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/BasicAliasAnalysis.h"
48 #include "llvm/Analysis/AssumptionCache.h"
49 #include "llvm/Analysis/DependenceAnalysis.h"
50 #include "llvm/Analysis/GlobalsModRef.h"
51 #include "llvm/Analysis/InstructionSimplify.h"
52 #include "llvm/Analysis/LoopInfo.h"
53 #include "llvm/Analysis/ScalarEvolution.h"
54 #include "llvm/Analysis/ScalarEvolutionAliasAnalysis.h"
55 #include "llvm/IR/CFG.h"
56 #include "llvm/IR/Constants.h"
57 #include "llvm/IR/DataLayout.h"
58 #include "llvm/IR/Dominators.h"
59 #include "llvm/IR/Function.h"
60 #include "llvm/IR/Instructions.h"
61 #include "llvm/IR/IntrinsicInst.h"
62 #include "llvm/IR/LLVMContext.h"
63 #include "llvm/IR/Module.h"
64 #include "llvm/IR/Type.h"
65 #include "llvm/Support/Debug.h"
66 #include "llvm/Support/raw_ostream.h"
67 #include "llvm/Transforms/Utils/BasicBlockUtils.h"
68 #include "llvm/Transforms/Utils/Local.h"
69 #include "llvm/Transforms/Utils/LoopUtils.h"
70 using namespace llvm;
71
72 #define DEBUG_TYPE "loop-simplify"
73
74 STATISTIC(NumInserted, "Number of pre-header or exit blocks inserted");
75 STATISTIC(NumNested , "Number of nested loops split out");
76
77 // If the block isn't already, move the new block to right after some 'outside
78 // block' block. This prevents the preheader from being placed inside the loop
79 // body, e.g. when the loop hasn't been rotated.
placeSplitBlockCarefully(BasicBlock * NewBB,SmallVectorImpl<BasicBlock * > & SplitPreds,Loop * L)80 static void placeSplitBlockCarefully(BasicBlock *NewBB,
81 SmallVectorImpl<BasicBlock *> &SplitPreds,
82 Loop *L) {
83 // Check to see if NewBB is already well placed.
84 Function::iterator BBI = --NewBB->getIterator();
85 for (unsigned i = 0, e = SplitPreds.size(); i != e; ++i) {
86 if (&*BBI == SplitPreds[i])
87 return;
88 }
89
90 // If it isn't already after an outside block, move it after one. This is
91 // always good as it makes the uncond branch from the outside block into a
92 // fall-through.
93
94 // Figure out *which* outside block to put this after. Prefer an outside
95 // block that neighbors a BB actually in the loop.
96 BasicBlock *FoundBB = nullptr;
97 for (unsigned i = 0, e = SplitPreds.size(); i != e; ++i) {
98 Function::iterator BBI = SplitPreds[i]->getIterator();
99 if (++BBI != NewBB->getParent()->end() && L->contains(&*BBI)) {
100 FoundBB = SplitPreds[i];
101 break;
102 }
103 }
104
105 // If our heuristic for a *good* bb to place this after doesn't find
106 // anything, just pick something. It's likely better than leaving it within
107 // the loop.
108 if (!FoundBB)
109 FoundBB = SplitPreds[0];
110 NewBB->moveAfter(FoundBB);
111 }
112
113 /// InsertPreheaderForLoop - Once we discover that a loop doesn't have a
114 /// preheader, this method is called to insert one. This method has two phases:
115 /// preheader insertion and analysis updating.
116 ///
InsertPreheaderForLoop(Loop * L,DominatorTree * DT,LoopInfo * LI,bool PreserveLCSSA)117 BasicBlock *llvm::InsertPreheaderForLoop(Loop *L, DominatorTree *DT,
118 LoopInfo *LI, bool PreserveLCSSA) {
119 BasicBlock *Header = L->getHeader();
120
121 // Compute the set of predecessors of the loop that are not in the loop.
122 SmallVector<BasicBlock*, 8> OutsideBlocks;
123 for (pred_iterator PI = pred_begin(Header), PE = pred_end(Header);
124 PI != PE; ++PI) {
125 BasicBlock *P = *PI;
126 if (!L->contains(P)) { // Coming in from outside the loop?
127 // If the loop is branched to from an indirect branch, we won't
128 // be able to fully transform the loop, because it prohibits
129 // edge splitting.
130 if (isa<IndirectBrInst>(P->getTerminator())) return nullptr;
131
132 // Keep track of it.
133 OutsideBlocks.push_back(P);
134 }
135 }
136
137 // Split out the loop pre-header.
