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/AssumptionCache.h"
48 #include "llvm/Analysis/DependenceAnalysis.h"
49 #include "llvm/Analysis/InstructionSimplify.h"
50 #include "llvm/Analysis/LoopInfo.h"
51 #include "llvm/Analysis/ScalarEvolution.h"
52 #include "llvm/IR/CFG.h"
53 #include "llvm/IR/Constants.h"
54 #include "llvm/IR/DataLayout.h"
55 #include "llvm/IR/Dominators.h"
56 #include "llvm/IR/Function.h"
57 #include "llvm/IR/Instructions.h"
58 #include "llvm/IR/IntrinsicInst.h"
59 #include "llvm/IR/LLVMContext.h"
60 #include "llvm/IR/Module.h"
61 #include "llvm/IR/Type.h"
62 #include "llvm/Support/Debug.h"
63 #include "llvm/Support/raw_ostream.h"
64 #include "llvm/Transforms/Utils/BasicBlockUtils.h"
65 #include "llvm/Transforms/Utils/Local.h"
66 #include "llvm/Transforms/Utils/LoopUtils.h"
69 #define DEBUG_TYPE "loop-simplify"
71 STATISTIC(NumInserted, "Number of pre-header or exit blocks inserted");
72 STATISTIC(NumNested , "Number of nested loops split out");
74 // If the block isn't already, move the new block to right after some 'outside
75 // block' block. This prevents the preheader from being placed inside the loop
76 // body, e.g. when the loop hasn't been rotated.
77 static void placeSplitBlockCarefully(BasicBlock *NewBB,
78 SmallVectorImpl<BasicBlock *> &SplitPreds,
80 // Check to see if NewBB is already well placed.
81 Function::iterator BBI = NewBB; --BBI;
82 for (unsigned i = 0, e = SplitPreds.size(); i != e; ++i) {
83 if (&*BBI == SplitPreds[i])
87 // If it isn't already after an outside block, move it after one. This is
88 // always good as it makes the uncond branch from the outside block into a
91 // Figure out *which* outside block to put this after. Prefer an outside
92 // block that neighbors a BB actually in the loop.
93 BasicBlock *FoundBB = nullptr;
94 for (unsigned i = 0, e = SplitPreds.size(); i != e; ++i) {
95 Function::iterator BBI = SplitPreds[i];
96 if (++BBI != NewBB->getParent()->end() &&
98 FoundBB = SplitPreds[i];
103 // If our heuristic for a *good* bb to place this after doesn't find
104 // anything, just pick something. It's likely better than leaving it within
107 FoundBB = SplitPreds[0];
108 NewBB->moveAfter(FoundBB);
111 /// InsertPreheaderForLoop - Once we discover that a loop doesn't have a
112 /// preheader, this method is called to insert one. This method has two phases:
113 /// preheader insertion and analysis updating.
115 BasicBlock *llvm::InsertPreheaderForLoop(Loop *L, Pass *PP) {
116 BasicBlock *Header = L->getHeader();
118 // Get analyses that we try to update.
119 auto *DTWP = PP->getAnalysisIfAvailable<DominatorTreeWrapperPass>();
120 auto *DT = DTWP ? &DTWP->getDomTree() : nullptr;
121 auto *LIWP = PP->getAnalysisIfAvailable<LoopInfoWrapperPass>();
122 auto *LI = LIWP ? &LIWP->getLoopInfo() : nullptr;
123 bool PreserveLCSSA = PP->mustPreserveAnalysisID(LCSSAID);
125 // Compute the set of predecessors of the loop that are not in the loop.
126 SmallVector<BasicBlock*, 8> OutsideBlocks;
127 for (pred_iterator PI = pred_begin(Header), PE = pred_end(Header);
130 if (!L->contains(P)) { // Coming in from outside the loop?
131 // If the loop is branched to from an indirect branch, we won't
132 // be able to fully transform the loop, because it prohibits
134 if (isa<IndirectBrInst>(P->getTerminator())) return nullptr;
137 OutsideBlocks.push_back(P);
141 // Split out the loop pre-header.
