1 //===- MachineSSAUpdater.cpp - Unstructured SSA Update Tool ---------------===//
2 //
3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4 // See https://llvm.org/LICENSE.txt for license information.
5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6 //
7 //===----------------------------------------------------------------------===//
8 //
9 // This file implements the MachineSSAUpdater class. It's based on SSAUpdater
10 // class in lib/Transforms/Utils.
11 //
12 //===----------------------------------------------------------------------===//
13 
14 #include "llvm/CodeGen/MachineSSAUpdater.h"
15 #include "llvm/ADT/DenseMap.h"
16 #include "llvm/ADT/SmallVector.h"
17 #include "llvm/CodeGen/MachineBasicBlock.h"
18 #include "llvm/CodeGen/MachineFunction.h"
19 #include "llvm/CodeGen/MachineInstr.h"
20 #include "llvm/CodeGen/MachineInstrBuilder.h"
21 #include "llvm/CodeGen/MachineOperand.h"
22 #include "llvm/CodeGen/MachineRegisterInfo.h"
23 #include "llvm/CodeGen/TargetInstrInfo.h"
24 #include "llvm/CodeGen/TargetOpcodes.h"
25 #include "llvm/CodeGen/TargetSubtargetInfo.h"
26 #include "llvm/IR/DebugLoc.h"
27 #include "llvm/Support/Debug.h"
28 #include "llvm/Support/ErrorHandling.h"
29 #include "llvm/Support/raw_ostream.h"
30 #include "llvm/Transforms/Utils/SSAUpdaterImpl.h"
31 #include <utility>
32 
33 using namespace llvm;
34 
35 #define DEBUG_TYPE "machine-ssaupdater"
36 
37 using AvailableValsTy = DenseMap<MachineBasicBlock *, Register>;
38 
39 static AvailableValsTy &getAvailableVals(void *AV) {
40   return *static_cast<AvailableValsTy*>(AV);
41 }
42 
43 MachineSSAUpdater::MachineSSAUpdater(MachineFunction &MF,
44                                      SmallVectorImpl<MachineInstr*> *NewPHI)
45   : InsertedPHIs(NewPHI), TII(MF.getSubtarget().getInstrInfo()),
46     MRI(&MF.getRegInfo()) {}
47 
48 MachineSSAUpdater::~MachineSSAUpdater() {
49   delete static_cast<AvailableValsTy*>(AV);
50 }
51 
52 /// Initialize - Reset this object to get ready for a new set of SSA
53 /// updates.
54 void MachineSSAUpdater::Initialize(const TargetRegisterClass *RC) {
55   if (!AV)
56     AV = new AvailableValsTy();
57   else
58     getAvailableVals(AV).clear();
59 
60   VRC = RC;
61 }
62 
63 void MachineSSAUpdater::Initialize(Register V) {
64   Initialize(MRI->getRegClass(V));
65 }
66 
67 /// HasValueForBlock - Return true if the MachineSSAUpdater already has a value for
68 /// the specified block.
69 bool MachineSSAUpdater::HasValueForBlock(MachineBasicBlock *BB) const {
70   return getAvailableVals(AV).count(BB);
71 }
72 
73 /// AddAvailableValue - Indicate that a rewritten value is available in the
74 /// specified block with the specified value.
75 void MachineSSAUpdater::AddAvailableValue(MachineBasicBlock *BB, Register V) {
76   getAvailableVals(AV)[BB] = V;
77 }
78 
79 /// GetValueAtEndOfBlock - Construct SSA form, materializing a value that is
80 /// live at the end of the specified block.
