1 //===- MachineSSAUpdater.cpp - Unstructured SSA Update Tool ---------------===//
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 file implements the MachineSSAUpdater class. It's based on SSAUpdater
11 // class in lib/Transforms/Utils.
13 //===----------------------------------------------------------------------===//
15 #include "llvm/CodeGen/MachineSSAUpdater.h"
16 #include "llvm/CodeGen/MachineInstr.h"
17 #include "llvm/CodeGen/MachineInstrBuilder.h"
18 #include "llvm/CodeGen/MachineRegisterInfo.h"
19 #include "llvm/Target/TargetInstrInfo.h"
20 #include "llvm/Target/TargetMachine.h"
21 #include "llvm/Target/TargetRegisterInfo.h"
22 #include "llvm/ADT/DenseMap.h"
23 #include "llvm/Support/Debug.h"
24 #include "llvm/Support/ErrorHandling.h"
25 #include "llvm/Support/raw_ostream.h"
28 typedef DenseMap<MachineBasicBlock*, unsigned> AvailableValsTy;
29 typedef std::vector<std::pair<MachineBasicBlock*, unsigned> >
32 static AvailableValsTy &getAvailableVals(void *AV) {
33 return *static_cast<AvailableValsTy*>(AV);
36 static IncomingPredInfoTy &getIncomingPredInfo(void *IPI) {
37 return *static_cast<IncomingPredInfoTy*>(IPI);
41 MachineSSAUpdater::MachineSSAUpdater(MachineFunction &MF,
42 SmallVectorImpl<MachineInstr*> *NewPHI)
43 : AV(0), IPI(0), InsertedPHIs(NewPHI) {
44 TII = MF.getTarget().getInstrInfo();
45 MRI = &MF.getRegInfo();
48 MachineSSAUpdater::~MachineSSAUpdater() {
49 delete &getAvailableVals(AV);
50 delete &getIncomingPredInfo(IPI);
53 /// Initialize - Reset this object to get ready for a new set of SSA
54 /// updates. ProtoValue is the value used to name PHI nodes.
55 void MachineSSAUpdater::Initialize(unsigned V) {
57 AV = new AvailableValsTy();
59 getAvailableVals(AV).clear();
62 IPI = new IncomingPredInfoTy();
64 getIncomingPredInfo(IPI).clear();
67 VRC = MRI->getRegClass(VR);
70 /// HasValueForBlock - Return true if the MachineSSAUpdater already has a value for
71 /// the specified block.
72 bool MachineSSAUpdater::HasValueForBlock(MachineBasicBlock *BB) const {
73 return getAvailableVals(AV).count(BB);
76 /// AddAvailableValue - Indicate that a rewritten value is available in the
77 /// specified block with the specified value.
78 void MachineSSAUpdater::AddAvailableValue(MachineBasicBlock *BB, unsigned V) {
79 getAvailableVals(AV)[BB] = V;
82 /// GetValueAtEndOfBlock - Construct SSA form, materializing a value that is
83 /// live at the end of the specified block.
84 unsigned MachineSSAUpdater::GetValueAtEndOfBlock(MachineBasicBlock *BB) {
85 return GetValueAtEndOfBlockInternal(BB);
89 unsigned LookForIdenticalPHI(MachineBasicBlock *BB,
90 SmallVector<std::pair<MachineBasicBlock*, unsigned>, 8> &PredValues) {
94 MachineBasicBlock::iterator I = BB->front();
95 if (I->getOpcode() != TargetInstrInfo::PHI)
98 AvailableValsTy AVals;
99 for (unsigned i = 0, e = PredValues.size(); i != e; ++i)
100 AVals[PredValues[i].first] = PredValues[i].second;
101 while (I != BB->end() && I->getOpcode() == TargetInstrInfo::PHI) {
103 for (unsigned i = 1, e = I->getNumOperands(); i != e; i += 2) {
104 unsigned SrcReg = I->getOperand(i).getReg();
105 MachineBasicBlock *SrcBB = I->getOperand(i+1).getMBB();
106 if (AVals[SrcBB] != SrcReg) {
112 return I->getOperand(0).getReg();
118 /// InsertNewDef - Insert an empty PHI or IMPLICIT_DEF instruction which define
119 /// a value of the given register class at the start of the specified basic
120 /// block. It returns the virtual register defined by the instruction.
