
"Number of high latency instructions hoisted");
STATISTIC(NumCSEed,
"Number of hoisted machine instructions CSEed");
STATISTIC(NumPostRAHoisted,
"Number of machine instructions hoisted out of loops post regalloc");
namespace {
class MachineLICM : public MachineFunctionPass {
bool PreRegAlloc;
const TargetMachine *TM;
const TargetInstrInfo *TII;
const TargetLowering *TLI;
const TargetRegisterInfo *TRI;
const MachineFrameInfo *MFI;
MachineRegisterInfo *MRI;
const InstrItineraryData *InstrItins;
// Various analyses that we use...
AliasAnalysis *AA; // Alias analysis info.
MachineLoopInfo *MLI; // Current MachineLoopInfo
MachineDominatorTree *DT; // Machine dominator tree for the cur loop
// State that is updated as we process loops
bool Changed; // True if a loop is changed.
bool FirstInLoop; // True if it's the first LICM in the loop.
MachineLoop *CurLoop; // The current loop we are working on.
MachineBasicBlock *CurPreheader; // The preheader for CurLoop.
BitVector AllocatableSet;
// Track 'estimated' register pressure.
SmallSet<unsigned, 32> RegSeen;
SmallVector<unsigned, 8> RegPressure;
// Register pressure "limit" per register class. If the pressure
// is higher than the limit, then it's considered high.
SmallVector<unsigned, 8> RegLimit;
// Register pressure on path leading from loop preheader to current BB.
SmallVector<SmallVector<unsigned, 8>, 16> BackTrace;
// For each opcode, keep a list of potential CSE instructions.
DenseMap<unsigned, std::vector<const MachineInstr*> > CSEMap;
public:
static char ID; // Pass identification, replacement for typeid
MachineLICM() :
MachineFunctionPass(ID), PreRegAlloc(true) {
initializeMachineLICMPass(*PassRegistry::getPassRegistry());
}
explicit MachineLICM(bool PreRA) :
MachineFunctionPass(ID), PreRegAlloc(PreRA) {
initializeMachineLICMPass(*PassRegistry::getPassRegistry());
}
virtual bool runOnMachineFunction(MachineFunction &MF);
const char *getPassName() const { return "Machine Instruction LICM"; }
virtual void getAnalysisUsage(AnalysisUsage &AU) const {
AU.addRequired<MachineLoopInfo>();
AU.addRequired<MachineDominatorTree>();
AU.addRequired<AliasAnalysis>();
AU.addPreserved<MachineLoopInfo>();
AU.addPreserved<MachineDominatorTree>();
MachineFunctionPass::getAnalysisUsage(AU);
}
virtual void releaseMemory() {
RegSeen.clear();
RegPressure.clear();
RegLimit.clear();
BackTrace.clear();
for (DenseMap<unsigned,std::vector<const MachineInstr*> >::iterator
CI = CSEMap.begin(), CE = CSEMap.end(); CI != CE; ++CI)
CI->second.clear();
CSEMap.clear();
}
private:
/// CandidateInfo - Keep track of information about hoisting candidates.
struct CandidateInfo {
MachineInstr *MI;
unsigned Def;
int FI;
CandidateInfo(MachineInstr *mi, unsigned def, int fi)
: MI(mi), Def(def), FI(fi) {}
};
/// HoistRegionPostRA - Walk the specified region of the CFG and hoist loop
/// invariants out to the preheader.
void HoistRegionPostRA();
/// HoistPostRA - When an instruction is found to only use loop invariant
/// operands that is safe to hoist, this instruction is called to do the
/// dirty work.
void HoistPostRA(MachineInstr *MI, unsigned Def);
/// ProcessMI - Examine the instruction for potentai LICM candidate. Also
/// gather register def and frame object update information.
void ProcessMI(MachineInstr *MI, unsigned *PhysRegDefs,
SmallSet<int, 32> &StoredFIs,
SmallVector<CandidateInfo, 32> &Candidates);
/// AddToLiveIns - Add register 'Reg' to the livein sets of BBs in the
/// current loop.
void AddToLiveIns(unsigned Reg);
/// IsLICMCandidate - Returns true if the instruction may be a suitable
/// candidate for LICM. e.g. If the instruction is a call, then it's
/// obviously not safe to hoist it.
bool IsLICMCandidate(MachineInstr &I);
/// IsLoopInvariantInst - Returns true if the instruction is loop
/// invariant. I.e., all virtual register operands are defined outside of
/// the loop, physical registers aren't accessed (explicitly or implicitly),
/// and the instruction is hoistable.
///
bool IsLoopInvariantInst(MachineInstr &I);
/// HasHighOperandLatency - Compute operand latency between a def of 'Reg'
/// and an use in the current loop, return true if the target considered
/// it 'high'.
bool HasHighOperandLatency(MachineInstr &MI, unsigned DefIdx,
unsigned Reg) const;
bool IsCheapInstruction(MachineInstr &MI) const;
/// CanCauseHighRegPressure - Visit BBs from header to current BB,
/// check if hoisting an instruction of the given cost matrix can cause high
/// register pressure.
bool CanCauseHighRegPressure(DenseMap<unsigned, int> &Cost);
/// UpdateBackTraceRegPressure - Traverse the back trace from header to
/// the current block and update their register pressures to reflect the
/// effect of hoisting MI from the current block to the preheader.
void UpdateBackTraceRegPressure(const MachineInstr *MI);
/// IsProfitableToHoist - Return true if it is potentially profitable to
/// hoist the given loop invariant.
bool IsProfitableToHoist(MachineInstr &MI);
/// HoistRegion - Walk the specified region of the CFG (defined by all
/// blocks dominated by the specified block, and that are in the current
/// loop) in depth first order w.r.t the DominatorTree. This allows us to
/// visit definitions before uses, allowing us to hoist a loop body in one
/// pass without iteration.
