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 "loop-simplify"
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/InstructionSimplify.h"
51 #include "llvm/Analysis/LoopPass.h"
52 #include "llvm/Analysis/ScalarEvolution.h"
53 #include "llvm/Transforms/Utils/BasicBlockUtils.h"
54 #include "llvm/Transforms/Utils/Local.h"
55 #include "llvm/Support/CFG.h"
56 #include "llvm/Support/Debug.h"
57 #include "llvm/ADT/SetOperations.h"
58 #include "llvm/ADT/SetVector.h"
59 #include "llvm/ADT/Statistic.h"
60 #include "llvm/ADT/DepthFirstIterator.h"
63 STATISTIC(NumInserted, "Number of pre-header or exit blocks inserted");
64 STATISTIC(NumNested , "Number of nested loops split out");
67 struct LoopSimplify : public LoopPass {
68 static char ID; // Pass identification, replacement for typeid
69 LoopSimplify() : LoopPass(ID) {
70 initializeLoopSimplifyPass(*PassRegistry::getPassRegistry());
73 // AA - If we have an alias analysis object to update, this is it, otherwise
80 virtual bool runOnLoop(Loop *L, LPPassManager &LPM);
82 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
83 // We need loop information to identify the loops...
84 AU.addRequired<DominatorTree>();
85 AU.addPreserved<DominatorTree>();
87 AU.addRequired<LoopInfo>();
88 AU.addPreserved<LoopInfo>();
90 AU.addPreserved<AliasAnalysis>();
91 AU.addPreserved<ScalarEvolution>();
92 AU.addPreservedID(BreakCriticalEdgesID); // No critical edges added.
95 /// verifyAnalysis() - Verify LoopSimplifyForm's guarantees.
96 void verifyAnalysis() const;
99 bool ProcessLoop(Loop *L, LPPassManager &LPM);
100 BasicBlock *RewriteLoopExitBlock(Loop *L, BasicBlock *Exit);
101 BasicBlock *InsertPreheaderForLoop(Loop *L);
102 Loop *SeparateNestedLoop(Loop *L, LPPassManager &LPM,
103 BasicBlock *Preheader);
104 BasicBlock *InsertUniqueBackedgeBlock(Loop *L, BasicBlock *Preheader);
105 void PlaceSplitBlockCarefully(BasicBlock *NewBB,
106 SmallVectorImpl<BasicBlock*> &SplitPreds,
111 char LoopSimplify::ID = 0;
112 INITIALIZE_PASS_BEGIN(LoopSimplify, "loop-simplify",
113 "Canonicalize natural loops", true, false)
114 INITIALIZE_PASS_DEPENDENCY(DominatorTree)
115 INITIALIZE_PASS_DEPENDENCY(LoopInfo)
116 INITIALIZE_PASS_END(LoopSimplify, "loop-simplify",
117 "Canonicalize natural loops", true, false)
119 // Publicly exposed interface to pass...
120 char &llvm::LoopSimplifyID = LoopSimplify::ID;
121 Pass *llvm::createLoopSimplifyPass() { return new LoopSimplify(); }
123 /// runOnLoop - Run down all loops in the CFG (recursively, but we could do
124 /// it in any convenient order) inserting preheaders...
126 bool LoopSimplify::runOnLoop(Loop *l, LPPassManager &LPM) {
128 bool Changed = false;
129 LI = &getAnalysis<LoopInfo>();
130 AA = getAnalysisIfAvailable<AliasAnalysis>();
131 DT = &getAnalysis<DominatorTree>();
132 SE = getAnalysisIfAvailable<ScalarEvolution>();
134 Changed |= ProcessLoop(L, LPM);
139 /// ProcessLoop - Walk the loop structure in depth first order, ensuring that
140 /// all loops have preheaders.
142 bool LoopSimplify::ProcessLoop(Loop *L, LPPassManager &LPM) {
143 bool Changed = false;
146 // Check to see that no blocks (other than the header) in this loop have
147 // predecessors that are not in the loop. This is not valid for natural
148 // loops, but can occur if the blocks are unreachable. Since they are
149 // unreachable we can just shamelessly delete those CFG edges!