138 BasicBlock *PreheaderBB;
139 PreheaderBB = SplitBlockPredecessors(Header, OutsideBlocks, ".preheader", DT,
140 LI, PreserveLCSSA);
141 if (!PreheaderBB)
142 return nullptr;
143
144 DEBUG(dbgs() << "LoopSimplify: Creating pre-header "
145 << PreheaderBB->getName() << "\n");
146
147 // Make sure that NewBB is put someplace intelligent, which doesn't mess up
148 // code layout too horribly.
149 placeSplitBlockCarefully(PreheaderBB, OutsideBlocks, L);
150
151 return PreheaderBB;
152 }
153
154 /// \brief Ensure that the loop preheader dominates all exit blocks.
155 ///
156 /// This method is used to split exit blocks that have predecessors outside of
157 /// the loop.
rewriteLoopExitBlock(Loop * L,BasicBlock * Exit,DominatorTree * DT,LoopInfo * LI,bool PreserveLCSSA)158 static BasicBlock *rewriteLoopExitBlock(Loop *L, BasicBlock *Exit,
159 DominatorTree *DT, LoopInfo *LI,
160 bool PreserveLCSSA) {
161 SmallVector<BasicBlock*, 8> LoopBlocks;
162 for (pred_iterator I = pred_begin(Exit), E = pred_end(Exit); I != E; ++I) {
163 BasicBlock *P = *I;
164 if (L->contains(P)) {
165 // Don't do this if the loop is exited via an indirect branch.
166 if (isa<IndirectBrInst>(P->getTerminator())) return nullptr;
167
168 LoopBlocks.push_back(P);
169 }
170 }
171
172 assert(!LoopBlocks.empty() && "No edges coming in from outside the loop?");
173 BasicBlock *NewExitBB = nullptr;
174
175 NewExitBB = SplitBlockPredecessors(Exit, LoopBlocks, ".loopexit", DT, LI,
176 PreserveLCSSA);
177 if (!NewExitBB)
178 return nullptr;
179
180 DEBUG(dbgs() << "LoopSimplify: Creating dedicated exit block "
181 << NewExitBB->getName() << "\n");
182 return NewExitBB;
183 }
184
185 /// Add the specified block, and all of its predecessors, to the specified set,
186 /// if it's not already in there. Stop predecessor traversal when we reach
187 /// StopBlock.
addBlockAndPredsToSet(BasicBlock * InputBB,BasicBlock * StopBlock,std::set<BasicBlock * > & Blocks)188 static void addBlockAndPredsToSet(BasicBlock *InputBB, BasicBlock *StopBlock,
189 std::set<BasicBlock*> &Blocks) {
190 SmallVector<BasicBlock *, 8> Worklist;
191 Worklist.push_back(InputBB);
192 do {
193 BasicBlock *BB = Worklist.pop_back_val();
194 if (Blocks.insert(BB).second && BB != StopBlock)
195 // If BB is not already processed and it is not a stop block then
196 // insert its predecessor in the work list
197 for (pred_iterator I = pred_begin(BB), E = pred_end(BB); I != E; ++I) {
198 BasicBlock *WBB = *I;
199 Worklist.push_back(WBB);
200 }
201 } while (!Worklist.empty());
202 }
203
204 /// \brief The first part of loop-nestification is to find a PHI node that tells
205 /// us how to partition the loops.
findPHIToPartitionLoops(Loop * L,DominatorTree * DT,AssumptionCache * AC)206 static PHINode *findPHIToPartitionLoops(Loop *L, DominatorTree *DT,
207 AssumptionCache *AC) {
208 const DataLayout &DL = L->getHeader()->getModule()->getDataLayout();
209 for (BasicBlock::iterator I = L->getHeader()->begin(); isa<PHINode>(I); ) {
210 PHINode *PN = cast<PHINode>(I);
211 ++I;
212 if (Value *V = SimplifyInstruction(PN, DL, nullptr, DT, AC)) {
213 // This is a degenerate PHI already, don't modify it!
214 PN->replaceAllUsesWith(V);
215 PN->eraseFromParent();
216 continue;
217 }
218
219 // Scan this PHI node looking for a use of the PHI node by itself.
220 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
221 if (PN->getIncomingValue(i) == PN &&
222 L->contains(PN->getIncomingBlock(i)))
223 // We found something tasty to remove.
224 return PN;
225 }
226 return nullptr;
227 }
228
229 /// \brief If this loop has multiple backedges, try to pull one of them out into
230 /// a nested loop.
231 ///
232 /// This is important for code that looks like
233 /// this:
234 ///
235 /// Loop:
236 /// ...
237 /// br cond, Loop, Next
238 /// ...
239 /// br cond2, Loop, Out
240 ///
241 /// To identify this common case, we look at the PHI nodes in the header of the
242 /// loop. PHI nodes with unchanging values on one backedge correspond to values
243 /// that change in the "outer" loop, but not in the "inner" loop.
