1 //===- LoopSimplify.cpp - Loop Canonicalization Pass ----------------------===//
3 // The LLVM Compiler Infrastructure
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
8 //===----------------------------------------------------------------------===//
10 // This pass performs several transformations to transform natural loops into a
11 // simpler form, which makes subsequent analyses and transformations simpler and
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
35 // This pass obviously modifies the CFG, but updates loop information and
36 // dominator information.
38 //===----------------------------------------------------------------------===//
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"
62 STATISTIC(NumInserted, "Number of pre-header or exit blocks inserted");
63 STATISTIC(NumNested , "Number of nested loops split out");
66 struct LoopSimplify : public LoopPass {
67 static char ID; // Pass identification, replacement for typeid
68 LoopSimplify() : LoopPass(ID) {
69 initializeLoopSimplifyPass(*PassRegistry::getPassRegistry());
72 // AA - If we have an alias analysis object to update, this is it, otherwise
79 virtual bool runOnLoop(Loop *L, LPPassManager &LPM);
81 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
82 // We need loop information to identify the loops...
83 AU.addRequired<DominatorTree>();
84 AU.addPreserved<DominatorTree>();
86 AU.addRequired<LoopInfo>();
87 AU.addPreserved<LoopInfo>();
89 AU.addPreserved<AliasAnalysis>();
90 AU.addPreserved<ScalarEvolution>();
91 AU.addPreservedID(BreakCriticalEdgesID); // No critical edges added.
92 AU.addPreserved<DominanceFrontier>();
93 AU.addPreservedID(LCSSAID);
96 /// verifyAnalysis() - Verify LoopSimplifyForm's guarantees.
97 void verifyAnalysis() const;
100 bool ProcessLoop(Loop *L, LPPassManager &LPM);
101 BasicBlock *RewriteLoopExitBlock(Loop *L, BasicBlock *Exit);
102 BasicBlock *InsertPreheaderForLoop(Loop *L);
103 Loop *SeparateNestedLoop(Loop *L, LPPassManager &LPM);
104 BasicBlock *InsertUniqueBackedgeBlock(Loop *L, BasicBlock *Preheader);
105 void PlaceSplitBlockCarefully(BasicBlock *NewBB,
106 SmallVectorImpl<BasicBlock*> &SplitPreds,
111 char LoopSimplify::ID = 0;
112 INITIALIZE_PASS_BEGIN(LoopSimplify, "loopsimplify",
113 "Canonicalize natural loops", true, false)
114 INITIALIZE_PASS_DEPENDENCY(DominatorTree)
115 INITIALIZE_PASS_DEPENDENCY(LoopInfo)
116 INITIALIZE_PASS_END(LoopSimplify, "loopsimplify",
117 "Canonicalize natural loops", true, false)
119 // Publically exposed interface to pass...
120 char &llvm::LoopSimplifyID = LoopSimplify::ID;
121 Pass *llvm::createLoopSimplifyPass() { return new LoopSimplify(); }
123 /// runOnLoop - Run down all loops in the CFG (recursively, but we could do
124 /// it in any convenient order) inserting preheaders...
126 bool LoopSimplify::runOnLoop(Loop *l, LPPassManager &LPM) {
128 bool Changed = false;
129 LI = &getAnalysis<LoopInfo>();
130 AA = getAnalysisIfAvailable<AliasAnalysis>();
131 DT = &getAnalysis<DominatorTree>();
132 SE = getAnalysisIfAvailable<ScalarEvolution>();
134 Changed |= ProcessLoop(L, LPM);
139 /// ProcessLoop - Walk the loop structure in depth first order, ensuring that
140 /// all loops have preheaders.
142 bool LoopSimplify::ProcessLoop(Loop *L, LPPassManager &LPM) {
143 bool Changed = false;
146 // Check to see that no blocks (other than the header) in this loop have
147 // predecessors that are not in the loop. This is not valid for natural
148 // loops, but can occur if the blocks are unreachable. Since they are
149 // unreachable we can just shamelessly delete those CFG edges!
