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 #include "llvm/Transforms/Scalar.h"
41 #include "llvm/ADT/DepthFirstIterator.h"
42 #include "llvm/ADT/SetOperations.h"
43 #include "llvm/ADT/SetVector.h"
44 #include "llvm/ADT/SmallVector.h"
45 #include "llvm/ADT/Statistic.h"
46 #include "llvm/Analysis/AliasAnalysis.h"
47 #include "llvm/Analysis/DependenceAnalysis.h"
48 #include "llvm/Analysis/InstructionSimplify.h"
49 #include "llvm/Analysis/LoopInfo.h"
50 #include "llvm/Analysis/ScalarEvolution.h"
51 #include "llvm/IR/CFG.h"
52 #include "llvm/IR/Constants.h"
53 #include "llvm/IR/DataLayout.h"
54 #include "llvm/IR/Dominators.h"
55 #include "llvm/IR/Function.h"
56 #include "llvm/IR/Instructions.h"
57 #include "llvm/IR/IntrinsicInst.h"
58 #include "llvm/IR/LLVMContext.h"
59 #include "llvm/IR/Type.h"
60 #include "llvm/Support/Debug.h"
61 #include "llvm/Transforms/Utils/BasicBlockUtils.h"
62 #include "llvm/Transforms/Utils/Local.h"
63 #include "llvm/Transforms/Utils/LoopUtils.h"
66 #define DEBUG_TYPE "loop-simplify"
68 STATISTIC(NumInserted, "Number of pre-header or exit blocks inserted");
69 STATISTIC(NumNested , "Number of nested loops split out");
71 // If the block isn't already, move the new block to right after some 'outside
72 // block' block. This prevents the preheader from being placed inside the loop
73 // body, e.g. when the loop hasn't been rotated.
74 static void placeSplitBlockCarefully(BasicBlock *NewBB,
75 SmallVectorImpl<BasicBlock *> &SplitPreds,
77 // Check to see if NewBB is already well placed.
78 Function::iterator BBI = NewBB; --BBI;
79 for (unsigned i = 0, e = SplitPreds.size(); i != e; ++i) {
80 if (&*BBI == SplitPreds[i])
84 // If it isn't already after an outside block, move it after one. This is
85 // always good as it makes the uncond branch from the outside block into a
88 // Figure out *which* outside block to put this after. Prefer an outside
89 // block that neighbors a BB actually in the loop.
90 BasicBlock *FoundBB = nullptr;
91 for (unsigned i = 0, e = SplitPreds.size(); i != e; ++i) {
92 Function::iterator BBI = SplitPreds[i];
93 if (++BBI != NewBB->getParent()->end() &&
95 FoundBB = SplitPreds[i];
100 // If our heuristic for a *good* bb to place this after doesn't find
101 // anything, just pick something. It's likely better than leaving it within
104 FoundBB = SplitPreds[0];
105 NewBB->moveAfter(FoundBB);
108 /// InsertPreheaderForLoop - Once we discover that a loop doesn't have a
109 /// preheader, this method is called to insert one. This method has two phases:
110 /// preheader insertion and analysis updating.
112 BasicBlock *llvm::InsertPreheaderForLoop(Loop *L, Pass *PP) {
113 BasicBlock *Header = L->getHeader();
115 // Compute the set of predecessors of the loop that are not in the loop.
116 SmallVector<BasicBlock*, 8> OutsideBlocks;
117 for (BasicBlock *P : predecessors(Header)) {
118 if (!L->contains(P)) { // Coming in from outside the loop?
119 // If the loop is branched to from an indirect branch, we won't
120 // be able to fully transform the loop, because it prohibits
122 if (isa<IndirectBrInst>(P->getTerminator())) return nullptr;
125 OutsideBlocks.push_back(P);
129 // Split out the loop pre-header.