142 BasicBlock *PreheaderBB;
143 PreheaderBB = SplitBlockPredecessors(Header, OutsideBlocks, ".preheader", DT,
148 DEBUG(dbgs() << "LoopSimplify: Creating pre-header "
149 << PreheaderBB->getName() << "\n");
151 // Make sure that NewBB is put someplace intelligent, which doesn't mess up
152 // code layout too horribly.
153 placeSplitBlockCarefully(PreheaderBB, OutsideBlocks, L);
158 /// \brief Ensure that the loop preheader dominates all exit blocks.
160 /// This method is used to split exit blocks that have predecessors outside of
162 static BasicBlock *rewriteLoopExitBlock(Loop *L, BasicBlock *Exit,
163 DominatorTree *DT, LoopInfo *LI,
165 SmallVector<BasicBlock*, 8> LoopBlocks;
166 for (pred_iterator I = pred_begin(Exit), E = pred_end(Exit); I != E; ++I) {
168 if (L->contains(P)) {
169 // Don't do this if the loop is exited via an indirect branch.
170 if (isa<IndirectBrInst>(P->getTerminator())) return nullptr;
172 LoopBlocks.push_back(P);
176 assert(!LoopBlocks.empty() && "No edges coming in from outside the loop?");
177 BasicBlock *NewExitBB = nullptr;
179 bool PreserveLCSSA = PP->mustPreserveAnalysisID(LCSSAID);
181 NewExitBB = SplitBlockPredecessors(Exit, LoopBlocks, ".loopexit", DT, LI,
186 DEBUG(dbgs() << "LoopSimplify: Creating dedicated exit block "
187 << NewExitBB->getName() << "\n");
191 /// Add the specified block, and all of its predecessors, to the specified set,
192 /// if it's not already in there. Stop predecessor traversal when we reach
194 static void addBlockAndPredsToSet(BasicBlock *InputBB, BasicBlock *StopBlock,
195 std::set<BasicBlock*> &Blocks) {
196 SmallVector<BasicBlock *, 8> Worklist;
197 Worklist.push_back(InputBB);
199 BasicBlock *BB = Worklist.pop_back_val();
200 if (Blocks.insert(BB).second && BB != StopBlock)
201 // If BB is not already processed and it is not a stop block then
202 // insert its predecessor in the work list
203 for (pred_iterator I = pred_begin(BB), E = pred_end(BB); I != E; ++I) {
204 BasicBlock *WBB = *I;
205 Worklist.push_back(WBB);
207 } while (!Worklist.empty());
210 /// \brief The first part of loop-nestification is to find a PHI node that tells
211 /// us how to partition the loops.
212 static PHINode *findPHIToPartitionLoops(Loop *L, DominatorTree *DT,
213 AssumptionCache *AC) {
214 const DataLayout &DL = L->getHeader()->getModule()->getDataLayout();
215 for (BasicBlock::iterator I = L->getHeader()->begin(); isa<PHINode>(I); ) {
216 PHINode *PN = cast<PHINode>(I);
218 if (Value *V = SimplifyInstruction(PN, DL, nullptr, DT, AC)) {
219 // This is a degenerate PHI already, don't modify it!
220 PN->replaceAllUsesWith(V);
221 PN->eraseFromParent();
225 // Scan this PHI node looking for a use of the PHI node by itself.
226 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
227 if (PN->getIncomingValue(i) == PN &&
228 L->contains(PN->getIncomingBlock(i)))
229 // We found something tasty to remove.
235 /// \brief If this loop has multiple backedges, try to pull one of them out into
238 /// This is important for code that looks like
243 /// br cond, Loop, Next
245 /// br cond2, Loop, Out
247 /// To identify this common case, we look at the PHI nodes in the header of the
248 /// loop. PHI nodes with unchanging values on one backedge correspond to values
249 /// that change in the "outer" loop, but not in the "inner" loop.