150 for (Loop::block_iterator BB = L->block_begin(), E = L->block_end();
152 if (*BB == L->getHeader()) continue;
154 SmallPtrSet<BasicBlock*, 4> BadPreds;
155 for (pred_iterator PI = pred_begin(*BB),
156 PE = pred_end(*BB); PI != PE; ++PI) {
162 // Delete each unique out-of-loop (and thus dead) predecessor.
163 for (SmallPtrSet<BasicBlock*, 4>::iterator I = BadPreds.begin(),
164 E = BadPreds.end(); I != E; ++I) {
166 DEBUG(dbgs() << "LoopSimplify: Deleting edge from dead predecessor ";
167 WriteAsOperand(dbgs(), *I, false);
170 // Inform each successor of each dead pred.
171 for (succ_iterator SI = succ_begin(*I), SE = succ_end(*I); SI != SE; ++SI)
172 (*SI)->removePredecessor(*I);
173 // Zap the dead pred's terminator and replace it with unreachable.
174 TerminatorInst *TI = (*I)->getTerminator();
175 TI->replaceAllUsesWith(UndefValue::get(TI->getType()));
176 (*I)->getTerminator()->eraseFromParent();
177 new UnreachableInst((*I)->getContext(), *I);
182 // If there are exiting blocks with branches on undef, resolve the undef in
183 // the direction which will exit the loop. This will help simplify loop
184 // trip count computations.
185 SmallVector<BasicBlock*, 8> ExitingBlocks;
186 L->getExitingBlocks(ExitingBlocks);
187 for (SmallVectorImpl<BasicBlock *>::iterator I = ExitingBlocks.begin(),
188 E = ExitingBlocks.end(); I != E; ++I)
189 if (BranchInst *BI = dyn_cast<BranchInst>((*I)->getTerminator()))
190 if (BI->isConditional()) {
191 if (UndefValue *Cond = dyn_cast<UndefValue>(BI->getCondition())) {
193 DEBUG(dbgs() << "LoopSimplify: Resolving \"br i1 undef\" to exit in ";
194 WriteAsOperand(dbgs(), *I, false);
197 BI->setCondition(ConstantInt::get(Cond->getType(),
198 !L->contains(BI->getSuccessor(0))));
203 // Does the loop already have a preheader? If so, don't insert one.
204 BasicBlock *Preheader = L->getLoopPreheader();
206 Preheader = InsertPreheaderForLoop(L);
213 // Next, check to make sure that all exit nodes of the loop only have
214 // predecessors that are inside of the loop. This check guarantees that the
215 // loop preheader/header will dominate the exit blocks. If the exit block has
216 // predecessors from outside of the loop, split the edge now.
217 SmallVector<BasicBlock*, 8> ExitBlocks;
218 L->getExitBlocks(ExitBlocks);
220 SmallSetVector<BasicBlock *, 8> ExitBlockSet(ExitBlocks.begin(),
222 for (SmallSetVector<BasicBlock *, 8>::iterator I = ExitBlockSet.begin(),
223 E = ExitBlockSet.end(); I != E; ++I) {
224 BasicBlock *ExitBlock = *I;
225 for (pred_iterator PI = pred_begin(ExitBlock), PE = pred_end(ExitBlock);
227 // Must be exactly this loop: no subloops, parent loops, or non-loop preds
229 if (!L->contains(*PI)) {
230 if (RewriteLoopExitBlock(L, ExitBlock)) {
238 // If the header has more than two predecessors at this point (from the
239 // preheader and from multiple backedges), we must adjust the loop.
240 BasicBlock *LoopLatch = L->getLoopLatch();
242 // If this is really a nested loop, rip it out into a child loop. Don't do
243 // this for loops with a giant number of backedges, just factor them into a
244 // common backedge instead.
245 if (L->getNumBackEdges() < 8) {
246 if (SeparateNestedLoop(L, LPM)) {
248 // This is a big restructuring change, reprocess the whole loop.
250 // GCC doesn't tail recursion eliminate this.
