MachineCSE.cpp revision e837dead3c8dc3445ef6a0e2322179c57e264a13 
1//===-- MachineCSE.cpp - Machine Common Subexpression Elimination Pass ----===//
2//
3//                     The LLVM Compiler Infrastructure
4//
5// This file is distributed under the University of Illinois Open Source
6// License. See LICENSE.TXT for details.
7//
8//===----------------------------------------------------------------------===//
9//
10// This pass performs global common subexpression elimination on machine
11// instructions using a scoped hash table based value numbering scheme. It
12// must be run while the machine function is still in SSA form.
13//
14//===----------------------------------------------------------------------===//
15
16#define DEBUG_TYPE "machine-cse"
17#include "llvm/CodeGen/Passes.h"
18#include "llvm/CodeGen/MachineDominators.h"
19#include "llvm/CodeGen/MachineInstr.h"
20#include "llvm/CodeGen/MachineRegisterInfo.h"
21#include "llvm/Analysis/AliasAnalysis.h"
22#include "llvm/Target/TargetInstrInfo.h"
23#include "llvm/ADT/DenseMap.h"
24#include "llvm/ADT/ScopedHashTable.h"
25#include "llvm/ADT/SmallSet.h"
26#include "llvm/ADT/Statistic.h"
27#include "llvm/Support/Debug.h"
28#include "llvm/Support/RecyclingAllocator.h"
29
30using namespace llvm;
31
32STATISTIC(NumCoalesces, "Number of copies coalesced");
33STATISTIC(NumCSEs,      "Number of common subexpression eliminated");
34STATISTIC(NumPhysCSEs,
35          "Number of physreg referencing common subexpr eliminated");
36STATISTIC(NumCommutes,  "Number of copies coalesced after commuting");
37
38namespace {
39  class MachineCSE : public MachineFunctionPass {
40    const TargetInstrInfo *TII;
41    const TargetRegisterInfo *TRI;
42    AliasAnalysis *AA;
43    MachineDominatorTree *DT;
44    MachineRegisterInfo *MRI;
45  public:
46    static char ID; // Pass identification
47    MachineCSE() : MachineFunctionPass(ID), LookAheadLimit(5), CurrVN(0) {
48      initializeMachineCSEPass(*PassRegistry::getPassRegistry());
49    }
50
51    virtual bool runOnMachineFunction(MachineFunction &MF);
52
53    virtual void getAnalysisUsage(AnalysisUsage &AU) const {
54      AU.setPreservesCFG();
55      MachineFunctionPass::getAnalysisUsage(AU);
56      AU.addRequired<AliasAnalysis>();
57      AU.addPreservedID(MachineLoopInfoID);
58      AU.addRequired<MachineDominatorTree>();
59      AU.addPreserved<MachineDominatorTree>();
60    }
61
62    virtual void releaseMemory() {
63      ScopeMap.clear();
64      Exps.clear();
65    }
66
67  private:
68    const unsigned LookAheadLimit;
69    typedef RecyclingAllocator<BumpPtrAllocator,
70        ScopedHashTableVal<MachineInstr*, unsigned> > AllocatorTy;
71    typedef ScopedHashTable<MachineInstr*, unsigned,
72        MachineInstrExpressionTrait, AllocatorTy> ScopedHTType;
73    typedef ScopedHTType::ScopeTy ScopeType;
74    DenseMap<MachineBasicBlock*, ScopeType*> ScopeMap;
75    ScopedHTType VNT;
76    SmallVector<MachineInstr*, 64> Exps;
77    unsigned CurrVN;
78
79    bool PerformTrivialCoalescing(MachineInstr *MI, MachineBasicBlock *MBB);
80    bool isPhysDefTriviallyDead(unsigned Reg,
81                                MachineBasicBlock::const_iterator I,
82                                MachineBasicBlock::const_iterator E) const ;
83    bool hasLivePhysRegDefUses(const MachineInstr *MI,
84                               const MachineBasicBlock *MBB,
85                               SmallSet<unsigned,8> &PhysRefs) const;
86    bool PhysRegDefsReach(MachineInstr *CSMI, MachineInstr *MI,
87                          SmallSet<unsigned,8> &PhysRefs) const;
88    bool isCSECandidate(MachineInstr *MI);
89    bool isProfitableToCSE(unsigned CSReg, unsigned Reg,
90                           MachineInstr *CSMI, MachineInstr *MI);
91    void EnterScope(MachineBasicBlock *MBB);
92    void ExitScope(MachineBasicBlock *MBB);
93    bool ProcessBlock(MachineBasicBlock *MBB);
94    void ExitScopeIfDone(MachineDomTreeNode *Node,
95                 DenseMap<MachineDomTreeNode*, unsigned> &OpenChildren,
96                 DenseMap<MachineDomTreeNode*, MachineDomTreeNode*> &ParentMap);