122 MachineInstr *InsertNewDef(unsigned Opcode,
123 MachineBasicBlock *BB, MachineBasicBlock::iterator I,
124 const TargetRegisterClass *RC,
125 MachineRegisterInfo *MRI, const TargetInstrInfo *TII) {
126 unsigned NewVR = MRI->createVirtualRegister(RC);
127 return BuildMI(*BB, I, DebugLoc::getUnknownLoc(), TII->get(Opcode), NewVR);
130 /// GetValueInMiddleOfBlock - Construct SSA form, materializing a value that
131 /// is live in the middle of the specified block.
133 /// GetValueInMiddleOfBlock is the same as GetValueAtEndOfBlock except in one
134 /// important case: if there is a definition of the rewritten value after the
135 /// 'use' in BB. Consider code like this:
141 /// br Cond, SomeBB, OutBB
143 /// In this case, there are two values (X1 and X2) added to the AvailableVals
144 /// set by the client of the rewriter, and those values are both live out of
145 /// their respective blocks. However, the use of X happens in the *middle* of
146 /// a block. Because of this, we need to insert a new PHI node in SomeBB to
147 /// merge the appropriate values, and this value isn't live out of the block.
149 unsigned MachineSSAUpdater::GetValueInMiddleOfBlock(MachineBasicBlock *BB) {
150 // If there is no definition of the renamed variable in this block, just use
151 // GetValueAtEndOfBlock to do our work.
152 if (!getAvailableVals(AV).count(BB))
153 return GetValueAtEndOfBlockInternal(BB);
155 // If there are no predecessors, just return undef.
156 if (BB->pred_empty()) {
157 // Insert an implicit_def to represent an undef value.
158 MachineInstr *NewDef = InsertNewDef(TargetInstrInfo::IMPLICIT_DEF,
159 BB, BB->getFirstTerminator(),
161 return NewDef->getOperand(0).getReg();
164 // Otherwise, we have the hard case. Get the live-in values for each
166 SmallVector<std::pair<MachineBasicBlock*, unsigned>, 8> PredValues;
167 unsigned SingularValue = 0;
169 bool isFirstPred = true;
170 for (MachineBasicBlock::pred_iterator PI = BB->pred_begin(),
171 E = BB->pred_end(); PI != E; ++PI) {
172 MachineBasicBlock *PredBB = *PI;
173 unsigned PredVal = GetValueAtEndOfBlockInternal(PredBB);
174 PredValues.push_back(std::make_pair(PredBB, PredVal));
176 // Compute SingularValue.
178 SingularValue = PredVal;
180 } else if (PredVal != SingularValue)
184 // Otherwise, if all the merged values are the same, just use it.
185 if (SingularValue != 0)
186 return SingularValue;
188 // If an identical PHI is already in BB, just reuse it.
189 unsigned DupPHI = LookForIdenticalPHI(BB, PredValues);
193 // Otherwise, we do need a PHI: insert one now.
194 MachineBasicBlock::iterator Loc = BB->empty() ? BB->end() : BB->front();
195 MachineInstr *InsertedPHI = InsertNewDef(TargetInstrInfo::PHI, BB,
198 // Fill in all the predecessors of the PHI.
199 MachineInstrBuilder MIB(InsertedPHI);
200 for (unsigned i = 0, e = PredValues.size(); i != e; ++i)
201 MIB.addReg(PredValues[i].second).addMBB(PredValues[i].first);
203 // See if the PHI node can be merged to a single value. This can happen in
204 // loop cases when we get a PHI of itself and one other value.