81 Register MachineSSAUpdater::GetValueAtEndOfBlock(MachineBasicBlock *BB) {
82   return GetValueAtEndOfBlockInternal(BB);
83 }
84 
85 static
86 Register LookForIdenticalPHI(MachineBasicBlock *BB,
87         SmallVectorImpl<std::pair<MachineBasicBlock *, Register>> &PredValues) {
88   if (BB->empty())
89     return Register();
90 
91   MachineBasicBlock::iterator I = BB->begin();
92   if (!I->isPHI())
93     return Register();
94 
95   AvailableValsTy AVals;
96   for (unsigned i = 0, e = PredValues.size(); i != e; ++i)
97     AVals[PredValues[i].first] = PredValues[i].second;
98   while (I != BB->end() && I->isPHI()) {
99     bool Same = true;
100     for (unsigned i = 1, e = I->getNumOperands(); i != e; i += 2) {
101       Register SrcReg = I->getOperand(i).getReg();
102       MachineBasicBlock *SrcBB = I->getOperand(i+1).getMBB();
103       if (AVals[SrcBB] != SrcReg) {
104         Same = false;
105         break;
106       }
107     }
108     if (Same)
109       return I->getOperand(0).getReg();
110     ++I;
111   }
112   return Register();
113 }
114 
115 /// InsertNewDef - Insert an empty PHI or IMPLICIT_DEF instruction which define
116 /// a value of the given register class at the start of the specified basic
117 /// block. It returns the virtual register defined by the instruction.
118 static
119 MachineInstrBuilder InsertNewDef(unsigned Opcode,
120                            MachineBasicBlock *BB, MachineBasicBlock::iterator I,
121                            const TargetRegisterClass *RC,
122                            MachineRegisterInfo *MRI,
123                            const TargetInstrInfo *TII) {
124   Register NewVR = MRI->createVirtualRegister(RC);
125   return BuildMI(*BB, I, DebugLoc(), TII->get(Opcode), NewVR);
126 }
127 
128 /// GetValueInMiddleOfBlock - Construct SSA form, materializing a value that
129 /// is live in the middle of the specified block.
130 ///
131 /// GetValueInMiddleOfBlock is the same as GetValueAtEndOfBlock except in one
132 /// important case: if there is a definition of the rewritten value after the
133 /// 'use' in BB.  Consider code like this:
134 ///
135 ///      X1 = ...
136 ///   SomeBB:
137 ///      use(X)
138 ///      X2 = ...
139 ///      br Cond, SomeBB, OutBB
140 ///
141 /// In this case, there are two values (X1 and X2) added to the AvailableVals
142 /// set by the client of the rewriter, and those values are both live out of
143 /// their respective blocks.  However, the use of X happens in the *middle* of
144 /// a block.  Because of this, we need to insert a new PHI node in SomeBB to
145 /// merge the appropriate values, and this value isn't live out of the block.
146 Register MachineSSAUpdater::GetValueInMiddleOfBlock(MachineBasicBlock *BB) {
147   // If there is no definition of the renamed variable in this block, just use
148   // GetValueAtEndOfBlock to do our work.
149   if (!HasValueForBlock(BB))
150     return GetValueAtEndOfBlockInternal(BB);
151 
152   // If there are no predecessors, just return undef.
153   if (BB->pred_empty()) {
154     // Insert an implicit_def to represent an undef value.
155     MachineInstr *NewDef = InsertNewDef(TargetOpcode::IMPLICIT_DEF,
156                                         BB, BB->getFirstTerminator(),
157                                         VRC, MRI, TII);
158     return NewDef->getOperand(0).getReg();
159   }
160 
161   // Otherwise, we have the hard case.  Get the live-in values for each
162   // predecessor.
163   SmallVector<std::pair<MachineBasicBlock*, Register>, 8> PredValues;
164   Register SingularValue;
165 
166   bool isFirstPred = true;
167   for (MachineBasicBlock *PredBB : BB->predecessors()) {
168     Register PredVal = GetValueAtEndOfBlockInternal(PredBB);
169     PredValues.push_back(std::make_pair(PredBB, PredVal));
170 
171     // Compute SingularValue.
172     if (isFirstPred) {
173       SingularValue = PredVal;
174       isFirstPred = false;
175     } else if (PredVal != SingularValue)
176       SingularValue = Register();
177   }
178 
179   // Otherwise, if all the merged values are the same, just use it.
180   if (SingularValue)
181     return SingularValue;
182 
183   // If an identical PHI is already in BB, just reuse it.
