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<LoopInfo>();
81 AU.addRequiredTransitive<DominatorTree>();
83 AU.addPreserved<LoopInfo>();
84 AU.addPreserved<DominatorTree>();
85 AU.addPreserved<DominanceFrontier>();
86 AU.addPreserved<AliasAnalysis>();
87 AU.addPreserved<ScalarEvolution>();
88 AU.addPreservedID(BreakCriticalEdgesID); // No critical edges added.
91 /// verifyAnalysis() - Verify LoopSimplifyForm's guarantees.
92 void verifyAnalysis() const;
95 bool ProcessLoop(Loop *L, LPPassManager &LPM);
96 BasicBlock *RewriteLoopExitBlock(Loop *L, BasicBlock *Exit);
97 BasicBlock *InsertPreheaderForLoop(Loop *L);
98 Loop *SeparateNestedLoop(Loop *L, LPPassManager &LPM);
99 BasicBlock *InsertUniqueBackedgeBlock(Loop *L, BasicBlock *Preheader);
100 void PlaceSplitBlockCarefully(BasicBlock *NewBB,
101 SmallVectorImpl<BasicBlock*> &SplitPreds,
106 char LoopSimplify::ID = 0;
107 static RegisterPass<LoopSimplify>
108 X("loopsimplify", "Canonicalize natural loops", true);
110 // Publically exposed interface to pass...
111 const PassInfo *const llvm::LoopSimplifyID = &X;
112 Pass *llvm::createLoopSimplifyPass() { return new LoopSimplify(); }
114 /// runOnLoop - Run down all loops in the CFG (recursively, but we could do
115 /// it in any convenient order) inserting preheaders...
117 bool LoopSimplify::runOnLoop(Loop *l, LPPassManager &LPM) {
119 bool Changed = false;
120 LI = &getAnalysis<LoopInfo>();
121 AA = getAnalysisIfAvailable<AliasAnalysis>();
122 DT = &getAnalysis<DominatorTree>();
124 Changed |= ProcessLoop(L, LPM);
129 /// ProcessLoop - Walk the loop structure in depth first order, ensuring that
130 /// all loops have preheaders.
132 bool LoopSimplify::ProcessLoop(Loop *L, LPPassManager &LPM) {
133 bool Changed = false;
136 // Check to see that no blocks (other than the header) in this loop have
137 // predecessors that are not in the loop. This is not valid for natural
138 // loops, but can occur if the blocks are unreachable. Since they are
139 // unreachable we can just shamelessly delete those CFG edges!
140 for (Loop::block_iterator BB = L->block_begin(), E = L->block_end();
142 if (*BB == L->getHeader()) continue;
144 SmallPtrSet<BasicBlock *, 4> BadPreds;
145 for (pred_iterator PI = pred_begin(*BB), PE = pred_end(*BB); PI != PE; ++PI){
151 // Delete each unique out-of-loop (and thus dead) predecessor.
152 for (SmallPtrSet<BasicBlock *, 4>::iterator I = BadPreds.begin(),
153 E = BadPreds.end(); I != E; ++I) {
155 DEBUG(dbgs() << "LoopSimplify: Deleting edge from dead predecessor ";
156 WriteAsOperand(dbgs(), *I, false);
159 // Inform each successor of each dead pred.
160 for (succ_iterator SI = succ_begin(*I), SE = succ_end(*I); SI != SE; ++SI)
161 (*SI)->removePredecessor(*I);
162 // Zap the dead pred's terminator and replace it with unreachable.
163 TerminatorInst *TI = (*I)->getTerminator();
164 TI->replaceAllUsesWith(UndefValue::get(TI->getType()));
165 (*I)->getTerminator()->eraseFromParent();
166 new UnreachableInst((*I)->getContext(), *I);
171 // If there are exiting blocks with branches on undef, resolve the undef in
172 // the direction which will exit the loop. This will help simplify loop
173 // trip count computations.
174 SmallVector<BasicBlock*, 8> ExitingBlocks;
175 L->getExitingBlocks(ExitingBlocks);
176 for (SmallVectorImpl<BasicBlock *>::iterator I = ExitingBlocks.begin(),
177 E = ExitingBlocks.end(); I != E; ++I)
178 if (BranchInst *BI = dyn_cast<BranchInst>((*I)->getTerminator()))
179 if (BI->isConditional()) {
180 if (UndefValue *Cond = dyn_cast<UndefValue>(BI->getCondition())) {
182 DEBUG(dbgs() << "LoopSimplify: Resolving \"br i1 undef\" to exit in ";
183 WriteAsOperand(dbgs(), *I, false);
186 BI->setCondition(ConstantInt::get(Cond->getType(),
187 !L->contains(BI->getSuccessor(0))));
192 // Does the loop already have a preheader? If so, don't insert one.