130 BasicBlock *PreheaderBB;
131 if (!Header->isLandingPad()) {
132 PreheaderBB = SplitBlockPredecessors(Header, OutsideBlocks, ".preheader",
135 SmallVector<BasicBlock*, 2> NewBBs;
136 SplitLandingPadPredecessors(Header, OutsideBlocks, ".preheader",
137 ".split-lp", PP, NewBBs);
138 PreheaderBB = NewBBs[0];
141 PreheaderBB->getTerminator()->setDebugLoc(
142 Header->getFirstNonPHI()->getDebugLoc());
143 DEBUG(dbgs() << "LoopSimplify: Creating pre-header "
144 << PreheaderBB->getName() << "\n");
146 // Make sure that NewBB is put someplace intelligent, which doesn't mess up
147 // code layout too horribly.
148 placeSplitBlockCarefully(PreheaderBB, OutsideBlocks, L);
153 /// \brief Ensure that the loop preheader dominates all exit blocks.
155 /// This method is used to split exit blocks that have predecessors outside of
157 static BasicBlock *rewriteLoopExitBlock(Loop *L, BasicBlock *Exit, Pass *PP) {
158 SmallVector<BasicBlock*, 8> LoopBlocks;
159 for (BasicBlock *P : predecessors(Exit)) {
160 if (L->contains(P)) {
161 // Don't do this if the loop is exited via an indirect branch.
162 if (isa<IndirectBrInst>(P->getTerminator())) return nullptr;
164 LoopBlocks.push_back(P);
168 assert(!LoopBlocks.empty() && "No edges coming in from outside the loop?");
169 BasicBlock *NewExitBB = nullptr;
171 if (Exit->isLandingPad()) {
172 SmallVector<BasicBlock*, 2> NewBBs;
173 SplitLandingPadPredecessors(Exit, ArrayRef<BasicBlock*>(&LoopBlocks[0],
175 ".loopexit", ".nonloopexit",
177 NewExitBB = NewBBs[0];
179 NewExitBB = SplitBlockPredecessors(Exit, LoopBlocks, ".loopexit", PP);
182 DEBUG(dbgs() << "LoopSimplify: Creating dedicated exit block "
183 << NewExitBB->getName() << "\n");
187 /// Add the specified block, and all of its predecessors, to the specified set,
188 /// if it's not already in there. Stop predecessor traversal when we reach
190 static void addBlockAndPredsToSet(BasicBlock *InputBB, BasicBlock *StopBlock,
191 std::set<BasicBlock*> &Blocks) {
192 SmallVector<BasicBlock *, 8> Worklist;
193 Worklist.push_back(InputBB);
195 BasicBlock *BB = Worklist.pop_back_val();
196 if (Blocks.insert(BB).second && BB != StopBlock)
197 // If BB is not already processed and it is not a stop block then
198 // insert its predecessor in the work list
199 for (BasicBlock *WBB : predecessors(BB))
200 Worklist.push_back(WBB);
201 } while (!Worklist.empty());
204 /// \brief The first part of loop-nestification is to find a PHI node that tells
205 /// us how to partition the loops.
206 static PHINode *findPHIToPartitionLoops(Loop *L, AliasAnalysis *AA,
208 for (BasicBlock::iterator I = L->getHeader()->begin(); isa<PHINode>(I); ) {
209 PHINode *PN = cast<PHINode>(I);
211 if (Value *V = SimplifyInstruction(PN, nullptr, nullptr, DT)) {
212 // This is a degenerate PHI already, don't modify it!
213 PN->replaceAllUsesWith(V);
214 if (AA) AA->deleteValue(PN);
215 PN->eraseFromParent();
219 // Scan this PHI node looking for a use of the PHI node by itself.
220 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
221 if (PN->getIncomingValue(i) == PN &&
222 L->contains(PN->getIncomingBlock(i)))
223 // We found something tasty to remove.
229 /// \brief If this loop has multiple backedges, try to pull one of them out into
232 /// This is important for code that looks like
237 /// br cond, Loop, Next
239 /// br cond2, Loop, Out
241 /// To identify this common case, we look at the PHI nodes in the header of the
242 /// loop. PHI nodes with unchanging values on one backedge correspond to values
243 /// that change in the "outer" loop, but not in the "inner" loop.