150 for (Loop::block_iterator BB = L->block_begin(), E = L->block_end();
152 if (*BB == L->getHeader()) continue;
154 SmallPtrSet<BasicBlock*, 4> BadPreds;
155 for (pred_iterator PI = pred_begin(*BB),
156 PE = pred_end(*BB); PI != PE; ++PI) {
162 // Delete each unique out-of-loop (and thus dead) predecessor.
163 for (SmallPtrSet<BasicBlock*, 4>::iterator I = BadPreds.begin(),
164 E = BadPreds.end(); I != E; ++I) {
166 DEBUG(dbgs() << "LoopSimplify: Deleting edge from dead predecessor "
167 << (*I)->getName() << "\n");
169 // Inform each successor of each dead pred.
170 for (succ_iterator SI = succ_begin(*I), SE = succ_end(*I); SI != SE; ++SI)
171 (*SI)->removePredecessor(*I);
172 // Zap the dead pred's terminator and replace it with unreachable.
173 TerminatorInst *TI = (*I)->getTerminator();
174 TI->replaceAllUsesWith(UndefValue::get(TI->getType()));
175 (*I)->getTerminator()->eraseFromParent();
176 new UnreachableInst((*I)->getContext(), *I);
181 // If there are exiting blocks with branches on undef, resolve the undef in
182 // the direction which will exit the loop. This will help simplify loop
183 // trip count computations.
184 SmallVector<BasicBlock*, 8> ExitingBlocks;
185 L->getExitingBlocks(ExitingBlocks);
186 for (SmallVectorImpl<BasicBlock *>::iterator I = ExitingBlocks.begin(),
187 E = ExitingBlocks.end(); I != E; ++I)
188 if (BranchInst *BI = dyn_cast<BranchInst>((*I)->getTerminator()))
189 if (BI->isConditional()) {
190 if (UndefValue *Cond = dyn_cast<UndefValue>(BI->getCondition())) {
192 DEBUG(dbgs() << "LoopSimplify: Resolving \"br i1 undef\" to exit in "
193 << (*I)->getName() << "\n");
195 BI->setCondition(ConstantInt::get(Cond->getType(),
196 !L->contains(BI->getSuccessor(0))));
198 // This may make the loop analyzable, force SCEV recomputation.
206 // Does the loop already have a preheader? If so, don't insert one.
207 BasicBlock *Preheader = L->getLoopPreheader();
209 Preheader = InsertPreheaderForLoop(L);
216 // Next, check to make sure that all exit nodes of the loop only have
217 // predecessors that are inside of the loop. This check guarantees that the
218 // loop preheader/header will dominate the exit blocks. If the exit block has
219 // predecessors from outside of the loop, split the edge now.
220 SmallVector<BasicBlock*, 8> ExitBlocks;
221 L->getExitBlocks(ExitBlocks);
223 SmallSetVector<BasicBlock *, 8> ExitBlockSet(ExitBlocks.begin(),
225 for (SmallSetVector<BasicBlock *, 8>::iterator I = ExitBlockSet.begin(),
226 E = ExitBlockSet.end(); I != E; ++I) {
227 BasicBlock *ExitBlock = *I;
228 for (pred_iterator PI = pred_begin(ExitBlock), PE = pred_end(ExitBlock);
230 // Must be exactly this loop: no subloops, parent loops, or non-loop preds
232 if (!L->contains(*PI)) {
233 if (RewriteLoopExitBlock(L, ExitBlock)) {
241 // If the header has more than two predecessors at this point (from the
242 // preheader and from multiple backedges), we must adjust the loop.
243 BasicBlock *LoopLatch = L->getLoopLatch();
245 // If this is really a nested loop, rip it out into a child loop. Don't do
246 // this for loops with a giant number of backedges, just factor them into a
247 // common backedge instead.
248 if (L->getNumBackEdges() < 8) {
249 if (SeparateNestedLoop(L, LPM, Preheader)) {
251 // This is a big restructuring change, reprocess the whole loop.
253 // GCC doesn't tail recursion eliminate this.
