llvm.org GIT mirror llvm / 732d1a4 lib / Transforms / Scalar / LICM.cpp
732d1a4

Tree @732d1a4 (Download .tar.gz)

LICM.cpp @732d1a4raw · history · blame

  1
  2
  3
  4
  5
  6
  7
  8
  9
 10
 11
 12
 13
 14
 15
 16
 17
 18
 19
 20
 21
 22
 23
 24
 25
 26
 27
 28
 29
 30
 31
 32
 33
 34
 35
 36
 37
 38
 39
 40
 41
 42
 43
 44
 45
 46
 47
 48
 49
 50
 51
 52
 53
 54
 55
 56
 57
 58
 59
 60
 61
 62
 63
 64
 65
 66
 67
 68
 69
 70
 71
 72
 73
 74
 75
 76
 77
 78
 79
 80
 81
 82
 83
 84
 85
 86
 87
 88
 89
 90
 91
 92
 93
 94
 95
 96
 97
 98
 99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
//===-- LICM.cpp - Loop Invariant Code Motion Pass ------------------------===//
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This pass performs loop invariant code motion, attempting to remove as much
// code from the body of a loop as possible.  It does this by either hoisting
// code into the preheader block, or by sinking code to the exit blocks if it is
// safe.  This pass also promotes must-aliased memory locations in the loop to
// live in registers, thus hoisting and sinking "invariant" loads and stores.
//
// This pass uses alias analysis for two purposes:
//
//  1. Moving loop invariant loads and calls out of loops.  If we can determine
//     that a load or call inside of a loop never aliases anything stored to,
//     we can hoist it or sink it like any other instruction.
//  2. Scalar Promotion of Memory - If there is a store instruction inside of
//     the loop, we try to move the store to happen AFTER the loop instead of
//     inside of the loop.  This can only happen if a few conditions are true:
//       A. The pointer stored through is loop invariant
//       B. There are no stores or loads in the loop which _may_ alias the
//          pointer.  There are no calls in the loop which mod/ref the pointer.
//     If these conditions are true, we can promote the loads and stores in the
//     loop of the pointer to use a temporary alloca'd variable.  We then use
//     the SSAUpdater to construct the appropriate SSA form for the value.
//
//===----------------------------------------------------------------------===//

#define DEBUG_TYPE "licm"
#include "llvm/Transforms/Scalar.h"
#include "llvm/Constants.h"
#include "llvm/DerivedTypes.h"
#include "llvm/IntrinsicInst.h"
#include "llvm/Instructions.h"
#include "llvm/Analysis/AliasAnalysis.h"
#include "llvm/Analysis/AliasSetTracker.h"
#include "llvm/Analysis/ConstantFolding.h"
#include "llvm/Analysis/LoopInfo.h"
#include "llvm/Analysis/LoopPass.h"
#include "llvm/Analysis/Dominators.h"
#include "llvm/Analysis/ScalarEvolution.h"
#include "llvm/Transforms/Utils/Local.h"
#include "llvm/Transforms/Utils/SSAUpdater.h"
#include "llvm/Support/CFG.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/Support/Debug.h"
#include "llvm/ADT/Statistic.h"
#include <algorithm>
using namespace llvm;

STATISTIC(NumSunk      , "Number of instructions sunk out of loop");
STATISTIC(NumHoisted   , "Number of instructions hoisted out of loop");
STATISTIC(NumMovedLoads, "Number of load insts hoisted or sunk");
STATISTIC(NumMovedCalls, "Number of call insts hoisted or sunk");
STATISTIC(NumPromoted  , "Number of memory locations promoted to registers");

static cl::opt<bool>
DisablePromotion("disable-licm-promotion", cl::Hidden,
                 cl::desc("Disable memory promotion in LICM pass"));

namespace {
  struct LICM : public LoopPass {
    static char ID; // Pass identification, replacement for typeid
    LICM() : LoopPass(ID) {}

    virtual bool runOnLoop(Loop *L, LPPassManager &LPM);

    /// This transformation requires natural loop information & requires that
    /// loop preheaders be inserted into the CFG...
    ///
    virtual void getAnalysisUsage(AnalysisUsage &AU) const {
      AU.setPreservesCFG();
      AU.addRequired<DominatorTree>();
      AU.addRequired<LoopInfo>();
      AU.addRequiredID(LoopSimplifyID);
      AU.addRequired<AliasAnalysis>();
      AU.addPreserved<AliasAnalysis>();
      AU.addPreserved<ScalarEvolution>();
      AU.addPreservedID(LoopSimplifyID);
    }

    bool doFinalization() {
      assert(LoopToAliasSetMap.empty() && "Didn't free loop alias sets");
      return false;
    }

  private:
    AliasAnalysis *AA;       // Current AliasAnalysis information
    LoopInfo      *LI;       // Current LoopInfo
    DominatorTree *DT;       // Dominator Tree for the current Loop.

