1//===- LoopSimplify.cpp - Loop Canonicalization Pass ----------------------===//
2//
3// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4// See https://llvm.org/LICENSE.txt for license information.
5// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6//
7//===----------------------------------------------------------------------===//
8//
9// This pass performs several transformations to transform natural loops into a
10// simpler form, which makes subsequent analyses and transformations simpler and
11// more effective.
12//
13// Loop pre-header insertion guarantees that there is a single, non-critical
14// entry edge from outside of the loop to the loop header. This simplifies a
15// number of analyses and transformations, such as LICM.
16//
17// Loop exit-block insertion guarantees that all exit blocks from the loop
18// (blocks which are outside of the loop that have predecessors inside of the
19// loop) only have predecessors from inside of the loop (and are thus dominated
20// by the loop header). This simplifies transformations such as store-sinking
21// that are built into LICM.
22//
23// This pass also guarantees that loops will have exactly one backedge.
24//
25// Indirectbr instructions introduce several complications. If the loop
26// contains or is entered by an indirectbr instruction, it may not be possible
27// to transform the loop and make these guarantees. Client code should check
28// that these conditions are true before relying on them.
29//
30// Similar complications arise from callbr instructions, particularly in
31// asm-goto where blockaddress expressions are used.
32//
33// Note that the simplifycfg pass will clean up blocks which are split out but
34// end up being unnecessary, so usage of this pass should not pessimize
35// generated code.
36//
37// This pass obviously modifies the CFG, but updates loop information and
38// dominator information.
39//
40//===----------------------------------------------------------------------===//
41
42#include "llvm/Transforms/Utils/LoopSimplify.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/BasicAliasAnalysis.h"
49#include "llvm/Analysis/BranchProbabilityInfo.h"
50#include "llvm/Analysis/DependenceAnalysis.h"
51#include "llvm/Analysis/GlobalsModRef.h"
52#include "llvm/Analysis/InstructionSimplify.h"
53#include "llvm/Analysis/LoopInfo.h"
54#include "llvm/Analysis/MemorySSA.h"
55#include "llvm/Analysis/MemorySSAUpdater.h"
56#include "llvm/Analysis/ScalarEvolution.h"
57#include "llvm/Analysis/ScalarEvolutionAliasAnalysis.h"
58#include "llvm/IR/CFG.h"
59#include "llvm/IR/Constants.h"
60#include "llvm/IR/Dominators.h"
61#include "llvm/IR/Function.h"
62#include "llvm/IR/Instructions.h"
63#include "llvm/IR/LLVMContext.h"
64#include "llvm/IR/Module.h"
65#include "llvm/InitializePasses.h"
66#include "llvm/Support/Debug.h"
67#include "llvm/Support/raw_ostream.h"
68#include "llvm/Transforms/Utils.h"
69#include "llvm/Transforms/Utils/BasicBlockUtils.h"
70#include "llvm/Transforms/Utils/Local.h"
71#include "llvm/Transforms/Utils/LoopUtils.h"
72using namespace llvm;
73
74#define DEBUG_TYPE "loop-simplify"
75
76STATISTIC(NumNested , "Number of nested loops split out");
77
78// If the block isn't already, move the new block to right after some 'outside
79// block' block. This prevents the preheader from being placed inside the loop
80// body, e.g. when the loop hasn't been rotated.
81static void placeSplitBlockCarefully(BasicBlock *NewBB,
82 SmallVectorImpl<BasicBlock *> &SplitPreds,
83 Loop *L) {
84 // Check to see if NewBB is already well placed.
85 Function::iterator BBI = --NewBB->getIterator();
86 for (unsigned i = 0, e = SplitPreds.size(); i != e; ++i) {
87 if (&*BBI == SplitPreds[i])
88 return;
89 }
90
91 // If it isn't already after an outside block, move it after one. This is
92 // always good as it makes the uncond branch from the outside block into a
93 // fall-through.
94
95 // Figure out *which* outside block to put this after. Prefer an outside
96 // block that neighbors a BB actually in the loop.
97 BasicBlock *FoundBB = nullptr;
98 for (unsigned i = 0, e = SplitPreds.size(); i != e; ++i) {
99 Function::iterator BBI = SplitPreds[i]->getIterator();
100 if (++BBI != NewBB->getParent()->end() && L->contains(BB: &*BBI)) {
101 FoundBB = SplitPreds[i];
102 break;
103 }
104 }
105
106 // If our heuristic for a *good* bb to place this after doesn't find
107 // anything, just pick something. It's likely better than leaving it within
108 // the loop.
109 if (!FoundBB)
110 FoundBB = SplitPreds[0];
111 NewBB->moveAfter(MovePos: FoundBB);
112}
113
114/// InsertPreheaderForLoop - Once we discover that a loop doesn't have a
115/// preheader, this method is called to insert one. This method has two phases:
116/// preheader insertion and analysis updating.
117///
118BasicBlock *llvm::InsertPreheaderForLoop(Loop *L, DominatorTree *DT,
119 LoopInfo *LI, MemorySSAUpdater *MSSAU,
120 bool PreserveLCSSA) {
121 BasicBlock *Header = L->getHeader();
122
123 // Compute the set of predecessors of the loop that are not in the loop.