184   Register DupPHI = LookForIdenticalPHI(BB, PredValues);
185   if (DupPHI)
186     return DupPHI;
187 
188   // Otherwise, we do need a PHI: insert one now.
189   MachineBasicBlock::iterator Loc = BB->empty() ? BB->end() : BB->begin();
190   MachineInstrBuilder InsertedPHI = InsertNewDef(TargetOpcode::PHI, BB,
191                                                  Loc, VRC, MRI, TII);
192 
193   // Fill in all the predecessors of the PHI.
194   for (unsigned i = 0, e = PredValues.size(); i != e; ++i)
195     InsertedPHI.addReg(PredValues[i].second).addMBB(PredValues[i].first);
196 
197   // See if the PHI node can be merged to a single value.  This can happen in
198   // loop cases when we get a PHI of itself and one other value.
199   if (unsigned ConstVal = InsertedPHI->isConstantValuePHI()) {
200     InsertedPHI->eraseFromParent();
201     return ConstVal;
202   }
203 
204   // If the client wants to know about all new instructions, tell it.
205   if (InsertedPHIs) InsertedPHIs->push_back(InsertedPHI);
206 
207   LLVM_DEBUG(dbgs() << "  Inserted PHI: " << *InsertedPHI << "\n");
208   return InsertedPHI.getReg(0);
209 }
210 
211 static
212 MachineBasicBlock *findCorrespondingPred(const MachineInstr *MI,
213                                          MachineOperand *U) {
214   for (unsigned i = 1, e = MI->getNumOperands(); i != e; i += 2) {
215     if (&MI->getOperand(i) == U)
216       return MI->getOperand(i+1).getMBB();
217   }
218 
219   llvm_unreachable("MachineOperand::getParent() failure?");
220 }
221 
222 /// RewriteUse - Rewrite a use of the symbolic value.  This handles PHI nodes,
223 /// which use their value in the corresponding predecessor.
224 void MachineSSAUpdater::RewriteUse(MachineOperand &U) {
225   MachineInstr *UseMI = U.getParent();
226   Register NewVR;
227   if (UseMI->isPHI()) {
228     MachineBasicBlock *SourceBB = findCorrespondingPred(UseMI, &U);
229     NewVR = GetValueAtEndOfBlockInternal(SourceBB);
230   } else {
231     NewVR = GetValueInMiddleOfBlock(UseMI->getParent());
232   }
233 
234   U.setReg(NewVR);
235 }
236 
237 namespace llvm {
238 
239 /// SSAUpdaterTraits<MachineSSAUpdater> - Traits for the SSAUpdaterImpl
240 /// template, specialized for MachineSSAUpdater.
241 template<>
242 class SSAUpdaterTraits<MachineSSAUpdater> {
243 public:
244   using BlkT = MachineBasicBlock;
245   using ValT = Register;
246   using PhiT = MachineInstr;
247   using BlkSucc_iterator = MachineBasicBlock::succ_iterator;
248 
249   static BlkSucc_iterator BlkSucc_begin(BlkT *BB) { return BB->succ_begin(); }
250   static BlkSucc_iterator BlkSucc_end(BlkT *BB) { return BB->succ_end(); }
251 
252   /// Iterator for PHI operands.
253   class PHI_iterator {
254   private:
255     MachineInstr *PHI;
256     unsigned idx;
257 
258   public:
259     explicit PHI_iterator(MachineInstr *P) // begin iterator
260       : PHI(P), idx(1) {}
261     PHI_iterator(MachineInstr *P, bool) // end iterator
262       : PHI(P), idx(PHI->getNumOperands()) {}
263 
264     PHI_iterator &operator++() { idx += 2; return *this; }
265     bool operator==(const PHI_iterator& x) const { return idx == x.idx; }
266     bool operator!=(const PHI_iterator& x) const { return !operator==(x); }
267 
268     unsigned getIncomingValue() { return PHI->getOperand(idx).getReg(); }
269 
270     MachineBasicBlock *getIncomingBlock() {
271       return PHI->getOperand(idx+1).getMBB();
272     }
273   };
274 
275   static inline PHI_iterator PHI_begin(PhiT *PHI) { return PHI_iterator(PHI); }
276 
277   static inline PHI_iterator PHI_end(PhiT *PHI) {
278     return PHI_iterator(PHI, true);
279   }
280 
281   /// FindPredecessorBlocks - Put the predecessors of BB into the Preds
282   /// vector.
