1//===-- LoopPredication.cpp - Guard based loop predication 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// The LoopPredication pass tries to convert loop variant range checks to loop
10// invariant by widening checks across loop iterations. For example, it will
11// convert
12//
13// for (i = 0; i < n; i++) {
14// guard(i < len);
15// ...
16// }
17//
18// to
19//
20// for (i = 0; i < n; i++) {
21// guard(n - 1 < len);
22// ...
23// }
24//
25// After this transformation the condition of the guard is loop invariant, so
26// loop-unswitch can later unswitch the loop by this condition which basically
27// predicates the loop by the widened condition:
28//
29// if (n - 1 < len)
30// for (i = 0; i < n; i++) {
31// ...
32// }
33// else
34// deoptimize
35//
36// It's tempting to rely on SCEV here, but it has proven to be problematic.
37// Generally the facts SCEV provides about the increment step of add
38// recurrences are true if the backedge of the loop is taken, which implicitly
39// assumes that the guard doesn't fail. Using these facts to optimize the
40// guard results in a circular logic where the guard is optimized under the
41// assumption that it never fails.
42//
43// For example, in the loop below the induction variable will be marked as nuw
44// basing on the guard. Basing on nuw the guard predicate will be considered
45// monotonic. Given a monotonic condition it's tempting to replace the induction
46// variable in the condition with its value on the last iteration. But this
47// transformation is not correct, e.g. e = 4, b = 5 breaks the loop.
48//
49// for (int i = b; i != e; i++)
50// guard(i u< len)
51//
52// One of the ways to reason about this problem is to use an inductive proof
53// approach. Given the loop:
54//
55// if (B(0)) {
56// do {
57// I = PHI(0, I.INC)
58// I.INC = I + Step
59// guard(G(I));
60// } while (B(I));
61// }
62//
63// where B(x) and G(x) are predicates that map integers to booleans, we want a
64// loop invariant expression M such the following program has the same semantics
65// as the above:
66//
67// if (B(0)) {
68// do {
69// I = PHI(0, I.INC)
70// I.INC = I + Step
71// guard(G(0) && M);
72// } while (B(I));
73// }
74//
75// One solution for M is M = forall X . (G(X) && B(X)) => G(X + Step)
76//
77// Informal proof that the transformation above is correct:
78//
79// By the definition of guards we can rewrite the guard condition to:
80// G(I) && G(0) && M
81//
82// Let's prove that for each iteration of the loop:
83// G(0) && M => G(I)
84// And the condition above can be simplified to G(Start) && M.
85//
86// Induction base.
87// G(0) && M => G(0)
88//
89// Induction step. Assuming G(0) && M => G(I) on the subsequent
90// iteration:
91//
92// B(I) is true because it's the backedge condition.
93// G(I) is true because the backedge is guarded by this condition.
94//
95// So M = forall X . (G(X) && B(X)) => G(X + Step) implies G(I + Step).
96//
97// Note that we can use anything stronger than M, i.e. any condition which
98// implies M.
99//
100// When S = 1 (i.e. forward iterating loop), the transformation is supported
101// when:
102// * The loop has a single latch with the condition of the form:
103// B(X) = latchStart + X <pred> latchLimit,
104// where <pred> is u<, u<=, s<, or s<=.
105// * The guard condition is of the form
106// G(X) = guardStart + X u< guardLimit
107//
108// For the ult latch comparison case M is:
109// forall X . guardStart + X u< guardLimit && latchStart + X <u latchLimit =>
110// guardStart + X + 1 u< guardLimit
111//
112// The only way the antecedent can be true and the consequent can be false is
113// if
114// X == guardLimit - 1 - guardStart
115// (and guardLimit is non-zero, but we won't use this latter fact).
116// If X == guardLimit - 1 - guardStart then the second half of the antecedent is
117// latchStart + guardLimit - 1 - guardStart u< latchLimit
118// and its negation is
119// latchStart + guardLimit - 1 - guardStart u>= latchLimit
120//
121// In other words, if
122// latchLimit u<= latchStart + guardLimit - 1 - guardStart
123// then:
124// (the ranges below are written in ConstantRange notation, where [A, B) is the
125// set for (I = A; I != B; I++ /*maywrap*/) yield(I);)
126//
127// forall X . guardStart + X u< guardLimit &&
128// latchStart + X u< latchLimit =>
129// guardStart + X + 1 u< guardLimit
130// == forall X . guardStart + X u< guardLimit &&
131// latchStart + X u< latchStart + guardLimit - 1 - guardStart =>
132// guardStart + X + 1 u< guardLimit
133// == forall X . (guardStart + X) in [0, guardLimit) &&
134// (latchStart + X) in [0, latchStart + guardLimit - 1 - guardStart) =>
135// (guardStart + X + 1) in [0, guardLimit)
136// == forall X . X in [-guardStart, guardLimit - guardStart) &&
137// X in [-latchStart, guardLimit - 1 - guardStart) =>
138// X in [-guardStart - 1, guardLimit - guardStart - 1)
139// == true
140//
141// So the widened condition is:
142// guardStart u< guardLimit &&
143// latchStart + guardLimit - 1 - guardStart u>= latchLimit
144// Similarly for ule condition the widened condition is:
145// guardStart u< guardLimit &&
146// latchStart + guardLimit - 1 - guardStart u> latchLimit
147// For slt condition the widened condition is:
148// guardStart u< guardLimit &&
149// latchStart + guardLimit - 1 - guardStart s>= latchLimit
150// For sle condition the widened condition is:
151// guardStart u< guardLimit &&
152// latchStart + guardLimit - 1 - guardStart s> latchLimit
153//
154// When S = -1 (i.e. reverse iterating loop), the transformation is supported
155// when:
156// * The loop has a single latch with the condition of the form:
157// B(X) = X <pred> latchLimit, where <pred> is u>, u>=, s>, or s>=.