97    bool PerformCSE(MachineDomTreeNode *Node);
98  };
99} // end anonymous namespace
100
101char MachineCSE::ID = 0;
102INITIALIZE_PASS_BEGIN(MachineCSE, "machine-cse",
103                "Machine Common Subexpression Elimination", false, false)
104INITIALIZE_PASS_DEPENDENCY(MachineDominatorTree)
105INITIALIZE_AG_DEPENDENCY(AliasAnalysis)
106INITIALIZE_PASS_END(MachineCSE, "machine-cse",
107                "Machine Common Subexpression Elimination", false, false)
108
109FunctionPass *llvm::createMachineCSEPass() { return new MachineCSE(); }
110
111bool MachineCSE::PerformTrivialCoalescing(MachineInstr *MI,
112                                          MachineBasicBlock *MBB) {
113  bool Changed = false;
114  for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) {
115    MachineOperand &MO = MI->getOperand(i);
116    if (!MO.isReg() || !MO.isUse())
117      continue;
118    unsigned Reg = MO.getReg();
119    if (!TargetRegisterInfo::isVirtualRegister(Reg))
120      continue;
121    if (!MRI->hasOneNonDBGUse(Reg))
122      // Only coalesce single use copies. This ensure the copy will be
123      // deleted.
124      continue;
125    MachineInstr *DefMI = MRI->getVRegDef(Reg);
126    if (DefMI->getParent() != MBB)
127      continue;
128    if (!DefMI->isCopy())
129      continue;
130    unsigned SrcReg = DefMI->getOperand(1).getReg();
131    if (!TargetRegisterInfo::isVirtualRegister(SrcReg))
132      continue;
133    if (DefMI->getOperand(0).getSubReg() || DefMI->getOperand(1).getSubReg())
134      continue;
135    if (!MRI->constrainRegClass(SrcReg, MRI->getRegClass(Reg)))
136      continue;
137    DEBUG(dbgs() << "Coalescing: " << *DefMI);
138    DEBUG(dbgs() << "***     to: " << *MI);
139    MO.setReg(SrcReg);
140    MRI->clearKillFlags(SrcReg);
141    DefMI->eraseFromParent();
142    ++NumCoalesces;
143    Changed = true;
144  }
145
146  return Changed;
147}
148
149bool
150MachineCSE::isPhysDefTriviallyDead(unsigned Reg,
151                                   MachineBasicBlock::const_iterator I,
152                                   MachineBasicBlock::const_iterator E) const {
153  unsigned LookAheadLeft = LookAheadLimit;
154  while (LookAheadLeft) {
155    // Skip over dbg_value's.
156    while (I != E && I->isDebugValue())
157      ++I;
158
159    if (I == E)
160      // Reached end of block, register is obviously dead.
161      return true;
162
163    bool SeenDef = false;
164    for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i) {
165      const MachineOperand &MO = I->getOperand(i);
166      if (!MO.isReg() || !MO.getReg())
167        continue;
168      if (!TRI->regsOverlap(MO.getReg(), Reg))
169        continue;
170      if (MO.isUse())
171        // Found a use!
172        return false;
173      SeenDef = true;
174    }
175    if (SeenDef)
176      // See a def of Reg (or an alias) before encountering any use, it's
177      // trivially dead.
178      return true;
179
180    --LookAheadLeft;
181    ++I;
182  }
183  return false;
184}
185
186/// hasLivePhysRegDefUses - Return true if the specified instruction read/write
187/// physical registers (except for dead defs of physical registers). It also
188/// returns the physical register def by reference if it's the only one and the
189/// instruction does not uses a physical register.
190bool MachineCSE::hasLivePhysRegDefUses(const MachineInstr *MI,
191                                       const MachineBasicBlock *MBB,
192                                       SmallSet<unsigned,8> &PhysRefs) const {
193  MachineBasicBlock::const_iterator I = MI; I = llvm::next(I);
194  for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) {
195    const MachineOperand &MO = MI->getOperand(i);
196    if (!MO.isReg())
197      continue;
198    unsigned Reg = MO.getReg();
199    if (!Reg)
200      continue;
201    if (TargetRegisterInfo::isVirtualRegister(Reg))
202      continue;
203    // If the def is dead, it's ok. But the def may not marked "dead". That's
204    // common since this pass is run before livevariables. We can scan
205    // forward a few instructions and check if it is obviously dead.
