1 //===-- MachineLICM.cpp - Machine Loop Invariant Code Motion Pass ---------===//
3 // The LLVM Compiler Infrastructure
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
8 //===----------------------------------------------------------------------===//
10 // This pass performs loop invariant code motion on machine instructions. We
11 // attempt to remove as much code from the body of a loop as possible.
13 // This pass does not attempt to throttle itself to limit register pressure.
14 // The register allocation phases are expected to perform rematerialization
15 // to recover when register pressure is high.
17 // This pass is not intended to be a replacement or a complete alternative
18 // for the LLVM-IR-level LICM pass. It is only designed to hoist simple
19 // constructs that are not exposed before lowering and instruction selection.
21 //===----------------------------------------------------------------------===//
23 #define DEBUG_TYPE "machine-licm"
24 #include "llvm/CodeGen/Passes.h"
25 #include "llvm/CodeGen/MachineConstantPool.h"
26 #include "llvm/CodeGen/MachineDominators.h"
27 #include "llvm/CodeGen/MachineLoopInfo.h"
28 #include "llvm/CodeGen/MachineMemOperand.h"
29 #include "llvm/CodeGen/MachineRegisterInfo.h"
30 #include "llvm/CodeGen/PseudoSourceValue.h"
31 #include "llvm/Target/TargetRegisterInfo.h"
32 #include "llvm/Target/TargetInstrInfo.h"
33 #include "llvm/Target/TargetMachine.h"
34 #include "llvm/Analysis/AliasAnalysis.h"
35 #include "llvm/ADT/DenseMap.h"
36 #include "llvm/ADT/Statistic.h"
37 #include "llvm/Support/Debug.h"
38 #include "llvm/Support/raw_ostream.h"
42 STATISTIC(NumHoisted, "Number of machine instructions hoisted out of loops");
43 STATISTIC(NumCSEed, "Number of hoisted machine instructions CSEed");
46 class MachineLICM : public MachineFunctionPass {
47 MachineConstantPool *MCP;
48 const TargetMachine *TM;
49 const TargetInstrInfo *TII;
50 const TargetRegisterInfo *TRI;
51 BitVector AllocatableSet;
53 // Various analyses that we use...
54 AliasAnalysis *AA; // Alias analysis info.
55 MachineLoopInfo *LI; // Current MachineLoopInfo
56 MachineDominatorTree *DT; // Machine dominator tree for the cur loop
57 MachineRegisterInfo *RegInfo; // Machine register information
59 // State that is updated as we process loops
60 bool Changed; // True if a loop is changed.
61 bool FirstInLoop; // True if it's the first LICM in the loop.
62 MachineLoop *CurLoop; // The current loop we are working on.
63 MachineBasicBlock *CurPreheader; // The preheader for CurLoop.
65 // For each opcode, keep a list of potentail CSE instructions.
66 DenseMap<unsigned, std::vector<const MachineInstr*> > CSEMap;
68 static char ID; // Pass identification, replacement for typeid
69 MachineLICM() : MachineFunctionPass(&ID) {}
71 virtual bool runOnMachineFunction(MachineFunction &MF);
73 const char *getPassName() const { return "Machine Instruction LICM"; }
75 // FIXME: Loop preheaders?
76 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
78 AU.addRequired<MachineLoopInfo>();
79 AU.addRequired<MachineDominatorTree>();
80 AU.addRequired<AliasAnalysis>();
81 AU.addPreserved<MachineLoopInfo>();
82 AU.addPreserved<MachineDominatorTree>();
83 MachineFunctionPass::getAnalysisUsage(AU);
86 virtual void releaseMemory() {
91 /// IsLoopInvariantInst - Returns true if the instruction is loop
92 /// invariant. I.e., all virtual register operands are defined outside of
93 /// the loop, physical registers aren't accessed (explicitly or implicitly),
94 /// and the instruction is hoistable.
96 bool IsLoopInvariantInst(MachineInstr &I);
98 /// IsProfitableToHoist - Return true if it is potentially profitable to
99 /// hoist the given loop invariant.
