1//===-- LICM.cpp - Loop Invariant Code Motion Pass ------------------------===//
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 pass performs loop invariant code motion, attempting to remove as much
11// code from the body of a loop as possible.  It does this by either hoisting
12// code into the preheader block, or by sinking code to the exit blocks if it is
13// safe.  This pass also promotes must-aliased memory locations in the loop to
14// live in registers, thus hoisting and sinking "invariant" loads and stores.
15//
16// This pass uses alias analysis for two purposes:
17//
18//  1. Moving loop invariant loads and calls out of loops.  If we can determine
19//     that a load or call inside of a loop never aliases anything stored to,
20//     we can hoist it or sink it like any other instruction.
21//  2. Scalar Promotion of Memory - If there is a store instruction inside of
22//     the loop, we try to move the store to happen AFTER the loop instead of
23//     inside of the loop.  This can only happen if a few conditions are true:
24//       A. The pointer stored through is loop invariant
25//       B. There are no stores or loads in the loop which _may_ alias the
26//          pointer.  There are no calls in the loop which mod/ref the pointer.
27//     If these conditions are true, we can promote the loads and stores in the
28//     loop of the pointer to use a temporary alloca'd variable.  We then use
29//     the SSAUpdater to construct the appropriate SSA form for the value.
30//
31//===----------------------------------------------------------------------===//
32
33#define DEBUG_TYPE "licm"
34#include "llvm/Transforms/Scalar.h"
35#include "llvm/Constants.h"
36#include "llvm/DerivedTypes.h"
37#include "llvm/IntrinsicInst.h"
38#include "llvm/Instructions.h"
39#include "llvm/LLVMContext.h"
40#include "llvm/Analysis/AliasAnalysis.h"
41#include "llvm/Analysis/AliasSetTracker.h"
42#include "llvm/Analysis/ConstantFolding.h"
43#include "llvm/Analysis/LoopInfo.h"
44#include "llvm/Analysis/LoopPass.h"
45#include "llvm/Analysis/Dominators.h"
46#include "llvm/Transforms/Utils/Local.h"
47#include "llvm/Transforms/Utils/SSAUpdater.h"
48#include "llvm/Support/CFG.h"
49#include "llvm/Support/CommandLine.h"
50#include "llvm/Support/raw_ostream.h"
51#include "llvm/Support/Debug.h"
52#include "llvm/ADT/Statistic.h"
53#include <algorithm>
54using namespace llvm;
55
56STATISTIC(NumSunk      , "Number of instructions sunk out of loop");
57STATISTIC(NumHoisted   , "Number of instructions hoisted out of loop");
58STATISTIC(NumMovedLoads, "Number of load insts hoisted or sunk");
59STATISTIC(NumMovedCalls, "Number of call insts hoisted or sunk");
60STATISTIC(NumPromoted  , "Number of memory locations promoted to registers");
61
62static cl::opt<bool>
63DisablePromotion("disable-licm-promotion", cl::Hidden,
64                 cl::desc("Disable memory promotion in LICM pass"));
65
66namespace {
67  struct LICM : public LoopPass {
68    static char ID; // Pass identification, replacement for typeid
69    LICM() : LoopPass(ID) {
70      initializeLICMPass(*PassRegistry::getPassRegistry());
71    }
72
73    virtual bool runOnLoop(Loop *L, LPPassManager &LPM);
74
75    /// This transformation requires natural loop information & requires that
76    /// loop preheaders be inserted into the CFG...
77    ///
78    virtual void getAnalysisUsage(AnalysisUsage &AU) const {
79      AU.setPreservesCFG();
80      AU.addRequired<DominatorTree>();
81      AU.addRequired<LoopInfo>();
82      AU.addRequiredID(LoopSimplifyID);
83      AU.addRequired<AliasAnalysis>();
84      AU.addPreserved<AliasAnalysis>();
85      AU.addPreserved("scalar-evolution");
86      AU.addPreservedID(LoopSimplifyID);
87    }
88
89    bool doFinalization() {
90      assert(LoopToAliasSetMap.empty() && "Didn't free loop alias sets");
91      return false;
92    }
93
94  private:
95    AliasAnalysis *AA;       // Current AliasAnalysis information
96    LoopInfo      *LI;       // Current LoopInfo
97    DominatorTree *DT;       // Dominator Tree for the current Loop.
98
99    // State that is updated as we process loops.
100    bool Changed;            // Set to true when we change anything.