    // State that is updated as we process loops.
    bool Changed;            // Set to true when we change anything.
    BasicBlock *Preheader;   // The preheader block of the current loop...
    Loop *CurLoop;           // The current loop we are working on...
    AliasSetTracker *CurAST; // AliasSet information for the current loop...
    DenseMap<Loop*, AliasSetTracker*> LoopToAliasSetMap;

    /// cloneBasicBlockAnalysis - Simple Analysis hook. Clone alias set info.
    void cloneBasicBlockAnalysis(BasicBlock *From, BasicBlock *To, Loop *L);

    /// deleteAnalysisValue - Simple Analysis hook. Delete value V from alias
    /// set.
    void deleteAnalysisValue(Value *V, Loop *L);

    /// SinkRegion - Walk the specified region of the CFG (defined by all blocks
    /// dominated by the specified block, and that are in the current loop) in
    /// reverse depth first order w.r.t the DominatorTree.  This allows us to
    /// visit uses before definitions, allowing us to sink a loop body in one
    /// pass without iteration.
    ///
    void SinkRegion(DomTreeNode *N);

    /// HoistRegion - Walk the specified region of the CFG (defined by all
    /// blocks dominated by the specified block, and that are in the current
    /// loop) in depth first order w.r.t the DominatorTree.  This allows us to
    /// visit definitions before uses, allowing us to hoist a loop body in one
    /// pass without iteration.
    ///
    void HoistRegion(DomTreeNode *N);

    /// inSubLoop - Little predicate that returns true if the specified basic
    /// block is in a subloop of the current one, not the current one itself.
    ///
    bool inSubLoop(BasicBlock *BB) {
      assert(CurLoop->contains(BB) && "Only valid if BB is IN the loop");
      for (Loop::iterator I = CurLoop->begin(), E = CurLoop->end(); I != E; ++I)
        if ((*I)->contains(BB))
          return true;  // A subloop actually contains this block!
      return false;
    }

    /// isExitBlockDominatedByBlockInLoop - This method checks to see if the
    /// specified exit block of the loop is dominated by the specified block
    /// that is in the body of the loop.  We use these constraints to
    /// dramatically limit the amount of the dominator tree that needs to be
    /// searched.
    bool isExitBlockDominatedByBlockInLoop(BasicBlock *ExitBlock,
                                           BasicBlock *BlockInLoop) const {
      // If the block in the loop is the loop header, it must be dominated!
      BasicBlock *LoopHeader = CurLoop->getHeader();
      if (BlockInLoop == LoopHeader)
        return true;

      DomTreeNode *BlockInLoopNode = DT->getNode(BlockInLoop);
      DomTreeNode *IDom            = DT->getNode(ExitBlock);

      // Because the exit block is not in the loop, we know we have to get _at
      // least_ its immediate dominator.
      IDom = IDom->getIDom();
      
      while (IDom && IDom != BlockInLoopNode) {
        // If we have got to the header of the loop, then the instructions block
        // did not dominate the exit node, so we can't hoist it.
        if (IDom->getBlock() == LoopHeader)
          return false;

        // Get next Immediate Dominator.
        IDom = IDom->getIDom();
      };

      return true;
    }

    /// sink - When an instruction is found to only be used outside of the loop,
    /// this function moves it to the exit blocks and patches up SSA form as
    /// needed.
    ///
    void sink(Instruction &I);

    /// hoist - When an instruction is found to only use loop invariant operands
    /// that is safe to hoist, this instruction is called to do the dirty work.
    ///
    void hoist(Instruction &I);

    /// isSafeToExecuteUnconditionally - Only sink or hoist an instruction if it
    /// is not a trapping instruction or if it is a trapping instruction and is
    /// guaranteed to execute.
    ///
    bool isSafeToExecuteUnconditionally(Instruction &I);

    /// pointerInvalidatedByLoop - Return true if the body of this loop may
    /// store into the memory location pointed to by V.
    ///
    bool pointerInvalidatedByLoop(Value *V, unsigned Size) {
      // Check to see if any of the basic blocks in CurLoop invalidate *V.
      return CurAST->getAliasSetForPointer(V, Size).isMod();
    }

    bool canSinkOrHoistInst(Instruction &I);
    bool isLoopInvariantInst(Instruction &I);
    bool isNotUsedInLoop(Instruction &I);

    void PromoteAliasSet(AliasSet &AS);
  };
}

char LICM::ID = 0;
INITIALIZE_PASS(LICM, "licm", "Loop Invariant Code Motion", false, false);