124 SmallVector<BasicBlock*, 8> OutsideBlocks;
125 for (BasicBlock *P : predecessors(BB: Header)) {
126 if (!L->contains(BB: P)) { // Coming in from outside the loop?
127 // If the loop is branched to from an indirect terminator, we won't
128 // be able to fully transform the loop, because it prohibits
129 // edge splitting.
130 if (isa<IndirectBrInst>(Val: P->getTerminator()))
131 return nullptr;
132
133 // Keep track of it.
134 OutsideBlocks.push_back(Elt: P);
135 }
136 }
137
138 // Split out the loop pre-header.
139 BasicBlock *PreheaderBB;
140 PreheaderBB = SplitBlockPredecessors(BB: Header, Preds: OutsideBlocks, Suffix: ".preheader", DT,
141 LI, MSSAU, PreserveLCSSA);
142 if (!PreheaderBB)
143 return nullptr;
144
145 LLVM_DEBUG(dbgs() << "LoopSimplify: Creating pre-header "
146 << PreheaderBB->getName() << "\n");
147
148 // Make sure that NewBB is put someplace intelligent, which doesn't mess up
149 // code layout too horribly.
150 placeSplitBlockCarefully(NewBB: PreheaderBB, SplitPreds&: OutsideBlocks, L);
151
152 return PreheaderBB;
153}
154
155/// Add the specified block, and all of its predecessors, to the specified set,
156/// if it's not already in there. Stop predecessor traversal when we reach
157/// StopBlock.
158static void addBlockAndPredsToSet(BasicBlock *InputBB, BasicBlock *StopBlock,
159 SmallPtrSetImpl<BasicBlock *> &Blocks) {
160 SmallVector<BasicBlock *, 8> Worklist;
161 Worklist.push_back(Elt: InputBB);
162 do {
163 BasicBlock *BB = Worklist.pop_back_val();
164 if (Blocks.insert(Ptr: BB).second && BB != StopBlock)
165 // If BB is not already processed and it is not a stop block then
166 // insert its predecessor in the work list
167 append_range(C&: Worklist, R: predecessors(BB));
168 } while (!Worklist.empty());
169}
170
171/// The first part of loop-nestification is to find a PHI node that tells
172/// us how to partition the loops.
173static PHINode *findPHIToPartitionLoops(Loop *L, DominatorTree *DT,
174 AssumptionCache *AC) {
175 const DataLayout &DL = L->getHeader()->getModule()->getDataLayout();
176 for (BasicBlock::iterator I = L->getHeader()->begin(); isa<PHINode>(Val: I); ) {
177 PHINode *PN = cast<PHINode>(Val&: I);
178 ++I;
179 if (Value *V = simplifyInstruction(I: PN, Q: {DL, nullptr, DT, AC})) {
180 // This is a degenerate PHI already, don't modify it!
181 PN->replaceAllUsesWith(V);
182 PN->eraseFromParent();
183 continue;
184 }
185
186 // Scan this PHI node looking for a use of the PHI node by itself.
187 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
188 if (PN->getIncomingValue(i) == PN &&
189 L->contains(BB: PN->getIncomingBlock(i)))
190 // We found something tasty to remove.
191 return PN;
192 }
193 return nullptr;
194}
195
196/// If this loop has multiple backedges, try to pull one of them out into
197/// a nested loop.
198///
199/// This is important for code that looks like
200/// this:
201///
202/// Loop:
203/// ...
204/// br cond, Loop, Next
205/// ...
206/// br cond2, Loop, Out
207///
208/// To identify this common case, we look at the PHI nodes in the header of the
209/// loop. PHI nodes with unchanging values on one backedge correspond to values
210/// that change in the "outer" loop, but not in the "inner" loop.
211///
212/// If we are able to separate out a loop, return the new outer loop that was
213/// created.
214///
215static Loop *separateNestedLoop(Loop *L, BasicBlock *Preheader,
216 DominatorTree *DT, LoopInfo *LI,
217 ScalarEvolution *SE, bool PreserveLCSSA,
218 AssumptionCache *AC, MemorySSAUpdater *MSSAU) {
219 // Don't try to separate loops without a preheader.
220 if (!Preheader)
221 return nullptr;
222
223 // Treat the presence of convergent functions conservatively. The
224 // transformation is invalid if calls to certain convergent
225 // functions (like an AMDGPU barrier) get included in the resulting
226 // inner loop. But blocks meant for the inner loop will be
227 // identified later at a point where it's too late to abort the
228 // transformation. Also, the convergent attribute is not really
229 // sufficient to express the semantics of functions that are
230 // affected by this transformation. So we choose to back off if such
231 // a function call is present until a better alternative becomes
232 // available. This is similar to the conservative treatment of
233 // convergent function calls in GVNHoist and JumpThreading.
234 for (auto *BB : L->blocks()) {
235 for (auto &II : *BB) {
236 if (auto CI = dyn_cast<CallBase>(Val: &II)) {
237 if (CI->isConvergent()) {
238 return nullptr;
239 }
240 }
241 }
242 }
243
244 // The header is not a landing pad; preheader insertion should ensure this.
245 BasicBlock *Header = L->getHeader();
246 assert(!Header->isEHPad() && "Can't insert backedge to EH pad");
247
248 PHINode *PN = findPHIToPartitionLoops(L, DT, AC);
249 if (!PN) return nullptr; // No known way to partition.
