1 //===- LoopSimplify.cpp - Loop Canonicalization 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 several transformations to transform natural loops into a
11 // simpler form, which makes subsequent analyses and transformations simpler and
12 // more effective.
13 //
14 // Loop pre-header insertion guarantees that there is a single, non-critical
15 // entry edge from outside of the loop to the loop header. This simplifies a
16 // number of analyses and transformations, such as LICM.
17 //
18 // Loop exit-block insertion guarantees that all exit blocks from the loop
19 // (blocks which are outside of the loop that have predecessors inside of the
20 // loop) only have predecessors from inside of the loop (and are thus dominated
21 // by the loop header). This simplifies transformations such as store-sinking
22 // that are built into LICM.
23 //
24 // This pass also guarantees that loops will have exactly one backedge.
25 //
26 // Indirectbr instructions introduce several complications. If the loop
27 // contains or is entered by an indirectbr instruction, it may not be possible
28 // to transform the loop and make these guarantees. Client code should check
29 // that these conditions are true before relying on them.
30 //
31 // Note that the simplifycfg pass will clean up blocks which are split out but
32 // end up being unnecessary, so usage of this pass should not pessimize
33 // generated code.
34 //
35 // This pass obviously modifies the CFG, but updates loop information and
36 // dominator information.
37 //
38 //===----------------------------------------------------------------------===//
39
40 #include "llvm/Transforms/Scalar.h"
41 #include "llvm/ADT/DepthFirstIterator.h"
42 #include "llvm/ADT/SetOperations.h"
43 #include "llvm/ADT/SetVector.h"
44 #include "llvm/ADT/SmallVector.h"
45 #include "llvm/ADT/Statistic.h"
46 #include "llvm/Analysis/AliasAnalysis.h"
47 #include "llvm/Analysis/DependenceAnalysis.h"
48 #include "llvm/Analysis/InstructionSimplify.h"
49 #include "llvm/Analysis/LoopInfo.h"
50 #include "llvm/Analysis/ScalarEvolution.h"
51 #include "llvm/IR/CFG.h"
52 #include "llvm/IR/Constants.h"
53 #include "llvm/IR/DataLayout.h"
54 #include "llvm/IR/Dominators.h"
55 #include "llvm/IR/Function.h"
56 #include "llvm/IR/Instructions.h"
57 #include "llvm/IR/IntrinsicInst.h"
58 #include "llvm/IR/LLVMContext.h"
59 #include "llvm/IR/Type.h"
60 #include "llvm/Support/Debug.h"
61 #include "llvm/Transforms/Utils/BasicBlockUtils.h"
62 #include "llvm/Transforms/Utils/Local.h"
63 #include "llvm/Transforms/Utils/LoopUtils.h"
64 using namespace llvm;
65
66 #define DEBUG_TYPE "loop-simplify"
67
68 STATISTIC(NumInserted, "Number of pre-header or exit blocks inserted");
69 STATISTIC(NumNested , "Number of nested loops split out");
70
71 // If the block isn't already, move the new block to right after some 'outside
72 // block' block. This prevents the preheader from being placed inside the loop
73 // body, e.g. when the loop hasn't been rotated.
placeSplitBlockCarefully(BasicBlock * NewBB,SmallVectorImpl<BasicBlock * > & SplitPreds,Loop * L)74 static void placeSplitBlockCarefully(BasicBlock *NewBB,
75 SmallVectorImpl<BasicBlock *> &SplitPreds,
76 Loop *L) {
77 // Check to see if NewBB is already well placed.
78 Function::iterator BBI = NewBB; --BBI;
79 for (unsigned i = 0, e = SplitPreds.size(); i != e; ++i) {
80 if (&*BBI == SplitPreds[i])
81 return;
82 }
83
84 // If it isn't already after an outside block, move it after one. This is
85 // always good as it makes the uncond branch from the outside block into a
86 // fall-through.
87
88 // Figure out *which* outside block to put this after. Prefer an outside
89 // block that neighbors a BB actually in the loop.
90 BasicBlock *FoundBB = nullptr;
91 for (unsigned i = 0, e = SplitPreds.size(); i != e; ++i) {
92 Function::iterator BBI = SplitPreds[i];
93 if (++BBI != NewBB->getParent()->end() &&
94 L->contains(BBI)) {
95 FoundBB = SplitPreds[i];
96 break;
97 }
98 }
99
100 // If our heuristic for a *good* bb to place this after doesn't find
101 // anything, just pick something. It's likely better than leaving it within
102 // the loop.
103 if (!FoundBB)
104 FoundBB = SplitPreds[0];
105 NewBB->moveAfter(FoundBB);
106 }
107
108 /// InsertPreheaderForLoop - Once we discover that a loop doesn't have a
109 /// preheader, this method is called to insert one. This method has two phases:
110 /// preheader insertion and analysis updating.
