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.addRequired<DominatorTree>();
81 AU.addPreserved<DominatorTree>();
83 AU.addRequired<LoopInfo>();
84 AU.addPreserved<LoopInfo>();
86 AU.addPreserved<AliasAnalysis>();
87 AU.addPreserved<ScalarEvolution>();
88 AU.addPreservedID(BreakCriticalEdgesID); // No critical edges added.
89 AU.addPreserved<DominanceFrontier>();
90 AU.addPreservedID(LCSSAID);
93 /// verifyAnalysis() - Verify LoopSimplifyForm's guarantees.
94 void verifyAnalysis() const;
97 bool ProcessLoop(Loop *L, LPPassManager &LPM);
98 BasicBlock *RewriteLoopExitBlock(Loop *L, BasicBlock *Exit);
99 BasicBlock *InsertPreheaderForLoop(Loop *L);
100 Loop *SeparateNestedLoop(Loop *L, LPPassManager &LPM);
101 BasicBlock *InsertUniqueBackedgeBlock(Loop *L, BasicBlock *Preheader);
102 void PlaceSplitBlockCarefully(BasicBlock *NewBB,
103 SmallVectorImpl<BasicBlock*> &SplitPreds,
108 char LoopSimplify::ID = 0;
109 static RegisterPass<LoopSimplify>
110 X("loopsimplify", "Canonicalize natural loops", true);
112 // Publically exposed interface to pass...
113 char &llvm::LoopSimplifyID = LoopSimplify::ID;
114 Pass *llvm::createLoopSimplifyPass() { return new LoopSimplify(); }
116 /// runOnLoop - Run down all loops in the CFG (recursively, but we could do
117 /// it in any convenient order) inserting preheaders...
119 bool LoopSimplify::runOnLoop(Loop *l, LPPassManager &LPM) {
121 bool Changed = false;
122 LI = &getAnalysis<LoopInfo>();
123 AA = getAnalysisIfAvailable<AliasAnalysis>();
124 DT = &getAnalysis<DominatorTree>();
126 Changed |= ProcessLoop(L, LPM);
131 /// ProcessLoop - Walk the loop structure in depth first order, ensuring that
132 /// all loops have preheaders.
134 bool LoopSimplify::ProcessLoop(Loop *L, LPPassManager &LPM) {
135 bool Changed = false;
138 // Check to see that no blocks (other than the header) in this loop have
139 // predecessors that are not in the loop. This is not valid for natural
140 // loops, but can occur if the blocks are unreachable. Since they are
141 // unreachable we can just shamelessly delete those CFG edges!
142 for (Loop::block_iterator BB = L->block_begin(), E = L->block_end();
144 if (*BB == L->getHeader()) continue;
146 SmallPtrSet<BasicBlock*, 4> BadPreds;
147 for (pred_iterator PI = pred_begin(*BB),
148 PE = pred_end(*BB); PI != PE; ++PI) {
154 // Delete each unique out-of-loop (and thus dead) predecessor.
155 for (SmallPtrSet<BasicBlock*, 4>::iterator I = BadPreds.begin(),
156 E = BadPreds.end(); I != E; ++I) {
158 DEBUG(dbgs() << "LoopSimplify: Deleting edge from dead predecessor ";
159 WriteAsOperand(dbgs(), *I, false);
162 // Inform each successor of each dead pred.
163 for (succ_iterator SI = succ_begin(*I), SE = succ_end(*I); SI != SE; ++SI)
164 (*SI)->removePredecessor(*I);
165 // Zap the dead pred's terminator and replace it with unreachable.
166 TerminatorInst *TI = (*I)->getTerminator();
167 TI->replaceAllUsesWith(UndefValue::get(TI->getType()));
168 (*I)->getTerminator()->eraseFromParent();
169 new UnreachableInst((*I)->getContext(), *I);
174 // If there are exiting blocks with branches on undef, resolve the undef in
175 // the direction which will exit the loop. This will help simplify loop
176 // trip count computations.
177 SmallVector<BasicBlock*, 8> ExitingBlocks;
178 L->getExitingBlocks(ExitingBlocks);
179 for (SmallVectorImpl<BasicBlock *>::iterator I = ExitingBlocks.begin(),
180 E = ExitingBlocks.end(); I != E; ++I)
181 if (BranchInst *BI = dyn_cast<BranchInst>((*I)->getTerminator()))
182 if (BI->isConditional()) {
183 if (UndefValue *Cond = dyn_cast<UndefValue>(BI->getCondition())) {
185 DEBUG(dbgs() << "LoopSimplify: Resolving \"br i1 undef\" to exit in ";
186 WriteAsOperand(dbgs(), *I, false);
189 BI->setCondition(ConstantInt::get(Cond->getType(),
190 !L->contains(BI->getSuccessor(0))));
195 // Does the loop already have a preheader? If so, don't insert one.