250
251 // Pull out all predecessors that have varying values in the loop. This
252 // handles the case when a PHI node has multiple instances of itself as
253 // arguments.
254 SmallVector<BasicBlock*, 8> OuterLoopPreds;
255 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
256 if (PN->getIncomingValue(i) != PN ||
257 !L->contains(BB: PN->getIncomingBlock(i))) {
258 // We can't split indirect control flow edges.
259 if (isa<IndirectBrInst>(Val: PN->getIncomingBlock(i)->getTerminator()))
260 return nullptr;
261 OuterLoopPreds.push_back(Elt: PN->getIncomingBlock(i));
262 }
263 }
264 LLVM_DEBUG(dbgs() << "LoopSimplify: Splitting out a new outer loop\n");
265
266 // If ScalarEvolution is around and knows anything about values in
267 // this loop, tell it to forget them, because we're about to
268 // substantially change it.
269 if (SE)
270 SE->forgetLoop(L);
271
272 BasicBlock *NewBB = SplitBlockPredecessors(BB: Header, Preds: OuterLoopPreds, Suffix: ".outer",
273 DT, LI, MSSAU, PreserveLCSSA);
274
275 // Make sure that NewBB is put someplace intelligent, which doesn't mess up
276 // code layout too horribly.
277 placeSplitBlockCarefully(NewBB, SplitPreds&: OuterLoopPreds, L);
278
279 // Create the new outer loop.
280 Loop *NewOuter = LI->AllocateLoop();
281
282 // Change the parent loop to use the outer loop as its child now.
283 if (Loop *Parent = L->getParentLoop())
284 Parent->replaceChildLoopWith(OldChild: L, NewChild: NewOuter);
285 else
286 LI->changeTopLevelLoop(OldLoop: L, NewLoop: NewOuter);
287
288 // L is now a subloop of our outer loop.
289 NewOuter->addChildLoop(NewChild: L);
290
291 for (BasicBlock *BB : L->blocks())
292 NewOuter->addBlockEntry(BB);
293
294 // Now reset the header in L, which had been moved by
295 // SplitBlockPredecessors for the outer loop.
296 L->moveToHeader(BB: Header);
297
298 // Determine which blocks should stay in L and which should be moved out to
299 // the Outer loop now.
300 SmallPtrSet<BasicBlock *, 4> BlocksInL;
301 for (BasicBlock *P : predecessors(BB: Header)) {
302 if (DT->dominates(A: Header, B: P))
303 addBlockAndPredsToSet(InputBB: P, StopBlock: Header, Blocks&: BlocksInL);
304 }
305
306 // Scan all of the loop children of L, moving them to OuterLoop if they are
307 // not part of the inner loop.
308 const std::vector<Loop*> &SubLoops = L->getSubLoops();
309 for (size_t I = 0; I != SubLoops.size(); )
310 if (BlocksInL.count(Ptr: SubLoops[I]->getHeader()))
311 ++I; // Loop remains in L
312 else
313 NewOuter->addChildLoop(NewChild: L->removeChildLoop(I: SubLoops.begin() + I));
314
315 SmallVector<BasicBlock *, 8> OuterLoopBlocks;
316 OuterLoopBlocks.push_back(Elt: NewBB);
317 // Now that we know which blocks are in L and which need to be moved to
318 // OuterLoop, move any blocks that need it.
319 for (unsigned i = 0; i != L->getBlocks().size(); ++i) {
320 BasicBlock *BB = L->getBlocks()[i];
321 if (!BlocksInL.count(Ptr: BB)) {
322 // Move this block to the parent, updating the exit blocks sets
323 L->removeBlockFromLoop(BB);
324 if ((*LI)[BB] == L) {
325 LI->changeLoopFor(BB, L: NewOuter);
326 OuterLoopBlocks.push_back(Elt: BB);
327 }
328 --i;
329 }
330 }
331
332 // Split edges to exit blocks from the inner loop, if they emerged in the
333 // process of separating the outer one.
334 formDedicatedExitBlocks(L, DT, LI, MSSAU, PreserveLCSSA);
335
336 if (PreserveLCSSA) {
337 // Fix LCSSA form for L. Some values, which previously were only used inside
338 // L, can now be used in NewOuter loop. We need to insert phi-nodes for them
339 // in corresponding exit blocks.
340 // We don't need to form LCSSA recursively, because there cannot be uses
341 // inside a newly created loop of defs from inner loops as those would
342 // already be a use of an LCSSA phi node.
343 formLCSSA(L&: *L, DT: *DT, LI, SE);
344
345 assert(NewOuter->isRecursivelyLCSSAForm(*DT, *LI) &&
346 "LCSSA is broken after separating nested loops!");
347 }
348
349 return NewOuter;
350}
351
352/// This method is called when the specified loop has more than one
353/// backedge in it.
354///
355/// If this occurs, revector all of these backedges to target a new basic block
356/// and have that block branch to the loop header. This ensures that loops
357/// have exactly one backedge.
358static BasicBlock *insertUniqueBackedgeBlock(Loop *L, BasicBlock *Preheader,
359 DominatorTree *DT, LoopInfo *LI,
360 MemorySSAUpdater *MSSAU) {
361 assert(L->getNumBackEdges() > 1 && "Must have > 1 backedge!");
362
363 // Get information about the loop
364 BasicBlock *Header = L->getHeader();
365 Function *F = Header->getParent();
366
367 // Unique backedge insertion currently depends on having a preheader.