///
void HoistRegion(MachineDomTreeNode *N, bool IsHeader = false);
/// InitRegPressure - Find all virtual register references that are liveout
/// of the preheader to initialize the starting "register pressure". Note
/// this does not count live through (livein but not used) registers.
void InitRegPressure(MachineBasicBlock *BB);
/// UpdateRegPressure - Update estimate of register pressure after the
/// specified instruction.
void UpdateRegPressure(const MachineInstr *MI);
/// isLoadFromConstantMemory - Return true if the given instruction is a
/// load from constant memory.
bool isLoadFromConstantMemory(MachineInstr *MI);
/// ExtractHoistableLoad - Unfold a load from the given machineinstr if
/// the load itself could be hoisted. Return the unfolded and hoistable
/// load, or null if the load couldn't be unfolded or if it wouldn't
/// be hoistable.
MachineInstr *ExtractHoistableLoad(MachineInstr *MI);
/// LookForDuplicate - Find an instruction amount PrevMIs that is a
/// duplicate of MI. Return this instruction if it's found.
const MachineInstr *LookForDuplicate(const MachineInstr *MI,
std::vector<const MachineInstr*> &PrevMIs);
/// EliminateCSE - Given a LICM'ed instruction, look for an instruction on
/// the preheader that compute the same value. If it's found, do a RAU on
/// with the definition of the existing instruction rather than hoisting
/// the instruction to the preheader.
bool EliminateCSE(MachineInstr *MI,
DenseMap<unsigned, std::vector<const MachineInstr*> >::iterator &CI);
/// Hoist - When an instruction is found to only use loop invariant operands
/// that is safe to hoist, this instruction is called to do the dirty work.
/// It returns true if the instruction is hoisted.
bool Hoist(MachineInstr *MI, MachineBasicBlock *Preheader);
/// InitCSEMap - Initialize the CSE map with instructions that are in the
/// current loop preheader that may become duplicates of instructions that
/// are hoisted out of the loop.
void InitCSEMap(MachineBasicBlock *BB);
/// getCurPreheader - Get the preheader for the current loop, splitting
/// a critical edge if needed.
MachineBasicBlock *getCurPreheader();
};
} // end anonymous namespace
char MachineLICM::ID = 0;
INITIALIZE_PASS_BEGIN(MachineLICM, "machinelicm",
"Machine Loop Invariant Code Motion", false, false)
INITIALIZE_PASS_DEPENDENCY(MachineLoopInfo)
INITIALIZE_PASS_DEPENDENCY(MachineDominatorTree)
INITIALIZE_AG_DEPENDENCY(AliasAnalysis)
INITIALIZE_PASS_END(MachineLICM, "machinelicm",
"Machine Loop Invariant Code Motion", false, false)
FunctionPass *llvm::createMachineLICMPass(bool PreRegAlloc) {
return new MachineLICM(PreRegAlloc);
}
/// LoopIsOuterMostWithPredecessor - Test if the given loop is the outer-most
/// loop that has a unique predecessor.
static bool LoopIsOuterMostWithPredecessor(MachineLoop *CurLoop) {
// Check whether this loop even has a unique predecessor.
if (!CurLoop->getLoopPredecessor())
return false;
// Ok, now check to see if any of its outer loops do.
for (MachineLoop *L = CurLoop->getParentLoop(); L; L = L->getParentLoop())
if (L->getLoopPredecessor())
return false;
// None of them did, so this is the outermost with a unique predecessor.
return true;
}
bool MachineLICM::runOnMachineFunction(MachineFunction &MF) {
if (PreRegAlloc)
DEBUG(dbgs() << "******** Pre-regalloc Machine LICM: ");
else
DEBUG(dbgs() << "******** Post-regalloc Machine LICM: ");
DEBUG(dbgs() << MF.getFunction()->getName() << " ********\n");
Changed = FirstInLoop = false;
TM = &MF.getTarget();
TII = TM->getInstrInfo();
TLI = TM->getTargetLowering();
TRI = TM->getRegisterInfo();
MFI = MF.getFrameInfo();
MRI = &MF.getRegInfo();
InstrItins = TM->getInstrItineraryData();
AllocatableSet = TRI->getAllocatableSet(MF);
if (PreRegAlloc) {
// Estimate register pressure during pre-regalloc pass.
unsigned NumRC = TRI->getNumRegClasses();
RegPressure.resize(NumRC);
std::fill(RegPressure.begin(), RegPressure.end(), 0);
RegLimit.resize(NumRC);
for (TargetRegisterInfo::regclass_iterator I = TRI->regclass_begin(),
E = TRI->regclass_end(); I != E; ++I)
RegLimit[(*I)->getID()] = TLI->getRegPressureLimit(*I, MF);
}
// Get our Loop information...
MLI = &getAnalysis<MachineLoopInfo>();
DT = &getAnalysis<MachineDominatorTree>();
AA = &getAnalysis<AliasAnalysis>();
SmallVector<MachineLoop *, 8> Worklist(MLI->begin(), MLI->end());
while (!Worklist.empty()) {
CurLoop = Worklist.pop_back_val();
CurPreheader = 0;
// If this is done before regalloc, only visit outer-most preheader-sporting
// loops.
if (PreRegAlloc && !LoopIsOuterMostWithPredecessor(CurLoop)) {
Worklist.append(CurLoop->begin(), CurLoop->end());
continue;
}
if (!PreRegAlloc)
HoistRegionPostRA();
else {
// CSEMap is initialized for loop header when the first instruction is
// being hoisted.
