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/Dominators.h"
50 #include "llvm/Analysis/LoopPass.h"
51 #include "llvm/Analysis/ScalarEvolution.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) {}
70 // AA - If we have an alias analysis object to update, this is it, otherwise
76 virtual bool runOnLoop(Loop *L, LPPassManager &LPM);
78 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
79 // We need loop information to identify the loops...
80 AU.addRequiredTransitive<DominatorTree>();
81 AU.addPreserved<DominatorTree>();
83 // Request DominanceFrontier now, even though LoopSimplify does
84 // not use it. This allows Pass Manager to schedule Dominance
85 // Frontier early enough such that one LPPassManager can handle
86 // multiple loop transformation passes.
87 AU.addRequired<DominanceFrontier>();
88 AU.addPreserved<DominanceFrontier>();
90 AU.addRequiredTransitive<LoopInfo>();
91 AU.addPreserved<LoopInfo>();
93 AU.addPreserved<AliasAnalysis>();
94 AU.addPreserved<ScalarEvolution>();
95 AU.addPreservedID(BreakCriticalEdgesID); // No critical edges added.
98 /// verifyAnalysis() - Verify LoopSimplifyForm's guarantees.
99 void verifyAnalysis() const;
102 bool ProcessLoop(Loop *L, LPPassManager &LPM);
103 BasicBlock *RewriteLoopExitBlock(Loop *L, BasicBlock *Exit);
104 BasicBlock *InsertPreheaderForLoop(Loop *L);
105 Loop *SeparateNestedLoop(Loop *L, LPPassManager &LPM);
106 BasicBlock *InsertUniqueBackedgeBlock(Loop *L, BasicBlock *Preheader);
107 void PlaceSplitBlockCarefully(BasicBlock *NewBB,
108 SmallVectorImpl<BasicBlock*> &SplitPreds,
113 char LoopSimplify::ID = 0;
114 static RegisterPass<LoopSimplify>
115 X("loopsimplify", "Canonicalize natural loops", true);
117 // Publically exposed interface to pass...
118 const PassInfo *const llvm::LoopSimplifyID = &X;
119 Pass *llvm::createLoopSimplifyPass() { return new LoopSimplify(); }
121 /// runOnLoop - Run down all loops in the CFG (recursively, but we could do
122 /// it in any convenient order) inserting preheaders...
124 bool LoopSimplify::runOnLoop(Loop *l, LPPassManager &LPM) {
126 bool Changed = false;
127 LI = &getAnalysis<LoopInfo>();
128 AA = getAnalysisIfAvailable<AliasAnalysis>();
129 DT = &getAnalysis<DominatorTree>();
131 Changed |= ProcessLoop(L, LPM);
136 /// ProcessLoop - Walk the loop structure in depth first order, ensuring that
137 /// all loops have preheaders.
139 bool LoopSimplify::ProcessLoop(Loop *L, LPPassManager &LPM) {
140 bool Changed = false;
143 // Check to see that no blocks (other than the header) in this loop have
144 // predecessors that are not in the loop. This is not valid for natural
145 // loops, but can occur if the blocks are unreachable. Since they are
146 // unreachable we can just shamelessly delete those CFG edges!
147 for (Loop::block_iterator BB = L->block_begin(), E = L->block_end();
149 if (*BB == L->getHeader()) continue;
151 SmallPtrSet<BasicBlock *, 4> BadPreds;
152 for (pred_iterator PI = pred_begin(*BB), PE = pred_end(*BB); PI != PE; ++PI){
158 // Delete each unique out-of-loop (and thus dead) predecessor.
159 for (SmallPtrSet<BasicBlock *, 4>::iterator I = BadPreds.begin(),
160 E = BadPreds.end(); I != E; ++I) {
162 DEBUG(dbgs() << "LoopSimplify: Deleting edge from dead predecessor ";
163 WriteAsOperand(dbgs(), *I, false);
166 // Inform each successor of each dead pred.
167 for (succ_iterator SI = succ_begin(*I), SE = succ_end(*I); SI != SE; ++SI)
168 (*SI)->removePredecessor(*I);
169 // Zap the dead pred's terminator and replace it with unreachable.
