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 #define DEBUG_TYPE "loopsimplify"
41 #include "llvm/Transforms/Scalar.h"
42 #include "llvm/Constants.h"
43 #include "llvm/Instructions.h"
44 #include "llvm/IntrinsicInst.h"
45 #include "llvm/Function.h"
46 #include "llvm/LLVMContext.h"
47 #include "llvm/Type.h"
48 #include "llvm/Analysis/AliasAnalysis.h"
49 #include "llvm/Analysis/ScalarEvolution.h"
50 #include "llvm/Analysis/Dominators.h"
51 #include "llvm/Analysis/LoopPass.h"
52 #include "llvm/Transforms/Utils/BasicBlockUtils.h"
53 #include "llvm/Transforms/Utils/Local.h"
54 #include "llvm/Support/CFG.h"
55 #include "llvm/Support/Debug.h"
56 #include "llvm/ADT/SetOperations.h"
57 #include "llvm/ADT/SetVector.h"
58 #include "llvm/ADT/Statistic.h"
59 #include "llvm/ADT/DepthFirstIterator.h"
60 using namespace llvm;
61
62 STATISTIC(NumInserted, "Number of pre-header or exit blocks inserted");
63 STATISTIC(NumNested , "Number of nested loops split out");
64
65 namespace {
66 struct LoopSimplify : public LoopPass {
67 static char ID; // Pass identification, replacement for typeid
LoopSimplify__anoneaa3f0410111::LoopSimplify68 LoopSimplify() : LoopPass(ID) {}
69
70 // AA - If we have an alias analysis object to update, this is it, otherwise
71 // this is null.
72 AliasAnalysis *AA;
73 LoopInfo *LI;
74 DominatorTree *DT;
75 ScalarEvolution *SE;
76 Loop *L;
77 virtual bool runOnLoop(Loop *L, LPPassManager &LPM);
78
getAnalysisUsage__anoneaa3f0410111::LoopSimplify79 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
80 // We need loop information to identify the loops...
81 AU.addRequired<DominatorTree>();
82 AU.addPreserved<DominatorTree>();
83
84 AU.addRequired<LoopInfo>();
85 AU.addPreserved<LoopInfo>();
86
87 AU.addPreserved<AliasAnalysis>();
88 AU.addPreserved<ScalarEvolution>();
89 AU.addPreservedID(BreakCriticalEdgesID); // No critical edges added.
90 AU.addPreserved<DominanceFrontier>();
91 AU.addPreservedID(LCSSAID);
92 }
93
94 /// verifyAnalysis() - Verify LoopSimplifyForm's guarantees.
95 void verifyAnalysis() const;
96
97 private:
98 bool ProcessLoop(Loop *L, LPPassManager &LPM);
99 BasicBlock *RewriteLoopExitBlock(Loop *L, BasicBlock *Exit);
100 BasicBlock *InsertPreheaderForLoop(Loop *L);
101 Loop *SeparateNestedLoop(Loop *L, LPPassManager &LPM);
102 BasicBlock *InsertUniqueBackedgeBlock(Loop *L, BasicBlock *Preheader);
103 void PlaceSplitBlockCarefully(BasicBlock *NewBB,
104 SmallVectorImpl<BasicBlock*> &SplitPreds,
105 Loop *L);
106 };
107 }
108
109 char LoopSimplify::ID = 0;
110 INITIALIZE_PASS(LoopSimplify, "loopsimplify",
111 "Canonicalize natural loops", true, false);
112
113 // Publically exposed interface to pass...
114 char &llvm::LoopSimplifyID = LoopSimplify::ID;
createLoopSimplifyPass()115 Pass *llvm::createLoopSimplifyPass() { return new LoopSimplify(); }
116
117 /// runOnLoop - Run down all loops in the CFG (recursively, but we could do
118 /// it in any convenient order) inserting preheaders...
