1//===- LoopFlatten.cpp - Loop flattening 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 flattens pairs nested loops into a single loop.
10//
11// The intention is to optimise loop nests like this, which together access an
12// array linearly:
13//
14// for (int i = 0; i < N; ++i)
15// for (int j = 0; j < M; ++j)
16// f(A[i*M+j]);
17//
18// into one loop:
19//
20// for (int i = 0; i < (N*M); ++i)
21// f(A[i]);
22//
23// It can also flatten loops where the induction variables are not used in the
24// loop. This is only worth doing if the induction variables are only used in an
25// expression like i*M+j. If they had any other uses, we would have to insert a
26// div/mod to reconstruct the original values, so this wouldn't be profitable.
27//
28// We also need to prove that N*M will not overflow. The preferred solution is
29// to widen the IV, which avoids overflow checks, so that is tried first. If
30// the IV cannot be widened, then we try to determine that this new tripcount
31// expression won't overflow.
32//
33// Q: Does LoopFlatten use SCEV?
34// Short answer: Yes and no.
35//
36// Long answer:
37// For this transformation to be valid, we require all uses of the induction
38// variables to be linear expressions of the form i*M+j. The different Loop
39// APIs are used to get some loop components like the induction variable,
40// compare statement, etc. In addition, we do some pattern matching to find the
41// linear expressions and other loop components like the loop increment. The
42// latter are examples of expressions that do use the induction variable, but
43// are safe to ignore when we check all uses to be of the form i*M+j. We keep
44// track of all of this in bookkeeping struct FlattenInfo.
45// We assume the loops to be canonical, i.e. starting at 0 and increment with
46// 1. This makes RHS of the compare the loop tripcount (with the right
47// predicate). We use SCEV to then sanity check that this tripcount matches
48// with the tripcount as computed by SCEV.
49//
50//===----------------------------------------------------------------------===//
51
52#include "llvm/Transforms/Scalar/LoopFlatten.h"
53
54#include "llvm/ADT/Statistic.h"
55#include "llvm/Analysis/AssumptionCache.h"
56#include "llvm/Analysis/LoopInfo.h"
57#include "llvm/Analysis/LoopNestAnalysis.h"
58#include "llvm/Analysis/MemorySSAUpdater.h"
59#include "llvm/Analysis/OptimizationRemarkEmitter.h"
60#include "llvm/Analysis/ScalarEvolution.h"
61#include "llvm/Analysis/TargetTransformInfo.h"
62#include "llvm/Analysis/ValueTracking.h"
63#include "llvm/IR/Dominators.h"
64#include "llvm/IR/Function.h"
65#include "llvm/IR/IRBuilder.h"
66#include "llvm/IR/Module.h"
67#include "llvm/IR/PatternMatch.h"
68#include "llvm/Support/Debug.h"
69#include "llvm/Support/raw_ostream.h"
70#include "llvm/Transforms/Scalar/LoopPassManager.h"
71#include "llvm/Transforms/Utils/Local.h"
72#include "llvm/Transforms/Utils/LoopUtils.h"
73#include "llvm/Transforms/Utils/LoopVersioning.h"
74#include "llvm/Transforms/Utils/ScalarEvolutionExpander.h"
75#include "llvm/Transforms/Utils/SimplifyIndVar.h"
76#include <optional>
77
78using namespace llvm;
79using namespace llvm::PatternMatch;
80
81#define DEBUG_TYPE "loop-flatten"
82
83STATISTIC(NumFlattened, "Number of loops flattened");
84
85static cl::opt<unsigned> RepeatedInstructionThreshold(
86 "loop-flatten-cost-threshold", cl::Hidden, cl::init(Val: 2),
87 cl::desc("Limit on the cost of instructions that can be repeated due to "
88 "loop flattening"));
89
90static cl::opt<bool>
91 AssumeNoOverflow("loop-flatten-assume-no-overflow", cl::Hidden,
92 cl::init(Val: false),
93 cl::desc("Assume that the product of the two iteration "
94 "trip counts will never overflow"));
95
96static cl::opt<bool>
97 WidenIV("loop-flatten-widen-iv", cl::Hidden, cl::init(Val: true),
98 cl::desc("Widen the loop induction variables, if possible, so "
99 "overflow checks won't reject flattening"));
100
101static cl::opt<bool>
102 VersionLoops("loop-flatten-version-loops", cl::Hidden, cl::init(Val: true),
103 cl::desc("Version loops if flattened loop could overflow"));
104
105namespace {
106// We require all uses of both induction variables to match this pattern:
107//
108// (OuterPHI * InnerTripCount) + InnerPHI
109//
110// I.e., it needs to be a linear expression of the induction variables and the
111// inner loop trip count. We keep track of all different expressions on which
112// checks will be performed in this bookkeeping struct.
113//
114struct FlattenInfo {
115 Loop *OuterLoop = nullptr; // The loop pair to be flattened.
116 Loop *InnerLoop = nullptr;
117
118 PHINode *InnerInductionPHI = nullptr; // These PHINodes correspond to loop
119 PHINode *OuterInductionPHI = nullptr; // induction variables, which are
120 // expected to start at zero and
121 // increment by one on each loop.
122
123 Value *InnerTripCount = nullptr; // The product of these two tripcounts
124 Value *OuterTripCount = nullptr; // will be the new flattened loop
125 // tripcount. Also used to recognise a
126 // linear expression that will be replaced.
127
128 SmallPtrSet<Value *, 4> LinearIVUses; // Contains the linear expressions
129 // of the form i*M+j that will be
130 // replaced.
131
132 BinaryOperator *InnerIncrement = nullptr; // Uses of induction variables in
133 BinaryOperator *OuterIncrement = nullptr; // loop control statements that
134 BranchInst *InnerBranch = nullptr; // are safe to ignore.
135
136 BranchInst *OuterBranch = nullptr; // The instruction that needs to be
137 // updated with new tripcount.
138
139 SmallPtrSet<PHINode *, 4> InnerPHIsToTransform;
140
141 bool Widened = false; // Whether this holds the flatten info before or after
142 // widening.
143
144 PHINode *NarrowInnerInductionPHI = nullptr; // Holds the old/narrow induction
145 PHINode *NarrowOuterInductionPHI = nullptr; // phis, i.e. the Phis before IV
146 // has been applied. Used to skip
147 // checks on phi nodes.