193 BasicBlock *Preheader = L->getLoopPreheader();
195 Preheader = InsertPreheaderForLoop(L);
202 // Next, check to make sure that all exit nodes of the loop only have
203 // predecessors that are inside of the loop. This check guarantees that the
204 // loop preheader/header will dominate the exit blocks. If the exit block has
205 // predecessors from outside of the loop, split the edge now.
206 SmallVector<BasicBlock*, 8> ExitBlocks;
207 L->getExitBlocks(ExitBlocks);
209 SmallSetVector<BasicBlock *, 8> ExitBlockSet(ExitBlocks.begin(),
211 for (SmallSetVector<BasicBlock *, 8>::iterator I = ExitBlockSet.begin(),
212 E = ExitBlockSet.end(); I != E; ++I) {
213 BasicBlock *ExitBlock = *I;
214 for (pred_iterator PI = pred_begin(ExitBlock), PE = pred_end(ExitBlock);
216 // Must be exactly this loop: no subloops, parent loops, or non-loop preds
218 if (!L->contains(*PI)) {
219 if (RewriteLoopExitBlock(L, ExitBlock)) {
227 // If the header has more than two predecessors at this point (from the
228 // preheader and from multiple backedges), we must adjust the loop.
229 BasicBlock *LoopLatch = L->getLoopLatch();
231 // If this is really a nested loop, rip it out into a child loop. Don't do
232 // this for loops with a giant number of backedges, just factor them into a
233 // common backedge instead.
234 if (L->getNumBackEdges() < 8) {
235 if (SeparateNestedLoop(L, LPM)) {
237 // This is a big restructuring change, reprocess the whole loop.
239 // GCC doesn't tail recursion eliminate this.
244 // If we either couldn't, or didn't want to, identify nesting of the loops,
245 // insert a new block that all backedges target, then make it jump to the
247 LoopLatch = InsertUniqueBackedgeBlock(L, Preheader);
254 // Scan over the PHI nodes in the loop header. Since they now have only two
255 // incoming values (the loop is canonicalized), we may have simplified the PHI
256 // down to 'X = phi [X, Y]', which should be replaced with 'Y'.
258 for (BasicBlock::iterator I = L->getHeader()->begin();
259 (PN = dyn_cast<PHINode>(I++)); )
260 if (Value *V = PN->hasConstantValue(DT)) {
261 if (AA) AA->deleteValue(PN);
262 PN->replaceAllUsesWith(V);
263 PN->eraseFromParent();
266 // If this loop has multiple exits and the exits all go to the same
267 // block, attempt to merge the exits. This helps several passes, such
268 // as LoopRotation, which do not support loops with multiple exits.
269 // SimplifyCFG also does this (and this code uses the same utility
270 // function), however this code is loop-aware, where SimplifyCFG is
271 // not. That gives it the advantage of being able to hoist
272 // loop-invariant instructions out of the way to open up more
273 // opportunities, and the disadvantage of having the responsibility
274 // to preserve dominator information.
275 bool UniqueExit = true;
276 if (!ExitBlocks.empty())
277 for (unsigned i = 1, e = ExitBlocks.size(); i != e; ++i)
278 if (ExitBlocks[i] != ExitBlocks[0]) {
283 for (unsigned i = 0, e = ExitingBlocks.size(); i != e; ++i) {
284 BasicBlock *ExitingBlock = ExitingBlocks[i];
285 if (!ExitingBlock->getSinglePredecessor()) continue;
286 BranchInst *BI = dyn_cast<BranchInst>(ExitingBlock->getTerminator());
287 if (!BI || !BI->isConditional()) continue;
288 CmpInst *CI = dyn_cast<CmpInst>(BI->getCondition());
289 if (!CI || CI->getParent() != ExitingBlock) continue;
291 // Attempt to hoist out all instructions except for the
292 // comparison and the branch.
293 bool AllInvariant = true;
294 for (BasicBlock::iterator I = ExitingBlock->begin(); &*I != BI; ) {
295 Instruction *Inst = I++;
296 // Skip debug info intrinsics.