119 ///
runOnLoop(Loop * l,LPPassManager & LPM)120 bool LoopSimplify::runOnLoop(Loop *l, LPPassManager &LPM) {
121 L = l;
122 bool Changed = false;
123 LI = &getAnalysis<LoopInfo>();
124 AA = getAnalysisIfAvailable<AliasAnalysis>();
125 DT = &getAnalysis<DominatorTree>();
126 SE = getAnalysisIfAvailable<ScalarEvolution>();
127
128 Changed |= ProcessLoop(L, LPM);
129
130 return Changed;
131 }
132
133 /// ProcessLoop - Walk the loop structure in depth first order, ensuring that
134 /// all loops have preheaders.
135 ///
ProcessLoop(Loop * L,LPPassManager & LPM)136 bool LoopSimplify::ProcessLoop(Loop *L, LPPassManager &LPM) {
137 bool Changed = false;
138 ReprocessLoop:
139
140 // Check to see that no blocks (other than the header) in this loop have
141 // predecessors that are not in the loop. This is not valid for natural
142 // loops, but can occur if the blocks are unreachable. Since they are
143 // unreachable we can just shamelessly delete those CFG edges!
144 for (Loop::block_iterator BB = L->block_begin(), E = L->block_end();
145 BB != E; ++BB) {
146 if (*BB == L->getHeader()) continue;
147
148 SmallPtrSet<BasicBlock*, 4> BadPreds;
149 for (pred_iterator PI = pred_begin(*BB),
150 PE = pred_end(*BB); PI != PE; ++PI) {
151 BasicBlock *P = *PI;
152 if (!L->contains(P))
153 BadPreds.insert(P);
154 }
155
156 // Delete each unique out-of-loop (and thus dead) predecessor.
157 for (SmallPtrSet<BasicBlock*, 4>::iterator I = BadPreds.begin(),
158 E = BadPreds.end(); I != E; ++I) {
159
160 DEBUG(dbgs() << "LoopSimplify: Deleting edge from dead predecessor ";
161 WriteAsOperand(dbgs(), *I, false);
162 dbgs() << "\n");
163
164 // Inform each successor of each dead pred.
165 for (succ_iterator SI = succ_begin(*I), SE = succ_end(*I); SI != SE; ++SI)
166 (*SI)->removePredecessor(*I);
167 // Zap the dead pred's terminator and replace it with unreachable.
168 TerminatorInst *TI = (*I)->getTerminator();
169 TI->replaceAllUsesWith(UndefValue::get(TI->getType()));
170 (*I)->getTerminator()->eraseFromParent();
171 new UnreachableInst((*I)->getContext(), *I);
172 Changed = true;
173 }
174 }
175
176 // If there are exiting blocks with branches on undef, resolve the undef in
177 // the direction which will exit the loop. This will help simplify loop
178 // trip count computations.
179 SmallVector<BasicBlock*, 8> ExitingBlocks;
180 L->getExitingBlocks(ExitingBlocks);
181 for (SmallVectorImpl<BasicBlock *>::iterator I = ExitingBlocks.begin(),
182 E = ExitingBlocks.end(); I != E; ++I)
183 if (BranchInst *BI = dyn_cast<BranchInst>((*I)->getTerminator()))
184 if (BI->isConditional()) {
185 if (UndefValue *Cond = dyn_cast<UndefValue>(BI->getCondition())) {
186
187 DEBUG(dbgs() << "LoopSimplify: Resolving \"br i1 undef\" to exit in ";
188 WriteAsOperand(dbgs(), *I, false);
189 dbgs() << "\n");
190
191 BI->setCondition(ConstantInt::get(Cond->getType(),
192 !L->contains(BI->getSuccessor(0))));
193 Changed = true;
194 }
195 }
196
197 // Does the loop already have a preheader? If so, don't insert one.
198 BasicBlock *Preheader = L->getLoopPreheader();
199 if (!Preheader) {
200 Preheader = InsertPreheaderForLoop(L);
201 if (Preheader) {
202 ++NumInserted;
203 Changed = true;
204 }
205 }
206
207 // Next, check to make sure that all exit nodes of the loop only have
208 // predecessors that are inside of the loop. This check guarantees that the
209 // loop preheader/header will dominate the exit blocks. If the exit block has
210 // predecessors from outside of the loop, split the edge now.