368 if (!Preheader)
369 return nullptr;
370
371 // The header is not an EH pad; preheader insertion should ensure this.
372 assert(!Header->isEHPad() && "Can't insert backedge to EH pad");
373
374 // Figure out which basic blocks contain back-edges to the loop header.
375 std::vector<BasicBlock*> BackedgeBlocks;
376 for (BasicBlock *P : predecessors(BB: Header)) {
377 // Indirect edges cannot be split, so we must fail if we find one.
378 if (isa<IndirectBrInst>(Val: P->getTerminator()))
379 return nullptr;
380
381 if (P != Preheader) BackedgeBlocks.push_back(x: P);
382 }
383
384 // Create and insert the new backedge block...
385 BasicBlock *BEBlock = BasicBlock::Create(Context&: Header->getContext(),
386 Name: Header->getName() + ".backedge", Parent: F);
387 BranchInst *BETerminator = BranchInst::Create(IfTrue: Header, InsertAtEnd: BEBlock);
388 BETerminator->setDebugLoc(Header->getFirstNonPHI()->getDebugLoc());
389
390 LLVM_DEBUG(dbgs() << "LoopSimplify: Inserting unique backedge block "
391 << BEBlock->getName() << "\n");
392
393 // Move the new backedge block to right after the last backedge block.
394 Function::iterator InsertPos = ++BackedgeBlocks.back()->getIterator();
395 F->splice(ToIt: InsertPos, FromF: F, FromIt: BEBlock->getIterator());
396
397 // Now that the block has been inserted into the function, create PHI nodes in
398 // the backedge block which correspond to any PHI nodes in the header block.
399 for (BasicBlock::iterator I = Header->begin(); isa<PHINode>(Val: I); ++I) {
400 PHINode *PN = cast<PHINode>(Val&: I);
401 PHINode *NewPN = PHINode::Create(Ty: PN->getType(), NumReservedValues: BackedgeBlocks.size(),
402 NameStr: PN->getName()+".be", InsertBefore: BETerminator->getIterator());
403
404 // Loop over the PHI node, moving all entries except the one for the
405 // preheader over to the new PHI node.
406 unsigned PreheaderIdx = ~0U;
407 bool HasUniqueIncomingValue = true;
408 Value *UniqueValue = nullptr;
409 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
410 BasicBlock *IBB = PN->getIncomingBlock(i);
411 Value *IV = PN->getIncomingValue(i);
412 if (IBB == Preheader) {
413 PreheaderIdx = i;
414 } else {
415 NewPN->addIncoming(V: IV, BB: IBB);
416 if (HasUniqueIncomingValue) {
417 if (!UniqueValue)
418 UniqueValue = IV;
419 else if (UniqueValue != IV)
420 HasUniqueIncomingValue = false;
421 }
422 }
423 }
424
425 // Delete all of the incoming values from the old PN except the preheader's
426 assert(PreheaderIdx != ~0U && "PHI has no preheader entry??");
427 if (PreheaderIdx != 0) {
428 PN->setIncomingValue(i: 0, V: PN->getIncomingValue(i: PreheaderIdx));
429 PN->setIncomingBlock(i: 0, BB: PN->getIncomingBlock(i: PreheaderIdx));
430 }
431 // Nuke all entries except the zero'th.
432 PN->removeIncomingValueIf(Predicate: [](unsigned Idx) { return Idx != 0; },
433 /* DeletePHIIfEmpty */ false);
434
435 // Finally, add the newly constructed PHI node as the entry for the BEBlock.
436 PN->addIncoming(V: NewPN, BB: BEBlock);
437
438 // As an optimization, if all incoming values in the new PhiNode (which is a
439 // subset of the incoming values of the old PHI node) have the same value,
440 // eliminate the PHI Node.
441 if (HasUniqueIncomingValue) {
442 NewPN->replaceAllUsesWith(V: UniqueValue);
443 NewPN->eraseFromParent();
444 }
445 }
446
447 // Now that all of the PHI nodes have been inserted and adjusted, modify the
448 // backedge blocks to jump to the BEBlock instead of the header.
449 // If one of the backedges has llvm.loop metadata attached, we remove
450 // it from the backedge and add it to BEBlock.
451 MDNode *LoopMD = nullptr;
452 for (BasicBlock *BB : BackedgeBlocks) {
453 Instruction *TI = BB->getTerminator();
454 if (!LoopMD)
455 LoopMD = TI->getMetadata(KindID: LLVMContext::MD_loop);
456 TI->setMetadata(KindID: LLVMContext::MD_loop, Node: nullptr);
457 TI->replaceSuccessorWith(OldBB: Header, NewBB: BEBlock);
458 }
459 BEBlock->getTerminator()->setMetadata(KindID: LLVMContext::MD_loop, Node: LoopMD);
460
461 //===--- Update all analyses which we must preserve now -----------------===//
462
463 // Update Loop Information - we know that this block is now in the current
464 // loop and all parent loops.