255 // If we either couldn't, or didn't want to, identify nesting of the loops,
256 // insert a new block that all backedges target, then make it jump to the
258 LoopLatch = InsertUniqueBackedgeBlock(L, Preheader);
265 // Scan over the PHI nodes in the loop header. Since they now have only two
266 // incoming values (the loop is canonicalized), we may have simplified the PHI
267 // down to 'X = phi [X, Y]', which should be replaced with 'Y'.
269 for (BasicBlock::iterator I = L->getHeader()->begin();
270 (PN = dyn_cast<PHINode>(I++)); )
271 if (Value *V = PN->hasConstantValue(DT)) {
272 if (AA) AA->deleteValue(PN);
273 PN->replaceAllUsesWith(V);
274 PN->eraseFromParent();
277 // If this loop has multiple exits and the exits all go to the same
278 // block, attempt to merge the exits. This helps several passes, such
279 // as LoopRotation, which do not support loops with multiple exits.
280 // SimplifyCFG also does this (and this code uses the same utility
281 // function), however this code is loop-aware, where SimplifyCFG is
282 // not. That gives it the advantage of being able to hoist
283 // loop-invariant instructions out of the way to open up more
284 // opportunities, and the disadvantage of having the responsibility
285 // to preserve dominator information.
286 bool UniqueExit = true;
287 if (!ExitBlocks.empty())
288 for (unsigned i = 1, e = ExitBlocks.size(); i != e; ++i)
289 if (ExitBlocks[i] != ExitBlocks[0]) {
294 for (unsigned i = 0, e = ExitingBlocks.size(); i != e; ++i) {
295 BasicBlock *ExitingBlock = ExitingBlocks[i];
296 if (!ExitingBlock->getSinglePredecessor()) continue;
297 BranchInst *BI = dyn_cast<BranchInst>(ExitingBlock->getTerminator());
298 if (!BI || !BI->isConditional()) continue;
299 CmpInst *CI = dyn_cast<CmpInst>(BI->getCondition());
300 if (!CI || CI->getParent() != ExitingBlock) continue;
302 // Attempt to hoist out all instructions except for the
303 // comparison and the branch.
304 bool AllInvariant = true;
305 for (BasicBlock::iterator I = ExitingBlock->begin(); &*I != BI; ) {
306 Instruction *Inst = I++;
307 // Skip debug info intrinsics.
308 if (isa<DbgInfoIntrinsic>(Inst))
312 if (!L->makeLoopInvariant(Inst, Changed,
313 Preheader ? Preheader->getTerminator() : 0)) {
314 AllInvariant = false;
318 if (!AllInvariant) continue;
320 // The block has now been cleared of all instructions except for
321 // a comparison and a conditional branch. SimplifyCFG may be able
323 if (!FoldBranchToCommonDest(BI)) continue;
325 // Success. The block is now dead, so remove it from the loop,
326 // update the dominator tree and dominance frontier, and delete it.
328 DEBUG(dbgs() << "LoopSimplify: Eliminating exiting block ";
329 WriteAsOperand(dbgs(), ExitingBlock, false);
332 assert(pred_begin(ExitingBlock) == pred_end(ExitingBlock));
334 LI->removeBlock(ExitingBlock);
336 DominanceFrontier *DF = getAnalysisIfAvailable<DominanceFrontier>();
337 DomTreeNode *Node = DT->getNode(ExitingBlock);
338 const std::vector<DomTreeNodeBase<BasicBlock> *> &Children =
340 while (!Children.empty()) {
341 DomTreeNode *Child = Children.front();
342 DT->changeImmediateDominator(Child, Node->getIDom());
343 if (DF) DF->changeImmediateDominator(Child->getBlock(),
344 Node->getIDom()->getBlock(),
347 DT->eraseNode(ExitingBlock);
348 if (DF) DF->removeBlock(ExitingBlock);
350 BI->getSuccessor(0)->removePredecessor(ExitingBlock);
351 BI->getSuccessor(1)->removePredecessor(ExitingBlock);
352 ExitingBlock->eraseFromParent();
359 /// InsertPreheaderForLoop - Once we discover that a loop doesn't have a
360 /// preheader, this method is called to insert one. This method has two phases:
361 /// preheader insertion and analysis updating.
