1 //===- LoopSimplify.cpp - Loop Canonicalization Pass ----------------------===//
3 // The LLVM Compiler Infrastructure
5 // This file was developed by the LLVM research group and is distributed under
6 // the University of Illinois Open Source 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/Constant.h"
38 #include "llvm/Instructions.h"
39 #include "llvm/Function.h"
40 #include "llvm/Type.h"
41 #include "llvm/Analysis/AliasAnalysis.h"
42 #include "llvm/Analysis/Dominators.h"
43 #include "llvm/Analysis/LoopInfo.h"
44 #include "llvm/Support/CFG.h"
45 #include "llvm/Support/Compiler.h"
46 #include "llvm/ADT/SetOperations.h"
47 #include "llvm/ADT/SetVector.h"
48 #include "llvm/ADT/Statistic.h"
49 #include "llvm/ADT/DepthFirstIterator.h"
52 STATISTIC(NumInserted, "Number of pre-header or exit blocks inserted");
53 STATISTIC(NumNested , "Number of nested loops split out");
56 struct VISIBILITY_HIDDEN LoopSimplify : public FunctionPass {
57 static char ID; // Pass identification, replacement for typeid
58 LoopSimplify() : FunctionPass((intptr_t)&ID) {}
60 // AA - If we have an alias analysis object to update, this is it, otherwise
65 virtual bool runOnFunction(Function &F);
67 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
68 // We need loop information to identify the loops...
69 AU.addRequired<LoopInfo>();
70 AU.addRequired<DominatorTree>();
72 AU.addPreserved<LoopInfo>();
73 AU.addPreserved<DominatorTree>();
74 AU.addPreserved<DominanceFrontier>();
75 AU.addPreservedID(BreakCriticalEdgesID); // No critical edges added.
78 /// verifyAnalysis() - Verify loop nest.
79 void verifyAnalysis() const {
81 LoopInfo *NLI = &getAnalysis<LoopInfo>();
82 for (LoopInfo::iterator I = NLI->begin(), E = NLI->end(); I != E; ++I)
88 bool ProcessLoop(Loop *L);
89 BasicBlock *SplitBlockPredecessors(BasicBlock *BB, const char *Suffix,
90 const std::vector<BasicBlock*> &Preds);
91 BasicBlock *RewriteLoopExitBlock(Loop *L, BasicBlock *Exit);
92 void InsertPreheaderForLoop(Loop *L);
93 Loop *SeparateNestedLoop(Loop *L);
94 void InsertUniqueBackedgeBlock(Loop *L);
95 void PlaceSplitBlockCarefully(BasicBlock *NewBB,
96 std::vector<BasicBlock*> &SplitPreds,
100 char LoopSimplify::ID = 0;
101 RegisterPass<LoopSimplify>
102 X("loopsimplify", "Canonicalize natural loops", true);
105 // Publically exposed interface to pass...
106 const PassInfo *llvm::LoopSimplifyID = X.getPassInfo();
107 FunctionPass *llvm::createLoopSimplifyPass() { return new LoopSimplify(); }
109 /// runOnFunction - Run down all loops in the CFG (recursively, but we could do
110 /// it in any convenient order) inserting preheaders...
112 bool LoopSimplify::runOnFunction(Function &F) {
113 bool Changed = false;
114 LI = &getAnalysis<LoopInfo>();
115 AA = getAnalysisToUpdate<AliasAnalysis>();
116 DT = &getAnalysis<DominatorTree>();
118 // Check to see that no blocks (other than the header) in loops have
119 // predecessors that are not in loops. This is not valid for natural loops,
120 // but can occur if the blocks are unreachable. Since they are unreachable we
121 // can just shamelessly destroy their terminators to make them not branch into
123 for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB) {
124 // This case can only occur for unreachable blocks. Blocks that are
125 // unreachable can't be in loops, so filter those blocks out.