465 L->addBasicBlockToLoop(NewBB: BEBlock, LI&: *LI);
466
467 // Update dominator information
468 DT->splitBlock(NewBB: BEBlock);
469
470 if (MSSAU)
471 MSSAU->updatePhisWhenInsertingUniqueBackedgeBlock(LoopHeader: Header, LoopPreheader: Preheader,
472 BackedgeBlock: BEBlock);
473
474 return BEBlock;
475}
476
477/// Simplify one loop and queue further loops for simplification.
478static bool simplifyOneLoop(Loop *L, SmallVectorImpl<Loop *> &Worklist,
479 DominatorTree *DT, LoopInfo *LI,
480 ScalarEvolution *SE, AssumptionCache *AC,
481 MemorySSAUpdater *MSSAU, bool PreserveLCSSA) {
482 bool Changed = false;
483 if (MSSAU && VerifyMemorySSA)
484 MSSAU->getMemorySSA()->verifyMemorySSA();
485
486ReprocessLoop:
487
488 // Check to see that no blocks (other than the header) in this loop have
489 // predecessors that are not in the loop. This is not valid for natural
490 // loops, but can occur if the blocks are unreachable. Since they are
491 // unreachable we can just shamelessly delete those CFG edges!
492 for (BasicBlock *BB : L->blocks()) {
493 if (BB == L->getHeader())
494 continue;
495
496 SmallPtrSet<BasicBlock*, 4> BadPreds;
497 for (BasicBlock *P : predecessors(BB))
498 if (!L->contains(BB: P))
499 BadPreds.insert(Ptr: P);
500
501 // Delete each unique out-of-loop (and thus dead) predecessor.
502 for (BasicBlock *P : BadPreds) {
503
504 LLVM_DEBUG(dbgs() << "LoopSimplify: Deleting edge from dead predecessor "
505 << P->getName() << "\n");
506
507 // Zap the dead pred's terminator and replace it with unreachable.
508 Instruction *TI = P->getTerminator();
509 changeToUnreachable(I: TI, PreserveLCSSA,
510 /*DTU=*/nullptr, MSSAU);
511 Changed = true;
512 }
513 }
514
515 if (MSSAU && VerifyMemorySSA)
516 MSSAU->getMemorySSA()->verifyMemorySSA();
517
518 // If there are exiting blocks with branches on undef, resolve the undef in
519 // the direction which will exit the loop. This will help simplify loop
520 // trip count computations.
521 SmallVector<BasicBlock*, 8> ExitingBlocks;
522 L->getExitingBlocks(ExitingBlocks);
523 for (BasicBlock *ExitingBlock : ExitingBlocks)
524 if (BranchInst *BI = dyn_cast<BranchInst>(Val: ExitingBlock->getTerminator()))
525 if (BI->isConditional()) {
526 if (UndefValue *Cond = dyn_cast<UndefValue>(Val: BI->getCondition())) {
527
528 LLVM_DEBUG(dbgs()
529 << "LoopSimplify: Resolving \"br i1 undef\" to exit in "
530 << ExitingBlock->getName() << "\n");
531
532 BI->setCondition(ConstantInt::get(Ty: Cond->getType(),
533 V: !L->contains(BB: BI->getSuccessor(i: 0))));
534
535 Changed = true;
536 }
537 }
538
539 // Does the loop already have a preheader? If so, don't insert one.
540 BasicBlock *Preheader = L->getLoopPreheader();
541 if (!Preheader) {
542 Preheader = InsertPreheaderForLoop(L, DT, LI, MSSAU, PreserveLCSSA);
543 if (Preheader)
544 Changed = true;
545 }
546
547 // Next, check to make sure that all exit nodes of the loop only have
548 // predecessors that are inside of the loop. This check guarantees that the
549 // loop preheader/header will dominate the exit blocks. If the exit block has
550 // predecessors from outside of the loop, split the edge now.
551 if (formDedicatedExitBlocks(L, DT, LI, MSSAU, PreserveLCSSA))
552 Changed = true;
553
554 if (MSSAU && VerifyMemorySSA)
555 MSSAU->getMemorySSA()->verifyMemorySSA();
556
557 // If the header has more than two predecessors at this point (from the
558 // preheader and from multiple backedges), we must adjust the loop.
559 BasicBlock *LoopLatch = L->getLoopLatch();
560 if (!LoopLatch) {
561 // If this is really a nested loop, rip it out into a child loop. Don't do
562 // this for loops with a giant number of backedges, just factor them into a
563 // common backedge instead.
564 if (L->getNumBackEdges() < 8) {
565 if (Loop *OuterL = separateNestedLoop(L, Preheader, DT, LI, SE,
566 PreserveLCSSA, AC, MSSAU)) {
567 ++NumNested;
568 // Enqueue the outer loop as it should be processed next in our
569 // depth-first nest walk.
570 Worklist.push_back(Elt: OuterL);
571
572 // This is a big restructuring change, reprocess the whole loop.
573 Changed = true;
574 // GCC doesn't tail recursion eliminate this.
575 // FIXME: It isn't clear we can't rely on LLVM to TRE this.
576 goto ReprocessLoop;
577 }
578 }
579
580 // If we either couldn't, or didn't want to, identify nesting of the loops,
581 // insert a new block that all backedges target, then make it jump to the
582 // loop header.