170 TerminatorInst *TI = (*I)->getTerminator();
171 TI->replaceAllUsesWith(UndefValue::get(TI->getType()));
172 (*I)->getTerminator()->eraseFromParent();
173 new UnreachableInst((*I)->getContext(), *I);
178 // If there are exiting blocks with branches on undef, resolve the undef in
179 // the direction which will exit the loop. This will help simplify loop
180 // trip count computations.
181 SmallVector<BasicBlock*, 8> ExitingBlocks;
182 L->getExitingBlocks(ExitingBlocks);
183 for (SmallVectorImpl<BasicBlock *>::iterator I = ExitingBlocks.begin(),
184 E = ExitingBlocks.end(); I != E; ++I)
185 if (BranchInst *BI = dyn_cast<BranchInst>((*I)->getTerminator()))
186 if (BI->isConditional()) {
187 if (UndefValue *Cond = dyn_cast<UndefValue>(BI->getCondition())) {
189 DEBUG(dbgs() << "LoopSimplify: Resolving \"br i1 undef\" to exit in ";
190 WriteAsOperand(dbgs(), *I, false);
193 BI->setCondition(ConstantInt::get(Cond->getType(),
194 !L->contains(BI->getSuccessor(0))));
199 // Does the loop already have a preheader? If so, don't insert one.
200 BasicBlock *Preheader = L->getLoopPreheader();
202 Preheader = InsertPreheaderForLoop(L);
209 // Next, check to make sure that all exit nodes of the loop only have
210 // predecessors that are inside of the loop. This check guarantees that the
211 // loop preheader/header will dominate the exit blocks. If the exit block has
212 // predecessors from outside of the loop, split the edge now.
213 SmallVector<BasicBlock*, 8> ExitBlocks;
214 L->getExitBlocks(ExitBlocks);
216 SmallSetVector<BasicBlock *, 8> ExitBlockSet(ExitBlocks.begin(),
218 for (SmallSetVector<BasicBlock *, 8>::iterator I = ExitBlockSet.begin(),
219 E = ExitBlockSet.end(); I != E; ++I) {
220 BasicBlock *ExitBlock = *I;
221 for (pred_iterator PI = pred_begin(ExitBlock), PE = pred_end(ExitBlock);
223 // Must be exactly this loop: no subloops, parent loops, or non-loop preds
225 if (!L->contains(*PI)) {
226 if (RewriteLoopExitBlock(L, ExitBlock)) {
234 // If the header has more than two predecessors at this point (from the
235 // preheader and from multiple backedges), we must adjust the loop.
236 BasicBlock *LoopLatch = L->getLoopLatch();
238 // If this is really a nested loop, rip it out into a child loop. Don't do
239 // this for loops with a giant number of backedges, just factor them into a
240 // common backedge instead.
241 if (L->getNumBackEdges() < 8) {
242 if (SeparateNestedLoop(L, LPM)) {
244 // This is a big restructuring change, reprocess the whole loop.
246 // GCC doesn't tail recursion eliminate this.
251 // If we either couldn't, or didn't want to, identify nesting of the loops,
252 // insert a new block that all backedges target, then make it jump to the
254 LoopLatch = InsertUniqueBackedgeBlock(L, Preheader);
261 // Scan over the PHI nodes in the loop header. Since they now have only two
262 // incoming values (the loop is canonicalized), we may have simplified the PHI
263 // down to 'X = phi [X, Y]', which should be replaced with 'Y'.
265 for (BasicBlock::iterator I = L->getHeader()->begin();
266 (PN = dyn_cast<PHINode>(I++)); )
267 if (Value *V = PN->hasConstantValue(DT)) {
268 if (AA) AA->deleteValue(PN);
269 PN->replaceAllUsesWith(V);
270 PN->eraseFromParent();
273 // If this loop has multiple exits and the exits all go to the same
274 // block, attempt to merge the exits. This helps several passes, such
275 // as LoopRotation, which do not support loops with multiple exits.
276 // SimplifyCFG also does this (and this code uses the same utility
277 // function), however this code is loop-aware, where SimplifyCFG is
278 // not. That gives it the advantage of being able to hoist
279 // loop-invariant instructions out of the way to open up more
280 // opportunities, and the disadvantage of having the responsibility
281 // to preserve dominator information.
