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[llvm/stm8.git] / lib / Transforms / Utils / LoopSimplify.cpp
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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.
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.
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.
24 // This pass also guarantees that loops will have exactly one backedge.
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.
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.
35 // This pass obviously modifies the CFG, but updates loop information and
36 // dominator information.
38 //===----------------------------------------------------------------------===//
40 #define DEBUG_TYPE "loop-simplify"
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/Dominators.h"
50 #include "llvm/Analysis/InstructionSimplify.h"
51 #include "llvm/Analysis/LoopPass.h"
52 #include "llvm/Analysis/ScalarEvolution.h"
53 #include "llvm/Transforms/Utils/BasicBlockUtils.h"
54 #include "llvm/Transforms/Utils/Local.h"
55 #include "llvm/Support/CFG.h"
56 #include "llvm/Support/Debug.h"
57 #include "llvm/ADT/SetOperations.h"
58 #include "llvm/ADT/SetVector.h"
59 #include "llvm/ADT/Statistic.h"
60 #include "llvm/ADT/DepthFirstIterator.h"
61 using namespace llvm;
63 STATISTIC(NumInserted, "Number of pre-header or exit blocks inserted");
64 STATISTIC(NumNested , "Number of nested loops split out");
66 namespace {
67 struct LoopSimplify : public LoopPass {
68 static char ID; // Pass identification, replacement for typeid
69 LoopSimplify() : LoopPass(ID) {
70 initializeLoopSimplifyPass(*PassRegistry::getPassRegistry());
73 // AA - If we have an alias analysis object to update, this is it, otherwise
74 // this is null.
75 AliasAnalysis *AA;
76 LoopInfo *LI;
77 DominatorTree *DT;
78 ScalarEvolution *SE;
79 Loop *L;
80 virtual bool runOnLoop(Loop *L, LPPassManager &LPM);
82 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
83 // We need loop information to identify the loops...
84 AU.addRequired<DominatorTree>();
85 AU.addPreserved<DominatorTree>();
87 AU.addRequired<LoopInfo>();
88 AU.addPreserved<LoopInfo>();
90 AU.addPreserved<AliasAnalysis>();
91 AU.addPreserved<ScalarEvolution>();
92 AU.addPreservedID(BreakCriticalEdgesID); // No critical edges added.
95 /// verifyAnalysis() - Verify LoopSimplifyForm's guarantees.
96 void verifyAnalysis() const;
98 private:
99 bool ProcessLoop(Loop *L, LPPassManager &LPM);
100 BasicBlock *RewriteLoopExitBlock(Loop *L, BasicBlock *Exit);
101 BasicBlock *InsertPreheaderForLoop(Loop *L);
102 Loop *SeparateNestedLoop(Loop *L, LPPassManager &LPM);
103 BasicBlock *InsertUniqueBackedgeBlock(Loop *L, BasicBlock *Preheader);
104 void PlaceSplitBlockCarefully(BasicBlock *NewBB,
105 SmallVectorImpl<BasicBlock*> &SplitPreds,
106 Loop *L);
110 char LoopSimplify::ID = 0;
111 INITIALIZE_PASS_BEGIN(LoopSimplify, "loop-simplify",
112 "Canonicalize natural loops", true, false)
113 INITIALIZE_PASS_DEPENDENCY(DominatorTree)
114 INITIALIZE_PASS_DEPENDENCY(LoopInfo)
115 INITIALIZE_PASS_END(LoopSimplify, "loop-simplify",
116 "Canonicalize natural loops", true, false)
118 // Publicly exposed interface to pass...
119 char &llvm::LoopSimplifyID = LoopSimplify::ID;
120 Pass *llvm::createLoopSimplifyPass() { return new LoopSimplify(); }
122 /// runOnLoop - Run down all loops in the CFG (recursively, but we could do
123 /// it in any convenient order) inserting preheaders...
125 bool LoopSimplify::runOnLoop(Loop *l, LPPassManager &LPM) {
126 L = l;
127 bool Changed = false;
128 LI = &getAnalysis<LoopInfo>();
129 AA = getAnalysisIfAvailable<AliasAnalysis>();
130 DT = &getAnalysis<DominatorTree>();
131 SE = getAnalysisIfAvailable<ScalarEvolution>();
133 Changed |= ProcessLoop(L, LPM);
135 return Changed;
138 /// ProcessLoop - Walk the loop structure in depth first order, ensuring that
139 /// all loops have preheaders.