MachineDomTreeNode *N = DT->getNode(CurLoop->getHeader());
FirstInLoop = true;
HoistRegion(N, true);
CSEMap.clear();
}
}
return Changed;
}
/// InstructionStoresToFI - Return true if instruction stores to the
/// specified frame.
static bool InstructionStoresToFI(const MachineInstr *MI, int FI) {
for (MachineInstr::mmo_iterator o = MI->memoperands_begin(),
oe = MI->memoperands_end(); o != oe; ++o) {
if (!(*o)->isStore() || !(*o)->getValue())
continue;
if (const FixedStackPseudoSourceValue *Value =
dyn_cast<const FixedStackPseudoSourceValue>((*o)->getValue())) {
if (Value->getFrameIndex() == FI)
return true;
}
}
return false;
}
/// ProcessMI - Examine the instruction for potentai LICM candidate. Also
/// gather register def and frame object update information.
void MachineLICM::ProcessMI(MachineInstr *MI,
unsigned *PhysRegDefs,
SmallSet<int, 32> &StoredFIs,
SmallVector<CandidateInfo, 32> &Candidates) {
bool RuledOut = false;
bool HasNonInvariantUse = false;
unsigned Def = 0;
for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) {
const MachineOperand &MO = MI->getOperand(i);
if (MO.isFI()) {
// Remember if the instruction stores to the frame index.
int FI = MO.getIndex();
if (!StoredFIs.count(FI) &&
MFI->isSpillSlotObjectIndex(FI) &&
InstructionStoresToFI(MI, FI))
StoredFIs.insert(FI);
HasNonInvariantUse = true;
continue;
}
if (!MO.isReg())
continue;
unsigned Reg = MO.getReg();
if (!Reg)
continue;
assert(TargetRegisterInfo::isPhysicalRegister(Reg) &&
"Not expecting virtual register!");
if (!MO.isDef()) {
if (Reg && PhysRegDefs[Reg])
// If it's using a non-loop-invariant register, then it's obviously not
// safe to hoist.
HasNonInvariantUse = true;
continue;
}
if (MO.isImplicit()) {
++PhysRegDefs[Reg];
for (const unsigned *AS = TRI->getAliasSet(Reg); *AS; ++AS)
++PhysRegDefs[*AS];
if (!MO.isDead())
// Non-dead implicit def? This cannot be hoisted.
RuledOut = true;
// No need to check if a dead implicit def is also defined by
// another instruction.
continue;
}
// FIXME: For now, avoid instructions with multiple defs, unless
// it's a dead implicit def.
if (Def)
RuledOut = true;
else
Def = Reg;
// If we have already seen another instruction that defines the same
// register, then this is not safe.
if (++PhysRegDefs[Reg] > 1)
// MI defined register is seen defined by another instruction in
// the loop, it cannot be a LICM candidate.
RuledOut = true;
for (const unsigned *AS = TRI->getAliasSet(Reg); *AS; ++AS)
if (++PhysRegDefs[*AS] > 1)
RuledOut = true;
}
// Only consider reloads for now and remats which do not have register
// operands. FIXME: Consider unfold load folding instructions.
if (Def && !RuledOut) {
int FI = INT_MIN;
if ((!HasNonInvariantUse && IsLICMCandidate(*MI)) ||
(TII->isLoadFromStackSlot(MI, FI) && MFI->isSpillSlotObjectIndex(FI)))
Candidates.push_back(CandidateInfo(MI, Def, FI));
}
}
/// HoistRegionPostRA - Walk the specified region of the CFG and hoist loop
/// invariants out to the preheader.
void MachineLICM::HoistRegionPostRA() {
unsigned NumRegs = TRI->getNumRegs();
unsigned *PhysRegDefs = new unsigned[NumRegs];
std::fill(PhysRegDefs, PhysRegDefs + NumRegs, 0);
SmallVector<CandidateInfo, 32> Candidates;
SmallSet<int, 32> StoredFIs;
// Walk the entire region, count number of defs for each register, and
// collect potential LICM candidates.
const std::vector<MachineBasicBlock*> Blocks = CurLoop->getBlocks();
for (unsigned i = 0, e = Blocks.size(); i != e; ++i) {
MachineBasicBlock *BB = Blocks[i];
// Conservatively treat live-in's as an external def.
// FIXME: That means a reload that're reused in successor block(s) will not
// be LICM'ed.
for (MachineBasicBlock::livein_iterator I = BB->livein_begin(),
E = BB->livein_end(); I != E; ++I) {
unsigned Reg = *I;
++PhysRegDefs[Reg];
for (const unsigned *AS = TRI->getAliasSet(Reg); *AS; ++AS)
++PhysRegDefs[*AS];
}
for (MachineBasicBlock::iterator
MII = BB->begin(), E = BB->end(); MII != E; ++MII) {
MachineInstr *MI = &*MII;
ProcessMI(MI, PhysRegDefs, StoredFIs, Candidates);
}
}
// Now evaluate whether the potential candidates qualify.
// 1. Check if the candidate defined register is defined by another
// instruction in the loop.
// 2. If the candidate is a load from stack slot (always true for now),
// check if the slot is stored anywhere in the loop.
for (unsigned i = 0, e = Candidates.size(); i != e; ++i) {
if (Candidates[i].FI != INT_MIN &&
StoredFIs.count(Candidates[i].FI))
continue;
if (PhysRegDefs[Candidates[i].Def] == 1) {
bool Safe = true;
MachineInstr *MI = Candidates[i].MI;
for (unsigned j = 0, ee = MI->getNumOperands(); j != ee; ++j) {
const MachineOperand &MO = MI->getOperand(j);
if (!MO.isReg() || MO.isDef() || !MO.getReg())
continue;
if (PhysRegDefs[MO.getReg()]) {
// If it's using a non-loop-invariant register, then it's obviously
// not safe to hoist.