583 LoopLatch = insertUniqueBackedgeBlock(L, Preheader, DT, LI, MSSAU);
584 if (LoopLatch)
585 Changed = true;
586 }
587
588 if (MSSAU && VerifyMemorySSA)
589 MSSAU->getMemorySSA()->verifyMemorySSA();
590
591 const DataLayout &DL = L->getHeader()->getModule()->getDataLayout();
592
593 // Scan over the PHI nodes in the loop header. Since they now have only two
594 // incoming values (the loop is canonicalized), we may have simplified the PHI
595 // down to 'X = phi [X, Y]', which should be replaced with 'Y'.
596 PHINode *PN;
597 for (BasicBlock::iterator I = L->getHeader()->begin();
598 (PN = dyn_cast<PHINode>(Val: I++)); )
599 if (Value *V = simplifyInstruction(I: PN, Q: {DL, nullptr, DT, AC})) {
600 if (SE) SE->forgetValue(V: PN);
601 if (!PreserveLCSSA || LI->replacementPreservesLCSSAForm(From: PN, To: V)) {
602 PN->replaceAllUsesWith(V);
603 PN->eraseFromParent();
604 Changed = true;
605 }
606 }
607
608 // If this loop has multiple exits and the exits all go to the same
609 // block, attempt to merge the exits. This helps several passes, such
610 // as LoopRotation, which do not support loops with multiple exits.
611 // SimplifyCFG also does this (and this code uses the same utility
612 // function), however this code is loop-aware, where SimplifyCFG is
613 // not. That gives it the advantage of being able to hoist
614 // loop-invariant instructions out of the way to open up more
615 // opportunities, and the disadvantage of having the responsibility
616 // to preserve dominator information.
617 auto HasUniqueExitBlock = [&]() {
618 BasicBlock *UniqueExit = nullptr;
619 for (auto *ExitingBB : ExitingBlocks)
620 for (auto *SuccBB : successors(BB: ExitingBB)) {
621 if (L->contains(BB: SuccBB))
622 continue;
623
624 if (!UniqueExit)
625 UniqueExit = SuccBB;
626 else if (UniqueExit != SuccBB)
627 return false;
628 }
629
630 return true;
631 };
632 if (HasUniqueExitBlock()) {
633 for (unsigned i = 0, e = ExitingBlocks.size(); i != e; ++i) {
634 BasicBlock *ExitingBlock = ExitingBlocks[i];
635 if (!ExitingBlock->getSinglePredecessor()) continue;
636 BranchInst *BI = dyn_cast<BranchInst>(Val: ExitingBlock->getTerminator());
637 if (!BI || !BI->isConditional()) continue;
638 CmpInst *CI = dyn_cast<CmpInst>(Val: BI->getCondition());
639 if (!CI || CI->getParent() != ExitingBlock) continue;
640
641 // Attempt to hoist out all instructions except for the
642 // comparison and the branch.
643 bool AllInvariant = true;
644 bool AnyInvariant = false;
645 for (auto I = ExitingBlock->instructionsWithoutDebug().begin(); &*I != BI; ) {
646 Instruction *Inst = &*I++;
647 if (Inst == CI)
648 continue;
649 if (!L->makeLoopInvariant(
650 I: Inst, Changed&: AnyInvariant,
651 InsertPt: Preheader ? Preheader->getTerminator() : nullptr, MSSAU, SE)) {
652 AllInvariant = false;
653 break;
654 }
655 }
656 if (AnyInvariant)
657 Changed = true;
658 if (!AllInvariant) continue;
659
660 // The block has now been cleared of all instructions except for
661 // a comparison and a conditional branch. SimplifyCFG may be able
662 // to fold it now.
663 if (!FoldBranchToCommonDest(BI, /*DTU=*/nullptr, MSSAU))
664 continue;
665
666 // Success. The block is now dead, so remove it from the loop,
667 // update the dominator tree and delete it.
668 LLVM_DEBUG(dbgs() << "LoopSimplify: Eliminating exiting block "
669 << ExitingBlock->getName() << "\n");
670
671 assert(pred_empty(ExitingBlock));
672 Changed = true;
673 LI->removeBlock(BB: ExitingBlock);
674
675 DomTreeNode *Node = DT->getNode(BB: ExitingBlock);
676 while (!Node->isLeaf()) {
677 DomTreeNode *Child = Node->back();
678 DT->changeImmediateDominator(N: Child, NewIDom: Node->getIDom());
679 }
680 DT->eraseNode(BB: ExitingBlock);
681 if (MSSAU) {
682 SmallSetVector<BasicBlock *, 8> ExitBlockSet;
683 ExitBlockSet.insert(X: ExitingBlock);
684 MSSAU->removeBlocks(DeadBlocks: ExitBlockSet);
685 }
686
687 BI->getSuccessor(i: 0)->removePredecessor(
688 Pred: ExitingBlock, /* KeepOneInputPHIs */ PreserveLCSSA);
689 BI->getSuccessor(i: 1)->removePredecessor(
690 Pred: ExitingBlock, /* KeepOneInputPHIs */ PreserveLCSSA);
691 ExitingBlock->eraseFromParent();
692 }
693 }
694
695 if (MSSAU && VerifyMemorySSA)
696 MSSAU->getMemorySSA()->verifyMemorySSA();
697
698 return Changed;
699}
700
701bool llvm::simplifyLoop(Loop *L, DominatorTree *DT, LoopInfo *LI,
702 ScalarEvolution *SE, AssumptionCache *AC,
703 MemorySSAUpdater *MSSAU, bool PreserveLCSSA) {
704 bool Changed = false;
705
706#ifndef NDEBUG
707 // If we're asked to preserve LCSSA, the loop nest needs to start in LCSSA
708 // form.