101    BasicBlock *Preheader;   // The preheader block of the current loop...
102    Loop *CurLoop;           // The current loop we are working on...
103    AliasSetTracker *CurAST; // AliasSet information for the current loop...
104    DenseMap<Loop*, AliasSetTracker*> LoopToAliasSetMap;
105
106    /// cloneBasicBlockAnalysis - Simple Analysis hook. Clone alias set info.
107    void cloneBasicBlockAnalysis(BasicBlock *From, BasicBlock *To, Loop *L);
108
109    /// deleteAnalysisValue - Simple Analysis hook. Delete value V from alias
110    /// set.
111    void deleteAnalysisValue(Value *V, Loop *L);
112
113    /// SinkRegion - Walk the specified region of the CFG (defined by all blocks
114    /// dominated by the specified block, and that are in the current loop) in
115    /// reverse depth first order w.r.t the DominatorTree.  This allows us to
116    /// visit uses before definitions, allowing us to sink a loop body in one
117    /// pass without iteration.
118    ///
119    void SinkRegion(DomTreeNode *N);
120
121    /// HoistRegion - Walk the specified region of the CFG (defined by all
122    /// blocks dominated by the specified block, and that are in the current
123    /// loop) in depth first order w.r.t the DominatorTree.  This allows us to
124    /// visit definitions before uses, allowing us to hoist a loop body in one
125    /// pass without iteration.
126    ///
127    void HoistRegion(DomTreeNode *N);
128
129    /// inSubLoop - Little predicate that returns true if the specified basic
130    /// block is in a subloop of the current one, not the current one itself.
131    ///
132    bool inSubLoop(BasicBlock *BB) {
133      assert(CurLoop->contains(BB) && "Only valid if BB is IN the loop");
134      return LI->getLoopFor(BB) != CurLoop;
135    }
136
137    /// sink - When an instruction is found to only be used outside of the loop,
138    /// this function moves it to the exit blocks and patches up SSA form as
139    /// needed.
140    ///
141    void sink(Instruction &I);
142
143    /// hoist - When an instruction is found to only use loop invariant operands
144    /// that is safe to hoist, this instruction is called to do the dirty work.
145    ///
146    void hoist(Instruction &I);
147
148    /// isSafeToExecuteUnconditionally - Only sink or hoist an instruction if it
149    /// is not a trapping instruction or if it is a trapping instruction and is
150    /// guaranteed to execute.
151    ///
152    bool isSafeToExecuteUnconditionally(Instruction &I);
153
154    /// isGuaranteedToExecute - Check that the instruction is guaranteed to
155    /// execute.
156    ///
157    bool isGuaranteedToExecute(Instruction &I);
158
159    /// pointerInvalidatedByLoop - Return true if the body of this loop may
160    /// store into the memory location pointed to by V.
161    ///
162    bool pointerInvalidatedByLoop(Value *V, uint64_t Size,
163                                  const MDNode *TBAAInfo) {
164      // Check to see if any of the basic blocks in CurLoop invalidate *V.
165      return CurAST->getAliasSetForPointer(V, Size, TBAAInfo).isMod();
166    }
167
168    bool canSinkOrHoistInst(Instruction &I);
169    bool isNotUsedInLoop(Instruction &I);
170
171    void PromoteAliasSet(AliasSet &AS);
172  };
173}
174
175char LICM::ID = 0;
176INITIALIZE_PASS_BEGIN(LICM, "licm", "Loop Invariant Code Motion", false, false)
177INITIALIZE_PASS_DEPENDENCY(DominatorTree)
178INITIALIZE_PASS_DEPENDENCY(LoopInfo)
179INITIALIZE_PASS_DEPENDENCY(LoopSimplify)
180INITIALIZE_AG_DEPENDENCY(AliasAnalysis)
181INITIALIZE_PASS_END(LICM, "licm", "Loop Invariant Code Motion", false, false)
182
183Pass *llvm::createLICMPass() { return new LICM(); }
184
185/// Hoist expressions out of the specified loop. Note, alias info for inner
186/// loop is not preserved so it is not a good idea to run LICM multiple
187/// times on one loop.
188///
189bool LICM::runOnLoop(Loop *L, LPPassManager &LPM) {
190  Changed = false;
191
192  // Get our Loop and Alias Analysis information...
193  LI = &getAnalysis<LoopInfo>();
194  AA = &getAnalysis<AliasAnalysis>();
195  DT = &getAnalysis<DominatorTree>();
196
197  CurAST = new AliasSetTracker(*AA);
198  // Collect Alias info from subloops.