244 ///
245 /// If we are able to separate out a loop, return the new outer loop that was
246 /// created.
247 ///
separateNestedLoop(Loop * L,BasicBlock * Preheader,DominatorTree * DT,LoopInfo * LI,ScalarEvolution * SE,bool PreserveLCSSA,AssumptionCache * AC)248 static Loop *separateNestedLoop(Loop *L, BasicBlock *Preheader,
249 DominatorTree *DT, LoopInfo *LI,
250 ScalarEvolution *SE, bool PreserveLCSSA,
251 AssumptionCache *AC) {
252 // Don't try to separate loops without a preheader.
253 if (!Preheader)
254 return nullptr;
255
256 // The header is not a landing pad; preheader insertion should ensure this.
257 BasicBlock *Header = L->getHeader();
258 assert(!Header->isEHPad() && "Can't insert backedge to EH pad");
259
260 PHINode *PN = findPHIToPartitionLoops(L, DT, AC);
261 if (!PN) return nullptr; // No known way to partition.
262
263 // Pull out all predecessors that have varying values in the loop. This
264 // handles the case when a PHI node has multiple instances of itself as
265 // arguments.
266 SmallVector<BasicBlock*, 8> OuterLoopPreds;
267 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
268 if (PN->getIncomingValue(i) != PN ||
269 !L->contains(PN->getIncomingBlock(i))) {
270 // We can't split indirectbr edges.
271 if (isa<IndirectBrInst>(PN->getIncomingBlock(i)->getTerminator()))
272 return nullptr;
273 OuterLoopPreds.push_back(PN->getIncomingBlock(i));
274 }
275 }
276 DEBUG(dbgs() << "LoopSimplify: Splitting out a new outer loop\n");
277
278 // If ScalarEvolution is around and knows anything about values in
279 // this loop, tell it to forget them, because we're about to
280 // substantially change it.
281 if (SE)
282 SE->forgetLoop(L);
283
284 BasicBlock *NewBB = SplitBlockPredecessors(Header, OuterLoopPreds, ".outer",
285 DT, LI, PreserveLCSSA);
286
287 // Make sure that NewBB is put someplace intelligent, which doesn't mess up
288 // code layout too horribly.
289 placeSplitBlockCarefully(NewBB, OuterLoopPreds, L);
290
291 // Create the new outer loop.
292 Loop *NewOuter = new Loop();
293
294 // Change the parent loop to use the outer loop as its child now.
295 if (Loop *Parent = L->getParentLoop())
296 Parent->replaceChildLoopWith(L, NewOuter);
297 else
298 LI->changeTopLevelLoop(L, NewOuter);
299
300 // L is now a subloop of our outer loop.
301 NewOuter->addChildLoop(L);
302
303 for (Loop::block_iterator I = L->block_begin(), E = L->block_end();
304 I != E; ++I)
305 NewOuter->addBlockEntry(*I);
306
307 // Now reset the header in L, which had been moved by
308 // SplitBlockPredecessors for the outer loop.
309 L->moveToHeader(Header);
310
311 // Determine which blocks should stay in L and which should be moved out to
312 // the Outer loop now.
313 std::set<BasicBlock*> BlocksInL;
314 for (pred_iterator PI=pred_begin(Header), E = pred_end(Header); PI!=E; ++PI) {
315 BasicBlock *P = *PI;
316 if (DT->dominates(Header, P))
317 addBlockAndPredsToSet(P, Header, BlocksInL);
318 }
319
320 // Scan all of the loop children of L, moving them to OuterLoop if they are
321 // not part of the inner loop.
322 const std::vector<Loop*> &SubLoops = L->getSubLoops();
323 for (size_t I = 0; I != SubLoops.size(); )
324 if (BlocksInL.count(SubLoops[I]->getHeader()))
325 ++I; // Loop remains in L
326 else
327 NewOuter->addChildLoop(L->removeChildLoop(SubLoops.begin() + I));
328
329 // Now that we know which blocks are in L and which need to be moved to
330 // OuterLoop, move any blocks that need it.
331 for (unsigned i = 0; i != L->getBlocks().size(); ++i) {
332 BasicBlock *BB = L->getBlocks()[i];
333 if (!BlocksInL.count(BB)) {
334 // Move this block to the parent, updating the exit blocks sets
335 L->removeBlockFromLoop(BB);
336 if ((*LI)[BB] == L)
337 LI->changeLoopFor(BB, NewOuter);
338 --i;
339 }
340 }
341
342 return NewOuter;
343 }
344
345 /// \brief This method is called when the specified loop has more than one
346 /// backedge in it.