251 /// If we are able to separate out a loop, return the new outer loop that was
254 static Loop *separateNestedLoop(Loop *L, BasicBlock *Preheader,
255 DominatorTree *DT, LoopInfo *LI,
256 ScalarEvolution *SE, Pass *PP,
257 AssumptionCache *AC) {
258 // Don't try to separate loops without a preheader.
262 // The header is not a landing pad; preheader insertion should ensure this.
263 BasicBlock *Header = L->getHeader();
264 assert(!Header->isEHPad() && "Can't insert backedge to EH pad");
266 PHINode *PN = findPHIToPartitionLoops(L, DT, AC);
267 if (!PN) return nullptr; // No known way to partition.
269 // Pull out all predecessors that have varying values in the loop. This
270 // handles the case when a PHI node has multiple instances of itself as
272 SmallVector<BasicBlock*, 8> OuterLoopPreds;
273 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
274 if (PN->getIncomingValue(i) != PN ||
275 !L->contains(PN->getIncomingBlock(i))) {
276 // We can't split indirectbr edges.
277 if (isa<IndirectBrInst>(PN->getIncomingBlock(i)->getTerminator()))
279 OuterLoopPreds.push_back(PN->getIncomingBlock(i));
282 DEBUG(dbgs() << "LoopSimplify: Splitting out a new outer loop\n");
284 // If ScalarEvolution is around and knows anything about values in
285 // this loop, tell it to forget them, because we're about to
286 // substantially change it.
290 bool PreserveLCSSA = PP->mustPreserveAnalysisID(LCSSAID);
292 BasicBlock *NewBB = SplitBlockPredecessors(Header, OuterLoopPreds, ".outer",
293 DT, LI, PreserveLCSSA);
295 // Make sure that NewBB is put someplace intelligent, which doesn't mess up
296 // code layout too horribly.
297 placeSplitBlockCarefully(NewBB, OuterLoopPreds, L);
299 // Create the new outer loop.
300 Loop *NewOuter = new Loop();
302 // Change the parent loop to use the outer loop as its child now.
303 if (Loop *Parent = L->getParentLoop())
304 Parent->replaceChildLoopWith(L, NewOuter);
306 LI->changeTopLevelLoop(L, NewOuter);
308 // L is now a subloop of our outer loop.
309 NewOuter->addChildLoop(L);
311 for (Loop::block_iterator I = L->block_begin(), E = L->block_end();
313 NewOuter->addBlockEntry(*I);
315 // Now reset the header in L, which had been moved by
316 // SplitBlockPredecessors for the outer loop.
317 L->moveToHeader(Header);
319 // Determine which blocks should stay in L and which should be moved out to
320 // the Outer loop now.
321 std::set<BasicBlock*> BlocksInL;
322 for (pred_iterator PI=pred_begin(Header), E = pred_end(Header); PI!=E; ++PI) {
324 if (DT->dominates(Header, P))
325 addBlockAndPredsToSet(P, Header, BlocksInL);
328 // Scan all of the loop children of L, moving them to OuterLoop if they are
329 // not part of the inner loop.
330 const std::vector<Loop*> &SubLoops = L->getSubLoops();
331 for (size_t I = 0; I != SubLoops.size(); )
332 if (BlocksInL.count(SubLoops[I]->getHeader()))
333 ++I; // Loop remains in L
335 NewOuter->addChildLoop(L->removeChildLoop(SubLoops.begin() + I));
337 // Now that we know which blocks are in L and which need to be moved to
338 // OuterLoop, move any blocks that need it.
339 for (unsigned i = 0; i != L->getBlocks().size(); ++i) {
340 BasicBlock *BB = L->getBlocks()[i];
341 if (!BlocksInL.count(BB)) {
342 // Move this block to the parent, updating the exit blocks sets
343 L->removeBlockFromLoop(BB);
345 LI->changeLoopFor(BB, NewOuter);
353 /// \brief This method is called when the specified loop has more than one
356 /// If this occurs, revector all of these backedges to target a new basic block
357 /// and have that block branch to the loop header. This ensures that loops
358 /// have exactly one backedge.