199  for (Loop::iterator LoopItr = L->begin(), LoopItrE = L->end();
200       LoopItr != LoopItrE; ++LoopItr) {
201    Loop *InnerL = *LoopItr;
202    AliasSetTracker *InnerAST = LoopToAliasSetMap[InnerL];
203    assert(InnerAST && "Where is my AST?");
204
205    // What if InnerLoop was modified by other passes ?
206    CurAST->add(*InnerAST);
207
208    // Once we've incorporated the inner loop's AST into ours, we don't need the
209    // subloop's anymore.
210    delete InnerAST;
211    LoopToAliasSetMap.erase(InnerL);
212  }
213
214  CurLoop = L;
215
216  // Get the preheader block to move instructions into...
217  Preheader = L->getLoopPreheader();
218
219  // Loop over the body of this loop, looking for calls, invokes, and stores.
220  // Because subloops have already been incorporated into AST, we skip blocks in
221  // subloops.
222  //
223  for (Loop::block_iterator I = L->block_begin(), E = L->block_end();
224       I != E; ++I) {
225    BasicBlock *BB = *I;
226    if (LI->getLoopFor(BB) == L)        // Ignore blocks in subloops.
227      CurAST->add(*BB);                 // Incorporate the specified basic block
228  }
229
230  // We want to visit all of the instructions in this loop... that are not parts
231  // of our subloops (they have already had their invariants hoisted out of
232  // their loop, into this loop, so there is no need to process the BODIES of
233  // the subloops).
234  //
235  // Traverse the body of the loop in depth first order on the dominator tree so
236  // that we are guaranteed to see definitions before we see uses.  This allows
237  // us to sink instructions in one pass, without iteration.  After sinking
238  // instructions, we perform another pass to hoist them out of the loop.
239  //
240  if (L->hasDedicatedExits())
241    SinkRegion(DT->getNode(L->getHeader()));
242  if (Preheader)
243    HoistRegion(DT->getNode(L->getHeader()));
244
245  // Now that all loop invariants have been removed from the loop, promote any
246  // memory references to scalars that we can.
247  if (!DisablePromotion && Preheader && L->hasDedicatedExits()) {
248    // Loop over all of the alias sets in the tracker object.
249    for (AliasSetTracker::iterator I = CurAST->begin(), E = CurAST->end();
250         I != E; ++I)
251      PromoteAliasSet(*I);
252  }
253
254  // Clear out loops state information for the next iteration
255  CurLoop = 0;
256  Preheader = 0;
257
258  // If this loop is nested inside of another one, save the alias information
259  // for when we process the outer loop.
260  if (L->getParentLoop())
261    LoopToAliasSetMap[L] = CurAST;
262  else
263    delete CurAST;
264  return Changed;
265}
266
267/// SinkRegion - Walk the specified region of the CFG (defined by all blocks
268/// dominated by the specified block, and that are in the current loop) in
269/// reverse depth first order w.r.t the DominatorTree.  This allows us to visit
270/// uses before definitions, allowing us to sink a loop body in one pass without
271/// iteration.
272///
273void LICM::SinkRegion(DomTreeNode *N) {
274  assert(N != 0 && "Null dominator tree node?");
275  BasicBlock *BB = N->getBlock();
276
277  // If this subregion is not in the top level loop at all, exit.
278  if (!CurLoop->contains(BB)) return;
279
280  // We are processing blocks in reverse dfo, so process children first.
281  const std::vector<DomTreeNode*> &Children = N->getChildren();
282  for (unsigned i = 0, e = Children.size(); i != e; ++i)
283    SinkRegion(Children[i]);
284
285  // Only need to process the contents of this block if it is not part of a
286  // subloop (which would already have been processed).
287  if (inSubLoop(BB)) return;
288
289  for (BasicBlock::iterator II = BB->end(); II != BB->begin(); ) {
290    Instruction &I = *--II;
291
292    // If the instruction is dead, we would try to sink it because it isn't used
293    // in the loop, instead, just delete it.
294    if (isInstructionTriviallyDead(&I)) {
295      DEBUG(dbgs() << "LICM deleting dead inst: " << I << '\n');
296      ++II;
297      CurAST->deleteValue(&I);
298      I.eraseFromParent();
299      Changed = true;
300      continue;
301    }
302
303    // Check to see if we can sink this instruction to the exit blocks
304    // of the loop.  We can do this if the all users of the instruction are
305    // outside of the loop.  In this case, it doesn't even matter if the
306    // operands of the instruction are loop invariant.