111 ///
InsertPreheaderForLoop(Loop * L,Pass * PP)112 BasicBlock *llvm::InsertPreheaderForLoop(Loop *L, Pass *PP) {
113 BasicBlock *Header = L->getHeader();
114
115 // Compute the set of predecessors of the loop that are not in the loop.
116 SmallVector<BasicBlock*, 8> OutsideBlocks;
117 for (pred_iterator PI = pred_begin(Header), PE = pred_end(Header);
118 PI != PE; ++PI) {
119 BasicBlock *P = *PI;
120 if (!L->contains(P)) { // Coming in from outside the loop?
121 // If the loop is branched to from an indirect branch, we won't
122 // be able to fully transform the loop, because it prohibits
123 // edge splitting.
124 if (isa<IndirectBrInst>(P->getTerminator())) return nullptr;
125
126 // Keep track of it.
127 OutsideBlocks.push_back(P);
128 }
129 }
130
131 // Split out the loop pre-header.
132 BasicBlock *PreheaderBB;
133 if (!Header->isLandingPad()) {
134 PreheaderBB = SplitBlockPredecessors(Header, OutsideBlocks, ".preheader",
135 PP);
136 } else {
137 SmallVector<BasicBlock*, 2> NewBBs;
138 SplitLandingPadPredecessors(Header, OutsideBlocks, ".preheader",
139 ".split-lp", PP, NewBBs);
140 PreheaderBB = NewBBs[0];
141 }
142
143 PreheaderBB->getTerminator()->setDebugLoc(
144 Header->getFirstNonPHI()->getDebugLoc());
145 DEBUG(dbgs() << "LoopSimplify: Creating pre-header "
146 << PreheaderBB->getName() << "\n");
147
148 // Make sure that NewBB is put someplace intelligent, which doesn't mess up
149 // code layout too horribly.
150 placeSplitBlockCarefully(PreheaderBB, OutsideBlocks, L);
151
152 return PreheaderBB;
153 }
154
155 /// \brief Ensure that the loop preheader dominates all exit blocks.
156 ///
157 /// This method is used to split exit blocks that have predecessors outside of
158 /// the loop.
rewriteLoopExitBlock(Loop * L,BasicBlock * Exit,Pass * PP)159 static BasicBlock *rewriteLoopExitBlock(Loop *L, BasicBlock *Exit, Pass *PP) {
160 SmallVector<BasicBlock*, 8> LoopBlocks;
161 for (pred_iterator I = pred_begin(Exit), E = pred_end(Exit); I != E; ++I) {
162 BasicBlock *P = *I;
163 if (L->contains(P)) {
164 // Don't do this if the loop is exited via an indirect branch.
165 if (isa<IndirectBrInst>(P->getTerminator())) return nullptr;
166
167 LoopBlocks.push_back(P);
168 }
169 }
170
171 assert(!LoopBlocks.empty() && "No edges coming in from outside the loop?");
172 BasicBlock *NewExitBB = nullptr;
173
174 if (Exit->isLandingPad()) {
175 SmallVector<BasicBlock*, 2> NewBBs;
176 SplitLandingPadPredecessors(Exit, ArrayRef<BasicBlock*>(&LoopBlocks[0],
177 LoopBlocks.size()),
178 ".loopexit", ".nonloopexit",
179 PP, NewBBs);
180 NewExitBB = NewBBs[0];
181 } else {
182 NewExitBB = SplitBlockPredecessors(Exit, LoopBlocks, ".loopexit", PP);
183 }
184
185 DEBUG(dbgs() << "LoopSimplify: Creating dedicated exit block "
186 << NewExitBB->getName() << "\n");
187 return NewExitBB;
188 }
189
190 /// Add the specified block, and all of its predecessors, to the specified set,
191 /// if it's not already in there. Stop predecessor traversal when we reach
192 /// StopBlock.
addBlockAndPredsToSet(BasicBlock * InputBB,BasicBlock * StopBlock,std::set<BasicBlock * > & Blocks)193 static void addBlockAndPredsToSet(BasicBlock *InputBB, BasicBlock *StopBlock,
194 std::set<BasicBlock*> &Blocks) {
195 SmallVector<BasicBlock *, 8> Worklist;
196 Worklist.push_back(InputBB);
197 do {
198 BasicBlock *BB = Worklist.pop_back_val();
199 if (Blocks.insert(BB).second && BB != StopBlock)
200 // If BB is not already processed and it is not a stop block then
201 // insert its predecessor in the work list
202 for (pred_iterator I = pred_begin(BB), E = pred_end(BB); I != E; ++I) {
203 BasicBlock *WBB = *I;
204 Worklist.push_back(WBB);
205 }
206 } while (!Worklist.empty());
207 }
208
209 /// \brief The first part of loop-nestification is to find a PHI node that tells
210 /// us how to partition the loops.
findPHIToPartitionLoops(Loop * L,AliasAnalysis * AA,DominatorTree * DT)211 static PHINode *findPHIToPartitionLoops(Loop *L, AliasAnalysis *AA,
212 DominatorTree *DT) {
213 for (BasicBlock::iterator I = L->getHeader()->begin(); isa<PHINode>(I); ) {
214 PHINode *PN = cast<PHINode>(I);
215 ++I;
216 if (Value *V = SimplifyInstruction(PN, nullptr, nullptr, DT)) {
217 // This is a degenerate PHI already, don't modify it!