347 ///
348 /// If this occurs, revector all of these backedges to target a new basic block
349 /// and have that block branch to the loop header. This ensures that loops
350 /// have exactly one backedge.
insertUniqueBackedgeBlock(Loop * L,BasicBlock * Preheader,DominatorTree * DT,LoopInfo * LI)351 static BasicBlock *insertUniqueBackedgeBlock(Loop *L, BasicBlock *Preheader,
352 DominatorTree *DT, LoopInfo *LI) {
353 assert(L->getNumBackEdges() > 1 && "Must have > 1 backedge!");
354
355 // Get information about the loop
356 BasicBlock *Header = L->getHeader();
357 Function *F = Header->getParent();
358
359 // Unique backedge insertion currently depends on having a preheader.
360 if (!Preheader)
361 return nullptr;
362
363 // The header is not an EH pad; preheader insertion should ensure this.
364 assert(!Header->isEHPad() && "Can't insert backedge to EH pad");
365
366 // Figure out which basic blocks contain back-edges to the loop header.
367 std::vector<BasicBlock*> BackedgeBlocks;
368 for (pred_iterator I = pred_begin(Header), E = pred_end(Header); I != E; ++I){
369 BasicBlock *P = *I;
370
371 // Indirectbr edges cannot be split, so we must fail if we find one.
372 if (isa<IndirectBrInst>(P->getTerminator()))
373 return nullptr;
374
375 if (P != Preheader) BackedgeBlocks.push_back(P);
376 }
377
378 // Create and insert the new backedge block...
379 BasicBlock *BEBlock = BasicBlock::Create(Header->getContext(),
380 Header->getName() + ".backedge", F);
381 BranchInst *BETerminator = BranchInst::Create(Header, BEBlock);
382 BETerminator->setDebugLoc(Header->getFirstNonPHI()->getDebugLoc());
383
384 DEBUG(dbgs() << "LoopSimplify: Inserting unique backedge block "
385 << BEBlock->getName() << "\n");
386
387 // Move the new backedge block to right after the last backedge block.
388 Function::iterator InsertPos = ++BackedgeBlocks.back()->getIterator();
389 F->getBasicBlockList().splice(InsertPos, F->getBasicBlockList(), BEBlock);
390
391 // Now that the block has been inserted into the function, create PHI nodes in
392 // the backedge block which correspond to any PHI nodes in the header block.
393 for (BasicBlock::iterator I = Header->begin(); isa<PHINode>(I); ++I) {
394 PHINode *PN = cast<PHINode>(I);
395 PHINode *NewPN = PHINode::Create(PN->getType(), BackedgeBlocks.size(),
396 PN->getName()+".be", BETerminator);
397
398 // Loop over the PHI node, moving all entries except the one for the
399 // preheader over to the new PHI node.
400 unsigned PreheaderIdx = ~0U;
401 bool HasUniqueIncomingValue = true;
402 Value *UniqueValue = nullptr;
403 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
404 BasicBlock *IBB = PN->getIncomingBlock(i);
405 Value *IV = PN->getIncomingValue(i);
406 if (IBB == Preheader) {
407 PreheaderIdx = i;
408 } else {
409 NewPN->addIncoming(IV, IBB);
410 if (HasUniqueIncomingValue) {
411 if (!UniqueValue)
412 UniqueValue = IV;
413 else if (UniqueValue != IV)
414 HasUniqueIncomingValue = false;
415 }
416 }
417 }
418
419 // Delete all of the incoming values from the old PN except the preheader's
420 assert(PreheaderIdx != ~0U && "PHI has no preheader entry??");
421 if (PreheaderIdx != 0) {
422 PN->setIncomingValue(0, PN->getIncomingValue(PreheaderIdx));
423 PN->setIncomingBlock(0, PN->getIncomingBlock(PreheaderIdx));
424 }
425 // Nuke all entries except the zero'th.
426 for (unsigned i = 0, e = PN->getNumIncomingValues()-1; i != e; ++i)
427 PN->removeIncomingValue(e-i, false);
428
429 // Finally, add the newly constructed PHI node as the entry for the BEBlock.
430 PN->addIncoming(NewPN, BEBlock);
431
432 // As an optimization, if all incoming values in the new PhiNode (which is a
433 // subset of the incoming values of the old PHI node) have the same value,
434 // eliminate the PHI Node.
435 if (HasUniqueIncomingValue) {
436 NewPN->replaceAllUsesWith(UniqueValue);
437 BEBlock->getInstList().erase(NewPN);
438 }
439 }
440
441 // Now that all of the PHI nodes have been inserted and adjusted, modify the
442 // backedge blocks to just to the BEBlock instead of the header.