100 bool IsProfitableToHoist(MachineInstr &MI);
102 /// HoistRegion - Walk the specified region of the CFG (defined by all
103 /// blocks dominated by the specified block, and that are in the current
104 /// loop) in depth first order w.r.t the DominatorTree. This allows us to
105 /// visit definitions before uses, allowing us to hoist a loop body in one
106 /// pass without iteration.
108 void HoistRegion(MachineDomTreeNode *N);
110 /// isLoadFromConstantMemory - Return true if the given instruction is a
111 /// load from constant memory.
112 bool isLoadFromConstantMemory(MachineInstr *MI);
114 /// ExtractHoistableLoad - Unfold a load from the given machineinstr if
115 /// the load itself could be hoisted. Return the unfolded and hoistable
116 /// load, or null if the load couldn't be unfolded or if it wouldn't
118 MachineInstr *ExtractHoistableLoad(MachineInstr *MI);
120 /// LookForDuplicate - Find an instruction amount PrevMIs that is a
121 /// duplicate of MI. Return this instruction if it's found.
122 const MachineInstr *LookForDuplicate(const MachineInstr *MI,
123 std::vector<const MachineInstr*> &PrevMIs);
125 /// EliminateCSE - Given a LICM'ed instruction, look for an instruction on
126 /// the preheader that compute the same value. If it's found, do a RAU on
127 /// with the definition of the existing instruction rather than hoisting
128 /// the instruction to the preheader.
129 bool EliminateCSE(MachineInstr *MI,
130 DenseMap<unsigned, std::vector<const MachineInstr*> >::iterator &CI);
132 /// Hoist - When an instruction is found to only use loop invariant operands
133 /// that is safe to hoist, this instruction is called to do the dirty work.
135 void Hoist(MachineInstr *MI);
137 /// InitCSEMap - Initialize the CSE map with instructions that are in the
138 /// current loop preheader that may become duplicates of instructions that
139 /// are hoisted out of the loop.
140 void InitCSEMap(MachineBasicBlock *BB);
142 } // end anonymous namespace
144 char MachineLICM::ID = 0;
145 static RegisterPass<MachineLICM>
146 X("machinelicm", "Machine Loop Invariant Code Motion");
148 FunctionPass *llvm::createMachineLICMPass() { return new MachineLICM(); }
150 /// LoopIsOuterMostWithPreheader - Test if the given loop is the outer-most
151 /// loop that has a preheader.
152 static bool LoopIsOuterMostWithPreheader(MachineLoop *CurLoop) {
153 for (MachineLoop *L = CurLoop->getParentLoop(); L; L = L->getParentLoop())
154 if (L->getLoopPreheader())
159 /// Hoist expressions out of the specified loop. Note, alias info for inner loop
160 /// is not preserved so it is not a good idea to run LICM multiple times on one
163 bool MachineLICM::runOnMachineFunction(MachineFunction &MF) {
164 DEBUG(dbgs() << "******** Machine LICM ********\n");
166 Changed = FirstInLoop = false;
167 MCP = MF.getConstantPool();
168 TM = &MF.getTarget();
169 TII = TM->getInstrInfo();
170 TRI = TM->getRegisterInfo();
171 RegInfo = &MF.getRegInfo();
172 AllocatableSet = TRI->getAllocatableSet(MF);
174 // Get our Loop information...
175 LI = &getAnalysis<MachineLoopInfo>();
176 DT = &getAnalysis<MachineDominatorTree>();
177 AA = &getAnalysis<AliasAnalysis>();
179 for (MachineLoopInfo::iterator I = LI->begin(), E = LI->end(); I != E; ++I) {
182 // Only visit outer-most preheader-sporting loops.
183 if (!LoopIsOuterMostWithPreheader(CurLoop))
186 // Determine the block to which to hoist instructions. If we can't find a
187 // suitable loop preheader, we can't do any hoisting.
189 // FIXME: We are only hoisting if the basic block coming into this loop
190 // has only one successor. This isn't the case in general because we haven't
191 // broken critical edges or added preheaders.
