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 // Note that the simplifycfg pass will clean up blocks which are split out but
27 // end up being unnecessary, so usage of this pass should not pessimize
30 // This pass obviously modifies the CFG, but updates loop information and
31 // dominator information.
33 //===----------------------------------------------------------------------===//
35 #define DEBUG_TYPE "loopsimplify"
36 #include "llvm/Transforms/Scalar.h"
37 #include "llvm/Constants.h"
38 #include "llvm/Instructions.h"
39 #include "llvm/Function.h"
40 #include "llvm/LLVMContext.h"
41 #include "llvm/Type.h"
42 #include "llvm/Analysis/AliasAnalysis.h"
43 #include "llvm/Analysis/Dominators.h"
44 #include "llvm/Analysis/LoopInfo.h"
45 #include "llvm/Transforms/Utils/BasicBlockUtils.h"
46 #include "llvm/Transforms/Utils/Local.h"
47 #include "llvm/Support/CFG.h"
48 #include "llvm/Support/Compiler.h"
49 #include "llvm/ADT/SetOperations.h"
50 #include "llvm/ADT/SetVector.h"
51 #include "llvm/ADT/Statistic.h"
52 #include "llvm/ADT/DepthFirstIterator.h"
55 STATISTIC(NumInserted, "Number of pre-header or exit blocks inserted");
56 STATISTIC(NumNested , "Number of nested loops split out");
59 struct VISIBILITY_HIDDEN LoopSimplify : public FunctionPass {
60 static char ID; // Pass identification, replacement for typeid
61 LoopSimplify() : FunctionPass(&ID) {}
63 // AA - If we have an alias analysis object to update, this is it, otherwise
68 virtual bool runOnFunction(Function &F);
70 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
71 // We need loop information to identify the loops...
72 AU.addRequired<LoopInfo>();
73 AU.addRequired<DominatorTree>();
75 AU.addPreserved<LoopInfo>();
76 AU.addPreserved<DominatorTree>();
77 AU.addPreserved<DominanceFrontier>();
78 AU.addPreserved<AliasAnalysis>();
79 AU.addPreservedID(BreakCriticalEdgesID); // No critical edges added.
82 /// verifyAnalysis() - Verify loop nest.
83 void verifyAnalysis() const {
85 LoopInfo *NLI = &getAnalysis<LoopInfo>();
86 for (LoopInfo::iterator I = NLI->begin(), E = NLI->end(); I != E; ++I)
92 bool ProcessLoop(Loop *L);
93 BasicBlock *RewriteLoopExitBlock(Loop *L, BasicBlock *Exit);
94 BasicBlock *InsertPreheaderForLoop(Loop *L);
95 Loop *SeparateNestedLoop(Loop *L);
96 void InsertUniqueBackedgeBlock(Loop *L);
97 void PlaceSplitBlockCarefully(BasicBlock *NewBB,
98 SmallVectorImpl<BasicBlock*> &SplitPreds,
103 char LoopSimplify::ID = 0;
104 static RegisterPass<LoopSimplify>
105 X("loopsimplify", "Canonicalize natural loops", true);
107 // Publically exposed interface to pass...
108 const PassInfo *const llvm::LoopSimplifyID = &X;
109 FunctionPass *llvm::createLoopSimplifyPass() { return new LoopSimplify(); }
111 /// runOnFunction - Run down all loops in the CFG (recursively, but we could do
112 /// it in any convenient order) inserting preheaders...
114 bool LoopSimplify::runOnFunction(Function &F) {
115 bool Changed = false;
116 LI = &getAnalysis<LoopInfo>();
117 AA = getAnalysisIfAvailable<AliasAnalysis>();
118 DT = &getAnalysis<DominatorTree>();
120 // Check to see that no blocks (other than the header) in loops have
121 // predecessors that are not in loops. This is not valid for natural loops,
122 // but can occur if the blocks are unreachable. Since they are unreachable we
123 // can just shamelessly destroy their terminators to make them not branch into
125 for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB) {
126 // This case can only occur for unreachable blocks. Blocks that are
127 // unreachable can't be in loops, so filter those blocks out.
128 if (LI->getLoopFor(BB)) continue;
130 bool BlockUnreachable = false;
131 TerminatorInst *TI = BB->getTerminator();
133 // Check to see if any successors of this block are non-loop-header loops
134 // that are not the header.