359 static BasicBlock *insertUniqueBackedgeBlock(Loop *L, BasicBlock *Preheader,
360 DominatorTree *DT, LoopInfo *LI) {
361 assert(L->getNumBackEdges() > 1 && "Must have > 1 backedge!");
363 // Get information about the loop
364 BasicBlock *Header = L->getHeader();
365 Function *F = Header->getParent();
367 // Unique backedge insertion currently depends on having a preheader.
371 // The header is not an EH pad; preheader insertion should ensure this.
372 assert(!Header->isEHPad() && "Can't insert backedge to EH pad");
374 // Figure out which basic blocks contain back-edges to the loop header.
375 std::vector<BasicBlock*> BackedgeBlocks;
376 for (pred_iterator I = pred_begin(Header), E = pred_end(Header); I != E; ++I){
379 // Indirectbr edges cannot be split, so we must fail if we find one.
380 if (isa<IndirectBrInst>(P->getTerminator()))
383 if (P != Preheader) BackedgeBlocks.push_back(P);
386 // Create and insert the new backedge block...
387 BasicBlock *BEBlock = BasicBlock::Create(Header->getContext(),
388 Header->getName() + ".backedge", F);
389 BranchInst *BETerminator = BranchInst::Create(Header, BEBlock);
390 BETerminator->setDebugLoc(Header->getFirstNonPHI()->getDebugLoc());
392 DEBUG(dbgs() << "LoopSimplify: Inserting unique backedge block "
393 << BEBlock->getName() << "\n");
395 // Move the new backedge block to right after the last backedge block.
396 Function::iterator InsertPos = BackedgeBlocks.back(); ++InsertPos;
397 F->getBasicBlockList().splice(InsertPos, F->getBasicBlockList(), BEBlock);
399 // Now that the block has been inserted into the function, create PHI nodes in
400 // the backedge block which correspond to any PHI nodes in the header block.
401 for (BasicBlock::iterator I = Header->begin(); isa<PHINode>(I); ++I) {
402 PHINode *PN = cast<PHINode>(I);
403 PHINode *NewPN = PHINode::Create(PN->getType(), BackedgeBlocks.size(),
404 PN->getName()+".be", BETerminator);
406 // Loop over the PHI node, moving all entries except the one for the
407 // preheader over to the new PHI node.
408 unsigned PreheaderIdx = ~0U;
409 bool HasUniqueIncomingValue = true;
410 Value *UniqueValue = nullptr;
411 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
412 BasicBlock *IBB = PN->getIncomingBlock(i);
413 Value *IV = PN->getIncomingValue(i);
414 if (IBB == Preheader) {
417 NewPN->addIncoming(IV, IBB);
418 if (HasUniqueIncomingValue) {
421 else if (UniqueValue != IV)
422 HasUniqueIncomingValue = false;
427 // Delete all of the incoming values from the old PN except the preheader's
428 assert(PreheaderIdx != ~0U && "PHI has no preheader entry??");
429 if (PreheaderIdx != 0) {
430 PN->setIncomingValue(0, PN->getIncomingValue(PreheaderIdx));
431 PN->setIncomingBlock(0, PN->getIncomingBlock(PreheaderIdx));
433 // Nuke all entries except the zero'th.
434 for (unsigned i = 0, e = PN->getNumIncomingValues()-1; i != e; ++i)
435 PN->removeIncomingValue(e-i, false);
437 // Finally, add the newly constructed PHI node as the entry for the BEBlock.
438 PN->addIncoming(NewPN, BEBlock);
440 // As an optimization, if all incoming values in the new PhiNode (which is a
441 // subset of the incoming values of the old PHI node) have the same value,
442 // eliminate the PHI Node.