218 PN->replaceAllUsesWith(V);
219 if (AA) AA->deleteValue(PN);
220 PN->eraseFromParent();
221 continue;
222 }
223
224 // Scan this PHI node looking for a use of the PHI node by itself.
225 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
226 if (PN->getIncomingValue(i) == PN &&
227 L->contains(PN->getIncomingBlock(i)))
228 // We found something tasty to remove.
229 return PN;
230 }
231 return nullptr;
232 }
233
234 /// \brief If this loop has multiple backedges, try to pull one of them out into
235 /// a nested loop.
236 ///
237 /// This is important for code that looks like
238 /// this:
239 ///
240 /// Loop:
241 /// ...
242 /// br cond, Loop, Next
243 /// ...
244 /// br cond2, Loop, Out
245 ///
246 /// To identify this common case, we look at the PHI nodes in the header of the
247 /// loop. PHI nodes with unchanging values on one backedge correspond to values
248 /// that change in the "outer" loop, but not in the "inner" loop.
249 ///
250 /// If we are able to separate out a loop, return the new outer loop that was
251 /// created.
252 ///
separateNestedLoop(Loop * L,BasicBlock * Preheader,AliasAnalysis * AA,DominatorTree * DT,LoopInfo * LI,ScalarEvolution * SE,Pass * PP)253 static Loop *separateNestedLoop(Loop *L, BasicBlock *Preheader,
254 AliasAnalysis *AA, DominatorTree *DT,
255 LoopInfo *LI, ScalarEvolution *SE, Pass *PP) {
256 // Don't try to separate loops without a preheader.
257 if (!Preheader)
258 return nullptr;
259
260 // The header is not a landing pad; preheader insertion should ensure this.
261 assert(!L->getHeader()->isLandingPad() &&
262 "Can't insert backedge to landing pad");
263
264 PHINode *PN = findPHIToPartitionLoops(L, AA, DT);
265 if (!PN) return nullptr; // No known way to partition.
266
267 // Pull out all predecessors that have varying values in the loop. This
268 // handles the case when a PHI node has multiple instances of itself as
269 // arguments.
270 SmallVector<BasicBlock*, 8> OuterLoopPreds;
271 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
272 if (PN->getIncomingValue(i) != PN ||
273 !L->contains(PN->getIncomingBlock(i))) {
274 // We can't split indirectbr edges.
275 if (isa<IndirectBrInst>(PN->getIncomingBlock(i)->getTerminator()))
276 return nullptr;
277 OuterLoopPreds.push_back(PN->getIncomingBlock(i));
278 }
279 }
280 DEBUG(dbgs() << "LoopSimplify: Splitting out a new outer loop\n");
281
282 // If ScalarEvolution is around and knows anything about values in
283 // this loop, tell it to forget them, because we're about to
284 // substantially change it.
285 if (SE)
286 SE->forgetLoop(L);
287
288 BasicBlock *Header = L->getHeader();
289 BasicBlock *NewBB =
290 SplitBlockPredecessors(Header, OuterLoopPreds, ".outer", PP);
291
292 // Make sure that NewBB is put someplace intelligent, which doesn't mess up
293 // code layout too horribly.
294 placeSplitBlockCarefully(NewBB, OuterLoopPreds, L);
295
296 // Create the new outer loop.
297 Loop *NewOuter = new Loop();
298
299 // Change the parent loop to use the outer loop as its child now.
300 if (Loop *Parent = L->getParentLoop())
301 Parent->replaceChildLoopWith(L, NewOuter);
302 else
303 LI->changeTopLevelLoop(L, NewOuter);
304
305 // L is now a subloop of our outer loop.
306 NewOuter->addChildLoop(L);
307
308 for (Loop::block_iterator I = L->block_begin(), E = L->block_end();
309 I != E; ++I)
310 NewOuter->addBlockEntry(*I);
311
312 // Now reset the header in L, which had been moved by
313 // SplitBlockPredecessors for the outer loop.
314 L->moveToHeader(Header);
315
316 // Determine which blocks should stay in L and which should be moved out to
317 // the Outer loop now.
318 std::set<BasicBlock*> BlocksInL;
319 for (pred_iterator PI=pred_begin(Header), E = pred_end(Header); PI!=E; ++PI) {
320 BasicBlock *P = *PI;
321 if (DT->dominates(Header, P))
322 addBlockAndPredsToSet(P, Header, BlocksInL);
323 }
324
325 // Scan all of the loop children of L, moving them to OuterLoop if they are
326 // not part of the inner loop.