206    if (MO.isDef() &&
207        (MO.isDead() || isPhysDefTriviallyDead(Reg, I, MBB->end())))
208      continue;
209    PhysRefs.insert(Reg);
210    for (const unsigned *Alias = TRI->getAliasSet(Reg); *Alias; ++Alias)
211      PhysRefs.insert(*Alias);
212  }
213
214  return !PhysRefs.empty();
215}
216
217bool MachineCSE::PhysRegDefsReach(MachineInstr *CSMI, MachineInstr *MI,
218                                  SmallSet<unsigned,8> &PhysRefs) const {
219  // For now conservatively returns false if the common subexpression is
220  // not in the same basic block as the given instruction.
221  MachineBasicBlock *MBB = MI->getParent();
222  if (CSMI->getParent() != MBB)
223    return false;
224  MachineBasicBlock::const_iterator I = CSMI; I = llvm::next(I);
225  MachineBasicBlock::const_iterator E = MI;
226  unsigned LookAheadLeft = LookAheadLimit;
227  while (LookAheadLeft) {
228    // Skip over dbg_value's.
229    while (I != E && I->isDebugValue())
230      ++I;
231
232    if (I == E)
233      return true;
234
235    for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i) {
236      const MachineOperand &MO = I->getOperand(i);
237      if (!MO.isReg() || !MO.isDef())
238        continue;
239      unsigned MOReg = MO.getReg();
240      if (TargetRegisterInfo::isVirtualRegister(MOReg))
241        continue;
242      if (PhysRefs.count(MOReg))
243        return false;
244    }
245
246    --LookAheadLeft;
247    ++I;
248  }
249
250  return false;
251}
252
253bool MachineCSE::isCSECandidate(MachineInstr *MI) {
254  if (MI->isLabel() || MI->isPHI() || MI->isImplicitDef() ||
255      MI->isKill() || MI->isInlineAsm() || MI->isDebugValue())
256    return false;
257
258  // Ignore copies.
259  if (MI->isCopyLike())
260    return false;
261
262  // Ignore stuff that we obviously can't move.
263  const MCInstrDesc &MCID = MI->getDesc();
264  if (MCID.mayStore() || MCID.isCall() || MCID.isTerminator() ||
265      MI->hasUnmodeledSideEffects())
266    return false;
267
268  if (MCID.mayLoad()) {
269    // Okay, this instruction does a load. As a refinement, we allow the target
270    // to decide whether the loaded value is actually a constant. If so, we can
271    // actually use it as a load.
272    if (!MI->isInvariantLoad(AA))
273      // FIXME: we should be able to hoist loads with no other side effects if
274      // there are no other instructions which can change memory in this loop.
275      // This is a trivial form of alias analysis.
276      return false;
277  }
278  return true;
279}
280
281/// isProfitableToCSE - Return true if it's profitable to eliminate MI with a
282/// common expression that defines Reg.
283bool MachineCSE::isProfitableToCSE(unsigned CSReg, unsigned Reg,
284                                   MachineInstr *CSMI, MachineInstr *MI) {
285  // FIXME: Heuristics that works around the lack the live range splitting.
286
287  // Heuristics #1: Don't CSE "cheap" computation if the def is not local or in
288  // an immediate predecessor. We don't want to increase register pressure and
289  // end up causing other computation to be spilled.
290  if (MI->getDesc().isAsCheapAsAMove()) {
291    MachineBasicBlock *CSBB = CSMI->getParent();
292    MachineBasicBlock *BB = MI->getParent();
293    if (CSBB != BB && !CSBB->isSuccessor(BB))
294      return false;
295  }
296
297  // Heuristics #2: If the expression doesn't not use a vr and the only use
298  // of the redundant computation are copies, do not cse.
299  bool HasVRegUse = false;
300  for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) {
301    const MachineOperand &MO = MI->getOperand(i);
302    if (MO.isReg() && MO.isUse() &&
303        TargetRegisterInfo::isVirtualRegister(MO.getReg())) {
304      HasVRegUse = true;
305      break;
306    }
307  }
308  if (!HasVRegUse) {
309    bool HasNonCopyUse = false;
310    for (MachineRegisterInfo::use_nodbg_iterator I =  MRI->use_nodbg_begin(Reg),
311           E = MRI->use_nodbg_end(); I != E; ++I) {
312      MachineInstr *Use = &*I;
313      // Ignore copies.