245 /// If we are able to separate out a loop, return the new outer loop that was
248 static Loop *separateNestedLoop(Loop *L, BasicBlock *Preheader,
249 AliasAnalysis *AA, DominatorTree *DT,
250 LoopInfo *LI, ScalarEvolution *SE, Pass *PP) {
251 // Don't try to separate loops without a preheader.
255 // The header is not a landing pad; preheader insertion should ensure this.
256 assert(!L->getHeader()->isLandingPad() &&
257 "Can't insert backedge to landing pad");
259 PHINode *PN = findPHIToPartitionLoops(L, AA, DT);
260 if (!PN) return nullptr; // No known way to partition.
262 // Pull out all predecessors that have varying values in the loop. This
263 // handles the case when a PHI node has multiple instances of itself as
265 SmallVector<BasicBlock*, 8> OuterLoopPreds;
266 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
267 if (PN->getIncomingValue(i) != PN ||
268 !L->contains(PN->getIncomingBlock(i))) {
269 // We can't split indirectbr edges.
270 if (isa<IndirectBrInst>(PN->getIncomingBlock(i)->getTerminator()))
272 OuterLoopPreds.push_back(PN->getIncomingBlock(i));
275 DEBUG(dbgs() << "LoopSimplify: Splitting out a new outer loop\n");
277 // If ScalarEvolution is around and knows anything about values in
278 // this loop, tell it to forget them, because we're about to
279 // substantially change it.
283 BasicBlock *Header = L->getHeader();
285 SplitBlockPredecessors(Header, OuterLoopPreds, ".outer", PP);
287 // Make sure that NewBB is put someplace intelligent, which doesn't mess up
288 // code layout too horribly.
289 placeSplitBlockCarefully(NewBB, OuterLoopPreds, L);
291 // Create the new outer loop.
292 Loop *NewOuter = new Loop();
294 // Change the parent loop to use the outer loop as its child now.
295 if (Loop *Parent = L->getParentLoop())
296 Parent->replaceChildLoopWith(L, NewOuter);
298 LI->changeTopLevelLoop(L, NewOuter);
300 // L is now a subloop of our outer loop.
301 NewOuter->addChildLoop(L);
303 for (Loop::block_iterator I = L->block_begin(), E = L->block_end();
305 NewOuter->addBlockEntry(*I);
307 // Now reset the header in L, which had been moved by
308 // SplitBlockPredecessors for the outer loop.
309 L->moveToHeader(Header);
311 // Determine which blocks should stay in L and which should be moved out to
312 // the Outer loop now.
313 std::set<BasicBlock*> BlocksInL;
314 for (BasicBlock *P : predecessors(Header)) {
315 if (DT->dominates(Header, P))
316 addBlockAndPredsToSet(P, Header, BlocksInL);
319 // Scan all of the loop children of L, moving them to OuterLoop if they are
320 // not part of the inner loop.
321 const std::vector<Loop*> &SubLoops = L->getSubLoops();
322 for (size_t I = 0; I != SubLoops.size(); )
323 if (BlocksInL.count(SubLoops[I]->getHeader()))
324 ++I; // Loop remains in L
326 NewOuter->addChildLoop(L->removeChildLoop(SubLoops.begin() + I));
328 // Now that we know which blocks are in L and which need to be moved to
329 // OuterLoop, move any blocks that need it.
330 for (unsigned i = 0; i != L->getBlocks().size(); ++i) {
331 BasicBlock *BB = L->getBlocks()[i];
332 if (!BlocksInL.count(BB)) {
333 // Move this block to the parent, updating the exit blocks sets
334 L->removeBlockFromLoop(BB);
336 LI->changeLoopFor(BB, NewOuter);
344 /// \brief This method is called when the specified loop has more than one
347 /// If this occurs, revector all of these backedges to target a new basic block
348 /// and have that block branch to the loop header. This ensures that loops
349 /// have exactly one backedge.