196 BasicBlock *Preheader = L->getLoopPreheader();
198 Preheader = InsertPreheaderForLoop(L);
205 // Next, check to make sure that all exit nodes of the loop only have
206 // predecessors that are inside of the loop. This check guarantees that the
207 // loop preheader/header will dominate the exit blocks. If the exit block has
208 // predecessors from outside of the loop, split the edge now.
209 SmallVector<BasicBlock*, 8> ExitBlocks;
210 L->getExitBlocks(ExitBlocks);
212 SmallSetVector<BasicBlock *, 8> ExitBlockSet(ExitBlocks.begin(),
214 for (SmallSetVector<BasicBlock *, 8>::iterator I = ExitBlockSet.begin(),
215 E = ExitBlockSet.end(); I != E; ++I) {
216 BasicBlock *ExitBlock = *I;
217 for (pred_iterator PI = pred_begin(ExitBlock), PE = pred_end(ExitBlock);
219 // Must be exactly this loop: no subloops, parent loops, or non-loop preds
221 if (!L->contains(*PI)) {
222 if (RewriteLoopExitBlock(L, ExitBlock)) {
230 // If the header has more than two predecessors at this point (from the
231 // preheader and from multiple backedges), we must adjust the loop.
232 BasicBlock *LoopLatch = L->getLoopLatch();
234 // If this is really a nested loop, rip it out into a child loop. Don't do
235 // this for loops with a giant number of backedges, just factor them into a
236 // common backedge instead.
237 if (L->getNumBackEdges() < 8) {
238 if (SeparateNestedLoop(L, LPM)) {
240 // This is a big restructuring change, reprocess the whole loop.
242 // GCC doesn't tail recursion eliminate this.
247 // If we either couldn't, or didn't want to, identify nesting of the loops,
248 // insert a new block that all backedges target, then make it jump to the
250 LoopLatch = InsertUniqueBackedgeBlock(L, Preheader);
257 // Scan over the PHI nodes in the loop header. Since they now have only two
258 // incoming values (the loop is canonicalized), we may have simplified the PHI
259 // down to 'X = phi [X, Y]', which should be replaced with 'Y'.
261 for (BasicBlock::iterator I = L->getHeader()->begin();
262 (PN = dyn_cast<PHINode>(I++)); )
263 if (Value *V = PN->hasConstantValue(DT)) {
264 if (AA) AA->deleteValue(PN);
265 PN->replaceAllUsesWith(V);
266 PN->eraseFromParent();
269 // If this loop has multiple exits and the exits all go to the same
270 // block, attempt to merge the exits. This helps several passes, such
271 // as LoopRotation, which do not support loops with multiple exits.
272 // SimplifyCFG also does this (and this code uses the same utility
273 // function), however this code is loop-aware, where SimplifyCFG is
274 // not. That gives it the advantage of being able to hoist
275 // loop-invariant instructions out of the way to open up more
276 // opportunities, and the disadvantage of having the responsibility
277 // to preserve dominator information.
278 bool UniqueExit = true;
279 if (!ExitBlocks.empty())
280 for (unsigned i = 1, e = ExitBlocks.size(); i != e; ++i)
281 if (ExitBlocks[i] != ExitBlocks[0]) {
286 for (unsigned i = 0, e = ExitingBlocks.size(); i != e; ++i) {
287 BasicBlock *ExitingBlock = ExitingBlocks[i];
288 if (!ExitingBlock->getSinglePredecessor()) continue;
289 BranchInst *BI = dyn_cast<BranchInst>(ExitingBlock->getTerminator());
290 if (!BI || !BI->isConditional()) continue;
291 CmpInst *CI = dyn_cast<CmpInst>(BI->getCondition());
292 if (!CI || CI->getParent() != ExitingBlock) continue;
294 // Attempt to hoist out all instructions except for the
295 // comparison and the branch.
296 bool AllInvariant = true;
297 for (BasicBlock::iterator I = ExitingBlock->begin(); &*I != BI; ) {
298 Instruction *Inst = I++;
299 // Skip debug info intrinsics.