141 bool LoopSimplify::ProcessLoop(Loop *L, LPPassManager &LPM) {
142 bool Changed = false;
143 ReprocessLoop:
145 // Check to see that no blocks (other than the header) in this loop have
146 // predecessors that are not in the loop. This is not valid for natural
147 // loops, but can occur if the blocks are unreachable. Since they are
148 // unreachable we can just shamelessly delete those CFG edges!
149 for (Loop::block_iterator BB = L->block_begin(), E = L->block_end();
150 BB != E; ++BB) {
151 if (*BB == L->getHeader()) continue;
153 SmallPtrSet<BasicBlock*, 4> BadPreds;
154 for (pred_iterator PI = pred_begin(*BB),
155 PE = pred_end(*BB); PI != PE; ++PI) {
156 BasicBlock *P = *PI;
157 if (!L->contains(P))
158 BadPreds.insert(P);
161 // Delete each unique out-of-loop (and thus dead) predecessor.
162 for (SmallPtrSet<BasicBlock*, 4>::iterator I = BadPreds.begin(),
163 E = BadPreds.end(); I != E; ++I) {
165 DEBUG(dbgs() << "LoopSimplify: Deleting edge from dead predecessor "
166 << (*I)->getName() << "\n");
168 // Inform each successor of each dead pred.
169 for (succ_iterator SI = succ_begin(*I), SE = succ_end(*I); SI != SE; ++SI)
170 (*SI)->removePredecessor(*I);
171 // Zap the dead pred's terminator and replace it with unreachable.
172 TerminatorInst *TI = (*I)->getTerminator();
173 TI->replaceAllUsesWith(UndefValue::get(TI->getType()));
174 (*I)->getTerminator()->eraseFromParent();
175 new UnreachableInst((*I)->getContext(), *I);
176 Changed = true;
180 // If there are exiting blocks with branches on undef, resolve the undef in
181 // the direction which will exit the loop. This will help simplify loop
182 // trip count computations.
183 SmallVector<BasicBlock*, 8> ExitingBlocks;
184 L->getExitingBlocks(ExitingBlocks);
185 for (SmallVectorImpl<BasicBlock *>::iterator I = ExitingBlocks.begin(),
186 E = ExitingBlocks.end(); I != E; ++I)
187 if (BranchInst *BI = dyn_cast<BranchInst>((*I)->getTerminator()))
188 if (BI->isConditional()) {
189 if (UndefValue *Cond = dyn_cast<UndefValue>(BI->getCondition())) {
191 DEBUG(dbgs() << "LoopSimplify: Resolving \"br i1 undef\" to exit in "
192 << (*I)->getName() << "\n");
194 BI->setCondition(ConstantInt::get(Cond->getType(),
195 !L->contains(BI->getSuccessor(0))));
196 Changed = true;
200 // Does the loop already have a preheader? If so, don't insert one.
201 BasicBlock *Preheader = L->getLoopPreheader();
202 if (!Preheader) {
203 Preheader = InsertPreheaderForLoop(L);
204 if (Preheader) {
205 ++NumInserted;
206 Changed = true;
210 // Next, check to make sure that all exit nodes of the loop only have
211 // predecessors that are inside of the loop. This check guarantees that the
212 // loop preheader/header will dominate the exit blocks. If the exit block has
213 // predecessors from outside of the loop, split the edge now.
214 SmallVector<BasicBlock*, 8> ExitBlocks;
215 L->getExitBlocks(ExitBlocks);
217 SmallSetVector<BasicBlock *, 8> ExitBlockSet(ExitBlocks.begin(),
218 ExitBlocks.end());
219 for (SmallSetVector<BasicBlock *, 8>::iterator I = ExitBlockSet.begin(),
220 E = ExitBlockSet.end(); I != E; ++I) {
221 BasicBlock *ExitBlock = *I;
222 for (pred_iterator PI = pred_begin(ExitBlock), PE = pred_end(ExitBlock);
223 PI != PE; ++PI)
224 // Must be exactly this loop: no subloops, parent loops, or non-loop preds
225 // allowed.
226 if (!L->contains(*PI)) {
227 if (RewriteLoopExitBlock(L, ExitBlock)) {
228 ++NumInserted;
229 Changed = true;
231 break;
235 // If the header has more than two predecessors at this point (from the
236 // preheader and from multiple backedges), we must adjust the loop.