211 SmallVector<BasicBlock*, 8> ExitBlocks;
212 L->getExitBlocks(ExitBlocks);
213
214 SmallSetVector<BasicBlock *, 8> ExitBlockSet(ExitBlocks.begin(),
215 ExitBlocks.end());
216 for (SmallSetVector<BasicBlock *, 8>::iterator I = ExitBlockSet.begin(),
217 E = ExitBlockSet.end(); I != E; ++I) {
218 BasicBlock *ExitBlock = *I;
219 for (pred_iterator PI = pred_begin(ExitBlock), PE = pred_end(ExitBlock);
220 PI != PE; ++PI)
221 // Must be exactly this loop: no subloops, parent loops, or non-loop preds
222 // allowed.
223 if (!L->contains(*PI)) {
224 if (RewriteLoopExitBlock(L, ExitBlock)) {
225 ++NumInserted;
226 Changed = true;
227 }
228 break;
229 }
230 }
231
232 // If the header has more than two predecessors at this point (from the
233 // preheader and from multiple backedges), we must adjust the loop.
234 BasicBlock *LoopLatch = L->getLoopLatch();
235 if (!LoopLatch) {
236 // If this is really a nested loop, rip it out into a child loop. Don't do
237 // this for loops with a giant number of backedges, just factor them into a
238 // common backedge instead.
239 if (L->getNumBackEdges() < 8) {
240 if (SeparateNestedLoop(L, LPM)) {
241 ++NumNested;
242 // This is a big restructuring change, reprocess the whole loop.
243 Changed = true;
244 // GCC doesn't tail recursion eliminate this.
245 goto ReprocessLoop;
246 }
247 }
248
249 // If we either couldn't, or didn't want to, identify nesting of the loops,
250 // insert a new block that all backedges target, then make it jump to the
251 // loop header.
252 LoopLatch = InsertUniqueBackedgeBlock(L, Preheader);
253 if (LoopLatch) {
254 ++NumInserted;
255 Changed = true;
256 }
257 }
258
259 // Scan over the PHI nodes in the loop header. Since they now have only two
260 // incoming values (the loop is canonicalized), we may have simplified the PHI
261 // down to 'X = phi [X, Y]', which should be replaced with 'Y'.
262 PHINode *PN;
263 for (BasicBlock::iterator I = L->getHeader()->begin();
264 (PN = dyn_cast<PHINode>(I++)); )
265 if (Value *V = PN->hasConstantValue(DT)) {
266 if (AA) AA->deleteValue(PN);
267 PN->replaceAllUsesWith(V);
268 PN->eraseFromParent();
269 }
270
271 // If this loop has multiple exits and the exits all go to the same
272 // block, attempt to merge the exits. This helps several passes, such
273 // as LoopRotation, which do not support loops with multiple exits.
274 // SimplifyCFG also does this (and this code uses the same utility
275 // function), however this code is loop-aware, where SimplifyCFG is
276 // not. That gives it the advantage of being able to hoist
277 // loop-invariant instructions out of the way to open up more
278 // opportunities, and the disadvantage of having the responsibility
279 // to preserve dominator information.
280 bool UniqueExit = true;
281 if (!ExitBlocks.empty())
282 for (unsigned i = 1, e = ExitBlocks.size(); i != e; ++i)
283 if (ExitBlocks[i] != ExitBlocks[0]) {
284 UniqueExit = false;
285 break;
286 }
287 if (UniqueExit) {
288 for (unsigned i = 0, e = ExitingBlocks.size(); i != e; ++i) {
289 BasicBlock *ExitingBlock = ExitingBlocks[i];
290 if (!ExitingBlock->getSinglePredecessor()) continue;
291 BranchInst *BI = dyn_cast<BranchInst>(ExitingBlock->getTerminator());
292 if (!BI || !BI->isConditional()) continue;
293 CmpInst *CI = dyn_cast<CmpInst>(BI->getCondition());
294 if (!CI || CI->getParent() != ExitingBlock) continue;
295
296 // Attempt to hoist out all instructions except for the
297 // comparison and the branch.
298 bool AllInvariant = true;
299 for (BasicBlock::iterator I = ExitingBlock->begin(); &*I != BI; ) {
300 Instruction *Inst = I++;
301 // Skip debug info intrinsics.