258 // If we either couldn't, or didn't want to, identify nesting of the loops,
259 // insert a new block that all backedges target, then make it jump to the
261 LoopLatch = InsertUniqueBackedgeBlock(L, Preheader);
268 // Scan over the PHI nodes in the loop header. Since they now have only two
269 // incoming values (the loop is canonicalized), we may have simplified the PHI
270 // down to 'X = phi [X, Y]', which should be replaced with 'Y'.
272 for (BasicBlock::iterator I = L->getHeader()->begin();
273 (PN = dyn_cast<PHINode>(I++)); )
274 if (Value *V = SimplifyInstruction(PN, 0, 0, DT)) {
275 if (AA) AA->deleteValue(PN);
276 if (SE) SE->forgetValue(PN);
277 PN->replaceAllUsesWith(V);
278 PN->eraseFromParent();
281 // If this loop has multiple exits and the exits all go to the same
282 // block, attempt to merge the exits. This helps several passes, such
283 // as LoopRotation, which do not support loops with multiple exits.
284 // SimplifyCFG also does this (and this code uses the same utility
285 // function), however this code is loop-aware, where SimplifyCFG is
286 // not. That gives it the advantage of being able to hoist
287 // loop-invariant instructions out of the way to open up more
288 // opportunities, and the disadvantage of having the responsibility
289 // to preserve dominator information.
290 bool UniqueExit = true;
291 if (!ExitBlocks.empty())
292 for (unsigned i = 1, e = ExitBlocks.size(); i != e; ++i)
293 if (ExitBlocks[i] != ExitBlocks[0]) {
298 for (unsigned i = 0, e = ExitingBlocks.size(); i != e; ++i) {
299 BasicBlock *ExitingBlock = ExitingBlocks[i];
300 if (!ExitingBlock->getSinglePredecessor()) continue;
301 BranchInst *BI = dyn_cast<BranchInst>(ExitingBlock->getTerminator());
302 if (!BI || !BI->isConditional()) continue;
303 CmpInst *CI = dyn_cast<CmpInst>(BI->getCondition());
304 if (!CI || CI->getParent() != ExitingBlock) continue;
306 // Attempt to hoist out all instructions except for the
307 // comparison and the branch.
308 bool AllInvariant = true;
309 for (BasicBlock::iterator I = ExitingBlock->begin(); &*I != BI; ) {
310 Instruction *Inst = I++;
311 // Skip debug info intrinsics.
312 if (isa<DbgInfoIntrinsic>(Inst))
316 if (!L->makeLoopInvariant(Inst, Changed,
317 Preheader ? Preheader->getTerminator() : 0)) {
318 AllInvariant = false;
322 if (!AllInvariant) continue;
324 // The block has now been cleared of all instructions except for
325 // a comparison and a conditional branch. SimplifyCFG may be able
327 if (!FoldBranchToCommonDest(BI)) continue;
329 // Success. The block is now dead, so remove it from the loop,
330 // update the dominator tree and delete it.
331 DEBUG(dbgs() << "LoopSimplify: Eliminating exiting block "
332 << ExitingBlock->getName() << "\n");
334 // If any reachable control flow within this loop has changed, notify
335 // ScalarEvolution. Currently assume the parent loop doesn't change
336 // (spliting edges doesn't count). If blocks, CFG edges, or other values
337 // in the parent loop change, then we need call to forgetLoop() for the
342 assert(pred_begin(ExitingBlock) == pred_end(ExitingBlock));
344 LI->removeBlock(ExitingBlock);
346 DomTreeNode *Node = DT->getNode(ExitingBlock);
347 const std::vector<DomTreeNodeBase<BasicBlock> *> &Children =
349 while (!Children.empty()) {
350 DomTreeNode *Child = Children.front();
351 DT->changeImmediateDominator(Child, Node->getIDom());
353 DT->eraseNode(ExitingBlock);
355 BI->getSuccessor(0)->removePredecessor(ExitingBlock);
356 BI->getSuccessor(1)->removePredecessor(ExitingBlock);
357 ExitingBlock->eraseFromParent();
364 /// InsertPreheaderForLoop - Once we discover that a loop doesn't have a
365 /// preheader, this method is called to insert one. This method has two phases:
366 /// preheader insertion and analysis updating.
368 BasicBlock *LoopSimplify::InsertPreheaderForLoop(Loop *L) {
369 BasicBlock *Header = L->getHeader();
371 // Compute the set of predecessors of the loop that are not in the loop.