300 if (isa<DbgInfoIntrinsic>(Inst))
304 if (!L->makeLoopInvariant(Inst, Changed,
305 Preheader ? Preheader->getTerminator() : 0)) {
306 AllInvariant = false;
310 if (!AllInvariant) continue;
312 // The block has now been cleared of all instructions except for
313 // a comparison and a conditional branch. SimplifyCFG may be able
315 if (!FoldBranchToCommonDest(BI)) continue;
317 // Success. The block is now dead, so remove it from the loop,
318 // update the dominator tree and dominance frontier, and delete it.
320 DEBUG(dbgs() << "LoopSimplify: Eliminating exiting block ";
321 WriteAsOperand(dbgs(), ExitingBlock, false);
324 assert(pred_begin(ExitingBlock) == pred_end(ExitingBlock));
326 LI->removeBlock(ExitingBlock);
328 DominanceFrontier *DF = getAnalysisIfAvailable<DominanceFrontier>();
329 DomTreeNode *Node = DT->getNode(ExitingBlock);
330 const std::vector<DomTreeNodeBase<BasicBlock> *> &Children =
332 while (!Children.empty()) {
333 DomTreeNode *Child = Children.front();
334 DT->changeImmediateDominator(Child, Node->getIDom());
335 if (DF) DF->changeImmediateDominator(Child->getBlock(),
336 Node->getIDom()->getBlock(),
339 DT->eraseNode(ExitingBlock);
340 if (DF) DF->removeBlock(ExitingBlock);
342 BI->getSuccessor(0)->removePredecessor(ExitingBlock);
343 BI->getSuccessor(1)->removePredecessor(ExitingBlock);
344 ExitingBlock->eraseFromParent();
351 /// InsertPreheaderForLoop - Once we discover that a loop doesn't have a
352 /// preheader, this method is called to insert one. This method has two phases:
353 /// preheader insertion and analysis updating.
355 BasicBlock *LoopSimplify::InsertPreheaderForLoop(Loop *L) {
356 BasicBlock *Header = L->getHeader();
358 // Compute the set of predecessors of the loop that are not in the loop.
359 SmallVector<BasicBlock*, 8> OutsideBlocks;
360 for (pred_iterator PI = pred_begin(Header), PE = pred_end(Header);
363 if (!L->contains(P)) { // Coming in from outside the loop?
364 // If the loop is branched to from an indirect branch, we won't
365 // be able to fully transform the loop, because it prohibits
367 if (isa<IndirectBrInst>(P->getTerminator())) return 0;
370 OutsideBlocks.push_back(P);
374 // Split out the loop pre-header.
376 SplitBlockPredecessors(Header, &OutsideBlocks[0], OutsideBlocks.size(),
379 DEBUG(dbgs() << "LoopSimplify: Creating pre-header ";
380 WriteAsOperand(dbgs(), NewBB, false);
383 // Make sure that NewBB is put someplace intelligent, which doesn't mess up
384 // code layout too horribly.
385 PlaceSplitBlockCarefully(NewBB, OutsideBlocks, L);
390 /// RewriteLoopExitBlock - Ensure that the loop preheader dominates all exit
391 /// blocks. This method is used to split exit blocks that have predecessors
392 /// outside of the loop.
393 BasicBlock *LoopSimplify::RewriteLoopExitBlock(Loop *L, BasicBlock *Exit) {
394 SmallVector<BasicBlock*, 8> LoopBlocks;
395 for (pred_iterator I = pred_begin(Exit), E = pred_end(Exit); I != E; ++I) {
397 if (L->contains(P)) {
398 // Don't do this if the loop is exited via an indirect branch.
399 if (isa<IndirectBrInst>(P->getTerminator())) return 0;
401 LoopBlocks.push_back(P);
405 assert(!LoopBlocks.empty() && "No edges coming in from outside the loop?");
406 BasicBlock *NewBB = SplitBlockPredecessors(Exit, &LoopBlocks[0],
407 LoopBlocks.size(), ".loopexit",
410 DEBUG(dbgs() << "LoopSimplify: Creating dedicated exit block ";
411 WriteAsOperand(dbgs(), NewBB, false);
417 /// AddBlockAndPredsToSet - Add the specified block, and all of its
418 /// predecessors, to the specified set, if it's not already in there. Stop
419 /// predecessor traversal when we reach StopBlock.