297 if (isa<DbgInfoIntrinsic>(Inst))
301 if (!L->makeLoopInvariant(Inst, Changed,
302 Preheader ? Preheader->getTerminator() : 0)) {
303 AllInvariant = false;
307 if (!AllInvariant) continue;
309 // The block has now been cleared of all instructions except for
310 // a comparison and a conditional branch. SimplifyCFG may be able
312 if (!FoldBranchToCommonDest(BI)) continue;
314 // Success. The block is now dead, so remove it from the loop,
315 // update the dominator tree and dominance frontier, and delete it.
317 DEBUG(dbgs() << "LoopSimplify: Eliminating exiting block ";
318 WriteAsOperand(dbgs(), ExitingBlock, false);
321 assert(pred_begin(ExitingBlock) == pred_end(ExitingBlock));
323 LI->removeBlock(ExitingBlock);
325 DominanceFrontier *DF = getAnalysisIfAvailable<DominanceFrontier>();
326 DomTreeNode *Node = DT->getNode(ExitingBlock);
327 const std::vector<DomTreeNodeBase<BasicBlock> *> &Children =
329 while (!Children.empty()) {
330 DomTreeNode *Child = Children.front();
331 DT->changeImmediateDominator(Child, Node->getIDom());
332 if (DF) DF->changeImmediateDominator(Child->getBlock(),
333 Node->getIDom()->getBlock(),
336 DT->eraseNode(ExitingBlock);
337 if (DF) DF->removeBlock(ExitingBlock);
339 BI->getSuccessor(0)->removePredecessor(ExitingBlock);
340 BI->getSuccessor(1)->removePredecessor(ExitingBlock);
341 ExitingBlock->eraseFromParent();
348 /// InsertPreheaderForLoop - Once we discover that a loop doesn't have a
349 /// preheader, this method is called to insert one. This method has two phases:
350 /// preheader insertion and analysis updating.
352 BasicBlock *LoopSimplify::InsertPreheaderForLoop(Loop *L) {
353 BasicBlock *Header = L->getHeader();
355 // Compute the set of predecessors of the loop that are not in the loop.
356 SmallVector<BasicBlock*, 8> OutsideBlocks;
357 for (pred_iterator PI = pred_begin(Header), PE = pred_end(Header);
360 if (!L->contains(P)) { // Coming in from outside the loop?
361 // If the loop is branched to from an indirect branch, we won't
362 // be able to fully transform the loop, because it prohibits
364 if (isa<IndirectBrInst>(P->getTerminator())) return 0;
367 OutsideBlocks.push_back(P);
371 // Split out the loop pre-header.
373 SplitBlockPredecessors(Header, &OutsideBlocks[0], OutsideBlocks.size(),
376 DEBUG(dbgs() << "LoopSimplify: Creating pre-header ";
377 WriteAsOperand(dbgs(), NewBB, false);
380 // Make sure that NewBB is put someplace intelligent, which doesn't mess up
381 // code layout too horribly.
382 PlaceSplitBlockCarefully(NewBB, OutsideBlocks, L);
387 /// RewriteLoopExitBlock - Ensure that the loop preheader dominates all exit
388 /// blocks. This method is used to split exit blocks that have predecessors
389 /// outside of the loop.
390 BasicBlock *LoopSimplify::RewriteLoopExitBlock(Loop *L, BasicBlock *Exit) {
391 SmallVector<BasicBlock*, 8> LoopBlocks;
392 for (pred_iterator I = pred_begin(Exit), E = pred_end(Exit); I != E; ++I) {
394 if (L->contains(P)) {
395 // Don't do this if the loop is exited via an indirect branch.
396 if (isa<IndirectBrInst>(P->getTerminator())) return 0;
398 LoopBlocks.push_back(P);
402 assert(!LoopBlocks.empty() && "No edges coming in from outside the loop?");
403 BasicBlock *NewBB = SplitBlockPredecessors(Exit, &LoopBlocks[0],
404 LoopBlocks.size(), ".loopexit",
407 DEBUG(dbgs() << "LoopSimplify: Creating dedicated exit block ";
408 WriteAsOperand(dbgs(), NewBB, false);
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);
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,
437 for (BasicBlock::iterator I = L->getHeader()->begin(); isa<PHINode>(I); ) {
438 PHINode *PN = cast<PHINode>(I);
440 if (Value *V = PN->hasConstantValue(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();
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.