192 CurPreheader = CurLoop->getLoopPreheader();
196 // CSEMap is initialized for loop header when the first instruction is
199 HoistRegion(DT->getNode(CurLoop->getHeader()));
206 /// HoistRegion - Walk the specified region of the CFG (defined by all blocks
207 /// dominated by the specified block, and that are in the current loop) in depth
208 /// first order w.r.t the DominatorTree. This allows us to visit definitions
209 /// before uses, allowing us to hoist a loop body in one pass without iteration.
211 void MachineLICM::HoistRegion(MachineDomTreeNode *N) {
212 assert(N != 0 && "Null dominator tree node?");
213 MachineBasicBlock *BB = N->getBlock();
215 // If this subregion is not in the top level loop at all, exit.
216 if (!CurLoop->contains(BB)) return;
218 for (MachineBasicBlock::iterator
219 MII = BB->begin(), E = BB->end(); MII != E; ) {
220 MachineBasicBlock::iterator NextMII = MII; ++NextMII;
225 const std::vector<MachineDomTreeNode*> &Children = N->getChildren();
227 for (unsigned I = 0, E = Children.size(); I != E; ++I)
228 HoistRegion(Children[I]);
231 /// IsLoopInvariantInst - Returns true if the instruction is loop
232 /// invariant. I.e., all virtual register operands are defined outside of the
233 /// loop, physical registers aren't accessed explicitly, and there are no side
234 /// effects that aren't captured by the operands or other flags.
236 bool MachineLICM::IsLoopInvariantInst(MachineInstr &I) {
237 const TargetInstrDesc &TID = I.getDesc();
239 // Ignore stuff that we obviously can't hoist.
240 if (TID.mayStore() || TID.isCall() || TID.isTerminator() ||
241 TID.hasUnmodeledSideEffects())
245 // Okay, this instruction does a load. As a refinement, we allow the target
246 // to decide whether the loaded value is actually a constant. If so, we can
247 // actually use it as a load.
248 if (!I.isInvariantLoad(AA))
249 // FIXME: we should be able to hoist loads with no other side effects if
250 // there are no other instructions which can change memory in this loop.
251 // This is a trivial form of alias analysis.
255 // The instruction is loop invariant if all of its operands are.
256 for (unsigned i = 0, e = I.getNumOperands(); i != e; ++i) {
257 const MachineOperand &MO = I.getOperand(i);
262 unsigned Reg = MO.getReg();
263 if (Reg == 0) continue;
265 // Don't hoist an instruction that uses or defines a physical register.
266 if (TargetRegisterInfo::isPhysicalRegister(Reg)) {
268 // If the physreg has no defs anywhere, it's just an ambient register
269 // and we can freely move its uses. Alternatively, if it's allocatable,
270 // it could get allocated to something with a def during allocation.
271 if (!RegInfo->def_empty(Reg))
273 if (AllocatableSet.test(Reg))
275 // Check for a def among the register's aliases too.
276 for (const unsigned *Alias = TRI->getAliasSet(Reg); *Alias; ++Alias) {
277 unsigned AliasReg = *Alias;
278 if (!RegInfo->def_empty(AliasReg))
280 if (AllocatableSet.test(AliasReg))
283 // Otherwise it's safe to move.
285 } else if (!MO.isDead()) {
286 // A def that isn't dead. We can't move it.
288 } else if (CurLoop->getHeader()->isLiveIn(Reg)) {
289 // If the reg is live into the loop, we can't hoist an instruction
290 // which would clobber it.
298 assert(RegInfo->getVRegDef(Reg) &&
299 "Machine instr not mapped for this vreg?!");
301 // If the loop contains the definition of an operand, then the instruction
302 // isn't loop invariant.
303 if (CurLoop->contains(RegInfo->getVRegDef(Reg)))
307 // If we got this far, the instruction is loop invariant!
312 /// HasPHIUses - Return true if the specified register has any PHI use.