Safe = false;
break;
}
}
if (Safe)
HoistPostRA(MI, Candidates[i].Def);
}
}
delete[] PhysRegDefs;
}
/// AddToLiveIns - Add register 'Reg' to the livein sets of BBs in the current
/// loop, and make sure it is not killed by any instructions in the loop.
void MachineLICM::AddToLiveIns(unsigned Reg) {
const std::vector<MachineBasicBlock*> Blocks = CurLoop->getBlocks();
for (unsigned i = 0, e = Blocks.size(); i != e; ++i) {
MachineBasicBlock *BB = Blocks[i];
if (!BB->isLiveIn(Reg))
BB->addLiveIn(Reg);
for (MachineBasicBlock::iterator
MII = BB->begin(), E = BB->end(); MII != E; ++MII) {
MachineInstr *MI = &*MII;
for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) {
MachineOperand &MO = MI->getOperand(i);
if (!MO.isReg() || !MO.getReg() || MO.isDef()) continue;
if (MO.getReg() == Reg || TRI->isSuperRegister(Reg, MO.getReg()))
MO.setIsKill(false);
}
}
}
}
/// HoistPostRA - When an instruction is found to only use loop invariant
/// operands that is safe to hoist, this instruction is called to do the
/// dirty work.
void MachineLICM::HoistPostRA(MachineInstr *MI, unsigned Def) {
MachineBasicBlock *Preheader = getCurPreheader();
if (!Preheader) return;
// Now move the instructions to the predecessor, inserting it before any
// terminator instructions.
DEBUG({
dbgs() << "Hoisting " << *MI;
if (Preheader->getBasicBlock())
dbgs() << " to MachineBasicBlock "
<< Preheader->getName();
if (MI->getParent()->getBasicBlock())
dbgs() << " from MachineBasicBlock "
<< MI->getParent()->getName();
dbgs() << "\n";
});
// Splice the instruction to the preheader.
MachineBasicBlock *MBB = MI->getParent();
Preheader->splice(Preheader->getFirstTerminator(), MBB, MI);
// Add register to livein list to all the BBs in the current loop since a
// loop invariant must be kept live throughout the whole loop. This is
// important to ensure later passes do not scavenge the def register.
AddToLiveIns(Def);
++NumPostRAHoisted;
Changed = true;
}
/// HoistRegion - Walk the specified region of the CFG (defined by all blocks
/// dominated by the specified block, and that are in the current loop) in depth
/// first order w.r.t the DominatorTree. This allows us to visit definitions
/// before uses, allowing us to hoist a loop body in one pass without iteration.
///
void MachineLICM::HoistRegion(MachineDomTreeNode *N, bool IsHeader) {
assert(N != 0 && "Null dominator tree node?");
MachineBasicBlock *BB = N->getBlock();
// If this subregion is not in the top level loop at all, exit.
if (!CurLoop->contains(BB)) return;
MachineBasicBlock *Preheader = getCurPreheader();
if (!Preheader)
return;
if (IsHeader) {
// Compute registers which are livein into the loop headers.
RegSeen.clear();
BackTrace.clear();
InitRegPressure(Preheader);
}
// Remember livein register pressure.
BackTrace.push_back(RegPressure);
for (MachineBasicBlock::iterator
MII = BB->begin(), E = BB->end(); MII != E; ) {
MachineBasicBlock::iterator NextMII = MII; ++NextMII;
MachineInstr *MI = &*MII;
if (!Hoist(MI, Preheader))
UpdateRegPressure(MI);
MII = NextMII;
}
// Don't hoist things out of a large switch statement. This often causes
// code to be hoisted that wasn't going to be executed, and increases
// register pressure in a situation where it's likely to matter.
if (BB->succ_size() < 25) {
const std::vector<MachineDomTreeNode*> &Children = N->getChildren();
for (unsigned I = 0, E = Children.size(); I != E; ++I)
HoistRegion(Children[I]);
}
BackTrace.pop_back();
}
static bool isOperandKill(const MachineOperand &MO, MachineRegisterInfo *MRI) {
return MO.isKill() || MRI->hasOneNonDBGUse(MO.getReg());
}
/// InitRegPressure - Find all virtual register references that are liveout of
/// the preheader to initialize the starting "register pressure". Note this
/// does not count live through (livein but not used) registers.
void MachineLICM::InitRegPressure(MachineBasicBlock *BB) {
std::fill(RegPressure.begin(), RegPressure.end(), 0);
// If the preheader has only a single predecessor and it ends with a
// fallthrough or an unconditional branch, then scan its predecessor for live
// defs as well. This happens whenever the preheader is created by splitting
// the critical edge from the loop predecessor to the loop header.
if (BB->pred_size() == 1) {
MachineBasicBlock *TBB = 0, *FBB = 0;
SmallVector<MachineOperand, 4> Cond;
if (!TII->AnalyzeBranch(*BB, TBB, FBB, Cond, false) && Cond.empty())
InitRegPressure(*BB->pred_begin());
}
for (MachineBasicBlock::iterator MII = BB->begin(), E = BB->end();
MII != E; ++MII) {
MachineInstr *MI = &*MII;
for (unsigned i = 0, e = MI->getDesc().getNumOperands(); i != e; ++i) {
const MachineOperand &MO = MI->getOperand(i);
if (!MO.isReg() || MO.isImplicit())
continue;
unsigned Reg = MO.getReg();
if (!Reg || TargetRegisterInfo::isPhysicalRegister(Reg))
continue;
bool isNew = RegSeen.insert(Reg);
const TargetRegisterClass *RC = MRI->getRegClass(Reg);
EVT VT = *RC->vt_begin();
unsigned RCId = TLI->getRepRegClassFor(VT)->getID();
if (MO.isDef())
RegPressure[RCId] += TLI->getRepRegClassCostFor(VT);
else {
bool isKill = isOperandKill(MO, MRI);
if (isNew && !isKill)
// Haven't seen this, it must be a livein.