158// * The guard condition is of the form
159// G(X) = X - 1 u< guardLimit
160//
161// For the ugt latch comparison case M is:
162// forall X. X-1 u< guardLimit and X u> latchLimit => X-2 u< guardLimit
163//
164// The only way the antecedent can be true and the consequent can be false is if
165// X == 1.
166// If X == 1 then the second half of the antecedent is
167// 1 u> latchLimit, and its negation is latchLimit u>= 1.
168//
169// So the widened condition is:
170// guardStart u< guardLimit && latchLimit u>= 1.
171// Similarly for sgt condition the widened condition is:
172// guardStart u< guardLimit && latchLimit s>= 1.
173// For uge condition the widened condition is:
174// guardStart u< guardLimit && latchLimit u> 1.
175// For sge condition the widened condition is:
176// guardStart u< guardLimit && latchLimit s> 1.
177//===----------------------------------------------------------------------===//
178
179#include "llvm/Transforms/Scalar/LoopPredication.h"
180#include "llvm/ADT/Statistic.h"
181#include "llvm/Analysis/AliasAnalysis.h"
182#include "llvm/Analysis/BranchProbabilityInfo.h"
183#include "llvm/Analysis/GuardUtils.h"
184#include "llvm/Analysis/LoopInfo.h"
185#include "llvm/Analysis/LoopPass.h"
186#include "llvm/Analysis/MemorySSA.h"
187#include "llvm/Analysis/MemorySSAUpdater.h"
188#include "llvm/Analysis/ScalarEvolution.h"
189#include "llvm/Analysis/ScalarEvolutionExpressions.h"
190#include "llvm/IR/Function.h"
191#include "llvm/IR/IntrinsicInst.h"
192#include "llvm/IR/Module.h"
193#include "llvm/IR/PatternMatch.h"
194#include "llvm/IR/ProfDataUtils.h"
195#include "llvm/InitializePasses.h"
196#include "llvm/Pass.h"
197#include "llvm/Support/CommandLine.h"
198#include "llvm/Support/Debug.h"
199#include "llvm/Transforms/Scalar.h"
200#include "llvm/Transforms/Utils/GuardUtils.h"
201#include "llvm/Transforms/Utils/Local.h"
202#include "llvm/Transforms/Utils/LoopUtils.h"
203#include "llvm/Transforms/Utils/ScalarEvolutionExpander.h"
204#include <optional>
205
206#define DEBUG_TYPE "loop-predication"
207
208STATISTIC(TotalConsidered, "Number of guards considered");
209STATISTIC(TotalWidened, "Number of checks widened");
210
211using namespace llvm;
212
213static cl::opt<bool> EnableIVTruncation("loop-predication-enable-iv-truncation",
214 cl::Hidden, cl::init(Val: true));
215
216static cl::opt<bool> EnableCountDownLoop("loop-predication-enable-count-down-loop",
217 cl::Hidden, cl::init(Val: true));
218
219static cl::opt<bool>
220 SkipProfitabilityChecks("loop-predication-skip-profitability-checks",
221 cl::Hidden, cl::init(Val: false));
222
223// This is the scale factor for the latch probability. We use this during
224// profitability analysis to find other exiting blocks that have a much higher
225// probability of exiting the loop instead of loop exiting via latch.
226// This value should be greater than 1 for a sane profitability check.
227static cl::opt<float> LatchExitProbabilityScale(
228 "loop-predication-latch-probability-scale", cl::Hidden, cl::init(Val: 2.0),
229 cl::desc("scale factor for the latch probability. Value should be greater "
230 "than 1. Lower values are ignored"));
231
232static cl::opt<bool> PredicateWidenableBranchGuards(
233 "loop-predication-predicate-widenable-branches-to-deopt", cl::Hidden,
234 cl::desc("Whether or not we should predicate guards "
235 "expressed as widenable branches to deoptimize blocks"),
236 cl::init(Val: true));
237
238static cl::opt<bool> InsertAssumesOfPredicatedGuardsConditions(
239 "loop-predication-insert-assumes-of-predicated-guards-conditions",
240 cl::Hidden,
241 cl::desc("Whether or not we should insert assumes of conditions of "
242 "predicated guards"),
243 cl::init(Val: true));
244
245namespace {
246/// Represents an induction variable check:
247/// icmp Pred, <induction variable>, <loop invariant limit>
248struct LoopICmp {
249 ICmpInst::Predicate Pred;
250 const SCEVAddRecExpr *IV;
251 const SCEV *Limit;
252 LoopICmp(ICmpInst::Predicate Pred, const SCEVAddRecExpr *IV,
253 const SCEV *Limit)
254 : Pred(Pred), IV(IV), Limit(Limit) {}
255 LoopICmp() = default;
256 void dump() {
257 dbgs() << "LoopICmp Pred = " << Pred << ", IV = " << *IV
258 << ", Limit = " << *Limit << "\n";
259 }
260};
261
262class LoopPredication {
263 AliasAnalysis *AA;
264 DominatorTree *DT;
265 ScalarEvolution *SE;
266 LoopInfo *LI;
267 MemorySSAUpdater *MSSAU;
268
269 Loop *L;
270 const DataLayout *DL;
271 BasicBlock *Preheader;
272 LoopICmp LatchCheck;
273
274 bool isSupportedStep(const SCEV* Step);
275 std::optional<LoopICmp> parseLoopICmp(ICmpInst *ICI);
276 std::optional<LoopICmp> parseLoopLatchICmp();
277
278 /// Return an insertion point suitable for inserting a safe to speculate
279 /// instruction whose only user will be 'User' which has operands 'Ops'. A
280 /// trivial result would be the at the User itself, but we try to return a
281 /// loop invariant location if possible.