Pass *llvm::createLICMPass() { return new LICM(); }

/// Hoist expressions out of the specified loop. Note, alias info for inner
/// loop is not preserved so it is not a good idea to run LICM multiple 
/// times on one loop.
///
bool LICM::runOnLoop(Loop *L, LPPassManager &LPM) {
  Changed = false;

  // Get our Loop and Alias Analysis information...
  LI = &getAnalysis<LoopInfo>();
  AA = &getAnalysis<AliasAnalysis>();
  DT = &getAnalysis<DominatorTree>();

  CurAST = new AliasSetTracker(*AA);
  // Collect Alias info from subloops.
  for (Loop::iterator LoopItr = L->begin(), LoopItrE = L->end();
       LoopItr != LoopItrE; ++LoopItr) {
    Loop *InnerL = *LoopItr;
    AliasSetTracker *InnerAST = LoopToAliasSetMap[InnerL];
    assert(InnerAST && "Where is my AST?");

    // What if InnerLoop was modified by other passes ?
    CurAST->add(*InnerAST);
    
    // Once we've incorporated the inner loop's AST into ours, we don't need the
    // subloop's anymore.
    delete InnerAST;
    LoopToAliasSetMap.erase(InnerL);
  }
  
  CurLoop = L;

  // Get the preheader block to move instructions into...
  Preheader = L->getLoopPreheader();

  // Loop over the body of this loop, looking for calls, invokes, and stores.
  // Because subloops have already been incorporated into AST, we skip blocks in
  // subloops.
  //
  for (Loop::block_iterator I = L->block_begin(), E = L->block_end();
       I != E; ++I) {
    BasicBlock *BB = *I;
    if (LI->getLoopFor(BB) == L)        // Ignore blocks in subloops.
      CurAST->add(*BB);                 // Incorporate the specified basic block
  }

  // We want to visit all of the instructions in this loop... that are not parts
  // of our subloops (they have already had their invariants hoisted out of
  // their loop, into this loop, so there is no need to process the BODIES of
  // the subloops).
  //
  // Traverse the body of the loop in depth first order on the dominator tree so
  // that we are guaranteed to see definitions before we see uses.  This allows
  // us to sink instructions in one pass, without iteration.  After sinking
  // instructions, we perform another pass to hoist them out of the loop.
  //
  if (L->hasDedicatedExits())
    SinkRegion(DT->getNode(L->getHeader()));
  if (Preheader)
    HoistRegion(DT->getNode(L->getHeader()));

  // Now that all loop invariants have been removed from the loop, promote any
  // memory references to scalars that we can.
  if (!DisablePromotion && Preheader && L->hasDedicatedExits()) {
    // Loop over all of the alias sets in the tracker object.
    for (AliasSetTracker::iterator I = CurAST->begin(), E = CurAST->end();
         I != E; ++I)
      PromoteAliasSet(*I);
  }
  
  // Clear out loops state information for the next iteration
  CurLoop = 0;
  Preheader = 0;

  // If this loop is nested inside of another one, save the alias information
  // for when we process the outer loop.
  if (L->getParentLoop())
    LoopToAliasSetMap[L] = CurAST;
  else
    delete CurAST;
  return Changed;
}

/// SinkRegion - Walk the specified region of the CFG (defined by all blocks
/// dominated by the specified block, and that are in the current loop) in
/// reverse depth first order w.r.t the DominatorTree.  This allows us to visit
/// uses before definitions, allowing us to sink a loop body in one pass without
/// iteration.
///
void LICM::SinkRegion(DomTreeNode *N) {
  assert(N != 0 && "Null dominator tree node?");
  BasicBlock *BB = N->getBlock();

  // If this subregion is not in the top level loop at all, exit.
  if (!CurLoop->contains(BB)) return;

  // We are processing blocks in reverse dfo, so process children first.
  const std::vector<DomTreeNode*> &Children = N->getChildren();
  for (unsigned i = 0, e = Children.size(); i != e; ++i)
    SinkRegion(Children[i]);

  // Only need to process the contents of this block if it is not part of a
  // subloop (which would already have been processed).
  if (inSubLoop(BB)) return;

  for (BasicBlock::iterator II = BB->end(); II != BB->begin(); ) {
    Instruction &I = *--II;
    
    // If the instruction is dead, we would try to sink it because it isn't used
    // in the loop, instead, just delete it.
    if (isInstructionTriviallyDead(&I)) {
      DEBUG(dbgs() << "LICM deleting dead inst: " << I << '\n');
      ++II;
      CurAST->deleteValue(&I);
      I.eraseFromParent();
      Changed = true;
      continue;
    }

    // Check to see if we can sink this instruction to the exit blocks
    // of the loop.  We can do this if the all users of the instruction are
    // outside of the loop.  In this case, it doesn't even matter if the
    // operands of the instruction are loop invariant.
    //
    if (isNotUsedInLoop(I) && canSinkOrHoistInst(I)) {
      ++II;
      sink(I);
    }
  }
}