307    //
308    if (isNotUsedInLoop(I) && canSinkOrHoistInst(I)) {
309      ++II;
310      sink(I);
311    }
312  }
313}
314
315/// HoistRegion - Walk the specified region of the CFG (defined by all blocks
316/// dominated by the specified block, and that are in the current loop) in depth
317/// first order w.r.t the DominatorTree.  This allows us to visit definitions
318/// before uses, allowing us to hoist a loop body in one pass without iteration.
319///
320void LICM::HoistRegion(DomTreeNode *N) {
321  assert(N != 0 && "Null dominator tree node?");
322  BasicBlock *BB = N->getBlock();
323
324  // If this subregion is not in the top level loop at all, exit.
325  if (!CurLoop->contains(BB)) return;
326
327  // Only need to process the contents of this block if it is not part of a
328  // subloop (which would already have been processed).
329  if (!inSubLoop(BB))
330    for (BasicBlock::iterator II = BB->begin(), E = BB->end(); II != E; ) {
331      Instruction &I = *II++;
332
333      // Try constant folding this instruction.  If all the operands are
334      // constants, it is technically hoistable, but it would be better to just
335      // fold it.
336      if (Constant *C = ConstantFoldInstruction(&I)) {
337        DEBUG(dbgs() << "LICM folding inst: " << I << "  --> " << *C << '\n');
338        CurAST->copyValue(&I, C);
339        CurAST->deleteValue(&I);
340        I.replaceAllUsesWith(C);
341        I.eraseFromParent();
342        continue;
343      }
344
345      // Try hoisting the instruction out to the preheader.  We can only do this
346      // if all of the operands of the instruction are loop invariant and if it
347      // is safe to hoist the instruction.
348      //
349      if (CurLoop->hasLoopInvariantOperands(&I) && canSinkOrHoistInst(I) &&
350          isSafeToExecuteUnconditionally(I))
351        hoist(I);
352    }
353
354  const std::vector<DomTreeNode*> &Children = N->getChildren();
355  for (unsigned i = 0, e = Children.size(); i != e; ++i)
356    HoistRegion(Children[i]);
357}
358
359/// canSinkOrHoistInst - Return true if the hoister and sinker can handle this
360/// instruction.
361///
362bool LICM::canSinkOrHoistInst(Instruction &I) {
363  // Loads have extra constraints we have to verify before we can hoist them.
364  if (LoadInst *LI = dyn_cast<LoadInst>(&I)) {
365    if (!LI->isUnordered())
366      return false;        // Don't hoist volatile/atomic loads!
367
368    // Loads from constant memory are always safe to move, even if they end up
369    // in the same alias set as something that ends up being modified.
370    if (AA->pointsToConstantMemory(LI->getOperand(0)))
371      return true;
372
373    // Don't hoist loads which have may-aliased stores in loop.
374    uint64_t Size = 0;
375    if (LI->getType()->isSized())
376      Size = AA->getTypeStoreSize(LI->getType());
377    return !pointerInvalidatedByLoop(LI->getOperand(0), Size,
378                                     LI->getMetadata(LLVMContext::MD_tbaa));
379  } else if (CallInst *CI = dyn_cast<CallInst>(&I)) {
380    // Don't sink or hoist dbg info; it's legal, but not useful.
381    if (isa<DbgInfoIntrinsic>(I))
382      return false;
383
384    // Handle simple cases by querying alias analysis.
385    AliasAnalysis::ModRefBehavior Behavior = AA->getModRefBehavior(CI);
386    if (Behavior == AliasAnalysis::DoesNotAccessMemory)
387      return true;
388    if (AliasAnalysis::onlyReadsMemory(Behavior)) {
389      // If this call only reads from memory and there are no writes to memory
390      // in the loop, we can hoist or sink the call as appropriate.
391      bool FoundMod = false;
392      for (AliasSetTracker::iterator I = CurAST->begin(), E = CurAST->end();
393           I != E; ++I) {
394        AliasSet &AS = *I;
395        if (!AS.isForwardingAliasSet() && AS.isMod()) {
396          FoundMod = true;
397          break;
398        }
399      }
400      if (!FoundMod) return true;
401    }
402
403    // FIXME: This should use mod/ref information to see if we can hoist or sink
404    // the call.
