1 //===-- PhiElimination.cpp - Eliminate PHI nodes by inserting copies ------===//
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 eliminates machine instruction PHI nodes by inserting copy
11 // instructions. This destroys SSA information, but is the desired input for
12 // some register allocators.
14 //===----------------------------------------------------------------------===//
16 #define DEBUG_TYPE "phielim"
17 #include "PHIElimination.h"
18 #include "llvm/CodeGen/LiveVariables.h"
19 #include "llvm/CodeGen/Passes.h"
20 #include "llvm/CodeGen/MachineDominators.h"
21 #include "llvm/CodeGen/MachineInstr.h"
22 #include "llvm/CodeGen/MachineInstrBuilder.h"
23 #include "llvm/CodeGen/MachineRegisterInfo.h"
24 #include "llvm/Target/TargetInstrInfo.h"
25 #include "llvm/Function.h"
26 #include "llvm/Target/TargetMachine.h"
27 #include "llvm/ADT/SmallPtrSet.h"
28 #include "llvm/ADT/STLExtras.h"
29 #include "llvm/ADT/Statistic.h"
30 #include "llvm/Support/CommandLine.h"
31 #include "llvm/Support/Compiler.h"
32 #include "llvm/Support/Debug.h"
37 STATISTIC(NumAtomic, "Number of atomic phis lowered");
38 STATISTIC(NumSplits, "Number of critical edges split on demand");
39 STATISTIC(NumReused, "Number of reused lowered phis");
41 char PHIElimination::ID = 0;
42 static RegisterPass<PHIElimination>
43 X("phi-node-elimination", "Eliminate PHI nodes for register allocation");
45 const PassInfo *const llvm::PHIEliminationID = &X;
47 void llvm::PHIElimination::getAnalysisUsage(AnalysisUsage &AU) const {
48 AU.addPreserved<LiveVariables>();
49 AU.addPreserved<MachineDominatorTree>();
50 // rdar://7401784 This would be nice:
51 // AU.addPreservedID(MachineLoopInfoID);
52 MachineFunctionPass::getAnalysisUsage(AU);
55 bool llvm::PHIElimination::runOnMachineFunction(MachineFunction &Fn) {
56 MRI = &Fn.getRegInfo();
60 // Split critical edges to help the coalescer
61 if (LiveVariables *LV = getAnalysisIfAvailable<LiveVariables>())
62 for (MachineFunction::iterator I = Fn.begin(), E = Fn.end(); I != E; ++I)
63 Changed |= SplitPHIEdges(Fn, *I, *LV);
65 // Populate VRegPHIUseCount
68 // Eliminate PHI instructions by inserting copies into predecessor blocks.
69 for (MachineFunction::iterator I = Fn.begin(), E = Fn.end(); I != E; ++I)
70 Changed |= EliminatePHINodes(Fn, *I);
72 // Remove dead IMPLICIT_DEF instructions.
73 for (SmallPtrSet<MachineInstr*, 4>::iterator I = ImpDefs.begin(),
74 E = ImpDefs.end(); I != E; ++I) {
75 MachineInstr *DefMI = *I;
76 unsigned DefReg = DefMI->getOperand(0).getReg();
77 if (MRI->use_empty(DefReg))
78 DefMI->eraseFromParent();
81 // Clean up the lowered PHI instructions.
82 for (LoweredPHIMap::iterator I = LoweredPHIs.begin(), E = LoweredPHIs.end();
84 Fn.DeleteMachineInstr(I->first);
88 VRegPHIUseCount.clear();
92 /// EliminatePHINodes - Eliminate phi nodes by inserting copy instructions in
93 /// predecessor basic blocks.
95 bool llvm::PHIElimination::EliminatePHINodes(MachineFunction &MF,
96 MachineBasicBlock &MBB) {
97 if (MBB.empty() || !MBB.front().isPHI())
98 return false; // Quick exit for basic blocks without PHIs.
100 // Get an iterator to the first instruction after the last PHI node (this may
101 // also be the end of the basic block).
102 MachineBasicBlock::iterator AfterPHIsIt = SkipPHIsAndLabels(MBB, MBB.begin());
104 while (MBB.front().isPHI())
105 LowerAtomicPHINode(MBB, AfterPHIsIt);
110 /// isSourceDefinedByImplicitDef - Return true if all sources of the phi node
111 /// are implicit_def's.