372 SmallVector<BasicBlock*, 8> OutsideBlocks;
373 for (pred_iterator PI = pred_begin(Header), PE = pred_end(Header);
376 if (!L->contains(P)) { // Coming in from outside the loop?
377 // If the loop is branched to from an indirect branch, we won't
378 // be able to fully transform the loop, because it prohibits
380 if (isa<IndirectBrInst>(P->getTerminator())) return 0;
383 OutsideBlocks.push_back(P);
387 // Split out the loop pre-header.
388 BasicBlock *PreheaderBB;
389 if (!Header->isLandingPad()) {
390 PreheaderBB = SplitBlockPredecessors(Header, OutsideBlocks, ".preheader",
393 SmallVector<BasicBlock*, 2> NewBBs;
394 SplitLandingPadPredecessors(Header, OutsideBlocks, ".preheader",
395 ".split-lp", this, NewBBs);
396 PreheaderBB = NewBBs[0];
399 PreheaderBB->getTerminator()->setDebugLoc(
400 Header->getFirstNonPHI()->getDebugLoc());
401 DEBUG(dbgs() << "LoopSimplify: Creating pre-header "
402 << PreheaderBB->getName() << "\n");
404 // Make sure that NewBB is put someplace intelligent, which doesn't mess up
405 // code layout too horribly.
406 PlaceSplitBlockCarefully(PreheaderBB, OutsideBlocks, L);
411 /// RewriteLoopExitBlock - Ensure that the loop preheader dominates all exit
412 /// blocks. This method is used to split exit blocks that have predecessors
413 /// outside of the loop.
414 BasicBlock *LoopSimplify::RewriteLoopExitBlock(Loop *L, BasicBlock *Exit) {
415 SmallVector<BasicBlock*, 8> LoopBlocks;
416 for (pred_iterator I = pred_begin(Exit), E = pred_end(Exit); I != E; ++I) {
418 if (L->contains(P)) {
419 // Don't do this if the loop is exited via an indirect branch.
420 if (isa<IndirectBrInst>(P->getTerminator())) return 0;
422 LoopBlocks.push_back(P);
426 assert(!LoopBlocks.empty() && "No edges coming in from outside the loop?");
427 BasicBlock *NewExitBB = 0;
429 if (Exit->isLandingPad()) {
430 SmallVector<BasicBlock*, 2> NewBBs;
431 SplitLandingPadPredecessors(Exit, ArrayRef<BasicBlock*>(&LoopBlocks[0],
433 ".loopexit", ".nonloopexit",
435 NewExitBB = NewBBs[0];
437 NewExitBB = SplitBlockPredecessors(Exit, LoopBlocks, ".loopexit", this);
440 DEBUG(dbgs() << "LoopSimplify: Creating dedicated exit block "
441 << NewExitBB->getName() << "\n");
445 /// AddBlockAndPredsToSet - Add the specified block, and all of its
446 /// predecessors, to the specified set, if it's not already in there. Stop
447 /// predecessor traversal when we reach StopBlock.
448 static void AddBlockAndPredsToSet(BasicBlock *InputBB, BasicBlock *StopBlock,
449 std::set<BasicBlock*> &Blocks) {
450 std::vector<BasicBlock *> WorkList;
451 WorkList.push_back(InputBB);
453 BasicBlock *BB = WorkList.back(); WorkList.pop_back();
454 if (Blocks.insert(BB).second && BB != StopBlock)
455 // If BB is not already processed and it is not a stop block then
456 // insert its predecessor in the work list
457 for (pred_iterator I = pred_begin(BB), E = pred_end(BB); I != E; ++I) {
458 BasicBlock *WBB = *I;
459 WorkList.push_back(WBB);
461 } while(!WorkList.empty());
464 /// FindPHIToPartitionLoops - The first part of loop-nestification is to find a
465 /// PHI node that tells us how to partition the loops.
466 static PHINode *FindPHIToPartitionLoops(Loop *L, DominatorTree *DT,
467 AliasAnalysis *AA, LoopInfo *LI) {
468 for (BasicBlock::iterator I = L->getHeader()->begin(); isa<PHINode>(I); ) {
469 PHINode *PN = cast<PHINode>(I);
471 if (Value *V = SimplifyInstruction(PN, 0, 0, DT)) {
472 // This is a degenerate PHI already, don't modify it!