443 if (HasUniqueIncomingValue) {
444 NewPN->replaceAllUsesWith(UniqueValue);
445 BEBlock->getInstList().erase(NewPN);
449 // Now that all of the PHI nodes have been inserted and adjusted, modify the
450 // backedge blocks to just to the BEBlock instead of the header.
451 for (unsigned i = 0, e = BackedgeBlocks.size(); i != e; ++i) {
452 TerminatorInst *TI = BackedgeBlocks[i]->getTerminator();
453 for (unsigned Op = 0, e = TI->getNumSuccessors(); Op != e; ++Op)
454 if (TI->getSuccessor(Op) == Header)
455 TI->setSuccessor(Op, BEBlock);
458 //===--- Update all analyses which we must preserve now -----------------===//
460 // Update Loop Information - we know that this block is now in the current
461 // loop and all parent loops.
462 L->addBasicBlockToLoop(BEBlock, *LI);
464 // Update dominator information
465 DT->splitBlock(BEBlock);
470 /// \brief Simplify one loop and queue further loops for simplification.
472 /// FIXME: Currently this accepts both lots of analyses that it uses and a raw
473 /// Pass pointer. The Pass pointer is used by numerous utilities to update
474 /// specific analyses. Rather than a pass it would be much cleaner and more
475 /// explicit if they accepted the analysis directly and then updated it.
476 static bool simplifyOneLoop(Loop *L, SmallVectorImpl<Loop *> &Worklist,
477 DominatorTree *DT, LoopInfo *LI,
478 ScalarEvolution *SE, Pass *PP,
479 AssumptionCache *AC) {
480 bool Changed = false;
483 // Check to see that no blocks (other than the header) in this loop have
484 // predecessors that are not in the loop. This is not valid for natural
485 // loops, but can occur if the blocks are unreachable. Since they are
486 // unreachable we can just shamelessly delete those CFG edges!
487 for (Loop::block_iterator BB = L->block_begin(), E = L->block_end();
489 if (*BB == L->getHeader()) continue;
491 SmallPtrSet<BasicBlock*, 4> BadPreds;
492 for (pred_iterator PI = pred_begin(*BB),
493 PE = pred_end(*BB); PI != PE; ++PI) {
499 // Delete each unique out-of-loop (and thus dead) predecessor.
500 for (BasicBlock *P : BadPreds) {
502 DEBUG(dbgs() << "LoopSimplify: Deleting edge from dead predecessor "
503 << P->getName() << "\n");
505 // Inform each successor of each dead pred.
506 for (succ_iterator SI = succ_begin(P), SE = succ_end(P); SI != SE; ++SI)
507 (*SI)->removePredecessor(P);
508 // Zap the dead pred's terminator and replace it with unreachable.
509 TerminatorInst *TI = P->getTerminator();
510 TI->replaceAllUsesWith(UndefValue::get(TI->getType()));
511 P->getTerminator()->eraseFromParent();
512 new UnreachableInst(P->getContext(), P);
517 // If there are exiting blocks with branches on undef, resolve the undef in
518 // the direction which will exit the loop. This will help simplify loop
519 // trip count computations.
520 SmallVector<BasicBlock*, 8> ExitingBlocks;
521 L->getExitingBlocks(ExitingBlocks);
522 for (SmallVectorImpl<BasicBlock *>::iterator I = ExitingBlocks.begin(),
523 E = ExitingBlocks.end(); I != E; ++I)
524 if (BranchInst *BI = dyn_cast<BranchInst>((*I)->getTerminator()))
525 if (BI->isConditional()) {
526 if (UndefValue *Cond = dyn_cast<UndefValue>(BI->getCondition())) {
528 DEBUG(dbgs() << "LoopSimplify: Resolving \"br i1 undef\" to exit in "
529 << (*I)->getName() << "\n");
531 BI->setCondition(ConstantInt::get(Cond->getType(),
532 !L->contains(BI->getSuccessor(0))));
534 // This may make the loop analyzable, force SCEV recomputation.
542 // Does the loop already have a preheader? If so, don't insert one.
