1 //===-- MachineCSE.cpp - Machine Common Subexpression Elimination 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 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.
14 //===----------------------------------------------------------------------===//
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"
31 STATISTIC(NumCoalesces, "Number of copies coalesced");
32 STATISTIC(NumCSEs, "Number of common subexpression eliminated");
33 STATISTIC(NumPhysCSEs,
34 "Number of physreg referencing common subexpr eliminated");
35 STATISTIC(NumCrossBBCSEs,
36 "Number of cross-MBB physreg referencing CS eliminated");
37 STATISTIC(NumCommutes, "Number of copies coalesced after commuting");
40 class MachineCSE : public MachineFunctionPass {
41 const TargetInstrInfo *TII;
42 const TargetRegisterInfo *TRI;
44 MachineDominatorTree *DT;
45 MachineRegisterInfo *MRI;
47 static char ID; // Pass identification
48 MachineCSE() : MachineFunctionPass(ID), LookAheadLimit(5), CurrVN(0) {
49 initializeMachineCSEPass(*PassRegistry::getPassRegistry());
52 virtual bool runOnMachineFunction(MachineFunction &MF);
54 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
56 MachineFunctionPass::getAnalysisUsage(AU);
57 AU.addRequired<AliasAnalysis>();
58 AU.addPreservedID(MachineLoopInfoID);
59 AU.addRequired<MachineDominatorTree>();
60 AU.addPreserved<MachineDominatorTree>();
63 virtual void releaseMemory() {
69 const unsigned LookAheadLimit;
70 typedef RecyclingAllocator<BumpPtrAllocator,
71 ScopedHashTableVal<MachineInstr*, unsigned> > AllocatorTy;
72 typedef ScopedHashTable<MachineInstr*, unsigned,
73 MachineInstrExpressionTrait, AllocatorTy> ScopedHTType;
74 typedef ScopedHTType::ScopeTy ScopeType;
75 DenseMap<MachineBasicBlock*, ScopeType*> ScopeMap;
77 SmallVector<MachineInstr*, 64> Exps;
80 bool PerformTrivialCoalescing(MachineInstr *MI, MachineBasicBlock *MBB);
81 bool isPhysDefTriviallyDead(unsigned Reg,
82 MachineBasicBlock::const_iterator I,
83 MachineBasicBlock::const_iterator E) const ;
84 bool hasLivePhysRegDefUses(const MachineInstr *MI,
85 const MachineBasicBlock *MBB,
86 SmallSet<unsigned,8> &PhysRefs,
87 SmallVector<unsigned,2> &PhysDefs) const;
88 bool PhysRegDefsReach(MachineInstr *CSMI, MachineInstr *MI,
89 SmallSet<unsigned,8> &PhysRefs,
90 SmallVector<unsigned,2> &PhysDefs,
91 bool &NonLocal) const;
92 bool isCSECandidate(MachineInstr *MI);
93 bool isProfitableToCSE(unsigned CSReg, unsigned Reg,
94 MachineInstr *CSMI, MachineInstr *MI);
95 void EnterScope(MachineBasicBlock *MBB);
96 void ExitScope(MachineBasicBlock *MBB);
97 bool ProcessBlock(MachineBasicBlock *MBB);
98 void ExitScopeIfDone(MachineDomTreeNode *Node,
99 DenseMap<MachineDomTreeNode*, unsigned> &OpenChildren,
100 DenseMap<MachineDomTreeNode*, MachineDomTreeNode*> &ParentMap);
101 bool PerformCSE(MachineDomTreeNode *Node);
103 } // end anonymous namespace
105 char MachineCSE::ID = 0;
106 char &llvm::MachineCSEID = MachineCSE::ID;
107 INITIALIZE_PASS_BEGIN(MachineCSE, "machine-cse",
108 "Machine Common Subexpression Elimination", false, false)
109 INITIALIZE_PASS_DEPENDENCY(MachineDominatorTree)
110 INITIALIZE_AG_DEPENDENCY(AliasAnalysis)
111 INITIALIZE_PASS_END(MachineCSE, "machine-cse",
112 "Machine Common Subexpression Elimination", false, false)