473 PN->replaceAllUsesWith(V);
474 if (AA) AA->deleteValue(PN);
475 PN->eraseFromParent();
479 // Scan this PHI node looking for a use of the PHI node by itself.
480 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
481 if (PN->getIncomingValue(i) == PN &&
482 L->contains(PN->getIncomingBlock(i)))
483 // We found something tasty to remove.
489 // PlaceSplitBlockCarefully - If the block isn't already, move the new block to
490 // right after some 'outside block' block. This prevents the preheader from
491 // being placed inside the loop body, e.g. when the loop hasn't been rotated.
492 void LoopSimplify::PlaceSplitBlockCarefully(BasicBlock *NewBB,
493 SmallVectorImpl<BasicBlock*> &SplitPreds,
495 // Check to see if NewBB is already well placed.
496 Function::iterator BBI = NewBB; --BBI;
497 for (unsigned i = 0, e = SplitPreds.size(); i != e; ++i) {
498 if (&*BBI == SplitPreds[i])
502 // If it isn't already after an outside block, move it after one. This is
503 // always good as it makes the uncond branch from the outside block into a
506 // Figure out *which* outside block to put this after. Prefer an outside
507 // block that neighbors a BB actually in the loop.
508 BasicBlock *FoundBB = 0;
509 for (unsigned i = 0, e = SplitPreds.size(); i != e; ++i) {
510 Function::iterator BBI = SplitPreds[i];
511 if (++BBI != NewBB->getParent()->end() &&
513 FoundBB = SplitPreds[i];
518 // If our heuristic for a *good* bb to place this after doesn't find
519 // anything, just pick something. It's likely better than leaving it within
522 FoundBB = SplitPreds[0];
523 NewBB->moveAfter(FoundBB);
527 /// SeparateNestedLoop - If this loop has multiple backedges, try to pull one of
528 /// them out into a nested loop. This is important for code that looks like
533 /// br cond, Loop, Next
535 /// br cond2, Loop, Out
537 /// To identify this common case, we look at the PHI nodes in the header of the
538 /// loop. PHI nodes with unchanging values on one backedge correspond to values
539 /// that change in the "outer" loop, but not in the "inner" loop.
541 /// If we are able to separate out a loop, return the new outer loop that was
544 Loop *LoopSimplify::SeparateNestedLoop(Loop *L, LPPassManager &LPM,
545 BasicBlock *Preheader) {
546 // Don't try to separate loops without a preheader.
550 // The header is not a landing pad; preheader insertion should ensure this.
551 assert(!L->getHeader()->isLandingPad() &&
552 "Can't insert backedge to landing pad");
554 PHINode *PN = FindPHIToPartitionLoops(L, DT, AA, LI);
555 if (PN == 0) return 0; // No known way to partition.
557 // Pull out all predecessors that have varying values in the loop. This
558 // handles the case when a PHI node has multiple instances of itself as
560 SmallVector<BasicBlock*, 8> OuterLoopPreds;
561 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
562 if (PN->getIncomingValue(i) != PN ||
563 !L->contains(PN->getIncomingBlock(i))) {
564 // We can't split indirectbr edges.
565 if (isa<IndirectBrInst>(PN->getIncomingBlock(i)->getTerminator()))
567 OuterLoopPreds.push_back(PN->getIncomingBlock(i));
570 DEBUG(dbgs() << "LoopSimplify: Splitting out a new outer loop\n");
572 // If ScalarEvolution is around and knows anything about values in
573 // this loop, tell it to forget them, because we're about to
574 // substantially change it.
578 BasicBlock *Header = L->getHeader();
580 SplitBlockPredecessors(Header, OuterLoopPreds, ".outer", this);
582 // Make sure that NewBB is put someplace intelligent, which doesn't mess up
583 // code layout too horribly.
584 PlaceSplitBlockCarefully(NewBB, OuterLoopPreds, L);
586 // Create the new outer loop.
587 Loop *NewOuter = new Loop();
589 // Change the parent loop to use the outer loop as its child now.