543 BasicBlock *Preheader = L->getLoopPreheader();
545 Preheader = InsertPreheaderForLoop(L, PP);
552 // Next, check to make sure that all exit nodes of the loop only have
553 // predecessors that are inside of the loop. This check guarantees that the
554 // loop preheader/header will dominate the exit blocks. If the exit block has
555 // predecessors from outside of the loop, split the edge now.
556 SmallVector<BasicBlock*, 8> ExitBlocks;
557 L->getExitBlocks(ExitBlocks);
559 SmallSetVector<BasicBlock *, 8> ExitBlockSet(ExitBlocks.begin(),
561 for (SmallSetVector<BasicBlock *, 8>::iterator I = ExitBlockSet.begin(),
562 E = ExitBlockSet.end(); I != E; ++I) {
563 BasicBlock *ExitBlock = *I;
564 for (pred_iterator PI = pred_begin(ExitBlock), PE = pred_end(ExitBlock);
566 // Must be exactly this loop: no subloops, parent loops, or non-loop preds
568 if (!L->contains(*PI)) {
569 if (rewriteLoopExitBlock(L, ExitBlock, DT, LI, PP)) {
577 // If the header has more than two predecessors at this point (from the
578 // preheader and from multiple backedges), we must adjust the loop.
579 BasicBlock *LoopLatch = L->getLoopLatch();
581 // If this is really a nested loop, rip it out into a child loop. Don't do
582 // this for loops with a giant number of backedges, just factor them into a
583 // common backedge instead.
584 if (L->getNumBackEdges() < 8) {
585 if (Loop *OuterL = separateNestedLoop(L, Preheader, DT, LI, SE, PP, AC)) {
587 // Enqueue the outer loop as it should be processed next in our
588 // depth-first nest walk.
589 Worklist.push_back(OuterL);
591 // This is a big restructuring change, reprocess the whole loop.
593 // GCC doesn't tail recursion eliminate this.
594 // FIXME: It isn't clear we can't rely on LLVM to TRE this.
599 // If we either couldn't, or didn't want to, identify nesting of the loops,
600 // insert a new block that all backedges target, then make it jump to the
602 LoopLatch = insertUniqueBackedgeBlock(L, Preheader, DT, LI);
609 const DataLayout &DL = L->getHeader()->getModule()->getDataLayout();
611 // Scan over the PHI nodes in the loop header. Since they now have only two
612 // incoming values (the loop is canonicalized), we may have simplified the PHI
613 // down to 'X = phi [X, Y]', which should be replaced with 'Y'.
615 for (BasicBlock::iterator I = L->getHeader()->begin();
616 (PN = dyn_cast<PHINode>(I++)); )
617 if (Value *V = SimplifyInstruction(PN, DL, nullptr, DT, AC)) {
618 if (SE) SE->forgetValue(PN);
619 PN->replaceAllUsesWith(V);
620 PN->eraseFromParent();
623 // If this loop has multiple exits and the exits all go to the same
624 // block, attempt to merge the exits. This helps several passes, such
625 // as LoopRotation, which do not support loops with multiple exits.
626 // SimplifyCFG also does this (and this code uses the same utility
627 // function), however this code is loop-aware, where SimplifyCFG is
628 // not. That gives it the advantage of being able to hoist
629 // loop-invariant instructions out of the way to open up more
630 // opportunities, and the disadvantage of having the responsibility
631 // to preserve dominator information.
632 bool UniqueExit = true;
633 if (!ExitBlocks.empty())
634 for (unsigned i = 1, e = ExitBlocks.size(); i != e; ++i)
635 if (ExitBlocks[i] != ExitBlocks[0]) {
640 for (unsigned i = 0, e = ExitingBlocks.size(); i != e; ++i) {
641 BasicBlock *ExitingBlock = ExitingBlocks[i];
642 if (!ExitingBlock->getSinglePredecessor()) continue;
643 BranchInst *BI = dyn_cast<BranchInst>(ExitingBlock->getTerminator());
644 if (!BI || !BI->isConditional()) continue;
645 CmpInst *CI = dyn_cast<CmpInst>(BI->getCondition());
646 if (!CI || CI->getParent() != ExitingBlock) continue;
648 // Attempt to hoist out all instructions except for the
649 // comparison and the branch.