/// HoistRegion - Walk the specified region of the CFG (defined by all blocks
/// dominated by the specified block, and that are in the current loop) in depth
/// first order w.r.t the DominatorTree.  This allows us to visit definitions
/// before uses, allowing us to hoist a loop body in one pass without iteration.
///
void LICM::HoistRegion(DomTreeNode *N) {
  assert(N != 0 && "Null dominator tree node?");
  BasicBlock *BB = N->getBlock();

  // If this subregion is not in the top level loop at all, exit.
  if (!CurLoop->contains(BB)) return;

  // Only need to process the contents of this block if it is not part of a
  // subloop (which would already have been processed).
  if (!inSubLoop(BB))
    for (BasicBlock::iterator II = BB->begin(), E = BB->end(); II != E; ) {
      Instruction &I = *II++;

      // Try constant folding this instruction.  If all the operands are
      // constants, it is technically hoistable, but it would be better to just
      // fold it.
      if (Constant *C = ConstantFoldInstruction(&I)) {
        DEBUG(dbgs() << "LICM folding inst: " << I << "  --> " << *C << '\n');
        CurAST->copyValue(&I, C);
        CurAST->deleteValue(&I);
        I.replaceAllUsesWith(C);
        I.eraseFromParent();
        continue;
      }
      
      // Try hoisting the instruction out to the preheader.  We can only do this
      // if all of the operands of the instruction are loop invariant and if it
      // is safe to hoist the instruction.
      //
      if (isLoopInvariantInst(I) && canSinkOrHoistInst(I) &&
          isSafeToExecuteUnconditionally(I))
        hoist(I);
    }

  const std::vector<DomTreeNode*> &Children = N->getChildren();
  for (unsigned i = 0, e = Children.size(); i != e; ++i)
    HoistRegion(Children[i]);
}

/// canSinkOrHoistInst - Return true if the hoister and sinker can handle this
/// instruction.
///
bool LICM::canSinkOrHoistInst(Instruction &I) {
  // Loads have extra constraints we have to verify before we can hoist them.
  if (LoadInst *LI = dyn_cast<LoadInst>(&I)) {
    if (LI->isVolatile())
      return false;        // Don't hoist volatile loads!

    // Loads from constant memory are always safe to move, even if they end up
    // in the same alias set as something that ends up being modified.
    if (AA->pointsToConstantMemory(LI->getOperand(0)))
      return true;
    
    // Don't hoist loads which have may-aliased stores in loop.
    unsigned Size = 0;
    if (LI->getType()->isSized())
      Size = AA->getTypeStoreSize(LI->getType());
    return !pointerInvalidatedByLoop(LI->getOperand(0), Size);
  } else if (CallInst *CI = dyn_cast<CallInst>(&I)) {
    // Handle obvious cases efficiently.
    AliasAnalysis::ModRefBehavior Behavior = AA->getModRefBehavior(CI);
    if (Behavior == AliasAnalysis::DoesNotAccessMemory)
      return true;
    else if (Behavior == AliasAnalysis::OnlyReadsMemory) {
      // If this call only reads from memory and there are no writes to memory
      // in the loop, we can hoist or sink the call as appropriate.
      bool FoundMod = false;
      for (AliasSetTracker::iterator I = CurAST->begin(), E = CurAST->end();
           I != E; ++I) {
        AliasSet &AS = *I;
        if (!AS.isForwardingAliasSet() && AS.isMod()) {
          FoundMod = true;
          break;
        }
      }
      if (!FoundMod) return true;
    }

    // FIXME: This should use mod/ref information to see if we can hoist or sink
    // the call.

    return false;
  }

  // Otherwise these instructions are hoistable/sinkable
  return isa<BinaryOperator>(I) || isa<CastInst>(I) ||
         isa<SelectInst>(I) || isa<GetElementPtrInst>(I) || isa<CmpInst>(I) ||
         isa<InsertElementInst>(I) || isa<ExtractElementInst>(I) ||
         isa<ShuffleVectorInst>(I);
}

/// isNotUsedInLoop - Return true if the only users of this instruction are
/// outside of the loop.  If this is true, we can sink the instruction to the
/// exit blocks of the loop.
///
bool LICM::isNotUsedInLoop(Instruction &I) {
  for (Value::use_iterator UI = I.use_begin(), E = I.use_end(); UI != E; ++UI) {
    Instruction *User = cast<Instruction>(*UI);
    if (PHINode *PN = dyn_cast<PHINode>(User)) {
      // PHI node uses occur in predecessor blocks!
      for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
        if (PN->getIncomingValue(i) == &I)
          if (CurLoop->contains(PN->getIncomingBlock(i)))
            return false;
    } else if (CurLoop->contains(User)) {
      return false;
    }
  }
  return true;
}