302 if (isa<DbgInfoIntrinsic>(Inst))
303 continue;
304 if (Inst == CI)
305 continue;
306 if (!L->makeLoopInvariant(Inst, Changed,
307 Preheader ? Preheader->getTerminator() : 0)) {
308 AllInvariant = false;
309 break;
310 }
311 }
312 if (!AllInvariant) continue;
313
314 // The block has now been cleared of all instructions except for
315 // a comparison and a conditional branch. SimplifyCFG may be able
316 // to fold it now.
317 if (!FoldBranchToCommonDest(BI)) continue;
318
319 // Success. The block is now dead, so remove it from the loop,
320 // update the dominator tree and dominance frontier, and delete it.
321
322 DEBUG(dbgs() << "LoopSimplify: Eliminating exiting block ";
323 WriteAsOperand(dbgs(), ExitingBlock, false);
324 dbgs() << "\n");
325
326 assert(pred_begin(ExitingBlock) == pred_end(ExitingBlock));
327 Changed = true;
328 LI->removeBlock(ExitingBlock);
329
330 DominanceFrontier *DF = getAnalysisIfAvailable<DominanceFrontier>();
331 DomTreeNode *Node = DT->getNode(ExitingBlock);
332 const std::vector<DomTreeNodeBase<BasicBlock> *> &Children =
333 Node->getChildren();
334 while (!Children.empty()) {
335 DomTreeNode *Child = Children.front();
336 DT->changeImmediateDominator(Child, Node->getIDom());
337 if (DF) DF->changeImmediateDominator(Child->getBlock(),
338 Node->getIDom()->getBlock(),
339 DT);
340 }
341 DT->eraseNode(ExitingBlock);
342 if (DF) DF->removeBlock(ExitingBlock);
343
344 BI->getSuccessor(0)->removePredecessor(ExitingBlock);
345 BI->getSuccessor(1)->removePredecessor(ExitingBlock);
346 ExitingBlock->eraseFromParent();
347 }
348 }
349
350 return Changed;
351 }
352
353 /// InsertPreheaderForLoop - Once we discover that a loop doesn't have a
354 /// preheader, this method is called to insert one. This method has two phases:
355 /// preheader insertion and analysis updating.
356 ///
InsertPreheaderForLoop(Loop * L)357 BasicBlock *LoopSimplify::InsertPreheaderForLoop(Loop *L) {
358 BasicBlock *Header = L->getHeader();
359
360 // Compute the set of predecessors of the loop that are not in the loop.
361 SmallVector<BasicBlock*, 8> OutsideBlocks;
362 for (pred_iterator PI = pred_begin(Header), PE = pred_end(Header);
363 PI != PE; ++PI) {
364 BasicBlock *P = *PI;
365 if (!L->contains(P)) { // Coming in from outside the loop?
366 // If the loop is branched to from an indirect branch, we won't
367 // be able to fully transform the loop, because it prohibits
368 // edge splitting.
369 if (isa<IndirectBrInst>(P->getTerminator())) return 0;
370
371 // Keep track of it.
372 OutsideBlocks.push_back(P);
373 }
374 }
375
376 // Split out the loop pre-header.
377 BasicBlock *NewBB =
378 SplitBlockPredecessors(Header, &OutsideBlocks[0], OutsideBlocks.size(),
379 ".preheader", this);
380
381 DEBUG(dbgs() << "LoopSimplify: Creating pre-header ";
382 WriteAsOperand(dbgs(), NewBB, false);
383 dbgs() << "\n");
384
385 // Make sure that NewBB is put someplace intelligent, which doesn't mess up
386 // code layout too horribly.
387 PlaceSplitBlockCarefully(NewBB, OutsideBlocks, L);
388
389 return NewBB;
390 }
391
392 /// RewriteLoopExitBlock - Ensure that the loop preheader dominates all exit
393 /// blocks. This method is used to split exit blocks that have predecessors
394 /// outside of the loop.
RewriteLoopExitBlock(Loop * L,BasicBlock * Exit)395 BasicBlock *LoopSimplify::RewriteLoopExitBlock(Loop *L, BasicBlock *Exit) {
396 SmallVector<BasicBlock*, 8> LoopBlocks;
397 for (pred_iterator I = pred_begin(Exit), E = pred_end(Exit); I != E; ++I) {
398 BasicBlock *P = *I;
399 if (L->contains(P)) {
400 // Don't do this if the loop is exited via an indirect branch.
