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/ADT/DepthFirstIterator.h"
43 #include "llvm/ADT/SetOperations.h"
44 #include "llvm/ADT/SetVector.h"
45 #include "llvm/ADT/Statistic.h"
46 #include "llvm/Analysis/AliasAnalysis.h"
47 #include "llvm/Analysis/DependenceAnalysis.h"
48 #include "llvm/Analysis/Dominators.h"
49 #include "llvm/Analysis/InstructionSimplify.h"
50 #include "llvm/Analysis/LoopPass.h"
51 #include "llvm/Analysis/ScalarEvolution.h"
52 #include "llvm/Constants.h"
53 #include "llvm/Function.h"
54 #include "llvm/Instructions.h"
55 #include "llvm/IntrinsicInst.h"
56 #include "llvm/LLVMContext.h"
57 #include "llvm/Support/CFG.h"
58 #include "llvm/Support/Debug.h"
59 #include "llvm/Transforms/Utils/BasicBlockUtils.h"
60 #include "llvm/Transforms/Utils/Local.h"
61 #include "llvm/Type.h"
64 STATISTIC(NumInserted, "Number of pre-header or exit blocks inserted");
65 STATISTIC(NumNested , "Number of nested loops split out");
68 struct LoopSimplify : public LoopPass {
69 static char ID; // Pass identification, replacement for typeid
70 LoopSimplify() : LoopPass(ID) {
71 initializeLoopSimplifyPass(*PassRegistry::getPassRegistry());
74 // AA - If we have an alias analysis object to update, this is it, otherwise
81 virtual bool runOnLoop(Loop *L, LPPassManager &LPM);
83 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
84 // We need loop information to identify the loops...
85 AU.addRequired<DominatorTree>();
86 AU.addPreserved<DominatorTree>();
88 AU.addRequired<LoopInfo>();
89 AU.addPreserved<LoopInfo>();
91 AU.addPreserved<AliasAnalysis>();
92 AU.addPreserved<ScalarEvolution>();
93 AU.addPreserved<DependenceAnalysis>();
94 AU.addPreservedID(BreakCriticalEdgesID); // No critical edges added.
97 /// verifyAnalysis() - Verify LoopSimplifyForm's guarantees.
98 void verifyAnalysis() const;
101 bool ProcessLoop(Loop *L, LPPassManager &LPM);
102 BasicBlock *RewriteLoopExitBlock(Loop *L, BasicBlock *Exit);
103 BasicBlock *InsertPreheaderForLoop(Loop *L);
104 Loop *SeparateNestedLoop(Loop *L, LPPassManager &LPM,
105 BasicBlock *Preheader);
106 BasicBlock *InsertUniqueBackedgeBlock(Loop *L, BasicBlock *Preheader);
107 void PlaceSplitBlockCarefully(BasicBlock *NewBB,
108 SmallVectorImpl<BasicBlock*> &SplitPreds,
113 char LoopSimplify::ID = 0;
114 INITIALIZE_PASS_BEGIN(LoopSimplify, "loop-simplify",
115 "Canonicalize natural loops", true, false)
116 INITIALIZE_PASS_DEPENDENCY(DominatorTree)
117 INITIALIZE_PASS_DEPENDENCY(LoopInfo)
118 INITIALIZE_PASS_END(LoopSimplify, "loop-simplify",
119 "Canonicalize natural loops", true, false)
121 // Publicly exposed interface to pass...
122 char &llvm::LoopSimplifyID = LoopSimplify::ID;
123 Pass *llvm::createLoopSimplifyPass() { return new LoopSimplify(); }
125 /// runOnLoop - Run down all loops in the CFG (recursively, but we could do
126 /// it in any convenient order) inserting preheaders...
128 bool LoopSimplify::runOnLoop(Loop *l, LPPassManager &LPM) {
130 bool Changed = false;
131 LI = &getAnalysis<LoopInfo>();
132 AA = getAnalysisIfAvailable<AliasAnalysis>();
133 DT = &getAnalysis<DominatorTree>();
134 SE = getAnalysisIfAvailable<ScalarEvolution>();
136 Changed |= ProcessLoop(L, LPM);
141 /// ProcessLoop - Walk the loop structure in depth first order, ensuring that
142 /// all loops have preheaders.