590 if (Loop *Parent = L->getParentLoop())
591 Parent->replaceChildLoopWith(L, NewOuter);
593 LI->changeTopLevelLoop(L, NewOuter);
595 // L is now a subloop of our outer loop.
596 NewOuter->addChildLoop(L);
598 // Add the new loop to the pass manager queue.
599 LPM.insertLoopIntoQueue(NewOuter);
601 for (Loop::block_iterator I = L->block_begin(), E = L->block_end();
603 NewOuter->addBlockEntry(*I);
605 // Now reset the header in L, which had been moved by
606 // SplitBlockPredecessors for the outer loop.
607 L->moveToHeader(Header);
609 // Determine which blocks should stay in L and which should be moved out to
610 // the Outer loop now.
611 std::set<BasicBlock*> BlocksInL;
612 for (pred_iterator PI=pred_begin(Header), E = pred_end(Header); PI!=E; ++PI) {
614 if (DT->dominates(Header, P))
615 AddBlockAndPredsToSet(P, Header, BlocksInL);
618 // Scan all of the loop children of L, moving them to OuterLoop if they are
619 // not part of the inner loop.
620 const std::vector<Loop*> &SubLoops = L->getSubLoops();
621 for (size_t I = 0; I != SubLoops.size(); )
622 if (BlocksInL.count(SubLoops[I]->getHeader()))
623 ++I; // Loop remains in L
625 NewOuter->addChildLoop(L->removeChildLoop(SubLoops.begin() + I));
627 // Now that we know which blocks are in L and which need to be moved to
628 // OuterLoop, move any blocks that need it.
629 for (unsigned i = 0; i != L->getBlocks().size(); ++i) {
630 BasicBlock *BB = L->getBlocks()[i];
631 if (!BlocksInL.count(BB)) {
632 // Move this block to the parent, updating the exit blocks sets
633 L->removeBlockFromLoop(BB);
635 LI->changeLoopFor(BB, NewOuter);
645 /// InsertUniqueBackedgeBlock - This method is called when the specified loop
646 /// has more than one backedge in it. If this occurs, revector all of these
647 /// backedges to target a new basic block and have that block branch to the loop
648 /// header. This ensures that loops have exactly one backedge.
651 LoopSimplify::InsertUniqueBackedgeBlock(Loop *L, BasicBlock *Preheader) {
652 assert(L->getNumBackEdges() > 1 && "Must have > 1 backedge!");
654 // Get information about the loop
655 BasicBlock *Header = L->getHeader();
656 Function *F = Header->getParent();
658 // Unique backedge insertion currently depends on having a preheader.
662 // The header is not a landing pad; preheader insertion should ensure this.
663 assert(!Header->isLandingPad() && "Can't insert backedge to landing pad");
665 // Figure out which basic blocks contain back-edges to the loop header.
666 std::vector<BasicBlock*> BackedgeBlocks;
667 for (pred_iterator I = pred_begin(Header), E = pred_end(Header); I != E; ++I){
670 // Indirectbr edges cannot be split, so we must fail if we find one.
671 if (isa<IndirectBrInst>(P->getTerminator()))
674 if (P != Preheader) BackedgeBlocks.push_back(P);
677 // Create and insert the new backedge block...
678 BasicBlock *BEBlock = BasicBlock::Create(Header->getContext(),
679 Header->getName()+".backedge", F);
680 BranchInst *BETerminator = BranchInst::Create(Header, BEBlock);
682 DEBUG(dbgs() << "LoopSimplify: Inserting unique backedge block "
683 << BEBlock->getName() << "\n");
685 // Move the new backedge block to right after the last backedge block.
686 Function::iterator InsertPos = BackedgeBlocks.back(); ++InsertPos;
687 F->getBasicBlockList().splice(InsertPos, F->getBasicBlockList(), BEBlock);
689 // Now that the block has been inserted into the function, create PHI nodes in
690 // the backedge block which correspond to any PHI nodes in the header block.