126 if (LI->getLoopFor(BB)) continue;
128 bool BlockUnreachable = false;
129 TerminatorInst *TI = BB->getTerminator();
131 // Check to see if any successors of this block are non-loop-header loops
132 // that are not the header.
133 for (unsigned i = 0, e = TI->getNumSuccessors(); i != e; ++i) {
134 // If this successor is not in a loop, BB is clearly ok.
135 Loop *L = LI->getLoopFor(TI->getSuccessor(i));
138 // If the succ is the loop header, and if L is a top-level loop, then this
139 // is an entrance into a loop through the header, which is also ok.
140 if (L->getHeader() == TI->getSuccessor(i) && L->getParentLoop() == 0)
143 // Otherwise, this is an entrance into a loop from some place invalid.
144 // Either the loop structure is invalid and this is not a natural loop (in
145 // which case the compiler is buggy somewhere else) or BB is unreachable.
146 BlockUnreachable = true;
150 // If this block is ok, check the next one.
151 if (!BlockUnreachable) continue;
153 // Otherwise, this block is dead. To clean up the CFG and to allow later
154 // loop transformations to ignore this case, we delete the edges into the
155 // loop by replacing the terminator.
157 // Remove PHI entries from the successors.
158 for (unsigned i = 0, e = TI->getNumSuccessors(); i != e; ++i)
159 TI->getSuccessor(i)->removePredecessor(BB);
161 // Add a new unreachable instruction.
162 new UnreachableInst(TI);
164 // Delete the dead terminator.
165 if (AA) AA->deleteValue(&BB->back());
166 BB->getInstList().pop_back();
170 for (LoopInfo::iterator I = LI->begin(), E = LI->end(); I != E; ++I)
171 Changed |= ProcessLoop(*I);
176 /// ProcessLoop - Walk the loop structure in depth first order, ensuring that
177 /// all loops have preheaders.
179 bool LoopSimplify::ProcessLoop(Loop *L) {
180 bool Changed = false;
183 // Canonicalize inner loops before outer loops. Inner loop canonicalization
184 // can provide work for the outer loop to canonicalize.
185 for (Loop::iterator I = L->begin(), E = L->end(); I != E; ++I)
186 Changed |= ProcessLoop(*I);
188 assert(L->getBlocks()[0] == L->getHeader() &&
189 "Header isn't first block in loop?");
191 // Does the loop already have a preheader? If so, don't insert one.
192 if (L->getLoopPreheader() == 0) {
193 InsertPreheaderForLoop(L);
198 // Next, check to make sure that all exit nodes of the loop only have
199 // predecessors that are inside of the loop. This check guarantees that the
200 // loop preheader/header will dominate the exit blocks. If the exit block has
201 // predecessors from outside of the loop, split the edge now.
202 std::vector<BasicBlock*> ExitBlocks;
203 L->getExitBlocks(ExitBlocks);
205 SetVector<BasicBlock*> ExitBlockSet(ExitBlocks.begin(), ExitBlocks.end());
206 for (SetVector<BasicBlock*>::iterator I = ExitBlockSet.begin(),
207 E = ExitBlockSet.end(); I != E; ++I) {
208 BasicBlock *ExitBlock = *I;
209 for (pred_iterator PI = pred_begin(ExitBlock), PE = pred_end(ExitBlock);
211 // Must be exactly this loop: no subloops, parent loops, or non-loop preds
213 if (!L->contains(*PI)) {
214 RewriteLoopExitBlock(L, ExitBlock);
221 // If the header has more than two predecessors at this point (from the
222 // preheader and from multiple backedges), we must adjust the loop.
223 unsigned NumBackedges = L->getNumBackEdges();
224 if (NumBackedges != 1) {
225 // If this is really a nested loop, rip it out into a child loop. Don't do
226 // this for loops with a giant number of backedges, just factor them into a
227 // common backedge instead.
