1//===-- SSAUpdaterImpl.h - SSA Updater Implementation -----------*- C++ -*-===//
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 provides a template that implements the core algorithm for the
11// SSAUpdater and MachineSSAUpdater.
12//
13//===----------------------------------------------------------------------===//
14
15#ifndef LLVM_TRANSFORMS_UTILS_SSAUPDATERIMPL_H
16#define LLVM_TRANSFORMS_UTILS_SSAUPDATERIMPL_H
17
18#include "llvm/ADT/DenseMap.h"
19#include "llvm/ADT/SmallVector.h"
20#include "llvm/IR/ValueHandle.h"
21#include "llvm/Support/Allocator.h"
22#include "llvm/Support/Debug.h"
23
24namespace llvm {
25
26#define DEBUG_TYPE "ssaupdater"
27
28class CastInst;
29class PHINode;
30template<typename T> class SSAUpdaterTraits;
31
32template<typename UpdaterT>
33class SSAUpdaterImpl {
34private:
35  UpdaterT *Updater;
36
37  typedef SSAUpdaterTraits<UpdaterT> Traits;
38  typedef typename Traits::BlkT BlkT;
39  typedef typename Traits::ValT ValT;
40  typedef typename Traits::PhiT PhiT;
41
42  /// BBInfo - Per-basic block information used internally by SSAUpdaterImpl.
43  /// The predecessors of each block are cached here since pred_iterator is
44  /// slow and we need to iterate over the blocks at least a few times.
45  class BBInfo {
46  public:
47    BlkT *BB;          // Back-pointer to the corresponding block.
48    ValT AvailableVal; // Value to use in this block.
49    BBInfo *DefBB;     // Block that defines the available value.
50    int BlkNum;        // Postorder number.
51    BBInfo *IDom;      // Immediate dominator.
52    unsigned NumPreds; // Number of predecessor blocks.
53    BBInfo **Preds;    // Array[NumPreds] of predecessor blocks.
54    PhiT *PHITag;      // Marker for existing PHIs that match.
55
56    BBInfo(BlkT *ThisBB, ValT V)
57      : BB(ThisBB), AvailableVal(V), DefBB(V ? this : nullptr), BlkNum(0),
58        IDom(nullptr), NumPreds(0), Preds(nullptr), PHITag(nullptr) {}
59  };
60
61  typedef DenseMap<BlkT*, ValT> AvailableValsTy;
62  AvailableValsTy *AvailableVals;
63
64  SmallVectorImpl<PhiT*> *InsertedPHIs;
65
66  typedef SmallVectorImpl<BBInfo*> BlockListTy;
67  typedef DenseMap<BlkT*, BBInfo*> BBMapTy;
68  BBMapTy BBMap;
69  BumpPtrAllocator Allocator;
70
71public:
72  explicit SSAUpdaterImpl(UpdaterT *U, AvailableValsTy *A,
73                          SmallVectorImpl<PhiT*> *Ins) :
74    Updater(U), AvailableVals(A), InsertedPHIs(Ins) { }
75
76  /// GetValue - Check to see if AvailableVals has an entry for the specified
77  /// BB and if so, return it.  If not, construct SSA form by first
78  /// calculating the required placement of PHIs and then inserting new PHIs
79  /// where needed.
80  ValT GetValue(BlkT *BB) {
81    SmallVector<BBInfo*, 100> BlockList;
82    BBInfo *PseudoEntry = BuildBlockList(BB, &BlockList);
83
84    // Special case: bail out if BB is unreachable.
85    if (BlockList.size() == 0) {
86      ValT V = Traits::GetUndefVal(BB, Updater);
87      (*AvailableVals)[BB] = V;
88      return V;
89    }
90
91    FindDominators(&BlockList, PseudoEntry);
92    FindPHIPlacement(&BlockList);
93    FindAvailableVals(&BlockList);
94
95    return BBMap[BB]->DefBB->AvailableVal;
96  }
97
98  /// BuildBlockList - Starting from the specified basic block, traverse back
99  /// through its predecessors until reaching blocks with known values.
100  /// Create BBInfo structures for the blocks and append them to the block
101  /// list.
102  BBInfo *BuildBlockList(BlkT *BB, BlockListTy *BlockList) {
103    SmallVector<BBInfo*, 10> RootList;
104    SmallVector<BBInfo*, 64> WorkList;
105
106    BBInfo *Info = new (Allocator) BBInfo(BB, 0);
107    BBMap[BB] = Info;
108    WorkList.push_back(Info);
109
110    // Search backward from BB, creating BBInfos along the way and stopping
111    // when reaching blocks that define the value.  Record those defining
112    // blocks on the RootList.