114 bool MachineCSE::PerformTrivialCoalescing(MachineInstr *MI,
115 MachineBasicBlock *MBB) {
116 bool Changed = false;
117 for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) {
118 MachineOperand &MO = MI->getOperand(i);
119 if (!MO.isReg() || !MO.isUse())
121 unsigned Reg = MO.getReg();
122 if (!TargetRegisterInfo::isVirtualRegister(Reg))
124 if (!MRI->hasOneNonDBGUse(Reg))
125 // Only coalesce single use copies. This ensure the copy will be
128 MachineInstr *DefMI = MRI->getVRegDef(Reg);
129 if (DefMI->getParent() != MBB)
131 if (!DefMI->isCopy())
133 unsigned SrcReg = DefMI->getOperand(1).getReg();
134 if (!TargetRegisterInfo::isVirtualRegister(SrcReg))
136 if (DefMI->getOperand(0).getSubReg() || DefMI->getOperand(1).getSubReg())
138 if (!MRI->constrainRegClass(SrcReg, MRI->getRegClass(Reg)))
140 DEBUG(dbgs() << "Coalescing: " << *DefMI);
141 DEBUG(dbgs() << "*** to: " << *MI);
143 MRI->clearKillFlags(SrcReg);
144 DefMI->eraseFromParent();
153 MachineCSE::isPhysDefTriviallyDead(unsigned Reg,
154 MachineBasicBlock::const_iterator I,
155 MachineBasicBlock::const_iterator E) const {
156 unsigned LookAheadLeft = LookAheadLimit;
157 while (LookAheadLeft) {
158 // Skip over dbg_value's.
159 while (I != E && I->isDebugValue())
163 // Reached end of block, register is obviously dead.
166 bool SeenDef = false;
167 for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i) {
168 const MachineOperand &MO = I->getOperand(i);
169 if (!MO.isReg() || !MO.getReg())
171 if (!TRI->regsOverlap(MO.getReg(), Reg))
179 // See a def of Reg (or an alias) before encountering any use, it's
189 /// hasLivePhysRegDefUses - Return true if the specified instruction read/write
190 /// physical registers (except for dead defs of physical registers). It also
191 /// returns the physical register def by reference if it's the only one and the
192 /// instruction does not uses a physical register.
193 bool MachineCSE::hasLivePhysRegDefUses(const MachineInstr *MI,
194 const MachineBasicBlock *MBB,
195 SmallSet<unsigned,8> &PhysRefs,
196 SmallVector<unsigned,2> &PhysDefs) const{
197 MachineBasicBlock::const_iterator I = MI; I = llvm::next(I);
198 for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) {
199 const MachineOperand &MO = MI->getOperand(i);
202 unsigned Reg = MO.getReg();
205 if (TargetRegisterInfo::isVirtualRegister(Reg))
207 // If the def is dead, it's ok. But the def may not marked "dead". That's
208 // common since this pass is run before livevariables. We can scan
209 // forward a few instructions and check if it is obviously dead.
211 (MO.isDead() || isPhysDefTriviallyDead(Reg, I, MBB->end())))
213 PhysRefs.insert(Reg);
215 PhysDefs.push_back(Reg);
216 for (const unsigned *Alias = TRI->getAliasSet(Reg); *Alias; ++Alias)
217 PhysRefs.insert(*Alias);
220 return !PhysRefs.empty();
223 bool MachineCSE::PhysRegDefsReach(MachineInstr *CSMI, MachineInstr *MI,
224 SmallSet<unsigned,8> &PhysRefs,
225 SmallVector<unsigned,2> &PhysDefs,
226 bool &NonLocal) const {
227 // For now conservatively returns false if the common subexpression is
228 // not in the same basic block as the given instruction. The only exception
229 // is if the common subexpression is in the sole predecessor block.