/// isLoopInvariantInst - Return true if all operands of this instruction are
/// loop invariant.  We also filter out non-hoistable instructions here just for
/// efficiency.
///
bool LICM::isLoopInvariantInst(Instruction &I) {
  // The instruction is loop invariant if all of its operands are loop-invariant
  for (unsigned i = 0, e = I.getNumOperands(); i != e; ++i)
    if (!CurLoop->isLoopInvariant(I.getOperand(i)))
      return false;

  // If we got this far, the instruction is loop invariant!
  return true;
}

/// sink - When an instruction is found to only be used outside of the loop,
/// this function moves it to the exit blocks and patches up SSA form as needed.
/// This method is guaranteed to remove the original instruction from its
/// position, and may either delete it or move it to outside of the loop.
///
void LICM::sink(Instruction &I) {
  DEBUG(dbgs() << "LICM sinking instruction: " << I << "\n");

  SmallVector<BasicBlock*, 8> ExitBlocks;
  CurLoop->getUniqueExitBlocks(ExitBlocks);

  if (isa<LoadInst>(I)) ++NumMovedLoads;
  else if (isa<CallInst>(I)) ++NumMovedCalls;
  ++NumSunk;
  Changed = true;

  // The case where there is only a single exit node of this loop is common
  // enough that we handle it as a special (more efficient) case.  It is more
  // efficient to handle because there are no PHI nodes that need to be placed.
  if (ExitBlocks.size() == 1) {
    if (!isExitBlockDominatedByBlockInLoop(ExitBlocks[0], I.getParent())) {
      // Instruction is not used, just delete it.
      CurAST->deleteValue(&I);
      // If I has users in unreachable blocks, eliminate.
      // If I is not void type then replaceAllUsesWith undef.
      // This allows ValueHandlers and custom metadata to adjust itself.
      if (!I.use_empty())
        I.replaceAllUsesWith(UndefValue::get(I.getType()));
      I.eraseFromParent();
    } else {
      // Move the instruction to the start of the exit block, after any PHI
      // nodes in it.
      I.moveBefore(ExitBlocks[0]->getFirstNonPHI());

      // This instruction is no longer in the AST for the current loop, because
      // we just sunk it out of the loop.  If we just sunk it into an outer
      // loop, we will rediscover the operation when we process it.
      CurAST->deleteValue(&I);
    }
    return;
  }
  
  if (ExitBlocks.empty()) {
    // The instruction is actually dead if there ARE NO exit blocks.
    CurAST->deleteValue(&I);
    // If I has users in unreachable blocks, eliminate.
    // If I is not void type then replaceAllUsesWith undef.
    // This allows ValueHandlers and custom metadata to adjust itself.
    if (!I.use_empty())
      I.replaceAllUsesWith(UndefValue::get(I.getType()));
    I.eraseFromParent();
    return;
  }
  
  // Otherwise, if we have multiple exits, use the SSAUpdater to do all of the
  // hard work of inserting PHI nodes as necessary.
  SmallVector<PHINode*, 8> NewPHIs;
  SSAUpdater SSA(&NewPHIs);
  
  if (!I.use_empty())
    SSA.Initialize(I.getType(), I.getName());
  
  // Insert a copy of the instruction in each exit block of the loop that is
  // dominated by the instruction.  Each exit block is known to only be in the
  // ExitBlocks list once.
  BasicBlock *InstOrigBB = I.getParent();
  unsigned NumInserted = 0;
  
  for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i) {
    BasicBlock *ExitBlock = ExitBlocks[i];
    
    if (!isExitBlockDominatedByBlockInLoop(ExitBlock, InstOrigBB))
      continue;
    
    // Insert the code after the last PHI node.
    BasicBlock::iterator InsertPt = ExitBlock->getFirstNonPHI();
    
    // If this is the first exit block processed, just move the original
    // instruction, otherwise clone the original instruction and insert
    // the copy.
    Instruction *New;
    if (NumInserted++ == 0) {
      I.moveBefore(InsertPt);
      New = &I;
    } else {
      New = I.clone();
      if (!I.getName().empty())
        New->setName(I.getName()+".le");
      ExitBlock->getInstList().insert(InsertPt, New);
    }
    
    // Now that we have inserted the instruction, inform SSAUpdater.
    if (!I.use_empty())
      SSA.AddAvailableValue(ExitBlock, New);
  }
  
  // If the instruction doesn't dominate any exit blocks, it must be dead.
  if (NumInserted == 0) {
    CurAST->deleteValue(&I);
    if (!I.use_empty())
      I.replaceAllUsesWith(UndefValue::get(I.getType()));
    I.eraseFromParent();
    return;
  }
  