420 static void AddBlockAndPredsToSet(BasicBlock *InputBB, BasicBlock *StopBlock,
421 std::set<BasicBlock*> &Blocks) {
422 std::vector<BasicBlock *> WorkList;
423 WorkList.push_back(InputBB);
425 BasicBlock *BB = WorkList.back(); WorkList.pop_back();
426 if (Blocks.insert(BB).second && BB != StopBlock)
427 // If BB is not already processed and it is not a stop block then
428 // insert its predecessor in the work list
429 for (pred_iterator I = pred_begin(BB), E = pred_end(BB); I != E; ++I) {
430 BasicBlock *WBB = *I;
431 WorkList.push_back(WBB);
433 } while(!WorkList.empty());
436 /// FindPHIToPartitionLoops - The first part of loop-nestification is to find a
437 /// PHI node that tells us how to partition the loops.
438 static PHINode *FindPHIToPartitionLoops(Loop *L, DominatorTree *DT,
440 for (BasicBlock::iterator I = L->getHeader()->begin(); isa<PHINode>(I); ) {
441 PHINode *PN = cast<PHINode>(I);
443 if (Value *V = PN->hasConstantValue(DT)) {
444 // This is a degenerate PHI already, don't modify it!
445 PN->replaceAllUsesWith(V);
446 if (AA) AA->deleteValue(PN);
447 PN->eraseFromParent();
451 // Scan this PHI node looking for a use of the PHI node by itself.
452 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
453 if (PN->getIncomingValue(i) == PN &&
454 L->contains(PN->getIncomingBlock(i)))
455 // We found something tasty to remove.
461 // PlaceSplitBlockCarefully - If the block isn't already, move the new block to
462 // right after some 'outside block' block. This prevents the preheader from
463 // being placed inside the loop body, e.g. when the loop hasn't been rotated.
464 void LoopSimplify::PlaceSplitBlockCarefully(BasicBlock *NewBB,
465 SmallVectorImpl<BasicBlock*> &SplitPreds,
467 // Check to see if NewBB is already well placed.
468 Function::iterator BBI = NewBB; --BBI;
469 for (unsigned i = 0, e = SplitPreds.size(); i != e; ++i) {
470 if (&*BBI == SplitPreds[i])
474 // If it isn't already after an outside block, move it after one. This is
475 // always good as it makes the uncond branch from the outside block into a
478 // Figure out *which* outside block to put this after. Prefer an outside
479 // block that neighbors a BB actually in the loop.
480 BasicBlock *FoundBB = 0;
481 for (unsigned i = 0, e = SplitPreds.size(); i != e; ++i) {
482 Function::iterator BBI = SplitPreds[i];
483 if (++BBI != NewBB->getParent()->end() &&
485 FoundBB = SplitPreds[i];
490 // If our heuristic for a *good* bb to place this after doesn't find
491 // anything, just pick something. It's likely better than leaving it within
494 FoundBB = SplitPreds[0];
495 NewBB->moveAfter(FoundBB);
499 /// SeparateNestedLoop - If this loop has multiple backedges, try to pull one of
500 /// them out into a nested loop. This is important for code that looks like
505 /// br cond, Loop, Next
507 /// br cond2, Loop, Out
509 /// To identify this common case, we look at the PHI nodes in the header of the
510 /// loop. PHI nodes with unchanging values on one backedge correspond to values
511 /// that change in the "outer" loop, but not in the "inner" loop.
513 /// If we are able to separate out a loop, return the new outer loop that was
516 Loop *LoopSimplify::SeparateNestedLoop(Loop *L, LPPassManager &LPM) {
517 PHINode *PN = FindPHIToPartitionLoops(L, DT, AA);
518 if (PN == 0) return 0; // No known way to partition.
520 // Pull out all predecessors that have varying values in the loop. This
521 // handles the case when a PHI node has multiple instances of itself as
523 SmallVector<BasicBlock*, 8> OuterLoopPreds;
524 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
525 if (PN->getIncomingValue(i) != PN ||
526 !L->contains(PN->getIncomingBlock(i))) {
527 // We can't split indirectbr edges.
528 if (isa<IndirectBrInst>(PN->getIncomingBlock(i)->getTerminator()))
531 OuterLoopPreds.push_back(PN->getIncomingBlock(i));
534 DEBUG(dbgs() << "LoopSimplify: Splitting out a new outer loop\n");
536 BasicBlock *Header = L->getHeader();
537 BasicBlock *NewBB = SplitBlockPredecessors(Header, &OuterLoopPreds[0],
538 OuterLoopPreds.size(),
541 // Make sure that NewBB is put someplace intelligent, which doesn't mess up
542 // code layout too horribly.