405
406    return false;
407  }
408
409  // Otherwise these instructions are hoistable/sinkable
410  return isa<BinaryOperator>(I) || isa<CastInst>(I) ||
411         isa<SelectInst>(I) || isa<GetElementPtrInst>(I) || isa<CmpInst>(I) ||
412         isa<InsertElementInst>(I) || isa<ExtractElementInst>(I) ||
413         isa<ShuffleVectorInst>(I);
414}
415
416/// isNotUsedInLoop - Return true if the only users of this instruction are
417/// outside of the loop.  If this is true, we can sink the instruction to the
418/// exit blocks of the loop.
419///
420bool LICM::isNotUsedInLoop(Instruction &I) {
421  for (Value::use_iterator UI = I.use_begin(), E = I.use_end(); UI != E; ++UI) {
422    Instruction *User = cast<Instruction>(*UI);
423    if (PHINode *PN = dyn_cast<PHINode>(User)) {
424      // PHI node uses occur in predecessor blocks!
425      for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
426        if (PN->getIncomingValue(i) == &I)
427          if (CurLoop->contains(PN->getIncomingBlock(i)))
428            return false;
429    } else if (CurLoop->contains(User)) {
430      return false;
431    }
432  }
433  return true;
434}
435
436
437/// sink - When an instruction is found to only be used outside of the loop,
438/// this function moves it to the exit blocks and patches up SSA form as needed.
439/// This method is guaranteed to remove the original instruction from its
440/// position, and may either delete it or move it to outside of the loop.
441///
442void LICM::sink(Instruction &I) {
443  DEBUG(dbgs() << "LICM sinking instruction: " << I << "\n");
444
445  SmallVector<BasicBlock*, 8> ExitBlocks;
446  CurLoop->getUniqueExitBlocks(ExitBlocks);
447
448  if (isa<LoadInst>(I)) ++NumMovedLoads;
449  else if (isa<CallInst>(I)) ++NumMovedCalls;
450  ++NumSunk;
451  Changed = true;
452
453  // The case where there is only a single exit node of this loop is common
454  // enough that we handle it as a special (more efficient) case.  It is more
455  // efficient to handle because there are no PHI nodes that need to be placed.
456  if (ExitBlocks.size() == 1) {
457    if (!DT->dominates(I.getParent(), ExitBlocks[0])) {
458      // Instruction is not used, just delete it.
459      CurAST->deleteValue(&I);
460      // If I has users in unreachable blocks, eliminate.
461      // If I is not void type then replaceAllUsesWith undef.
462      // This allows ValueHandlers and custom metadata to adjust itself.
463      if (!I.use_empty())
464        I.replaceAllUsesWith(UndefValue::get(I.getType()));
465      I.eraseFromParent();
466    } else {
467      // Move the instruction to the start of the exit block, after any PHI
468      // nodes in it.
469      I.moveBefore(ExitBlocks[0]->getFirstInsertionPt());
470
471      // This instruction is no longer in the AST for the current loop, because
472      // we just sunk it out of the loop.  If we just sunk it into an outer
473      // loop, we will rediscover the operation when we process it.
474      CurAST->deleteValue(&I);
475    }
476    return;
477  }
478
479  if (ExitBlocks.empty()) {
480    // The instruction is actually dead if there ARE NO exit blocks.
481    CurAST->deleteValue(&I);
482    // If I has users in unreachable blocks, eliminate.
483    // If I is not void type then replaceAllUsesWith undef.
484    // This allows ValueHandlers and custom metadata to adjust itself.
485    if (!I.use_empty())
486      I.replaceAllUsesWith(UndefValue::get(I.getType()));
487    I.eraseFromParent();
488    return;
489  }
490
491  // Otherwise, if we have multiple exits, use the SSAUpdater to do all of the
492  // hard work of inserting PHI nodes as necessary.
493  SmallVector<PHINode*, 8> NewPHIs;
494  SSAUpdater SSA(&NewPHIs);
495
496  if (!I.use_empty())
497    SSA.Initialize(I.getType(), I.getName());
498
499  // Insert a copy of the instruction in each exit block of the loop that is
500  // dominated by the instruction.  Each exit block is known to only be in the
501  // ExitBlocks list once.
502  BasicBlock *InstOrigBB = I.getParent();
503  unsigned NumInserted = 0;
504
505  for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i) {
506    BasicBlock *ExitBlock = ExitBlocks[i];
507
508    if (!DT->dominates(InstOrigBB, ExitBlock))
509      continue;
510
511    // Insert the code after the last PHI node.