237 BasicBlock *LoopLatch = L->getLoopLatch();
238 if (!LoopLatch) {
239 // If this is really a nested loop, rip it out into a child loop. Don't do
240 // this for loops with a giant number of backedges, just factor them into a
241 // common backedge instead.
242 if (L->getNumBackEdges() < 8) {
243 if (SeparateNestedLoop(L, LPM)) {
244 ++NumNested;
245 // This is a big restructuring change, reprocess the whole loop.
246 Changed = true;
247 // GCC doesn't tail recursion eliminate this.
248 goto ReprocessLoop;
252 // If we either couldn't, or didn't want to, identify nesting of the loops,
253 // insert a new block that all backedges target, then make it jump to the
254 // loop header.
255 LoopLatch = InsertUniqueBackedgeBlock(L, Preheader);
256 if (LoopLatch) {
257 ++NumInserted;
258 Changed = true;
262 // Scan over the PHI nodes in the loop header. Since they now have only two
263 // incoming values (the loop is canonicalized), we may have simplified the PHI
264 // down to 'X = phi [X, Y]', which should be replaced with 'Y'.
265 PHINode *PN;
266 for (BasicBlock::iterator I = L->getHeader()->begin();
267 (PN = dyn_cast<PHINode>(I++)); )
268 if (Value *V = SimplifyInstruction(PN, 0, DT)) {
269 if (AA) AA->deleteValue(PN);
270 if (SE) SE->forgetValue(PN);
271 PN->replaceAllUsesWith(V);
272 PN->eraseFromParent();
275 // If this loop has multiple exits and the exits all go to the same
276 // block, attempt to merge the exits. This helps several passes, such
277 // as LoopRotation, which do not support loops with multiple exits.
278 // SimplifyCFG also does this (and this code uses the same utility
279 // function), however this code is loop-aware, where SimplifyCFG is
280 // not. That gives it the advantage of being able to hoist
281 // loop-invariant instructions out of the way to open up more
282 // opportunities, and the disadvantage of having the responsibility
283 // to preserve dominator information.
284 bool UniqueExit = true;
285 if (!ExitBlocks.empty())
286 for (unsigned i = 1, e = ExitBlocks.size(); i != e; ++i)
287 if (ExitBlocks[i] != ExitBlocks[0]) {
288 UniqueExit = false;
289 break;
291 if (UniqueExit) {
292 for (unsigned i = 0, e = ExitingBlocks.size(); i != e; ++i) {
293 BasicBlock *ExitingBlock = ExitingBlocks[i];
294 if (!ExitingBlock->getSinglePredecessor()) continue;
295 BranchInst *BI = dyn_cast<BranchInst>(ExitingBlock->getTerminator());
296 if (!BI || !BI->isConditional()) continue;
297 CmpInst *CI = dyn_cast<CmpInst>(BI->getCondition());
298 if (!CI || CI->getParent() != ExitingBlock) continue;
300 // Attempt to hoist out all instructions except for the
301 // comparison and the branch.
302 bool AllInvariant = true;
303 for (BasicBlock::iterator I = ExitingBlock->begin(); &*I != BI; ) {
304 Instruction *Inst = I++;
305 // Skip debug info intrinsics.
306 if (isa<DbgInfoIntrinsic>(Inst))
307 continue;
308 if (Inst == CI)
309 continue;
310 if (!L->makeLoopInvariant(Inst, Changed,
311 Preheader ? Preheader->getTerminator() : 0)) {
312 AllInvariant = false;
313 break;
316 if (!AllInvariant) continue;
318 // The block has now been cleared of all instructions except for
319 // a comparison and a conditional branch. SimplifyCFG may be able
320 // to fold it now.
321 if (!FoldBranchToCommonDest(BI)) continue;
323 // Success. The block is now dead, so remove it from the loop,
324 // update the dominator tree and delete it.
325 DEBUG(dbgs() << "LoopSimplify: Eliminating exiting block "
326 << ExitingBlock->getName() << "\n");
328 assert(pred_begin(ExitingBlock) == pred_end(ExitingBlock));
329 Changed = true;
330 LI->removeBlock(ExitingBlock);
332 DomTreeNode *Node = DT->getNode(ExitingBlock);
333 const std::vector<DomTreeNodeBase<BasicBlock> *> &Children =
334 Node->getChildren();
335 while (!Children.empty()) {
336 DomTreeNode *Child = Children.front();
337 DT->changeImmediateDominator(Child, Node->getIDom());
339 DT->eraseNode(ExitingBlock);
341 BI->getSuccessor(0)->removePredecessor(ExitingBlock);
342 BI->getSuccessor(1)->removePredecessor(ExitingBlock);
343 ExitingBlock->eraseFromParent();
347 return Changed;
350 /// InsertPreheaderForLoop - Once we discover that a loop doesn't have a
351 /// preheader, this method is called to insert one. This method has two phases:
352 /// preheader insertion and analysis updating.