  // Next, rewrite uses of the instruction, inserting PHI nodes as needed.
  for (Value::use_iterator UI = I.use_begin(), UE = I.use_end(); UI != UE; ) {
    // Grab the use before incrementing the iterator.
    Use &U = UI.getUse();
    // Increment the iterator before removing the use from the list.
    ++UI;
    SSA.RewriteUseAfterInsertions(U);
  }
  
  // Update CurAST for NewPHIs if I had pointer type.
  if (I.getType()->isPointerTy())
    for (unsigned i = 0, e = NewPHIs.size(); i != e; ++i)
      CurAST->copyValue(&I, NewPHIs[i]);
  
  // Finally, remove the instruction from CurAST.  It is no longer in the loop.
  CurAST->deleteValue(&I);
}

/// hoist - When an instruction is found to only use loop invariant operands
/// that is safe to hoist, this instruction is called to do the dirty work.
///
void LICM::hoist(Instruction &I) {
  DEBUG(dbgs() << "LICM hoisting to " << Preheader->getName() << ": "
        << I << "\n");

  // Move the new node to the Preheader, before its terminator.
  I.moveBefore(Preheader->getTerminator());

  if (isa<LoadInst>(I)) ++NumMovedLoads;
  else if (isa<CallInst>(I)) ++NumMovedCalls;
  ++NumHoisted;
  Changed = true;
}

/// isSafeToExecuteUnconditionally - Only sink or hoist an instruction if it is
/// not a trapping instruction or if it is a trapping instruction and is
/// guaranteed to execute.
///
bool LICM::isSafeToExecuteUnconditionally(Instruction &Inst) {
  // If it is not a trapping instruction, it is always safe to hoist.
  if (Inst.isSafeToSpeculativelyExecute())
    return true;

  // Otherwise we have to check to make sure that the instruction dominates all
  // of the exit blocks.  If it doesn't, then there is a path out of the loop
  // which does not execute this instruction, so we can't hoist it.

  // If the instruction is in the header block for the loop (which is very
  // common), it is always guaranteed to dominate the exit blocks.  Since this
  // is a common case, and can save some work, check it now.
  if (Inst.getParent() == CurLoop->getHeader())
    return true;

  // Get the exit blocks for the current loop.
  SmallVector<BasicBlock*, 8> ExitBlocks;
  CurLoop->getExitBlocks(ExitBlocks);

  // For each exit block, get the DT node and walk up the DT until the
  // instruction's basic block is found or we exit the loop.
  for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i)
    if (!isExitBlockDominatedByBlockInLoop(ExitBlocks[i], Inst.getParent()))
      return false;

  return true;
}

/// PromoteAliasSet - Try to promote memory values to scalars by sinking
/// stores out of the loop and moving loads to before the loop.  We do this by
/// looping over the stores in the loop, looking for stores to Must pointers
/// which are loop invariant.
///
void LICM::PromoteAliasSet(AliasSet &AS) {
  // We can promote this alias set if it has a store, if it is a "Must" alias
  // set, if the pointer is loop invariant, and if we are not eliminating any
  // volatile loads or stores.
  if (AS.isForwardingAliasSet() || !AS.isMod() || !AS.isMustAlias() ||
      AS.isVolatile() || !CurLoop->isLoopInvariant(AS.begin()->getValue()))
    return;
  
  assert(!AS.empty() &&
         "Must alias set should have at least one pointer element in it!");
  Value *SomePtr = AS.begin()->getValue();

  // It isn't safe to promote a load/store from the loop if the load/store is
  // conditional.  For example, turning:
  //
  //    for () { if (c) *P += 1; }
  //
  // into:
  //
  //    tmp = *P;  for () { if (c) tmp +=1; } *P = tmp;
  //
  // is not safe, because *P may only be valid to access if 'c' is true.
  // 
  // It is safe to promote P if all uses are direct load/stores and if at
  // least one is guaranteed to be executed.
  bool GuaranteedToExecute = false;
  
  SmallVector<Instruction*, 64> LoopUses;
  SmallPtrSet<Value*, 4> PointerMustAliases;

  // Check that all of the pointers in the alias set have the same type.  We
  // cannot (yet) promote a memory location that is loaded and stored in
  // different sizes.
  for (AliasSet::iterator ASI = AS.begin(), E = AS.end(); ASI != E; ++ASI) {
    Value *ASIV = ASI->getValue();
    PointerMustAliases.insert(ASIV);
    
    // Check that all of the pointers in the alias set have the same type.  We
    // cannot (yet) promote a memory location that is loaded and stored in
    // different sizes.
    if (SomePtr->getType() != ASIV->getType())
      return;
    
    for (Value::use_iterator UI = ASIV->use_begin(), UE = ASIV->use_end();
         UI != UE; ++UI) {
      // Ignore instructions that are outside the loop.
      Instruction *Use = dyn_cast<Instruction>(*UI);
      if (!Use || !CurLoop->contains(Use))
        continue;
      