709 if (PreserveLCSSA) {
710 assert(DT && "DT not available.");
711 assert(LI && "LI not available.");
712 assert(L->isRecursivelyLCSSAForm(*DT, *LI) &&
713 "Requested to preserve LCSSA, but it's already broken.");
714 }
715#endif
716
717 // Worklist maintains our depth-first queue of loops in this nest to process.
718 SmallVector<Loop *, 4> Worklist;
719 Worklist.push_back(Elt: L);
720
721 // Walk the worklist from front to back, pushing newly found sub loops onto
722 // the back. This will let us process loops from back to front in depth-first
723 // order. We can use this simple process because loops form a tree.
724 for (unsigned Idx = 0; Idx != Worklist.size(); ++Idx) {
725 Loop *L2 = Worklist[Idx];
726 Worklist.append(in_start: L2->begin(), in_end: L2->end());
727 }
728
729 while (!Worklist.empty())
730 Changed |= simplifyOneLoop(L: Worklist.pop_back_val(), Worklist, DT, LI, SE,
731 AC, MSSAU, PreserveLCSSA);
732
733 // Changing exit conditions for blocks may affect exit counts of this loop and
734 // any of its parents, so we must invalidate the entire subtree if we've made
735 // any changes. Do this here rather than in simplifyOneLoop() as the top-most
736 // loop is going to be the same for all child loops.
737 if (Changed && SE)
738 SE->forgetTopmostLoop(L);
739
740 return Changed;
741}
742
743namespace {
744 struct LoopSimplify : public FunctionPass {
745 static char ID; // Pass identification, replacement for typeid
746 LoopSimplify() : FunctionPass(ID) {
747 initializeLoopSimplifyPass(*PassRegistry::getPassRegistry());
748 }
749
750 bool runOnFunction(Function &F) override;
751
752 void getAnalysisUsage(AnalysisUsage &AU) const override {
753 AU.addRequired<AssumptionCacheTracker>();
754
755 // We need loop information to identify the loops...
756 AU.addRequired<DominatorTreeWrapperPass>();
757 AU.addPreserved<DominatorTreeWrapperPass>();
758
759 AU.addRequired<LoopInfoWrapperPass>();
760 AU.addPreserved<LoopInfoWrapperPass>();
761
762 AU.addPreserved<BasicAAWrapperPass>();
763 AU.addPreserved<AAResultsWrapperPass>();
764 AU.addPreserved<GlobalsAAWrapperPass>();
765 AU.addPreserved<ScalarEvolutionWrapperPass>();
766 AU.addPreserved<SCEVAAWrapperPass>();
767 AU.addPreservedID(ID&: LCSSAID);
768 AU.addPreserved<DependenceAnalysisWrapperPass>();
769 AU.addPreservedID(ID&: BreakCriticalEdgesID); // No critical edges added.
770 AU.addPreserved<BranchProbabilityInfoWrapperPass>();
771 AU.addPreserved<MemorySSAWrapperPass>();
772 }
773
774 /// verifyAnalysis() - Verify LoopSimplifyForm's guarantees.
775 void verifyAnalysis() const override;
776 };
777}
778
779char LoopSimplify::ID = 0;
780INITIALIZE_PASS_BEGIN(LoopSimplify, "loop-simplify",
781 "Canonicalize natural loops", false, false)
782INITIALIZE_PASS_DEPENDENCY(AssumptionCacheTracker)
783INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
784INITIALIZE_PASS_DEPENDENCY(LoopInfoWrapperPass)
785INITIALIZE_PASS_END(LoopSimplify, "loop-simplify",
786 "Canonicalize natural loops", false, false)
787
788// Publicly exposed interface to pass...
789char &llvm::LoopSimplifyID = LoopSimplify::ID;
790Pass *llvm::createLoopSimplifyPass() { return new LoopSimplify(); }
791
792/// runOnFunction - Run down all loops in the CFG (recursively, but we could do
793/// it in any convenient order) inserting preheaders...
794///
795bool LoopSimplify::runOnFunction(Function &F) {
796 bool Changed = false;
797 LoopInfo *LI = &getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
798 DominatorTree *DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
799 auto *SEWP = getAnalysisIfAvailable<ScalarEvolutionWrapperPass>();
800 ScalarEvolution *SE = SEWP ? &SEWP->getSE() : nullptr;
801 AssumptionCache *AC =
802 &getAnalysis<AssumptionCacheTracker>().getAssumptionCache(F);
803 MemorySSA *MSSA = nullptr;
804 std::unique_ptr<MemorySSAUpdater> MSSAU;
805 auto *MSSAAnalysis = getAnalysisIfAvailable<MemorySSAWrapperPass>();
806 if (MSSAAnalysis) {
807 MSSA = &MSSAAnalysis->getMSSA();
808 MSSAU = std::make_unique<MemorySSAUpdater>(args&: MSSA);
809 }
810
811 bool PreserveLCSSA = mustPreserveAnalysisID(AID&: LCSSAID);
812
813 // Simplify each loop nest in the function.
