/3rd_party/llvm/include/llvm/Transforms/Utils/SSAUpdaterImpl.h

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