282 Instruction *findInsertPt(Instruction *User, ArrayRef<Value*> Ops);
283 /// Same as above, *except* that this uses the SCEV definition of invariant
284 /// which is that an expression *can be made* invariant via SCEVExpander.
285 /// Thus, this version is only suitable for finding an insert point to be
286 /// passed to SCEVExpander!
287 Instruction *findInsertPt(const SCEVExpander &Expander, Instruction *User,
288 ArrayRef<const SCEV *> Ops);
289
290 /// Return true if the value is known to produce a single fixed value across
291 /// all iterations on which it executes. Note that this does not imply
292 /// speculation safety. That must be established separately.
293 bool isLoopInvariantValue(const SCEV* S);
294
295 Value *expandCheck(SCEVExpander &Expander, Instruction *Guard,
296 ICmpInst::Predicate Pred, const SCEV *LHS,
297 const SCEV *RHS);
298
299 std::optional<Value *> widenICmpRangeCheck(ICmpInst *ICI,
300 SCEVExpander &Expander,
301 Instruction *Guard);
302 std::optional<Value *>
303 widenICmpRangeCheckIncrementingLoop(LoopICmp LatchCheck, LoopICmp RangeCheck,
304 SCEVExpander &Expander,
305 Instruction *Guard);
306 std::optional<Value *>
307 widenICmpRangeCheckDecrementingLoop(LoopICmp LatchCheck, LoopICmp RangeCheck,
308 SCEVExpander &Expander,
309 Instruction *Guard);
310 void widenChecks(SmallVectorImpl<Value *> &Checks,
311 SmallVectorImpl<Value *> &WidenedChecks,
312 SCEVExpander &Expander, Instruction *Guard);
313 bool widenGuardConditions(IntrinsicInst *II, SCEVExpander &Expander);
314 bool widenWidenableBranchGuardConditions(BranchInst *Guard, SCEVExpander &Expander);
315 // If the loop always exits through another block in the loop, we should not
316 // predicate based on the latch check. For example, the latch check can be a
317 // very coarse grained check and there can be more fine grained exit checks
318 // within the loop.
319 bool isLoopProfitableToPredicate();
320
321 bool predicateLoopExits(Loop *L, SCEVExpander &Rewriter);
322
323public:
324 LoopPredication(AliasAnalysis *AA, DominatorTree *DT, ScalarEvolution *SE,
325 LoopInfo *LI, MemorySSAUpdater *MSSAU)
326 : AA(AA), DT(DT), SE(SE), LI(LI), MSSAU(MSSAU){};
327 bool runOnLoop(Loop *L);
328};
329
330} // end namespace
331
332PreservedAnalyses LoopPredicationPass::run(Loop &L, LoopAnalysisManager &AM,
333 LoopStandardAnalysisResults &AR,
334 LPMUpdater &U) {
335 std::unique_ptr<MemorySSAUpdater> MSSAU;
336 if (AR.MSSA)
337 MSSAU = std::make_unique<MemorySSAUpdater>(args&: AR.MSSA);
338 LoopPredication LP(&AR.AA, &AR.DT, &AR.SE, &AR.LI,
339 MSSAU ? MSSAU.get() : nullptr);
340 if (!LP.runOnLoop(L: &L))
341 return PreservedAnalyses::all();
342
343 auto PA = getLoopPassPreservedAnalyses();
344 if (AR.MSSA)
345 PA.preserve<MemorySSAAnalysis>();
346 return PA;
347}
348
349std::optional<LoopICmp> LoopPredication::parseLoopICmp(ICmpInst *ICI) {
350 auto Pred = ICI->getPredicate();
351 auto *LHS = ICI->getOperand(i_nocapture: 0);
352 auto *RHS = ICI->getOperand(i_nocapture: 1);
353
354 const SCEV *LHSS = SE->getSCEV(V: LHS);
355 if (isa<SCEVCouldNotCompute>(Val: LHSS))
356 return std::nullopt;
357 const SCEV *RHSS = SE->getSCEV(V: RHS);
358 if (isa<SCEVCouldNotCompute>(Val: RHSS))
359 return std::nullopt;
360
361 // Canonicalize RHS to be loop invariant bound, LHS - a loop computable IV
362 if (SE->isLoopInvariant(S: LHSS, L)) {
363 std::swap(a&: LHS, b&: RHS);
364 std::swap(a&: LHSS, b&: RHSS);
365 Pred = ICmpInst::getSwappedPredicate(pred: Pred);
366 }
367
368 const SCEVAddRecExpr *AR = dyn_cast<SCEVAddRecExpr>(Val: LHSS);
369 if (!AR || AR->getLoop() != L)
370 return std::nullopt;
371
372 return LoopICmp(Pred, AR, RHSS);
373}
374
375Value *LoopPredication::expandCheck(SCEVExpander &Expander,
376 Instruction *Guard,
377 ICmpInst::Predicate Pred, const SCEV *LHS,
378 const SCEV *RHS) {
379 Type *Ty = LHS->getType();
380 assert(Ty == RHS->getType() && "expandCheck operands have different types?");
381
382 if (SE->isLoopInvariant(S: LHS, L) && SE->isLoopInvariant(S: RHS, L)) {
383 IRBuilder<> Builder(Guard);
384 if (SE->isLoopEntryGuardedByCond(L, Pred, LHS, RHS))
385 return Builder.getTrue();
386 if (SE->isLoopEntryGuardedByCond(L, Pred: ICmpInst::getInversePredicate(pred: Pred),
387 LHS, RHS))
388 return Builder.getFalse();
389 }
390
391 Value *LHSV =
392 Expander.expandCodeFor(SH: LHS, Ty, I: findInsertPt(Expander, User: Guard, Ops: {LHS}));
393 Value *RHSV =
394 Expander.expandCodeFor(SH: RHS, Ty, I: findInsertPt(Expander, User: Guard, Ops: {RHS}));
395 IRBuilder<> Builder(findInsertPt(User: Guard, Ops: {LHSV, RHSV}));
396 return Builder.CreateICmp(P: Pred, LHS: LHSV, RHS: RHSV);
397}
398
399// Returns true if its safe to truncate the IV to RangeCheckType.