354 BasicBlock *LoopSimplify::InsertPreheaderForLoop(Loop *L) {
355 BasicBlock *Header = L->getHeader();
357 // Compute the set of predecessors of the loop that are not in the loop.
358 SmallVector<BasicBlock*, 8> OutsideBlocks;
359 for (pred_iterator PI = pred_begin(Header), PE = pred_end(Header);
360 PI != PE; ++PI) {
361 BasicBlock *P = *PI;
362 if (!L->contains(P)) { // Coming in from outside the loop?
363 // If the loop is branched to from an indirect branch, we won't
364 // be able to fully transform the loop, because it prohibits
365 // edge splitting.
366 if (isa<IndirectBrInst>(P->getTerminator())) return 0;
368 // Keep track of it.
369 OutsideBlocks.push_back(P);
373 // Split out the loop pre-header.
374 BasicBlock *NewBB =
375 SplitBlockPredecessors(Header, &OutsideBlocks[0], OutsideBlocks.size(),
376 ".preheader", this);
378 NewBB->getTerminator()->setDebugLoc(Header->getFirstNonPHI()->getDebugLoc());
379 DEBUG(dbgs() << "LoopSimplify: Creating pre-header " << NewBB->getName()
380 << "\n");
382 // Make sure that NewBB is put someplace intelligent, which doesn't mess up
383 // code layout too horribly.
384 PlaceSplitBlockCarefully(NewBB, OutsideBlocks, L);
386 return NewBB;
389 /// RewriteLoopExitBlock - Ensure that the loop preheader dominates all exit
390 /// blocks. This method is used to split exit blocks that have predecessors
391 /// outside of the loop.
392 BasicBlock *LoopSimplify::RewriteLoopExitBlock(Loop *L, BasicBlock *Exit) {
393 SmallVector<BasicBlock*, 8> LoopBlocks;
394 for (pred_iterator I = pred_begin(Exit), E = pred_end(Exit); I != E; ++I) {
395 BasicBlock *P = *I;
396 if (L->contains(P)) {
397 // Don't do this if the loop is exited via an indirect branch.
398 if (isa<IndirectBrInst>(P->getTerminator())) return 0;
400 LoopBlocks.push_back(P);
404 assert(!LoopBlocks.empty() && "No edges coming in from outside the loop?");
405 BasicBlock *NewBB = SplitBlockPredecessors(Exit, &LoopBlocks[0],
406 LoopBlocks.size(), ".loopexit",
407 this);
409 DEBUG(dbgs() << "LoopSimplify: Creating dedicated exit block "
410 << NewBB->getName() << "\n");
411 return NewBB;
414 /// AddBlockAndPredsToSet - Add the specified block, and all of its
415 /// predecessors, to the specified set, if it's not already in there. Stop
416 /// predecessor traversal when we reach StopBlock.
417 static void AddBlockAndPredsToSet(BasicBlock *InputBB, BasicBlock *StopBlock,
418 std::set<BasicBlock*> &Blocks) {
419 std::vector<BasicBlock *> WorkList;
420 WorkList.push_back(InputBB);
421 do {
422 BasicBlock *BB = WorkList.back(); WorkList.pop_back();
423 if (Blocks.insert(BB).second && BB != StopBlock)
424 // If BB is not already processed and it is not a stop block then
425 // insert its predecessor in the work list
426 for (pred_iterator I = pred_begin(BB), E = pred_end(BB); I != E; ++I) {
427 BasicBlock *WBB = *I;
428 WorkList.push_back(WBB);
430 } while(!WorkList.empty());
433 /// FindPHIToPartitionLoops - The first part of loop-nestification is to find a
434 /// PHI node that tells us how to partition the loops.