401 if (isa<IndirectBrInst>(P->getTerminator())) return 0;
402
403 LoopBlocks.push_back(P);
404 }
405 }
406
407 assert(!LoopBlocks.empty() && "No edges coming in from outside the loop?");
408 BasicBlock *NewBB = SplitBlockPredecessors(Exit, &LoopBlocks[0],
409 LoopBlocks.size(), ".loopexit",
410 this);
411
412 DEBUG(dbgs() << "LoopSimplify: Creating dedicated exit block ";
413 WriteAsOperand(dbgs(), NewBB, false);
414 dbgs() << "\n");
415
416 return NewBB;
417 }
418
419 /// AddBlockAndPredsToSet - Add the specified block, and all of its
420 /// predecessors, to the specified set, if it's not already in there. Stop
421 /// predecessor traversal when we reach StopBlock.
AddBlockAndPredsToSet(BasicBlock * InputBB,BasicBlock * StopBlock,std::set<BasicBlock * > & Blocks)422 static void AddBlockAndPredsToSet(BasicBlock *InputBB, BasicBlock *StopBlock,
423 std::set<BasicBlock*> &Blocks) {
424 std::vector<BasicBlock *> WorkList;
425 WorkList.push_back(InputBB);
426 do {
427 BasicBlock *BB = WorkList.back(); WorkList.pop_back();
428 if (Blocks.insert(BB).second && BB != StopBlock)
429 // If BB is not already processed and it is not a stop block then
430 // insert its predecessor in the work list
431 for (pred_iterator I = pred_begin(BB), E = pred_end(BB); I != E; ++I) {
432 BasicBlock *WBB = *I;
433 WorkList.push_back(WBB);
434 }
435 } while(!WorkList.empty());
436 }
437
438 /// FindPHIToPartitionLoops - The first part of loop-nestification is to find a
439 /// PHI node that tells us how to partition the loops.
FindPHIToPartitionLoops(Loop * L,DominatorTree * DT,AliasAnalysis * AA)440 static PHINode *FindPHIToPartitionLoops(Loop *L, DominatorTree *DT,
441 AliasAnalysis *AA) {
442 for (BasicBlock::iterator I = L->getHeader()->begin(); isa<PHINode>(I); ) {
443 PHINode *PN = cast<PHINode>(I);
444 ++I;
445 if (Value *V = PN->hasConstantValue(DT)) {
446 // This is a degenerate PHI already, don't modify it!
447 PN->replaceAllUsesWith(V);
448 if (AA) AA->deleteValue(PN);
449 PN->eraseFromParent();
450 continue;
451 }
452
453 // Scan this PHI node looking for a use of the PHI node by itself.
454 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
455 if (PN->getIncomingValue(i) == PN &&
456 L->contains(PN->getIncomingBlock(i)))
457 // We found something tasty to remove.
458 return PN;
459 }
460 return 0;
461 }
462
463 // PlaceSplitBlockCarefully - If the block isn't already, move the new block to
464 // right after some 'outside block' block. This prevents the preheader from
465 // being placed inside the loop body, e.g. when the loop hasn't been rotated.
PlaceSplitBlockCarefully(BasicBlock * NewBB,SmallVectorImpl<BasicBlock * > & SplitPreds,Loop * L)466 void LoopSimplify::PlaceSplitBlockCarefully(BasicBlock *NewBB,
467 SmallVectorImpl<BasicBlock*> &SplitPreds,
468 Loop *L) {
469 // Check to see if NewBB is already well placed.
470 Function::iterator BBI = NewBB; --BBI;
471 for (unsigned i = 0, e = SplitPreds.size(); i != e; ++i) {
472 if (&*BBI == SplitPreds[i])
473 return;
474 }
475
476 // If it isn't already after an outside block, move it after one. This is
477 // always good as it makes the uncond branch from the outside block into a
478 // fall-through.
479
480 // Figure out *which* outside block to put this after. Prefer an outside
481 // block that neighbors a BB actually in the loop.