400// When the IV type is wider than the range operand type, we can still do loop
401// predication, by generating SCEVs for the range and latch that are of the
402// same type. We achieve this by generating a SCEV truncate expression for the
403// latch IV. This is done iff truncation of the IV is a safe operation,
404// without loss of information.
405// Another way to achieve this is by generating a wider type SCEV for the
406// range check operand, however, this needs a more involved check that
407// operands do not overflow. This can lead to loss of information when the
408// range operand is of the form: add i32 %offset, %iv. We need to prove that
409// sext(x + y) is same as sext(x) + sext(y).
410// This function returns true if we can safely represent the IV type in
411// the RangeCheckType without loss of information.
412static bool isSafeToTruncateWideIVType(const DataLayout &DL,
413 ScalarEvolution &SE,
414 const LoopICmp LatchCheck,
415 Type *RangeCheckType) {
416 if (!EnableIVTruncation)
417 return false;
418 assert(DL.getTypeSizeInBits(LatchCheck.IV->getType()).getFixedValue() >
419 DL.getTypeSizeInBits(RangeCheckType).getFixedValue() &&
420 "Expected latch check IV type to be larger than range check operand "
421 "type!");
422 // The start and end values of the IV should be known. This is to guarantee
423 // that truncating the wide type will not lose information.
424 auto *Limit = dyn_cast<SCEVConstant>(Val: LatchCheck.Limit);
425 auto *Start = dyn_cast<SCEVConstant>(Val: LatchCheck.IV->getStart());
426 if (!Limit || !Start)
427 return false;
428 // This check makes sure that the IV does not change sign during loop
429 // iterations. Consider latchType = i64, LatchStart = 5, Pred = ICMP_SGE,
430 // LatchEnd = 2, rangeCheckType = i32. If it's not a monotonic predicate, the
431 // IV wraps around, and the truncation of the IV would lose the range of
432 // iterations between 2^32 and 2^64.
433 if (!SE.getMonotonicPredicateType(LHS: LatchCheck.IV, Pred: LatchCheck.Pred))
434 return false;
435 // The active bits should be less than the bits in the RangeCheckType. This
436 // guarantees that truncating the latch check to RangeCheckType is a safe
437 // operation.
438 auto RangeCheckTypeBitSize =
439 DL.getTypeSizeInBits(Ty: RangeCheckType).getFixedValue();
440 return Start->getAPInt().getActiveBits() < RangeCheckTypeBitSize &&
441 Limit->getAPInt().getActiveBits() < RangeCheckTypeBitSize;
442}
443
444
445// Return an LoopICmp describing a latch check equivlent to LatchCheck but with
446// the requested type if safe to do so. May involve the use of a new IV.
447static std::optional<LoopICmp> generateLoopLatchCheck(const DataLayout &DL,
448 ScalarEvolution &SE,
449 const LoopICmp LatchCheck,
450 Type *RangeCheckType) {
451
452 auto *LatchType = LatchCheck.IV->getType();
453 if (RangeCheckType == LatchType)
454 return LatchCheck;
455 // For now, bail out if latch type is narrower than range type.
456 if (DL.getTypeSizeInBits(Ty: LatchType).getFixedValue() <
457 DL.getTypeSizeInBits(Ty: RangeCheckType).getFixedValue())
458 return std::nullopt;
459 if (!isSafeToTruncateWideIVType(DL, SE, LatchCheck, RangeCheckType))
460 return std::nullopt;
461 // We can now safely identify the truncated version of the IV and limit for
462 // RangeCheckType.
463 LoopICmp NewLatchCheck;
464 NewLatchCheck.Pred = LatchCheck.Pred;
465 NewLatchCheck.IV = dyn_cast<SCEVAddRecExpr>(
466 Val: SE.getTruncateExpr(Op: LatchCheck.IV, Ty: RangeCheckType));
467 if (!NewLatchCheck.IV)
468 return std::nullopt;
469 NewLatchCheck.Limit = SE.getTruncateExpr(Op: LatchCheck.Limit, Ty: RangeCheckType);
470 LLVM_DEBUG(dbgs() << "IV of type: " << *LatchType
471 << "can be represented as range check type:"
472 << *RangeCheckType << "\n");
473 LLVM_DEBUG(dbgs() << "LatchCheck.IV: " << *NewLatchCheck.IV << "\n");
474 LLVM_DEBUG(dbgs() << "LatchCheck.Limit: " << *NewLatchCheck.Limit << "\n");
475 return NewLatchCheck;
476}
477
478bool LoopPredication::isSupportedStep(const SCEV* Step) {
479 return Step->isOne() || (Step->isAllOnesValue() && EnableCountDownLoop);
480}
481
482Instruction *LoopPredication::findInsertPt(Instruction *Use,
483 ArrayRef<Value*> Ops) {
484 for (Value *Op : Ops)
485 if (!L->isLoopInvariant(V: Op))
486 return Use;
487 return Preheader->getTerminator();
488}
489
490Instruction *LoopPredication::findInsertPt(const SCEVExpander &Expander,
491 Instruction *Use,
492 ArrayRef<const SCEV *> Ops) {
493 // Subtlety: SCEV considers things to be invariant if the value produced is
494 // the same across iterations. This is not the same as being able to
495 // evaluate outside the loop, which is what we actually need here.