814 for (auto *L : *LI)
815 Changed |= simplifyLoop(L, DT, LI, SE, AC, MSSAU: MSSAU.get(), PreserveLCSSA);
816
817#ifndef NDEBUG
818 if (PreserveLCSSA) {
819 bool InLCSSA = all_of(
820 Range&: *LI, P: [&](Loop *L) { return L->isRecursivelyLCSSAForm(DT: *DT, LI: *LI); });
821 assert(InLCSSA && "LCSSA is broken after loop-simplify.");
822 }
823#endif
824 return Changed;
825}
826
827PreservedAnalyses LoopSimplifyPass::run(Function &F,
828 FunctionAnalysisManager &AM) {
829 bool Changed = false;
830 LoopInfo *LI = &AM.getResult<LoopAnalysis>(IR&: F);
831 DominatorTree *DT = &AM.getResult<DominatorTreeAnalysis>(IR&: F);
832 ScalarEvolution *SE = AM.getCachedResult<ScalarEvolutionAnalysis>(IR&: F);
833 AssumptionCache *AC = &AM.getResult<AssumptionAnalysis>(IR&: F);
834 auto *MSSAAnalysis = AM.getCachedResult<MemorySSAAnalysis>(IR&: F);
835 std::unique_ptr<MemorySSAUpdater> MSSAU;
836 if (MSSAAnalysis) {
837 auto *MSSA = &MSSAAnalysis->getMSSA();
838 MSSAU = std::make_unique<MemorySSAUpdater>(args&: MSSA);
839 }
840
841
842 // Note that we don't preserve LCSSA in the new PM, if you need it run LCSSA
843 // after simplifying the loops. MemorySSA is preserved if it exists.
844 for (auto *L : *LI)
845 Changed |=
846 simplifyLoop(L, DT, LI, SE, AC, MSSAU: MSSAU.get(), /*PreserveLCSSA*/ false);
847
848 if (!Changed)
849 return PreservedAnalyses::all();
850
851 PreservedAnalyses PA;
852 PA.preserve<DominatorTreeAnalysis>();
853 PA.preserve<LoopAnalysis>();
854 PA.preserve<ScalarEvolutionAnalysis>();
855 PA.preserve<DependenceAnalysis>();
856 if (MSSAAnalysis)
857 PA.preserve<MemorySSAAnalysis>();
858 // BPI maps conditional terminators to probabilities, LoopSimplify can insert
859 // blocks, but it does so only by splitting existing blocks and edges. This
860 // results in the interesting property that all new terminators inserted are
861 // unconditional branches which do not appear in BPI. All deletions are
862 // handled via ValueHandle callbacks w/in BPI.
863 PA.preserve<BranchProbabilityAnalysis>();
864 return PA;
865}
866
867// FIXME: Restore this code when we re-enable verification in verifyAnalysis
868// below.
869#if 0
870static void verifyLoop(Loop *L) {
871 // Verify subloops.
872 for (Loop::iterator I = L->begin(), E = L->end(); I != E; ++I)
873 verifyLoop(*I);
874
875 // It used to be possible to just assert L->isLoopSimplifyForm(), however
876 // with the introduction of indirectbr, there are now cases where it's
877 // not possible to transform a loop as necessary. We can at least check
878 // that there is an indirectbr near any time there's trouble.
879
880 // Indirectbr can interfere with preheader and unique backedge insertion.
881 if (!L->getLoopPreheader() || !L->getLoopLatch()) {
882 bool HasIndBrPred = false;
883 for (BasicBlock *Pred : predecessors(L->getHeader()))
884 if (isa<IndirectBrInst>(Pred->getTerminator())) {
885 HasIndBrPred = true;
886 break;
887 }
888 assert(HasIndBrPred &&
889 "LoopSimplify has no excuse for missing loop header info!");
890 (void)HasIndBrPred;
891 }
892
893 // Indirectbr can interfere with exit block canonicalization.
894 if (!L->hasDedicatedExits()) {
895 bool HasIndBrExiting = false;
896 SmallVector<BasicBlock*, 8> ExitingBlocks;
897 L->getExitingBlocks(ExitingBlocks);
898 for (unsigned i = 0, e = ExitingBlocks.size(); i != e; ++i) {
899 if (isa<IndirectBrInst>((ExitingBlocks[i])->getTerminator())) {
900 HasIndBrExiting = true;
901 break;
902 }
903 }
904
905 assert(HasIndBrExiting &&
906 "LoopSimplify has no excuse for missing exit block info!");
907 (void)HasIndBrExiting;
908 }
909}
910#endif
911
912void LoopSimplify::verifyAnalysis() const {
913 // FIXME: This routine is being called mid-way through the loop pass manager
914 // as loop passes destroy this analysis. That's actually fine, but we have no
915 // way of expressing that here. Once all of the passes that destroy this are
916 // hoisted out of the loop pass manager we can add back verification here.
917#if 0
918 for (LoopInfo::iterator I = LI->begin(), E = LI->end(); I != E; ++I)
919 verifyLoop(*I);
920#endif
921}
922

source code of llvm/lib/Transforms/Utils/LoopSimplify.cpp