228 if (NumBackedges < 8) {
229 if (Loop *NL = SeparateNestedLoop(L)) {
231 // This is a big restructuring change, reprocess the whole loop.
234 // GCC doesn't tail recursion eliminate this.
239 // If we either couldn't, or didn't want to, identify nesting of the loops,
240 // insert a new block that all backedges target, then make it jump to the
242 InsertUniqueBackedgeBlock(L);
247 // Scan over the PHI nodes in the loop header. Since they now have only two
248 // incoming values (the loop is canonicalized), we may have simplified the PHI
249 // down to 'X = phi [X, Y]', which should be replaced with 'Y'.
251 for (BasicBlock::iterator I = L->getHeader()->begin();
252 (PN = dyn_cast<PHINode>(I++)); )
253 if (Value *V = PN->hasConstantValue()) {
254 PN->replaceAllUsesWith(V);
255 PN->eraseFromParent();
261 /// SplitBlockPredecessors - Split the specified block into two blocks. We want
262 /// to move the predecessors specified in the Preds list to point to the new
263 /// block, leaving the remaining predecessors pointing to BB. This method
264 /// updates the SSA PHINode's, but no other analyses.
266 BasicBlock *LoopSimplify::SplitBlockPredecessors(BasicBlock *BB,
268 const std::vector<BasicBlock*> &Preds) {
270 // Create new basic block, insert right before the original block...
271 BasicBlock *NewBB = new BasicBlock(BB->getName()+Suffix, BB->getParent(), BB);
273 // The preheader first gets an unconditional branch to the loop header...
274 BranchInst *BI = new BranchInst(BB, NewBB);
276 // For every PHI node in the block, insert a PHI node into NewBB where the
277 // incoming values from the out of loop edges are moved to NewBB. We have two
278 // possible cases here. If the loop is dead, we just insert dummy entries
279 // into the PHI nodes for the new edge. If the loop is not dead, we move the
280 // incoming edges in BB into new PHI nodes in NewBB.
282 if (!Preds.empty()) { // Is the loop not obviously dead?
283 // Check to see if the values being merged into the new block need PHI
284 // nodes. If so, insert them.
285 for (BasicBlock::iterator I = BB->begin(); isa<PHINode>(I); ) {
286 PHINode *PN = cast<PHINode>(I);
289 // Check to see if all of the values coming in are the same. If so, we
290 // don't need to create a new PHI node.
291 Value *InVal = PN->getIncomingValueForBlock(Preds[0]);
292 for (unsigned i = 1, e = Preds.size(); i != e; ++i)
293 if (InVal != PN->getIncomingValueForBlock(Preds[i])) {
298 // If the values coming into the block are not the same, we need a PHI.
300 // Create the new PHI node, insert it into NewBB at the end of the block
301 PHINode *NewPHI = new PHINode(PN->getType(), PN->getName()+".ph", BI);
302 if (AA) AA->copyValue(PN, NewPHI);
304 // Move all of the edges from blocks outside the loop to the new PHI
305 for (unsigned i = 0, e = Preds.size(); i != e; ++i) {
306 Value *V = PN->removeIncomingValue(Preds[i], false);
307 NewPHI->addIncoming(V, Preds[i]);
311 // Remove all of the edges coming into the PHI nodes from outside of the
313 for (unsigned i = 0, e = Preds.size(); i != e; ++i)
314 PN->removeIncomingValue(Preds[i], false);
317 // Add an incoming value to the PHI node in the loop for the preheader
319 PN->addIncoming(InVal, NewBB);
321 // Can we eliminate this phi node now?
322 if (Value *V = PN->hasConstantValue(true)) {
323 Instruction *I = dyn_cast<Instruction>(V);
324 // If I is in NewBB, the Dominator call will fail, because NewBB isn't
325 // registered in DominatorTree yet. Handle this case explicitly.
