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);
88 /// InsertNewPHI - Insert an empty PHI instruction which define a value of the
89 /// given register class at the start of the specified basic block. It returns
90 /// the virtual register defined by the PHI instruction.
92 MachineInstr *InsertNewPHI(MachineBasicBlock *BB, const TargetRegisterClass *RC,
93 MachineRegisterInfo *MRI, const TargetInstrInfo *TII) {
94 unsigned NewVR = MRI->createVirtualRegister(RC);
95 return BuildMI(*BB, BB->front(), BB->front().getDebugLoc(),
96 TII->get(TargetInstrInfo::PHI), NewVR);
100 /// GetValueInMiddleOfBlock - Construct SSA form, materializing a value that
101 /// is live in the middle of the specified block.
103 /// GetValueInMiddleOfBlock is the same as GetValueAtEndOfBlock except in one
104 /// important case: if there is a definition of the rewritten value after the
105 /// 'use' in BB. Consider code like this:
111 /// br Cond, SomeBB, OutBB
113 /// In this case, there are two values (X1 and X2) added to the AvailableVals
114 /// set by the client of the rewriter, and those values are both live out of
115 /// their respective blocks. However, the use of X happens in the *middle* of
116 /// a block. Because of this, we need to insert a new PHI node in SomeBB to
117 /// merge the appropriate values, and this value isn't live out of the block.
119 unsigned MachineSSAUpdater::GetValueInMiddleOfBlock(MachineBasicBlock *BB) {
120 // If there is no definition of the renamed variable in this block, just use
121 // GetValueAtEndOfBlock to do our work.
122 if (!getAvailableVals(AV).count(BB))
123 return GetValueAtEndOfBlock(BB);
125 if (BB->pred_empty())
126 llvm_unreachable("Unreachable block!");
128 // Otherwise, we have the hard case. Get the live-in values for each
130 SmallVector<std::pair<MachineBasicBlock*, unsigned>, 8> PredValues;
131 unsigned SingularValue = 0;
133 bool isFirstPred = true;
134 for (MachineBasicBlock::pred_iterator PI = BB->pred_begin(),
135 E = BB->pred_end(); PI != E; ++PI) {
136 MachineBasicBlock *PredBB = *PI;
137 unsigned PredVal = GetValueAtEndOfBlockInternal(PredBB);
138 PredValues.push_back(std::make_pair(PredBB, PredVal));
140 // Compute SingularValue.
142 SingularValue = PredVal;
144 } else if (PredVal != SingularValue)
148 // Otherwise, if all the merged values are the same, just use it.
149 if (SingularValue != 0)
150 return SingularValue;
152 // Otherwise, we do need a PHI: insert one now.
153 MachineInstr *InsertedPHI = InsertNewPHI(BB, VRC, MRI, TII);
155 // Fill in all the predecessors of the PHI.
156 MachineInstrBuilder MIB(InsertedPHI);
157 for (unsigned i = 0, e = PredValues.size(); i != e; ++i)
158 MIB.addReg(PredValues[i].second).addMBB(PredValues[i].first);
160 // See if the PHI node can be merged to a single value. This can happen in
161 // loop cases when we get a PHI of itself and one other value.
162 if (unsigned ConstVal = InsertedPHI->isConstantValuePHI()) {
163 InsertedPHI->eraseFromParent();
167 // If the client wants to know about all new instructions, tell it.
168 if (InsertedPHIs) InsertedPHIs->push_back(InsertedPHI);
170 DEBUG(errs() << " Inserted PHI: " << *InsertedPHI << "\n");
171 return InsertedPHI->getOperand(0).getReg();
175 MachineBasicBlock *findCorrespondingPred(const MachineInstr *MI,
177 for (unsigned i = 1, e = MI->getNumOperands(); i != e; i += 2) {
178 if (&MI->getOperand(i) == U)
179 return MI->getOperand(i+1).getMBB();
182 llvm_unreachable("MachineOperand::getParent() failure?");
186 /// RewriteUse - Rewrite a use of the symbolic value. This handles PHI nodes,
187 /// which use their value in the corresponding predecessor.
188 void MachineSSAUpdater::RewriteUse(MachineOperand &U) {
189 MachineInstr *UseMI = U.getParent();
191 if (UseMI->getOpcode() == TargetInstrInfo::PHI) {
192 MachineBasicBlock *SourceBB = findCorrespondingPred(UseMI, &U);
193 NewVR = GetValueAtEndOfBlock(SourceBB);
195 NewVR = GetValueInMiddleOfBlock(UseMI->getParent());
201 /// GetValueAtEndOfBlockInternal - Check to see if AvailableVals has an entry
202 /// for the specified BB and if so, return it. If not, construct SSA form by
203 /// walking predecessors inserting PHI nodes as needed until we get to a block
204 /// where the value is available.
206 unsigned MachineSSAUpdater::GetValueAtEndOfBlockInternal(MachineBasicBlock *BB){
207 AvailableValsTy &AvailableVals = getAvailableVals(AV);
209 // Query AvailableVals by doing an insertion of null.