283   static void FindPredecessorBlocks(MachineBasicBlock *BB,
284                                     SmallVectorImpl<MachineBasicBlock*> *Preds){
285     append_range(*Preds, BB->predecessors());
286   }
287 
288   /// GetUndefVal - Create an IMPLICIT_DEF instruction with a new register.
289   /// Add it into the specified block and return the register.
290   static Register GetUndefVal(MachineBasicBlock *BB,
291                               MachineSSAUpdater *Updater) {
292     // Insert an implicit_def to represent an undef value.
293     MachineInstr *NewDef = InsertNewDef(TargetOpcode::IMPLICIT_DEF,
294                                         BB, BB->getFirstNonPHI(),
295                                         Updater->VRC, Updater->MRI,
296                                         Updater->TII);
297     return NewDef->getOperand(0).getReg();
298   }
299 
300   /// CreateEmptyPHI - Create a PHI instruction that defines a new register.
301   /// Add it into the specified block and return the register.
302   static Register CreateEmptyPHI(MachineBasicBlock *BB, unsigned NumPreds,
303                                  MachineSSAUpdater *Updater) {
304     MachineBasicBlock::iterator Loc = BB->empty() ? BB->end() : BB->begin();
305     MachineInstr *PHI = InsertNewDef(TargetOpcode::PHI, BB, Loc,
306                                      Updater->VRC, Updater->MRI,
307                                      Updater->TII);
308     return PHI->getOperand(0).getReg();
309   }
310 
311   /// AddPHIOperand - Add the specified value as an operand of the PHI for
312   /// the specified predecessor block.
313   static void AddPHIOperand(MachineInstr *PHI, Register Val,
314                             MachineBasicBlock *Pred) {
315     MachineInstrBuilder(*Pred->getParent(), PHI).addReg(Val).addMBB(Pred);
316   }
317 
318   /// InstrIsPHI - Check if an instruction is a PHI.
319   static MachineInstr *InstrIsPHI(MachineInstr *I) {
320     if (I && I->isPHI())
321       return I;
322     return nullptr;
323   }
324 
325   /// ValueIsPHI - Check if the instruction that defines the specified register
326   /// is a PHI instruction.
327   static MachineInstr *ValueIsPHI(Register Val, MachineSSAUpdater *Updater) {
328     return InstrIsPHI(Updater->MRI->getVRegDef(Val));
329   }
330 
331   /// ValueIsNewPHI - Like ValueIsPHI but also check if the PHI has no source
332   /// operands, i.e., it was just added.
333   static MachineInstr *ValueIsNewPHI(Register Val, MachineSSAUpdater *Updater) {
334     MachineInstr *PHI = ValueIsPHI(Val, Updater);
335     if (PHI && PHI->getNumOperands() <= 1)
336       return PHI;
337     return nullptr;
338   }
339 
340   /// GetPHIValue - For the specified PHI instruction, return the register
341   /// that it defines.
342   static Register GetPHIValue(MachineInstr *PHI) {
343     return PHI->getOperand(0).getReg();
344   }
345 };
346 
347 } // end namespace llvm
348 
349 /// GetValueAtEndOfBlockInternal - Check to see if AvailableVals has an entry
350 /// for the specified BB and if so, return it.  If not, construct SSA form by
351 /// first calculating the required placement of PHIs and then inserting new
352 /// PHIs where needed.
353 Register MachineSSAUpdater::GetValueAtEndOfBlockInternal(MachineBasicBlock *BB){
354   AvailableValsTy &AvailableVals = getAvailableVals(AV);
355   if (Register V = AvailableVals[BB])
356     return V;
357 
358   SSAUpdaterImpl<MachineSSAUpdater> Impl(this, &AvailableVals, InsertedPHIs);
359   return Impl.GetValue(BB);
360 }
361