435 static PHINode *FindPHIToPartitionLoops(Loop *L, DominatorTree *DT,
436 AliasAnalysis *AA, LoopInfo *LI) {
437 for (BasicBlock::iterator I = L->getHeader()->begin(); isa<PHINode>(I); ) {
438 PHINode *PN = cast<PHINode>(I);
439 ++I;
440 if (Value *V = SimplifyInstruction(PN, 0, DT)) {
441 // This is a degenerate PHI already, don't modify it!
442 PN->replaceAllUsesWith(V);
443 if (AA) AA->deleteValue(PN);
444 PN->eraseFromParent();
445 continue;
448 // Scan this PHI node looking for a use of the PHI node by itself.
449 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
450 if (PN->getIncomingValue(i) == PN &&
451 L->contains(PN->getIncomingBlock(i)))
452 // We found something tasty to remove.
453 return PN;
455 return 0;
458 // PlaceSplitBlockCarefully - If the block isn't already, move the new block to
459 // right after some 'outside block' block. This prevents the preheader from
460 // being placed inside the loop body, e.g. when the loop hasn't been rotated.
461 void LoopSimplify::PlaceSplitBlockCarefully(BasicBlock *NewBB,
462 SmallVectorImpl<BasicBlock*> &SplitPreds,
463 Loop *L) {
464 // Check to see if NewBB is already well placed.
465 Function::iterator BBI = NewBB; --BBI;
466 for (unsigned i = 0, e = SplitPreds.size(); i != e; ++i) {
467 if (&*BBI == SplitPreds[i])
468 return;
471 // If it isn't already after an outside block, move it after one. This is
472 // always good as it makes the uncond branch from the outside block into a
473 // fall-through.
475 // Figure out *which* outside block to put this after. Prefer an outside
476 // block that neighbors a BB actually in the loop.
477 BasicBlock *FoundBB = 0;
478 for (unsigned i = 0, e = SplitPreds.size(); i != e; ++i) {
479 Function::iterator BBI = SplitPreds[i];
480 if (++BBI != NewBB->getParent()->end() &&
481 L->contains(BBI)) {
482 FoundBB = SplitPreds[i];
483 break;
487 // If our heuristic for a *good* bb to place this after doesn't find
488 // anything, just pick something. It's likely better than leaving it within
489 // the loop.
490 if (!FoundBB)
491 FoundBB = SplitPreds[0];
492 NewBB->moveAfter(FoundBB);
496 /// SeparateNestedLoop - If this loop has multiple backedges, try to pull one of
497 /// them out into a nested loop. This is important for code that looks like
498 /// this:
500 /// Loop:
501 /// ...
502 /// br cond, Loop, Next
503 /// ...
504 /// br cond2, Loop, Out
506 /// To identify this common case, we look at the PHI nodes in the header of the
507 /// loop. PHI nodes with unchanging values on one backedge correspond to values
508 /// that change in the "outer" loop, but not in the "inner" loop.
510 /// If we are able to separate out a loop, return the new outer loop that was
511 /// created.
513 Loop *LoopSimplify::SeparateNestedLoop(Loop *L, LPPassManager &LPM) {
514 PHINode *PN = FindPHIToPartitionLoops(L, DT, AA, LI);
515 if (PN == 0) return 0; // No known way to partition.
517 // Pull out all predecessors that have varying values in the loop. This
518 // handles the case when a PHI node has multiple instances of itself as
519 // arguments.
520 SmallVector<BasicBlock*, 8> OuterLoopPreds;
521 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
522 if (PN->getIncomingValue(i) != PN ||
523 !L->contains(PN->getIncomingBlock(i))) {
524 // We can't split indirectbr edges.
525 if (isa<IndirectBrInst>(PN->getIncomingBlock(i)->getTerminator()))
526 return 0;
528 OuterLoopPreds.push_back(PN->getIncomingBlock(i));
531 DEBUG(dbgs() << "LoopSimplify: Splitting out a new outer loop\n");
533 // If ScalarEvolution is around and knows anything about values in
534 // this loop, tell it to forget them, because we're about to
535 // substantially change it.
536 if (SE)
537 SE->forgetLoop(L);
539 BasicBlock *Header = L->getHeader();
540 BasicBlock *NewBB = SplitBlockPredecessors(Header, &OuterLoopPreds[0],
541 OuterLoopPreds.size(),
542 ".outer", this);
544 // Make sure that NewBB is put someplace intelligent, which doesn't mess up
545 // code layout too horribly.
546 PlaceSplitBlockCarefully(NewBB, OuterLoopPreds, L);
548 // Create the new outer loop.