443 for (unsigned i = 0, e = BackedgeBlocks.size(); i != e; ++i) {
444 TerminatorInst *TI = BackedgeBlocks[i]->getTerminator();
445 for (unsigned Op = 0, e = TI->getNumSuccessors(); Op != e; ++Op)
446 if (TI->getSuccessor(Op) == Header)
447 TI->setSuccessor(Op, BEBlock);
448 }
449
450 //===--- Update all analyses which we must preserve now -----------------===//
451
452 // Update Loop Information - we know that this block is now in the current
453 // loop and all parent loops.
454 L->addBasicBlockToLoop(BEBlock, *LI);
455
456 // Update dominator information
457 DT->splitBlock(BEBlock);
458
459 return BEBlock;
460 }
461
462 /// \brief Simplify one loop and queue further loops for simplification.
simplifyOneLoop(Loop * L,SmallVectorImpl<Loop * > & Worklist,DominatorTree * DT,LoopInfo * LI,ScalarEvolution * SE,AssumptionCache * AC,bool PreserveLCSSA)463 static bool simplifyOneLoop(Loop *L, SmallVectorImpl<Loop *> &Worklist,
464 DominatorTree *DT, LoopInfo *LI,
465 ScalarEvolution *SE, AssumptionCache *AC,
466 bool PreserveLCSSA) {
467 bool Changed = false;
468 ReprocessLoop:
469
470 // Check to see that no blocks (other than the header) in this loop have
471 // predecessors that are not in the loop. This is not valid for natural
472 // loops, but can occur if the blocks are unreachable. Since they are
473 // unreachable we can just shamelessly delete those CFG edges!
474 for (Loop::block_iterator BB = L->block_begin(), E = L->block_end();
475 BB != E; ++BB) {
476 if (*BB == L->getHeader()) continue;
477
478 SmallPtrSet<BasicBlock*, 4> BadPreds;
479 for (pred_iterator PI = pred_begin(*BB),
480 PE = pred_end(*BB); PI != PE; ++PI) {
481 BasicBlock *P = *PI;
482 if (!L->contains(P))
483 BadPreds.insert(P);
484 }
485
486 // Delete each unique out-of-loop (and thus dead) predecessor.
487 for (BasicBlock *P : BadPreds) {
488
489 DEBUG(dbgs() << "LoopSimplify: Deleting edge from dead predecessor "
490 << P->getName() << "\n");
491
492 // Inform each successor of each dead pred.
493 for (succ_iterator SI = succ_begin(P), SE = succ_end(P); SI != SE; ++SI)
494 (*SI)->removePredecessor(P);
495 // Zap the dead pred's terminator and replace it with unreachable.
496 TerminatorInst *TI = P->getTerminator();
497 TI->replaceAllUsesWith(UndefValue::get(TI->getType()));
498 P->getTerminator()->eraseFromParent();
499 new UnreachableInst(P->getContext(), P);
500 Changed = true;
501 }
502 }
503
504 // If there are exiting blocks with branches on undef, resolve the undef in
505 // the direction which will exit the loop. This will help simplify loop
506 // trip count computations.
507 SmallVector<BasicBlock*, 8> ExitingBlocks;
508 L->getExitingBlocks(ExitingBlocks);
509 for (SmallVectorImpl<BasicBlock *>::iterator I = ExitingBlocks.begin(),
510 E = ExitingBlocks.end(); I != E; ++I)
511 if (BranchInst *BI = dyn_cast<BranchInst>((*I)->getTerminator()))
512 if (BI->isConditional()) {
513 if (UndefValue *Cond = dyn_cast<UndefValue>(BI->getCondition())) {
514
515 DEBUG(dbgs() << "LoopSimplify: Resolving \"br i1 undef\" to exit in "
516 << (*I)->getName() << "\n");
517
518 BI->setCondition(ConstantInt::get(Cond->getType(),
519 !L->contains(BI->getSuccessor(0))));
520
521 // This may make the loop analyzable, force SCEV recomputation.
522 if (SE)
523 SE->forgetLoop(L);
524
525 Changed = true;
526 }
527 }
528
529 // Does the loop already have a preheader? If so, don't insert one.
530 BasicBlock *Preheader = L->getLoopPreheader();
531 if (!Preheader) {
532 Preheader = InsertPreheaderForLoop(L, DT, LI, PreserveLCSSA);
533 if (Preheader) {
534 ++NumInserted;
535 Changed = true;
536 }
537 }
538
539 // Next, check to make sure that all exit nodes of the loop only have
540 // predecessors that are inside of the loop. This check guarantees that the
541 // loop preheader/header will dominate the exit blocks. If the exit block has
542 // predecessors from outside of the loop, split the edge now.