144 bool LoopSimplify::ProcessLoop(Loop *L, LPPassManager &LPM) {
145 bool Changed = false;
148 // Check to see that no blocks (other than the header) in this loop have
149 // predecessors that are not in the loop. This is not valid for natural
150 // loops, but can occur if the blocks are unreachable. Since they are
151 // unreachable we can just shamelessly delete those CFG edges!
152 for (Loop::block_iterator BB = L->block_begin(), E = L->block_end();
154 if (*BB == L->getHeader()) continue;
156 SmallPtrSet<BasicBlock*, 4> BadPreds;
157 for (pred_iterator PI = pred_begin(*BB),
158 PE = pred_end(*BB); PI != PE; ++PI) {
164 // Delete each unique out-of-loop (and thus dead) predecessor.
165 for (SmallPtrSet<BasicBlock*, 4>::iterator I = BadPreds.begin(),
166 E = BadPreds.end(); I != E; ++I) {
168 DEBUG(dbgs() << "LoopSimplify: Deleting edge from dead predecessor "
169 << (*I)->getName() << "\n");
171 // Inform each successor of each dead pred.
172 for (succ_iterator SI = succ_begin(*I), SE = succ_end(*I); SI != SE; ++SI)
173 (*SI)->removePredecessor(*I);
174 // Zap the dead pred's terminator and replace it with unreachable.
175 TerminatorInst *TI = (*I)->getTerminator();
176 TI->replaceAllUsesWith(UndefValue::get(TI->getType()));
177 (*I)->getTerminator()->eraseFromParent();
178 new UnreachableInst((*I)->getContext(), *I);
183 // If there are exiting blocks with branches on undef, resolve the undef in
184 // the direction which will exit the loop. This will help simplify loop
185 // trip count computations.
186 SmallVector<BasicBlock*, 8> ExitingBlocks;
187 L->getExitingBlocks(ExitingBlocks);
188 for (SmallVectorImpl<BasicBlock *>::iterator I = ExitingBlocks.begin(),
189 E = ExitingBlocks.end(); I != E; ++I)
190 if (BranchInst *BI = dyn_cast<BranchInst>((*I)->getTerminator()))
191 if (BI->isConditional()) {
192 if (UndefValue *Cond = dyn_cast<UndefValue>(BI->getCondition())) {
194 DEBUG(dbgs() << "LoopSimplify: Resolving \"br i1 undef\" to exit in "
195 << (*I)->getName() << "\n");
197 BI->setCondition(ConstantInt::get(Cond->getType(),
198 !L->contains(BI->getSuccessor(0))));
200 // This may make the loop analyzable, force SCEV recomputation.
208 // Does the loop already have a preheader? If so, don't insert one.
209 BasicBlock *Preheader = L->getLoopPreheader();
211 Preheader = InsertPreheaderForLoop(L);
218 // Next, check to make sure that all exit nodes of the loop only have
219 // predecessors that are inside of the loop. This check guarantees that the
220 // loop preheader/header will dominate the exit blocks. If the exit block has
221 // predecessors from outside of the loop, split the edge now.
222 SmallVector<BasicBlock*, 8> ExitBlocks;
223 L->getExitBlocks(ExitBlocks);
225 SmallSetVector<BasicBlock *, 8> ExitBlockSet(ExitBlocks.begin(),
227 for (SmallSetVector<BasicBlock *, 8>::iterator I = ExitBlockSet.begin(),
228 E = ExitBlockSet.end(); I != E; ++I) {
229 BasicBlock *ExitBlock = *I;
230 for (pred_iterator PI = pred_begin(ExitBlock), PE = pred_end(ExitBlock);
232 // Must be exactly this loop: no subloops, parent loops, or non-loop preds
234 if (!L->contains(*PI)) {
235 if (RewriteLoopExitBlock(L, ExitBlock)) {
243 // If the header has more than two predecessors at this point (from the
244 // preheader and from multiple backedges), we must adjust the loop.
245 BasicBlock *LoopLatch = L->getLoopLatch();
247 // If this is really a nested loop, rip it out into a child loop. Don't do
248 // this for loops with a giant number of backedges, just factor them into a
249 // common backedge instead.
250 if (L->getNumBackEdges() < 8) {
251 if (SeparateNestedLoop(L, LPM, Preheader)) {
253 // This is a big restructuring change, reprocess the whole loop.