650 bool AllInvariant = true;
651 bool AnyInvariant = false;
652 for (BasicBlock::iterator I = ExitingBlock->begin(); &*I != BI; ) {
653 Instruction *Inst = I++;
654 // Skip debug info intrinsics.
655 if (isa<DbgInfoIntrinsic>(Inst))
659 if (!L->makeLoopInvariant(Inst, AnyInvariant,
660 Preheader ? Preheader->getTerminator()
662 AllInvariant = false;
668 // The loop disposition of all SCEV expressions that depend on any
669 // hoisted values have also changed.
671 SE->forgetLoopDispositions(L);
673 if (!AllInvariant) continue;
675 // The block has now been cleared of all instructions except for
676 // a comparison and a conditional branch. SimplifyCFG may be able
678 if (!FoldBranchToCommonDest(BI))
681 // Success. The block is now dead, so remove it from the loop,
682 // update the dominator tree and delete it.
683 DEBUG(dbgs() << "LoopSimplify: Eliminating exiting block "
684 << ExitingBlock->getName() << "\n");
686 // Notify ScalarEvolution before deleting this block. Currently assume the
687 // parent loop doesn't change (spliting edges doesn't count). If blocks,
688 // CFG edges, or other values in the parent loop change, then we need call
689 // to forgetLoop() for the parent instead.
693 assert(pred_begin(ExitingBlock) == pred_end(ExitingBlock));
695 LI->removeBlock(ExitingBlock);
697 DomTreeNode *Node = DT->getNode(ExitingBlock);
698 const std::vector<DomTreeNodeBase<BasicBlock> *> &Children =
700 while (!Children.empty()) {
701 DomTreeNode *Child = Children.front();
702 DT->changeImmediateDominator(Child, Node->getIDom());
704 DT->eraseNode(ExitingBlock);
706 BI->getSuccessor(0)->removePredecessor(ExitingBlock);
707 BI->getSuccessor(1)->removePredecessor(ExitingBlock);
708 ExitingBlock->eraseFromParent();
715 bool llvm::simplifyLoop(Loop *L, DominatorTree *DT, LoopInfo *LI, Pass *PP,
716 ScalarEvolution *SE, AssumptionCache *AC) {
717 bool Changed = false;
719 // Worklist maintains our depth-first queue of loops in this nest to process.
720 SmallVector<Loop *, 4> Worklist;
721 Worklist.push_back(L);
723 // Walk the worklist from front to back, pushing newly found sub loops onto
724 // the back. This will let us process loops from back to front in depth-first
725 // order. We can use this simple process because loops form a tree.
726 for (unsigned Idx = 0; Idx != Worklist.size(); ++Idx) {
727 Loop *L2 = Worklist[Idx];
728 Worklist.append(L2->begin(), L2->end());
731 while (!Worklist.empty())
733 simplifyOneLoop(Worklist.pop_back_val(), Worklist, DT, LI, SE, PP, AC);
739 struct LoopSimplify : public FunctionPass {
740 static char ID; // Pass identification, replacement for typeid
741 LoopSimplify() : FunctionPass(ID) {
742 initializeLoopSimplifyPass(*PassRegistry::getPassRegistry());
750 bool runOnFunction(Function &F) override;
752 void getAnalysisUsage(AnalysisUsage &AU) const override {
753 AU.addRequired<AssumptionCacheTracker>();
755 // We need loop information to identify the loops...