496 for (const SCEV *Op : Ops)
497 if (!SE->isLoopInvariant(S: Op, L) ||
498 !Expander.isSafeToExpandAt(S: Op, InsertionPoint: Preheader->getTerminator()))
499 return Use;
500 return Preheader->getTerminator();
501}
502
503bool LoopPredication::isLoopInvariantValue(const SCEV* S) {
504 // Handling expressions which produce invariant results, but *haven't* yet
505 // been removed from the loop serves two important purposes.
506 // 1) Most importantly, it resolves a pass ordering cycle which would
507 // otherwise need us to iteration licm, loop-predication, and either
508 // loop-unswitch or loop-peeling to make progress on examples with lots of
509 // predicable range checks in a row. (Since, in the general case, we can't
510 // hoist the length checks until the dominating checks have been discharged
511 // as we can't prove doing so is safe.)
512 // 2) As a nice side effect, this exposes the value of peeling or unswitching
513 // much more obviously in the IR. Otherwise, the cost modeling for other
514 // transforms would end up needing to duplicate all of this logic to model a
515 // check which becomes predictable based on a modeled peel or unswitch.
516 //
517 // The cost of doing so in the worst case is an extra fill from the stack in
518 // the loop to materialize the loop invariant test value instead of checking
519 // against the original IV which is presumable in a register inside the loop.
520 // Such cases are presumably rare, and hint at missing oppurtunities for
521 // other passes.
522
523 if (SE->isLoopInvariant(S, L))
524 // Note: This the SCEV variant, so the original Value* may be within the
525 // loop even though SCEV has proven it is loop invariant.
526 return true;
527
528 // Handle a particular important case which SCEV doesn't yet know about which
529 // shows up in range checks on arrays with immutable lengths.
530 // TODO: This should be sunk inside SCEV.
531 if (const SCEVUnknown *U = dyn_cast<SCEVUnknown>(Val: S))
532 if (const auto *LI = dyn_cast<LoadInst>(Val: U->getValue()))
533 if (LI->isUnordered() && L->hasLoopInvariantOperands(I: LI))
534 if (!isModSet(MRI: AA->getModRefInfoMask(P: LI->getOperand(i_nocapture: 0))) ||
535 LI->hasMetadata(KindID: LLVMContext::MD_invariant_load))
536 return true;
537 return false;
538}
539
540std::optional<Value *> LoopPredication::widenICmpRangeCheckIncrementingLoop(
541 LoopICmp LatchCheck, LoopICmp RangeCheck, SCEVExpander &Expander,
542 Instruction *Guard) {
543 auto *Ty = RangeCheck.IV->getType();
544 // Generate the widened condition for the forward loop:
545 // guardStart u< guardLimit &&
546 // latchLimit <pred> guardLimit - 1 - guardStart + latchStart
547 // where <pred> depends on the latch condition predicate. See the file
548 // header comment for the reasoning.
549 // guardLimit - guardStart + latchStart - 1
550 const SCEV *GuardStart = RangeCheck.IV->getStart();
551 const SCEV *GuardLimit = RangeCheck.Limit;
552 const SCEV *LatchStart = LatchCheck.IV->getStart();
553 const SCEV *LatchLimit = LatchCheck.Limit;
554 // Subtlety: We need all the values to be *invariant* across all iterations,
555 // but we only need to check expansion safety for those which *aren't*
556 // already guaranteed to dominate the guard.
557 if (!isLoopInvariantValue(S: GuardStart) ||
558 !isLoopInvariantValue(S: GuardLimit) ||
559 !isLoopInvariantValue(S: LatchStart) ||
560 !isLoopInvariantValue(S: LatchLimit)) {
561 LLVM_DEBUG(dbgs() << "Can't expand limit check!\n");
562 return std::nullopt;
563 }
564 if (!Expander.isSafeToExpandAt(S: LatchStart, InsertionPoint: Guard) ||
565 !Expander.isSafeToExpandAt(S: LatchLimit, InsertionPoint: Guard)) {
566 LLVM_DEBUG(dbgs() << "Can't expand limit check!\n");
567 return std::nullopt;
568 }
569
570 // guardLimit - guardStart + latchStart - 1
571 const SCEV *RHS =
572 SE->getAddExpr(LHS: SE->getMinusSCEV(LHS: GuardLimit, RHS: GuardStart),
573 RHS: SE->getMinusSCEV(LHS: LatchStart, RHS: SE->getOne(Ty)));
574 auto LimitCheckPred =
575 ICmpInst::getFlippedStrictnessPredicate(pred: LatchCheck.Pred);
576
577 LLVM_DEBUG(dbgs() << "LHS: " << *LatchLimit << "\n");
578 LLVM_DEBUG(dbgs() << "RHS: " << *RHS << "\n");
579 LLVM_DEBUG(dbgs() << "Pred: " << LimitCheckPred << "\n");
580
581 auto *LimitCheck =
582 expandCheck(Expander, Guard, Pred: LimitCheckPred, LHS: LatchLimit, RHS);
583 auto *FirstIterationCheck = expandCheck(Expander, Guard, Pred: RangeCheck.Pred,
584 LHS: GuardStart, RHS: GuardLimit);
585 IRBuilder<> Builder(findInsertPt(Use: Guard, Ops: {FirstIterationCheck, LimitCheck}));
586 return Builder.CreateFreeze(
587 V: Builder.CreateAnd(LHS: FirstIterationCheck, RHS: LimitCheck));
588}
589
590std::optional<Value *> LoopPredication::widenICmpRangeCheckDecrementingLoop(
591 LoopICmp LatchCheck, LoopICmp RangeCheck, SCEVExpander &Expander,
592 Instruction *Guard) {
593 auto *Ty = RangeCheck.IV->getType();
594 const SCEV *GuardStart = RangeCheck.IV->getStart();
595 const SCEV *GuardLimit = RangeCheck.Limit;
596 const SCEV *LatchStart = LatchCheck.IV->getStart();
597 const SCEV *LatchLimit = LatchCheck.Limit;
598 // Subtlety: We need all the values to be *invariant* across all iterations,
599 // but we only need to check expansion safety for those which *aren't*
600 // already guaranteed to dominate the guard.