282 bool UniqueExit = true;
283 if (!ExitBlocks.empty())
284 for (unsigned i = 1, e = ExitBlocks.size(); i != e; ++i)
285 if (ExitBlocks[i] != ExitBlocks[0]) {
290 for (unsigned i = 0, e = ExitingBlocks.size(); i != e; ++i) {
291 BasicBlock *ExitingBlock = ExitingBlocks[i];
292 if (!ExitingBlock->getSinglePredecessor()) continue;
293 BranchInst *BI = dyn_cast<BranchInst>(ExitingBlock->getTerminator());
294 if (!BI || !BI->isConditional()) continue;
295 CmpInst *CI = dyn_cast<CmpInst>(BI->getCondition());
296 if (!CI || CI->getParent() != ExitingBlock) continue;
298 // Attempt to hoist out all instructions except for the
299 // comparison and the branch.
300 bool AllInvariant = true;
301 for (BasicBlock::iterator I = ExitingBlock->begin(); &*I != BI; ) {
302 Instruction *Inst = I++;
303 // Skip debug info intrinsics.
304 if (isa<DbgInfoIntrinsic>(Inst))
308 if (!L->makeLoopInvariant(Inst, Changed,
309 Preheader ? Preheader->getTerminator() : 0)) {
310 AllInvariant = false;
314 if (!AllInvariant) continue;
316 // The block has now been cleared of all instructions except for
317 // a comparison and a conditional branch. SimplifyCFG may be able
319 if (!FoldBranchToCommonDest(BI)) continue;
321 // Success. The block is now dead, so remove it from the loop,
322 // update the dominator tree and dominance frontier, and delete it.
324 DEBUG(dbgs() << "LoopSimplify: Eliminating exiting block ";
325 WriteAsOperand(dbgs(), ExitingBlock, false);
328 assert(pred_begin(ExitingBlock) == pred_end(ExitingBlock));
330 LI->removeBlock(ExitingBlock);
332 DominanceFrontier *DF = getAnalysisIfAvailable<DominanceFrontier>();
333 DomTreeNode *Node = DT->getNode(ExitingBlock);
334 const std::vector<DomTreeNodeBase<BasicBlock> *> &Children =
336 while (!Children.empty()) {
337 DomTreeNode *Child = Children.front();
338 DT->changeImmediateDominator(Child, Node->getIDom());
339 if (DF) DF->changeImmediateDominator(Child->getBlock(),
340 Node->getIDom()->getBlock(),
343 DT->eraseNode(ExitingBlock);
344 if (DF) DF->removeBlock(ExitingBlock);
346 BI->getSuccessor(0)->removePredecessor(ExitingBlock);
347 BI->getSuccessor(1)->removePredecessor(ExitingBlock);
348 ExitingBlock->eraseFromParent();
355 /// InsertPreheaderForLoop - Once we discover that a loop doesn't have a
356 /// preheader, this method is called to insert one. This method has two phases:
357 /// preheader insertion and analysis updating.
359 BasicBlock *LoopSimplify::InsertPreheaderForLoop(Loop *L) {
360 BasicBlock *Header = L->getHeader();
362 // Compute the set of predecessors of the loop that are not in the loop.
363 SmallVector<BasicBlock*, 8> OutsideBlocks;
364 for (pred_iterator PI = pred_begin(Header), PE = pred_end(Header);
367 if (!L->contains(P)) { // Coming in from outside the loop?
368 // If the loop is branched to from an indirect branch, we won't
369 // be able to fully transform the loop, because it prohibits
371 if (isa<IndirectBrInst>(P->getTerminator())) return 0;
374 OutsideBlocks.push_back(P);
378 // Split out the loop pre-header.
380 SplitBlockPredecessors(Header, &OutsideBlocks[0], OutsideBlocks.size(),
383 DEBUG(dbgs() << "LoopSimplify: Creating pre-header ";
384 WriteAsOperand(dbgs(), NewBB, false);
387 // Make sure that NewBB is put someplace intelligent, which doesn't mess up
388 // code layout too horribly.
389 PlaceSplitBlockCarefully(NewBB, OutsideBlocks, L);
394 /// RewriteLoopExitBlock - Ensure that the loop preheader dominates all exit
395 /// blocks. This method is used to split exit blocks that have predecessors
396 /// outside of the loop.