350 static BasicBlock *insertUniqueBackedgeBlock(Loop *L, BasicBlock *Preheader,
352 DominatorTree *DT, LoopInfo *LI) {
353 assert(L->getNumBackEdges() > 1 && "Must have > 1 backedge!");
355 // Get information about the loop
356 BasicBlock *Header = L->getHeader();
357 Function *F = Header->getParent();
359 // Unique backedge insertion currently depends on having a preheader.
363 // The header is not a landing pad; preheader insertion should ensure this.
364 assert(!Header->isLandingPad() && "Can't insert backedge to landing pad");
366 // Figure out which basic blocks contain back-edges to the loop header.
367 std::vector<BasicBlock*> BackedgeBlocks;
368 for (BasicBlock *P : predecessors(Header)) {
369 // Indirectbr edges cannot be split, so we must fail if we find one.
370 if (isa<IndirectBrInst>(P->getTerminator()))
373 if (P != Preheader) BackedgeBlocks.push_back(P);
376 // Create and insert the new backedge block...
377 BasicBlock *BEBlock = BasicBlock::Create(Header->getContext(),
378 Header->getName()+".backedge", F);
379 BranchInst *BETerminator = BranchInst::Create(Header, BEBlock);
381 DEBUG(dbgs() << "LoopSimplify: Inserting unique backedge block "
382 << BEBlock->getName() << "\n");
384 // Move the new backedge block to right after the last backedge block.
385 Function::iterator InsertPos = BackedgeBlocks.back(); ++InsertPos;
386 F->getBasicBlockList().splice(InsertPos, F->getBasicBlockList(), BEBlock);
388 // Now that the block has been inserted into the function, create PHI nodes in
389 // the backedge block which correspond to any PHI nodes in the header block.
390 for (BasicBlock::iterator I = Header->begin(); isa<PHINode>(I); ++I) {
391 PHINode *PN = cast<PHINode>(I);
392 PHINode *NewPN = PHINode::Create(PN->getType(), BackedgeBlocks.size(),
393 PN->getName()+".be", BETerminator);
394 if (AA) AA->copyValue(PN, NewPN);
396 // Loop over the PHI node, moving all entries except the one for the
397 // preheader over to the new PHI node.
398 unsigned PreheaderIdx = ~0U;
399 bool HasUniqueIncomingValue = true;
400 Value *UniqueValue = nullptr;
401 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
402 BasicBlock *IBB = PN->getIncomingBlock(i);
403 Value *IV = PN->getIncomingValue(i);
404 if (IBB == Preheader) {
407 NewPN->addIncoming(IV, IBB);
408 if (HasUniqueIncomingValue) {
411 else if (UniqueValue != IV)
412 HasUniqueIncomingValue = false;
417 // Delete all of the incoming values from the old PN except the preheader's
418 assert(PreheaderIdx != ~0U && "PHI has no preheader entry??");
419 if (PreheaderIdx != 0) {
420 PN->setIncomingValue(0, PN->getIncomingValue(PreheaderIdx));
421 PN->setIncomingBlock(0, PN->getIncomingBlock(PreheaderIdx));
423 // Nuke all entries except the zero'th.
424 for (unsigned i = 0, e = PN->getNumIncomingValues()-1; i != e; ++i)
425 PN->removeIncomingValue(e-i, false);
427 // Finally, add the newly constructed PHI node as the entry for the BEBlock.
428 PN->addIncoming(NewPN, BEBlock);
430 // As an optimization, if all incoming values in the new PhiNode (which is a
431 // subset of the incoming values of the old PHI node) have the same value,
432 // eliminate the PHI Node.
433 if (HasUniqueIncomingValue) {
434 NewPN->replaceAllUsesWith(UniqueValue);
435 if (AA) AA->deleteValue(NewPN);
436 BEBlock->getInstList().erase(NewPN);
440 // Now that all of the PHI nodes have been inserted and adjusted, modify the
441 // backedge blocks to just to the BEBlock instead of the header.