363 BasicBlock *LoopSimplify::InsertPreheaderForLoop(Loop *L) {
364 BasicBlock *Header = L->getHeader();
366 // Compute the set of predecessors of the loop that are not in the loop.
367 SmallVector<BasicBlock*, 8> OutsideBlocks;
368 for (pred_iterator PI = pred_begin(Header), PE = pred_end(Header);
371 if (!L->contains(P)) { // Coming in from outside the loop?
372 // If the loop is branched to from an indirect branch, we won't
373 // be able to fully transform the loop, because it prohibits
375 if (isa<IndirectBrInst>(P->getTerminator())) return 0;
378 OutsideBlocks.push_back(P);
382 // Split out the loop pre-header.
384 SplitBlockPredecessors(Header, &OutsideBlocks[0], OutsideBlocks.size(),
387 DEBUG(dbgs() << "LoopSimplify: Creating pre-header ";
388 WriteAsOperand(dbgs(), NewBB, false);
391 // Make sure that NewBB is put someplace intelligent, which doesn't mess up
392 // code layout too horribly.
393 PlaceSplitBlockCarefully(NewBB, OutsideBlocks, L);
398 /// RewriteLoopExitBlock - Ensure that the loop preheader dominates all exit
399 /// blocks. This method is used to split exit blocks that have predecessors
400 /// outside of the loop.
401 BasicBlock *LoopSimplify::RewriteLoopExitBlock(Loop *L, BasicBlock *Exit) {
402 SmallVector<BasicBlock*, 8> LoopBlocks;
403 for (pred_iterator I = pred_begin(Exit), E = pred_end(Exit); I != E; ++I) {
405 if (L->contains(P)) {
406 // Don't do this if the loop is exited via an indirect branch.
407 if (isa<IndirectBrInst>(P->getTerminator())) return 0;
409 LoopBlocks.push_back(P);
413 assert(!LoopBlocks.empty() && "No edges coming in from outside the loop?");
414 BasicBlock *NewBB = SplitBlockPredecessors(Exit, &LoopBlocks[0],
415 LoopBlocks.size(), ".loopexit",
418 DEBUG(dbgs() << "LoopSimplify: Creating dedicated exit block ";
419 WriteAsOperand(dbgs(), NewBB, false);
425 /// AddBlockAndPredsToSet - Add the specified block, and all of its
426 /// predecessors, to the specified set, if it's not already in there. Stop
427 /// predecessor traversal when we reach StopBlock.
428 static void AddBlockAndPredsToSet(BasicBlock *InputBB, BasicBlock *StopBlock,
429 std::set<BasicBlock*> &Blocks) {
430 std::vector<BasicBlock *> WorkList;
431 WorkList.push_back(InputBB);
433 BasicBlock *BB = WorkList.back(); WorkList.pop_back();
434 if (Blocks.insert(BB).second && BB != StopBlock)
435 // If BB is not already processed and it is not a stop block then
436 // insert its predecessor in the work list
437 for (pred_iterator I = pred_begin(BB), E = pred_end(BB); I != E; ++I) {
438 BasicBlock *WBB = *I;
439 WorkList.push_back(WBB);
441 } while(!WorkList.empty());
444 /// FindPHIToPartitionLoops - The first part of loop-nestification is to find a
445 /// PHI node that tells us how to partition the loops.
446 static PHINode *FindPHIToPartitionLoops(Loop *L, DominatorTree *DT,
448 for (BasicBlock::iterator I = L->getHeader()->begin(); isa<PHINode>(I); ) {
449 PHINode *PN = cast<PHINode>(I);
451 if (Value *V = PN->hasConstantValue(DT)) {
452 // This is a degenerate PHI already, don't modify it!
453 PN->replaceAllUsesWith(V);
454 if (AA) AA->deleteValue(PN);
455 PN->eraseFromParent();
459 // Scan this PHI node looking for a use of the PHI node by itself.