112 static bool isSourceDefinedByImplicitDef(const MachineInstr *MPhi,
113 const MachineRegisterInfo *MRI) {
114 for (unsigned i = 1; i != MPhi->getNumOperands(); i += 2) {
115 unsigned SrcReg = MPhi->getOperand(i).getReg();
116 const MachineInstr *DefMI = MRI->getVRegDef(SrcReg);
117 if (!DefMI || !DefMI->isImplicitDef())
123 // FindCopyInsertPoint - Find a safe place in MBB to insert a copy from SrcReg
124 // when following the CFG edge to SuccMBB. This needs to be after any def of
125 // SrcReg, but before any subsequent point where control flow might jump out of
127 MachineBasicBlock::iterator
128 llvm::PHIElimination::FindCopyInsertPoint(MachineBasicBlock &MBB,
129 MachineBasicBlock &SuccMBB,
131 // Handle the trivial case trivially.
135 // Usually, we just want to insert the copy before the first terminator
136 // instruction. However, for the edge going to a landing pad, we must insert
137 // the copy before the call/invoke instruction.
138 if (!SuccMBB.isLandingPad())
139 return MBB.getFirstTerminator();
141 // Discover any defs/uses in this basic block.
142 SmallPtrSet<MachineInstr*, 8> DefUsesInMBB;
143 for (MachineRegisterInfo::reg_iterator RI = MRI->reg_begin(SrcReg),
144 RE = MRI->reg_end(); RI != RE; ++RI) {
145 MachineInstr *DefUseMI = &*RI;
146 if (DefUseMI->getParent() == &MBB)
147 DefUsesInMBB.insert(DefUseMI);
150 MachineBasicBlock::iterator InsertPoint;
151 if (DefUsesInMBB.empty()) {
152 // No defs. Insert the copy at the start of the basic block.
153 InsertPoint = MBB.begin();
154 } else if (DefUsesInMBB.size() == 1) {
155 // Insert the copy immediately after the def/use.
156 InsertPoint = *DefUsesInMBB.begin();
159 // Insert the copy immediately after the last def/use.
160 InsertPoint = MBB.end();
161 while (!DefUsesInMBB.count(&*--InsertPoint)) {}
165 // Make sure the copy goes after any phi nodes however.
166 return SkipPHIsAndLabels(MBB, InsertPoint);
169 /// LowerAtomicPHINode - Lower the PHI node at the top of the specified block,
170 /// under the assuption that it needs to be lowered in a way that supports
171 /// atomic execution of PHIs. This lowering method is always correct all of the
174 void llvm::PHIElimination::LowerAtomicPHINode(
175 MachineBasicBlock &MBB,
176 MachineBasicBlock::iterator AfterPHIsIt) {
178 // Unlink the PHI node from the basic block, but don't delete the PHI yet.
179 MachineInstr *MPhi = MBB.remove(MBB.begin());
181 unsigned NumSrcs = (MPhi->getNumOperands() - 1) / 2;
182 unsigned DestReg = MPhi->getOperand(0).getReg();
183 bool isDead = MPhi->getOperand(0).isDead();
185 // Create a new register for the incoming PHI arguments.
186 MachineFunction &MF = *MBB.getParent();
187 const TargetRegisterClass *RC = MF.getRegInfo().getRegClass(DestReg);
188 unsigned IncomingReg = 0;
189 bool reusedIncoming = false; // Is IncomingReg reused from an earlier PHI?
191 // Insert a register to register copy at the top of the current block (but
192 // after any remaining phi nodes) which copies the new incoming register
193 // into the phi node destination.
194 const TargetInstrInfo *TII = MF.getTarget().getInstrInfo();
195 if (isSourceDefinedByImplicitDef(MPhi, MRI))
196 // If all sources of a PHI node are implicit_def, just emit an
197 // implicit_def instead of a copy.
198 BuildMI(MBB, AfterPHIsIt, MPhi->getDebugLoc(),
199 TII->get(TargetOpcode::IMPLICIT_DEF), DestReg);
201 // Can we reuse an earlier PHI node? This only happens for critical edges,
202 // typically those created by tail duplication.
203 unsigned &entry = LoweredPHIs[MPhi];
205 // An identical PHI node was already lowered. Reuse the incoming register.
207 reusedIncoming = true;
209 DEBUG(dbgs() << "Reusing %reg" << IncomingReg << " for " << *MPhi);
211 entry = IncomingReg = MF.getRegInfo().createVirtualRegister(RC);
213 TII->copyRegToReg(MBB, AfterPHIsIt, DestReg, IncomingReg, RC, RC);
216 // Update live variable information if there is any.
217 LiveVariables *LV = getAnalysisIfAvailable<LiveVariables>();
219 MachineInstr *PHICopy = prior(AfterPHIsIt);
222 LiveVariables::VarInfo &VI = LV->getVarInfo(IncomingReg);
224 // Increment use count of the newly created virtual register.
226 LV->setPHIJoin(IncomingReg);
228 // When we are reusing the incoming register, it may already have been
229 // killed in this block. The old kill will also have been inserted at
230 // AfterPHIsIt, so it appears before the current PHICopy.