543 PlaceSplitBlockCarefully(NewBB, OuterLoopPreds, L);
545 // Create the new outer loop.
546 Loop *NewOuter = new Loop();
548 // Change the parent loop to use the outer loop as its child now.
549 if (Loop *Parent = L->getParentLoop())
550 Parent->replaceChildLoopWith(L, NewOuter);
552 LI->changeTopLevelLoop(L, NewOuter);
554 // L is now a subloop of our outer loop.
555 NewOuter->addChildLoop(L);
557 // Add the new loop to the pass manager queue.
558 LPM.insertLoopIntoQueue(NewOuter);
560 for (Loop::block_iterator I = L->block_begin(), E = L->block_end();
562 NewOuter->addBlockEntry(*I);
564 // Now reset the header in L, which had been moved by
565 // SplitBlockPredecessors for the outer loop.
566 L->moveToHeader(Header);
568 // Determine which blocks should stay in L and which should be moved out to
569 // the Outer loop now.
570 std::set<BasicBlock*> BlocksInL;
571 for (pred_iterator PI=pred_begin(Header), E = pred_end(Header); PI!=E; ++PI) {
573 if (DT->dominates(Header, P))
574 AddBlockAndPredsToSet(P, Header, BlocksInL);
577 // Scan all of the loop children of L, moving them to OuterLoop if they are
578 // not part of the inner loop.
579 const std::vector<Loop*> &SubLoops = L->getSubLoops();
580 for (size_t I = 0; I != SubLoops.size(); )
581 if (BlocksInL.count(SubLoops[I]->getHeader()))
582 ++I; // Loop remains in L
584 NewOuter->addChildLoop(L->removeChildLoop(SubLoops.begin() + I));
586 // Now that we know which blocks are in L and which need to be moved to
587 // OuterLoop, move any blocks that need it.
588 for (unsigned i = 0; i != L->getBlocks().size(); ++i) {
589 BasicBlock *BB = L->getBlocks()[i];
590 if (!BlocksInL.count(BB)) {
591 // Move this block to the parent, updating the exit blocks sets
592 L->removeBlockFromLoop(BB);
594 LI->changeLoopFor(BB, NewOuter);
604 /// InsertUniqueBackedgeBlock - This method is called when the specified loop
605 /// has more than one backedge in it. If this occurs, revector all of these
606 /// backedges to target a new basic block and have that block branch to the loop
607 /// header. This ensures that loops have exactly one backedge.
610 LoopSimplify::InsertUniqueBackedgeBlock(Loop *L, BasicBlock *Preheader) {
611 assert(L->getNumBackEdges() > 1 && "Must have > 1 backedge!");
613 // Get information about the loop
614 BasicBlock *Header = L->getHeader();
615 Function *F = Header->getParent();
617 // Unique backedge insertion currently depends on having a preheader.
621 // Figure out which basic blocks contain back-edges to the loop header.
622 std::vector<BasicBlock*> BackedgeBlocks;
623 for (pred_iterator I = pred_begin(Header), E = pred_end(Header); I != E; ++I){
626 // Indirectbr edges cannot be split, so we must fail if we find one.
627 if (isa<IndirectBrInst>(P->getTerminator()))
630 if (P != Preheader) BackedgeBlocks.push_back(P);
633 // Create and insert the new backedge block...
634 BasicBlock *BEBlock = BasicBlock::Create(Header->getContext(),
635 Header->getName()+".backedge", F);
636 BranchInst *BETerminator = BranchInst::Create(Header, BEBlock);
638 DEBUG(dbgs() << "LoopSimplify: Inserting unique backedge block ";
639 WriteAsOperand(dbgs(), BEBlock, false);
642 // Move the new backedge block to right after the last backedge block.
643 Function::iterator InsertPos = BackedgeBlocks.back(); ++InsertPos;
644 F->getBasicBlockList().splice(InsertPos, F->getBasicBlockList(), BEBlock);
646 // Now that the block has been inserted into the function, create PHI nodes in
647 // the backedge block which correspond to any PHI nodes in the header block.