113    SmallVector<BlkT*, 10> Preds;
114    while (!WorkList.empty()) {
115      Info = WorkList.pop_back_val();
116      Preds.clear();
117      Traits::FindPredecessorBlocks(Info->BB, &Preds);
118      Info->NumPreds = Preds.size();
119      if (Info->NumPreds == 0)
120        Info->Preds = nullptr;
121      else
122        Info->Preds = static_cast<BBInfo**>
123          (Allocator.Allocate(Info->NumPreds * sizeof(BBInfo*),
124                              AlignOf<BBInfo*>::Alignment));
125
126      for (unsigned p = 0; p != Info->NumPreds; ++p) {
127        BlkT *Pred = Preds[p];
128        // Check if BBMap already has a BBInfo for the predecessor block.
129        typename BBMapTy::value_type &BBMapBucket =
130          BBMap.FindAndConstruct(Pred);
131        if (BBMapBucket.second) {
132          Info->Preds[p] = BBMapBucket.second;
133          continue;
134        }
135
136        // Create a new BBInfo for the predecessor.
137        ValT PredVal = AvailableVals->lookup(Pred);
138        BBInfo *PredInfo = new (Allocator) BBInfo(Pred, PredVal);
139        BBMapBucket.second = PredInfo;
140        Info->Preds[p] = PredInfo;
141
142        if (PredInfo->AvailableVal) {
143          RootList.push_back(PredInfo);
144          continue;
145        }
146        WorkList.push_back(PredInfo);
147      }
148    }
149
150    // Now that we know what blocks are backwards-reachable from the starting
151    // block, do a forward depth-first traversal to assign postorder numbers
152    // to those blocks.
153    BBInfo *PseudoEntry = new (Allocator) BBInfo(nullptr, 0);
154    unsigned BlkNum = 1;
155
156    // Initialize the worklist with the roots from the backward traversal.
157    while (!RootList.empty()) {
158      Info = RootList.pop_back_val();
159      Info->IDom = PseudoEntry;
160      Info->BlkNum = -1;
161      WorkList.push_back(Info);
162    }
163
164    while (!WorkList.empty()) {
165      Info = WorkList.back();
166
167      if (Info->BlkNum == -2) {
168        // All the successors have been handled; assign the postorder number.
169        Info->BlkNum = BlkNum++;
170        // If not a root, put it on the BlockList.
171        if (!Info->AvailableVal)
172          BlockList->push_back(Info);
173        WorkList.pop_back();
174        continue;
175      }
176
177      // Leave this entry on the worklist, but set its BlkNum to mark that its
178      // successors have been put on the worklist.  When it returns to the top
179      // the list, after handling its successors, it will be assigned a
180      // number.
181      Info->BlkNum = -2;
182
183      // Add unvisited successors to the work list.
184      for (typename Traits::BlkSucc_iterator SI =
185             Traits::BlkSucc_begin(Info->BB),
186             E = Traits::BlkSucc_end(Info->BB); SI != E; ++SI) {
187        BBInfo *SuccInfo = BBMap[*SI];
188        if (!SuccInfo || SuccInfo->BlkNum)
189          continue;
190        SuccInfo->BlkNum = -1;
191        WorkList.push_back(SuccInfo);
192      }
193    }
194    PseudoEntry->BlkNum = BlkNum;
195    return PseudoEntry;
196  }
197
198  /// IntersectDominators - This is the dataflow lattice "meet" operation for
199  /// finding dominators.  Given two basic blocks, it walks up the dominator
200  /// tree until it finds a common dominator of both.  It uses the postorder
201  /// number of the blocks to determine how to do that.
202  BBInfo *IntersectDominators(BBInfo *Blk1, BBInfo *Blk2) {
203    while (Blk1 != Blk2) {
204      while (Blk1->BlkNum < Blk2->BlkNum) {
205        Blk1 = Blk1->IDom;
206        if (!Blk1)
207          return Blk2;
208      }
209      while (Blk2->BlkNum < Blk1->BlkNum) {
210        Blk2 = Blk2->IDom;
211        if (!Blk2)
212          return Blk1;
213      }
214    }
215    return Blk1;
216  }
217
218  /// FindDominators - Calculate the dominator tree for the subset of the CFG
219  /// corresponding to the basic blocks on the BlockList.  This uses the
220  /// algorithm from: "A Simple, Fast Dominance Algorithm" by Cooper, Harvey
221  /// and Kennedy, published in Software--Practice and Experience, 2001,
222  /// 4:1-10.  Because the CFG subset does not include any edges leading into
223  /// blocks that define the value, the results are not the usual dominator
224  /// tree.  The CFG subset has a single pseudo-entry node with edges to a set
225  /// of root nodes for blocks that define the value.  The dominators for this
226  /// subset CFG are not the standard dominators but they are adequate for
227  /// placing PHIs within the subset CFG.