691 for (BasicBlock::iterator I = Header->begin(); isa<PHINode>(I); ++I) {
692 PHINode *PN = cast<PHINode>(I);
693 PHINode *NewPN = PHINode::Create(PN->getType(), BackedgeBlocks.size(),
694 PN->getName()+".be", BETerminator);
695 if (AA) AA->copyValue(PN, NewPN);
697 // Loop over the PHI node, moving all entries except the one for the
698 // preheader over to the new PHI node.
699 unsigned PreheaderIdx = ~0U;
700 bool HasUniqueIncomingValue = true;
701 Value *UniqueValue = 0;
702 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
703 BasicBlock *IBB = PN->getIncomingBlock(i);
704 Value *IV = PN->getIncomingValue(i);
705 if (IBB == Preheader) {
708 NewPN->addIncoming(IV, IBB);
709 if (HasUniqueIncomingValue) {
710 if (UniqueValue == 0)
712 else if (UniqueValue != IV)
713 HasUniqueIncomingValue = false;
718 // Delete all of the incoming values from the old PN except the preheader's
719 assert(PreheaderIdx != ~0U && "PHI has no preheader entry??");
720 if (PreheaderIdx != 0) {
721 PN->setIncomingValue(0, PN->getIncomingValue(PreheaderIdx));
722 PN->setIncomingBlock(0, PN->getIncomingBlock(PreheaderIdx));
724 // Nuke all entries except the zero'th.
725 for (unsigned i = 0, e = PN->getNumIncomingValues()-1; i != e; ++i)
726 PN->removeIncomingValue(e-i, false);
728 // Finally, add the newly constructed PHI node as the entry for the BEBlock.
729 PN->addIncoming(NewPN, BEBlock);
731 // As an optimization, if all incoming values in the new PhiNode (which is a
732 // subset of the incoming values of the old PHI node) have the same value,
733 // eliminate the PHI Node.
734 if (HasUniqueIncomingValue) {
735 NewPN->replaceAllUsesWith(UniqueValue);
736 if (AA) AA->deleteValue(NewPN);
737 BEBlock->getInstList().erase(NewPN);
741 // Now that all of the PHI nodes have been inserted and adjusted, modify the
742 // backedge blocks to just to the BEBlock instead of the header.
743 for (unsigned i = 0, e = BackedgeBlocks.size(); i != e; ++i) {
744 TerminatorInst *TI = BackedgeBlocks[i]->getTerminator();
745 for (unsigned Op = 0, e = TI->getNumSuccessors(); Op != e; ++Op)
746 if (TI->getSuccessor(Op) == Header)
747 TI->setSuccessor(Op, BEBlock);
750 //===--- Update all analyses which we must preserve now -----------------===//
752 // Update Loop Information - we know that this block is now in the current
753 // loop and all parent loops.
754 L->addBasicBlockToLoop(BEBlock, LI->getBase());
756 // Update dominator information
757 DT->splitBlock(BEBlock);
762 void LoopSimplify::verifyAnalysis() const {
763 // It used to be possible to just assert L->isLoopSimplifyForm(), however
764 // with the introduction of indirectbr, there are now cases where it's
765 // not possible to transform a loop as necessary. We can at least check
766 // that there is an indirectbr near any time there's trouble.
768 // Indirectbr can interfere with preheader and unique backedge insertion.
769 if (!L->getLoopPreheader() || !L->getLoopLatch()) {
770 bool HasIndBrPred = false;
771 for (pred_iterator PI = pred_begin(L->getHeader()),
772 PE = pred_end(L->getHeader()); PI != PE; ++PI)
773 if (isa<IndirectBrInst>((*PI)->getTerminator())) {
777 assert(HasIndBrPred &&
778 "LoopSimplify has no excuse for missing loop header info!");
782 // Indirectbr can interfere with exit block canonicalization.
783 if (!L->hasDedicatedExits()) {
784 bool HasIndBrExiting = false;
785 SmallVector<BasicBlock*, 8> ExitingBlocks;
786 L->getExitingBlocks(ExitingBlocks);
787 for (unsigned i = 0, e = ExitingBlocks.size(); i != e; ++i) {
788 if (isa<IndirectBrInst>((ExitingBlocks[i])->getTerminator())) {
789 HasIndBrExiting = true;
794 assert(HasIndBrExiting &&
795 "LoopSimplify has no excuse for missing exit block info!");
796 (void)HasIndBrExiting;