648 for (BasicBlock::iterator I = Header->begin(); isa<PHINode>(I); ++I) {
649 PHINode *PN = cast<PHINode>(I);
650 PHINode *NewPN = PHINode::Create(PN->getType(), PN->getName()+".be",
652 NewPN->reserveOperandSpace(BackedgeBlocks.size());
653 if (AA) AA->copyValue(PN, NewPN);
655 // Loop over the PHI node, moving all entries except the one for the
656 // preheader over to the new PHI node.
657 unsigned PreheaderIdx = ~0U;
658 bool HasUniqueIncomingValue = true;
659 Value *UniqueValue = 0;
660 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
661 BasicBlock *IBB = PN->getIncomingBlock(i);
662 Value *IV = PN->getIncomingValue(i);
663 if (IBB == Preheader) {
666 NewPN->addIncoming(IV, IBB);
667 if (HasUniqueIncomingValue) {
668 if (UniqueValue == 0)
670 else if (UniqueValue != IV)
671 HasUniqueIncomingValue = false;
676 // Delete all of the incoming values from the old PN except the preheader's
677 assert(PreheaderIdx != ~0U && "PHI has no preheader entry??");
678 if (PreheaderIdx != 0) {
679 PN->setIncomingValue(0, PN->getIncomingValue(PreheaderIdx));
680 PN->setIncomingBlock(0, PN->getIncomingBlock(PreheaderIdx));
682 // Nuke all entries except the zero'th.
683 for (unsigned i = 0, e = PN->getNumIncomingValues()-1; i != e; ++i)
684 PN->removeIncomingValue(e-i, false);
686 // Finally, add the newly constructed PHI node as the entry for the BEBlock.
687 PN->addIncoming(NewPN, BEBlock);
689 // As an optimization, if all incoming values in the new PhiNode (which is a
690 // subset of the incoming values of the old PHI node) have the same value,
691 // eliminate the PHI Node.
692 if (HasUniqueIncomingValue) {
693 NewPN->replaceAllUsesWith(UniqueValue);
694 if (AA) AA->deleteValue(NewPN);
695 BEBlock->getInstList().erase(NewPN);
699 // Now that all of the PHI nodes have been inserted and adjusted, modify the
700 // backedge blocks to just to the BEBlock instead of the header.
701 for (unsigned i = 0, e = BackedgeBlocks.size(); i != e; ++i) {
702 TerminatorInst *TI = BackedgeBlocks[i]->getTerminator();
703 for (unsigned Op = 0, e = TI->getNumSuccessors(); Op != e; ++Op)
704 if (TI->getSuccessor(Op) == Header)
705 TI->setSuccessor(Op, BEBlock);
708 //===--- Update all analyses which we must preserve now -----------------===//
710 // Update Loop Information - we know that this block is now in the current
711 // loop and all parent loops.
712 L->addBasicBlockToLoop(BEBlock, LI->getBase());
714 // Update dominator information
715 DT->splitBlock(BEBlock);
716 if (DominanceFrontier *DF = getAnalysisIfAvailable<DominanceFrontier>())
717 DF->splitBlock(BEBlock);
722 void LoopSimplify::verifyAnalysis() const {
723 // It used to be possible to just assert L->isLoopSimplifyForm(), however
724 // with the introduction of indirectbr, there are now cases where it's
725 // not possible to transform a loop as necessary. We can at least check
726 // that there is an indirectbr near any time there's trouble.
728 // Indirectbr can interfere with preheader and unique backedge insertion.
729 if (!L->getLoopPreheader() || !L->getLoopLatch()) {
730 bool HasIndBrPred = false;
731 for (pred_iterator PI = pred_begin(L->getHeader()),
732 PE = pred_end(L->getHeader()); PI != PE; ++PI)
733 if (isa<IndirectBrInst>((*PI)->getTerminator())) {
737 assert(HasIndBrPred &&
738 "LoopSimplify has no excuse for missing loop header info!");
741 // Indirectbr can interfere with exit block canonicalization.
742 if (!L->hasDedicatedExits()) {
743 bool HasIndBrExiting = false;
744 SmallVector<BasicBlock*, 8> ExitingBlocks;
745 L->getExitingBlocks(ExitingBlocks);
746 for (unsigned i = 0, e = ExitingBlocks.size(); i != e; ++i)
747 if (isa<IndirectBrInst>((ExitingBlocks[i])->getTerminator())) {
748 HasIndBrExiting = true;
751 assert(HasIndBrExiting &&
752 "LoopSimplify has no excuse for missing exit block info!");