228  void FindDominators(BlockListTy *BlockList, BBInfo *PseudoEntry) {
229    bool Changed;
230    do {
231      Changed = false;
232      // Iterate over the list in reverse order, i.e., forward on CFG edges.
233      for (typename BlockListTy::reverse_iterator I = BlockList->rbegin(),
234             E = BlockList->rend(); I != E; ++I) {
235        BBInfo *Info = *I;
236        BBInfo *NewIDom = nullptr;
237
238        // Iterate through the block's predecessors.
239        for (unsigned p = 0; p != Info->NumPreds; ++p) {
240          BBInfo *Pred = Info->Preds[p];
241
242          // Treat an unreachable predecessor as a definition with 'undef'.
243          if (Pred->BlkNum == 0) {
244            Pred->AvailableVal = Traits::GetUndefVal(Pred->BB, Updater);
245            (*AvailableVals)[Pred->BB] = Pred->AvailableVal;
246            Pred->DefBB = Pred;
247            Pred->BlkNum = PseudoEntry->BlkNum;
248            PseudoEntry->BlkNum++;
249          }
250
251          if (!NewIDom)
252            NewIDom = Pred;
253          else
254            NewIDom = IntersectDominators(NewIDom, Pred);
255        }
256
257        // Check if the IDom value has changed.
258        if (NewIDom && NewIDom != Info->IDom) {
259          Info->IDom = NewIDom;
260          Changed = true;
261        }
262      }
263    } while (Changed);
264  }
265
266  /// IsDefInDomFrontier - Search up the dominator tree from Pred to IDom for
267  /// any blocks containing definitions of the value.  If one is found, then
268  /// the successor of Pred is in the dominance frontier for the definition,
269  /// and this function returns true.
270  bool IsDefInDomFrontier(const BBInfo *Pred, const BBInfo *IDom) {
271    for (; Pred != IDom; Pred = Pred->IDom) {
272      if (Pred->DefBB == Pred)
273        return true;
274    }
275    return false;
276  }
277
278  /// FindPHIPlacement - PHIs are needed in the iterated dominance frontiers
279  /// of the known definitions.  Iteratively add PHIs in the dom frontiers
280  /// until nothing changes.  Along the way, keep track of the nearest
281  /// dominating definitions for non-PHI blocks.
282  void FindPHIPlacement(BlockListTy *BlockList) {
283    bool Changed;
284    do {
285      Changed = false;
286      // Iterate over the list in reverse order, i.e., forward on CFG edges.
287      for (typename BlockListTy::reverse_iterator I = BlockList->rbegin(),
288             E = BlockList->rend(); I != E; ++I) {
289        BBInfo *Info = *I;
290
291        // If this block already needs a PHI, there is nothing to do here.
292        if (Info->DefBB == Info)
293          continue;
294
295        // Default to use the same def as the immediate dominator.
296        BBInfo *NewDefBB = Info->IDom->DefBB;
297        for (unsigned p = 0; p != Info->NumPreds; ++p) {
298          if (IsDefInDomFrontier(Info->Preds[p], Info->IDom)) {
299            // Need a PHI here.
300            NewDefBB = Info;
301            break;
302          }
303        }
304
305        // Check if anything changed.
306        if (NewDefBB != Info->DefBB) {
307          Info->DefBB = NewDefBB;
308          Changed = true;
309        }
310      }
311    } while (Changed);
312  }
313
314  /// FindAvailableVal - If this block requires a PHI, first check if an
315  /// existing PHI matches the PHI placement and reaching definitions computed
316  /// earlier, and if not, create a new PHI.  Visit all the block's
317  /// predecessors to calculate the available value for each one and fill in
318  /// the incoming values for a new PHI.
319  void FindAvailableVals(BlockListTy *BlockList) {
320    // Go through the worklist in forward order (i.e., backward through the CFG)
321    // and check if existing PHIs can be used.  If not, create empty PHIs where
322    // they are needed.
323    for (typename BlockListTy::iterator I = BlockList->begin(),
324           E = BlockList->end(); I != E; ++I) {
325      BBInfo *Info = *I;
326      // Check if there needs to be a PHI in BB.
327      if (Info->DefBB != Info)
328        continue;
329
330      // Look for an existing PHI.
331      FindExistingPHI(Info->BB, BlockList);
332      if (Info->AvailableVal)
333        continue;
334
335      ValT PHI = Traits::CreateEmptyPHI(Info->BB, Info->NumPreds, Updater);
336      Info->AvailableVal = PHI;
337      (*AvailableVals)[Info->BB] = PHI;
338    }
339
340    // Now go back through the worklist in reverse order to fill in the
341    // arguments for any new PHIs added in the forward traversal.