148
149 Value *NewTripCount = nullptr; // The tripcount of the flattened loop.
150
151 FlattenInfo(Loop *OL, Loop *IL) : OuterLoop(OL), InnerLoop(IL){};
152
153 bool isNarrowInductionPhi(PHINode *Phi) {
154 // This can't be the narrow phi if we haven't widened the IV first.
155 if (!Widened)
156 return false;
157 return NarrowInnerInductionPHI == Phi || NarrowOuterInductionPHI == Phi;
158 }
159 bool isInnerLoopIncrement(User *U) {
160 return InnerIncrement == U;
161 }
162 bool isOuterLoopIncrement(User *U) {
163 return OuterIncrement == U;
164 }
165 bool isInnerLoopTest(User *U) {
166 return InnerBranch->getCondition() == U;
167 }
168
169 bool checkOuterInductionPhiUsers(SmallPtrSet<Value *, 4> &ValidOuterPHIUses) {
170 for (User *U : OuterInductionPHI->users()) {
171 if (isOuterLoopIncrement(U))
172 continue;
173
174 auto IsValidOuterPHIUses = [&] (User *U) -> bool {
175 LLVM_DEBUG(dbgs() << "Found use of outer induction variable: "; U->dump());
176 if (!ValidOuterPHIUses.count(Ptr: U)) {
177 LLVM_DEBUG(dbgs() << "Did not match expected pattern, bailing\n");
178 return false;
179 }
180 LLVM_DEBUG(dbgs() << "Use is optimisable\n");
181 return true;
182 };
183
184 if (auto *V = dyn_cast<TruncInst>(Val: U)) {
185 for (auto *K : V->users()) {
186 if (!IsValidOuterPHIUses(K))
187 return false;
188 }
189 continue;
190 }
191
192 if (!IsValidOuterPHIUses(U))
193 return false;
194 }
195 return true;
196 }
197
198 bool matchLinearIVUser(User *U, Value *InnerTripCount,
199 SmallPtrSet<Value *, 4> &ValidOuterPHIUses) {
200 LLVM_DEBUG(dbgs() << "Checking linear i*M+j expression for: "; U->dump());
201 Value *MatchedMul = nullptr;
202 Value *MatchedItCount = nullptr;
203
204 bool IsAdd = match(V: U, P: m_c_Add(L: m_Specific(V: InnerInductionPHI),
205 R: m_Value(V&: MatchedMul))) &&
206 match(V: MatchedMul, P: m_c_Mul(L: m_Specific(V: OuterInductionPHI),
207 R: m_Value(V&: MatchedItCount)));
208
209 // Matches the same pattern as above, except it also looks for truncs
210 // on the phi, which can be the result of widening the induction variables.
211 bool IsAddTrunc =
212 match(V: U, P: m_c_Add(L: m_Trunc(Op: m_Specific(V: InnerInductionPHI)),
213 R: m_Value(V&: MatchedMul))) &&
214 match(V: MatchedMul, P: m_c_Mul(L: m_Trunc(Op: m_Specific(V: OuterInductionPHI)),
215 R: m_Value(V&: MatchedItCount)));
216
217 // Matches the pattern ptr+i*M+j, with the two additions being done via GEP.
218 bool IsGEP = match(V: U, P: m_GEP(Ops: m_GEP(Ops: m_Value(), Ops: m_Value(V&: MatchedMul)),
219 Ops: m_Specific(V: InnerInductionPHI))) &&
220 match(V: MatchedMul, P: m_c_Mul(L: m_Specific(V: OuterInductionPHI),
221 R: m_Value(V&: MatchedItCount)));
222
223 if (!MatchedItCount)
224 return false;
225
226 LLVM_DEBUG(dbgs() << "Matched multiplication: "; MatchedMul->dump());
227 LLVM_DEBUG(dbgs() << "Matched iteration count: "; MatchedItCount->dump());
228
229 // The mul should not have any other uses. Widening may leave trivially dead
230 // uses, which can be ignored.
231 if (count_if(Range: MatchedMul->users(), P: [](User *U) {
232 return !isInstructionTriviallyDead(I: cast<Instruction>(Val: U));
233 }) > 1) {
234 LLVM_DEBUG(dbgs() << "Multiply has more than one use\n");
235 return false;
236 }
237
238 // Look through extends if the IV has been widened. Don't look through
239 // extends if we already looked through a trunc.
240 if (Widened && (IsAdd || IsGEP) &&
241 (isa<SExtInst>(Val: MatchedItCount) || isa<ZExtInst>(Val: MatchedItCount))) {
242 assert(MatchedItCount->getType() == InnerInductionPHI->getType() &&
243 "Unexpected type mismatch in types after widening");
244 MatchedItCount = isa<SExtInst>(Val: MatchedItCount)
245 ? dyn_cast<SExtInst>(Val: MatchedItCount)->getOperand(i_nocapture: 0)
246 : dyn_cast<ZExtInst>(Val: MatchedItCount)->getOperand(i_nocapture: 0);
247 }
248
249 LLVM_DEBUG(dbgs() << "Looking for inner trip count: ";
250 InnerTripCount->dump());
251
252 if ((IsAdd || IsAddTrunc || IsGEP) && MatchedItCount == InnerTripCount) {
253 LLVM_DEBUG(dbgs() << "Found. This sse is optimisable\n");
254 ValidOuterPHIUses.insert(Ptr: MatchedMul);
255 LinearIVUses.insert(Ptr: U);
256 return true;
257 }
258
259 LLVM_DEBUG(dbgs() << "Did not match expected pattern, bailing\n");
260 return false;
261 }
262
263 bool checkInnerInductionPhiUsers(SmallPtrSet<Value *, 4> &ValidOuterPHIUses) {
264 Value *SExtInnerTripCount = InnerTripCount;
265 if (Widened &&
266 (isa<SExtInst>(Val: InnerTripCount) || isa<ZExtInst>(Val: InnerTripCount)))
267 SExtInnerTripCount = cast<Instruction>(Val: InnerTripCount)->getOperand(i: 0);
268
269 for (User *U : InnerInductionPHI->users()) {
270 LLVM_DEBUG(dbgs() << "Checking User: "; U->dump());
271 if (isInnerLoopIncrement(U)) {
272 LLVM_DEBUG(dbgs() << "Use is inner loop increment, continuing\n");
273 continue;
274 }
275
276 // After widening the IVs, a trunc instruction might have been introduced,
277 // so look through truncs.