135 for (unsigned i = 0, e = TI->getNumSuccessors(); i != e; ++i) {
136 // If this successor is not in a loop, BB is clearly ok.
137 Loop *L = LI->getLoopFor(TI->getSuccessor(i));
140 // If the succ is the loop header, and if L is a top-level loop, then this
141 // is an entrance into a loop through the header, which is also ok.
142 if (L->getHeader() == TI->getSuccessor(i) && L->getParentLoop() == 0)
145 // Otherwise, this is an entrance into a loop from some place invalid.
146 // Either the loop structure is invalid and this is not a natural loop (in
147 // which case the compiler is buggy somewhere else) or BB is unreachable.
148 BlockUnreachable = true;
152 // If this block is ok, check the next one.
153 if (!BlockUnreachable) continue;
155 // Otherwise, this block is dead. To clean up the CFG and to allow later
156 // loop transformations to ignore this case, we delete the edges into the
157 // loop by replacing the terminator.
159 // Remove PHI entries from the successors.
160 for (unsigned i = 0, e = TI->getNumSuccessors(); i != e; ++i)
161 TI->getSuccessor(i)->removePredecessor(BB);
163 // Add a new unreachable instruction before the old terminator.
164 new UnreachableInst(TI);
166 // Delete the dead terminator.
167 if (AA) AA->deleteValue(TI);
168 if (!TI->use_empty())
169 TI->replaceAllUsesWith(Context->getUndef(TI->getType()));
170 TI->eraseFromParent();
174 for (LoopInfo::iterator I = LI->begin(), E = LI->end(); I != E; ++I)
175 Changed |= ProcessLoop(*I);
180 /// ProcessLoop - Walk the loop structure in depth first order, ensuring that
181 /// all loops have preheaders.
183 bool LoopSimplify::ProcessLoop(Loop *L) {
184 bool Changed = false;
187 // Canonicalize inner loops before outer loops. Inner loop canonicalization
188 // can provide work for the outer loop to canonicalize.
189 for (Loop::iterator I = L->begin(), E = L->end(); I != E; ++I)
190 Changed |= ProcessLoop(*I);
192 assert(L->getBlocks()[0] == L->getHeader() &&
193 "Header isn't first block in loop?");
195 // Does the loop already have a preheader? If so, don't insert one.
196 BasicBlock *Preheader = L->getLoopPreheader();
198 Preheader = InsertPreheaderForLoop(L);
203 // Next, check to make sure that all exit nodes of the loop only have
204 // predecessors that are inside of the loop. This check guarantees that the
205 // loop preheader/header will dominate the exit blocks. If the exit block has
206 // predecessors from outside of the loop, split the edge now.
207 SmallVector<BasicBlock*, 8> ExitBlocks;
208 L->getExitBlocks(ExitBlocks);
210 SetVector<BasicBlock*> ExitBlockSet(ExitBlocks.begin(), ExitBlocks.end());
211 for (SetVector<BasicBlock*>::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 RewriteLoopExitBlock(L, ExitBlock);
226 // If the header has more than two predecessors at this point (from the
227 // preheader and from multiple backedges), we must adjust the loop.
228 unsigned NumBackedges = L->getNumBackEdges();
229 if (NumBackedges != 1) {
230 // If this is really a nested loop, rip it out into a child loop. Don't do
231 // this for loops with a giant number of backedges, just factor them into a
232 // common backedge instead.
233 if (NumBackedges < 8) {
234 if (Loop *NL = SeparateNestedLoop(L)) {
236 // 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 InsertUniqueBackedgeBlock(L);
252 // Scan over the PHI nodes in the loop header. Since they now have only two
253 // incoming values (the loop is canonicalized), we may have simplified the PHI
254 // down to 'X = phi [X, Y]', which should be replaced with 'Y'.
256 for (BasicBlock::iterator I = L->getHeader()->begin();
257 (PN = dyn_cast<PHINode>(I++)); )
258 if (Value *V = PN->hasConstantValue()) {
259 if (AA) AA->deleteValue(PN);
260 PN->replaceAllUsesWith(V);
261 PN->eraseFromParent();
264 // If this loop has muliple exits and the exits all go to the same
265 // block, attempt to merge the exits. This helps several passes, such
266 // as LoopRotation, which do not support loops with multiple exits.