313 static bool HasPHIUses(unsigned Reg, MachineRegisterInfo *RegInfo) {
314 for (MachineRegisterInfo::use_iterator UI = RegInfo->use_begin(Reg),
315 UE = RegInfo->use_end(); UI != UE; ++UI) {
316 MachineInstr *UseMI = &*UI;
323 /// isLoadFromConstantMemory - Return true if the given instruction is a
324 /// load from constant memory. Machine LICM will hoist these even if they are
325 /// not re-materializable.
326 bool MachineLICM::isLoadFromConstantMemory(MachineInstr *MI) {
327 if (!MI->getDesc().mayLoad()) return false;
328 if (!MI->hasOneMemOperand()) return false;
329 MachineMemOperand *MMO = *MI->memoperands_begin();
330 if (MMO->isVolatile()) return false;
331 if (!MMO->getValue()) return false;
332 const PseudoSourceValue *PSV = dyn_cast<PseudoSourceValue>(MMO->getValue());
334 MachineFunction &MF = *MI->getParent()->getParent();
335 return PSV->isConstant(MF.getFrameInfo());
337 return AA->pointsToConstantMemory(MMO->getValue());
341 /// IsProfitableToHoist - Return true if it is potentially profitable to hoist
342 /// the given loop invariant.
343 bool MachineLICM::IsProfitableToHoist(MachineInstr &MI) {
344 if (MI.isImplicitDef())
347 // FIXME: For now, only hoist re-materilizable instructions. LICM will
348 // increase register pressure. We want to make sure it doesn't increase
350 // Also hoist loads from constant memory, e.g. load from stubs, GOT. Hoisting
351 // these tend to help performance in low register pressure situation. The
352 // trade off is it may cause spill in high pressure situation. It will end up
353 // adding a store in the loop preheader. But the reload is no more expensive.
354 // The side benefit is these loads are frequently CSE'ed.
355 if (!TII->isTriviallyReMaterializable(&MI, AA)) {
356 if (!isLoadFromConstantMemory(&MI))
360 // If result(s) of this instruction is used by PHIs, then don't hoist it.
361 // The presence of joins makes it difficult for current register allocator
362 // implementation to perform remat.
363 for (unsigned i = 0, e = MI.getNumOperands(); i != e; ++i) {
364 const MachineOperand &MO = MI.getOperand(i);
365 if (!MO.isReg() || !MO.isDef())
367 if (HasPHIUses(MO.getReg(), RegInfo))
374 MachineInstr *MachineLICM::ExtractHoistableLoad(MachineInstr *MI) {
375 // If not, we may be able to unfold a load and hoist that.
376 // First test whether the instruction is loading from an amenable
378 if (!isLoadFromConstantMemory(MI))
381 // Next determine the register class for a temporary register.
382 unsigned LoadRegIndex;
384 TII->getOpcodeAfterMemoryUnfold(MI->getOpcode(),
386 /*UnfoldStore=*/false,
388 if (NewOpc == 0) return 0;
389 const TargetInstrDesc &TID = TII->get(NewOpc);
390 if (TID.getNumDefs() != 1) return 0;
391 const TargetRegisterClass *RC = TID.OpInfo[LoadRegIndex].getRegClass(TRI);
392 // Ok, we're unfolding. Create a temporary register and do the unfold.
393 unsigned Reg = RegInfo->createVirtualRegister(RC);
395 MachineFunction &MF = *MI->getParent()->getParent();
396 SmallVector<MachineInstr *, 2> NewMIs;
398 TII->unfoldMemoryOperand(MF, MI, Reg,
399 /*UnfoldLoad=*/true, /*UnfoldStore=*/false,
403 "unfoldMemoryOperand failed when getOpcodeAfterMemoryUnfold "
405 assert(NewMIs.size() == 2 &&
406 "Unfolded a load into multiple instructions!");
407 MachineBasicBlock *MBB = MI->getParent();
408 MBB->insert(MI, NewMIs[0]);
409 MBB->insert(MI, NewMIs[1]);
410 // If unfolding produced a load that wasn't loop-invariant or profitable to
411 // hoist, discard the new instructions and bail.