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,
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])
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
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() &&
482 FoundBB = SplitPreds[i];
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
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
502 /// br cond, Loop, Next
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
513 Loop *LoopSimplify::SeparateNestedLoop(Loop *L, LPPassManager &LPM) {
514 PHINode *PN = FindPHIToPartitionLoops(L, DT, AA);
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
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()))
528 OuterLoopPreds.push_back(PN->getIncomingBlock(i));
531 DEBUG(dbgs() << "LoopSimplify: Splitting out a new outer loop\n");
533 BasicBlock *Header = L->getHeader();
534 BasicBlock *NewBB = SplitBlockPredecessors(Header, &OuterLoopPreds[0],
535 OuterLoopPreds.size(),
538 // Make sure that NewBB is put someplace intelligent, which doesn't mess up
539 // code layout too horribly.
540 PlaceSplitBlockCarefully(NewBB, OuterLoopPreds, L);
542 // Create the new outer loop.
543 Loop *NewOuter = new Loop();
545 // Change the parent loop to use the outer loop as its child now.
546 if (Loop *Parent = L->getParentLoop())
547 Parent->replaceChildLoopWith(L, NewOuter);
549 LI->changeTopLevelLoop(L, NewOuter);
551 // L is now a subloop of our outer loop.
552 NewOuter->addChildLoop(L);
554 // Add the new loop to the pass manager queue.
555 LPM.insertLoopIntoQueue(NewOuter);
557 for (Loop::block_iterator I = L->block_begin(), E = L->block_end();
559 NewOuter->addBlockEntry(*I);
561 // Now reset the header in L, which had been moved by
562 // SplitBlockPredecessors for the outer loop.
563 L->moveToHeader(Header);
565 // Determine which blocks should stay in L and which should be moved out to
566 // the Outer loop now.
567 std::set<BasicBlock*> BlocksInL;
568 for (pred_iterator PI=pred_begin(Header), E = pred_end(Header); PI!=E; ++PI) {
570 if (DT->dominates(Header, P))
571 AddBlockAndPredsToSet(P, Header, BlocksInL);
574 // Scan all of the loop children of L, moving them to OuterLoop if they are
575 // not part of the inner loop.
576 const std::vector<Loop*> &SubLoops = L->getSubLoops();
577 for (size_t I = 0; I != SubLoops.size(); )
578 if (BlocksInL.count(SubLoops[I]->getHeader()))
579 ++I; // Loop remains in L
581 NewOuter->addChildLoop(L->removeChildLoop(SubLoops.begin() + I));
583 // Now that we know which blocks are in L and which need to be moved to
584 // OuterLoop, move any blocks that need it.
585 for (unsigned i = 0; i != L->getBlocks().size(); ++i) {
586 BasicBlock *BB = L->getBlocks()[i];
587 if (!BlocksInL.count(BB)) {
588 // Move this block to the parent, updating the exit blocks sets
589 L->removeBlockFromLoop(BB);
591 LI->changeLoopFor(BB, NewOuter);
601 /// InsertUniqueBackedgeBlock - This method is called when the specified loop
602 /// has more than one backedge in it. If this occurs, revector all of these
603 /// backedges to target a new basic block and have that block branch to the loop
604 /// header. This ensures that loops have exactly one backedge.
607 LoopSimplify::InsertUniqueBackedgeBlock(Loop *L, BasicBlock *Preheader) {
608 assert(L->getNumBackEdges() > 1 && "Must have > 1 backedge!");
610 // Get information about the loop
611 BasicBlock *Header = L->getHeader();
612 Function *F = Header->getParent();
614 // Unique backedge insertion currently depends on having a preheader.
618 // Figure out which basic blocks contain back-edges to the loop header.
619 std::vector<BasicBlock*> BackedgeBlocks;
620 for (pred_iterator I = pred_begin(Header), E = pred_end(Header); I != E; ++I)
621 if (*I != Preheader) BackedgeBlocks.push_back(*I);
623 // Create and insert the new backedge block...
624 BasicBlock *BEBlock = BasicBlock::Create(Header->getContext(),
625 Header->getName()+".backedge", F);
626 BranchInst *BETerminator = BranchInst::Create(Header, BEBlock);
628 DEBUG(dbgs() << "LoopSimplify: Inserting unique backedge block ";
629 WriteAsOperand(dbgs(), BEBlock, false);
632 // Move the new backedge block to right after the last backedge block.
633 Function::iterator InsertPos = BackedgeBlocks.back(); ++InsertPos;
634 F->getBasicBlockList().splice(InsertPos, F->getBasicBlockList(), BEBlock);
636 // Now that the block has been inserted into the function, create PHI nodes in
637 // the backedge block which correspond to any PHI nodes in the header block.