442 for (unsigned i = 0, e = BackedgeBlocks.size(); i != e; ++i) {
443 TerminatorInst *TI = BackedgeBlocks[i]->getTerminator();
444 for (unsigned Op = 0, e = TI->getNumSuccessors(); Op != e; ++Op)
445 if (TI->getSuccessor(Op) == Header)
446 TI->setSuccessor(Op, BEBlock);
449 //===--- Update all analyses which we must preserve now -----------------===//
451 // Update Loop Information - we know that this block is now in the current
452 // loop and all parent loops.
453 L->addBasicBlockToLoop(BEBlock, LI->getBase());
455 // Update dominator information
456 DT->splitBlock(BEBlock);
461 /// \brief Simplify one loop and queue further loops for simplification.
463 /// FIXME: Currently this accepts both lots of analyses that it uses and a raw
464 /// Pass pointer. The Pass pointer is used by numerous utilities to update
465 /// specific analyses. Rather than a pass it would be much cleaner and more
466 /// explicit if they accepted the analysis directly and then updated it.
467 static bool simplifyOneLoop(Loop *L, SmallVectorImpl<Loop *> &Worklist,
468 AliasAnalysis *AA, DominatorTree *DT, LoopInfo *LI,
469 ScalarEvolution *SE, Pass *PP,
470 const DataLayout *DL) {
471 bool Changed = false;
474 // Check to see that no blocks (other than the header) in this loop have
475 // predecessors that are not in the loop. This is not valid for natural
476 // loops, but can occur if the blocks are unreachable. Since they are
477 // unreachable we can just shamelessly delete those CFG edges!
478 for (Loop::block_iterator BB = L->block_begin(), E = L->block_end();
480 if (*BB == L->getHeader()) continue;
482 SmallPtrSet<BasicBlock*, 4> BadPreds;
483 for (BasicBlock *P : predecessors(*BB)) {
488 // Delete each unique out-of-loop (and thus dead) predecessor.
489 for (SmallPtrSet<BasicBlock*, 4>::iterator I = BadPreds.begin(),
490 E = BadPreds.end(); I != E; ++I) {
492 DEBUG(dbgs() << "LoopSimplify: Deleting edge from dead predecessor "
493 << (*I)->getName() << "\n");
495 // Inform each successor of each dead pred.
496 for (BasicBlock *Succ : successors(*I))
497 Succ->removePredecessor(*I);
498 // Zap the dead pred's terminator and replace it with unreachable.
499 TerminatorInst *TI = (*I)->getTerminator();
500 TI->replaceAllUsesWith(UndefValue::get(TI->getType()));
501 (*I)->getTerminator()->eraseFromParent();
502 new UnreachableInst((*I)->getContext(), *I);
507 // If there are exiting blocks with branches on undef, resolve the undef in
508 // the direction which will exit the loop. This will help simplify loop
509 // trip count computations.
510 SmallVector<BasicBlock*, 8> ExitingBlocks;
511 L->getExitingBlocks(ExitingBlocks);
512 for (SmallVectorImpl<BasicBlock *>::iterator I = ExitingBlocks.begin(),
513 E = ExitingBlocks.end(); I != E; ++I)
514 if (BranchInst *BI = dyn_cast<BranchInst>((*I)->getTerminator()))
515 if (BI->isConditional()) {
516 if (UndefValue *Cond = dyn_cast<UndefValue>(BI->getCondition())) {
518 DEBUG(dbgs() << "LoopSimplify: Resolving \"br i1 undef\" to exit in "
519 << (*I)->getName() << "\n");
521 BI->setCondition(ConstantInt::get(Cond->getType(),
522 !L->contains(BI->getSuccessor(0))));
524 // This may make the loop analyzable, force SCEV recomputation.
532 // Does the loop already have a preheader? If so, don't insert one.