482 BasicBlock *FoundBB = 0;
483 for (unsigned i = 0, e = SplitPreds.size(); i != e; ++i) {
484 Function::iterator BBI = SplitPreds[i];
485 if (++BBI != NewBB->getParent()->end() &&
486 L->contains(BBI)) {
487 FoundBB = SplitPreds[i];
488 break;
489 }
490 }
491
492 // If our heuristic for a *good* bb to place this after doesn't find
493 // anything, just pick something. It's likely better than leaving it within
494 // the loop.
495 if (!FoundBB)
496 FoundBB = SplitPreds[0];
497 NewBB->moveAfter(FoundBB);
498 }
499
500
501 /// SeparateNestedLoop - If this loop has multiple backedges, try to pull one of
502 /// them out into a nested loop. This is important for code that looks like
503 /// this:
504 ///
505 /// Loop:
506 /// ...
507 /// br cond, Loop, Next
508 /// ...
509 /// br cond2, Loop, Out
510 ///
511 /// To identify this common case, we look at the PHI nodes in the header of the
512 /// loop. PHI nodes with unchanging values on one backedge correspond to values
513 /// that change in the "outer" loop, but not in the "inner" loop.
514 ///
515 /// If we are able to separate out a loop, return the new outer loop that was
516 /// created.
517 ///
SeparateNestedLoop(Loop * L,LPPassManager & LPM)518 Loop *LoopSimplify::SeparateNestedLoop(Loop *L, LPPassManager &LPM) {
519 PHINode *PN = FindPHIToPartitionLoops(L, DT, AA);
520 if (PN == 0) return 0; // No known way to partition.
521
522 // Pull out all predecessors that have varying values in the loop. This
523 // handles the case when a PHI node has multiple instances of itself as
524 // arguments.
525 SmallVector<BasicBlock*, 8> OuterLoopPreds;
526 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
527 if (PN->getIncomingValue(i) != PN ||
528 !L->contains(PN->getIncomingBlock(i))) {
529 // We can't split indirectbr edges.
530 if (isa<IndirectBrInst>(PN->getIncomingBlock(i)->getTerminator()))
531 return 0;
532
533 OuterLoopPreds.push_back(PN->getIncomingBlock(i));
534 }
535
536 DEBUG(dbgs() << "LoopSimplify: Splitting out a new outer loop\n");
537
538 // If ScalarEvolution is around and knows anything about values in
539 // this loop, tell it to forget them, because we're about to
540 // substantially change it.
541 if (SE)
542 SE->forgetLoop(L);
543
544 BasicBlock *Header = L->getHeader();
545 BasicBlock *NewBB = SplitBlockPredecessors(Header, &OuterLoopPreds[0],
546 OuterLoopPreds.size(),
547 ".outer", this);
548
549 // Make sure that NewBB is put someplace intelligent, which doesn't mess up
550 // code layout too horribly.
551 PlaceSplitBlockCarefully(NewBB, OuterLoopPreds, L);
552
553 // Create the new outer loop.
554 Loop *NewOuter = new Loop();
555
556 // Change the parent loop to use the outer loop as its child now.
557 if (Loop *Parent = L->getParentLoop())
558 Parent->replaceChildLoopWith(L, NewOuter);
559 else
560 LI->changeTopLevelLoop(L, NewOuter);
561
562 // L is now a subloop of our outer loop.
563 NewOuter->addChildLoop(L);
564
565 // Add the new loop to the pass manager queue.
566 LPM.insertLoopIntoQueue(NewOuter);
567
568 for (Loop::block_iterator I = L->block_begin(), E = L->block_end();
569 I != E; ++I)
570 NewOuter->addBlockEntry(*I);
571
572 // Now reset the header in L, which had been moved by
573 // SplitBlockPredecessors for the outer loop.
574 L->moveToHeader(Header);
575
576 // Determine which blocks should stay in L and which should be moved out to
577 // the Outer loop now.