255 // GCC doesn't tail recursion eliminate this.
260 // If we either couldn't, or didn't want to, identify nesting of the loops,
261 // insert a new block that all backedges target, then make it jump to the
263 LoopLatch = InsertUniqueBackedgeBlock(L, Preheader);
270 // Scan over the PHI nodes in the loop header. Since they now have only two
271 // incoming values (the loop is canonicalized), we may have simplified the PHI
272 // down to 'X = phi [X, Y]', which should be replaced with 'Y'.
274 for (BasicBlock::iterator I = L->getHeader()->begin();
275 (PN = dyn_cast<PHINode>(I++)); )
276 if (Value *V = SimplifyInstruction(PN, 0, 0, DT)) {
277 if (AA) AA->deleteValue(PN);
278 if (SE) SE->forgetValue(PN);
279 PN->replaceAllUsesWith(V);
280 PN->eraseFromParent();
283 // If this loop has multiple exits and the exits all go to the same
284 // block, attempt to merge the exits. This helps several passes, such
285 // as LoopRotation, which do not support loops with multiple exits.
286 // SimplifyCFG also does this (and this code uses the same utility
287 // function), however this code is loop-aware, where SimplifyCFG is
288 // not. That gives it the advantage of being able to hoist
289 // loop-invariant instructions out of the way to open up more
290 // opportunities, and the disadvantage of having the responsibility
291 // to preserve dominator information.
292 bool UniqueExit = true;
293 if (!ExitBlocks.empty())
294 for (unsigned i = 1, e = ExitBlocks.size(); i != e; ++i)
295 if (ExitBlocks[i] != ExitBlocks[0]) {
300 for (unsigned i = 0, e = ExitingBlocks.size(); i != e; ++i) {
301 BasicBlock *ExitingBlock = ExitingBlocks[i];
302 if (!ExitingBlock->getSinglePredecessor()) continue;
303 BranchInst *BI = dyn_cast<BranchInst>(ExitingBlock->getTerminator());
304 if (!BI || !BI->isConditional()) continue;
305 CmpInst *CI = dyn_cast<CmpInst>(BI->getCondition());
306 if (!CI || CI->getParent() != ExitingBlock) continue;
308 // Attempt to hoist out all instructions except for the
309 // comparison and the branch.
310 bool AllInvariant = true;
311 for (BasicBlock::iterator I = ExitingBlock->begin(); &*I != BI; ) {
312 Instruction *Inst = I++;
313 // Skip debug info intrinsics.
314 if (isa<DbgInfoIntrinsic>(Inst))
318 if (!L->makeLoopInvariant(Inst, Changed,
319 Preheader ? Preheader->getTerminator() : 0)) {
320 AllInvariant = false;
324 if (!AllInvariant) continue;
326 // The block has now been cleared of all instructions except for
327 // a comparison and a conditional branch. SimplifyCFG may be able
329 if (!FoldBranchToCommonDest(BI)) continue;
331 // Success. The block is now dead, so remove it from the loop,
332 // update the dominator tree and delete it.
333 DEBUG(dbgs() << "LoopSimplify: Eliminating exiting block "
334 << ExitingBlock->getName() << "\n");
336 // If any reachable control flow within this loop has changed, notify
337 // ScalarEvolution. Currently assume the parent loop doesn't change
338 // (spliting edges doesn't count). If blocks, CFG edges, or other values
339 // in the parent loop change, then we need call to forgetLoop() for the
344 assert(pred_begin(ExitingBlock) == pred_end(ExitingBlock));
346 LI->removeBlock(ExitingBlock);
348 DomTreeNode *Node = DT->getNode(ExitingBlock);
349 const std::vector<DomTreeNodeBase<BasicBlock> *> &Children =
351 while (!Children.empty()) {
352 DomTreeNode *Child = Children.front();
353 DT->changeImmediateDominator(Child, Node->getIDom());
355 DT->eraseNode(ExitingBlock);
357 BI->getSuccessor(0)->removePredecessor(ExitingBlock);
358 BI->getSuccessor(1)->removePredecessor(ExitingBlock);
359 ExitingBlock->eraseFromParent();
366 /// InsertPreheaderForLoop - Once we discover that a loop doesn't have a
367 /// preheader, this method is called to insert one. This method has two phases:
368 /// preheader insertion and analysis updating.