533 BasicBlock *Preheader = L->getLoopPreheader();
535 Preheader = InsertPreheaderForLoop(L, PP);
542 // Next, check to make sure that all exit nodes of the loop only have
543 // predecessors that are inside of the loop. This check guarantees that the
544 // loop preheader/header will dominate the exit blocks. If the exit block has
545 // predecessors from outside of the loop, split the edge now.
546 SmallVector<BasicBlock*, 8> ExitBlocks;
547 L->getExitBlocks(ExitBlocks);
549 SmallSetVector<BasicBlock *, 8> ExitBlockSet(ExitBlocks.begin(),
551 for (SmallSetVector<BasicBlock *, 8>::iterator I = ExitBlockSet.begin(),
552 E = ExitBlockSet.end(); I != E; ++I) {
553 BasicBlock *ExitBlock = *I;
554 for (BasicBlock *Pred : predecessors(ExitBlock))
555 // Must be exactly this loop: no subloops, parent loops, or non-loop preds
557 if (!L->contains(Pred)) {
558 if (rewriteLoopExitBlock(L, ExitBlock, PP)) {
566 // If the header has more than two predecessors at this point (from the
567 // preheader and from multiple backedges), we must adjust the loop.
568 BasicBlock *LoopLatch = L->getLoopLatch();
570 // If this is really a nested loop, rip it out into a child loop. Don't do
571 // this for loops with a giant number of backedges, just factor them into a
572 // common backedge instead.
573 if (L->getNumBackEdges() < 8) {
574 if (Loop *OuterL = separateNestedLoop(L, Preheader, AA, DT, LI, SE, PP)) {
576 // Enqueue the outer loop as it should be processed next in our
577 // depth-first nest walk.
578 Worklist.push_back(OuterL);
580 // This is a big restructuring change, reprocess the whole loop.
582 // GCC doesn't tail recursion eliminate this.
583 // FIXME: It isn't clear we can't rely on LLVM to TRE this.
588 // If we either couldn't, or didn't want to, identify nesting of the loops,
589 // insert a new block that all backedges target, then make it jump to the
591 LoopLatch = insertUniqueBackedgeBlock(L, Preheader, AA, DT, LI);
598 // Scan over the PHI nodes in the loop header. Since they now have only two
599 // incoming values (the loop is canonicalized), we may have simplified the PHI
600 // down to 'X = phi [X, Y]', which should be replaced with 'Y'.
602 for (BasicBlock::iterator I = L->getHeader()->begin();
603 (PN = dyn_cast<PHINode>(I++)); )
604 if (Value *V = SimplifyInstruction(PN, nullptr, nullptr, DT)) {
605 if (AA) AA->deleteValue(PN);
606 if (SE) SE->forgetValue(PN);
607 PN->replaceAllUsesWith(V);
608 PN->eraseFromParent();
611 // If this loop has multiple exits and the exits all go to the same
612 // block, attempt to merge the exits. This helps several passes, such
613 // as LoopRotation, which do not support loops with multiple exits.
614 // SimplifyCFG also does this (and this code uses the same utility
615 // function), however this code is loop-aware, where SimplifyCFG is
616 // not. That gives it the advantage of being able to hoist
617 // loop-invariant instructions out of the way to open up more
618 // opportunities, and the disadvantage of having the responsibility
619 // to preserve dominator information.
620 bool UniqueExit = true;
621 if (!ExitBlocks.empty())
622 for (unsigned i = 1, e = ExitBlocks.size(); i != e; ++i)
623 if (ExitBlocks[i] != ExitBlocks[0]) {
628 for (unsigned i = 0, e = ExitingBlocks.size(); i != e; ++i) {
629 BasicBlock *ExitingBlock = ExitingBlocks[i];
630 if (!ExitingBlock->getSinglePredecessor()) continue;
631 BranchInst *BI = dyn_cast<BranchInst>(ExitingBlock->getTerminator());
632 if (!BI || !BI->isConditional()) continue;
633 CmpInst *CI = dyn_cast<CmpInst>(BI->getCondition());
634 if (!CI || CI->getParent() != ExitingBlock) continue;
636 // Attempt to hoist out all instructions except for the
637 // comparison and the branch.