342    for (typename BlockListTy::reverse_iterator I = BlockList->rbegin(),
343           E = BlockList->rend(); I != E; ++I) {
344      BBInfo *Info = *I;
345
346      if (Info->DefBB != Info) {
347        // Record the available value at join nodes to speed up subsequent
348        // uses of this SSAUpdater for the same value.
349        if (Info->NumPreds > 1)
350          (*AvailableVals)[Info->BB] = Info->DefBB->AvailableVal;
351        continue;
352      }
353
354      // Check if this block contains a newly added PHI.
355      PhiT *PHI = Traits::ValueIsNewPHI(Info->AvailableVal, Updater);
356      if (!PHI)
357        continue;
358
359      // Iterate through the block's predecessors.
360      for (unsigned p = 0; p != Info->NumPreds; ++p) {
361        BBInfo *PredInfo = Info->Preds[p];
362        BlkT *Pred = PredInfo->BB;
363        // Skip to the nearest preceding definition.
364        if (PredInfo->DefBB != PredInfo)
365          PredInfo = PredInfo->DefBB;
366        Traits::AddPHIOperand(PHI, PredInfo->AvailableVal, Pred);
367      }
368
369      DEBUG(dbgs() << "  Inserted PHI: " << *PHI << "\n");
370
371      // If the client wants to know about all new instructions, tell it.
372      if (InsertedPHIs) InsertedPHIs->push_back(PHI);
373    }
374  }
375
376  /// FindExistingPHI - Look through the PHI nodes in a block to see if any of
377  /// them match what is needed.
378  void FindExistingPHI(BlkT *BB, BlockListTy *BlockList) {
379    for (typename BlkT::iterator BBI = BB->begin(), BBE = BB->end();
380         BBI != BBE; ++BBI) {
381      PhiT *SomePHI = Traits::InstrIsPHI(&*BBI);
382      if (!SomePHI)
383        break;
384      if (CheckIfPHIMatches(SomePHI)) {
385        RecordMatchingPHIs(BlockList);
386        break;
387      }
388      // Match failed: clear all the PHITag values.
389      for (typename BlockListTy::iterator I = BlockList->begin(),
390             E = BlockList->end(); I != E; ++I)
391        (*I)->PHITag = nullptr;
392    }
393  }
394
395  /// CheckIfPHIMatches - Check if a PHI node matches the placement and values
396  /// in the BBMap.
397  bool CheckIfPHIMatches(PhiT *PHI) {
398    SmallVector<PhiT*, 20> WorkList;
399    WorkList.push_back(PHI);
400
401    // Mark that the block containing this PHI has been visited.
402    BBMap[PHI->getParent()]->PHITag = PHI;
403
404    while (!WorkList.empty()) {
405      PHI = WorkList.pop_back_val();
406
407      // Iterate through the PHI's incoming values.
408      for (typename Traits::PHI_iterator I = Traits::PHI_begin(PHI),
409             E = Traits::PHI_end(PHI); I != E; ++I) {
410        ValT IncomingVal = I.getIncomingValue();
411        BBInfo *PredInfo = BBMap[I.getIncomingBlock()];
412        // Skip to the nearest preceding definition.
413        if (PredInfo->DefBB != PredInfo)
414          PredInfo = PredInfo->DefBB;
415
416        // Check if it matches the expected value.
417        if (PredInfo->AvailableVal) {
418          if (IncomingVal == PredInfo->AvailableVal)
419            continue;
420          return false;
421        }
422
423        // Check if the value is a PHI in the correct block.
424        PhiT *IncomingPHIVal = Traits::ValueIsPHI(IncomingVal, Updater);
425        if (!IncomingPHIVal || IncomingPHIVal->getParent() != PredInfo->BB)
426          return false;
427
428        // If this block has already been visited, check if this PHI matches.
429        if (PredInfo->PHITag) {
430          if (IncomingPHIVal == PredInfo->PHITag)
431            continue;
432          return false;
433        }
434        PredInfo->PHITag = IncomingPHIVal;
435
436        WorkList.push_back(IncomingPHIVal);
437      }
438    }
439    return true;
440  }
441
442  /// RecordMatchingPHIs - For each PHI node that matches, record it in both
443  /// the BBMap and the AvailableVals mapping.
444  void RecordMatchingPHIs(BlockListTy *BlockList) {
445    for (typename BlockListTy::iterator I = BlockList->begin(),
446           E = BlockList->end(); I != E; ++I)
447      if (PhiT *PHI = (*I)->PHITag) {
448        BlkT *BB = PHI->getParent();
449        ValT PHIVal = Traits::GetPHIValue(PHI);
450        (*AvailableVals)[BB] = PHIVal;
451        BBMap[BB]->AvailableVal = PHIVal;
452      }
453  }
454};
455
456#undef DEBUG_TYPE // "ssaupdater"
457
458} // End llvm namespace
459
460#endif
461