278 if (isa<TruncInst>(Val: U)) {
279 if (!U->hasOneUse())
280 return false;
281 U = *U->user_begin();
282 }
283
284 // If the use is in the compare (which is also the condition of the inner
285 // branch) then the compare has been altered by another transformation e.g
286 // icmp ult %inc, tripcount -> icmp ult %j, tripcount-1, where tripcount is
287 // a constant. Ignore this use as the compare gets removed later anyway.
288 if (isInnerLoopTest(U)) {
289 LLVM_DEBUG(dbgs() << "Use is the inner loop test, continuing\n");
290 continue;
291 }
292
293 if (!matchLinearIVUser(U, InnerTripCount: SExtInnerTripCount, ValidOuterPHIUses)) {
294 LLVM_DEBUG(dbgs() << "Not a linear IV user\n");
295 return false;
296 }
297 LLVM_DEBUG(dbgs() << "Linear IV users found!\n");
298 }
299 return true;
300 }
301};
302} // namespace
303
304static bool
305setLoopComponents(Value *&TC, Value *&TripCount, BinaryOperator *&Increment,
306 SmallPtrSetImpl<Instruction *> &IterationInstructions) {
307 TripCount = TC;
308 IterationInstructions.insert(Ptr: Increment);
309 LLVM_DEBUG(dbgs() << "Found Increment: "; Increment->dump());
310 LLVM_DEBUG(dbgs() << "Found trip count: "; TripCount->dump());
311 LLVM_DEBUG(dbgs() << "Successfully found all loop components\n");
312 return true;
313}
314
315// Given the RHS of the loop latch compare instruction, verify with SCEV
316// that this is indeed the loop tripcount.
317// TODO: This used to be a straightforward check but has grown to be quite
318// complicated now. It is therefore worth revisiting what the additional
319// benefits are of this (compared to relying on canonical loops and pattern
320// matching).
321static bool verifyTripCount(Value *RHS, Loop *L,
322 SmallPtrSetImpl<Instruction *> &IterationInstructions,
323 PHINode *&InductionPHI, Value *&TripCount, BinaryOperator *&Increment,
324 BranchInst *&BackBranch, ScalarEvolution *SE, bool IsWidened) {
325 const SCEV *BackedgeTakenCount = SE->getBackedgeTakenCount(L);
326 if (isa<SCEVCouldNotCompute>(Val: BackedgeTakenCount)) {
327 LLVM_DEBUG(dbgs() << "Backedge-taken count is not predictable\n");
328 return false;
329 }
330
331 // Evaluating in the trip count's type can not overflow here as the overflow
332 // checks are performed in checkOverflow, but are first tried to avoid by
333 // widening the IV.
334 const SCEV *SCEVTripCount =
335 SE->getTripCountFromExitCount(ExitCount: BackedgeTakenCount,
336 EvalTy: BackedgeTakenCount->getType(), L);
337
338 const SCEV *SCEVRHS = SE->getSCEV(V: RHS);
339 if (SCEVRHS == SCEVTripCount)
340 return setLoopComponents(TC&: RHS, TripCount, Increment, IterationInstructions);
341 ConstantInt *ConstantRHS = dyn_cast<ConstantInt>(Val: RHS);
342 if (ConstantRHS) {
343 const SCEV *BackedgeTCExt = nullptr;
344 if (IsWidened) {
345 const SCEV *SCEVTripCountExt;
346 // Find the extended backedge taken count and extended trip count using
347 // SCEV. One of these should now match the RHS of the compare.
348 BackedgeTCExt = SE->getZeroExtendExpr(Op: BackedgeTakenCount, Ty: RHS->getType());
349 SCEVTripCountExt = SE->getTripCountFromExitCount(ExitCount: BackedgeTCExt,
350 EvalTy: RHS->getType(), L);
351 if (SCEVRHS != BackedgeTCExt && SCEVRHS != SCEVTripCountExt) {
352 LLVM_DEBUG(dbgs() << "Could not find valid trip count\n");
353 return false;
354 }
355 }
356 // If the RHS of the compare is equal to the backedge taken count we need
357 // to add one to get the trip count.
358 if (SCEVRHS == BackedgeTCExt || SCEVRHS == BackedgeTakenCount) {
359 Value *NewRHS = ConstantInt::get(Context&: ConstantRHS->getContext(),
360 V: ConstantRHS->getValue() + 1);
361 return setLoopComponents(TC&: NewRHS, TripCount, Increment,
362 IterationInstructions);
363 }
364 return setLoopComponents(TC&: RHS, TripCount, Increment, IterationInstructions);
365 }
366 // If the RHS isn't a constant then check that the reason it doesn't match
367 // the SCEV trip count is because the RHS is a ZExt or SExt instruction
368 // (and take the trip count to be the RHS).
369 if (!IsWidened) {
370 LLVM_DEBUG(dbgs() << "Could not find valid trip count\n");
371 return false;
372 }
373 auto *TripCountInst = dyn_cast<Instruction>(Val: RHS);
374 if (!TripCountInst) {
375 LLVM_DEBUG(dbgs() << "Could not find valid trip count\n");
376 return false;
377 }
378 if ((!isa<ZExtInst>(Val: TripCountInst) && !isa<SExtInst>(Val: TripCountInst)) ||
379 SE->getSCEV(V: TripCountInst->getOperand(i: 0)) != SCEVTripCount) {
380 LLVM_DEBUG(dbgs() << "Could not find valid extended trip count\n");
381 return false;
382 }
383 return setLoopComponents(TC&: RHS, TripCount, Increment, IterationInstructions);
384}
385
386// Finds the induction variable, increment and trip count for a simple loop that
387// we can flatten.
388static bool findLoopComponents(
389 Loop *L, SmallPtrSetImpl<Instruction *> &IterationInstructions,
390 PHINode *&InductionPHI, Value *&TripCount, BinaryOperator *&Increment,
391 BranchInst *&BackBranch, ScalarEvolution *SE, bool IsWidened) {
392 LLVM_DEBUG(dbgs() << "Finding components of loop: " << L->getName() << "\n");
393
394 if (!L->isLoopSimplifyForm()) {
395 LLVM_DEBUG(dbgs() << "Loop is not in normal form\n");
396 return false;
397 }
398
399 // Currently, to simplify the implementation, the Loop induction variable must
400 // start at zero and increment with a step size of one.
401 if (!L->isCanonical(SE&: *SE)) {
402 LLVM_DEBUG(dbgs() << "Loop is not canonical\n");
403 return false;
404 }
405
406 // There must be exactly one exiting block, and it must be the same at the
407 // latch.