327 const std::vector<Loop*> &SubLoops = L->getSubLoops();
328 for (size_t I = 0; I != SubLoops.size(); )
329 if (BlocksInL.count(SubLoops[I]->getHeader()))
330 ++I; // Loop remains in L
331 else
332 NewOuter->addChildLoop(L->removeChildLoop(SubLoops.begin() + I));
333
334 // Now that we know which blocks are in L and which need to be moved to
335 // OuterLoop, move any blocks that need it.
336 for (unsigned i = 0; i != L->getBlocks().size(); ++i) {
337 BasicBlock *BB = L->getBlocks()[i];
338 if (!BlocksInL.count(BB)) {
339 // Move this block to the parent, updating the exit blocks sets
340 L->removeBlockFromLoop(BB);
341 if ((*LI)[BB] == L)
342 LI->changeLoopFor(BB, NewOuter);
343 --i;
344 }
345 }
346
347 return NewOuter;
348 }
349
350 /// \brief This method is called when the specified loop has more than one
351 /// backedge in it.
352 ///
353 /// If this occurs, revector all of these backedges to target a new basic block
354 /// and have that block branch to the loop header. This ensures that loops
355 /// have exactly one backedge.
insertUniqueBackedgeBlock(Loop * L,BasicBlock * Preheader,AliasAnalysis * AA,DominatorTree * DT,LoopInfo * LI)356 static BasicBlock *insertUniqueBackedgeBlock(Loop *L, BasicBlock *Preheader,
357 AliasAnalysis *AA,
358 DominatorTree *DT, LoopInfo *LI) {
359 assert(L->getNumBackEdges() > 1 && "Must have > 1 backedge!");
360
361 // Get information about the loop
362 BasicBlock *Header = L->getHeader();
363 Function *F = Header->getParent();
364
365 // Unique backedge insertion currently depends on having a preheader.
366 if (!Preheader)
367 return nullptr;
368
369 // The header is not a landing pad; preheader insertion should ensure this.
370 assert(!Header->isLandingPad() && "Can't insert backedge to landing pad");
371
372 // Figure out which basic blocks contain back-edges to the loop header.
373 std::vector<BasicBlock*> BackedgeBlocks;
374 for (pred_iterator I = pred_begin(Header), E = pred_end(Header); I != E; ++I){
375 BasicBlock *P = *I;
376
377 // Indirectbr edges cannot be split, so we must fail if we find one.
378 if (isa<IndirectBrInst>(P->getTerminator()))
379 return nullptr;
380
381 if (P != Preheader) BackedgeBlocks.push_back(P);
382 }
383
384 // Create and insert the new backedge block...
385 BasicBlock *BEBlock = BasicBlock::Create(Header->getContext(),
386 Header->getName()+".backedge", F);
387 BranchInst *BETerminator = BranchInst::Create(Header, BEBlock);
388
389 DEBUG(dbgs() << "LoopSimplify: Inserting unique backedge block "
390 << BEBlock->getName() << "\n");
391
392 // Move the new backedge block to right after the last backedge block.
393 Function::iterator InsertPos = BackedgeBlocks.back(); ++InsertPos;
394 F->getBasicBlockList().splice(InsertPos, F->getBasicBlockList(), BEBlock);
395
396 // Now that the block has been inserted into the function, create PHI nodes in
397 // the backedge block which correspond to any PHI nodes in the header block.
398 for (BasicBlock::iterator I = Header->begin(); isa<PHINode>(I); ++I) {
399 PHINode *PN = cast<PHINode>(I);
400 PHINode *NewPN = PHINode::Create(PN->getType(), BackedgeBlocks.size(),
401 PN->getName()+".be", BETerminator);
402 if (AA) AA->copyValue(PN, NewPN);
403
404 // Loop over the PHI node, moving all entries except the one for the
405 // preheader over to the new PHI node.
406 unsigned PreheaderIdx = ~0U;
407 bool HasUniqueIncomingValue = true;
408 Value *UniqueValue = nullptr;
409 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
410 BasicBlock *IBB = PN->getIncomingBlock(i);
411 Value *IV = PN->getIncomingValue(i);
412 if (IBB == Preheader) {
413 PreheaderIdx = i;
414 } else {
415 NewPN->addIncoming(IV, IBB);
416 if (HasUniqueIncomingValue) {
417 if (!UniqueValue)
418 UniqueValue = IV;
419 else if (UniqueValue != IV)
420 HasUniqueIncomingValue = false;
421 }
422 }
423 }
424
425 // Delete all of the incoming values from the old PN except the preheader's
426 assert(PreheaderIdx != ~0U && "PHI has no preheader entry??");
427 if (PreheaderIdx != 0) {
428 PN->setIncomingValue(0, PN->getIncomingValue(PreheaderIdx));
429 PN->setIncomingBlock(0, PN->getIncomingBlock(PreheaderIdx));
430 }
431 // Nuke all entries except the zero'th.