232 if (MachineInstr *OldKill = VI.findKill(&MBB)) {
233 DEBUG(dbgs() << "Remove old kill from " << *OldKill);
234 LV->removeVirtualRegisterKilled(IncomingReg, OldKill);
238 // Add information to LiveVariables to know that the incoming value is
239 // killed. Note that because the value is defined in several places (once
240 // each for each incoming block), the "def" block and instruction fields
241 // for the VarInfo is not filled in.
242 LV->addVirtualRegisterKilled(IncomingReg, PHICopy);
245 // Since we are going to be deleting the PHI node, if it is the last use of
246 // any registers, or if the value itself is dead, we need to move this
247 // information over to the new copy we just inserted.
248 LV->removeVirtualRegistersKilled(MPhi);
250 // If the result is dead, update LV.
252 LV->addVirtualRegisterDead(DestReg, PHICopy);
253 LV->removeVirtualRegisterDead(DestReg, MPhi);
257 // Adjust the VRegPHIUseCount map to account for the removal of this PHI node.
258 for (unsigned i = 1; i != MPhi->getNumOperands(); i += 2)
259 --VRegPHIUseCount[BBVRegPair(MPhi->getOperand(i+1).getMBB()->getNumber(),
260 MPhi->getOperand(i).getReg())];
262 // Now loop over all of the incoming arguments, changing them to copy into the
263 // IncomingReg register in the corresponding predecessor basic block.
264 SmallPtrSet<MachineBasicBlock*, 8> MBBsInsertedInto;
265 for (int i = NumSrcs - 1; i >= 0; --i) {
266 unsigned SrcReg = MPhi->getOperand(i*2+1).getReg();
267 assert(TargetRegisterInfo::isVirtualRegister(SrcReg) &&
268 "Machine PHI Operands must all be virtual registers!");
270 // Get the MachineBasicBlock equivalent of the BasicBlock that is the source
272 MachineBasicBlock &opBlock = *MPhi->getOperand(i*2+2).getMBB();
274 // If source is defined by an implicit def, there is no need to insert a
276 MachineInstr *DefMI = MRI->getVRegDef(SrcReg);
277 if (DefMI->isImplicitDef()) {
278 ImpDefs.insert(DefMI);
282 // Check to make sure we haven't already emitted the copy for this block.
283 // This can happen because PHI nodes may have multiple entries for the same
285 if (!MBBsInsertedInto.insert(&opBlock))
286 continue; // If the copy has already been emitted, we're done.
288 // Find a safe location to insert the copy, this may be the first terminator
289 // in the block (or end()).
290 MachineBasicBlock::iterator InsertPos =
291 FindCopyInsertPoint(opBlock, MBB, SrcReg);
294 if (!reusedIncoming && IncomingReg)
295 TII->copyRegToReg(opBlock, InsertPos, IncomingReg, SrcReg, RC, RC);
297 // Now update live variable information if we have it. Otherwise we're done
300 // We want to be able to insert a kill of the register if this PHI (aka, the
301 // copy we just inserted) is the last use of the source value. Live
302 // variable analysis conservatively handles this by saying that the value is
303 // live until the end of the block the PHI entry lives in. If the value
304 // really is dead at the PHI copy, there will be no successor blocks which
305 // have the value live-in.
307 // Also check to see if this register is in use by another PHI node which
308 // has not yet been eliminated. If so, it will be killed at an appropriate
311 // Is it used by any PHI instructions in this block?
312 bool ValueIsUsed = VRegPHIUseCount[BBVRegPair(opBlock.getNumber(), SrcReg)];
314 // Okay, if we now know that the value is not live out of the block, we can
315 // add a kill marker in this block saying that it kills the incoming value!
316 if (!ValueIsUsed && !LV->isLiveOut(SrcReg, opBlock)) {
317 // In our final twist, we have to decide which instruction kills the
318 // register. In most cases this is the copy, however, the first
319 // terminator instruction at the end of the block may also use the value.
320 // In this case, we should mark *it* as being the killing block, not the
322 MachineBasicBlock::iterator KillInst;
323 MachineBasicBlock::iterator Term = opBlock.getFirstTerminator();
324 if (Term != opBlock.end() && Term->readsRegister(SrcReg)) {
327 // Check that no other terminators use values.
329 for (MachineBasicBlock::iterator TI = llvm::next(Term);
330 TI != opBlock.end(); ++TI) {
331 assert(!TI->readsRegister(SrcReg) &&
332 "Terminator instructions cannot use virtual registers unless"
333 "they are the first terminator in a block!");
336 } else if (reusedIncoming || !IncomingReg) {
337 // We may have to rewind a bit if we didn't insert a copy this time.
339 while (KillInst != opBlock.begin())
340 if ((--KillInst)->readsRegister(SrcReg))
343 // We just inserted this copy.