512    BasicBlock::iterator InsertPt = ExitBlock->getFirstInsertionPt();
513
514    // If this is the first exit block processed, just move the original
515    // instruction, otherwise clone the original instruction and insert
516    // the copy.
517    Instruction *New;
518    if (NumInserted++ == 0) {
519      I.moveBefore(InsertPt);
520      New = &I;
521    } else {
522      New = I.clone();
523      if (!I.getName().empty())
524        New->setName(I.getName()+".le");
525      ExitBlock->getInstList().insert(InsertPt, New);
526    }
527
528    // Now that we have inserted the instruction, inform SSAUpdater.
529    if (!I.use_empty())
530      SSA.AddAvailableValue(ExitBlock, New);
531  }
532
533  // If the instruction doesn't dominate any exit blocks, it must be dead.
534  if (NumInserted == 0) {
535    CurAST->deleteValue(&I);
536    if (!I.use_empty())
537      I.replaceAllUsesWith(UndefValue::get(I.getType()));
538    I.eraseFromParent();
539    return;
540  }
541
542  // Next, rewrite uses of the instruction, inserting PHI nodes as needed.
543  for (Value::use_iterator UI = I.use_begin(), UE = I.use_end(); UI != UE; ) {
544    // Grab the use before incrementing the iterator.
545    Use &U = UI.getUse();
546    // Increment the iterator before removing the use from the list.
547    ++UI;
548    SSA.RewriteUseAfterInsertions(U);
549  }
550
551  // Update CurAST for NewPHIs if I had pointer type.
552  if (I.getType()->isPointerTy())
553    for (unsigned i = 0, e = NewPHIs.size(); i != e; ++i)
554      CurAST->copyValue(&I, NewPHIs[i]);
555
556  // Finally, remove the instruction from CurAST.  It is no longer in the loop.
557  CurAST->deleteValue(&I);
558}
559
560/// hoist - When an instruction is found to only use loop invariant operands
561/// that is safe to hoist, this instruction is called to do the dirty work.
562///
563void LICM::hoist(Instruction &I) {
564  DEBUG(dbgs() << "LICM hoisting to " << Preheader->getName() << ": "
565        << I << "\n");
566
567  // Move the new node to the Preheader, before its terminator.
568  I.moveBefore(Preheader->getTerminator());
569
570  if (isa<LoadInst>(I)) ++NumMovedLoads;
571  else if (isa<CallInst>(I)) ++NumMovedCalls;
572  ++NumHoisted;
573  Changed = true;
574}
575
576/// isSafeToExecuteUnconditionally - Only sink or hoist an instruction if it is
577/// not a trapping instruction or if it is a trapping instruction and is
578/// guaranteed to execute.
579///
580bool LICM::isSafeToExecuteUnconditionally(Instruction &Inst) {
581  // If it is not a trapping instruction, it is always safe to hoist.
582  if (Inst.isSafeToSpeculativelyExecute())
583    return true;
584
585  return isGuaranteedToExecute(Inst);
586}
587
588bool LICM::isGuaranteedToExecute(Instruction &Inst) {
589  // Otherwise we have to check to make sure that the instruction dominates all
590  // of the exit blocks.  If it doesn't, then there is a path out of the loop
591  // which does not execute this instruction, so we can't hoist it.
592
593  // If the instruction is in the header block for the loop (which is very
594  // common), it is always guaranteed to dominate the exit blocks.  Since this
595  // is a common case, and can save some work, check it now.
596  if (Inst.getParent() == CurLoop->getHeader())
597    return true;
598
599  // Get the exit blocks for the current loop.
600  SmallVector<BasicBlock*, 8> ExitBlocks;
601  CurLoop->getExitBlocks(ExitBlocks);
602
603  // Verify that the block dominates each of the exit blocks of the loop.