RegPressure[RCId] += TLI->getRepRegClassCostFor(VT);
else if (!isNew && isKill)
RegPressure[RCId] -= TLI->getRepRegClassCostFor(VT);
}
}
}
}
/// UpdateRegPressure - Update estimate of register pressure after the
/// specified instruction.
void MachineLICM::UpdateRegPressure(const MachineInstr *MI) {
if (MI->isImplicitDef())
return;
SmallVector<unsigned, 4> Defs;
for (unsigned i = 0, e = MI->getDesc().getNumOperands(); i != e; ++i) {
const MachineOperand &MO = MI->getOperand(i);
if (!MO.isReg() || MO.isImplicit())
continue;
unsigned Reg = MO.getReg();
if (!Reg || TargetRegisterInfo::isPhysicalRegister(Reg))
continue;
bool isNew = RegSeen.insert(Reg);
if (MO.isDef())
Defs.push_back(Reg);
else if (!isNew && isOperandKill(MO, MRI)) {
const TargetRegisterClass *RC = MRI->getRegClass(Reg);
EVT VT = *RC->vt_begin();
unsigned RCId = TLI->getRepRegClassFor(VT)->getID();
unsigned RCCost = TLI->getRepRegClassCostFor(VT);
if (RCCost > RegPressure[RCId])
RegPressure[RCId] = 0;
else
RegPressure[RCId] -= RCCost;
}
}
while (!Defs.empty()) {
unsigned Reg = Defs.pop_back_val();
const TargetRegisterClass *RC = MRI->getRegClass(Reg);
EVT VT = *RC->vt_begin();
unsigned RCId = TLI->getRepRegClassFor(VT)->getID();
unsigned RCCost = TLI->getRepRegClassCostFor(VT);
RegPressure[RCId] += RCCost;
}
}
/// IsLICMCandidate - Returns true if the instruction may be a suitable
/// candidate for LICM. e.g. If the instruction is a call, then it's obviously
/// not safe to hoist it.
bool MachineLICM::IsLICMCandidate(MachineInstr &I) {
// Check if it's safe to move the instruction.
bool DontMoveAcrossStore = true;
if (!I.isSafeToMove(TII, AA, DontMoveAcrossStore))
return false;
return true;
}
/// IsLoopInvariantInst - Returns true if the instruction is loop
/// invariant. I.e., all virtual register operands are defined outside of the
/// loop, physical registers aren't accessed explicitly, and there are no side
/// effects that aren't captured by the operands or other flags.
///
bool MachineLICM::IsLoopInvariantInst(MachineInstr &I) {
if (!IsLICMCandidate(I))
return false;
// The instruction is loop invariant if all of its operands are.
for (unsigned i = 0, e = I.getNumOperands(); i != e; ++i) {
const MachineOperand &MO = I.getOperand(i);
if (!MO.isReg())
continue;
unsigned Reg = MO.getReg();
if (Reg == 0) continue;
// Don't hoist an instruction that uses or defines a physical register.
if (TargetRegisterInfo::isPhysicalRegister(Reg)) {
if (MO.isUse()) {
// If the physreg has no defs anywhere, it's just an ambient register
// and we can freely move its uses. Alternatively, if it's allocatable,
// it could get allocated to something with a def during allocation.
if (!MRI->def_empty(Reg))
return false;
if (AllocatableSet.test(Reg))
return false;
// Check for a def among the register's aliases too.
for (const unsigned *Alias = TRI->getAliasSet(Reg); *Alias; ++Alias) {
unsigned AliasReg = *Alias;
if (!MRI->def_empty(AliasReg))
return false;
if (AllocatableSet.test(AliasReg))
return false;
}
// Otherwise it's safe to move.
continue;
} else if (!MO.isDead()) {
// A def that isn't dead. We can't move it.
return false;
} else if (CurLoop->getHeader()->isLiveIn(Reg)) {
// If the reg is live into the loop, we can't hoist an instruction
// which would clobber it.
return false;
}
}
if (!MO.isUse())
continue;
assert(MRI->getVRegDef(Reg) &&
"Machine instr not mapped for this vreg?!");
// If the loop contains the definition of an operand, then the instruction
// isn't loop invariant.
if (CurLoop->contains(MRI->getVRegDef(Reg)))
return false;
}
// If we got this far, the instruction is loop invariant!
return true;
}
/// HasPHIUses - Return true if the specified register has any PHI use.
static bool HasPHIUses(unsigned Reg, MachineRegisterInfo *MRI) {
for (MachineRegisterInfo::use_iterator UI = MRI->use_begin(Reg),
UE = MRI->use_end(); UI != UE; ++UI) {
MachineInstr *UseMI = &*UI;
if (UseMI->isPHI())
return true;
}
return false;
}
/// isLoadFromConstantMemory - Return true if the given instruction is a
/// load from constant memory. Machine LICM will hoist these even if they are
/// not re-materializable.