432 for (unsigned i = 0, e = PN->getNumIncomingValues()-1; i != e; ++i)
433 PN->removeIncomingValue(e-i, false);
434
435 // Finally, add the newly constructed PHI node as the entry for the BEBlock.
436 PN->addIncoming(NewPN, BEBlock);
437
438 // As an optimization, if all incoming values in the new PhiNode (which is a
439 // subset of the incoming values of the old PHI node) have the same value,
440 // eliminate the PHI Node.
441 if (HasUniqueIncomingValue) {
442 NewPN->replaceAllUsesWith(UniqueValue);
443 if (AA) AA->deleteValue(NewPN);
444 BEBlock->getInstList().erase(NewPN);
445 }
446 }
447
448 // Now that all of the PHI nodes have been inserted and adjusted, modify the
449 // backedge blocks to just to the BEBlock instead of the header.
450 for (unsigned i = 0, e = BackedgeBlocks.size(); i != e; ++i) {
451 TerminatorInst *TI = BackedgeBlocks[i]->getTerminator();
452 for (unsigned Op = 0, e = TI->getNumSuccessors(); Op != e; ++Op)
453 if (TI->getSuccessor(Op) == Header)
454 TI->setSuccessor(Op, BEBlock);
455 }
456
457 //===--- Update all analyses which we must preserve now -----------------===//
458
459 // Update Loop Information - we know that this block is now in the current
460 // loop and all parent loops.
461 L->addBasicBlockToLoop(BEBlock, LI->getBase());
462
463 // Update dominator information
464 DT->splitBlock(BEBlock);
465
466 return BEBlock;
467 }
468
469 /// \brief Simplify one loop and queue further loops for simplification.
470 ///
471 /// FIXME: Currently this accepts both lots of analyses that it uses and a raw
472 /// Pass pointer. The Pass pointer is used by numerous utilities to update
473 /// specific analyses. Rather than a pass it would be much cleaner and more
474 /// explicit if they accepted the analysis directly and then updated it.
simplifyOneLoop(Loop * L,SmallVectorImpl<Loop * > & Worklist,AliasAnalysis * AA,DominatorTree * DT,LoopInfo * LI,ScalarEvolution * SE,Pass * PP,const DataLayout * DL)475 static bool simplifyOneLoop(Loop *L, SmallVectorImpl<Loop *> &Worklist,
476 AliasAnalysis *AA, DominatorTree *DT, LoopInfo *LI,
477 ScalarEvolution *SE, Pass *PP,
478 const DataLayout *DL) {
479 bool Changed = false;
480 ReprocessLoop:
481
482 // Check to see that no blocks (other than the header) in this loop have
483 // predecessors that are not in the loop. This is not valid for natural
484 // loops, but can occur if the blocks are unreachable. Since they are
485 // unreachable we can just shamelessly delete those CFG edges!
486 for (Loop::block_iterator BB = L->block_begin(), E = L->block_end();
487 BB != E; ++BB) {
488 if (*BB == L->getHeader()) continue;
489
490 SmallPtrSet<BasicBlock*, 4> BadPreds;
491 for (pred_iterator PI = pred_begin(*BB),
492 PE = pred_end(*BB); PI != PE; ++PI) {
493 BasicBlock *P = *PI;
494 if (!L->contains(P))
495 BadPreds.insert(P);
496 }
497
498 // Delete each unique out-of-loop (and thus dead) predecessor.
499 for (SmallPtrSet<BasicBlock*, 4>::iterator I = BadPreds.begin(),
500 E = BadPreds.end(); I != E; ++I) {
501
502 DEBUG(dbgs() << "LoopSimplify: Deleting edge from dead predecessor "
503 << (*I)->getName() << "\n");
504
505 // Inform each successor of each dead pred.
506 for (succ_iterator SI = succ_begin(*I), SE = succ_end(*I); SI != SE; ++SI)
507 (*SI)->removePredecessor(*I);
508 // Zap the dead pred's terminator and replace it with unreachable.
509 TerminatorInst *TI = (*I)->getTerminator();
510 TI->replaceAllUsesWith(UndefValue::get(TI->getType()));
511 (*I)->getTerminator()->eraseFromParent();
512 new UnreachableInst((*I)->getContext(), *I);
513 Changed = true;
514 }
515 }
516
517 // If there are exiting blocks with branches on undef, resolve the undef in
518 // the direction which will exit the loop. This will help simplify loop
519 // trip count computations.
520 SmallVector<BasicBlock*, 8> ExitingBlocks;
521 L->getExitingBlocks(ExitingBlocks);
522 for (SmallVectorImpl<BasicBlock *>::iterator I = ExitingBlocks.begin(),
523 E = ExitingBlocks.end(); I != E; ++I)
524 if (BranchInst *BI = dyn_cast<BranchInst>((*I)->getTerminator()))
525 if (BI->isConditional()) {
526 if (UndefValue *Cond = dyn_cast<UndefValue>(BI->getCondition())) {
527
528 DEBUG(dbgs() << "LoopSimplify: Resolving \"br i1 undef\" to exit in "
529 << (*I)->getName() << "\n");
530
531 BI->setCondition(ConstantInt::get(Cond->getType(),
532 !L->contains(BI->getSuccessor(0))));
533
534 // This may make the loop analyzable, force SCEV recomputation.