205 if (unsigned ConstVal = InsertedPHI->isConstantValuePHI()) {
206 InsertedPHI->eraseFromParent();
210 // If the client wants to know about all new instructions, tell it.
211 if (InsertedPHIs) InsertedPHIs->push_back(InsertedPHI);
213 DEBUG(dbgs() << " Inserted PHI: " << *InsertedPHI << "\n");
214 return InsertedPHI->getOperand(0).getReg();
218 MachineBasicBlock *findCorrespondingPred(const MachineInstr *MI,
220 for (unsigned i = 1, e = MI->getNumOperands(); i != e; i += 2) {
221 if (&MI->getOperand(i) == U)
222 return MI->getOperand(i+1).getMBB();
225 llvm_unreachable("MachineOperand::getParent() failure?");
229 /// RewriteUse - Rewrite a use of the symbolic value. This handles PHI nodes,
230 /// which use their value in the corresponding predecessor.
231 void MachineSSAUpdater::RewriteUse(MachineOperand &U) {
232 MachineInstr *UseMI = U.getParent();
234 if (UseMI->getOpcode() == TargetInstrInfo::PHI) {
235 MachineBasicBlock *SourceBB = findCorrespondingPred(UseMI, &U);
236 NewVR = GetValueAtEndOfBlockInternal(SourceBB);
238 NewVR = GetValueInMiddleOfBlock(UseMI->getParent());
244 void MachineSSAUpdater::ReplaceRegWith(unsigned OldReg, unsigned NewReg) {
245 MRI->replaceRegWith(OldReg, NewReg);
247 AvailableValsTy &AvailableVals = getAvailableVals(AV);
248 for (DenseMap<MachineBasicBlock*, unsigned>::iterator
249 I = AvailableVals.begin(), E = AvailableVals.end(); I != E; ++I)
250 if (I->second == OldReg)
254 /// GetValueAtEndOfBlockInternal - Check to see if AvailableVals has an entry
255 /// for the specified BB and if so, return it. If not, construct SSA form by
256 /// walking predecessors inserting PHI nodes as needed until we get to a block
257 /// where the value is available.
259 unsigned MachineSSAUpdater::GetValueAtEndOfBlockInternal(MachineBasicBlock *BB){
260 AvailableValsTy &AvailableVals = getAvailableVals(AV);
262 // Query AvailableVals by doing an insertion of null.
263 std::pair<AvailableValsTy::iterator, bool> InsertRes =
264 AvailableVals.insert(std::make_pair(BB, 0));
266 // Handle the case when the insertion fails because we have already seen BB.
267 if (!InsertRes.second) {
268 // If the insertion failed, there are two cases. The first case is that the
269 // value is already available for the specified block. If we get this, just
271 if (InsertRes.first->second != 0)
272 return InsertRes.first->second;
274 // Otherwise, if the value we find is null, then this is the value is not
275 // known but it is being computed elsewhere in our recursion. This means
276 // that we have a cycle. Handle this by inserting a PHI node and returning
277 // it. When we get back to the first instance of the recursion we will fill
279 MachineBasicBlock::iterator Loc = BB->empty() ? BB->end() : BB->front();
280 MachineInstr *NewPHI = InsertNewDef(TargetInstrInfo::PHI, BB, Loc,
282 unsigned NewVR = NewPHI->getOperand(0).getReg();
283 InsertRes.first->second = NewVR;
287 // If there are no predecessors, then we must have found an unreachable block
288 // just return 'undef'. Since there are no predecessors, InsertRes must not
290 if (BB->pred_empty()) {
291 // Insert an implicit_def to represent an undef value.
292 MachineInstr *NewDef = InsertNewDef(TargetInstrInfo::IMPLICIT_DEF,
293 BB, BB->getFirstTerminator(),
295 return InsertRes.first->second = NewDef->getOperand(0).getReg();
298 // Okay, the value isn't in the map and we just inserted a null in the entry
299 // to indicate that we're processing the block. Since we have no idea what
300 // value is in this block, we have to recurse through our predecessors.
