1 //===- MachineSSAUpdater.cpp - Unstructured SSA Update Tool ---------------===//
2 //
3 //                     The LLVM Compiler Infrastructure
4 //
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
7 //
8 //===----------------------------------------------------------------------===//
9 //
10 // This file implements the MachineSSAUpdater class. It's based on SSAUpdater
11 // class in lib/Transforms/Utils.
12 //
13 //===----------------------------------------------------------------------===//
14 
15 #include "llvm/CodeGen/MachineSSAUpdater.h"
16 #include "llvm/ADT/DenseMap.h"
17 #include "llvm/ADT/SmallVector.h"
18 #include "llvm/CodeGen/MachineInstr.h"
19 #include "llvm/CodeGen/MachineInstrBuilder.h"
20 #include "llvm/CodeGen/MachineRegisterInfo.h"
21 #include "llvm/Support/AlignOf.h"
22 #include "llvm/Support/Allocator.h"
23 #include "llvm/Support/Debug.h"
24 #include "llvm/Support/ErrorHandling.h"
25 #include "llvm/Support/raw_ostream.h"
26 #include "llvm/Target/TargetInstrInfo.h"
27 #include "llvm/Target/TargetRegisterInfo.h"
28 #include "llvm/Target/TargetSubtargetInfo.h"
29 #include "llvm/Transforms/Utils/SSAUpdaterImpl.h"
30 using namespace llvm;
31 
32 #define DEBUG_TYPE "machine-ssaupdater"
33 
34 typedef DenseMap<MachineBasicBlock*, unsigned> AvailableValsTy;
getAvailableVals(void * AV)35 static AvailableValsTy &getAvailableVals(void *AV) {
36   return *static_cast<AvailableValsTy*>(AV);
37 }
38 
MachineSSAUpdater(MachineFunction & MF,SmallVectorImpl<MachineInstr * > * NewPHI)39 MachineSSAUpdater::MachineSSAUpdater(MachineFunction &MF,
40                                      SmallVectorImpl<MachineInstr*> *NewPHI)
41   : AV(nullptr), InsertedPHIs(NewPHI) {
42   TII = MF.getSubtarget().getInstrInfo();
43   MRI = &MF.getRegInfo();
44 }
45 
~MachineSSAUpdater()46 MachineSSAUpdater::~MachineSSAUpdater() {
47   delete static_cast<AvailableValsTy*>(AV);
48 }
49 
50 /// Initialize - Reset this object to get ready for a new set of SSA
51 /// updates.  ProtoValue is the value used to name PHI nodes.
Initialize(unsigned V)52 void MachineSSAUpdater::Initialize(unsigned V) {
53   if (!AV)
54     AV = new AvailableValsTy();
55   else
56     getAvailableVals(AV).clear();
57 
58   VR = V;
59   VRC = MRI->getRegClass(VR);
60 }
61 
62 /// HasValueForBlock - Return true if the MachineSSAUpdater already has a value for
63 /// the specified block.
HasValueForBlock(MachineBasicBlock * BB) const64 bool MachineSSAUpdater::HasValueForBlock(MachineBasicBlock *BB) const {
65   return getAvailableVals(AV).count(BB);
66 }
67 
68 /// AddAvailableValue - Indicate that a rewritten value is available in the
69 /// specified block with the specified value.
AddAvailableValue(MachineBasicBlock * BB,unsigned V)70 void MachineSSAUpdater::AddAvailableValue(MachineBasicBlock *BB, unsigned V) {
71   getAvailableVals(AV)[BB] = V;
72 }
73 
74 /// GetValueAtEndOfBlock - Construct SSA form, materializing a value that is
75 /// live at the end of the specified block.
GetValueAtEndOfBlock(MachineBasicBlock * BB)76 unsigned MachineSSAUpdater::GetValueAtEndOfBlock(MachineBasicBlock *BB) {
77   return GetValueAtEndOfBlockInternal(BB);
78 }
79 
80 static
LookForIdenticalPHI(MachineBasicBlock * BB,SmallVectorImpl<std::pair<MachineBasicBlock *,unsigned>> & PredValues)81 unsigned LookForIdenticalPHI(MachineBasicBlock *BB,
82         SmallVectorImpl<std::pair<MachineBasicBlock*, unsigned> > &PredValues) {
83   if (BB->empty())
84     return 0;
85 
86   MachineBasicBlock::iterator I = BB->begin();
87   if (!I->isPHI())
88     return 0;
89 
90   AvailableValsTy AVals;
91   for (unsigned i = 0, e = PredValues.size(); i != e; ++i)
92     AVals[PredValues[i].first] = PredValues[i].second;
93   while (I != BB->end() && I->isPHI()) {
94     bool Same = true;
95     for (unsigned i = 1, e = I->getNumOperands(); i != e; i += 2) {
96       unsigned SrcReg = I->getOperand(i).getReg();
97       MachineBasicBlock *SrcBB = I->getOperand(i+1).getMBB();
98       if (AVals[SrcBB] != SrcReg) {
99         Same = false;
100         break;
101       }
102     }
103     if (Same)
104       return I->getOperand(0).getReg();
105     ++I;
106   }
107   return 0;
108 }
109 
110 /// InsertNewDef - Insert an empty PHI or IMPLICIT_DEF instruction which define
111 /// a value of the given register class at the start of the specified basic
112 /// block. It returns the virtual register defined by the instruction.