408 BasicBlock *Latch = L->getLoopLatch();
409 if (L->getExitingBlock() != Latch) {
410 LLVM_DEBUG(dbgs() << "Exiting and latch block are different\n");
411 return false;
412 }
413
414 // Find the induction PHI. If there is no induction PHI, we can't do the
415 // transformation. TODO: could other variables trigger this? Do we have to
416 // search for the best one?
417 InductionPHI = L->getInductionVariable(SE&: *SE);
418 if (!InductionPHI) {
419 LLVM_DEBUG(dbgs() << "Could not find induction PHI\n");
420 return false;
421 }
422 LLVM_DEBUG(dbgs() << "Found induction PHI: "; InductionPHI->dump());
423
424 bool ContinueOnTrue = L->contains(BB: Latch->getTerminator()->getSuccessor(Idx: 0));
425 auto IsValidPredicate = [&](ICmpInst::Predicate Pred) {
426 if (ContinueOnTrue)
427 return Pred == CmpInst::ICMP_NE || Pred == CmpInst::ICMP_ULT;
428 else
429 return Pred == CmpInst::ICMP_EQ;
430 };
431
432 // Find Compare and make sure it is valid. getLatchCmpInst checks that the
433 // back branch of the latch is conditional.
434 ICmpInst *Compare = L->getLatchCmpInst();
435 if (!Compare || !IsValidPredicate(Compare->getUnsignedPredicate()) ||
436 Compare->hasNUsesOrMore(N: 2)) {
437 LLVM_DEBUG(dbgs() << "Could not find valid comparison\n");
438 return false;
439 }
440 BackBranch = cast<BranchInst>(Val: Latch->getTerminator());
441 IterationInstructions.insert(Ptr: BackBranch);
442 LLVM_DEBUG(dbgs() << "Found back branch: "; BackBranch->dump());
443 IterationInstructions.insert(Ptr: Compare);
444 LLVM_DEBUG(dbgs() << "Found comparison: "; Compare->dump());
445
446 // Find increment and trip count.
447 // There are exactly 2 incoming values to the induction phi; one from the
448 // pre-header and one from the latch. The incoming latch value is the
449 // increment variable.
450 Increment =
451 cast<BinaryOperator>(Val: InductionPHI->getIncomingValueForBlock(BB: Latch));
452 if ((Compare->getOperand(i_nocapture: 0) != Increment || !Increment->hasNUses(N: 2)) &&
453 !Increment->hasNUses(N: 1)) {
454 LLVM_DEBUG(dbgs() << "Could not find valid increment\n");
455 return false;
456 }
457 // The trip count is the RHS of the compare. If this doesn't match the trip
458 // count computed by SCEV then this is because the trip count variable
459 // has been widened so the types don't match, or because it is a constant and
460 // another transformation has changed the compare (e.g. icmp ult %inc,
461 // tripcount -> icmp ult %j, tripcount-1), or both.
462 Value *RHS = Compare->getOperand(i_nocapture: 1);
463
464 return verifyTripCount(RHS, L, IterationInstructions, InductionPHI, TripCount,
465 Increment, BackBranch, SE, IsWidened);
466}
467
468static bool checkPHIs(FlattenInfo &FI, const TargetTransformInfo *TTI) {
469 // All PHIs in the inner and outer headers must either be:
470 // - The induction PHI, which we are going to rewrite as one induction in
471 // the new loop. This is already checked by findLoopComponents.
472 // - An outer header PHI with all incoming values from outside the loop.
473 // LoopSimplify guarantees we have a pre-header, so we don't need to
474 // worry about that here.
475 // - Pairs of PHIs in the inner and outer headers, which implement a
476 // loop-carried dependency that will still be valid in the new loop. To
477 // be valid, this variable must be modified only in the inner loop.
478
479 // The set of PHI nodes in the outer loop header that we know will still be
480 // valid after the transformation. These will not need to be modified (with
481 // the exception of the induction variable), but we do need to check that
482 // there are no unsafe PHI nodes.
483 SmallPtrSet<PHINode *, 4> SafeOuterPHIs;
484 SafeOuterPHIs.insert(Ptr: FI.OuterInductionPHI);
485
486 // Check that all PHI nodes in the inner loop header match one of the valid
487 // patterns.
488 for (PHINode &InnerPHI : FI.InnerLoop->getHeader()->phis()) {
489 // The induction PHIs break these rules, and that's OK because we treat
490 // them specially when doing the transformation.
491 if (&InnerPHI == FI.InnerInductionPHI)
492 continue;
493 if (FI.isNarrowInductionPhi(Phi: &InnerPHI))
494 continue;
495
496 // Each inner loop PHI node must have two incoming values/blocks - one
497 // from the pre-header, and one from the latch.
498 assert(InnerPHI.getNumIncomingValues() == 2);
499 Value *PreHeaderValue =
500 InnerPHI.getIncomingValueForBlock(BB: FI.InnerLoop->getLoopPreheader());
501 Value *LatchValue =
502 InnerPHI.getIncomingValueForBlock(BB: FI.InnerLoop->getLoopLatch());
503
504 // The incoming value from the outer loop must be the PHI node in the
505 // outer loop header, with no modifications made in the top of the outer
506 // loop.
507 PHINode *OuterPHI = dyn_cast<PHINode>(Val: PreHeaderValue);
508 if (!OuterPHI || OuterPHI->getParent() != FI.OuterLoop->getHeader()) {
509 LLVM_DEBUG(dbgs() << "value modified in top of outer loop\n");
510 return false;
511 }
512
513 // The other incoming value must come from the inner loop, without any
514 // modifications in the tail end of the outer loop. We are in LCSSA form,
515 // so this will actually be a PHI in the inner loop's exit block, which
516 // only uses values from inside the inner loop.
517 PHINode *LCSSAPHI = dyn_cast<PHINode>(
518 Val: OuterPHI->getIncomingValueForBlock(BB: FI.OuterLoop->getLoopLatch()));
519 if (!LCSSAPHI) {
520 LLVM_DEBUG(dbgs() << "could not find LCSSA PHI\n");
521 return false;
522 }
523
524 // The value used by the LCSSA PHI must be the same one that the inner
525 // loop's PHI uses.