326 if (!I || (I->getParent() != NewBB &&
327 getAnalysis<DominatorTree>().dominates(I, PN))) {
328 PN->replaceAllUsesWith(V);
329 if (AA) AA->deleteValue(PN);
330 BB->getInstList().erase(PN);
335 // Now that the PHI nodes are updated, actually move the edges from
336 // Preds to point to NewBB instead of BB.
338 for (unsigned i = 0, e = Preds.size(); i != e; ++i) {
339 TerminatorInst *TI = Preds[i]->getTerminator();
340 for (unsigned s = 0, e = TI->getNumSuccessors(); s != e; ++s)
341 if (TI->getSuccessor(s) == BB)
342 TI->setSuccessor(s, NewBB);
345 } else { // Otherwise the loop is dead...
346 for (BasicBlock::iterator I = BB->begin(); isa<PHINode>(I); ++I) {
347 PHINode *PN = cast<PHINode>(I);
348 // Insert dummy values as the incoming value...
349 PN->addIncoming(Constant::getNullValue(PN->getType()), NewBB);
356 /// InsertPreheaderForLoop - Once we discover that a loop doesn't have a
357 /// preheader, this method is called to insert one. This method has two phases:
358 /// preheader insertion and analysis updating.
360 void LoopSimplify::InsertPreheaderForLoop(Loop *L) {
361 BasicBlock *Header = L->getHeader();
363 // Compute the set of predecessors of the loop that are not in the loop.
364 std::vector<BasicBlock*> OutsideBlocks;
365 for (pred_iterator PI = pred_begin(Header), PE = pred_end(Header);
367 if (!L->contains(*PI)) // Coming in from outside the loop?
368 OutsideBlocks.push_back(*PI); // Keep track of it...
370 // Split out the loop pre-header.
372 SplitBlockPredecessors(Header, ".preheader", OutsideBlocks);
375 //===--------------------------------------------------------------------===//
376 // Update analysis results now that we have performed the transformation
379 // We know that we have loop information to update... update it now.
380 if (Loop *Parent = L->getParentLoop())
381 Parent->addBasicBlockToLoop(NewBB, *LI);
383 DT->splitBlock(NewBB);
384 if (DominanceFrontier *DF = getAnalysisToUpdate<DominanceFrontier>())
385 DF->splitBlock(NewBB);
387 // Make sure that NewBB is put someplace intelligent, which doesn't mess up
388 // code layout too horribly.
389 PlaceSplitBlockCarefully(NewBB, OutsideBlocks, L);
392 /// RewriteLoopExitBlock - Ensure that the loop preheader dominates all exit
393 /// blocks. This method is used to split exit blocks that have predecessors
394 /// outside of the loop.
395 BasicBlock *LoopSimplify::RewriteLoopExitBlock(Loop *L, BasicBlock *Exit) {
396 std::vector<BasicBlock*> LoopBlocks;
397 for (pred_iterator I = pred_begin(Exit), E = pred_end(Exit); I != E; ++I)
399 LoopBlocks.push_back(*I);
401 assert(!LoopBlocks.empty() && "No edges coming in from outside the loop?");
402 BasicBlock *NewBB = SplitBlockPredecessors(Exit, ".loopexit", LoopBlocks);
404 // Update Loop Information - we know that the new block will be in whichever
405 // loop the Exit block is in. Note that it may not be in that immediate loop,
406 // if the successor is some other loop header. In that case, we continue
407 // walking up the loop tree to find a loop that contains both the successor
408 // block and the predecessor block.
409 Loop *SuccLoop = LI->getLoopFor(Exit);
410 while (SuccLoop && !SuccLoop->contains(L->getHeader()))
411 SuccLoop = SuccLoop->getParentLoop();
413 SuccLoop->addBasicBlockToLoop(NewBB, *LI);
415 // Update Dominator Information
416 DT->splitBlock(NewBB);
417 if (DominanceFrontier *DF = getAnalysisToUpdate<DominanceFrontier>())
418 DF->splitBlock(NewBB);
423 /// AddBlockAndPredsToSet - Add the specified block, and all of its
424 /// predecessors, to the specified set, if it's not already in there. Stop
425 /// predecessor traversal when we reach StopBlock.