bool MachineLICM::isLoadFromConstantMemory(MachineInstr *MI) {
if (!MI->getDesc().mayLoad()) return false;
if (!MI->hasOneMemOperand()) return false;
MachineMemOperand *MMO = *MI->memoperands_begin();
if (MMO->isVolatile()) return false;
if (!MMO->getValue()) return false;
const PseudoSourceValue *PSV = dyn_cast<PseudoSourceValue>(MMO->getValue());
if (PSV) {
MachineFunction &MF = *MI->getParent()->getParent();
return PSV->isConstant(MF.getFrameInfo());
} else {
return AA->pointsToConstantMemory(AliasAnalysis::Location(MMO->getValue(),
MMO->getSize(),
MMO->getTBAAInfo()));
}
}
/// HasHighOperandLatency - Compute operand latency between a def of 'Reg'
/// and an use in the current loop, return true if the target considered
/// it 'high'.
bool MachineLICM::HasHighOperandLatency(MachineInstr &MI,
unsigned DefIdx, unsigned Reg) const {
if (!InstrItins || InstrItins->isEmpty() || MRI->use_nodbg_empty(Reg))
return false;
for (MachineRegisterInfo::use_nodbg_iterator I = MRI->use_nodbg_begin(Reg),
E = MRI->use_nodbg_end(); I != E; ++I) {
MachineInstr *UseMI = &*I;
if (UseMI->isCopyLike())
continue;
if (!CurLoop->contains(UseMI->getParent()))
continue;
for (unsigned i = 0, e = UseMI->getNumOperands(); i != e; ++i) {
const MachineOperand &MO = UseMI->getOperand(i);
if (!MO.isReg() || !MO.isUse())
continue;
unsigned MOReg = MO.getReg();
if (MOReg != Reg)
continue;
if (TII->hasHighOperandLatency(InstrItins, MRI, &MI, DefIdx, UseMI, i))
return true;
}
// Only look at the first in loop use.
break;
}
return false;
}
/// IsCheapInstruction - Return true if the instruction is marked "cheap" or
/// the operand latency between its def and a use is one or less.
bool MachineLICM::IsCheapInstruction(MachineInstr &MI) const {
if (MI.getDesc().isAsCheapAsAMove() || MI.isCopyLike())
return true;
if (!InstrItins || InstrItins->isEmpty())
return false;
bool isCheap = false;
unsigned NumDefs = MI.getDesc().getNumDefs();
for (unsigned i = 0, e = MI.getNumOperands(); NumDefs && i != e; ++i) {
MachineOperand &DefMO = MI.getOperand(i);
if (!DefMO.isReg() || !DefMO.isDef())
continue;
--NumDefs;
unsigned Reg = DefMO.getReg();
if (TargetRegisterInfo::isPhysicalRegister(Reg))
continue;
if (!TII->hasLowDefLatency(InstrItins, &MI, i))
return false;
isCheap = true;
}
return isCheap;
}
/// CanCauseHighRegPressure - Visit BBs from header to current BB, check
/// if hoisting an instruction of the given cost matrix can cause high
/// register pressure.
bool MachineLICM::CanCauseHighRegPressure(DenseMap<unsigned, int> &Cost) {
for (DenseMap<unsigned, int>::iterator CI = Cost.begin(), CE = Cost.end();
CI != CE; ++CI) {
if (CI->second <= 0)
continue;
unsigned RCId = CI->first;
for (unsigned i = BackTrace.size(); i != 0; --i) {
SmallVector<unsigned, 8> &RP = BackTrace[i-1];
if (RP[RCId] + CI->second >= RegLimit[RCId])
return true;
}
}
return false;
}
/// UpdateBackTraceRegPressure - Traverse the back trace from header to the
/// current block and update their register pressures to reflect the effect
/// of hoisting MI from the current block to the preheader.
void MachineLICM::UpdateBackTraceRegPressure(const MachineInstr *MI) {
if (MI->isImplicitDef())
return;
// First compute the 'cost' of the instruction, i.e. its contribution
// to register pressure.
DenseMap<unsigned, int> Cost;
for (unsigned i = 0, e = MI->getDesc().getNumOperands(); i != e; ++i) {
const MachineOperand &MO = MI->getOperand(i);
if (!MO.isReg() || MO.isImplicit())
continue;
unsigned Reg = MO.getReg();
if (!Reg || TargetRegisterInfo::isPhysicalRegister(Reg))
continue;
const TargetRegisterClass *RC = MRI->getRegClass(Reg);
EVT VT = *RC->vt_begin();
unsigned RCId = TLI->getRepRegClassFor(VT)->getID();
unsigned RCCost = TLI->getRepRegClassCostFor(VT);
if (MO.isDef()) {
DenseMap<unsigned, int>::iterator CI = Cost.find(RCId);
if (CI != Cost.end())
CI->second += RCCost;
else
Cost.insert(std::make_pair(RCId, RCCost));
} else if (isOperandKill(MO, MRI)) {
DenseMap<unsigned, int>::iterator CI = Cost.find(RCId);
if (CI != Cost.end())
CI->second -= RCCost;
else
Cost.insert(std::make_pair(RCId, -RCCost));
}
}
// Update register pressure of blocks from loop header to current block.
for (unsigned i = 0, e = BackTrace.size(); i != e; ++i) {
SmallVector<unsigned, 8> &RP = BackTrace[i];
for (DenseMap<unsigned, int>::iterator CI = Cost.begin(), CE = Cost.end();
CI != CE; ++CI) {
unsigned RCId = CI->first;
RP[RCId] += CI->second;
}
}
}
/// IsProfitableToHoist - Return true if it is potentially profitable to hoist
/// the given loop invariant.
bool MachineLICM::IsProfitableToHoist(MachineInstr &MI) {
if (MI.isImplicitDef())
return true;
// If the instruction is cheap, only hoist if it is re-materilizable. LICM
// will increase register pressure. It's probably not worth it if the
// instruction is cheap.