638 bool AllInvariant = true;
639 bool AnyInvariant = false;
640 for (BasicBlock::iterator I = ExitingBlock->begin(); &*I != BI; ) {
641 Instruction *Inst = I++;
642 // Skip debug info intrinsics.
643 if (isa<DbgInfoIntrinsic>(Inst))
647 if (!L->makeLoopInvariant(Inst, AnyInvariant,
648 Preheader ? Preheader->getTerminator()
650 AllInvariant = false;
656 // The loop disposition of all SCEV expressions that depend on any
657 // hoisted values have also changed.
659 SE->forgetLoopDispositions(L);
661 if (!AllInvariant) continue;
663 // The block has now been cleared of all instructions except for
664 // a comparison and a conditional branch. SimplifyCFG may be able
666 if (!FoldBranchToCommonDest(BI, DL)) continue;
668 // Success. The block is now dead, so remove it from the loop,
669 // update the dominator tree and delete it.
670 DEBUG(dbgs() << "LoopSimplify: Eliminating exiting block "
671 << ExitingBlock->getName() << "\n");
673 // Notify ScalarEvolution before deleting this block. Currently assume the
674 // parent loop doesn't change (spliting edges doesn't count). If blocks,
675 // CFG edges, or other values in the parent loop change, then we need call
676 // to forgetLoop() for the parent instead.
680 assert(pred_begin(ExitingBlock) == pred_end(ExitingBlock));
682 LI->removeBlock(ExitingBlock);
684 DomTreeNode *Node = DT->getNode(ExitingBlock);
685 const std::vector<DomTreeNodeBase<BasicBlock> *> &Children =
687 while (!Children.empty()) {
688 DomTreeNode *Child = Children.front();
689 DT->changeImmediateDominator(Child, Node->getIDom());
691 DT->eraseNode(ExitingBlock);
693 BI->getSuccessor(0)->removePredecessor(ExitingBlock);
694 BI->getSuccessor(1)->removePredecessor(ExitingBlock);
695 ExitingBlock->eraseFromParent();
702 bool llvm::simplifyLoop(Loop *L, DominatorTree *DT, LoopInfo *LI, Pass *PP,
703 AliasAnalysis *AA, ScalarEvolution *SE,
704 const DataLayout *DL) {
705 bool Changed = false;
707 // Worklist maintains our depth-first queue of loops in this nest to process.
708 SmallVector<Loop *, 4> Worklist;
709 Worklist.push_back(L);
711 // Walk the worklist from front to back, pushing newly found sub loops onto
712 // the back. This will let us process loops from back to front in depth-first
713 // order. We can use this simple process because loops form a tree.
714 for (unsigned Idx = 0; Idx != Worklist.size(); ++Idx) {
715 Loop *L2 = Worklist[Idx];
716 for (Loop::iterator I = L2->begin(), E = L2->end(); I != E; ++I)
717 Worklist.push_back(*I);
720 while (!Worklist.empty())
721 Changed |= simplifyOneLoop(Worklist.pop_back_val(), Worklist, AA, DT, LI,
728 struct LoopSimplify : public FunctionPass {
729 static char ID; // Pass identification, replacement for typeid
730 LoopSimplify() : FunctionPass(ID) {
731 initializeLoopSimplifyPass(*PassRegistry::getPassRegistry());
734 // AA - If we have an alias analysis object to update, this is it, otherwise
740 const DataLayout *DL;
742 bool runOnFunction(Function &F) override;
744 void getAnalysisUsage(AnalysisUsage &AU) const override {
745 // We need loop information to identify the loops...