460 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
461 if (PN->getIncomingValue(i) == PN &&
462 L->contains(PN->getIncomingBlock(i)))
463 // We found something tasty to remove.
469 // PlaceSplitBlockCarefully - If the block isn't already, move the new block to
470 // right after some 'outside block' block. This prevents the preheader from
471 // being placed inside the loop body, e.g. when the loop hasn't been rotated.
472 void LoopSimplify::PlaceSplitBlockCarefully(BasicBlock *NewBB,
473 SmallVectorImpl<BasicBlock*> &SplitPreds,
475 // Check to see if NewBB is already well placed.
476 Function::iterator BBI = NewBB; --BBI;
477 for (unsigned i = 0, e = SplitPreds.size(); i != e; ++i) {
478 if (&*BBI == SplitPreds[i])
482 // If it isn't already after an outside block, move it after one. This is
483 // always good as it makes the uncond branch from the outside block into a
486 // Figure out *which* outside block to put this after. Prefer an outside
487 // block that neighbors a BB actually in the loop.
488 BasicBlock *FoundBB = 0;
489 for (unsigned i = 0, e = SplitPreds.size(); i != e; ++i) {
490 Function::iterator BBI = SplitPreds[i];
491 if (++BBI != NewBB->getParent()->end() &&
493 FoundBB = SplitPreds[i];
498 // If our heuristic for a *good* bb to place this after doesn't find
499 // anything, just pick something. It's likely better than leaving it within
502 FoundBB = SplitPreds[0];
503 NewBB->moveAfter(FoundBB);
507 /// SeparateNestedLoop - If this loop has multiple backedges, try to pull one of
508 /// them out into a nested loop. This is important for code that looks like
513 /// br cond, Loop, Next
515 /// br cond2, Loop, Out
517 /// To identify this common case, we look at the PHI nodes in the header of the
518 /// loop. PHI nodes with unchanging values on one backedge correspond to values
519 /// that change in the "outer" loop, but not in the "inner" loop.
521 /// If we are able to separate out a loop, return the new outer loop that was
524 Loop *LoopSimplify::SeparateNestedLoop(Loop *L, LPPassManager &LPM) {
525 PHINode *PN = FindPHIToPartitionLoops(L, DT, AA);
526 if (PN == 0) return 0; // No known way to partition.
528 // Pull out all predecessors that have varying values in the loop. This
529 // handles the case when a PHI node has multiple instances of itself as
531 SmallVector<BasicBlock*, 8> OuterLoopPreds;
532 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
533 if (PN->getIncomingValue(i) != PN ||
534 !L->contains(PN->getIncomingBlock(i))) {
535 // We can't split indirectbr edges.
536 if (isa<IndirectBrInst>(PN->getIncomingBlock(i)->getTerminator()))
539 OuterLoopPreds.push_back(PN->getIncomingBlock(i));
542 DEBUG(dbgs() << "LoopSimplify: Splitting out a new outer loop\n");
544 // If ScalarEvolution is around and knows anything about values in
545 // this loop, tell it to forget them, because we're about to
546 // substantially change it.
550 BasicBlock *Header = L->getHeader();
551 BasicBlock *NewBB = SplitBlockPredecessors(Header, &OuterLoopPreds[0],
552 OuterLoopPreds.size(),
555 // Make sure that NewBB is put someplace intelligent, which doesn't mess up
556 // code layout too horribly.
557 PlaceSplitBlockCarefully(NewBB, OuterLoopPreds, L);
559 // Create the new outer loop.
560 Loop *NewOuter = new Loop();
562 // Change the parent loop to use the outer loop as its child now.
563 if (Loop *Parent = L->getParentLoop())
564 Parent->replaceChildLoopWith(L, NewOuter);
566 LI->changeTopLevelLoop(L, NewOuter);
568 // L is now a subloop of our outer loop.
569 NewOuter->addChildLoop(L);
571 // Add the new loop to the pass manager queue.