549 Loop *NewOuter = new Loop();
551 // Change the parent loop to use the outer loop as its child now.
552 if (Loop *Parent = L->getParentLoop())
553 Parent->replaceChildLoopWith(L, NewOuter);
554 else
555 LI->changeTopLevelLoop(L, NewOuter);
557 // L is now a subloop of our outer loop.
558 NewOuter->addChildLoop(L);
560 // Add the new loop to the pass manager queue.
561 LPM.insertLoopIntoQueue(NewOuter);
563 for (Loop::block_iterator I = L->block_begin(), E = L->block_end();
564 I != E; ++I)
565 NewOuter->addBlockEntry(*I);
567 // Now reset the header in L, which had been moved by
568 // SplitBlockPredecessors for the outer loop.
569 L->moveToHeader(Header);
571 // Determine which blocks should stay in L and which should be moved out to
572 // the Outer loop now.
573 std::set<BasicBlock*> BlocksInL;
574 for (pred_iterator PI=pred_begin(Header), E = pred_end(Header); PI!=E; ++PI) {
575 BasicBlock *P = *PI;
576 if (DT->dominates(Header, P))
577 AddBlockAndPredsToSet(P, Header, BlocksInL);
580 // Scan all of the loop children of L, moving them to OuterLoop if they are
581 // not part of the inner loop.
582 const std::vector<Loop*> &SubLoops = L->getSubLoops();
583 for (size_t I = 0; I != SubLoops.size(); )
584 if (BlocksInL.count(SubLoops[I]->getHeader()))
585 ++I; // Loop remains in L
586 else
587 NewOuter->addChildLoop(L->removeChildLoop(SubLoops.begin() + I));
589 // Now that we know which blocks are in L and which need to be moved to
590 // OuterLoop, move any blocks that need it.
591 for (unsigned i = 0; i != L->getBlocks().size(); ++i) {
592 BasicBlock *BB = L->getBlocks()[i];
593 if (!BlocksInL.count(BB)) {
594 // Move this block to the parent, updating the exit blocks sets
595 L->removeBlockFromLoop(BB);
596 if ((*LI)[BB] == L)
597 LI->changeLoopFor(BB, NewOuter);
598 --i;
602 return NewOuter;
607 /// InsertUniqueBackedgeBlock - This method is called when the specified loop
608 /// has more than one backedge in it. If this occurs, revector all of these
609 /// backedges to target a new basic block and have that block branch to the loop
610 /// header. This ensures that loops have exactly one backedge.
612 BasicBlock *
613 LoopSimplify::InsertUniqueBackedgeBlock(Loop *L, BasicBlock *Preheader) {
614 assert(L->getNumBackEdges() > 1 && "Must have > 1 backedge!");
616 // Get information about the loop
617 BasicBlock *Header = L->getHeader();
618 Function *F = Header->getParent();
620 // Unique backedge insertion currently depends on having a preheader.
621 if (!Preheader)
622 return 0;
624 // Figure out which basic blocks contain back-edges to the loop header.
625 std::vector<BasicBlock*> BackedgeBlocks;
626 for (pred_iterator I = pred_begin(Header), E = pred_end(Header); I != E; ++I){
627 BasicBlock *P = *I;
629 // Indirectbr edges cannot be split, so we must fail if we find one.
630 if (isa<IndirectBrInst>(P->getTerminator()))
631 return 0;
633 if (P != Preheader) BackedgeBlocks.push_back(P);
636 // Create and insert the new backedge block...
637 BasicBlock *BEBlock = BasicBlock::Create(Header->getContext(),
638 Header->getName()+".backedge", F);
639 BranchInst *BETerminator = BranchInst::Create(Header, BEBlock);
641 DEBUG(dbgs() << "LoopSimplify: Inserting unique backedge block "
642 << BEBlock->getName() << "\n");
644 // Move the new backedge block to right after the last backedge block.
645 Function::iterator InsertPos = BackedgeBlocks.back(); ++InsertPos;
646 F->getBasicBlockList().splice(InsertPos, F->getBasicBlockList(), BEBlock);
648 // Now that the block has been inserted into the function, create PHI nodes in
649 // the backedge block which correspond to any PHI nodes in the header block.