426 static void AddBlockAndPredsToSet(BasicBlock *InputBB, BasicBlock *StopBlock,
427 std::set<BasicBlock*> &Blocks) {
428 std::vector<BasicBlock *> WorkList;
429 WorkList.push_back(InputBB);
431 BasicBlock *BB = WorkList.back(); WorkList.pop_back();
432 if (Blocks.insert(BB).second && BB != StopBlock)
433 // If BB is not already processed and it is not a stop block then
434 // insert its predecessor in the work list
435 for (pred_iterator I = pred_begin(BB), E = pred_end(BB); I != E; ++I) {
436 BasicBlock *WBB = *I;
437 WorkList.push_back(WBB);
439 } while(!WorkList.empty());
442 /// FindPHIToPartitionLoops - The first part of loop-nestification is to find a
443 /// PHI node that tells us how to partition the loops.
444 static PHINode *FindPHIToPartitionLoops(Loop *L, DominatorTree *DT,
446 for (BasicBlock::iterator I = L->getHeader()->begin(); isa<PHINode>(I); ) {
447 PHINode *PN = cast<PHINode>(I);
449 if (Value *V = PN->hasConstantValue())
450 if (!isa<Instruction>(V) || DT->dominates(cast<Instruction>(V), PN)) {
451 // This is a degenerate PHI already, don't modify it!
452 PN->replaceAllUsesWith(V);
453 if (AA) AA->deleteValue(PN);
454 PN->eraseFromParent();
458 // Scan this PHI node looking for a use of the PHI node by itself.
459 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
460 if (PN->getIncomingValue(i) == PN &&
461 L->contains(PN->getIncomingBlock(i)))
462 // We found something tasty to remove.
468 // PlaceSplitBlockCarefully - If the block isn't already, move the new block to
469 // right after some 'outside block' block. This prevents the preheader from
470 // being placed inside the loop body, e.g. when the loop hasn't been rotated.
471 void LoopSimplify::PlaceSplitBlockCarefully(BasicBlock *NewBB,
472 std::vector<BasicBlock*>&SplitPreds,
474 // Check to see if NewBB is already well placed.
475 Function::iterator BBI = NewBB; --BBI;
476 for (unsigned i = 0, e = SplitPreds.size(); i != e; ++i) {
477 if (&*BBI == SplitPreds[i])
481 // If it isn't already after an outside block, move it after one. This is
482 // always good as it makes the uncond branch from the outside block into a
485 // Figure out *which* outside block to put this after. Prefer an outside
486 // block that neighbors a BB actually in the loop.
487 BasicBlock *FoundBB = 0;
488 for (unsigned i = 0, e = SplitPreds.size(); i != e; ++i) {
489 Function::iterator BBI = SplitPreds[i];
490 if (++BBI != NewBB->getParent()->end() &&
492 FoundBB = SplitPreds[i];
497 // If our heuristic for a *good* bb to place this after doesn't find
498 // anything, just pick something. It's likely better than leaving it within
501 FoundBB = SplitPreds[0];
502 NewBB->moveAfter(FoundBB);
506 /// SeparateNestedLoop - If this loop has multiple backedges, try to pull one of
507 /// them out into a nested loop. This is important for code that looks like
512 /// br cond, Loop, Next
514 /// br cond2, Loop, Out
516 /// To identify this common case, we look at the PHI nodes in the header of the
517 /// loop. PHI nodes with unchanging values on one backedge correspond to values
518 /// that change in the "outer" loop, but not in the "inner" loop.