113 static
InsertNewDef(unsigned Opcode,MachineBasicBlock * BB,MachineBasicBlock::iterator I,const TargetRegisterClass * RC,MachineRegisterInfo * MRI,const TargetInstrInfo * TII)114 MachineInstrBuilder InsertNewDef(unsigned Opcode,
115                            MachineBasicBlock *BB, MachineBasicBlock::iterator I,
116                            const TargetRegisterClass *RC,
117                            MachineRegisterInfo *MRI,
118                            const TargetInstrInfo *TII) {
119   unsigned NewVR = MRI->createVirtualRegister(RC);
120   return BuildMI(*BB, I, DebugLoc(), TII->get(Opcode), NewVR);
121 }
122 
123 /// GetValueInMiddleOfBlock - Construct SSA form, materializing a value that
124 /// is live in the middle of the specified block.
125 ///
126 /// GetValueInMiddleOfBlock is the same as GetValueAtEndOfBlock except in one
127 /// important case: if there is a definition of the rewritten value after the
128 /// 'use' in BB.  Consider code like this:
129 ///
130 ///      X1 = ...
131 ///   SomeBB:
132 ///      use(X)
133 ///      X2 = ...
134 ///      br Cond, SomeBB, OutBB
135 ///
136 /// In this case, there are two values (X1 and X2) added to the AvailableVals
137 /// set by the client of the rewriter, and those values are both live out of
138 /// their respective blocks.  However, the use of X happens in the *middle* of
139 /// a block.  Because of this, we need to insert a new PHI node in SomeBB to
140 /// merge the appropriate values, and this value isn't live out of the block.
141 ///
GetValueInMiddleOfBlock(MachineBasicBlock * BB)142 unsigned MachineSSAUpdater::GetValueInMiddleOfBlock(MachineBasicBlock *BB) {
143   // If there is no definition of the renamed variable in this block, just use
144   // GetValueAtEndOfBlock to do our work.
145   if (!HasValueForBlock(BB))
146     return GetValueAtEndOfBlockInternal(BB);
147 
148   // If there are no predecessors, just return undef.
149   if (BB->pred_empty()) {
150     // Insert an implicit_def to represent an undef value.
151     MachineInstr *NewDef = InsertNewDef(TargetOpcode::IMPLICIT_DEF,
152                                         BB, BB->getFirstTerminator(),
153                                         VRC, MRI, TII);
154     return NewDef->getOperand(0).getReg();
155   }
156 
157   // Otherwise, we have the hard case.  Get the live-in values for each
158   // predecessor.
159   SmallVector<std::pair<MachineBasicBlock*, unsigned>, 8> PredValues;
160   unsigned SingularValue = 0;
161 
162   bool isFirstPred = true;
163   for (MachineBasicBlock::pred_iterator PI = BB->pred_begin(),
164          E = BB->pred_end(); PI != E; ++PI) {
165     MachineBasicBlock *PredBB = *PI;
166     unsigned PredVal = GetValueAtEndOfBlockInternal(PredBB);
167     PredValues.push_back(std::make_pair(PredBB, PredVal));
168 
169     // Compute SingularValue.
170     if (isFirstPred) {
171       SingularValue = PredVal;
172       isFirstPred = false;
173     } else if (PredVal != SingularValue)
174       SingularValue = 0;
175   }
176 
177   // Otherwise, if all the merged values are the same, just use it.
178   if (SingularValue != 0)
179     return SingularValue;
180 
181   // If an identical PHI is already in BB, just reuse it.
182   unsigned DupPHI = LookForIdenticalPHI(BB, PredValues);
183   if (DupPHI)
184     return DupPHI;
185 
186   // Otherwise, we do need a PHI: insert one now.
187   MachineBasicBlock::iterator Loc = BB->empty() ? BB->end() : BB->begin();
188   MachineInstrBuilder InsertedPHI = InsertNewDef(TargetOpcode::PHI, BB,
189                                                  Loc, VRC, MRI, TII);
190 
191   // Fill in all the predecessors of the PHI.