756 AU.addRequired<DominatorTreeWrapperPass>();
757 AU.addPreserved<DominatorTreeWrapperPass>();
759 AU.addRequired<LoopInfoWrapperPass>();
760 AU.addPreserved<LoopInfoWrapperPass>();
762 AU.addPreserved<AliasAnalysis>();
763 AU.addPreserved<ScalarEvolutionWrapperPass>();
764 AU.addPreserved<DependenceAnalysis>();
765 AU.addPreservedID(BreakCriticalEdgesID); // No critical edges added.
768 /// verifyAnalysis() - Verify LoopSimplifyForm's guarantees.
769 void verifyAnalysis() const override;
773 char LoopSimplify::ID = 0;
774 INITIALIZE_PASS_BEGIN(LoopSimplify, "loop-simplify",
775 "Canonicalize natural loops", false, false)
776 INITIALIZE_PASS_DEPENDENCY(AssumptionCacheTracker)
777 INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
778 INITIALIZE_PASS_DEPENDENCY(LoopInfoWrapperPass)
779 INITIALIZE_PASS_END(LoopSimplify, "loop-simplify",
780 "Canonicalize natural loops", false, false)
782 // Publicly exposed interface to pass...
783 char &llvm::LoopSimplifyID = LoopSimplify::ID;
784 Pass *llvm::createLoopSimplifyPass() { return new LoopSimplify(); }
786 /// runOnFunction - Run down all loops in the CFG (recursively, but we could do
787 /// it in any convenient order) inserting preheaders...
789 bool LoopSimplify::runOnFunction(Function &F) {
790 bool Changed = false;
791 LI = &getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
792 DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
793 auto *SEWP = getAnalysisIfAvailable<ScalarEvolutionWrapperPass>();
794 SE = SEWP ? &SEWP->getSE() : nullptr;
795 AC = &getAnalysis<AssumptionCacheTracker>().getAssumptionCache(F);
797 // Simplify each loop nest in the function.
798 for (LoopInfo::iterator I = LI->begin(), E = LI->end(); I != E; ++I)
799 Changed |= simplifyLoop(*I, DT, LI, this, SE, AC);
804 // FIXME: Restore this code when we re-enable verification in verifyAnalysis
807 static void verifyLoop(Loop *L) {
809 for (Loop::iterator I = L->begin(), E = L->end(); I != E; ++I)
812 // It used to be possible to just assert L->isLoopSimplifyForm(), however
813 // with the introduction of indirectbr, there are now cases where it's
814 // not possible to transform a loop as necessary. We can at least check
815 // that there is an indirectbr near any time there's trouble.
817 // Indirectbr can interfere with preheader and unique backedge insertion.
818 if (!L->getLoopPreheader() || !L->getLoopLatch()) {
819 bool HasIndBrPred = false;
820 for (pred_iterator PI = pred_begin(L->getHeader()),
821 PE = pred_end(L->getHeader()); PI != PE; ++PI)
822 if (isa<IndirectBrInst>((*PI)->getTerminator())) {
826 assert(HasIndBrPred &&
827 "LoopSimplify has no excuse for missing loop header info!");
831 // Indirectbr can interfere with exit block canonicalization.
832 if (!L->hasDedicatedExits()) {
833 bool HasIndBrExiting = false;
834 SmallVector<BasicBlock*, 8> ExitingBlocks;
835 L->getExitingBlocks(ExitingBlocks);
836 for (unsigned i = 0, e = ExitingBlocks.size(); i != e; ++i) {
837 if (isa<IndirectBrInst>((ExitingBlocks[i])->getTerminator())) {
838 HasIndBrExiting = true;
843 assert(HasIndBrExiting &&
844 "LoopSimplify has no excuse for missing exit block info!");
845 (void)HasIndBrExiting;
850 void LoopSimplify::verifyAnalysis() const {
851 // FIXME: This routine is being called mid-way through the loop pass manager
852 // as loop passes destroy this analysis. That's actually fine, but we have no
853 // way of expressing that here. Once all of the passes that destroy this are
854 // hoisted out of the loop pass manager we can add back verification here.
856 for (LoopInfo::iterator I = LI->begin(), E = LI->end(); I != E; ++I)