601 if (!isLoopInvariantValue(S: GuardStart) ||
602 !isLoopInvariantValue(S: GuardLimit) ||
603 !isLoopInvariantValue(S: LatchStart) ||
604 !isLoopInvariantValue(S: LatchLimit)) {
605 LLVM_DEBUG(dbgs() << "Can't expand limit check!\n");
606 return std::nullopt;
607 }
608 if (!Expander.isSafeToExpandAt(S: LatchStart, InsertionPoint: Guard) ||
609 !Expander.isSafeToExpandAt(S: LatchLimit, InsertionPoint: Guard)) {
610 LLVM_DEBUG(dbgs() << "Can't expand limit check!\n");
611 return std::nullopt;
612 }
613 // The decrement of the latch check IV should be the same as the
614 // rangeCheckIV.
615 auto *PostDecLatchCheckIV = LatchCheck.IV->getPostIncExpr(SE&: *SE);
616 if (RangeCheck.IV != PostDecLatchCheckIV) {
617 LLVM_DEBUG(dbgs() << "Not the same. PostDecLatchCheckIV: "
618 << *PostDecLatchCheckIV
619 << " and RangeCheckIV: " << *RangeCheck.IV << "\n");
620 return std::nullopt;
621 }
622
623 // Generate the widened condition for CountDownLoop:
624 // guardStart u< guardLimit &&
625 // latchLimit <pred> 1.
626 // See the header comment for reasoning of the checks.
627 auto LimitCheckPred =
628 ICmpInst::getFlippedStrictnessPredicate(pred: LatchCheck.Pred);
629 auto *FirstIterationCheck = expandCheck(Expander, Guard,
630 Pred: ICmpInst::ICMP_ULT,
631 LHS: GuardStart, RHS: GuardLimit);
632 auto *LimitCheck = expandCheck(Expander, Guard, Pred: LimitCheckPred, LHS: LatchLimit,
633 RHS: SE->getOne(Ty));
634 IRBuilder<> Builder(findInsertPt(Use: Guard, Ops: {FirstIterationCheck, LimitCheck}));
635 return Builder.CreateFreeze(
636 V: Builder.CreateAnd(LHS: FirstIterationCheck, RHS: LimitCheck));
637}
638
639static void normalizePredicate(ScalarEvolution *SE, Loop *L,
640 LoopICmp& RC) {
641 // LFTR canonicalizes checks to the ICMP_NE/EQ form; normalize back to the
642 // ULT/UGE form for ease of handling by our caller.
643 if (ICmpInst::isEquality(P: RC.Pred) &&
644 RC.IV->getStepRecurrence(SE&: *SE)->isOne() &&
645 SE->isKnownPredicate(Pred: ICmpInst::ICMP_ULE, LHS: RC.IV->getStart(), RHS: RC.Limit))
646 RC.Pred = RC.Pred == ICmpInst::ICMP_NE ?
647 ICmpInst::ICMP_ULT : ICmpInst::ICMP_UGE;
648}
649
650/// If ICI can be widened to a loop invariant condition emits the loop
651/// invariant condition in the loop preheader and return it, otherwise
652/// returns std::nullopt.
653std::optional<Value *>
654LoopPredication::widenICmpRangeCheck(ICmpInst *ICI, SCEVExpander &Expander,
655 Instruction *Guard) {
656 LLVM_DEBUG(dbgs() << "Analyzing ICmpInst condition:\n");
657 LLVM_DEBUG(ICI->dump());
658
659 // parseLoopStructure guarantees that the latch condition is:
660 // ++i <pred> latchLimit, where <pred> is u<, u<=, s<, or s<=.
661 // We are looking for the range checks of the form:
662 // i u< guardLimit
663 auto RangeCheck = parseLoopICmp(ICI);
664 if (!RangeCheck) {
665 LLVM_DEBUG(dbgs() << "Failed to parse the loop latch condition!\n");
666 return std::nullopt;
667 }
668 LLVM_DEBUG(dbgs() << "Guard check:\n");
669 LLVM_DEBUG(RangeCheck->dump());
670 if (RangeCheck->Pred != ICmpInst::ICMP_ULT) {
671 LLVM_DEBUG(dbgs() << "Unsupported range check predicate("
672 << RangeCheck->Pred << ")!\n");
673 return std::nullopt;
674 }
675 auto *RangeCheckIV = RangeCheck->IV;
676 if (!RangeCheckIV->isAffine()) {
677 LLVM_DEBUG(dbgs() << "Range check IV is not affine!\n");
678 return std::nullopt;
679 }
680 auto *Step = RangeCheckIV->getStepRecurrence(SE&: *SE);
681 // We cannot just compare with latch IV step because the latch and range IVs
682 // may have different types.