192   for (unsigned i = 0, e = PredValues.size(); i != e; ++i)
193     InsertedPHI.addReg(PredValues[i].second).addMBB(PredValues[i].first);
194 
195   // See if the PHI node can be merged to a single value.  This can happen in
196   // loop cases when we get a PHI of itself and one other value.
197   if (unsigned ConstVal = InsertedPHI->isConstantValuePHI()) {
198     InsertedPHI->eraseFromParent();
199     return ConstVal;
200   }
201 
202   // If the client wants to know about all new instructions, tell it.
203   if (InsertedPHIs) InsertedPHIs->push_back(InsertedPHI);
204 
205   DEBUG(dbgs() << "  Inserted PHI: " << *InsertedPHI << "\n");
206   return InsertedPHI->getOperand(0).getReg();
207 }
208 
209 static
findCorrespondingPred(const MachineInstr * MI,MachineOperand * U)210 MachineBasicBlock *findCorrespondingPred(const MachineInstr *MI,
211                                          MachineOperand *U) {
212   for (unsigned i = 1, e = MI->getNumOperands(); i != e; i += 2) {
213     if (&MI->getOperand(i) == U)
214       return MI->getOperand(i+1).getMBB();
215   }
216 
217   llvm_unreachable("MachineOperand::getParent() failure?");
218 }
219 
220 /// RewriteUse - Rewrite a use of the symbolic value.  This handles PHI nodes,
221 /// which use their value in the corresponding predecessor.
RewriteUse(MachineOperand & U)222 void MachineSSAUpdater::RewriteUse(MachineOperand &U) {
223   MachineInstr *UseMI = U.getParent();
224   unsigned NewVR = 0;
225   if (UseMI->isPHI()) {
226     MachineBasicBlock *SourceBB = findCorrespondingPred(UseMI, &U);
227     NewVR = GetValueAtEndOfBlockInternal(SourceBB);
228   } else {
229     NewVR = GetValueInMiddleOfBlock(UseMI->getParent());
230   }
231 
232   U.setReg(NewVR);
233 }
234 
235 /// SSAUpdaterTraits<MachineSSAUpdater> - Traits for the SSAUpdaterImpl
236 /// template, specialized for MachineSSAUpdater.
237 namespace llvm {
238 template<>
239 class SSAUpdaterTraits<MachineSSAUpdater> {
240 public:
241   typedef MachineBasicBlock BlkT;
242   typedef unsigned ValT;
243   typedef MachineInstr PhiT;
244 
245   typedef MachineBasicBlock::succ_iterator BlkSucc_iterator;
BlkSucc_begin(BlkT * BB)246   static BlkSucc_iterator BlkSucc_begin(BlkT *BB) { return BB->succ_begin(); }
BlkSucc_end(BlkT * BB)247   static BlkSucc_iterator BlkSucc_end(BlkT *BB) { return BB->succ_end(); }
248 
249   /// Iterator for PHI operands.
250   class PHI_iterator {
251   private:
252     MachineInstr *PHI;
253     unsigned idx;
254 
255   public:
PHI_iterator(MachineInstr * P)256     explicit PHI_iterator(MachineInstr *P) // begin iterator
257       : PHI(P), idx(1) {}
PHI_iterator(MachineInstr * P,bool)258     PHI_iterator(MachineInstr *P, bool) // end iterator
259       : PHI(P), idx(PHI->getNumOperands()) {}
260 
operator ++()261     PHI_iterator &operator++() { idx += 2; return *this; }
operator ==(const PHI_iterator & x) const262     bool operator==(const PHI_iterator& x) const { return idx == x.idx; }
operator !=(const PHI_iterator & x) const263     bool operator!=(const PHI_iterator& x) const { return !operator==(x); }
getIncomingValue()264     unsigned getIncomingValue() { return PHI->getOperand(idx).getReg(); }
getIncomingBlock()265     MachineBasicBlock *getIncomingBlock() {
266       return PHI->getOperand(idx+1).getMBB();
267     }
268   };
PHI_begin(PhiT * PHI)269   static inline PHI_iterator PHI_begin(PhiT *PHI) { return PHI_iterator(PHI); }
PHI_end(PhiT * PHI)270   static inline PHI_iterator PHI_end(PhiT *PHI) {
271     return PHI_iterator(PHI, true);
272   }
273 
274   /// FindPredecessorBlocks - Put the predecessors of BB into the Preds
275   /// vector.