412 if (!IsLoopInvariantInst(*NewMIs[0]) || !IsProfitableToHoist(*NewMIs[0])) {
413 NewMIs[0]->eraseFromParent();
414 NewMIs[1]->eraseFromParent();
417 // Otherwise we successfully unfolded a load that we can hoist.
418 MI->eraseFromParent();
422 void MachineLICM::InitCSEMap(MachineBasicBlock *BB) {
423 for (MachineBasicBlock::iterator I = BB->begin(),E = BB->end(); I != E; ++I) {
424 const MachineInstr *MI = &*I;
425 // FIXME: For now, only hoist re-materilizable instructions. LICM will
426 // increase register pressure. We want to make sure it doesn't increase
428 if (TII->isTriviallyReMaterializable(MI, AA)) {
429 unsigned Opcode = MI->getOpcode();
430 DenseMap<unsigned, std::vector<const MachineInstr*> >::iterator
431 CI = CSEMap.find(Opcode);
432 if (CI != CSEMap.end())
433 CI->second.push_back(MI);
435 std::vector<const MachineInstr*> CSEMIs;
436 CSEMIs.push_back(MI);
437 CSEMap.insert(std::make_pair(Opcode, CSEMIs));
444 MachineLICM::LookForDuplicate(const MachineInstr *MI,
445 std::vector<const MachineInstr*> &PrevMIs) {
446 for (unsigned i = 0, e = PrevMIs.size(); i != e; ++i) {
447 const MachineInstr *PrevMI = PrevMIs[i];
448 if (TII->isIdentical(MI, PrevMI, RegInfo))
454 bool MachineLICM::EliminateCSE(MachineInstr *MI,
455 DenseMap<unsigned, std::vector<const MachineInstr*> >::iterator &CI) {
456 if (CI == CSEMap.end())
459 if (const MachineInstr *Dup = LookForDuplicate(MI, CI->second)) {
460 DEBUG(dbgs() << "CSEing " << *MI << " with " << *Dup);
461 for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) {
462 const MachineOperand &MO = MI->getOperand(i);
463 if (MO.isReg() && MO.isDef())
464 RegInfo->replaceRegWith(MO.getReg(), Dup->getOperand(i).getReg());
466 MI->eraseFromParent();
473 /// Hoist - When an instruction is found to use only loop invariant operands
474 /// that are safe to hoist, this instruction is called to do the dirty work.
476 void MachineLICM::Hoist(MachineInstr *MI) {
477 // First check whether we should hoist this instruction.
478 if (!IsLoopInvariantInst(*MI) || !IsProfitableToHoist(*MI)) {
479 // If not, try unfolding a hoistable load.
480 MI = ExtractHoistableLoad(MI);
484 // Now move the instructions to the predecessor, inserting it before any
485 // terminator instructions.
487 dbgs() << "Hoisting " << *MI;
488 if (CurPreheader->getBasicBlock())
489 dbgs() << " to MachineBasicBlock "
490 << CurPreheader->getName();
491 if (MI->getParent()->getBasicBlock())
492 dbgs() << " from MachineBasicBlock "
493 << MI->getParent()->getName();
497 // If this is the first instruction being hoisted to the preheader,
498 // initialize the CSE map with potential common expressions.
499 InitCSEMap(CurPreheader);
501 // Look for opportunity to CSE the hoisted instruction.
502 unsigned Opcode = MI->getOpcode();
503 DenseMap<unsigned, std::vector<const MachineInstr*> >::iterator
504 CI = CSEMap.find(Opcode);
505 if (!EliminateCSE(MI, CI)) {
506 // Otherwise, splice the instruction to the preheader.
507 CurPreheader->splice(CurPreheader->getFirstTerminator(),MI->getParent(),MI);
509 // Add to the CSE map.
510 if (CI != CSEMap.end())
511 CI->second.push_back(MI);
513 std::vector<const MachineInstr*> CSEMIs;
514 CSEMIs.push_back(MI);
515 CSEMap.insert(std::make_pair(Opcode, CSEMIs));