572 LPM.insertLoopIntoQueue(NewOuter);
574 for (Loop::block_iterator I = L->block_begin(), E = L->block_end();
576 NewOuter->addBlockEntry(*I);
578 // Now reset the header in L, which had been moved by
579 // SplitBlockPredecessors for the outer loop.
580 L->moveToHeader(Header);
582 // Determine which blocks should stay in L and which should be moved out to
583 // the Outer loop now.
584 std::set<BasicBlock*> BlocksInL;
585 for (pred_iterator PI=pred_begin(Header), E = pred_end(Header); PI!=E; ++PI) {
587 if (DT->dominates(Header, P))
588 AddBlockAndPredsToSet(P, Header, BlocksInL);
591 // Scan all of the loop children of L, moving them to OuterLoop if they are
592 // not part of the inner loop.
593 const std::vector<Loop*> &SubLoops = L->getSubLoops();
594 for (size_t I = 0; I != SubLoops.size(); )
595 if (BlocksInL.count(SubLoops[I]->getHeader()))
596 ++I; // Loop remains in L
598 NewOuter->addChildLoop(L->removeChildLoop(SubLoops.begin() + I));
600 // Now that we know which blocks are in L and which need to be moved to
601 // OuterLoop, move any blocks that need it.
602 for (unsigned i = 0; i != L->getBlocks().size(); ++i) {
603 BasicBlock *BB = L->getBlocks()[i];
604 if (!BlocksInL.count(BB)) {
605 // Move this block to the parent, updating the exit blocks sets
606 L->removeBlockFromLoop(BB);
608 LI->changeLoopFor(BB, NewOuter);
618 /// InsertUniqueBackedgeBlock - This method is called when the specified loop
619 /// has more than one backedge in it. If this occurs, revector all of these
620 /// backedges to target a new basic block and have that block branch to the loop
621 /// header. This ensures that loops have exactly one backedge.
624 LoopSimplify::InsertUniqueBackedgeBlock(Loop *L, BasicBlock *Preheader) {
625 assert(L->getNumBackEdges() > 1 && "Must have > 1 backedge!");
627 // Get information about the loop
628 BasicBlock *Header = L->getHeader();
629 Function *F = Header->getParent();
631 // Unique backedge insertion currently depends on having a preheader.
635 // Figure out which basic blocks contain back-edges to the loop header.
636 std::vector<BasicBlock*> BackedgeBlocks;
637 for (pred_iterator I = pred_begin(Header), E = pred_end(Header); I != E; ++I){
640 // Indirectbr edges cannot be split, so we must fail if we find one.
641 if (isa<IndirectBrInst>(P->getTerminator()))
644 if (P != Preheader) BackedgeBlocks.push_back(P);
647 // Create and insert the new backedge block...
648 BasicBlock *BEBlock = BasicBlock::Create(Header->getContext(),
649 Header->getName()+".backedge", F);
650 BranchInst *BETerminator = BranchInst::Create(Header, BEBlock);
652 DEBUG(dbgs() << "LoopSimplify: Inserting unique backedge block ";
653 WriteAsOperand(dbgs(), BEBlock, false);
656 // Move the new backedge block to right after the last backedge block.
657 Function::iterator InsertPos = BackedgeBlocks.back(); ++InsertPos;
658 F->getBasicBlockList().splice(InsertPos, F->getBasicBlockList(), BEBlock);
660 // Now that the block has been inserted into the function, create PHI nodes in
661 // the backedge block which correspond to any PHI nodes in the header block.
662 for (BasicBlock::iterator I = Header->begin(); isa<PHINode>(I); ++I) {
663 PHINode *PN = cast<PHINode>(I);
664 PHINode *NewPN = PHINode::Create(PN->getType(), PN->getName()+".be",
666 NewPN->reserveOperandSpace(BackedgeBlocks.size());
667 if (AA) AA->copyValue(PN, NewPN);
669 // Loop over the PHI node, moving all entries except the one for the
670 // preheader over to the new PHI node.