302 // While we're walking our predecessors, we keep track of them in a vector,
303 // then insert a PHI node in the end if we actually need one. We could use a
304 // smallvector here, but that would take a lot of stack space for every level
305 // of the recursion, just use IncomingPredInfo as an explicit stack.
306 IncomingPredInfoTy &IncomingPredInfo = getIncomingPredInfo(IPI);
307 unsigned FirstPredInfoEntry = IncomingPredInfo.size();
309 // As we're walking the predecessors, keep track of whether they are all
310 // producing the same value. If so, this value will capture it, if not, it
311 // will get reset to null. We distinguish the no-predecessor case explicitly
313 unsigned SingularValue = 0;
314 bool isFirstPred = true;
315 for (MachineBasicBlock::pred_iterator PI = BB->pred_begin(),
316 E = BB->pred_end(); PI != E; ++PI) {
317 MachineBasicBlock *PredBB = *PI;
318 unsigned PredVal = GetValueAtEndOfBlockInternal(PredBB);
319 IncomingPredInfo.push_back(std::make_pair(PredBB, PredVal));
321 // Compute SingularValue.
323 SingularValue = PredVal;
325 } else if (PredVal != SingularValue)
329 /// Look up BB's entry in AvailableVals. 'InsertRes' may be invalidated. If
330 /// this block is involved in a loop, a no-entry PHI node will have been
331 /// inserted as InsertedVal. Otherwise, we'll still have the null we inserted
333 unsigned &InsertedVal = AvailableVals[BB];
335 // If all the predecessor values are the same then we don't need to insert a
336 // PHI. This is the simple and common case.
338 // If a PHI node got inserted, replace it with the singlar value and delete
341 MachineInstr *OldVal = MRI->getVRegDef(InsertedVal);
342 // Be careful about dead loops. These RAUW's also update InsertedVal.
343 assert(InsertedVal != SingularValue && "Dead loop?");
344 ReplaceRegWith(InsertedVal, SingularValue);
345 OldVal->eraseFromParent();
348 InsertedVal = SingularValue;
350 // Drop the entries we added in IncomingPredInfo to restore the stack.
351 IncomingPredInfo.erase(IncomingPredInfo.begin()+FirstPredInfoEntry,
352 IncomingPredInfo.end());
357 // Otherwise, we do need a PHI: insert one now if we don't already have one.
358 MachineInstr *InsertedPHI;
359 if (InsertedVal == 0) {
360 MachineBasicBlock::iterator Loc = BB->empty() ? BB->end() : BB->front();
361 InsertedPHI = InsertNewDef(TargetInstrInfo::PHI, BB, Loc,
363 InsertedVal = InsertedPHI->getOperand(0).getReg();
365 InsertedPHI = MRI->getVRegDef(InsertedVal);
368 // Fill in all the predecessors of the PHI.
369 MachineInstrBuilder MIB(InsertedPHI);
370 for (IncomingPredInfoTy::iterator I =
371 IncomingPredInfo.begin()+FirstPredInfoEntry,
372 E = IncomingPredInfo.end(); I != E; ++I)
373 MIB.addReg(I->second).addMBB(I->first);
375 // Drop the entries we added in IncomingPredInfo to restore the stack.
376 IncomingPredInfo.erase(IncomingPredInfo.begin()+FirstPredInfoEntry,
377 IncomingPredInfo.end());
379 // See if the PHI node can be merged to a single value. This can happen in
380 // loop cases when we get a PHI of itself and one other value.
381 if (unsigned ConstVal = InsertedPHI->isConstantValuePHI()) {
382 MRI->replaceRegWith(InsertedVal, ConstVal);
383 InsertedPHI->eraseFromParent();
384 InsertedVal = ConstVal;
386 DEBUG(dbgs() << " Inserted PHI: " << *InsertedPHI << "\n");
388 // If the client wants to know about all new instructions, tell it.
389 if (InsertedPHIs) InsertedPHIs->push_back(InsertedPHI);