FindPredecessorBlocks(MachineBasicBlock * BB,SmallVectorImpl<MachineBasicBlock * > * Preds)276   static void FindPredecessorBlocks(MachineBasicBlock *BB,
277                                     SmallVectorImpl<MachineBasicBlock*> *Preds){
278     for (MachineBasicBlock::pred_iterator PI = BB->pred_begin(),
279            E = BB->pred_end(); PI != E; ++PI)
280       Preds->push_back(*PI);
281   }
282 
283   /// GetUndefVal - Create an IMPLICIT_DEF instruction with a new register.
284   /// Add it into the specified block and return the register.
GetUndefVal(MachineBasicBlock * BB,MachineSSAUpdater * Updater)285   static unsigned GetUndefVal(MachineBasicBlock *BB,
286                               MachineSSAUpdater *Updater) {
287     // Insert an implicit_def to represent an undef value.
288     MachineInstr *NewDef = InsertNewDef(TargetOpcode::IMPLICIT_DEF,
289                                         BB, BB->getFirstTerminator(),
290                                         Updater->VRC, Updater->MRI,
291                                         Updater->TII);
292     return NewDef->getOperand(0).getReg();
293   }
294 
295   /// CreateEmptyPHI - Create a PHI instruction that defines a new register.
296   /// Add it into the specified block and return the register.
CreateEmptyPHI(MachineBasicBlock * BB,unsigned NumPreds,MachineSSAUpdater * Updater)297   static unsigned CreateEmptyPHI(MachineBasicBlock *BB, unsigned NumPreds,
298                                  MachineSSAUpdater *Updater) {
299     MachineBasicBlock::iterator Loc = BB->empty() ? BB->end() : BB->begin();
300     MachineInstr *PHI = InsertNewDef(TargetOpcode::PHI, BB, Loc,
301                                      Updater->VRC, Updater->MRI,
302                                      Updater->TII);
303     return PHI->getOperand(0).getReg();
304   }
305 
306   /// AddPHIOperand - Add the specified value as an operand of the PHI for
307   /// the specified predecessor block.
AddPHIOperand(MachineInstr * PHI,unsigned Val,MachineBasicBlock * Pred)308   static void AddPHIOperand(MachineInstr *PHI, unsigned Val,
309                             MachineBasicBlock *Pred) {
310     MachineInstrBuilder(*Pred->getParent(), PHI).addReg(Val).addMBB(Pred);
311   }
312 
313   /// InstrIsPHI - Check if an instruction is a PHI.
314   ///
InstrIsPHI(MachineInstr * I)315   static MachineInstr *InstrIsPHI(MachineInstr *I) {
316     if (I && I->isPHI())
317       return I;
318     return nullptr;
319   }
320 
321   /// ValueIsPHI - Check if the instruction that defines the specified register
322   /// is a PHI instruction.
ValueIsPHI(unsigned Val,MachineSSAUpdater * Updater)323   static MachineInstr *ValueIsPHI(unsigned Val, MachineSSAUpdater *Updater) {
324     return InstrIsPHI(Updater->MRI->getVRegDef(Val));
325   }
326 
327   /// ValueIsNewPHI - Like ValueIsPHI but also check if the PHI has no source
328   /// operands, i.e., it was just added.
ValueIsNewPHI(unsigned Val,MachineSSAUpdater * Updater)329   static MachineInstr *ValueIsNewPHI(unsigned Val, MachineSSAUpdater *Updater) {
330     MachineInstr *PHI = ValueIsPHI(Val, Updater);
331     if (PHI && PHI->getNumOperands() <= 1)
332       return PHI;
333     return nullptr;
334   }
335 
336   /// GetPHIValue - For the specified PHI instruction, return the register
337   /// that it defines.
GetPHIValue(MachineInstr * PHI)338   static unsigned GetPHIValue(MachineInstr *PHI) {
339     return PHI->getOperand(0).getReg();
340   }
341 };
342 
343 } // End llvm namespace
344 
345 /// GetValueAtEndOfBlockInternal - Check to see if AvailableVals has an entry
346 /// for the specified BB and if so, return it.  If not, construct SSA form by
347 /// first calculating the required placement of PHIs and then inserting new
348 /// PHIs where needed.
GetValueAtEndOfBlockInternal(MachineBasicBlock * BB)349 unsigned MachineSSAUpdater::GetValueAtEndOfBlockInternal(MachineBasicBlock *BB){
350   AvailableValsTy &AvailableVals = getAvailableVals(AV);
351   if (unsigned V = AvailableVals[BB])
352     return V;
353 
354   SSAUpdaterImpl<MachineSSAUpdater> Impl(this, &AvailableVals, InsertedPHIs);
355   return Impl.GetValue(BB);
356 }
357