535 if (SE)
536 SE->forgetLoop(L);
537
538 Changed = true;
539 }
540 }
541
542 // Does the loop already have a preheader? If so, don't insert one.
543 BasicBlock *Preheader = L->getLoopPreheader();
544 if (!Preheader) {
545 Preheader = InsertPreheaderForLoop(L, PP);
546 if (Preheader) {
547 ++NumInserted;
548 Changed = true;
549 }
550 }
551
552 // Next, check to make sure that all exit nodes of the loop only have
553 // predecessors that are inside of the loop. This check guarantees that the
554 // loop preheader/header will dominate the exit blocks. If the exit block has
555 // predecessors from outside of the loop, split the edge now.
556 SmallVector<BasicBlock*, 8> ExitBlocks;
557 L->getExitBlocks(ExitBlocks);
558
559 SmallSetVector<BasicBlock *, 8> ExitBlockSet(ExitBlocks.begin(),
560 ExitBlocks.end());
561 for (SmallSetVector<BasicBlock *, 8>::iterator I = ExitBlockSet.begin(),
562 E = ExitBlockSet.end(); I != E; ++I) {
563 BasicBlock *ExitBlock = *I;
564 for (pred_iterator PI = pred_begin(ExitBlock), PE = pred_end(ExitBlock);
565 PI != PE; ++PI)
566 // Must be exactly this loop: no subloops, parent loops, or non-loop preds
567 // allowed.
568 if (!L->contains(*PI)) {
569 if (rewriteLoopExitBlock(L, ExitBlock, PP)) {
570 ++NumInserted;
571 Changed = true;
572 }
573 break;
574 }
575 }
576
577 // If the header has more than two predecessors at this point (from the
578 // preheader and from multiple backedges), we must adjust the loop.
579 BasicBlock *LoopLatch = L->getLoopLatch();
580 if (!LoopLatch) {
581 // If this is really a nested loop, rip it out into a child loop. Don't do
582 // this for loops with a giant number of backedges, just factor them into a
583 // common backedge instead.
584 if (L->getNumBackEdges() < 8) {
585 if (Loop *OuterL = separateNestedLoop(L, Preheader, AA, DT, LI, SE, PP)) {
586 ++NumNested;
587 // Enqueue the outer loop as it should be processed next in our
588 // depth-first nest walk.
589 Worklist.push_back(OuterL);
590
591 // This is a big restructuring change, reprocess the whole loop.
592 Changed = true;
593 // GCC doesn't tail recursion eliminate this.
594 // FIXME: It isn't clear we can't rely on LLVM to TRE this.
595 goto ReprocessLoop;
596 }
597 }
598
599 // If we either couldn't, or didn't want to, identify nesting of the loops,
600 // insert a new block that all backedges target, then make it jump to the
601 // loop header.
602 LoopLatch = insertUniqueBackedgeBlock(L, Preheader, AA, DT, LI);
603 if (LoopLatch) {
604 ++NumInserted;
605 Changed = true;
606 }
607 }
608
609 // Scan over the PHI nodes in the loop header. Since they now have only two
610 // incoming values (the loop is canonicalized), we may have simplified the PHI
611 // down to 'X = phi [X, Y]', which should be replaced with 'Y'.
612 PHINode *PN;
613 for (BasicBlock::iterator I = L->getHeader()->begin();
614 (PN = dyn_cast<PHINode>(I++)); )
615 if (Value *V = SimplifyInstruction(PN, nullptr, nullptr, DT)) {
616 if (AA) AA->deleteValue(PN);
617 if (SE) SE->forgetValue(PN);
618 PN->replaceAllUsesWith(V);
619 PN->eraseFromParent();
620 }
621
622 // If this loop has multiple exits and the exits all go to the same
623 // block, attempt to merge the exits. This helps several passes, such
624 // as LoopRotation, which do not support loops with multiple exits.
625 // SimplifyCFG also does this (and this code uses the same utility
626 // function), however this code is loop-aware, where SimplifyCFG is
627 // not. That gives it the advantage of being able to hoist
628 // loop-invariant instructions out of the way to open up more
629 // opportunities, and the disadvantage of having the responsibility
630 // to preserve dominator information.