230 const MachineBasicBlock *MBB = MI->getParent();
231 const MachineBasicBlock *CSMBB = CSMI->getParent();
233 bool CrossMBB = false;
235 if (MBB->pred_size() != 1 || *MBB->pred_begin() != CSMBB)
238 for (unsigned i = 0, e = PhysDefs.size(); i != e; ++i) {
239 if (TRI->isInAllocatableClass(PhysDefs[i]))
240 // Avoid extending live range of physical registers unless
241 // they are unallocatable.
246 MachineBasicBlock::const_iterator I = CSMI; I = llvm::next(I);
247 MachineBasicBlock::const_iterator E = MI;
248 MachineBasicBlock::const_iterator EE = CSMBB->end();
249 unsigned LookAheadLeft = LookAheadLimit;
250 while (LookAheadLeft) {
251 // Skip over dbg_value's.
252 while (I != E && I != EE && I->isDebugValue())
256 assert(CrossMBB && "Reaching end-of-MBB without finding MI?");
268 for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i) {
269 const MachineOperand &MO = I->getOperand(i);
270 if (!MO.isReg() || !MO.isDef())
272 unsigned MOReg = MO.getReg();
273 if (TargetRegisterInfo::isVirtualRegister(MOReg))
275 if (PhysRefs.count(MOReg))
286 bool MachineCSE::isCSECandidate(MachineInstr *MI) {
287 if (MI->isLabel() || MI->isPHI() || MI->isImplicitDef() ||
288 MI->isKill() || MI->isInlineAsm() || MI->isDebugValue())
292 if (MI->isCopyLike())
295 // Ignore stuff that we obviously can't move.
296 if (MI->mayStore() || MI->isCall() || MI->isTerminator() ||
297 MI->hasUnmodeledSideEffects())
301 // Okay, this instruction does a load. As a refinement, we allow the target
302 // to decide whether the loaded value is actually a constant. If so, we can
303 // actually use it as a load.
304 if (!MI->isInvariantLoad(AA))
305 // FIXME: we should be able to hoist loads with no other side effects if
306 // there are no other instructions which can change memory in this loop.
307 // This is a trivial form of alias analysis.
313 /// isProfitableToCSE - Return true if it's profitable to eliminate MI with a
314 /// common expression that defines Reg.
315 bool MachineCSE::isProfitableToCSE(unsigned CSReg, unsigned Reg,
316 MachineInstr *CSMI, MachineInstr *MI) {
317 // FIXME: Heuristics that works around the lack the live range splitting.
319 // Heuristics #1: Don't CSE "cheap" computation if the def is not local or in
320 // an immediate predecessor. We don't want to increase register pressure and
321 // end up causing other computation to be spilled.
322 if (MI->isAsCheapAsAMove()) {
323 MachineBasicBlock *CSBB = CSMI->getParent();
324 MachineBasicBlock *BB = MI->getParent();
325 if (CSBB != BB && !CSBB->isSuccessor(BB))
329 // Heuristics #2: If the expression doesn't not use a vr and the only use
330 // of the redundant computation are copies, do not cse.
331 bool HasVRegUse = false;
332 for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) {
333 const MachineOperand &MO = MI->getOperand(i);
334 if (MO.isReg() && MO.isUse() &&
335 TargetRegisterInfo::isVirtualRegister(MO.getReg())) {
341 bool HasNonCopyUse = false;
342 for (MachineRegisterInfo::use_nodbg_iterator I = MRI->use_nodbg_begin(Reg),
343 E = MRI->use_nodbg_end(); I != E; ++I) {
344 MachineInstr *Use = &*I;
346 if (!Use->isCopyLike()) {
347 HasNonCopyUse = true;
355 // Heuristics #3: If the common subexpression is used by PHIs, do not reuse
356 // it unless the defined value is already used in the BB of the new use.