650 for (BasicBlock::iterator I = Header->begin(); isa<PHINode>(I); ++I) {
651 PHINode *PN = cast<PHINode>(I);
652 PHINode *NewPN = PHINode::Create(PN->getType(), BackedgeBlocks.size(),
653 PN->getName()+".be", BETerminator);
654 if (AA) AA->copyValue(PN, NewPN);
656 // Loop over the PHI node, moving all entries except the one for the
657 // preheader over to the new PHI node.
658 unsigned PreheaderIdx = ~0U;
659 bool HasUniqueIncomingValue = true;
660 Value *UniqueValue = 0;
661 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
662 BasicBlock *IBB = PN->getIncomingBlock(i);
663 Value *IV = PN->getIncomingValue(i);
664 if (IBB == Preheader) {
665 PreheaderIdx = i;
666 } else {
667 NewPN->addIncoming(IV, IBB);
668 if (HasUniqueIncomingValue) {
669 if (UniqueValue == 0)
670 UniqueValue = IV;
671 else if (UniqueValue != IV)
672 HasUniqueIncomingValue = false;
677 // Delete all of the incoming values from the old PN except the preheader's
678 assert(PreheaderIdx != ~0U && "PHI has no preheader entry??");
679 if (PreheaderIdx != 0) {
680 PN->setIncomingValue(0, PN->getIncomingValue(PreheaderIdx));
681 PN->setIncomingBlock(0, PN->getIncomingBlock(PreheaderIdx));
683 // Nuke all entries except the zero'th.
684 for (unsigned i = 0, e = PN->getNumIncomingValues()-1; i != e; ++i)
685 PN->removeIncomingValue(e-i, false);
687 // Finally, add the newly constructed PHI node as the entry for the BEBlock.
688 PN->addIncoming(NewPN, BEBlock);
690 // As an optimization, if all incoming values in the new PhiNode (which is a
691 // subset of the incoming values of the old PHI node) have the same value,
692 // eliminate the PHI Node.
693 if (HasUniqueIncomingValue) {
694 NewPN->replaceAllUsesWith(UniqueValue);
695 if (AA) AA->deleteValue(NewPN);
696 BEBlock->getInstList().erase(NewPN);
700 // Now that all of the PHI nodes have been inserted and adjusted, modify the
701 // backedge blocks to just to the BEBlock instead of the header.
702 for (unsigned i = 0, e = BackedgeBlocks.size(); i != e; ++i) {
703 TerminatorInst *TI = BackedgeBlocks[i]->getTerminator();
704 for (unsigned Op = 0, e = TI->getNumSuccessors(); Op != e; ++Op)
705 if (TI->getSuccessor(Op) == Header)
706 TI->setSuccessor(Op, BEBlock);
709 //===--- Update all analyses which we must preserve now -----------------===//
711 // Update Loop Information - we know that this block is now in the current
712 // loop and all parent loops.
713 L->addBasicBlockToLoop(BEBlock, LI->getBase());
715 // Update dominator information
716 DT->splitBlock(BEBlock);
718 return BEBlock;
721 void LoopSimplify::verifyAnalysis() const {
722 // It used to be possible to just assert L->isLoopSimplifyForm(), however
723 // with the introduction of indirectbr, there are now cases where it's
724 // not possible to transform a loop as necessary. We can at least check
725 // that there is an indirectbr near any time there's trouble.
727 // Indirectbr can interfere with preheader and unique backedge insertion.
728 if (!L->getLoopPreheader() || !L->getLoopLatch()) {
729 bool HasIndBrPred = false;
730 for (pred_iterator PI = pred_begin(L->getHeader()),
731 PE = pred_end(L->getHeader()); PI != PE; ++PI)
732 if (isa<IndirectBrInst>((*PI)->getTerminator())) {
733 HasIndBrPred = true;
734 break;
736 assert(HasIndBrPred &&
737 "LoopSimplify has no excuse for missing loop header info!");
740 // Indirectbr can interfere with exit block canonicalization.
741 if (!L->hasDedicatedExits()) {
742 bool HasIndBrExiting = false;
743 SmallVector<BasicBlock*, 8> ExitingBlocks;
744 L->getExitingBlocks(ExitingBlocks);
745 for (unsigned i = 0, e = ExitingBlocks.size(); i != e; ++i)
746 if (isa<IndirectBrInst>((ExitingBlocks[i])->getTerminator())) {
747 HasIndBrExiting = true;
748 break;
750 assert(HasIndBrExiting &&
751 "LoopSimplify has no excuse for missing exit block info!");