631 bool UniqueExit = true;
632 if (!ExitBlocks.empty())
633 for (unsigned i = 1, e = ExitBlocks.size(); i != e; ++i)
634 if (ExitBlocks[i] != ExitBlocks[0]) {
635 UniqueExit = false;
636 break;
637 }
638 if (UniqueExit) {
639 for (unsigned i = 0, e = ExitingBlocks.size(); i != e; ++i) {
640 BasicBlock *ExitingBlock = ExitingBlocks[i];
641 if (!ExitingBlock->getSinglePredecessor()) continue;
642 BranchInst *BI = dyn_cast<BranchInst>(ExitingBlock->getTerminator());
643 if (!BI || !BI->isConditional()) continue;
644 CmpInst *CI = dyn_cast<CmpInst>(BI->getCondition());
645 if (!CI || CI->getParent() != ExitingBlock) continue;
646
647 // Attempt to hoist out all instructions except for the
648 // comparison and the branch.
649 bool AllInvariant = true;
650 bool AnyInvariant = false;
651 for (BasicBlock::iterator I = ExitingBlock->begin(); &*I != BI; ) {
652 Instruction *Inst = I++;
653 // Skip debug info intrinsics.
654 if (isa<DbgInfoIntrinsic>(Inst))
655 continue;
656 if (Inst == CI)
657 continue;
658 if (!L->makeLoopInvariant(Inst, AnyInvariant,
659 Preheader ? Preheader->getTerminator()
660 : nullptr)) {
661 AllInvariant = false;
662 break;
663 }
664 }
665 if (AnyInvariant) {
666 Changed = true;
667 // The loop disposition of all SCEV expressions that depend on any
668 // hoisted values have also changed.
669 if (SE)
670 SE->forgetLoopDispositions(L);
671 }
672 if (!AllInvariant) continue;
673
674 // The block has now been cleared of all instructions except for
675 // a comparison and a conditional branch. SimplifyCFG may be able
676 // to fold it now.
677 if (!FoldBranchToCommonDest(BI, DL)) continue;
678
679 // Success. The block is now dead, so remove it from the loop,
680 // update the dominator tree and delete it.
681 DEBUG(dbgs() << "LoopSimplify: Eliminating exiting block "
682 << ExitingBlock->getName() << "\n");
683
684 // Notify ScalarEvolution before deleting this block. Currently assume the
685 // parent loop doesn't change (spliting edges doesn't count). If blocks,
686 // CFG edges, or other values in the parent loop change, then we need call
687 // to forgetLoop() for the parent instead.
688 if (SE)
689 SE->forgetLoop(L);
690
691 assert(pred_begin(ExitingBlock) == pred_end(ExitingBlock));
692 Changed = true;
693 LI->removeBlock(ExitingBlock);
694
695 DomTreeNode *Node = DT->getNode(ExitingBlock);
696 const std::vector<DomTreeNodeBase<BasicBlock> *> &Children =
697 Node->getChildren();
698 while (!Children.empty()) {
699 DomTreeNode *Child = Children.front();
700 DT->changeImmediateDominator(Child, Node->getIDom());
701 }
702 DT->eraseNode(ExitingBlock);
703
704 BI->getSuccessor(0)->removePredecessor(ExitingBlock);
705 BI->getSuccessor(1)->removePredecessor(ExitingBlock);
706 ExitingBlock->eraseFromParent();
707 }
708 }
709
710 return Changed;
711 }
712
simplifyLoop(Loop * L,DominatorTree * DT,LoopInfo * LI,Pass * PP,AliasAnalysis * AA,ScalarEvolution * SE,const DataLayout * DL)713 bool llvm::simplifyLoop(Loop *L, DominatorTree *DT, LoopInfo *LI, Pass *PP,
714 AliasAnalysis *AA, ScalarEvolution *SE,
715 const DataLayout *DL) {
716 bool Changed = false;
717
718 // Worklist maintains our depth-first queue of loops in this nest to process.
719 SmallVector<Loop *, 4> Worklist;
720 Worklist.push_back(L);
721
722 // Walk the worklist from front to back, pushing newly found sub loops onto
723 // the back. This will let us process loops from back to front in depth-first
724 // order. We can use this simple process because loops form a tree.
725 for (unsigned Idx = 0; Idx != Worklist.size(); ++Idx) {
726 Loop *L2 = Worklist[Idx];
727 for (Loop::iterator I = L2->begin(), E = L2->end(); I != E; ++I)
728 Worklist.push_back(*I);
729 }
730
731 while (!Worklist.empty())
732 Changed |= simplifyOneLoop(Worklist.pop_back_val(), Worklist, AA, DT, LI,
733 SE, PP, DL);
734
735 return Changed;
736 }
737
738 namespace {
739 struct LoopSimplify : public FunctionPass {
740 static char ID; // Pass identification, replacement for typeid
LoopSimplify__anonfd6269ce0111::LoopSimplify741 LoopSimplify() : FunctionPass(ID) {
742 initializeLoopSimplifyPass(*PassRegistry::getPassRegistry());
743 }
744
745 // AA - If we have an alias analysis object to update, this is it, otherwise
746 // this is null.