543 SmallVector<BasicBlock*, 8> ExitBlocks;
544 L->getExitBlocks(ExitBlocks);
545
546 SmallSetVector<BasicBlock *, 8> ExitBlockSet(ExitBlocks.begin(),
547 ExitBlocks.end());
548 for (SmallSetVector<BasicBlock *, 8>::iterator I = ExitBlockSet.begin(),
549 E = ExitBlockSet.end(); I != E; ++I) {
550 BasicBlock *ExitBlock = *I;
551 for (pred_iterator PI = pred_begin(ExitBlock), PE = pred_end(ExitBlock);
552 PI != PE; ++PI)
553 // Must be exactly this loop: no subloops, parent loops, or non-loop preds
554 // allowed.
555 if (!L->contains(*PI)) {
556 if (rewriteLoopExitBlock(L, ExitBlock, DT, LI, PreserveLCSSA)) {
557 ++NumInserted;
558 Changed = true;
559 }
560 break;
561 }
562 }
563
564 // If the header has more than two predecessors at this point (from the
565 // preheader and from multiple backedges), we must adjust the loop.
566 BasicBlock *LoopLatch = L->getLoopLatch();
567 if (!LoopLatch) {
568 // If this is really a nested loop, rip it out into a child loop. Don't do
569 // this for loops with a giant number of backedges, just factor them into a
570 // common backedge instead.
571 if (L->getNumBackEdges() < 8) {
572 if (Loop *OuterL =
573 separateNestedLoop(L, Preheader, DT, LI, SE, PreserveLCSSA, AC)) {
574 ++NumNested;
575 // Enqueue the outer loop as it should be processed next in our
576 // depth-first nest walk.
577 Worklist.push_back(OuterL);
578
579 // This is a big restructuring change, reprocess the whole loop.
580 Changed = true;
581 // GCC doesn't tail recursion eliminate this.
582 // FIXME: It isn't clear we can't rely on LLVM to TRE this.
583 goto ReprocessLoop;
584 }
585 }
586
587 // If we either couldn't, or didn't want to, identify nesting of the loops,
588 // insert a new block that all backedges target, then make it jump to the
589 // loop header.
590 LoopLatch = insertUniqueBackedgeBlock(L, Preheader, DT, LI);
591 if (LoopLatch) {
592 ++NumInserted;
593 Changed = true;
594 }
595 }
596
597 const DataLayout &DL = L->getHeader()->getModule()->getDataLayout();
598
599 // Scan over the PHI nodes in the loop header. Since they now have only two
600 // incoming values (the loop is canonicalized), we may have simplified the PHI
601 // down to 'X = phi [X, Y]', which should be replaced with 'Y'.
602 PHINode *PN;
603 for (BasicBlock::iterator I = L->getHeader()->begin();
604 (PN = dyn_cast<PHINode>(I++)); )
605 if (Value *V = SimplifyInstruction(PN, DL, nullptr, DT, AC)) {
606 if (SE) SE->forgetValue(PN);
607 PN->replaceAllUsesWith(V);
608 PN->eraseFromParent();
609 }
610
611 // If this loop has multiple exits and the exits all go to the same
612 // block, attempt to merge the exits. This helps several passes, such
613 // as LoopRotation, which do not support loops with multiple exits.
614 // SimplifyCFG also does this (and this code uses the same utility
615 // function), however this code is loop-aware, where SimplifyCFG is
616 // not. That gives it the advantage of being able to hoist
617 // loop-invariant instructions out of the way to open up more
618 // opportunities, and the disadvantage of having the responsibility
619 // to preserve dominator information.
620 bool UniqueExit = true;
621 if (!ExitBlocks.empty())
622 for (unsigned i = 1, e = ExitBlocks.size(); i != e; ++i)
623 if (ExitBlocks[i] != ExitBlocks[0]) {
624 UniqueExit = false;
625 break;
626 }
627 if (UniqueExit) {
628 for (unsigned i = 0, e = ExitingBlocks.size(); i != e; ++i) {
629 BasicBlock *ExitingBlock = ExitingBlocks[i];
630 if (!ExitingBlock->getSinglePredecessor()) continue;
631 BranchInst *BI = dyn_cast<BranchInst>(ExitingBlock->getTerminator());
632 if (!BI || !BI->isConditional()) continue;
633 CmpInst *CI = dyn_cast<CmpInst>(BI->getCondition());
634 if (!CI || CI->getParent() != ExitingBlock) continue;
635
636 // Attempt to hoist out all instructions except for the
637 // comparison and the branch.
638 bool AllInvariant = true;
639 bool AnyInvariant = false;
640 for (BasicBlock::iterator I = ExitingBlock->begin(); &*I != BI; ) {
641 Instruction *Inst = &*I++;
642 // Skip debug info intrinsics.