// Also hoist loads from constant memory, e.g. load from stubs, GOT. Hoisting
// these tend to help performance in low register pressure situation. The
// trade off is it may cause spill in high pressure situation. It will end up
// adding a store in the loop preheader. But the reload is no more expensive.
// The side benefit is these loads are frequently CSE'ed.
if (IsCheapInstruction(MI)) {
if (!TII->isTriviallyReMaterializable(&MI, AA))
return false;
} else {
// Estimate register pressure to determine whether to LICM the instruction.
// In low register pressure situation, we can be more aggressive about
// hoisting. Also, favors hoisting long latency instructions even in
// moderately high pressure situation.
// FIXME: If there are long latency loop-invariant instructions inside the
// loop at this point, why didn't the optimizer's LICM hoist them?
DenseMap<unsigned, int> Cost;
for (unsigned i = 0, e = MI.getDesc().getNumOperands(); i != e; ++i) {
const MachineOperand &MO = MI.getOperand(i);
if (!MO.isReg() || MO.isImplicit())
continue;
unsigned Reg = MO.getReg();
if (!Reg || TargetRegisterInfo::isPhysicalRegister(Reg))
continue;
if (MO.isDef()) {
if (HasHighOperandLatency(MI, i, Reg)) {
++NumHighLatency;
return true;
}
const TargetRegisterClass *RC = MRI->getRegClass(Reg);
EVT VT = *RC->vt_begin();
unsigned RCId = TLI->getRepRegClassFor(VT)->getID();
unsigned RCCost = TLI->getRepRegClassCostFor(VT);
DenseMap<unsigned, int>::iterator CI = Cost.find(RCId);
if (CI != Cost.end())
CI->second += RCCost;
else
Cost.insert(std::make_pair(RCId, RCCost));
} else if (isOperandKill(MO, MRI)) {
// Is a virtual register use is a kill, hoisting it out of the loop
// may actually reduce register pressure or be register pressure
// neutral.
const TargetRegisterClass *RC = MRI->getRegClass(Reg);
EVT VT = *RC->vt_begin();
unsigned RCId = TLI->getRepRegClassFor(VT)->getID();
unsigned RCCost = TLI->getRepRegClassCostFor(VT);
DenseMap<unsigned, int>::iterator CI = Cost.find(RCId);
if (CI != Cost.end())
CI->second -= RCCost;
else
Cost.insert(std::make_pair(RCId, -RCCost));
}
}
// Visit BBs from header to current BB, if hoisting this doesn't cause
// high register pressure, then it's safe to proceed.
if (!CanCauseHighRegPressure(Cost)) {
++NumLowRP;
return true;
}
// High register pressure situation, only hoist if the instruction is going to
// be remat'ed.
if (!TII->isTriviallyReMaterializable(&MI, AA) &&
!isLoadFromConstantMemory(&MI))
return false;
}
// If result(s) of this instruction is used by PHIs, then don't hoist it.
// The presence of joins makes it difficult for current register allocator
// implementation to perform remat.
for (unsigned i = 0, e = MI.getNumOperands(); i != e; ++i) {
const MachineOperand &MO = MI.getOperand(i);
if (!MO.isReg() || !MO.isDef())
continue;
if (HasPHIUses(MO.getReg(), MRI))
return false;
}
return true;
}
MachineInstr *MachineLICM::ExtractHoistableLoad(MachineInstr *MI) {
// Don't unfold simple loads.
if (MI->getDesc().canFoldAsLoad())
return 0;
// If not, we may be able to unfold a load and hoist that.
// First test whether the instruction is loading from an amenable
// memory location.
if (!isLoadFromConstantMemory(MI))
return 0;
// Next determine the register class for a temporary register.
unsigned LoadRegIndex;
unsigned NewOpc =
TII->getOpcodeAfterMemoryUnfold(MI->getOpcode(),
/*UnfoldLoad=*/true,
/*UnfoldStore=*/false,
&LoadRegIndex);
if (NewOpc == 0) return 0;
const TargetInstrDesc &TID = TII->get(NewOpc);
if (TID.getNumDefs() != 1) return 0;
const TargetRegisterClass *RC = TID.OpInfo[LoadRegIndex].getRegClass(TRI);
// Ok, we're unfolding. Create a temporary register and do the unfold.
unsigned Reg = MRI->createVirtualRegister(RC);
MachineFunction &MF = *MI->getParent()->getParent();
SmallVector<MachineInstr *, 2> NewMIs;
bool Success =
TII->unfoldMemoryOperand(MF, MI, Reg,
/*UnfoldLoad=*/true, /*UnfoldStore=*/false,
NewMIs);
(void)Success;
assert(Success &&
"unfoldMemoryOperand failed when getOpcodeAfterMemoryUnfold "
"succeeded!");
assert(NewMIs.size() == 2 &&
"Unfolded a load into multiple instructions!");
MachineBasicBlock *MBB = MI->getParent();
MBB->insert(MI, NewMIs[0]);
MBB->insert(MI, NewMIs[1]);
// If unfolding produced a load that wasn't loop-invariant or profitable to
// hoist, discard the new instructions and bail.
if (!IsLoopInvariantInst(*NewMIs[0]) || !IsProfitableToHoist(*NewMIs[0])) {
NewMIs[0]->eraseFromParent();
NewMIs[1]->eraseFromParent();
return 0;
}
// Update register pressure for the unfolded instruction.
UpdateRegPressure(NewMIs[1]);
// Otherwise we successfully unfolded a load that we can hoist.