520 /// If we are able to separate out a loop, return the new outer loop that was
523 Loop *LoopSimplify::SeparateNestedLoop(Loop *L) {
524 PHINode *PN = FindPHIToPartitionLoops(L, DT, AA);
525 if (PN == 0) return 0; // No known way to partition.
527 // Pull out all predecessors that have varying values in the loop. This
528 // handles the case when a PHI node has multiple instances of itself as
530 std::vector<BasicBlock*> OuterLoopPreds;
531 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
532 if (PN->getIncomingValue(i) != PN ||
533 !L->contains(PN->getIncomingBlock(i)))
534 OuterLoopPreds.push_back(PN->getIncomingBlock(i));
536 BasicBlock *Header = L->getHeader();
537 BasicBlock *NewBB = SplitBlockPredecessors(Header, ".outer", OuterLoopPreds);
539 // Update dominator information
540 DT->splitBlock(NewBB);
541 if (DominanceFrontier *DF = getAnalysisToUpdate<DominanceFrontier>())
542 DF->splitBlock(NewBB);
544 // Make sure that NewBB is put someplace intelligent, which doesn't mess up
545 // code layout too horribly.
546 PlaceSplitBlockCarefully(NewBB, OuterLoopPreds, L);
548 // Create the new outer loop.
549 Loop *NewOuter = new Loop();
551 // Change the parent loop to use the outer loop as its child now.
552 if (Loop *Parent = L->getParentLoop())
553 Parent->replaceChildLoopWith(L, NewOuter);
555 LI->changeTopLevelLoop(L, NewOuter);
557 // This block is going to be our new header block: add it to this loop and all
559 NewOuter->addBasicBlockToLoop(NewBB, *LI);
561 // L is now a subloop of our outer loop.
562 NewOuter->addChildLoop(L);
564 for (unsigned i = 0, e = L->getBlocks().size(); i != e; ++i)
565 NewOuter->addBlockEntry(L->getBlocks()[i]);
567 // Determine which blocks should stay in L and which should be moved out to
568 // the Outer loop now.
569 std::set<BasicBlock*> BlocksInL;
570 for (pred_iterator PI = pred_begin(Header), E = pred_end(Header); PI!=E; ++PI)
571 if (DT->dominates(Header, *PI))
572 AddBlockAndPredsToSet(*PI, Header, BlocksInL);
575 // Scan all of the loop children of L, moving them to OuterLoop if they are
576 // not part of the inner loop.
577 const std::vector<Loop*> &SubLoops = L->getSubLoops();
578 for (size_t I = 0; I != SubLoops.size(); )
579 if (BlocksInL.count(SubLoops[I]->getHeader()))
580 ++I; // Loop remains in L
582 NewOuter->addChildLoop(L->removeChildLoop(SubLoops.begin() + I));
584 // Now that we know which blocks are in L and which need to be moved to
585 // OuterLoop, move any blocks that need it.
586 for (unsigned i = 0; i != L->getBlocks().size(); ++i) {
587 BasicBlock *BB = L->getBlocks()[i];
588 if (!BlocksInL.count(BB)) {
589 // Move this block to the parent, updating the exit blocks sets
590 L->removeBlockFromLoop(BB);
592 LI->changeLoopFor(BB, NewOuter);
602 /// InsertUniqueBackedgeBlock - This method is called when the specified loop
603 /// has more than one backedge in it. If this occurs, revector all of these
604 /// backedges to target a new basic block and have that block branch to the loop
605 /// header. This ensures that loops have exactly one backedge.
607 void LoopSimplify::InsertUniqueBackedgeBlock(Loop *L) {
608 assert(L->getNumBackEdges() > 1 && "Must have > 1 backedge!");
610 // Get information about the loop
611 BasicBlock *Preheader = L->getLoopPreheader();
612 BasicBlock *Header = L->getHeader();
613 Function *F = Header->getParent();
615 // Figure out which basic blocks contain back-edges to the loop header.