397 BasicBlock *LoopSimplify::RewriteLoopExitBlock(Loop *L, BasicBlock *Exit) {
398 SmallVector<BasicBlock*, 8> LoopBlocks;
399 for (pred_iterator I = pred_begin(Exit), E = pred_end(Exit); I != E; ++I) {
401 if (L->contains(P)) {
402 // Don't do this if the loop is exited via an indirect branch.
403 if (isa<IndirectBrInst>(P->getTerminator())) return 0;
405 LoopBlocks.push_back(P);
409 assert(!LoopBlocks.empty() && "No edges coming in from outside the loop?");
410 BasicBlock *NewBB = SplitBlockPredecessors(Exit, &LoopBlocks[0],
411 LoopBlocks.size(), ".loopexit",
414 DEBUG(dbgs() << "LoopSimplify: Creating dedicated exit block ";
415 WriteAsOperand(dbgs(), NewBB, false);
421 /// AddBlockAndPredsToSet - Add the specified block, and all of its
422 /// predecessors, to the specified set, if it's not already in there. Stop
423 /// predecessor traversal when we reach StopBlock.
424 static void AddBlockAndPredsToSet(BasicBlock *InputBB, BasicBlock *StopBlock,
425 std::set<BasicBlock*> &Blocks) {
426 std::vector<BasicBlock *> WorkList;
427 WorkList.push_back(InputBB);
429 BasicBlock *BB = WorkList.back(); WorkList.pop_back();
430 if (Blocks.insert(BB).second && BB != StopBlock)
431 // If BB is not already processed and it is not a stop block then
432 // insert its predecessor in the work list
433 for (pred_iterator I = pred_begin(BB), E = pred_end(BB); I != E; ++I) {
434 BasicBlock *WBB = *I;
435 WorkList.push_back(WBB);
437 } while(!WorkList.empty());
440 /// FindPHIToPartitionLoops - The first part of loop-nestification is to find a
441 /// PHI node that tells us how to partition the loops.
442 static PHINode *FindPHIToPartitionLoops(Loop *L, DominatorTree *DT,
444 for (BasicBlock::iterator I = L->getHeader()->begin(); isa<PHINode>(I); ) {
445 PHINode *PN = cast<PHINode>(I);
447 if (Value *V = PN->hasConstantValue(DT)) {
448 // This is a degenerate PHI already, don't modify it!
449 PN->replaceAllUsesWith(V);
450 if (AA) AA->deleteValue(PN);
451 PN->eraseFromParent();
455 // Scan this PHI node looking for a use of the PHI node by itself.
456 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
457 if (PN->getIncomingValue(i) == PN &&
458 L->contains(PN->getIncomingBlock(i)))
459 // We found something tasty to remove.
465 // PlaceSplitBlockCarefully - If the block isn't already, move the new block to
466 // right after some 'outside block' block. This prevents the preheader from
467 // being placed inside the loop body, e.g. when the loop hasn't been rotated.
468 void LoopSimplify::PlaceSplitBlockCarefully(BasicBlock *NewBB,
469 SmallVectorImpl<BasicBlock*> &SplitPreds,
471 // Check to see if NewBB is already well placed.
472 Function::iterator BBI = NewBB; --BBI;
473 for (unsigned i = 0, e = SplitPreds.size(); i != e; ++i) {
474 if (&*BBI == SplitPreds[i])
478 // If it isn't already after an outside block, move it after one. This is
479 // always good as it makes the uncond branch from the outside block into a
482 // Figure out *which* outside block to put this after. Prefer an outside
483 // block that neighbors a BB actually in the loop.
484 BasicBlock *FoundBB = 0;
485 for (unsigned i = 0, e = SplitPreds.size(); i != e; ++i) {
486 Function::iterator BBI = SplitPreds[i];
487 if (++BBI != NewBB->getParent()->end() &&
489 FoundBB = SplitPreds[i];
494 // If our heuristic for a *good* bb to place this after doesn't find
495 // anything, just pick something. It's likely better than leaving it within
498 FoundBB = SplitPreds[0];
499 NewBB->moveAfter(FoundBB);
503 /// SeparateNestedLoop - If this loop has multiple backedges, try to pull one of
504 /// them out into a nested loop. This is important for code that looks like
509 /// br cond, Loop, Next
511 /// br cond2, Loop, Out
513 /// To identify this common case, we look at the PHI nodes in the header of the
514 /// loop. PHI nodes with unchanging values on one backedge correspond to values
515 /// that change in the "outer" loop, but not in the "inner" loop.