643 if (isa<DbgInfoIntrinsic>(Inst))
644 continue;
645 if (Inst == CI)
646 continue;
647 if (!L->makeLoopInvariant(Inst, AnyInvariant,
648 Preheader ? Preheader->getTerminator()
649 : nullptr)) {
650 AllInvariant = false;
651 break;
652 }
653 }
654 if (AnyInvariant) {
655 Changed = true;
656 // The loop disposition of all SCEV expressions that depend on any
657 // hoisted values have also changed.
658 if (SE)
659 SE->forgetLoopDispositions(L);
660 }
661 if (!AllInvariant) continue;
662
663 // The block has now been cleared of all instructions except for
664 // a comparison and a conditional branch. SimplifyCFG may be able
665 // to fold it now.
666 if (!FoldBranchToCommonDest(BI))
667 continue;
668
669 // Success. The block is now dead, so remove it from the loop,
670 // update the dominator tree and delete it.
671 DEBUG(dbgs() << "LoopSimplify: Eliminating exiting block "
672 << ExitingBlock->getName() << "\n");
673
674 // Notify ScalarEvolution before deleting this block. Currently assume the
675 // parent loop doesn't change (spliting edges doesn't count). If blocks,
676 // CFG edges, or other values in the parent loop change, then we need call
677 // to forgetLoop() for the parent instead.
678 if (SE)
679 SE->forgetLoop(L);
680
681 assert(pred_begin(ExitingBlock) == pred_end(ExitingBlock));
682 Changed = true;
683 LI->removeBlock(ExitingBlock);
684
685 DomTreeNode *Node = DT->getNode(ExitingBlock);
686 const std::vector<DomTreeNodeBase<BasicBlock> *> &Children =
687 Node->getChildren();
688 while (!Children.empty()) {
689 DomTreeNode *Child = Children.front();
690 DT->changeImmediateDominator(Child, Node->getIDom());
691 }
692 DT->eraseNode(ExitingBlock);
693
694 BI->getSuccessor(0)->removePredecessor(ExitingBlock);
695 BI->getSuccessor(1)->removePredecessor(ExitingBlock);
696 ExitingBlock->eraseFromParent();
697 }
698 }
699
700 return Changed;
701 }
702
simplifyLoop(Loop * L,DominatorTree * DT,LoopInfo * LI,ScalarEvolution * SE,AssumptionCache * AC,bool PreserveLCSSA)703 bool llvm::simplifyLoop(Loop *L, DominatorTree *DT, LoopInfo *LI,
704 ScalarEvolution *SE, AssumptionCache *AC,
705 bool PreserveLCSSA) {
706 bool Changed = false;
707
708 // Worklist maintains our depth-first queue of loops in this nest to process.
709 SmallVector<Loop *, 4> Worklist;
710 Worklist.push_back(L);
711
712 // Walk the worklist from front to back, pushing newly found sub loops onto
713 // the back. This will let us process loops from back to front in depth-first
714 // order. We can use this simple process because loops form a tree.
715 for (unsigned Idx = 0; Idx != Worklist.size(); ++Idx) {
716 Loop *L2 = Worklist[Idx];
717 Worklist.append(L2->begin(), L2->end());
718 }
719
720 while (!Worklist.empty())
721 Changed |= simplifyOneLoop(Worklist.pop_back_val(), Worklist, DT, LI, SE,
722 AC, PreserveLCSSA);
723
724 return Changed;
725 }
726
727 namespace {
728 struct LoopSimplify : public FunctionPass {
729 static char ID; // Pass identification, replacement for typeid
LoopSimplify__anon439bab570111::LoopSimplify730 LoopSimplify() : FunctionPass(ID) {
731 initializeLoopSimplifyPass(*PassRegistry::getPassRegistry());
732 }
733
734 DominatorTree *DT;
735 LoopInfo *LI;
736 ScalarEvolution *SE;
737 AssumptionCache *AC;
738
739 bool runOnFunction(Function &F) override;
740
getAnalysisUsage__anon439bab570111::LoopSimplify741 void getAnalysisUsage(AnalysisUsage &AU) const override {
742 AU.addRequired<AssumptionCacheTracker>();
743
744 // We need loop information to identify the loops...
745 AU.addRequired<DominatorTreeWrapperPass>();
746 AU.addPreserved<DominatorTreeWrapperPass>();
747
748 AU.addRequired<LoopInfoWrapperPass>();
749 AU.addPreserved<LoopInfoWrapperPass>();
750
751 AU.addPreserved<BasicAAWrapperPass>();
752 AU.addPreserved<AAResultsWrapperPass>();
753 AU.addPreserved<GlobalsAAWrapperPass>();
754 AU.addPreserved<ScalarEvolutionWrapperPass>();
755 AU.addPreserved<SCEVAAWrapperPass>();
756 AU.addPreserved<DependenceAnalysis>();
757 AU.addPreservedID(BreakCriticalEdgesID); // No critical edges added.
758 }
759
760 /// verifyAnalysis() - Verify LoopSimplifyForm's guarantees.