638 for (BasicBlock::iterator I = Header->begin(); isa<PHINode>(I); ++I) {
639 PHINode *PN = cast<PHINode>(I);
640 PHINode *NewPN = PHINode::Create(PN->getType(), PN->getName()+".be",
642 NewPN->reserveOperandSpace(BackedgeBlocks.size());
643 if (AA) AA->copyValue(PN, NewPN);
645 // Loop over the PHI node, moving all entries except the one for the
646 // preheader over to the new PHI node.
647 unsigned PreheaderIdx = ~0U;
648 bool HasUniqueIncomingValue = true;
649 Value *UniqueValue = 0;
650 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
651 BasicBlock *IBB = PN->getIncomingBlock(i);
652 Value *IV = PN->getIncomingValue(i);
653 if (IBB == Preheader) {
656 NewPN->addIncoming(IV, IBB);
657 if (HasUniqueIncomingValue) {
658 if (UniqueValue == 0)
660 else if (UniqueValue != IV)
661 HasUniqueIncomingValue = false;
666 // Delete all of the incoming values from the old PN except the preheader's
667 assert(PreheaderIdx != ~0U && "PHI has no preheader entry??");
668 if (PreheaderIdx != 0) {
669 PN->setIncomingValue(0, PN->getIncomingValue(PreheaderIdx));
670 PN->setIncomingBlock(0, PN->getIncomingBlock(PreheaderIdx));
672 // Nuke all entries except the zero'th.
673 for (unsigned i = 0, e = PN->getNumIncomingValues()-1; i != e; ++i)
674 PN->removeIncomingValue(e-i, false);
676 // Finally, add the newly constructed PHI node as the entry for the BEBlock.
677 PN->addIncoming(NewPN, BEBlock);
679 // As an optimization, if all incoming values in the new PhiNode (which is a
680 // subset of the incoming values of the old PHI node) have the same value,
681 // eliminate the PHI Node.
682 if (HasUniqueIncomingValue) {
683 NewPN->replaceAllUsesWith(UniqueValue);
684 if (AA) AA->deleteValue(NewPN);
685 BEBlock->getInstList().erase(NewPN);
689 // Now that all of the PHI nodes have been inserted and adjusted, modify the
690 // backedge blocks to just to the BEBlock instead of the header.
691 for (unsigned i = 0, e = BackedgeBlocks.size(); i != e; ++i) {
692 TerminatorInst *TI = BackedgeBlocks[i]->getTerminator();
693 for (unsigned Op = 0, e = TI->getNumSuccessors(); Op != e; ++Op)
694 if (TI->getSuccessor(Op) == Header)
695 TI->setSuccessor(Op, BEBlock);
698 //===--- Update all analyses which we must preserve now -----------------===//
700 // Update Loop Information - we know that this block is now in the current
701 // loop and all parent loops.
702 L->addBasicBlockToLoop(BEBlock, LI->getBase());
704 // Update dominator information
705 DT->splitBlock(BEBlock);
706 if (DominanceFrontier *DF = getAnalysisIfAvailable<DominanceFrontier>())
707 DF->splitBlock(BEBlock);
712 void LoopSimplify::verifyAnalysis() const {
713 // It used to be possible to just assert L->isLoopSimplifyForm(), however
714 // with the introduction of indirectbr, there are now cases where it's
715 // not possible to transform a loop as necessary. We can at least check
716 // that there is an indirectbr near any time there's trouble.
718 // Indirectbr can interfere with preheader and unique backedge insertion.
719 if (!L->getLoopPreheader() || !L->getLoopLatch()) {
720 bool HasIndBrPred = false;
721 for (pred_iterator PI = pred_begin(L->getHeader()),
722 PE = pred_end(L->getHeader()); PI != PE; ++PI)
723 if (isa<IndirectBrInst>((*PI)->getTerminator())) {
727 assert(HasIndBrPred &&
728 "LoopSimplify has no excuse for missing loop header info!");
731 // Indirectbr can interfere with exit block canonicalization.
732 if (!L->hasDedicatedExits()) {
733 bool HasIndBrExiting = false;
734 SmallVector<BasicBlock*, 8> ExitingBlocks;
735 L->getExitingBlocks(ExitingBlocks);
736 for (unsigned i = 0, e = ExitingBlocks.size(); i != e; ++i)
737 if (isa<IndirectBrInst>((ExitingBlocks[i])->getTerminator())) {
738 HasIndBrExiting = true;
741 assert(HasIndBrExiting &&
742 "LoopSimplify has no excuse for missing exit block info!");