526 if (LCSSAPHI->hasConstantValue() != LatchValue) {
527 LLVM_DEBUG(
528 dbgs() << "LCSSA PHI incoming value does not match latch value\n");
529 return false;
530 }
531
532 LLVM_DEBUG(dbgs() << "PHI pair is safe:\n");
533 LLVM_DEBUG(dbgs() << " Inner: "; InnerPHI.dump());
534 LLVM_DEBUG(dbgs() << " Outer: "; OuterPHI->dump());
535 SafeOuterPHIs.insert(Ptr: OuterPHI);
536 FI.InnerPHIsToTransform.insert(Ptr: &InnerPHI);
537 }
538
539 for (PHINode &OuterPHI : FI.OuterLoop->getHeader()->phis()) {
540 if (FI.isNarrowInductionPhi(Phi: &OuterPHI))
541 continue;
542 if (!SafeOuterPHIs.count(Ptr: &OuterPHI)) {
543 LLVM_DEBUG(dbgs() << "found unsafe PHI in outer loop: "; OuterPHI.dump());
544 return false;
545 }
546 }
547
548 LLVM_DEBUG(dbgs() << "checkPHIs: OK\n");
549 return true;
550}
551
552static bool
553checkOuterLoopInsts(FlattenInfo &FI,
554 SmallPtrSetImpl<Instruction *> &IterationInstructions,
555 const TargetTransformInfo *TTI) {
556 // Check for instructions in the outer but not inner loop. If any of these
557 // have side-effects then this transformation is not legal, and if there is
558 // a significant amount of code here which can't be optimised out that it's
559 // not profitable (as these instructions would get executed for each
560 // iteration of the inner loop).
561 InstructionCost RepeatedInstrCost = 0;
562 for (auto *B : FI.OuterLoop->getBlocks()) {
563 if (FI.InnerLoop->contains(BB: B))
564 continue;
565
566 for (auto &I : *B) {
567 if (!isa<PHINode>(Val: &I) && !I.isTerminator() &&
568 !isSafeToSpeculativelyExecute(I: &I)) {
569 LLVM_DEBUG(dbgs() << "Cannot flatten because instruction may have "
570 "side effects: ";
571 I.dump());
572 return false;
573 }
574 // The execution count of the outer loop's iteration instructions
575 // (increment, compare and branch) will be increased, but the
576 // equivalent instructions will be removed from the inner loop, so
577 // they make a net difference of zero.
578 if (IterationInstructions.count(Ptr: &I))
579 continue;
580 // The unconditional branch to the inner loop's header will turn into
581 // a fall-through, so adds no cost.
582 BranchInst *Br = dyn_cast<BranchInst>(Val: &I);
583 if (Br && Br->isUnconditional() &&
584 Br->getSuccessor(i: 0) == FI.InnerLoop->getHeader())
585 continue;
586 // Multiplies of the outer iteration variable and inner iteration
587 // count will be optimised out.
588 if (match(V: &I, P: m_c_Mul(L: m_Specific(V: FI.OuterInductionPHI),
589 R: m_Specific(V: FI.InnerTripCount))))
590 continue;
591 InstructionCost Cost =
592 TTI->getInstructionCost(U: &I, CostKind: TargetTransformInfo::TCK_SizeAndLatency);
593 LLVM_DEBUG(dbgs() << "Cost " << Cost << ": "; I.dump());
594 RepeatedInstrCost += Cost;
595 }
596 }
597
598 LLVM_DEBUG(dbgs() << "Cost of instructions that will be repeated: "
599 << RepeatedInstrCost << "\n");
600 // Bail out if flattening the loops would cause instructions in the outer
601 // loop but not in the inner loop to be executed extra times.
602 if (RepeatedInstrCost > RepeatedInstructionThreshold) {
603 LLVM_DEBUG(dbgs() << "checkOuterLoopInsts: not profitable, bailing.\n");
604 return false;
605 }
606
607 LLVM_DEBUG(dbgs() << "checkOuterLoopInsts: OK\n");
608 return true;
609}
610
611
612
613// We require all uses of both induction variables to match this pattern:
614//
615// (OuterPHI * InnerTripCount) + InnerPHI
616//
617// Any uses of the induction variables not matching that pattern would
618// require a div/mod to reconstruct in the flattened loop, so the
619// transformation wouldn't be profitable.
620static bool checkIVUsers(FlattenInfo &FI) {
621 // Check that all uses of the inner loop's induction variable match the
622 // expected pattern, recording the uses of the outer IV.
623 SmallPtrSet<Value *, 4> ValidOuterPHIUses;
624 if (!FI.checkInnerInductionPhiUsers(ValidOuterPHIUses))
625 return false;
626
627 // Check that there are no uses of the outer IV other than the ones found
628 // as part of the pattern above.
629 if (!FI.checkOuterInductionPhiUsers(ValidOuterPHIUses))
630 return false;
631
632 LLVM_DEBUG(dbgs() << "checkIVUsers: OK\n";
633 dbgs() << "Found " << FI.LinearIVUses.size()
634 << " value(s) that can be replaced:\n";
635 for (Value *V : FI.LinearIVUses) {
636 dbgs() << " ";
637 V->dump();
638 });
639 return true;
640}
641
642// Return an OverflowResult dependant on if overflow of the multiplication of
643// InnerTripCount and OuterTripCount can be assumed not to happen.
644static OverflowResult checkOverflow(FlattenInfo &FI, DominatorTree *DT,
645 AssumptionCache *AC) {
646 Function *F = FI.OuterLoop->getHeader()->getParent();
647 const DataLayout &DL = F->getDataLayout();
648
649 // For debugging/testing.
650 if (AssumeNoOverflow)
651 return OverflowResult::NeverOverflows;
652
653 // Check if the multiply could not overflow due to known ranges of the
654 // input values.
655 OverflowResult OR = computeOverflowForUnsignedMul(
656 LHS: FI.InnerTripCount, RHS: FI.OuterTripCount,
657 SQ: SimplifyQuery(DL, DT, AC,
658 FI.OuterLoop->getLoopPreheader()->getTerminator()));
659 if (OR != OverflowResult::MayOverflow)
660 return OR;
661
662 auto CheckGEP = [&](GetElementPtrInst *GEP, Value *GEPOperand) {
663 for (Value *GEPUser : GEP->users()) {
664 auto *GEPUserInst = cast<Instruction>(Val: GEPUser);
665 if (!isa<LoadInst>(Val: GEPUserInst) &&
666 !(isa<StoreInst>(Val: GEPUserInst) && GEP == GEPUserInst->getOperand(i: 1)))
667 continue;
668 if (!isGuaranteedToExecuteForEveryIteration(I: GEPUserInst, L: FI.InnerLoop))
669 continue;
670 // The IV is used as the operand of a GEP which dominates the loop
671 // latch, and the IV is at least as wide as the address space of the
672 // GEP. In this case, the GEP would wrap around the address space
673 // before the IV increment wraps, which would be UB.