370 BasicBlock *LoopSimplify::InsertPreheaderForLoop(Loop *L) {
371 BasicBlock *Header = L->getHeader();
373 // Compute the set of predecessors of the loop that are not in the loop.
374 SmallVector<BasicBlock*, 8> OutsideBlocks;
375 for (pred_iterator PI = pred_begin(Header), PE = pred_end(Header);
378 if (!L->contains(P)) { // Coming in from outside the loop?
379 // If the loop is branched to from an indirect branch, we won't
380 // be able to fully transform the loop, because it prohibits
382 if (isa<IndirectBrInst>(P->getTerminator())) return 0;
385 OutsideBlocks.push_back(P);
389 // Split out the loop pre-header.
390 BasicBlock *PreheaderBB;
391 if (!Header->isLandingPad()) {
392 PreheaderBB = SplitBlockPredecessors(Header, OutsideBlocks, ".preheader",
395 SmallVector<BasicBlock*, 2> NewBBs;
396 SplitLandingPadPredecessors(Header, OutsideBlocks, ".preheader",
397 ".split-lp", this, NewBBs);
398 PreheaderBB = NewBBs[0];
401 PreheaderBB->getTerminator()->setDebugLoc(
402 Header->getFirstNonPHI()->getDebugLoc());
403 DEBUG(dbgs() << "LoopSimplify: Creating pre-header "
404 << PreheaderBB->getName() << "\n");
406 // Make sure that NewBB is put someplace intelligent, which doesn't mess up
407 // code layout too horribly.
408 PlaceSplitBlockCarefully(PreheaderBB, OutsideBlocks, L);
413 /// RewriteLoopExitBlock - Ensure that the loop preheader dominates all exit
414 /// blocks. This method is used to split exit blocks that have predecessors
415 /// outside of the loop.
416 BasicBlock *LoopSimplify::RewriteLoopExitBlock(Loop *L, BasicBlock *Exit) {
417 SmallVector<BasicBlock*, 8> LoopBlocks;
418 for (pred_iterator I = pred_begin(Exit), E = pred_end(Exit); I != E; ++I) {
420 if (L->contains(P)) {
421 // Don't do this if the loop is exited via an indirect branch.
422 if (isa<IndirectBrInst>(P->getTerminator())) return 0;
424 LoopBlocks.push_back(P);
428 assert(!LoopBlocks.empty() && "No edges coming in from outside the loop?");
429 BasicBlock *NewExitBB = 0;
431 if (Exit->isLandingPad()) {
432 SmallVector<BasicBlock*, 2> NewBBs;
433 SplitLandingPadPredecessors(Exit, ArrayRef<BasicBlock*>(&LoopBlocks[0],
435 ".loopexit", ".nonloopexit",
437 NewExitBB = NewBBs[0];
439 NewExitBB = SplitBlockPredecessors(Exit, LoopBlocks, ".loopexit", this);
442 DEBUG(dbgs() << "LoopSimplify: Creating dedicated exit block "
443 << NewExitBB->getName() << "\n");
447 /// AddBlockAndPredsToSet - Add the specified block, and all of its
448 /// predecessors, to the specified set, if it's not already in there. Stop
449 /// predecessor traversal when we reach StopBlock.
450 static void AddBlockAndPredsToSet(BasicBlock *InputBB, BasicBlock *StopBlock,
451 std::set<BasicBlock*> &Blocks) {
452 std::vector<BasicBlock *> WorkList;
453 WorkList.push_back(InputBB);
455 BasicBlock *BB = WorkList.back(); WorkList.pop_back();
456 if (Blocks.insert(BB).second && BB != StopBlock)
457 // If BB is not already processed and it is not a stop block then
458 // insert its predecessor in the work list
459 for (pred_iterator I = pred_begin(BB), E = pred_end(BB); I != E; ++I) {
460 BasicBlock *WBB = *I;
461 WorkList.push_back(WBB);
463 } while(!WorkList.empty());
466 /// FindPHIToPartitionLoops - The first part of loop-nestification is to find a
467 /// PHI node that tells us how to partition the loops.