MI->eraseFromParent();
return NewMIs[0];
}
void MachineLICM::InitCSEMap(MachineBasicBlock *BB) {
for (MachineBasicBlock::iterator I = BB->begin(),E = BB->end(); I != E; ++I) {
const MachineInstr *MI = &*I;
// FIXME: For now, only hoist re-materilizable instructions. LICM will
// increase register pressure. We want to make sure it doesn't increase
// spilling.
if (TII->isTriviallyReMaterializable(MI, AA)) {
unsigned Opcode = MI->getOpcode();
DenseMap<unsigned, std::vector<const MachineInstr*> >::iterator
CI = CSEMap.find(Opcode);
if (CI != CSEMap.end())
CI->second.push_back(MI);
else {
std::vector<const MachineInstr*> CSEMIs;
CSEMIs.push_back(MI);
CSEMap.insert(std::make_pair(Opcode, CSEMIs));
}
}
}
}
const MachineInstr*
MachineLICM::LookForDuplicate(const MachineInstr *MI,
std::vector<const MachineInstr*> &PrevMIs) {
for (unsigned i = 0, e = PrevMIs.size(); i != e; ++i) {
const MachineInstr *PrevMI = PrevMIs[i];
if (TII->produceSameValue(MI, PrevMI))
return PrevMI;
}
return 0;
}
bool MachineLICM::EliminateCSE(MachineInstr *MI,
DenseMap<unsigned, std::vector<const MachineInstr*> >::iterator &CI) {
// Do not CSE implicit_def so ProcessImplicitDefs can properly propagate
// the undef property onto uses.
if (CI == CSEMap.end() || MI->isImplicitDef())
return false;
if (const MachineInstr *Dup = LookForDuplicate(MI, CI->second)) {
DEBUG(dbgs() << "CSEing " << *MI << " with " << *Dup);
// Replace virtual registers defined by MI by their counterparts defined
// by Dup.
for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) {
const MachineOperand &MO = MI->getOperand(i);
// Physical registers may not differ here.
assert((!MO.isReg() || MO.getReg() == 0 ||
!TargetRegisterInfo::isPhysicalRegister(MO.getReg()) ||
MO.getReg() == Dup->getOperand(i).getReg()) &&
"Instructions with different phys regs are not identical!");
if (MO.isReg() && MO.isDef() &&
!TargetRegisterInfo::isPhysicalRegister(MO.getReg())) {
MRI->replaceRegWith(MO.getReg(), Dup->getOperand(i).getReg());
MRI->clearKillFlags(Dup->getOperand(i).getReg());
}
}
MI->eraseFromParent();
++NumCSEed;
return true;
}
return false;
}
/// Hoist - When an instruction is found to use only loop invariant operands
/// that are safe to hoist, this instruction is called to do the dirty work.
///
bool MachineLICM::Hoist(MachineInstr *MI, MachineBasicBlock *Preheader) {
// First check whether we should hoist this instruction.
if (!IsLoopInvariantInst(*MI) || !IsProfitableToHoist(*MI)) {
// If not, try unfolding a hoistable load.
MI = ExtractHoistableLoad(MI);
if (!MI) return false;
}
// Now move the instructions to the predecessor, inserting it before any
// terminator instructions.
DEBUG({
dbgs() << "Hoisting " << *MI;
if (Preheader->getBasicBlock())
dbgs() << " to MachineBasicBlock "
<< Preheader->getName();
if (MI->getParent()->getBasicBlock())
dbgs() << " from MachineBasicBlock "
<< MI->getParent()->getName();
dbgs() << "\n";
});
// If this is the first instruction being hoisted to the preheader,
// initialize the CSE map with potential common expressions.
if (FirstInLoop) {
InitCSEMap(Preheader);
FirstInLoop = false;
}
// Look for opportunity to CSE the hoisted instruction.
unsigned Opcode = MI->getOpcode();
DenseMap<unsigned, std::vector<const MachineInstr*> >::iterator
CI = CSEMap.find(Opcode);
if (!EliminateCSE(MI, CI)) {
// Otherwise, splice the instruction to the preheader.
Preheader->splice(Preheader->getFirstTerminator(),MI->getParent(),MI);
// Update register pressure for BBs from header to this block.
UpdateBackTraceRegPressure(MI);
// Clear the kill flags of any register this instruction defines,
// since they may need to be live throughout the entire loop
// rather than just live for part of it.
for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) {
MachineOperand &MO = MI->getOperand(i);
if (MO.isReg() && MO.isDef() && !MO.isDead())
MRI->clearKillFlags(MO.getReg());
}
// Add to the CSE map.
if (CI != CSEMap.end())
CI->second.push_back(MI);
else {
std::vector<const MachineInstr*> CSEMIs;
CSEMIs.push_back(MI);
CSEMap.insert(std::make_pair(Opcode, CSEMIs));
}
}
++NumHoisted;
Changed = true;
return true;
}
MachineBasicBlock *MachineLICM::getCurPreheader() {
// Determine the block to which to hoist instructions. If we can't find a
// suitable loop predecessor, we can't do any hoisting.
// If we've tried to get a preheader and failed, don't try again.
if (CurPreheader == reinterpret_cast<MachineBasicBlock *>(-1))
return 0;
if (!CurPreheader) {
CurPreheader = CurLoop->getLoopPreheader();
if (!CurPreheader) {
MachineBasicBlock *Pred = CurLoop->getLoopPredecessor();
if (!Pred) {
CurPreheader = reinterpret_cast<MachineBasicBlock *>(-1);
return 0;
}
CurPreheader = Pred->SplitCriticalEdge(CurLoop->getHeader(), this);
if (!CurPreheader) {
CurPreheader = reinterpret_cast<MachineBasicBlock *>(-1);
return 0;
}
}
}
return CurPreheader;
}
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