671 unsigned PreheaderIdx = ~0U;
672 bool HasUniqueIncomingValue = true;
673 Value *UniqueValue = 0;
674 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
675 BasicBlock *IBB = PN->getIncomingBlock(i);
676 Value *IV = PN->getIncomingValue(i);
677 if (IBB == Preheader) {
680 NewPN->addIncoming(IV, IBB);
681 if (HasUniqueIncomingValue) {
682 if (UniqueValue == 0)
684 else if (UniqueValue != IV)
685 HasUniqueIncomingValue = false;
690 // Delete all of the incoming values from the old PN except the preheader's
691 assert(PreheaderIdx != ~0U && "PHI has no preheader entry??");
692 if (PreheaderIdx != 0) {
693 PN->setIncomingValue(0, PN->getIncomingValue(PreheaderIdx));
694 PN->setIncomingBlock(0, PN->getIncomingBlock(PreheaderIdx));
696 // Nuke all entries except the zero'th.
697 for (unsigned i = 0, e = PN->getNumIncomingValues()-1; i != e; ++i)
698 PN->removeIncomingValue(e-i, false);
700 // Finally, add the newly constructed PHI node as the entry for the BEBlock.
701 PN->addIncoming(NewPN, BEBlock);
703 // As an optimization, if all incoming values in the new PhiNode (which is a
704 // subset of the incoming values of the old PHI node) have the same value,
705 // eliminate the PHI Node.
706 if (HasUniqueIncomingValue) {
707 NewPN->replaceAllUsesWith(UniqueValue);
708 if (AA) AA->deleteValue(NewPN);
709 BEBlock->getInstList().erase(NewPN);
713 // Now that all of the PHI nodes have been inserted and adjusted, modify the
714 // backedge blocks to just to the BEBlock instead of the header.
715 for (unsigned i = 0, e = BackedgeBlocks.size(); i != e; ++i) {
716 TerminatorInst *TI = BackedgeBlocks[i]->getTerminator();
717 for (unsigned Op = 0, e = TI->getNumSuccessors(); Op != e; ++Op)
718 if (TI->getSuccessor(Op) == Header)
719 TI->setSuccessor(Op, BEBlock);
722 //===--- Update all analyses which we must preserve now -----------------===//
724 // Update Loop Information - we know that this block is now in the current
725 // loop and all parent loops.
726 L->addBasicBlockToLoop(BEBlock, LI->getBase());
728 // Update dominator information
729 DT->splitBlock(BEBlock);
730 if (DominanceFrontier *DF = getAnalysisIfAvailable<DominanceFrontier>())
731 DF->splitBlock(BEBlock);
736 void LoopSimplify::verifyAnalysis() const {
737 // It used to be possible to just assert L->isLoopSimplifyForm(), however
738 // with the introduction of indirectbr, there are now cases where it's
739 // not possible to transform a loop as necessary. We can at least check
740 // that there is an indirectbr near any time there's trouble.
742 // Indirectbr can interfere with preheader and unique backedge insertion.
743 if (!L->getLoopPreheader() || !L->getLoopLatch()) {
744 bool HasIndBrPred = false;
745 for (pred_iterator PI = pred_begin(L->getHeader()),
746 PE = pred_end(L->getHeader()); PI != PE; ++PI)
747 if (isa<IndirectBrInst>((*PI)->getTerminator())) {
751 assert(HasIndBrPred &&
752 "LoopSimplify has no excuse for missing loop header info!");
755 // Indirectbr can interfere with exit block canonicalization.
756 if (!L->hasDedicatedExits()) {
757 bool HasIndBrExiting = false;
758 SmallVector<BasicBlock*, 8> ExitingBlocks;
759 L->getExitingBlocks(ExitingBlocks);
760 for (unsigned i = 0, e = ExitingBlocks.size(); i != e; ++i)
761 if (isa<IndirectBrInst>((ExitingBlocks[i])->getTerminator())) {
762 HasIndBrExiting = true;
765 assert(HasIndBrExiting &&
766 "LoopSimplify has no excuse for missing exit block info!");