683 if (!isSupportedStep(Step)) {
684 LLVM_DEBUG(dbgs() << "Range check and latch have IVs different steps!\n");
685 return std::nullopt;
686 }
687 auto *Ty = RangeCheckIV->getType();
688 auto CurrLatchCheckOpt = generateLoopLatchCheck(DL: *DL, SE&: *SE, LatchCheck, RangeCheckType: Ty);
689 if (!CurrLatchCheckOpt) {
690 LLVM_DEBUG(dbgs() << "Failed to generate a loop latch check "
691 "corresponding to range type: "
692 << *Ty << "\n");
693 return std::nullopt;
694 }
695
696 LoopICmp CurrLatchCheck = *CurrLatchCheckOpt;
697 // At this point, the range and latch step should have the same type, but need
698 // not have the same value (we support both 1 and -1 steps).
699 assert(Step->getType() ==
700 CurrLatchCheck.IV->getStepRecurrence(*SE)->getType() &&
701 "Range and latch steps should be of same type!");
702 if (Step != CurrLatchCheck.IV->getStepRecurrence(SE&: *SE)) {
703 LLVM_DEBUG(dbgs() << "Range and latch have different step values!\n");
704 return std::nullopt;
705 }
706
707 if (Step->isOne())
708 return widenICmpRangeCheckIncrementingLoop(LatchCheck: CurrLatchCheck, RangeCheck: *RangeCheck,
709 Expander, Guard);
710 else {
711 assert(Step->isAllOnesValue() && "Step should be -1!");
712 return widenICmpRangeCheckDecrementingLoop(LatchCheck: CurrLatchCheck, RangeCheck: *RangeCheck,
713 Expander, Guard);
714 }
715}
716
717void LoopPredication::widenChecks(SmallVectorImpl<Value *> &Checks,
718 SmallVectorImpl<Value *> &WidenedChecks,
719 SCEVExpander &Expander, Instruction *Guard) {
720 for (auto &Check : Checks)
721 if (ICmpInst *ICI = dyn_cast<ICmpInst>(Val: Check))
722 if (auto NewRangeCheck = widenICmpRangeCheck(ICI, Expander, Guard)) {
723 WidenedChecks.push_back(Elt: Check);
724 Check = *NewRangeCheck;
725 }
726}
727
728bool LoopPredication::widenGuardConditions(IntrinsicInst *Guard,
729 SCEVExpander &Expander) {
730 LLVM_DEBUG(dbgs() << "Processing guard:\n");
731 LLVM_DEBUG(Guard->dump());
732
733 TotalConsidered++;
734 SmallVector<Value *, 4> Checks;
735 SmallVector<Value *> WidenedChecks;
736 parseWidenableGuard(U: Guard, Checks);
737 widenChecks(Checks, WidenedChecks, Expander, Guard);
738 if (WidenedChecks.empty())
739 return false;
740
741 TotalWidened += WidenedChecks.size();
742
743 // Emit the new guard condition
744 IRBuilder<> Builder(findInsertPt(Use: Guard, Ops: Checks));
745 Value *AllChecks = Builder.CreateAnd(Ops: Checks);
746 auto *OldCond = Guard->getOperand(i_nocapture: 0);
747 Guard->setOperand(i_nocapture: 0, Val_nocapture: AllChecks);
748 if (InsertAssumesOfPredicatedGuardsConditions) {
749 Builder.SetInsertPoint(&*++BasicBlock::iterator(Guard));
750 Builder.CreateAssumption(Cond: OldCond);
751 }
752 RecursivelyDeleteTriviallyDeadInstructions(V: OldCond, TLI: nullptr /* TLI */, MSSAU);
753
754 LLVM_DEBUG(dbgs() << "Widened checks = " << WidenedChecks.size() << "\n");
755 return true;
756}
757
758bool LoopPredication::widenWidenableBranchGuardConditions(
759 BranchInst *BI, SCEVExpander &Expander) {
760 assert(isGuardAsWidenableBranch(BI) && "Must be!");
761 LLVM_DEBUG(dbgs() << "Processing guard:\n");
762 LLVM_DEBUG(BI->dump());
763
764 TotalConsidered++;
765 SmallVector<Value *, 4> Checks;
766 SmallVector<Value *> WidenedChecks;
767 parseWidenableGuard(U: BI, Checks);
768 // At the moment, our matching logic for wideable conditions implicitly
769 // assumes we preserve the form: (br (and Cond, WC())). FIXME
770 auto WC = extractWidenableCondition(U: BI);
771 Checks.push_back(Elt: WC);
772 widenChecks(Checks, WidenedChecks, Expander, Guard: BI);
773 if (WidenedChecks.empty())
774 return false;
775
776 TotalWidened += WidenedChecks.size();
777
778 // Emit the new guard condition
779 IRBuilder<> Builder(findInsertPt(Use: BI, Ops: Checks));
780 Value *AllChecks = Builder.CreateAnd(Ops: Checks);
781 auto *OldCond = BI->getCondition();
782 BI->setCondition(AllChecks);
783 if (InsertAssumesOfPredicatedGuardsConditions) {
784 BasicBlock *IfTrueBB = BI->getSuccessor(i: 0);
785 Builder.SetInsertPoint(TheBB: IfTrueBB, IP: IfTrueBB->getFirstInsertionPt());
786 // If this block has other predecessors, we might not be able to use Cond.