267 // SimplifyCFG also does this (and this code uses the same utility
268 // function), however this code is loop-aware, where SimplifyCFG is
269 // not. That gives it the advantage of being able to hoist
270 // loop-invariant instructions out of the way to open up more
271 // opportunities, and the disadvantage of having the responsibility
272 // to preserve dominator information.
273 if (ExitBlocks.size() > 1 && L->getUniqueExitBlock()) {
274 SmallVector<BasicBlock*, 8> ExitingBlocks;
275 L->getExitingBlocks(ExitingBlocks);
276 for (unsigned i = 0, e = ExitingBlocks.size(); i != e; ++i) {
277 BasicBlock *ExitingBlock = ExitingBlocks[i];
278 if (!ExitingBlock->getSinglePredecessor()) continue;
279 BranchInst *BI = dyn_cast<BranchInst>(ExitingBlock->getTerminator());
280 if (!BI || !BI->isConditional()) continue;
281 CmpInst *CI = dyn_cast<CmpInst>(BI->getCondition());
282 if (!CI || CI->getParent() != ExitingBlock) continue;
284 // Attempt to hoist out all instructions except for the
285 // comparison and the branch.
286 bool AllInvariant = true;
287 for (BasicBlock::iterator I = ExitingBlock->begin(); &*I != BI; ) {
288 Instruction *Inst = I++;
291 if (!L->makeLoopInvariant(Inst, Preheader->getTerminator())) {
292 AllInvariant = false;
297 if (!AllInvariant) continue;
299 // The block has now been cleared of all instructions except for
300 // a comparison and a conditional branch. SimplifyCFG may be able
302 if (!FoldBranchToCommonDest(BI)) continue;
304 // Success. The block is now dead, so remove it from the loop,
305 // update the dominator tree and dominance frontier, and delete it.
306 assert(pred_begin(ExitingBlock) == pred_end(ExitingBlock));
308 LI->removeBlock(ExitingBlock);
310 DominanceFrontier *DF = getAnalysisIfAvailable<DominanceFrontier>();
311 DomTreeNode *Node = DT->getNode(ExitingBlock);
312 const std::vector<DomTreeNodeBase<BasicBlock> *> &Children =
314 for (unsigned k = 0, g = Children.size(); k != g; ++k) {
315 DT->changeImmediateDominator(Children[k], Node->getIDom());
316 if (DF) DF->changeImmediateDominator(Children[k]->getBlock(),
317 Node->getIDom()->getBlock(),
320 DT->eraseNode(ExitingBlock);
321 if (DF) DF->removeBlock(ExitingBlock);
323 BI->getSuccessor(0)->removePredecessor(ExitingBlock);
324 BI->getSuccessor(1)->removePredecessor(ExitingBlock);
325 ExitingBlock->eraseFromParent();
332 /// InsertPreheaderForLoop - Once we discover that a loop doesn't have a
333 /// preheader, this method is called to insert one. This method has two phases:
334 /// preheader insertion and analysis updating.
336 BasicBlock *LoopSimplify::InsertPreheaderForLoop(Loop *L) {
337 BasicBlock *Header = L->getHeader();
339 // Compute the set of predecessors of the loop that are not in the loop.
340 SmallVector<BasicBlock*, 8> OutsideBlocks;
341 for (pred_iterator PI = pred_begin(Header), PE = pred_end(Header);
343 if (!L->contains(*PI)) // Coming in from outside the loop?
344 OutsideBlocks.push_back(*PI); // Keep track of it...
346 // Split out the loop pre-header.
348 SplitBlockPredecessors(Header, &OutsideBlocks[0], OutsideBlocks.size(),
352 //===--------------------------------------------------------------------===//
353 // Update analysis results now that we have performed the transformation
356 // We know that we have loop information to update... update it now.
357 if (Loop *Parent = L->getParentLoop())
358 Parent->addBasicBlockToLoop(NewBB, LI->getBase());
360 // Make sure that NewBB is put someplace intelligent, which doesn't mess up
361 // code layout too horribly.