314      if (!Use->isCopyLike()) {
315        HasNonCopyUse = true;
316        break;
317      }
318    }
319    if (!HasNonCopyUse)
320      return false;
321  }
322
323  // Heuristics #3: If the common subexpression is used by PHIs, do not reuse
324  // it unless the defined value is already used in the BB of the new use.
325  bool HasPHI = false;
326  SmallPtrSet<MachineBasicBlock*, 4> CSBBs;
327  for (MachineRegisterInfo::use_nodbg_iterator I =  MRI->use_nodbg_begin(CSReg),
328       E = MRI->use_nodbg_end(); I != E; ++I) {
329    MachineInstr *Use = &*I;
330    HasPHI |= Use->isPHI();
331    CSBBs.insert(Use->getParent());
332  }
333
334  if (!HasPHI)
335    return true;
336  return CSBBs.count(MI->getParent());
337}
338
339void MachineCSE::EnterScope(MachineBasicBlock *MBB) {
340  DEBUG(dbgs() << "Entering: " << MBB->getName() << '\n');
341  ScopeType *Scope = new ScopeType(VNT);
342  ScopeMap[MBB] = Scope;
343}
344
345void MachineCSE::ExitScope(MachineBasicBlock *MBB) {
346  DEBUG(dbgs() << "Exiting: " << MBB->getName() << '\n');
347  DenseMap<MachineBasicBlock*, ScopeType*>::iterator SI = ScopeMap.find(MBB);
348  assert(SI != ScopeMap.end());
349  ScopeMap.erase(SI);
350  delete SI->second;
351}
352
353bool MachineCSE::ProcessBlock(MachineBasicBlock *MBB) {
354  bool Changed = false;
355
356  SmallVector<std::pair<unsigned, unsigned>, 8> CSEPairs;
357  for (MachineBasicBlock::iterator I = MBB->begin(), E = MBB->end(); I != E; ) {
358    MachineInstr *MI = &*I;
359    ++I;
360
361    if (!isCSECandidate(MI))
362      continue;
363
364    bool FoundCSE = VNT.count(MI);
365    if (!FoundCSE) {
366      // Look for trivial copy coalescing opportunities.
367      if (PerformTrivialCoalescing(MI, MBB)) {
368        Changed = true;
369
370        // After coalescing MI itself may become a copy.
371        if (MI->isCopyLike())
372          continue;
373        FoundCSE = VNT.count(MI);
374      }
375    }
376
377    // Commute commutable instructions.
378    bool Commuted = false;
379    if (!FoundCSE && MI->getDesc().isCommutable()) {
380      MachineInstr *NewMI = TII->commuteInstruction(MI);
381      if (NewMI) {
382        Commuted = true;
383        FoundCSE = VNT.count(NewMI);
384        if (NewMI != MI) {
385          // New instruction. It doesn't need to be kept.
386          NewMI->eraseFromParent();
387          Changed = true;
388        } else if (!FoundCSE)
389          // MI was changed but it didn't help, commute it back!
390          (void)TII->commuteInstruction(MI);
391      }
392    }
393
394    // If the instruction defines physical registers and the values *may* be
395    // used, then it's not safe to replace it with a common subexpression.
396    // It's also not safe if the instruction uses physical registers.
397    SmallSet<unsigned,8> PhysRefs;
398    if (FoundCSE && hasLivePhysRegDefUses(MI, MBB, PhysRefs)) {
399      FoundCSE = false;
400
401      // ... Unless the CS is local and it also defines the physical register
402      // which is not clobbered in between and the physical register uses
403      // were not clobbered.
404      unsigned CSVN = VNT.lookup(MI);
405      MachineInstr *CSMI = Exps[CSVN];
406      if (PhysRegDefsReach(CSMI, MI, PhysRefs))
407        FoundCSE = true;
408    }
409
410    if (!FoundCSE) {
411      VNT.insert(MI, CurrVN++);
412      Exps.push_back(MI);
413      continue;
414    }
415
416    // Found a common subexpression, eliminate it.
417    unsigned CSVN = VNT.lookup(MI);
418    MachineInstr *CSMI = Exps[CSVN];
419    DEBUG(dbgs() << "Examining: " << *MI);
420    DEBUG(dbgs() << "*** Found a common subexpression: " << *CSMI);
421
422    // Check if it's profitable to perform this CSE.