468 static PHINode *FindPHIToPartitionLoops(Loop *L, DominatorTree *DT,
469 AliasAnalysis *AA, LoopInfo *LI) {
470 for (BasicBlock::iterator I = L->getHeader()->begin(); isa<PHINode>(I); ) {
471 PHINode *PN = cast<PHINode>(I);
473 if (Value *V = SimplifyInstruction(PN, 0, 0, DT)) {
474 // This is a degenerate PHI already, don't modify it!
475 PN->replaceAllUsesWith(V);
476 if (AA) AA->deleteValue(PN);
477 PN->eraseFromParent();
481 // Scan this PHI node looking for a use of the PHI node by itself.
482 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
483 if (PN->getIncomingValue(i) == PN &&
484 L->contains(PN->getIncomingBlock(i)))
485 // We found something tasty to remove.
491 // PlaceSplitBlockCarefully - If the block isn't already, move the new block to
492 // right after some 'outside block' block. This prevents the preheader from
493 // being placed inside the loop body, e.g. when the loop hasn't been rotated.
494 void LoopSimplify::PlaceSplitBlockCarefully(BasicBlock *NewBB,
495 SmallVectorImpl<BasicBlock*> &SplitPreds,
497 // Check to see if NewBB is already well placed.
498 Function::iterator BBI = NewBB; --BBI;
499 for (unsigned i = 0, e = SplitPreds.size(); i != e; ++i) {
500 if (&*BBI == SplitPreds[i])
504 // If it isn't already after an outside block, move it after one. This is
505 // always good as it makes the uncond branch from the outside block into a
508 // Figure out *which* outside block to put this after. Prefer an outside
509 // block that neighbors a BB actually in the loop.
510 BasicBlock *FoundBB = 0;
511 for (unsigned i = 0, e = SplitPreds.size(); i != e; ++i) {
512 Function::iterator BBI = SplitPreds[i];
513 if (++BBI != NewBB->getParent()->end() &&
515 FoundBB = SplitPreds[i];
520 // If our heuristic for a *good* bb to place this after doesn't find
521 // anything, just pick something. It's likely better than leaving it within
524 FoundBB = SplitPreds[0];
525 NewBB->moveAfter(FoundBB);
529 /// SeparateNestedLoop - If this loop has multiple backedges, try to pull one of
530 /// them out into a nested loop. This is important for code that looks like
535 /// br cond, Loop, Next
537 /// br cond2, Loop, Out
539 /// To identify this common case, we look at the PHI nodes in the header of the
540 /// loop. PHI nodes with unchanging values on one backedge correspond to values
541 /// that change in the "outer" loop, but not in the "inner" loop.
543 /// If we are able to separate out a loop, return the new outer loop that was
546 Loop *LoopSimplify::SeparateNestedLoop(Loop *L, LPPassManager &LPM,
547 BasicBlock *Preheader) {
548 // Don't try to separate loops without a preheader.
552 // The header is not a landing pad; preheader insertion should ensure this.
553 assert(!L->getHeader()->isLandingPad() &&
554 "Can't insert backedge to landing pad");
556 PHINode *PN = FindPHIToPartitionLoops(L, DT, AA, LI);
557 if (PN == 0) return 0; // No known way to partition.
559 // Pull out all predecessors that have varying values in the loop. This
560 // handles the case when a PHI node has multiple instances of itself as
562 SmallVector<BasicBlock*, 8> OuterLoopPreds;
563 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
564 if (PN->getIncomingValue(i) != PN ||
565 !L->contains(PN->getIncomingBlock(i))) {
566 // We can't split indirectbr edges.
567 if (isa<IndirectBrInst>(PN->getIncomingBlock(i)->getTerminator()))
569 OuterLoopPreds.push_back(PN->getIncomingBlock(i));
572 DEBUG(dbgs() << "LoopSimplify: Splitting out a new outer loop\n");
574 // If ScalarEvolution is around and knows anything about values in
575 // this loop, tell it to forget them, because we're about to
576 // substantially change it.
580 BasicBlock *Header = L->getHeader();
582 SplitBlockPredecessors(Header, OuterLoopPreds, ".outer", this);
584 // Make sure that NewBB is put someplace intelligent, which doesn't mess up
585 // code layout too horribly.
586 PlaceSplitBlockCarefully(NewBB, OuterLoopPreds, L);
588 // Create the new outer loop.