362 PlaceSplitBlockCarefully(NewBB, OutsideBlocks, L);
367 /// RewriteLoopExitBlock - Ensure that the loop preheader dominates all exit
368 /// blocks. This method is used to split exit blocks that have predecessors
369 /// outside of the loop.
370 BasicBlock *LoopSimplify::RewriteLoopExitBlock(Loop *L, BasicBlock *Exit) {
371 SmallVector<BasicBlock*, 8> LoopBlocks;
372 for (pred_iterator I = pred_begin(Exit), E = pred_end(Exit); I != E; ++I)
374 LoopBlocks.push_back(*I);
376 assert(!LoopBlocks.empty() && "No edges coming in from outside the loop?");
377 BasicBlock *NewBB = SplitBlockPredecessors(Exit, &LoopBlocks[0],
378 LoopBlocks.size(), ".loopexit",
381 // Update Loop Information - we know that the new block will be in whichever
382 // loop the Exit block is in. Note that it may not be in that immediate loop,
383 // if the successor is some other loop header. In that case, we continue
384 // walking up the loop tree to find a loop that contains both the successor
385 // block and the predecessor block.
386 Loop *SuccLoop = LI->getLoopFor(Exit);
387 while (SuccLoop && !SuccLoop->contains(L->getHeader()))
388 SuccLoop = SuccLoop->getParentLoop();
390 SuccLoop->addBasicBlockToLoop(NewBB, LI->getBase());
395 /// AddBlockAndPredsToSet - Add the specified block, and all of its
396 /// predecessors, to the specified set, if it's not already in there. Stop
397 /// predecessor traversal when we reach StopBlock.
398 static void AddBlockAndPredsToSet(BasicBlock *InputBB, BasicBlock *StopBlock,
399 std::set<BasicBlock*> &Blocks) {
400 std::vector<BasicBlock *> WorkList;
401 WorkList.push_back(InputBB);
403 BasicBlock *BB = WorkList.back(); WorkList.pop_back();
404 if (Blocks.insert(BB).second && BB != StopBlock)
405 // If BB is not already processed and it is not a stop block then
406 // insert its predecessor in the work list
407 for (pred_iterator I = pred_begin(BB), E = pred_end(BB); I != E; ++I) {
408 BasicBlock *WBB = *I;
409 WorkList.push_back(WBB);
411 } while(!WorkList.empty());
414 /// FindPHIToPartitionLoops - The first part of loop-nestification is to find a
415 /// PHI node that tells us how to partition the loops.
416 static PHINode *FindPHIToPartitionLoops(Loop *L, DominatorTree *DT,
418 for (BasicBlock::iterator I = L->getHeader()->begin(); isa<PHINode>(I); ) {
419 PHINode *PN = cast<PHINode>(I);
421 if (Value *V = PN->hasConstantValue())
422 if (!isa<Instruction>(V) || DT->dominates(cast<Instruction>(V), PN)) {
423 // This is a degenerate PHI already, don't modify it!
424 PN->replaceAllUsesWith(V);
425 if (AA) AA->deleteValue(PN);
426 PN->eraseFromParent();
430 // Scan this PHI node looking for a use of the PHI node by itself.
431 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
432 if (PN->getIncomingValue(i) == PN &&
433 L->contains(PN->getIncomingBlock(i)))
434 // We found something tasty to remove.
440 // PlaceSplitBlockCarefully - If the block isn't already, move the new block to
441 // right after some 'outside block' block. This prevents the preheader from
442 // being placed inside the loop body, e.g. when the loop hasn't been rotated.
443 void LoopSimplify::PlaceSplitBlockCarefully(BasicBlock *NewBB,
444 SmallVectorImpl<BasicBlock*> &SplitPreds,
446 // Check to see if NewBB is already well placed.
447 Function::iterator BBI = NewBB; --BBI;
448 for (unsigned i = 0, e = SplitPreds.size(); i != e; ++i) {
449 if (&*BBI == SplitPreds[i])
453 // If it isn't already after an outside block, move it after one. This is
454 // always good as it makes the uncond branch from the outside block into a
457 // Figure out *which* outside block to put this after. Prefer an outside
458 // block that neighbors a BB actually in the loop.