344 KillInst = prior(InsertPos);
346 assert(KillInst->readsRegister(SrcReg) && "Cannot find kill instruction");
348 // Finally, mark it killed.
349 LV->addVirtualRegisterKilled(SrcReg, KillInst);
351 // This vreg no longer lives all of the way through opBlock.
352 unsigned opBlockNum = opBlock.getNumber();
353 LV->getVarInfo(SrcReg).AliveBlocks.reset(opBlockNum);
357 // Really delete the PHI instruction now, if it is not in the LoweredPHIs map.
358 if (reusedIncoming || !IncomingReg)
359 MF.DeleteMachineInstr(MPhi);
362 /// analyzePHINodes - Gather information about the PHI nodes in here. In
363 /// particular, we want to map the number of uses of a virtual register which is
364 /// used in a PHI node. We map that to the BB the vreg is coming from. This is
365 /// used later to determine when the vreg is killed in the BB.
367 void llvm::PHIElimination::analyzePHINodes(const MachineFunction& Fn) {
368 for (MachineFunction::const_iterator I = Fn.begin(), E = Fn.end();
370 for (MachineBasicBlock::const_iterator BBI = I->begin(), BBE = I->end();
371 BBI != BBE && BBI->isPHI(); ++BBI)
372 for (unsigned i = 1, e = BBI->getNumOperands(); i != e; i += 2)
373 ++VRegPHIUseCount[BBVRegPair(BBI->getOperand(i+1).getMBB()->getNumber(),
374 BBI->getOperand(i).getReg())];
377 bool llvm::PHIElimination::SplitPHIEdges(MachineFunction &MF,
378 MachineBasicBlock &MBB,
380 if (MBB.empty() || !MBB.front().isPHI() || MBB.isLandingPad())
381 return false; // Quick exit for basic blocks without PHIs.
383 for (MachineBasicBlock::const_iterator BBI = MBB.begin(), BBE = MBB.end();
384 BBI != BBE && BBI->isPHI(); ++BBI) {
385 for (unsigned i = 1, e = BBI->getNumOperands(); i != e; i += 2) {
386 unsigned Reg = BBI->getOperand(i).getReg();
387 MachineBasicBlock *PreMBB = BBI->getOperand(i+1).getMBB();
388 // We break edges when registers are live out from the predecessor block
389 // (not considering PHI nodes). If the register is live in to this block
390 // anyway, we would gain nothing from splitting.
391 if (!LV.isLiveIn(Reg, MBB) && LV.isLiveOut(Reg, *PreMBB))
392 SplitCriticalEdge(PreMBB, &MBB);
398 MachineBasicBlock *PHIElimination::SplitCriticalEdge(MachineBasicBlock *A,
399 MachineBasicBlock *B) {
400 assert(A && B && "Missing MBB end point");
402 MachineFunction *MF = A->getParent();
404 // We may need to update A's terminator, but we can't do that if AnalyzeBranch
405 // fails. If A uses a jump table, we won't touch it.
406 const TargetInstrInfo *TII = MF->getTarget().getInstrInfo();
407 MachineBasicBlock *TBB = 0, *FBB = 0;
408 SmallVector<MachineOperand, 4> Cond;
409 if (TII->AnalyzeBranch(*A, TBB, FBB, Cond))
414 MachineBasicBlock *NMBB = MF->CreateMachineBasicBlock();
415 MF->insert(llvm::next(MachineFunction::iterator(A)), NMBB);
416 DEBUG(dbgs() << "PHIElimination splitting critical edge:"
417 " BB#" << A->getNumber()
418 << " -- BB#" << NMBB->getNumber()
419 << " -- BB#" << B->getNumber() << '\n');
421 A->ReplaceUsesOfBlockWith(B, NMBB);
422 A->updateTerminator();
424 // Insert unconditional "jump B" instruction in NMBB if necessary.
425 NMBB->addSuccessor(B);
426 if (!NMBB->isLayoutSuccessor(B)) {
428 MF->getTarget().getInstrInfo()->InsertBranch(*NMBB, B, NULL, Cond);
431 // Fix PHI nodes in B so they refer to NMBB instead of A
432 for (MachineBasicBlock::iterator i = B->begin(), e = B->end();
433 i != e && i->isPHI(); ++i)
434 for (unsigned ni = 1, ne = i->getNumOperands(); ni != ne; ni += 2)
435 if (i->getOperand(ni+1).getMBB() == A)
436 i->getOperand(ni+1).setMBB(NMBB);
438 if (LiveVariables *LV=getAnalysisIfAvailable<LiveVariables>())
439 LV->addNewBlock(NMBB, A, B);
441 if (MachineDominatorTree *MDT=getAnalysisIfAvailable<MachineDominatorTree>())
442 MDT->addNewBlock(NMBB, A);