578 std::set<BasicBlock*> BlocksInL;
579 for (pred_iterator PI=pred_begin(Header), E = pred_end(Header); PI!=E; ++PI) {
580 BasicBlock *P = *PI;
581 if (DT->dominates(Header, P))
582 AddBlockAndPredsToSet(P, Header, BlocksInL);
583 }
584
585 // Scan all of the loop children of L, moving them to OuterLoop if they are
586 // not part of the inner loop.
587 const std::vector<Loop*> &SubLoops = L->getSubLoops();
588 for (size_t I = 0; I != SubLoops.size(); )
589 if (BlocksInL.count(SubLoops[I]->getHeader()))
590 ++I; // Loop remains in L
591 else
592 NewOuter->addChildLoop(L->removeChildLoop(SubLoops.begin() + I));
593
594 // Now that we know which blocks are in L and which need to be moved to
595 // OuterLoop, move any blocks that need it.
596 for (unsigned i = 0; i != L->getBlocks().size(); ++i) {
597 BasicBlock *BB = L->getBlocks()[i];
598 if (!BlocksInL.count(BB)) {
599 // Move this block to the parent, updating the exit blocks sets
600 L->removeBlockFromLoop(BB);
601 if ((*LI)[BB] == L)
602 LI->changeLoopFor(BB, NewOuter);
603 --i;
604 }
605 }
606
607 return NewOuter;
608 }
609
610
611
612 /// InsertUniqueBackedgeBlock - This method is called when the specified loop
613 /// has more than one backedge in it. If this occurs, revector all of these
614 /// backedges to target a new basic block and have that block branch to the loop
615 /// header. This ensures that loops have exactly one backedge.
616 ///
617 BasicBlock *
InsertUniqueBackedgeBlock(Loop * L,BasicBlock * Preheader)618 LoopSimplify::InsertUniqueBackedgeBlock(Loop *L, BasicBlock *Preheader) {
619 assert(L->getNumBackEdges() > 1 && "Must have > 1 backedge!");
620
621 // Get information about the loop
622 BasicBlock *Header = L->getHeader();
623 Function *F = Header->getParent();
624
625 // Unique backedge insertion currently depends on having a preheader.
626 if (!Preheader)
627 return 0;
628
629 // Figure out which basic blocks contain back-edges to the loop header.
630 std::vector<BasicBlock*> BackedgeBlocks;
631 for (pred_iterator I = pred_begin(Header), E = pred_end(Header); I != E; ++I){
632 BasicBlock *P = *I;
633
634 // Indirectbr edges cannot be split, so we must fail if we find one.
635 if (isa<IndirectBrInst>(P->getTerminator()))
636 return 0;
637
638 if (P != Preheader) BackedgeBlocks.push_back(P);
639 }
640
641 // Create and insert the new backedge block...
642 BasicBlock *BEBlock = BasicBlock::Create(Header->getContext(),
643 Header->getName()+".backedge", F);
644 BranchInst *BETerminator = BranchInst::Create(Header, BEBlock);
645
646 DEBUG(dbgs() << "LoopSimplify: Inserting unique backedge block ";
647 WriteAsOperand(dbgs(), BEBlock, false);
648 dbgs() << "\n");
649
650 // Move the new backedge block to right after the last backedge block.
651 Function::iterator InsertPos = BackedgeBlocks.back(); ++InsertPos;
652 F->getBasicBlockList().splice(InsertPos, F->getBasicBlockList(), BEBlock);
653
654 // Now that the block has been inserted into the function, create PHI nodes in
655 // the backedge block which correspond to any PHI nodes in the header block.
656 for (BasicBlock::iterator I = Header->begin(); isa<PHINode>(I); ++I) {
657 PHINode *PN = cast<PHINode>(I);
658 PHINode *NewPN = PHINode::Create(PN->getType(), PN->getName()+".be",
659 BETerminator);
660 NewPN->reserveOperandSpace(BackedgeBlocks.size());
661 if (AA) AA->copyValue(PN, NewPN);
662
663 // Loop over the PHI node, moving all entries except the one for the
664 // preheader over to the new PHI node.