459 BasicBlock *FoundBB = 0;
460 for (unsigned i = 0, e = SplitPreds.size(); i != e; ++i) {
461 Function::iterator BBI = SplitPreds[i];
462 if (++BBI != NewBB->getParent()->end() &&
464 FoundBB = SplitPreds[i];
469 // If our heuristic for a *good* bb to place this after doesn't find
470 // anything, just pick something. It's likely better than leaving it within
473 FoundBB = SplitPreds[0];
474 NewBB->moveAfter(FoundBB);
478 /// SeparateNestedLoop - If this loop has multiple backedges, try to pull one of
479 /// them out into a nested loop. This is important for code that looks like
484 /// br cond, Loop, Next
486 /// br cond2, Loop, Out
488 /// To identify this common case, we look at the PHI nodes in the header of the
489 /// loop. PHI nodes with unchanging values on one backedge correspond to values
490 /// that change in the "outer" loop, but not in the "inner" loop.
492 /// If we are able to separate out a loop, return the new outer loop that was
495 Loop *LoopSimplify::SeparateNestedLoop(Loop *L) {
496 PHINode *PN = FindPHIToPartitionLoops(L, DT, AA);
497 if (PN == 0) return 0; // No known way to partition.
499 // Pull out all predecessors that have varying values in the loop. This
500 // handles the case when a PHI node has multiple instances of itself as
502 SmallVector<BasicBlock*, 8> OuterLoopPreds;
503 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
504 if (PN->getIncomingValue(i) != PN ||
505 !L->contains(PN->getIncomingBlock(i)))
506 OuterLoopPreds.push_back(PN->getIncomingBlock(i));
508 BasicBlock *Header = L->getHeader();
509 BasicBlock *NewBB = SplitBlockPredecessors(Header, &OuterLoopPreds[0],
510 OuterLoopPreds.size(),
513 // Make sure that NewBB is put someplace intelligent, which doesn't mess up
514 // code layout too horribly.
515 PlaceSplitBlockCarefully(NewBB, OuterLoopPreds, L);
517 // Create the new outer loop.
518 Loop *NewOuter = new Loop();
520 // Change the parent loop to use the outer loop as its child now.
521 if (Loop *Parent = L->getParentLoop())
522 Parent->replaceChildLoopWith(L, NewOuter);
524 LI->changeTopLevelLoop(L, NewOuter);
526 // This block is going to be our new header block: add it to this loop and all
528 NewOuter->addBasicBlockToLoop(NewBB, LI->getBase());
530 // L is now a subloop of our outer loop.
531 NewOuter->addChildLoop(L);
533 for (Loop::block_iterator I = L->block_begin(), E = L->block_end();
535 NewOuter->addBlockEntry(*I);
537 // Determine which blocks should stay in L and which should be moved out to
538 // the Outer loop now.
539 std::set<BasicBlock*> BlocksInL;
540 for (pred_iterator PI = pred_begin(Header), E = pred_end(Header); PI!=E; ++PI)
541 if (DT->dominates(Header, *PI))
542 AddBlockAndPredsToSet(*PI, Header, BlocksInL);
545 // Scan all of the loop children of L, moving them to OuterLoop if they are
546 // not part of the inner loop.
547 const std::vector<Loop*> &SubLoops = L->getSubLoops();
548 for (size_t I = 0; I != SubLoops.size(); )
549 if (BlocksInL.count(SubLoops[I]->getHeader()))
550 ++I; // Loop remains in L
552 NewOuter->addChildLoop(L->removeChildLoop(SubLoops.begin() + I));
554 // Now that we know which blocks are in L and which need to be moved to
555 // OuterLoop, move any blocks that need it.
556 for (unsigned i = 0; i != L->getBlocks().size(); ++i) {
557 BasicBlock *BB = L->getBlocks()[i];
558 if (!BlocksInL.count(BB)) {
559 // Move this block to the parent, updating the exit blocks sets
560 L->removeBlockFromLoop(BB);
562 LI->changeLoopFor(BB, NewOuter);
572 /// InsertUniqueBackedgeBlock - This method is called when the specified loop
573 /// has more than one backedge in it. If this occurs, revector all of these
574 /// backedges to target a new basic block and have that block branch to the loop
575 /// header. This ensures that loops have exactly one backedge.