358 SmallPtrSet<MachineBasicBlock*, 4> CSBBs;
359 for (MachineRegisterInfo::use_nodbg_iterator I = MRI->use_nodbg_begin(CSReg),
360 E = MRI->use_nodbg_end(); I != E; ++I) {
361 MachineInstr *Use = &*I;
362 HasPHI |= Use->isPHI();
363 CSBBs.insert(Use->getParent());
368 return CSBBs.count(MI->getParent());
371 void MachineCSE::EnterScope(MachineBasicBlock *MBB) {
372 DEBUG(dbgs() << "Entering: " << MBB->getName() << '\n');
373 ScopeType *Scope = new ScopeType(VNT);
374 ScopeMap[MBB] = Scope;
377 void MachineCSE::ExitScope(MachineBasicBlock *MBB) {
378 DEBUG(dbgs() << "Exiting: " << MBB->getName() << '\n');
379 DenseMap<MachineBasicBlock*, ScopeType*>::iterator SI = ScopeMap.find(MBB);
380 assert(SI != ScopeMap.end());
385 bool MachineCSE::ProcessBlock(MachineBasicBlock *MBB) {
386 bool Changed = false;
388 SmallVector<std::pair<unsigned, unsigned>, 8> CSEPairs;
389 for (MachineBasicBlock::iterator I = MBB->begin(), E = MBB->end(); I != E; ) {
390 MachineInstr *MI = &*I;
393 if (!isCSECandidate(MI))
396 bool FoundCSE = VNT.count(MI);
398 // Look for trivial copy coalescing opportunities.
399 if (PerformTrivialCoalescing(MI, MBB)) {
402 // After coalescing MI itself may become a copy.
403 if (MI->isCopyLike())
405 FoundCSE = VNT.count(MI);
409 // Commute commutable instructions.
410 bool Commuted = false;
411 if (!FoundCSE && MI->isCommutable()) {
412 MachineInstr *NewMI = TII->commuteInstruction(MI);
415 FoundCSE = VNT.count(NewMI);
417 // New instruction. It doesn't need to be kept.
418 NewMI->eraseFromParent();
420 } else if (!FoundCSE)
421 // MI was changed but it didn't help, commute it back!
422 (void)TII->commuteInstruction(MI);
426 // If the instruction defines physical registers and the values *may* be
427 // used, then it's not safe to replace it with a common subexpression.
428 // It's also not safe if the instruction uses physical registers.
429 bool CrossMBBPhysDef = false;
430 SmallSet<unsigned,8> PhysRefs;
431 SmallVector<unsigned, 2> PhysDefs;
432 if (FoundCSE && hasLivePhysRegDefUses(MI, MBB, PhysRefs, PhysDefs)) {
435 // ... Unless the CS is local or is in the sole predecessor block
436 // and it also defines the physical register which is not clobbered
437 // in between and the physical register uses were not clobbered.
438 unsigned CSVN = VNT.lookup(MI);
439 MachineInstr *CSMI = Exps[CSVN];
440 if (PhysRegDefsReach(CSMI, MI, PhysRefs, PhysDefs, CrossMBBPhysDef))
445 VNT.insert(MI, CurrVN++);
450 // Found a common subexpression, eliminate it.
451 unsigned CSVN = VNT.lookup(MI);
452 MachineInstr *CSMI = Exps[CSVN];
453 DEBUG(dbgs() << "Examining: " << *MI);
454 DEBUG(dbgs() << "*** Found a common subexpression: " << *CSMI);
456 // Check if it's profitable to perform this CSE.
458 unsigned NumDefs = MI->getDesc().getNumDefs();
459 for (unsigned i = 0, e = MI->getNumOperands(); NumDefs && i != e; ++i) {
460 MachineOperand &MO = MI->getOperand(i);
461 if (!MO.isReg() || !MO.isDef())
463 unsigned OldReg = MO.getReg();
464 unsigned NewReg = CSMI->getOperand(i).getReg();
465 if (OldReg == NewReg)
468 assert(TargetRegisterInfo::isVirtualRegister(OldReg) &&
469 TargetRegisterInfo::isVirtualRegister(NewReg) &&
470 "Do not CSE physical register defs!");
472 if (!isProfitableToCSE(NewReg, OldReg, CSMI, MI)) {
477 // Don't perform CSE if the result of the old instruction cannot exist
478 // within the register class of the new instruction.