665 unsigned PreheaderIdx = ~0U;
666 bool HasUniqueIncomingValue = true;
667 Value *UniqueValue = 0;
668 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
669 BasicBlock *IBB = PN->getIncomingBlock(i);
670 Value *IV = PN->getIncomingValue(i);
671 if (IBB == Preheader) {
672 PreheaderIdx = i;
673 } else {
674 NewPN->addIncoming(IV, IBB);
675 if (HasUniqueIncomingValue) {
676 if (UniqueValue == 0)
677 UniqueValue = IV;
678 else if (UniqueValue != IV)
679 HasUniqueIncomingValue = false;
680 }
681 }
682 }
683
684 // Delete all of the incoming values from the old PN except the preheader's
685 assert(PreheaderIdx != ~0U && "PHI has no preheader entry??");
686 if (PreheaderIdx != 0) {
687 PN->setIncomingValue(0, PN->getIncomingValue(PreheaderIdx));
688 PN->setIncomingBlock(0, PN->getIncomingBlock(PreheaderIdx));
689 }
690 // Nuke all entries except the zero'th.
691 for (unsigned i = 0, e = PN->getNumIncomingValues()-1; i != e; ++i)
692 PN->removeIncomingValue(e-i, false);
693
694 // Finally, add the newly constructed PHI node as the entry for the BEBlock.
695 PN->addIncoming(NewPN, BEBlock);
696
697 // As an optimization, if all incoming values in the new PhiNode (which is a
698 // subset of the incoming values of the old PHI node) have the same value,
699 // eliminate the PHI Node.
700 if (HasUniqueIncomingValue) {
701 NewPN->replaceAllUsesWith(UniqueValue);
702 if (AA) AA->deleteValue(NewPN);
703 BEBlock->getInstList().erase(NewPN);
704 }
705 }
706
707 // Now that all of the PHI nodes have been inserted and adjusted, modify the
708 // backedge blocks to just to the BEBlock instead of the header.
709 for (unsigned i = 0, e = BackedgeBlocks.size(); i != e; ++i) {
710 TerminatorInst *TI = BackedgeBlocks[i]->getTerminator();
711 for (unsigned Op = 0, e = TI->getNumSuccessors(); Op != e; ++Op)
712 if (TI->getSuccessor(Op) == Header)
713 TI->setSuccessor(Op, BEBlock);
714 }
715
716 //===--- Update all analyses which we must preserve now -----------------===//
717
718 // Update Loop Information - we know that this block is now in the current
719 // loop and all parent loops.
720 L->addBasicBlockToLoop(BEBlock, LI->getBase());
721
722 // Update dominator information
723 DT->splitBlock(BEBlock);
724 if (DominanceFrontier *DF = getAnalysisIfAvailable<DominanceFrontier>())
725 DF->splitBlock(BEBlock);
726
727 return BEBlock;
728 }
729
verifyAnalysis() const730 void LoopSimplify::verifyAnalysis() const {
731 // It used to be possible to just assert L->isLoopSimplifyForm(), however
732 // with the introduction of indirectbr, there are now cases where it's
733 // not possible to transform a loop as necessary. We can at least check
734 // that there is an indirectbr near any time there's trouble.
735
736 // Indirectbr can interfere with preheader and unique backedge insertion.
737 if (!L->getLoopPreheader() || !L->getLoopLatch()) {
738 bool HasIndBrPred = false;
739 for (pred_iterator PI = pred_begin(L->getHeader()),
740 PE = pred_end(L->getHeader()); PI != PE; ++PI)
741 if (isa<IndirectBrInst>((*PI)->getTerminator())) {
742 HasIndBrPred = true;
743 break;
744 }
745 assert(HasIndBrPred &&
746 "LoopSimplify has no excuse for missing loop header info!");
747 }
748
749 // Indirectbr can interfere with exit block canonicalization.
750 if (!L->hasDedicatedExits()) {
751 bool HasIndBrExiting = false;
752 SmallVector<BasicBlock*, 8> ExitingBlocks;
753 L->getExitingBlocks(ExitingBlocks);
754 for (unsigned i = 0, e = ExitingBlocks.size(); i != e; ++i)
755 if (isa<IndirectBrInst>((ExitingBlocks[i])->getTerminator())) {
756 HasIndBrExiting = true;
757 break;
758 }
759 assert(HasIndBrExiting &&
760 "LoopSimplify has no excuse for missing exit block info!");
761 }
762 }
763