616 std::vector<BasicBlock*> BackedgeBlocks;
617 for (pred_iterator I = pred_begin(Header), E = pred_end(Header); I != E; ++I)
618 if (*I != Preheader) BackedgeBlocks.push_back(*I);
620 // Create and insert the new backedge block...
621 BasicBlock *BEBlock = new BasicBlock(Header->getName()+".backedge", F);
622 BranchInst *BETerminator = new BranchInst(Header, BEBlock);
624 // Move the new backedge block to right after the last backedge block.
625 Function::iterator InsertPos = BackedgeBlocks.back(); ++InsertPos;
626 F->getBasicBlockList().splice(InsertPos, F->getBasicBlockList(), BEBlock);
628 // Now that the block has been inserted into the function, create PHI nodes in
629 // the backedge block which correspond to any PHI nodes in the header block.
630 for (BasicBlock::iterator I = Header->begin(); isa<PHINode>(I); ++I) {
631 PHINode *PN = cast<PHINode>(I);
632 PHINode *NewPN = new PHINode(PN->getType(), PN->getName()+".be",
634 NewPN->reserveOperandSpace(BackedgeBlocks.size());
635 if (AA) AA->copyValue(PN, NewPN);
637 // Loop over the PHI node, moving all entries except the one for the
638 // preheader over to the new PHI node.
639 unsigned PreheaderIdx = ~0U;
640 bool HasUniqueIncomingValue = true;
641 Value *UniqueValue = 0;
642 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
643 BasicBlock *IBB = PN->getIncomingBlock(i);
644 Value *IV = PN->getIncomingValue(i);
645 if (IBB == Preheader) {
648 NewPN->addIncoming(IV, IBB);
649 if (HasUniqueIncomingValue) {
650 if (UniqueValue == 0)
652 else if (UniqueValue != IV)
653 HasUniqueIncomingValue = false;
658 // Delete all of the incoming values from the old PN except the preheader's
659 assert(PreheaderIdx != ~0U && "PHI has no preheader entry??");
660 if (PreheaderIdx != 0) {
661 PN->setIncomingValue(0, PN->getIncomingValue(PreheaderIdx));
662 PN->setIncomingBlock(0, PN->getIncomingBlock(PreheaderIdx));
664 // Nuke all entries except the zero'th.
665 for (unsigned i = 0, e = PN->getNumIncomingValues()-1; i != e; ++i)
666 PN->removeIncomingValue(e-i, false);
668 // Finally, add the newly constructed PHI node as the entry for the BEBlock.
669 PN->addIncoming(NewPN, BEBlock);
671 // As an optimization, if all incoming values in the new PhiNode (which is a
672 // subset of the incoming values of the old PHI node) have the same value,
673 // eliminate the PHI Node.
674 if (HasUniqueIncomingValue) {
675 NewPN->replaceAllUsesWith(UniqueValue);
676 if (AA) AA->deleteValue(NewPN);
677 BEBlock->getInstList().erase(NewPN);
681 // Now that all of the PHI nodes have been inserted and adjusted, modify the
682 // backedge blocks to just to the BEBlock instead of the header.
683 for (unsigned i = 0, e = BackedgeBlocks.size(); i != e; ++i) {
684 TerminatorInst *TI = BackedgeBlocks[i]->getTerminator();
685 for (unsigned Op = 0, e = TI->getNumSuccessors(); Op != e; ++Op)
686 if (TI->getSuccessor(Op) == Header)
687 TI->setSuccessor(Op, BEBlock);
690 //===--- Update all analyses which we must preserve now -----------------===//
692 // Update Loop Information - we know that this block is now in the current
693 // loop and all parent loops.
694 L->addBasicBlockToLoop(BEBlock, *LI);
696 // Update dominator information
697 DT->splitBlock(BEBlock);
698 if (DominanceFrontier *DF = getAnalysisToUpdate<DominanceFrontier>())
699 DF->splitBlock(BEBlock);