589 Loop *NewOuter = new Loop();
591 // Change the parent loop to use the outer loop as its child now.
592 if (Loop *Parent = L->getParentLoop())
593 Parent->replaceChildLoopWith(L, NewOuter);
595 LI->changeTopLevelLoop(L, NewOuter);
597 // L is now a subloop of our outer loop.
598 NewOuter->addChildLoop(L);
600 // Add the new loop to the pass manager queue.
601 LPM.insertLoopIntoQueue(NewOuter);
603 for (Loop::block_iterator I = L->block_begin(), E = L->block_end();
605 NewOuter->addBlockEntry(*I);
607 // Now reset the header in L, which had been moved by
608 // SplitBlockPredecessors for the outer loop.
609 L->moveToHeader(Header);
611 // Determine which blocks should stay in L and which should be moved out to
612 // the Outer loop now.
613 std::set<BasicBlock*> BlocksInL;
614 for (pred_iterator PI=pred_begin(Header), E = pred_end(Header); PI!=E; ++PI) {
616 if (DT->dominates(Header, P))
617 AddBlockAndPredsToSet(P, Header, BlocksInL);
620 // Scan all of the loop children of L, moving them to OuterLoop if they are
621 // not part of the inner loop.
622 const std::vector<Loop*> &SubLoops = L->getSubLoops();
623 for (size_t I = 0; I != SubLoops.size(); )
624 if (BlocksInL.count(SubLoops[I]->getHeader()))
625 ++I; // Loop remains in L
627 NewOuter->addChildLoop(L->removeChildLoop(SubLoops.begin() + I));
629 // Now that we know which blocks are in L and which need to be moved to
630 // OuterLoop, move any blocks that need it.
631 for (unsigned i = 0; i != L->getBlocks().size(); ++i) {
632 BasicBlock *BB = L->getBlocks()[i];
633 if (!BlocksInL.count(BB)) {
634 // Move this block to the parent, updating the exit blocks sets
635 L->removeBlockFromLoop(BB);
637 LI->changeLoopFor(BB, NewOuter);
647 /// InsertUniqueBackedgeBlock - This method is called when the specified loop
648 /// has more than one backedge in it. If this occurs, revector all of these
649 /// backedges to target a new basic block and have that block branch to the loop
650 /// header. This ensures that loops have exactly one backedge.
653 LoopSimplify::InsertUniqueBackedgeBlock(Loop *L, BasicBlock *Preheader) {
654 assert(L->getNumBackEdges() > 1 && "Must have > 1 backedge!");
656 // Get information about the loop
657 BasicBlock *Header = L->getHeader();
658 Function *F = Header->getParent();
660 // Unique backedge insertion currently depends on having a preheader.
664 // The header is not a landing pad; preheader insertion should ensure this.
665 assert(!Header->isLandingPad() && "Can't insert backedge to landing pad");
667 // Figure out which basic blocks contain back-edges to the loop header.
668 std::vector<BasicBlock*> BackedgeBlocks;
669 for (pred_iterator I = pred_begin(Header), E = pred_end(Header); I != E; ++I){
672 // Indirectbr edges cannot be split, so we must fail if we find one.
673 if (isa<IndirectBrInst>(P->getTerminator()))
676 if (P != Preheader) BackedgeBlocks.push_back(P);
679 // Create and insert the new backedge block...
680 BasicBlock *BEBlock = BasicBlock::Create(Header->getContext(),
681 Header->getName()+".backedge", F);
682 BranchInst *BETerminator = BranchInst::Create(Header, BEBlock);
684 DEBUG(dbgs() << "LoopSimplify: Inserting unique backedge block "
685 << BEBlock->getName() << "\n");
687 // Move the new backedge block to right after the last backedge block.
688 Function::iterator InsertPos = BackedgeBlocks.back(); ++InsertPos;
689 F->getBasicBlockList().splice(InsertPos, F->getBasicBlockList(), BEBlock);
691 // Now that the block has been inserted into the function, create PHI nodes in
692 // the backedge block which correspond to any PHI nodes in the header block.