604  for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i)
605    if (!DT->dominates(Inst.getParent(), ExitBlocks[i]))
606      return false;
607
608  return true;
609}
610
611namespace {
612  class LoopPromoter : public LoadAndStorePromoter {
613    Value *SomePtr;  // Designated pointer to store to.
614    SmallPtrSet<Value*, 4> &PointerMustAliases;
615    SmallVectorImpl<BasicBlock*> &LoopExitBlocks;
616    AliasSetTracker &AST;
617    DebugLoc DL;
618    int Alignment;
619  public:
620    LoopPromoter(Value *SP,
621                 const SmallVectorImpl<Instruction*> &Insts, SSAUpdater &S,
622                 SmallPtrSet<Value*, 4> &PMA,
623                 SmallVectorImpl<BasicBlock*> &LEB, AliasSetTracker &ast,
624                 DebugLoc dl, int alignment)
625      : LoadAndStorePromoter(Insts, S), SomePtr(SP),
626        PointerMustAliases(PMA), LoopExitBlocks(LEB), AST(ast), DL(dl),
627        Alignment(alignment) {}
628
629    virtual bool isInstInList(Instruction *I,
630                              const SmallVectorImpl<Instruction*> &) const {
631      Value *Ptr;
632      if (LoadInst *LI = dyn_cast<LoadInst>(I))
633        Ptr = LI->getOperand(0);
634      else
635        Ptr = cast<StoreInst>(I)->getPointerOperand();
636      return PointerMustAliases.count(Ptr);
637    }
638
639    virtual void doExtraRewritesBeforeFinalDeletion() const {
640      // Insert stores after in the loop exit blocks.  Each exit block gets a
641      // store of the live-out values that feed them.  Since we've already told
642      // the SSA updater about the defs in the loop and the preheader
643      // definition, it is all set and we can start using it.
644      for (unsigned i = 0, e = LoopExitBlocks.size(); i != e; ++i) {
645        BasicBlock *ExitBlock = LoopExitBlocks[i];
646        Value *LiveInValue = SSA.GetValueInMiddleOfBlock(ExitBlock);
647        Instruction *InsertPos = ExitBlock->getFirstInsertionPt();
648        StoreInst *NewSI = new StoreInst(LiveInValue, SomePtr, InsertPos);
649        NewSI->setAlignment(Alignment);
650        NewSI->setDebugLoc(DL);
651      }
652    }
653
654    virtual void replaceLoadWithValue(LoadInst *LI, Value *V) const {
655      // Update alias analysis.
656      AST.copyValue(LI, V);
657    }
658    virtual void instructionDeleted(Instruction *I) const {
659      AST.deleteValue(I);
660    }
661  };
662} // end anon namespace
663
664/// PromoteAliasSet - Try to promote memory values to scalars by sinking
665/// stores out of the loop and moving loads to before the loop.  We do this by
666/// looping over the stores in the loop, looking for stores to Must pointers
667/// which are loop invariant.
668///
669void LICM::PromoteAliasSet(AliasSet &AS) {
670  // We can promote this alias set if it has a store, if it is a "Must" alias
671  // set, if the pointer is loop invariant, and if we are not eliminating any
672  // volatile loads or stores.
673  if (AS.isForwardingAliasSet() || !AS.isMod() || !AS.isMustAlias() ||
674      AS.isVolatile() || !CurLoop->isLoopInvariant(AS.begin()->getValue()))
675    return;
676
677  assert(!AS.empty() &&
678         "Must alias set should have at least one pointer element in it!");
679  Value *SomePtr = AS.begin()->getValue();
680
681  // It isn't safe to promote a load/store from the loop if the load/store is
682  // conditional.  For example, turning:
683  //
684  //    for () { if (c) *P += 1; }
685  //
686  // into:
687  //
688  //    tmp = *P;  for () { if (c) tmp +=1; } *P = tmp;
689  //
690  // is not safe, because *P may only be valid to access if 'c' is true.
691  //
692  // It is safe to promote P if all uses are direct load/stores and if at
693  // least one is guaranteed to be executed.
694  bool GuaranteedToExecute = false;
695
696  SmallVector<Instruction*, 64> LoopUses;
697  SmallPtrSet<Value*, 4> PointerMustAliases;
698
699  // We start with an alignment of one and try to find instructions that allow
700  // us to prove better alignment.
701  unsigned Alignment = 1;
702
703  // Check that all of the pointers in the alias set have the same type.  We
704  // cannot (yet) promote a memory location that is loaded and stored in
705  // different sizes.
706  for (AliasSet::iterator ASI = AS.begin(), E = AS.end(); ASI != E; ++ASI) {
707    Value *ASIV = ASI->getValue();
708    PointerMustAliases.insert(ASIV);
709
710    // Check that all of the pointers in the alias set have the same type.  We
711    // cannot (yet) promote a memory location that is loaded and stored in
712    // different sizes.
713    if (SomePtr->getType() != ASIV->getType())
714      return;
715
716    for (Value::use_iterator UI = ASIV->use_begin(), UE = ASIV->use_end();
717         UI != UE; ++UI) {
718      // Ignore instructions that are outside the loop.