761 void verifyAnalysis() const override;
762 };
763 }
764
765 char LoopSimplify::ID = 0;
766 INITIALIZE_PASS_BEGIN(LoopSimplify, "loop-simplify",
767 "Canonicalize natural loops", false, false)
768 INITIALIZE_PASS_DEPENDENCY(AssumptionCacheTracker)
769 INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
770 INITIALIZE_PASS_DEPENDENCY(LoopInfoWrapperPass)
771 INITIALIZE_PASS_DEPENDENCY(BasicAAWrapperPass)
772 INITIALIZE_PASS_DEPENDENCY(GlobalsAAWrapperPass)
773 INITIALIZE_PASS_DEPENDENCY(SCEVAAWrapperPass)
774 INITIALIZE_PASS_END(LoopSimplify, "loop-simplify",
775 "Canonicalize natural loops", false, false)
776
777 // Publicly exposed interface to pass...
778 char &llvm::LoopSimplifyID = LoopSimplify::ID;
createLoopSimplifyPass()779 Pass *llvm::createLoopSimplifyPass() { return new LoopSimplify(); }
780
781 /// runOnFunction - Run down all loops in the CFG (recursively, but we could do
782 /// it in any convenient order) inserting preheaders...
783 ///
runOnFunction(Function & F)784 bool LoopSimplify::runOnFunction(Function &F) {
785 bool Changed = false;
786 LI = &getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
787 DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
788 auto *SEWP = getAnalysisIfAvailable<ScalarEvolutionWrapperPass>();
789 SE = SEWP ? &SEWP->getSE() : nullptr;
790 AC = &getAnalysis<AssumptionCacheTracker>().getAssumptionCache(F);
791 bool PreserveLCSSA = mustPreserveAnalysisID(LCSSAID);
792
793 // Simplify each loop nest in the function.
794 for (LoopInfo::iterator I = LI->begin(), E = LI->end(); I != E; ++I)
795 Changed |= simplifyLoop(*I, DT, LI, SE, AC, PreserveLCSSA);
796
797 return Changed;
798 }
799
800 // FIXME: Restore this code when we re-enable verification in verifyAnalysis
801 // below.
802 #if 0
803 static void verifyLoop(Loop *L) {
804 // Verify subloops.
805 for (Loop::iterator I = L->begin(), E = L->end(); I != E; ++I)
806 verifyLoop(*I);
807
808 // It used to be possible to just assert L->isLoopSimplifyForm(), however
809 // with the introduction of indirectbr, there are now cases where it's
810 // not possible to transform a loop as necessary. We can at least check
811 // that there is an indirectbr near any time there's trouble.
812
813 // Indirectbr can interfere with preheader and unique backedge insertion.
814 if (!L->getLoopPreheader() || !L->getLoopLatch()) {
815 bool HasIndBrPred = false;
816 for (pred_iterator PI = pred_begin(L->getHeader()),
817 PE = pred_end(L->getHeader()); PI != PE; ++PI)
818 if (isa<IndirectBrInst>((*PI)->getTerminator())) {
819 HasIndBrPred = true;
820 break;
821 }
822 assert(HasIndBrPred &&
823 "LoopSimplify has no excuse for missing loop header info!");
824 (void)HasIndBrPred;
825 }
826
827 // Indirectbr can interfere with exit block canonicalization.
828 if (!L->hasDedicatedExits()) {
829 bool HasIndBrExiting = false;
830 SmallVector<BasicBlock*, 8> ExitingBlocks;
831 L->getExitingBlocks(ExitingBlocks);
832 for (unsigned i = 0, e = ExitingBlocks.size(); i != e; ++i) {
833 if (isa<IndirectBrInst>((ExitingBlocks[i])->getTerminator())) {
834 HasIndBrExiting = true;
835 break;
836 }
837 }
838
839 assert(HasIndBrExiting &&
840 "LoopSimplify has no excuse for missing exit block info!");
841 (void)HasIndBrExiting;
842 }
843 }
844 #endif
845
verifyAnalysis() const846 void LoopSimplify::verifyAnalysis() const {
847 // FIXME: This routine is being called mid-way through the loop pass manager
848 // as loop passes destroy this analysis. That's actually fine, but we have no
849 // way of expressing that here. Once all of the passes that destroy this are
850 // hoisted out of the loop pass manager we can add back verification here.
851 #if 0
852 for (LoopInfo::iterator I = LI->begin(), E = LI->end(); I != E; ++I)
853 verifyLoop(*I);
854 #endif
855 }
856