674 if (GEP->isInBounds() &&
675 GEPOperand->getType()->getIntegerBitWidth() >=
676 DL.getPointerTypeSizeInBits(GEP->getType())) {
677 LLVM_DEBUG(
678 dbgs() << "use of linear IV would be UB if overflow occurred: ";
679 GEP->dump());
680 return true;
681 }
682 }
683 return false;
684 };
685
686 // Check if any IV user is, or is used by, a GEP that would cause UB if the
687 // multiply overflows.
688 for (Value *V : FI.LinearIVUses) {
689 if (auto *GEP = dyn_cast<GetElementPtrInst>(Val: V))
690 if (GEP->getNumIndices() == 1 && CheckGEP(GEP, GEP->getOperand(i_nocapture: 1)))
691 return OverflowResult::NeverOverflows;
692 for (Value *U : V->users())
693 if (auto *GEP = dyn_cast<GetElementPtrInst>(Val: U))
694 if (CheckGEP(GEP, V))
695 return OverflowResult::NeverOverflows;
696 }
697
698 return OverflowResult::MayOverflow;
699}
700
701static bool CanFlattenLoopPair(FlattenInfo &FI, DominatorTree *DT, LoopInfo *LI,
702 ScalarEvolution *SE, AssumptionCache *AC,
703 const TargetTransformInfo *TTI) {
704 SmallPtrSet<Instruction *, 8> IterationInstructions;
705 if (!findLoopComponents(L: FI.InnerLoop, IterationInstructions,
706 InductionPHI&: FI.InnerInductionPHI, TripCount&: FI.InnerTripCount,
707 Increment&: FI.InnerIncrement, BackBranch&: FI.InnerBranch, SE, IsWidened: FI.Widened))
708 return false;
709 if (!findLoopComponents(L: FI.OuterLoop, IterationInstructions,
710 InductionPHI&: FI.OuterInductionPHI, TripCount&: FI.OuterTripCount,
711 Increment&: FI.OuterIncrement, BackBranch&: FI.OuterBranch, SE, IsWidened: FI.Widened))
712 return false;
713
714 // Both of the loop trip count values must be invariant in the outer loop
715 // (non-instructions are all inherently invariant).
716 if (!FI.OuterLoop->isLoopInvariant(V: FI.InnerTripCount)) {
717 LLVM_DEBUG(dbgs() << "inner loop trip count not invariant\n");
718 return false;
719 }
720 if (!FI.OuterLoop->isLoopInvariant(V: FI.OuterTripCount)) {
721 LLVM_DEBUG(dbgs() << "outer loop trip count not invariant\n");
722 return false;
723 }
724
725 if (!checkPHIs(FI, TTI))
726 return false;
727
728 // FIXME: it should be possible to handle different types correctly.
729 if (FI.InnerInductionPHI->getType() != FI.OuterInductionPHI->getType())
730 return false;
731
732 if (!checkOuterLoopInsts(FI, IterationInstructions, TTI))
733 return false;
734
735 // Find the values in the loop that can be replaced with the linearized
736 // induction variable, and check that there are no other uses of the inner
737 // or outer induction variable. If there were, we could still do this
738 // transformation, but we'd have to insert a div/mod to calculate the
739 // original IVs, so it wouldn't be profitable.
740 if (!checkIVUsers(FI))
741 return false;
742
743 LLVM_DEBUG(dbgs() << "CanFlattenLoopPair: OK\n");
744 return true;
745}
746
747static bool DoFlattenLoopPair(FlattenInfo &FI, DominatorTree *DT, LoopInfo *LI,
748 ScalarEvolution *SE, AssumptionCache *AC,
749 const TargetTransformInfo *TTI, LPMUpdater *U,
750 MemorySSAUpdater *MSSAU) {
751 Function *F = FI.OuterLoop->getHeader()->getParent();
752 LLVM_DEBUG(dbgs() << "Checks all passed, doing the transformation\n");
753 {
754 using namespace ore;
755 OptimizationRemark Remark(DEBUG_TYPE, "Flattened", FI.InnerLoop->getStartLoc(),
756 FI.InnerLoop->getHeader());
757 OptimizationRemarkEmitter ORE(F);
758 Remark << "Flattened into outer loop";
759 ORE.emit(OptDiag&: Remark);
760 }
761
762 if (!FI.NewTripCount) {
763 FI.NewTripCount = BinaryOperator::CreateMul(
764 V1: FI.InnerTripCount, V2: FI.OuterTripCount, Name: "flatten.tripcount",
765 InsertBefore: FI.OuterLoop->getLoopPreheader()->getTerminator()->getIterator());
766 LLVM_DEBUG(dbgs() << "Created new trip count in preheader: ";
767 FI.NewTripCount->dump());
768 }
769
770 // Fix up PHI nodes that take values from the inner loop back-edge, which
771 // we are about to remove.
772 FI.InnerInductionPHI->removeIncomingValue(BB: FI.InnerLoop->getLoopLatch());
773
774 // The old Phi will be optimised away later, but for now we can't leave
775 // leave it in an invalid state, so are updating them too.
776 for (PHINode *PHI : FI.InnerPHIsToTransform)
777 PHI->removeIncomingValue(BB: FI.InnerLoop->getLoopLatch());
778
779 // Modify the trip count of the outer loop to be the product of the two
780 // trip counts.
781 cast<User>(Val: FI.OuterBranch->getCondition())->setOperand(i: 1, Val: FI.NewTripCount);
782
783 // Replace the inner loop backedge with an unconditional branch to the exit.
784 BasicBlock *InnerExitBlock = FI.InnerLoop->getExitBlock();
785 BasicBlock *InnerExitingBlock = FI.InnerLoop->getExitingBlock();
786 Instruction *Term = InnerExitingBlock->getTerminator();
787 Instruction *BI = BranchInst::Create(IfTrue: InnerExitBlock, InsertBefore: InnerExitingBlock);
788 BI->setDebugLoc(Term->getDebugLoc());
789 Term->eraseFromParent();
790
791 // Update the DomTree and MemorySSA.