747 AliasAnalysis *AA;
748 DominatorTree *DT;
749 LoopInfo *LI;
750 ScalarEvolution *SE;
751 const DataLayout *DL;
752
753 bool runOnFunction(Function &F) override;
754
getAnalysisUsage__anonfd6269ce0111::LoopSimplify755 void getAnalysisUsage(AnalysisUsage &AU) const override {
756 // We need loop information to identify the loops...
757 AU.addRequired<DominatorTreeWrapperPass>();
758 AU.addPreserved<DominatorTreeWrapperPass>();
759
760 AU.addRequired<LoopInfo>();
761 AU.addPreserved<LoopInfo>();
762
763 AU.addPreserved<AliasAnalysis>();
764 AU.addPreserved<ScalarEvolution>();
765 AU.addPreserved<DependenceAnalysis>();
766 AU.addPreservedID(BreakCriticalEdgesID); // No critical edges added.
767 }
768
769 /// verifyAnalysis() - Verify LoopSimplifyForm's guarantees.
770 void verifyAnalysis() const override;
771 };
772 }
773
774 char LoopSimplify::ID = 0;
775 INITIALIZE_PASS_BEGIN(LoopSimplify, "loop-simplify",
776 "Canonicalize natural loops", true, false)
777 INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
778 INITIALIZE_PASS_DEPENDENCY(LoopInfo)
779 INITIALIZE_PASS_END(LoopSimplify, "loop-simplify",
780 "Canonicalize natural loops", true, false)
781
782 // Publicly exposed interface to pass...
783 char &llvm::LoopSimplifyID = LoopSimplify::ID;
createLoopSimplifyPass()784 Pass *llvm::createLoopSimplifyPass() { return new LoopSimplify(); }
785
786 /// runOnFunction - Run down all loops in the CFG (recursively, but we could do
787 /// it in any convenient order) inserting preheaders...
788 ///
runOnFunction(Function & F)789 bool LoopSimplify::runOnFunction(Function &F) {
790 bool Changed = false;
791 AA = getAnalysisIfAvailable<AliasAnalysis>();
792 LI = &getAnalysis<LoopInfo>();
793 DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
794 SE = getAnalysisIfAvailable<ScalarEvolution>();
795 DataLayoutPass *DLP = getAnalysisIfAvailable<DataLayoutPass>();
796 DL = DLP ? &DLP->getDataLayout() : nullptr;
797
798 // Simplify each loop nest in the function.
799 for (LoopInfo::iterator I = LI->begin(), E = LI->end(); I != E; ++I)
800 Changed |= simplifyLoop(*I, DT, LI, this, AA, SE, DL);
801
802 return Changed;
803 }
804
805 // FIXME: Restore this code when we re-enable verification in verifyAnalysis
806 // below.
807 #if 0
808 static void verifyLoop(Loop *L) {
809 // Verify subloops.
810 for (Loop::iterator I = L->begin(), E = L->end(); I != E; ++I)
811 verifyLoop(*I);
812
813 // It used to be possible to just assert L->isLoopSimplifyForm(), however
814 // with the introduction of indirectbr, there are now cases where it's
815 // not possible to transform a loop as necessary. We can at least check
816 // that there is an indirectbr near any time there's trouble.
817
818 // Indirectbr can interfere with preheader and unique backedge insertion.
819 if (!L->getLoopPreheader() || !L->getLoopLatch()) {
820 bool HasIndBrPred = false;
821 for (pred_iterator PI = pred_begin(L->getHeader()),
822 PE = pred_end(L->getHeader()); PI != PE; ++PI)
823 if (isa<IndirectBrInst>((*PI)->getTerminator())) {
824 HasIndBrPred = true;
825 break;
826 }
827 assert(HasIndBrPred &&
828 "LoopSimplify has no excuse for missing loop header info!");
829 (void)HasIndBrPred;
830 }
831
832 // Indirectbr can interfere with exit block canonicalization.
833 if (!L->hasDedicatedExits()) {
834 bool HasIndBrExiting = false;
835 SmallVector<BasicBlock*, 8> ExitingBlocks;
836 L->getExitingBlocks(ExitingBlocks);
837 for (unsigned i = 0, e = ExitingBlocks.size(); i != e; ++i) {
838 if (isa<IndirectBrInst>((ExitingBlocks[i])->getTerminator())) {
839 HasIndBrExiting = true;
840 break;
841 }
842 }
843
844 assert(HasIndBrExiting &&
845 "LoopSimplify has no excuse for missing exit block info!");
846 (void)HasIndBrExiting;
847 }
848 }
849 #endif
850
verifyAnalysis() const851 void LoopSimplify::verifyAnalysis() const {
852 // FIXME: This routine is being called mid-way through the loop pass manager
853 // as loop passes destroy this analysis. That's actually fine, but we have no
854 // way of expressing that here. Once all of the passes that destroy this are
855 // hoisted out of the loop pass manager we can add back verification here.
856 #if 0
857 for (LoopInfo::iterator I = LI->begin(), E = LI->end(); I != E; ++I)
858 verifyLoop(*I);
859 #endif
860 }
861