693 for (BasicBlock::iterator I = Header->begin(); isa<PHINode>(I); ++I) {
694 PHINode *PN = cast<PHINode>(I);
695 PHINode *NewPN = PHINode::Create(PN->getType(), BackedgeBlocks.size(),
696 PN->getName()+".be", BETerminator);
697 if (AA) AA->copyValue(PN, NewPN);
699 // Loop over the PHI node, moving all entries except the one for the
700 // preheader over to the new PHI node.
701 unsigned PreheaderIdx = ~0U;
702 bool HasUniqueIncomingValue = true;
703 Value *UniqueValue = 0;
704 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
705 BasicBlock *IBB = PN->getIncomingBlock(i);
706 Value *IV = PN->getIncomingValue(i);
707 if (IBB == Preheader) {
710 NewPN->addIncoming(IV, IBB);
711 if (HasUniqueIncomingValue) {
712 if (UniqueValue == 0)
714 else if (UniqueValue != IV)
715 HasUniqueIncomingValue = false;
720 // Delete all of the incoming values from the old PN except the preheader's
721 assert(PreheaderIdx != ~0U && "PHI has no preheader entry??");
722 if (PreheaderIdx != 0) {
723 PN->setIncomingValue(0, PN->getIncomingValue(PreheaderIdx));
724 PN->setIncomingBlock(0, PN->getIncomingBlock(PreheaderIdx));
726 // Nuke all entries except the zero'th.
727 for (unsigned i = 0, e = PN->getNumIncomingValues()-1; i != e; ++i)
728 PN->removeIncomingValue(e-i, false);
730 // Finally, add the newly constructed PHI node as the entry for the BEBlock.
731 PN->addIncoming(NewPN, BEBlock);
733 // As an optimization, if all incoming values in the new PhiNode (which is a
734 // subset of the incoming values of the old PHI node) have the same value,
735 // eliminate the PHI Node.
736 if (HasUniqueIncomingValue) {
737 NewPN->replaceAllUsesWith(UniqueValue);
738 if (AA) AA->deleteValue(NewPN);
739 BEBlock->getInstList().erase(NewPN);
743 // Now that all of the PHI nodes have been inserted and adjusted, modify the
744 // backedge blocks to just to the BEBlock instead of the header.
745 for (unsigned i = 0, e = BackedgeBlocks.size(); i != e; ++i) {
746 TerminatorInst *TI = BackedgeBlocks[i]->getTerminator();
747 for (unsigned Op = 0, e = TI->getNumSuccessors(); Op != e; ++Op)
748 if (TI->getSuccessor(Op) == Header)
749 TI->setSuccessor(Op, BEBlock);
752 //===--- Update all analyses which we must preserve now -----------------===//
754 // Update Loop Information - we know that this block is now in the current
755 // loop and all parent loops.
756 L->addBasicBlockToLoop(BEBlock, LI->getBase());
758 // Update dominator information
759 DT->splitBlock(BEBlock);
764 void LoopSimplify::verifyAnalysis() const {
765 // It used to be possible to just assert L->isLoopSimplifyForm(), however
766 // with the introduction of indirectbr, there are now cases where it's
767 // not possible to transform a loop as necessary. We can at least check
768 // that there is an indirectbr near any time there's trouble.
770 // Indirectbr can interfere with preheader and unique backedge insertion.
771 if (!L->getLoopPreheader() || !L->getLoopLatch()) {
772 bool HasIndBrPred = false;
773 for (pred_iterator PI = pred_begin(L->getHeader()),
774 PE = pred_end(L->getHeader()); PI != PE; ++PI)
775 if (isa<IndirectBrInst>((*PI)->getTerminator())) {
779 assert(HasIndBrPred &&
780 "LoopSimplify has no excuse for missing loop header info!");
784 // Indirectbr can interfere with exit block canonicalization.
785 if (!L->hasDedicatedExits()) {
786 bool HasIndBrExiting = false;
787 SmallVector<BasicBlock*, 8> ExitingBlocks;
788 L->getExitingBlocks(ExitingBlocks);
789 for (unsigned i = 0, e = ExitingBlocks.size(); i != e; ++i) {
790 if (isa<IndirectBrInst>((ExitingBlocks[i])->getTerminator())) {
791 HasIndBrExiting = true;
796 assert(HasIndBrExiting &&
797 "LoopSimplify has no excuse for missing exit block info!");
798 (void)HasIndBrExiting;