746 AU.addRequired<DominatorTreeWrapperPass>();
747 AU.addPreserved<DominatorTreeWrapperPass>();
749 AU.addRequired<LoopInfo>();
750 AU.addPreserved<LoopInfo>();
752 AU.addPreserved<AliasAnalysis>();
753 AU.addPreserved<ScalarEvolution>();
754 AU.addPreserved<DependenceAnalysis>();
755 AU.addPreservedID(BreakCriticalEdgesID); // No critical edges added.
758 /// verifyAnalysis() - Verify LoopSimplifyForm's guarantees.
759 void verifyAnalysis() const override;
763 char LoopSimplify::ID = 0;
764 INITIALIZE_PASS_BEGIN(LoopSimplify, "loop-simplify",
765 "Canonicalize natural loops", true, false)
766 INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
767 INITIALIZE_PASS_DEPENDENCY(LoopInfo)
768 INITIALIZE_PASS_END(LoopSimplify, "loop-simplify",
769 "Canonicalize natural loops", true, false)
771 // Publicly exposed interface to pass...
772 char &llvm::LoopSimplifyID = LoopSimplify::ID;
773 Pass *llvm::createLoopSimplifyPass() { return new LoopSimplify(); }
775 /// runOnFunction - Run down all loops in the CFG (recursively, but we could do
776 /// it in any convenient order) inserting preheaders...
778 bool LoopSimplify::runOnFunction(Function &F) {
779 bool Changed = false;
780 AA = getAnalysisIfAvailable<AliasAnalysis>();
781 LI = &getAnalysis<LoopInfo>();
782 DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
783 SE = getAnalysisIfAvailable<ScalarEvolution>();
784 DataLayoutPass *DLP = getAnalysisIfAvailable<DataLayoutPass>();
785 DL = DLP ? &DLP->getDataLayout() : nullptr;
787 // Simplify each loop nest in the function.
788 for (LoopInfo::iterator I = LI->begin(), E = LI->end(); I != E; ++I)
789 Changed |= simplifyLoop(*I, DT, LI, this, AA, SE, DL);
794 // FIXME: Restore this code when we re-enable verification in verifyAnalysis
797 static void verifyLoop(Loop *L) {
799 for (Loop::iterator I = L->begin(), E = L->end(); I != E; ++I)
802 // It used to be possible to just assert L->isLoopSimplifyForm(), however
803 // with the introduction of indirectbr, there are now cases where it's
804 // not possible to transform a loop as necessary. We can at least check
805 // that there is an indirectbr near any time there's trouble.
807 // Indirectbr can interfere with preheader and unique backedge insertion.
808 if (!L->getLoopPreheader() || !L->getLoopLatch()) {
809 bool HasIndBrPred = false;
810 for (pred_iterator PI = pred_begin(L->getHeader()),
811 PE = pred_end(L->getHeader()); PI != PE; ++PI)
812 if (isa<IndirectBrInst>((*PI)->getTerminator())) {
816 assert(HasIndBrPred &&
817 "LoopSimplify has no excuse for missing loop header info!");
821 // Indirectbr can interfere with exit block canonicalization.
822 if (!L->hasDedicatedExits()) {
823 bool HasIndBrExiting = false;
824 SmallVector<BasicBlock*, 8> ExitingBlocks;
825 L->getExitingBlocks(ExitingBlocks);
826 for (unsigned i = 0, e = ExitingBlocks.size(); i != e; ++i) {
827 if (isa<IndirectBrInst>((ExitingBlocks[i])->getTerminator())) {
828 HasIndBrExiting = true;
833 assert(HasIndBrExiting &&
834 "LoopSimplify has no excuse for missing exit block info!");
835 (void)HasIndBrExiting;
840 void LoopSimplify::verifyAnalysis() const {
841 // FIXME: This routine is being called mid-way through the loop pass manager
842 // as loop passes destroy this analysis. That's actually fine, but we have no
843 // way of expressing that here. Once all of the passes that destroy this are
844 // hoisted out of the loop pass manager we can add back verification here.
846 for (LoopInfo::iterator I = LI->begin(), E = LI->end(); I != E; ++I)