792 DT->deleteEdge(From: InnerExitingBlock, To: FI.InnerLoop->getHeader());
793 if (MSSAU)
794 MSSAU->removeEdge(From: InnerExitingBlock, To: FI.InnerLoop->getHeader());
795
796 // Replace all uses of the polynomial calculated from the two induction
797 // variables with the one new one.
798 IRBuilder<> Builder(FI.OuterInductionPHI->getParent()->getTerminator());
799 for (Value *V : FI.LinearIVUses) {
800 Value *OuterValue = FI.OuterInductionPHI;
801 if (FI.Widened)
802 OuterValue = Builder.CreateTrunc(V: FI.OuterInductionPHI, DestTy: V->getType(),
803 Name: "flatten.trunciv");
804
805 if (auto *GEP = dyn_cast<GetElementPtrInst>(Val: V)) {
806 // Replace the GEP with one that uses OuterValue as the offset.
807 auto *InnerGEP = cast<GetElementPtrInst>(Val: GEP->getOperand(i_nocapture: 0));
808 Value *Base = InnerGEP->getOperand(i_nocapture: 0);
809 // When the base of the GEP doesn't dominate the outer induction phi then
810 // we need to insert the new GEP where the old GEP was.
811 if (!DT->dominates(Def: Base, User: &*Builder.GetInsertPoint()))
812 Builder.SetInsertPoint(cast<Instruction>(Val: V));
813 OuterValue =
814 Builder.CreateGEP(Ty: GEP->getSourceElementType(), Ptr: Base, IdxList: OuterValue,
815 Name: "flatten." + V->getName(),
816 NW: GEP->isInBounds() && InnerGEP->isInBounds());
817 }
818
819 LLVM_DEBUG(dbgs() << "Replacing: "; V->dump(); dbgs() << "with: ";
820 OuterValue->dump());
821 V->replaceAllUsesWith(V: OuterValue);
822 }
823
824 // Tell LoopInfo, SCEV and the pass manager that the inner loop has been
825 // deleted, and invalidate any outer loop information.
826 SE->forgetLoop(L: FI.OuterLoop);
827 SE->forgetBlockAndLoopDispositions();
828 if (U)
829 U->markLoopAsDeleted(L&: *FI.InnerLoop, Name: FI.InnerLoop->getName());
830 LI->erase(L: FI.InnerLoop);
831
832 // Increment statistic value.
833 NumFlattened++;
834
835 return true;
836}
837
838static bool CanWidenIV(FlattenInfo &FI, DominatorTree *DT, LoopInfo *LI,
839 ScalarEvolution *SE, AssumptionCache *AC,
840 const TargetTransformInfo *TTI) {
841 if (!WidenIV) {
842 LLVM_DEBUG(dbgs() << "Widening the IVs is disabled\n");
843 return false;
844 }
845
846 LLVM_DEBUG(dbgs() << "Try widening the IVs\n");
847 Module *M = FI.InnerLoop->getHeader()->getParent()->getParent();
848 auto &DL = M->getDataLayout();
849 auto *InnerType = FI.InnerInductionPHI->getType();
850 auto *OuterType = FI.OuterInductionPHI->getType();
851 unsigned MaxLegalSize = DL.getLargestLegalIntTypeSizeInBits();
852 auto *MaxLegalType = DL.getLargestLegalIntType(C&: M->getContext());
853
854 // If both induction types are less than the maximum legal integer width,
855 // promote both to the widest type available so we know calculating
856 // (OuterTripCount * InnerTripCount) as the new trip count is safe.
857 if (InnerType != OuterType ||
858 InnerType->getScalarSizeInBits() >= MaxLegalSize ||
859 MaxLegalType->getScalarSizeInBits() <
860 InnerType->getScalarSizeInBits() * 2) {
861 LLVM_DEBUG(dbgs() << "Can't widen the IV\n");
862 return false;
863 }
864
865 SCEVExpander Rewriter(*SE, DL, "loopflatten");
866 SmallVector<WeakTrackingVH, 4> DeadInsts;
867 unsigned ElimExt = 0;
868 unsigned Widened = 0;
869
870 auto CreateWideIV = [&](WideIVInfo WideIV, bool &Deleted) -> bool {
871 PHINode *WidePhi =
872 createWideIV(WI: WideIV, LI, SE, Rewriter, DT, DeadInsts, NumElimExt&: ElimExt, NumWidened&: Widened,
873 HasGuards: true /* HasGuards */, UsePostIncrementRanges: true /* UsePostIncrementRanges */);
874 if (!WidePhi)
875 return false;
876 LLVM_DEBUG(dbgs() << "Created wide phi: "; WidePhi->dump());
877 LLVM_DEBUG(dbgs() << "Deleting old phi: "; WideIV.NarrowIV->dump());
878 Deleted = RecursivelyDeleteDeadPHINode(PN: WideIV.NarrowIV);
879 return true;
880 };
881
882 bool Deleted;
883 if (!CreateWideIV({.NarrowIV: FI.InnerInductionPHI, .WidestNativeType: MaxLegalType, .IsSigned: false}, Deleted))
884 return false;
885 // Add the narrow phi to list, so that it will be adjusted later when the
886 // the transformation is performed.
887 if (!Deleted)
888 FI.InnerPHIsToTransform.insert(Ptr: FI.InnerInductionPHI);
889
890 if (!CreateWideIV({.NarrowIV: FI.OuterInductionPHI, .WidestNativeType: MaxLegalType, .IsSigned: false}, Deleted))
891 return false;
892
893 assert(Widened && "Widened IV expected");
894 FI.Widened = true;
895
896 // Save the old/narrow induction phis, which we need to ignore in CheckPHIs.
897 FI.NarrowInnerInductionPHI = FI.InnerInductionPHI;
898 FI.NarrowOuterInductionPHI = FI.OuterInductionPHI;
899
900 // After widening, rediscover all the loop components.
901 return CanFlattenLoopPair(FI, DT, LI, SE, AC, TTI);
902}
903
904static bool FlattenLoopPair(FlattenInfo &FI, DominatorTree *DT, LoopInfo *LI,
905 ScalarEvolution *SE, AssumptionCache *AC,
906 const TargetTransformInfo *TTI, LPMUpdater *U,
907 MemorySSAUpdater *MSSAU,
908 const LoopAccessInfo &LAI) {
909 LLVM_DEBUG(
910 dbgs() << "Loop flattening running on outer loop "
911 << FI.OuterLoop->getHeader()->getName() << " and inner loop "
912 << FI.InnerLoop->getHeader()->getName() << " in "
913 << FI.OuterLoop->getHeader()->getParent()->getName() << "\n");
914
915 if (!CanFlattenLoopPair(FI, DT, LI, SE, AC, TTI))
916 return false;
917
918 // Check if we can widen the induction variables to avoid overflow checks.