210 std::pair<AvailableValsTy::iterator, bool> InsertRes =
211 AvailableVals.insert(std::make_pair(BB, 0));
213 // Handle the case when the insertion fails because we have already seen BB.
214 if (!InsertRes.second) {
215 // If the insertion failed, there are two cases. The first case is that the
216 // value is already available for the specified block. If we get this, just
218 if (InsertRes.first->second != 0)
219 return InsertRes.first->second;
221 // Otherwise, if the value we find is null, then this is the value is not
222 // known but it is being computed elsewhere in our recursion. This means
223 // that we have a cycle. Handle this by inserting a PHI node and returning
224 // it. When we get back to the first instance of the recursion we will fill
226 MachineInstr *NewPHI = InsertNewPHI(BB, VRC, MRI, TII);
227 unsigned NewVR = NewPHI->getOperand(0).getReg();
228 InsertRes.first->second = NewVR;
232 if (BB->pred_empty())
233 llvm_unreachable("Unreachable block!");
235 // Okay, the value isn't in the map and we just inserted a null in the entry
236 // to indicate that we're processing the block. Since we have no idea what
237 // value is in this block, we have to recurse through our predecessors.
239 // While we're walking our predecessors, we keep track of them in a vector,
240 // then insert a PHI node in the end if we actually need one. We could use a
241 // smallvector here, but that would take a lot of stack space for every level
242 // of the recursion, just use IncomingPredInfo as an explicit stack.
243 IncomingPredInfoTy &IncomingPredInfo = getIncomingPredInfo(IPI);
244 unsigned FirstPredInfoEntry = IncomingPredInfo.size();
246 // As we're walking the predecessors, keep track of whether they are all
247 // producing the same value. If so, this value will capture it, if not, it
248 // will get reset to null. We distinguish the no-predecessor case explicitly
250 unsigned SingularValue = 0;
251 bool isFirstPred = true;
252 for (MachineBasicBlock::pred_iterator PI = BB->pred_begin(),
253 E = BB->pred_end(); PI != E; ++PI) {
254 MachineBasicBlock *PredBB = *PI;
255 unsigned PredVal = GetValueAtEndOfBlockInternal(PredBB);
256 IncomingPredInfo.push_back(std::make_pair(PredBB, PredVal));
258 // Compute SingularValue.
260 SingularValue = PredVal;
262 } else if (PredVal != SingularValue)
266 /// Look up BB's entry in AvailableVals. 'InsertRes' may be invalidated. If
267 /// this block is involved in a loop, a no-entry PHI node will have been
268 /// inserted as InsertedVal. Otherwise, we'll still have the null we inserted
270 unsigned InsertedVal = AvailableVals[BB];
272 // If all the predecessor values are the same then we don't need to insert a
273 // PHI. This is the simple and common case.
275 // If a PHI node got inserted, replace it with the singlar value and delete
278 MachineInstr *OldVal = MRI->getVRegDef(InsertedVal);
279 // Be careful about dead loops. These RAUW's also update InsertedVal.
280 assert(InsertedVal != SingularValue && "Dead loop?");
281 MRI->replaceRegWith(InsertedVal, SingularValue);
282 OldVal->eraseFromParent();
284 InsertedVal = SingularValue;
287 // Drop the entries we added in IncomingPredInfo to restore the stack.
288 IncomingPredInfo.erase(IncomingPredInfo.begin()+FirstPredInfoEntry,
289 IncomingPredInfo.end());
294 // Otherwise, we do need a PHI: insert one now if we don't already have one.
295 MachineInstr *InsertedPHI;
296 if (InsertedVal == 0) {
297 InsertedPHI = InsertNewPHI(BB, VRC, MRI, TII);
298 InsertedVal = InsertedPHI->getOperand(0).getReg();
300 InsertedPHI = MRI->getVRegDef(InsertedVal);
303 // Fill in all the predecessors of the PHI.
304 MachineInstrBuilder MIB(InsertedPHI);
305 for (IncomingPredInfoTy::iterator I =
306 IncomingPredInfo.begin()+FirstPredInfoEntry,
307 E = IncomingPredInfo.end(); I != E; ++I)
308 MIB.addReg(I->second).addMBB(I->first);
310 // Drop the entries we added in IncomingPredInfo to restore the stack.
311 IncomingPredInfo.erase(IncomingPredInfo.begin()+FirstPredInfoEntry,
312 IncomingPredInfo.end());
314 // See if the PHI node can be merged to a single value. This can happen in
315 // loop cases when we get a PHI of itself and one other value.
316 if (unsigned ConstVal = InsertedPHI->isConstantValuePHI()) {
317 MRI->replaceRegWith(InsertedVal, ConstVal);
318 InsertedPHI->eraseFromParent();
319 InsertedVal = ConstVal;
321 DEBUG(errs() << " Inserted PHI: " << *InsertedPHI << "\n");
323 // If the client wants to know about all new instructions, tell it.
324 if (InsertedPHIs) InsertedPHIs->push_back(InsertedPHI);