423    bool DoCSE = true;
424    unsigned NumDefs = MI->getDesc().getNumDefs();
425    for (unsigned i = 0, e = MI->getNumOperands(); NumDefs && i != e; ++i) {
426      MachineOperand &MO = MI->getOperand(i);
427      if (!MO.isReg() || !MO.isDef())
428        continue;
429      unsigned OldReg = MO.getReg();
430      unsigned NewReg = CSMI->getOperand(i).getReg();
431      if (OldReg == NewReg)
432        continue;
433      assert(TargetRegisterInfo::isVirtualRegister(OldReg) &&
434             TargetRegisterInfo::isVirtualRegister(NewReg) &&
435             "Do not CSE physical register defs!");
436      if (!isProfitableToCSE(NewReg, OldReg, CSMI, MI)) {
437        DoCSE = false;
438        break;
439      }
440      CSEPairs.push_back(std::make_pair(OldReg, NewReg));
441      --NumDefs;
442    }
443
444    // Actually perform the elimination.
445    if (DoCSE) {
446      for (unsigned i = 0, e = CSEPairs.size(); i != e; ++i) {
447        MRI->replaceRegWith(CSEPairs[i].first, CSEPairs[i].second);
448        MRI->clearKillFlags(CSEPairs[i].second);
449      }
450      MI->eraseFromParent();
451      ++NumCSEs;
452      if (!PhysRefs.empty())
453        ++NumPhysCSEs;
454      if (Commuted)
455        ++NumCommutes;
456      Changed = true;
457    } else {
458      DEBUG(dbgs() << "*** Not profitable, avoid CSE!\n");
459      VNT.insert(MI, CurrVN++);
460      Exps.push_back(MI);
461    }
462    CSEPairs.clear();
463  }
464
465  return Changed;
466}
467
468/// ExitScopeIfDone - Destroy scope for the MBB that corresponds to the given
469/// dominator tree node if its a leaf or all of its children are done. Walk
470/// up the dominator tree to destroy ancestors which are now done.
471void
472MachineCSE::ExitScopeIfDone(MachineDomTreeNode *Node,
473                DenseMap<MachineDomTreeNode*, unsigned> &OpenChildren,
474                DenseMap<MachineDomTreeNode*, MachineDomTreeNode*> &ParentMap) {
475  if (OpenChildren[Node])
476    return;
477
478  // Pop scope.
479  ExitScope(Node->getBlock());
480
481  // Now traverse upwards to pop ancestors whose offsprings are all done.
482  while (MachineDomTreeNode *Parent = ParentMap[Node]) {
483    unsigned Left = --OpenChildren[Parent];
484    if (Left != 0)
485      break;
486    ExitScope(Parent->getBlock());
487    Node = Parent;
488  }
489}
490
491bool MachineCSE::PerformCSE(MachineDomTreeNode *Node) {
492  SmallVector<MachineDomTreeNode*, 32> Scopes;
493  SmallVector<MachineDomTreeNode*, 8> WorkList;
494  DenseMap<MachineDomTreeNode*, MachineDomTreeNode*> ParentMap;
495  DenseMap<MachineDomTreeNode*, unsigned> OpenChildren;
496
497  CurrVN = 0;
498
499  // Perform a DFS walk to determine the order of visit.
500  WorkList.push_back(Node);
501  do {
502    Node = WorkList.pop_back_val();
503    Scopes.push_back(Node);
504    const std::vector<MachineDomTreeNode*> &Children = Node->getChildren();
505    unsigned NumChildren = Children.size();
506    OpenChildren[Node] = NumChildren;
507    for (unsigned i = 0; i != NumChildren; ++i) {
508      MachineDomTreeNode *Child = Children[i];
509      ParentMap[Child] = Node;
510      WorkList.push_back(Child);
511    }
512  } while (!WorkList.empty());
513
514  // Now perform CSE.
515  bool Changed = false;
516  for (unsigned i = 0, e = Scopes.size(); i != e; ++i) {
517    MachineDomTreeNode *Node = Scopes[i];
518    MachineBasicBlock *MBB = Node->getBlock();
519    EnterScope(MBB);
520    Changed |= ProcessBlock(MBB);
521    // If it's a leaf node, it's done. Traverse upwards to pop ancestors.
522    ExitScopeIfDone(Node, OpenChildren, ParentMap);
523  }
524
525  return Changed;
526}
527
528bool MachineCSE::runOnMachineFunction(MachineFunction &MF) {
529  TII = MF.getTarget().getInstrInfo();
530  TRI = MF.getTarget().getRegisterInfo();
531  MRI = &MF.getRegInfo();
532  AA = &getAnalysis<AliasAnalysis>();
533  DT = &getAnalysis<MachineDominatorTree>();
534  return PerformCSE(DT->getRootNode());
535}
536