719      Instruction *Use = dyn_cast<Instruction>(*UI);
720      if (!Use || !CurLoop->contains(Use))
721        continue;
722
723      // If there is an non-load/store instruction in the loop, we can't promote
724      // it.
725      if (LoadInst *load = dyn_cast<LoadInst>(Use)) {
726        assert(!load->isVolatile() && "AST broken");
727        if (!load->isSimple())
728          return;
729      } else if (StoreInst *store = dyn_cast<StoreInst>(Use)) {
730        // Stores *of* the pointer are not interesting, only stores *to* the
731        // pointer.
732        if (Use->getOperand(1) != ASIV)
733          continue;
734        assert(!store->isVolatile() && "AST broken");
735        if (!store->isSimple())
736          return;
737
738        // Note that we only check GuaranteedToExecute inside the store case
739        // so that we do not introduce stores where they did not exist before
740        // (which would break the LLVM concurrency model).
741
742        // If the alignment of this instruction allows us to specify a more
743        // restrictive (and performant) alignment and if we are sure this
744        // instruction will be executed, update the alignment.
745        // Larger is better, with the exception of 0 being the best alignment.
746        unsigned InstAlignment = store->getAlignment();
747        if ((InstAlignment > Alignment || InstAlignment == 0)
748            && (Alignment != 0))
749          if (isGuaranteedToExecute(*Use)) {
750            GuaranteedToExecute = true;
751            Alignment = InstAlignment;
752          }
753
754        if (!GuaranteedToExecute)
755          GuaranteedToExecute = isGuaranteedToExecute(*Use);
756
757      } else
758        return; // Not a load or store.
759
760      LoopUses.push_back(Use);
761    }
762  }
763
764  // If there isn't a guaranteed-to-execute instruction, we can't promote.
765  if (!GuaranteedToExecute)
766    return;
767
768  // Otherwise, this is safe to promote, lets do it!
769  DEBUG(dbgs() << "LICM: Promoting value stored to in loop: " <<*SomePtr<<'\n');
770  Changed = true;
771  ++NumPromoted;
772
773  // Grab a debug location for the inserted loads/stores; given that the
774  // inserted loads/stores have little relation to the original loads/stores,
775  // this code just arbitrarily picks a location from one, since any debug
776  // location is better than none.
777  DebugLoc DL = LoopUses[0]->getDebugLoc();
778
779  SmallVector<BasicBlock*, 8> ExitBlocks;
780  CurLoop->getUniqueExitBlocks(ExitBlocks);
781
782  // We use the SSAUpdater interface to insert phi nodes as required.
783  SmallVector<PHINode*, 16> NewPHIs;
784  SSAUpdater SSA(&NewPHIs);
785  LoopPromoter Promoter(SomePtr, LoopUses, SSA, PointerMustAliases, ExitBlocks,
786                        *CurAST, DL, Alignment);
787
788  // Set up the preheader to have a definition of the value.  It is the live-out
789  // value from the preheader that uses in the loop will use.
790  LoadInst *PreheaderLoad =
791    new LoadInst(SomePtr, SomePtr->getName()+".promoted",
792                 Preheader->getTerminator());
793  PreheaderLoad->setAlignment(Alignment);
794  PreheaderLoad->setDebugLoc(DL);
795  SSA.AddAvailableValue(Preheader, PreheaderLoad);
796
797  // Rewrite all the loads in the loop and remember all the definitions from
798  // stores in the loop.
799  Promoter.run(LoopUses);
800
801  // If the SSAUpdater didn't use the load in the preheader, just zap it now.
802  if (PreheaderLoad->use_empty())
803    PreheaderLoad->eraseFromParent();
804}
805
806
807/// cloneBasicBlockAnalysis - Simple Analysis hook. Clone alias set info.
808void LICM::cloneBasicBlockAnalysis(BasicBlock *From, BasicBlock *To, Loop *L) {
809  AliasSetTracker *AST = LoopToAliasSetMap.lookup(L);
810  if (!AST)
811    return;
812
813  AST->copyValue(From, To);
814}
815
816/// deleteAnalysisValue - Simple Analysis hook. Delete value V from alias
817/// set.
818void LICM::deleteAnalysisValue(Value *V, Loop *L) {
819  AliasSetTracker *AST = LoopToAliasSetMap.lookup(L);
820  if (!AST)
821    return;
822
823  AST->deleteValue(V);
824}
825