      // If there is an non-load/store instruction in the loop, we can't promote
      // it.
      if (isa<LoadInst>(Use))
        assert(!cast<LoadInst>(Use)->isVolatile() && "AST broken");
      else if (isa<StoreInst>(Use)) {
        assert(!cast<StoreInst>(Use)->isVolatile() && "AST broken");
        if (Use->getOperand(0) == ASIV) return;
      } else
        return; // Not a load or store.
      
      if (!GuaranteedToExecute)
        GuaranteedToExecute = isSafeToExecuteUnconditionally(*Use);
      
      LoopUses.push_back(Use);
    }
  }
  
  // If there isn't a guaranteed-to-execute instruction, we can't promote.
  if (!GuaranteedToExecute)
    return;
  
  // Otherwise, this is safe to promote, lets do it!
  DEBUG(dbgs() << "LICM: Promoting value stored to in loop: " <<*SomePtr<<'\n');  
  Changed = true;
  ++NumPromoted;

  // We use the SSAUpdater interface to insert phi nodes as required.
  SmallVector<PHINode*, 16> NewPHIs;
  SSAUpdater SSA(&NewPHIs);
  
  // It wants to know some value of the same type as what we'll be inserting.
  Value *SomeValue;
  if (isa<LoadInst>(LoopUses[0]))
    SomeValue = LoopUses[0];
  else
    SomeValue = cast<StoreInst>(LoopUses[0])->getOperand(0);
  SSA.Initialize(SomeValue->getType(), SomeValue->getName());

  // First step: bucket up uses of the pointers by the block they occur in.
  // This is important because we have to handle multiple defs/uses in a block
  // ourselves: SSAUpdater is purely for cross-block references.
  // FIXME: Want a TinyVector<Instruction*> since there is usually 0/1 element.
  DenseMap<BasicBlock*, std::vector<Instruction*> > UsesByBlock;
  for (unsigned i = 0, e = LoopUses.size(); i != e; ++i) {
    Instruction *User = LoopUses[i];
    UsesByBlock[User->getParent()].push_back(User);
  }
  
  // Okay, now we can iterate over all the blocks in the loop with uses,
  // processing them.  Keep track of which loads are loading a live-in value.
  SmallVector<LoadInst*, 32> LiveInLoads;
  DenseMap<Value*, Value*> ReplacedLoads;
  
  for (unsigned LoopUse = 0, e = LoopUses.size(); LoopUse != e; ++LoopUse) {
    Instruction *User = LoopUses[LoopUse];
    std::vector<Instruction*> &BlockUses = UsesByBlock[User->getParent()];
    
    // If this block has already been processed, ignore this repeat use.
    if (BlockUses.empty()) continue;
    
    // Okay, this is the first use in the block.  If this block just has a
    // single user in it, we can rewrite it trivially.
    if (BlockUses.size() == 1) {
      // If it is a store, it is a trivial def of the value in the block.
      if (isa<StoreInst>(User)) {
        SSA.AddAvailableValue(User->getParent(),
                              cast<StoreInst>(User)->getOperand(0));
      } else {
        // Otherwise it is a load, queue it to rewrite as a live-in load.
        LiveInLoads.push_back(cast<LoadInst>(User));
      }
      BlockUses.clear();
      continue;
    }
    
    // Otherwise, check to see if this block is all loads.  If so, we can queue
    // them all as live in loads.
    bool HasStore = false;
    for (unsigned i = 0, e = BlockUses.size(); i != e; ++i) {
      if (isa<StoreInst>(BlockUses[i])) {
        HasStore = true;
        break;
      }
    }
    
    if (!HasStore) {
      for (unsigned i = 0, e = BlockUses.size(); i != e; ++i)
        LiveInLoads.push_back(cast<LoadInst>(BlockUses[i]));
      BlockUses.clear();
      continue;
    }

    // Otherwise, we have mixed loads and stores (or just a bunch of stores).
    // Since SSAUpdater is purely for cross-block values, we need to determine
    // the order of these instructions in the block.  If the first use in the
    // block is a load, then it uses the live in value.  The last store defines
    // the live out value.  We handle this by doing a linear scan of the block.
    BasicBlock *BB = User->getParent();
    Value *StoredValue = 0;
    for (BasicBlock::iterator II = BB->begin(), E = BB->end(); II != E; ++II) {
      if (LoadInst *L = dyn_cast<LoadInst>(II)) {
        // If this is a load from an unrelated pointer, ignore it.
        if (!PointerMustAliases.count(L->getOperand(0))) continue;