479 const TargetRegisterClass *OldRC = MRI->getRegClass(OldReg);
480 if (!MRI->constrainRegClass(NewReg, OldRC)) {
485 CSEPairs.push_back(std::make_pair(OldReg, NewReg));
489 // Actually perform the elimination.
491 for (unsigned i = 0, e = CSEPairs.size(); i != e; ++i) {
492 MRI->replaceRegWith(CSEPairs[i].first, CSEPairs[i].second);
493 MRI->clearKillFlags(CSEPairs[i].second);
496 if (CrossMBBPhysDef) {
497 // Add physical register defs now coming in from a predecessor to MBB
499 while (!PhysDefs.empty()) {
500 unsigned LiveIn = PhysDefs.pop_back_val();
501 if (!MBB->isLiveIn(LiveIn))
502 MBB->addLiveIn(LiveIn);
507 MI->eraseFromParent();
509 if (!PhysRefs.empty())
515 DEBUG(dbgs() << "*** Not profitable, avoid CSE!\n");
516 VNT.insert(MI, CurrVN++);
525 /// ExitScopeIfDone - Destroy scope for the MBB that corresponds to the given
526 /// dominator tree node if its a leaf or all of its children are done. Walk
527 /// up the dominator tree to destroy ancestors which are now done.
529 MachineCSE::ExitScopeIfDone(MachineDomTreeNode *Node,
530 DenseMap<MachineDomTreeNode*, unsigned> &OpenChildren,
531 DenseMap<MachineDomTreeNode*, MachineDomTreeNode*> &ParentMap) {
532 if (OpenChildren[Node])
536 ExitScope(Node->getBlock());
538 // Now traverse upwards to pop ancestors whose offsprings are all done.
539 while (MachineDomTreeNode *Parent = ParentMap[Node]) {
540 unsigned Left = --OpenChildren[Parent];
543 ExitScope(Parent->getBlock());
548 bool MachineCSE::PerformCSE(MachineDomTreeNode *Node) {
549 SmallVector<MachineDomTreeNode*, 32> Scopes;
550 SmallVector<MachineDomTreeNode*, 8> WorkList;
551 DenseMap<MachineDomTreeNode*, MachineDomTreeNode*> ParentMap;
552 DenseMap<MachineDomTreeNode*, unsigned> OpenChildren;
556 // Perform a DFS walk to determine the order of visit.
557 WorkList.push_back(Node);
559 Node = WorkList.pop_back_val();
560 Scopes.push_back(Node);
561 const std::vector<MachineDomTreeNode*> &Children = Node->getChildren();
562 unsigned NumChildren = Children.size();
563 OpenChildren[Node] = NumChildren;
564 for (unsigned i = 0; i != NumChildren; ++i) {
565 MachineDomTreeNode *Child = Children[i];
566 ParentMap[Child] = Node;
567 WorkList.push_back(Child);
569 } while (!WorkList.empty());
572 bool Changed = false;
573 for (unsigned i = 0, e = Scopes.size(); i != e; ++i) {
574 MachineDomTreeNode *Node = Scopes[i];
575 MachineBasicBlock *MBB = Node->getBlock();
577 Changed |= ProcessBlock(MBB);
578 // If it's a leaf node, it's done. Traverse upwards to pop ancestors.
579 ExitScopeIfDone(Node, OpenChildren, ParentMap);
585 bool MachineCSE::runOnMachineFunction(MachineFunction &MF) {
586 TII = MF.getTarget().getInstrInfo();
587 TRI = MF.getTarget().getRegisterInfo();
588 MRI = &MF.getRegInfo();
589 AA = &getAnalysis<AliasAnalysis>();
590 DT = &getAnalysis<MachineDominatorTree>();
591 return PerformCSE(DT->getRootNode());