577 void LoopSimplify::InsertUniqueBackedgeBlock(Loop *L) {
578 assert(L->getNumBackEdges() > 1 && "Must have > 1 backedge!");
580 // Get information about the loop
581 BasicBlock *Preheader = L->getLoopPreheader();
582 BasicBlock *Header = L->getHeader();
583 Function *F = Header->getParent();
585 // Figure out which basic blocks contain back-edges to the loop header.
586 std::vector<BasicBlock*> BackedgeBlocks;
587 for (pred_iterator I = pred_begin(Header), E = pred_end(Header); I != E; ++I)
588 if (*I != Preheader) BackedgeBlocks.push_back(*I);
590 // Create and insert the new backedge block...
591 BasicBlock *BEBlock = BasicBlock::Create(Header->getName()+".backedge", F);
592 BranchInst *BETerminator = BranchInst::Create(Header, BEBlock);
594 // Move the new backedge block to right after the last backedge block.
595 Function::iterator InsertPos = BackedgeBlocks.back(); ++InsertPos;
596 F->getBasicBlockList().splice(InsertPos, F->getBasicBlockList(), BEBlock);
598 // Now that the block has been inserted into the function, create PHI nodes in
599 // the backedge block which correspond to any PHI nodes in the header block.
600 for (BasicBlock::iterator I = Header->begin(); isa<PHINode>(I); ++I) {
601 PHINode *PN = cast<PHINode>(I);
602 PHINode *NewPN = PHINode::Create(PN->getType(), PN->getName()+".be",
604 NewPN->reserveOperandSpace(BackedgeBlocks.size());
605 if (AA) AA->copyValue(PN, NewPN);
607 // Loop over the PHI node, moving all entries except the one for the
608 // preheader over to the new PHI node.
609 unsigned PreheaderIdx = ~0U;
610 bool HasUniqueIncomingValue = true;
611 Value *UniqueValue = 0;
612 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
613 BasicBlock *IBB = PN->getIncomingBlock(i);
614 Value *IV = PN->getIncomingValue(i);
615 if (IBB == Preheader) {
618 NewPN->addIncoming(IV, IBB);
619 if (HasUniqueIncomingValue) {
620 if (UniqueValue == 0)
622 else if (UniqueValue != IV)
623 HasUniqueIncomingValue = false;
628 // Delete all of the incoming values from the old PN except the preheader's
629 assert(PreheaderIdx != ~0U && "PHI has no preheader entry??");
630 if (PreheaderIdx != 0) {
631 PN->setIncomingValue(0, PN->getIncomingValue(PreheaderIdx));
632 PN->setIncomingBlock(0, PN->getIncomingBlock(PreheaderIdx));
634 // Nuke all entries except the zero'th.
635 for (unsigned i = 0, e = PN->getNumIncomingValues()-1; i != e; ++i)
636 PN->removeIncomingValue(e-i, false);
638 // Finally, add the newly constructed PHI node as the entry for the BEBlock.
639 PN->addIncoming(NewPN, BEBlock);
641 // As an optimization, if all incoming values in the new PhiNode (which is a
642 // subset of the incoming values of the old PHI node) have the same value,
643 // eliminate the PHI Node.
644 if (HasUniqueIncomingValue) {
645 NewPN->replaceAllUsesWith(UniqueValue);
646 if (AA) AA->deleteValue(NewPN);
647 BEBlock->getInstList().erase(NewPN);
651 // Now that all of the PHI nodes have been inserted and adjusted, modify the
652 // backedge blocks to just to the BEBlock instead of the header.
653 for (unsigned i = 0, e = BackedgeBlocks.size(); i != e; ++i) {
654 TerminatorInst *TI = BackedgeBlocks[i]->getTerminator();
655 for (unsigned Op = 0, e = TI->getNumSuccessors(); Op != e; ++Op)
656 if (TI->getSuccessor(Op) == Header)
657 TI->setSuccessor(Op, BEBlock);
660 //===--- Update all analyses which we must preserve now -----------------===//
662 // Update Loop Information - we know that this block is now in the current
663 // loop and all parent loops.
664 L->addBasicBlockToLoop(BEBlock, LI->getBase());
666 // Update dominator information
667 DT->splitBlock(BEBlock);
668 if (DominanceFrontier *DF = getAnalysisIfAvailable<DominanceFrontier>())
669 DF->splitBlock(BEBlock);