        // If we haven't seen a store yet, this is a live in use, otherwise
        // use the stored value.
        if (StoredValue) {
          L->replaceAllUsesWith(StoredValue);
          ReplacedLoads[L] = StoredValue;
        } else {
          LiveInLoads.push_back(L);
        }
        continue;
      }
      
      if (StoreInst *S = dyn_cast<StoreInst>(II)) {
        // If this is a store to an unrelated pointer, ignore it.
        if (!PointerMustAliases.count(S->getOperand(1))) continue;

        // Remember that this is the active value in the block.
        StoredValue = S->getOperand(0);
      }
    }
    
    // The last stored value that happened is the live-out for the block.
    assert(StoredValue && "Already checked that there is a store in block");
    SSA.AddAvailableValue(BB, StoredValue);
    BlockUses.clear();
  }
  
  // Now that all the intra-loop values are classified, set up the preheader.
  // It gets a load of the pointer we're promoting, and it is the live-out value
  // from the preheader.
  LoadInst *PreheaderLoad = new LoadInst(SomePtr,SomePtr->getName()+".promoted",
                                         Preheader->getTerminator());
  SSA.AddAvailableValue(Preheader, PreheaderLoad);

  // Now that the preheader is good to go, set up the exit blocks.  Each exit
  // block gets a store of the live-out values that feed them.  Since we've
  // already told the SSA updater about the defs in the loop and the preheader
  // definition, it is all set and we can start using it.
  SmallVector<BasicBlock*, 8> ExitBlocks;
  CurLoop->getUniqueExitBlocks(ExitBlocks);
  for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i) {
    BasicBlock *ExitBlock = ExitBlocks[i];
    Value *LiveInValue = SSA.GetValueInMiddleOfBlock(ExitBlock);
    Instruction *InsertPos = ExitBlock->getFirstNonPHI();
    new StoreInst(LiveInValue, SomePtr, InsertPos);
  }

  // Okay, now we rewrite all loads that use live-in values in the loop,
  // inserting PHI nodes as necessary.
  for (unsigned i = 0, e = LiveInLoads.size(); i != e; ++i) {
    LoadInst *ALoad = LiveInLoads[i];
    Value *NewVal = SSA.GetValueInMiddleOfBlock(ALoad->getParent());
    ALoad->replaceAllUsesWith(NewVal);
    CurAST->copyValue(ALoad, NewVal);
    ReplacedLoads[ALoad] = NewVal;
  }
  
  // If the preheader load is itself a pointer, we need to tell alias analysis
  // about the new pointer we created in the preheader block and about any PHI
  // nodes that just got inserted.
  if (PreheaderLoad->getType()->isPointerTy()) {
    // Copy any value stored to or loaded from a must-alias of the pointer.
    CurAST->copyValue(SomeValue, PreheaderLoad);
    
    for (unsigned i = 0, e = NewPHIs.size(); i != e; ++i)
      CurAST->copyValue(SomeValue, NewPHIs[i]);
  }
  
  // Now that everything is rewritten, delete the old instructions from the body
  // of the loop.  They should all be dead now.
  for (unsigned i = 0, e = LoopUses.size(); i != e; ++i) {
    Instruction *User = LoopUses[i];
    
    // If this is a load that still has uses, then the load must have been added
    // as a live value in the SSAUpdate data structure for a block (e.g. because
    // the loaded value was stored later).  In this case, we need to recursively
    // propagate the updates until we get to the real value.
    if (!User->use_empty()) {
      Value *NewVal = ReplacedLoads[User];
      assert(NewVal && "not a replaced load?");
      
      // Propagate down to the ultimate replacee.  The intermediately loads
      // could theoretically already have been deleted, so we don't want to
      // dereference the Value*'s.
      DenseMap<Value*, Value*>::iterator RLI = ReplacedLoads.find(NewVal);
      while (RLI != ReplacedLoads.end()) {
        NewVal = RLI->second;
        RLI = ReplacedLoads.find(NewVal);
      }
      
      User->replaceAllUsesWith(NewVal);
      CurAST->copyValue(User, NewVal);
    }
    
    CurAST->deleteValue(User);
    User->eraseFromParent();
  }
  
  // fwew, we're done!
}


/// cloneBasicBlockAnalysis - Simple Analysis hook. Clone alias set info.
void LICM::cloneBasicBlockAnalysis(BasicBlock *From, BasicBlock *To, Loop *L) {
  AliasSetTracker *AST = LoopToAliasSetMap.lookup(L);
  if (!AST)
    return;

  AST->copyValue(From, To);
}

/// deleteAnalysisValue - Simple Analysis hook. Delete value V from alias
/// set.
void LICM::deleteAnalysisValue(Value *V, Loop *L) {
  AliasSetTracker *AST = LoopToAliasSetMap.lookup(L);
  if (!AST)
    return;

  AST->deleteValue(V);
}