517 /// If we are able to separate out a loop, return the new outer loop that was
520 Loop *LoopSimplify::SeparateNestedLoop(Loop *L, LPPassManager &LPM) {
521 PHINode *PN = FindPHIToPartitionLoops(L, DT, AA);
522 if (PN == 0) return 0; // No known way to partition.
524 // Pull out all predecessors that have varying values in the loop. This
525 // handles the case when a PHI node has multiple instances of itself as
527 SmallVector<BasicBlock*, 8> OuterLoopPreds;
528 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
529 if (PN->getIncomingValue(i) != PN ||
530 !L->contains(PN->getIncomingBlock(i))) {
531 // We can't split indirectbr edges.
532 if (isa<IndirectBrInst>(PN->getIncomingBlock(i)->getTerminator()))
535 OuterLoopPreds.push_back(PN->getIncomingBlock(i));
538 DEBUG(dbgs() << "LoopSimplify: Splitting out a new outer loop\n");
540 BasicBlock *Header = L->getHeader();
541 BasicBlock *NewBB = SplitBlockPredecessors(Header, &OuterLoopPreds[0],
542 OuterLoopPreds.size(),
545 // Make sure that NewBB is put someplace intelligent, which doesn't mess up
546 // code layout too horribly.
547 PlaceSplitBlockCarefully(NewBB, OuterLoopPreds, L);
549 // Create the new outer loop.
550 Loop *NewOuter = new Loop();
552 // Change the parent loop to use the outer loop as its child now.
553 if (Loop *Parent = L->getParentLoop())
554 Parent->replaceChildLoopWith(L, NewOuter);
556 LI->changeTopLevelLoop(L, NewOuter);
558 // L is now a subloop of our outer loop.
559 NewOuter->addChildLoop(L);
561 // Add the new loop to the pass manager queue.
562 LPM.insertLoopIntoQueue(NewOuter);
564 for (Loop::block_iterator I = L->block_begin(), E = L->block_end();
566 NewOuter->addBlockEntry(*I);
568 // Now reset the header in L, which had been moved by
569 // SplitBlockPredecessors for the outer loop.
570 L->moveToHeader(Header);
572 // Determine which blocks should stay in L and which should be moved out to
573 // the Outer loop now.
574 std::set<BasicBlock*> BlocksInL;
575 for (pred_iterator PI=pred_begin(Header), E = pred_end(Header); PI!=E; ++PI) {
577 if (DT->dominates(Header, P))
578 AddBlockAndPredsToSet(P, Header, BlocksInL);
581 // Scan all of the loop children of L, moving them to OuterLoop if they are
582 // not part of the inner loop.
583 const std::vector<Loop*> &SubLoops = L->getSubLoops();
584 for (size_t I = 0; I != SubLoops.size(); )
585 if (BlocksInL.count(SubLoops[I]->getHeader()))
586 ++I; // Loop remains in L
588 NewOuter->addChildLoop(L->removeChildLoop(SubLoops.begin() + I));
590 // Now that we know which blocks are in L and which need to be moved to
591 // OuterLoop, move any blocks that need it.
592 for (unsigned i = 0; i != L->getBlocks().size(); ++i) {
593 BasicBlock *BB = L->getBlocks()[i];
594 if (!BlocksInL.count(BB)) {
595 // Move this block to the parent, updating the exit blocks sets
596 L->removeBlockFromLoop(BB);
598 LI->changeLoopFor(BB, NewOuter);
608 /// InsertUniqueBackedgeBlock - This method is called when the specified loop
609 /// has more than one backedge in it. If this occurs, revector all of these
610 /// backedges to target a new basic block and have that block branch to the loop
611 /// header. This ensures that loops have exactly one backedge.
614 LoopSimplify::InsertUniqueBackedgeBlock(Loop *L, BasicBlock *Preheader) {
615 assert(L->getNumBackEdges() > 1 && "Must have > 1 backedge!");
617 // Get information about the loop
618 BasicBlock *Header = L->getHeader();
619 Function *F = Header->getParent();
621 // Unique backedge insertion currently depends on having a preheader.