919 bool CanFlatten = CanWidenIV(FI, DT, LI, SE, AC, TTI);
920
921 // It can happen that after widening of the IV, flattening may not be
922 // possible/happening, e.g. when it is deemed unprofitable. So bail here if
923 // that is the case.
924 // TODO: IV widening without performing the actual flattening transformation
925 // is not ideal. While this codegen change should not matter much, it is an
926 // unnecessary change which is better to avoid. It's unlikely this happens
927 // often, because if it's unprofitibale after widening, it should be
928 // unprofitabe before widening as checked in the first round of checks. But
929 // 'RepeatedInstructionThreshold' is set to only 2, which can probably be
930 // relaxed. Because this is making a code change (the IV widening, but not
931 // the flattening), we return true here.
932 if (FI.Widened && !CanFlatten)
933 return true;
934
935 // If we have widened and can perform the transformation, do that here.
936 if (CanFlatten)
937 return DoFlattenLoopPair(FI, DT, LI, SE, AC, TTI, U, MSSAU);
938
939 // Otherwise, if we haven't widened the IV, check if the new iteration
940 // variable might overflow. In this case, we need to version the loop, and
941 // select the original version at runtime if the iteration space is too
942 // large.
943 OverflowResult OR = checkOverflow(FI, DT, AC);
944 if (OR == OverflowResult::AlwaysOverflowsHigh ||
945 OR == OverflowResult::AlwaysOverflowsLow) {
946 LLVM_DEBUG(dbgs() << "Multiply would always overflow, so not profitable\n");
947 return false;
948 } else if (OR == OverflowResult::MayOverflow) {
949 Module *M = FI.OuterLoop->getHeader()->getParent()->getParent();
950 const DataLayout &DL = M->getDataLayout();
951 if (!VersionLoops) {
952 LLVM_DEBUG(dbgs() << "Multiply might overflow, not flattening\n");
953 return false;
954 } else if (!DL.isLegalInteger(
955 Width: FI.OuterTripCount->getType()->getScalarSizeInBits())) {
956 // If the trip count type isn't legal then it won't be possible to check
957 // for overflow using only a single multiply instruction, so don't
958 // flatten.
959 LLVM_DEBUG(
960 dbgs() << "Can't check overflow efficiently, not flattening\n");
961 return false;
962 }
963 LLVM_DEBUG(dbgs() << "Multiply might overflow, versioning loop\n");
964
965 // Version the loop. The overflow check isn't a runtime pointer check, so we
966 // pass an empty list of runtime pointer checks, causing LoopVersioning to
967 // emit 'false' as the branch condition, and add our own check afterwards.
968 BasicBlock *CheckBlock = FI.OuterLoop->getLoopPreheader();
969 ArrayRef<RuntimePointerCheck> Checks(nullptr, nullptr);
970 LoopVersioning LVer(LAI, Checks, FI.OuterLoop, LI, DT, SE);
971 LVer.versionLoop();
972
973 // Check for overflow by calculating the new tripcount using
974 // umul_with_overflow and then checking if it overflowed.
975 BranchInst *Br = cast<BranchInst>(Val: CheckBlock->getTerminator());
976 assert(Br->isConditional() &&
977 "Expected LoopVersioning to generate a conditional branch");
978 assert(match(Br->getCondition(), m_Zero()) &&
979 "Expected branch condition to be false");
980 IRBuilder<> Builder(Br);
981 Value *Call = Builder.CreateIntrinsic(
982 ID: Intrinsic::umul_with_overflow, Types: FI.OuterTripCount->getType(),
983 Args: {FI.OuterTripCount, FI.InnerTripCount},
984 /*FMFSource=*/nullptr, Name: "flatten.mul");
985 FI.NewTripCount = Builder.CreateExtractValue(Agg: Call, Idxs: 0, Name: "flatten.tripcount");
986 Value *Overflow = Builder.CreateExtractValue(Agg: Call, Idxs: 1, Name: "flatten.overflow");
987 Br->setCondition(Overflow);
988 } else {
989 LLVM_DEBUG(dbgs() << "Multiply cannot overflow, modifying loop in-place\n");
990 }
991
992 return DoFlattenLoopPair(FI, DT, LI, SE, AC, TTI, U, MSSAU);
993}
994
995PreservedAnalyses LoopFlattenPass::run(LoopNest &LN, LoopAnalysisManager &LAM,
996 LoopStandardAnalysisResults &AR,
997 LPMUpdater &U) {
998
999 bool Changed = false;
1000
1001 std::optional<MemorySSAUpdater> MSSAU;
1002 if (AR.MSSA) {
1003 MSSAU = MemorySSAUpdater(AR.MSSA);
1004 if (VerifyMemorySSA)
1005 AR.MSSA->verifyMemorySSA();
1006 }
1007
1008 // The loop flattening pass requires loops to be
1009 // in simplified form, and also needs LCSSA. Running
1010 // this pass will simplify all loops that contain inner loops,
1011 // regardless of whether anything ends up being flattened.
1012 LoopAccessInfoManager LAIM(AR.SE, AR.AA, AR.DT, AR.LI, &AR.TTI, nullptr);
1013 for (Loop *InnerLoop : LN.getLoops()) {
1014 auto *OuterLoop = InnerLoop->getParentLoop();
1015 if (!OuterLoop)
1016 continue;
1017 FlattenInfo FI(OuterLoop, InnerLoop);
1018 Changed |=
1019 FlattenLoopPair(FI, DT: &AR.DT, LI: &AR.LI, SE: &AR.SE, AC: &AR.AC, TTI: &AR.TTI, U: &U,
1020 MSSAU: MSSAU ? &*MSSAU : nullptr, LAI: LAIM.getInfo(L&: *OuterLoop));
1021 }
1022
1023 if (!Changed)
1024 return PreservedAnalyses::all();
1025
1026 if (AR.MSSA && VerifyMemorySSA)
1027 AR.MSSA->verifyMemorySSA();
1028
1029 auto PA = getLoopPassPreservedAnalyses();
1030 if (AR.MSSA)
1031 PA.preserve<MemorySSAAnalysis>();
1032 return PA;
1033}
1034