LLVM  3.7.0
SSAUpdaterImpl.h
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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 
24 namespace llvm {
25 
26 #define DEBUG_TYPE "ssaupdater"
27 
28 class CastInst;
29 class PHINode;
30 template<typename T> class SSAUpdaterTraits;
31 
32 template<typename UpdaterT>
34 private:
35  UpdaterT *Updater;
36 
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 
62  AvailableValsTy *AvailableVals;
63 
64  SmallVectorImpl<PhiT*> *InsertedPHIs;
65 
68  BBMapTy BBMap;
69  BumpPtrAllocator Allocator;
70 
71 public:
72  explicit SSAUpdaterImpl(UpdaterT *U, AvailableValsTy *A,
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.
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*),
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
std::reverse_iterator< iterator > reverse_iterator
Definition: SmallVector.h:104
AlignOf - A templated class that contains an enum value representing the alignment of the template ar...
Definition: AlignOf.h:46
ValueT lookup(const KeyT &Val) const
lookup - Return the entry for the specified key, or a default constructed value if no such entry exis...
Definition: DenseMap.h:159
ValT GetValue(BlkT *BB)
GetValue - Check to see if AvailableVals has an entry for the specified BB and if so...
void FindPHIPlacement(BlockListTy *BlockList)
FindPHIPlacement - PHIs are needed in the iterated dominance frontiers of the known definitions...
void FindExistingPHI(BlkT *BB, BlockListTy *BlockList)
FindExistingPHI - Look through the PHI nodes in a block to see if any of them match what is needed...
This file defines the MallocAllocator and BumpPtrAllocator interfaces.
T LLVM_ATTRIBUTE_UNUSED_RESULT pop_back_val()
Definition: SmallVector.h:406
bool LLVM_ATTRIBUTE_UNUSED_RESULT empty() const
Definition: SmallVector.h:57
bool CheckIfPHIMatches(PhiT *PHI)
CheckIfPHIMatches - Check if a PHI node matches the placement and values in the BBMap.
value_type & FindAndConstruct(const KeyT &Key)
Definition: DenseMap.h:225
Allocate memory in an ever growing pool, as if by bump-pointer.
Definition: Allocator.h:135
LLVM_ATTRIBUTE_RETURNS_NONNULL LLVM_ATTRIBUTE_RETURNS_NOALIAS void * Allocate(size_t Size, size_t Alignment)
Allocate space at the specified alignment.
Definition: Allocator.h:208
void FindDominators(BlockListTy *BlockList, BBInfo *PseudoEntry)
FindDominators - Calculate the dominator tree for the subset of the CFG corresponding to the basic bl...
bool IsDefInDomFrontier(const BBInfo *Pred, const BBInfo *IDom)
IsDefInDomFrontier - Search up the dominator tree from Pred to IDom for any blocks containing definit...
void RecordMatchingPHIs(BlockListTy *BlockList)
RecordMatchingPHIs - For each PHI node that matches, record it in both the BBMap and the AvailableVal...
SSAUpdaterImpl(UpdaterT *U, AvailableValsTy *A, SmallVectorImpl< PhiT * > *Ins)
This is a 'vector' (really, a variable-sized array), optimized for the case when the array is small...
Definition: SmallVector.h:861
void FindAvailableVals(BlockListTy *BlockList)
FindAvailableVal - If this block requires a PHI, first check if an existing PHI matches the PHI place...
raw_ostream & dbgs()
dbgs() - This returns a reference to a raw_ostream for debugging messages.
Definition: Debug.cpp:123
BBInfo * IntersectDominators(BBInfo *Blk1, BBInfo *Blk2)
IntersectDominators - This is the dataflow lattice "meet" operation for finding dominators.
#define I(x, y, z)
Definition: MD5.cpp:54
BBInfo * BuildBlockList(BlkT *BB, BlockListTy *BlockList)
BuildBlockList - Starting from the specified basic block, traverse back through its predecessors unti...
#define DEBUG(X)
Definition: Debug.h:92