625 // Figure out which basic blocks contain back-edges to the loop header.
626 std::vector<BasicBlock*> BackedgeBlocks;
627 for (pred_iterator I = pred_begin(Header), E = pred_end(Header); I != E; ++I){
629 if (P != Preheader) BackedgeBlocks.push_back(P);
632 // Create and insert the new backedge block...
633 BasicBlock *BEBlock = BasicBlock::Create(Header->getContext(),
634 Header->getName()+".backedge", F);
635 BranchInst *BETerminator = BranchInst::Create(Header, BEBlock);
637 DEBUG(dbgs() << "LoopSimplify: Inserting unique backedge block ";
638 WriteAsOperand(dbgs(), BEBlock, false);
641 // Move the new backedge block to right after the last backedge block.
642 Function::iterator InsertPos = BackedgeBlocks.back(); ++InsertPos;
643 F->getBasicBlockList().splice(InsertPos, F->getBasicBlockList(), BEBlock);
645 // Now that the block has been inserted into the function, create PHI nodes in
646 // the backedge block which correspond to any PHI nodes in the header block.
647 for (BasicBlock::iterator I = Header->begin(); isa<PHINode>(I); ++I) {
648 PHINode *PN = cast<PHINode>(I);
649 PHINode *NewPN = PHINode::Create(PN->getType(), PN->getName()+".be",
651 NewPN->reserveOperandSpace(BackedgeBlocks.size());
652 if (AA) AA->copyValue(PN, NewPN);
654 // Loop over the PHI node, moving all entries except the one for the
655 // preheader over to the new PHI node.
656 unsigned PreheaderIdx = ~0U;
657 bool HasUniqueIncomingValue = true;
658 Value *UniqueValue = 0;
659 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
660 BasicBlock *IBB = PN->getIncomingBlock(i);
661 Value *IV = PN->getIncomingValue(i);
662 if (IBB == Preheader) {
665 NewPN->addIncoming(IV, IBB);
666 if (HasUniqueIncomingValue) {
667 if (UniqueValue == 0)
669 else if (UniqueValue != IV)
670 HasUniqueIncomingValue = false;
675 // Delete all of the incoming values from the old PN except the preheader's
676 assert(PreheaderIdx != ~0U && "PHI has no preheader entry??");
677 if (PreheaderIdx != 0) {
678 PN->setIncomingValue(0, PN->getIncomingValue(PreheaderIdx));
679 PN->setIncomingBlock(0, PN->getIncomingBlock(PreheaderIdx));
681 // Nuke all entries except the zero'th.
682 for (unsigned i = 0, e = PN->getNumIncomingValues()-1; i != e; ++i)
683 PN->removeIncomingValue(e-i, false);
685 // Finally, add the newly constructed PHI node as the entry for the BEBlock.
686 PN->addIncoming(NewPN, BEBlock);
688 // As an optimization, if all incoming values in the new PhiNode (which is a
689 // subset of the incoming values of the old PHI node) have the same value,
690 // eliminate the PHI Node.
691 if (HasUniqueIncomingValue) {
692 NewPN->replaceAllUsesWith(UniqueValue);
693 if (AA) AA->deleteValue(NewPN);
694 BEBlock->getInstList().erase(NewPN);
698 // Now that all of the PHI nodes have been inserted and adjusted, modify the
699 // backedge blocks to just to the BEBlock instead of the header.
700 for (unsigned i = 0, e = BackedgeBlocks.size(); i != e; ++i) {
701 TerminatorInst *TI = BackedgeBlocks[i]->getTerminator();
702 for (unsigned Op = 0, e = TI->getNumSuccessors(); Op != e; ++Op)
703 if (TI->getSuccessor(Op) == Header)
704 TI->setSuccessor(Op, BEBlock);
707 //===--- Update all analyses which we must preserve now -----------------===//
709 // Update Loop Information - we know that this block is now in the current
710 // loop and all parent loops.
711 L->addBasicBlockToLoop(BEBlock, LI->getBase());
713 // Update dominator information
714 DT->splitBlock(BEBlock);
715 if (DominanceFrontier *DF = getAnalysisIfAvailable<DominanceFrontier>())
716 DF->splitBlock(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())) {
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;
750 assert(HasIndBrExiting &&
751 "LoopSimplify has no excuse for missing exit block info!");