787 // In this case, create a Phi where every other input is `true` and input
788 // from guard block is Cond.
789 Value *AssumeCond = Builder.CreateAnd(Ops: WidenedChecks);
790 if (!IfTrueBB->getUniquePredecessor()) {
791 auto *GuardBB = BI->getParent();
792 auto *PN = Builder.CreatePHI(Ty: AssumeCond->getType(), NumReservedValues: pred_size(BB: IfTrueBB),
793 Name: "assume.cond");
794 for (auto *Pred : predecessors(BB: IfTrueBB))
795 PN->addIncoming(V: Pred == GuardBB ? AssumeCond : Builder.getTrue(), BB: Pred);
796 AssumeCond = PN;
797 }
798 Builder.CreateAssumption(Cond: AssumeCond);
799 }
800 RecursivelyDeleteTriviallyDeadInstructions(V: OldCond, TLI: nullptr /* TLI */, MSSAU);
801 assert(isGuardAsWidenableBranch(BI) &&
802 "Stopped being a guard after transform?");
803
804 LLVM_DEBUG(dbgs() << "Widened checks = " << WidenedChecks.size() << "\n");
805 return true;
806}
807
808std::optional<LoopICmp> LoopPredication::parseLoopLatchICmp() {
809 using namespace PatternMatch;
810
811 BasicBlock *LoopLatch = L->getLoopLatch();
812 if (!LoopLatch) {
813 LLVM_DEBUG(dbgs() << "The loop doesn't have a single latch!\n");
814 return std::nullopt;
815 }
816
817 auto *BI = dyn_cast<BranchInst>(Val: LoopLatch->getTerminator());
818 if (!BI || !BI->isConditional()) {
819 LLVM_DEBUG(dbgs() << "Failed to match the latch terminator!\n");
820 return std::nullopt;
821 }
822 BasicBlock *TrueDest = BI->getSuccessor(i: 0);
823 assert(
824 (TrueDest == L->getHeader() || BI->getSuccessor(1) == L->getHeader()) &&
825 "One of the latch's destinations must be the header");
826
827 auto *ICI = dyn_cast<ICmpInst>(Val: BI->getCondition());
828 if (!ICI) {
829 LLVM_DEBUG(dbgs() << "Failed to match the latch condition!\n");
830 return std::nullopt;
831 }
832 auto Result = parseLoopICmp(ICI);
833 if (!Result) {
834 LLVM_DEBUG(dbgs() << "Failed to parse the loop latch condition!\n");
835 return std::nullopt;
836 }
837
838 if (TrueDest != L->getHeader())
839 Result->Pred = ICmpInst::getInversePredicate(pred: Result->Pred);
840
841 // Check affine first, so if it's not we don't try to compute the step
842 // recurrence.
843 if (!Result->IV->isAffine()) {
844 LLVM_DEBUG(dbgs() << "The induction variable is not affine!\n");
845 return std::nullopt;
846 }
847
848 auto *Step = Result->IV->getStepRecurrence(SE&: *SE);
849 if (!isSupportedStep(Step)) {
850 LLVM_DEBUG(dbgs() << "Unsupported loop stride(" << *Step << ")!\n");
851 return std::nullopt;
852 }
853
854 auto IsUnsupportedPredicate = [](const SCEV *Step, ICmpInst::Predicate Pred) {
855 if (Step->isOne()) {
856 return Pred != ICmpInst::ICMP_ULT && Pred != ICmpInst::ICMP_SLT &&
857 Pred != ICmpInst::ICMP_ULE && Pred != ICmpInst::ICMP_SLE;
858 } else {
859 assert(Step->isAllOnesValue() && "Step should be -1!");
860 return Pred != ICmpInst::ICMP_UGT && Pred != ICmpInst::ICMP_SGT &&
861 Pred != ICmpInst::ICMP_UGE && Pred != ICmpInst::ICMP_SGE;
862 }
863 };
864
865 normalizePredicate(SE, L, RC&: *Result);
866 if (IsUnsupportedPredicate(Step, Result->Pred)) {
867 LLVM_DEBUG(dbgs() << "Unsupported loop latch predicate(" << Result->Pred
868 << ")!\n");
869 return std::nullopt;
870 }
871
872 return Result;
873}
874
875bool LoopPredication::isLoopProfitableToPredicate() {
876 if (SkipProfitabilityChecks)
877 return true;
878
879 SmallVector<std::pair<BasicBlock *, BasicBlock *>, 8> ExitEdges;
880 L->getExitEdges(ExitEdges);
881 // If there is only one exiting edge in the loop, it is always profitable to
882 // predicate the loop.
883 if (ExitEdges.size() == 1)
884 return true;
885
886 // Calculate the exiting probabilities of all exiting edges from the loop,
887 // starting with the LatchExitProbability.
888 // Heuristic for profitability: If any of the exiting blocks' probability of
889 // exiting the loop is larger than exiting through the latch block, it's not
890 // profitable to predicate the loop.
891 auto *LatchBlock = L->getLoopLatch();
892 assert(LatchBlock && "Should have a single latch at this point!");
893 auto *LatchTerm = LatchBlock->getTerminator();
894 assert(LatchTerm->getNumSuccessors() == 2 &&
895 "expected to be an exiting block with 2 succs!");
896 unsigned LatchBrExitIdx =
897 LatchTerm->getSuccessor(Idx: 0) == L->getHeader() ? 1 : 0;
898 // We compute branch probabilities without BPI. We do not rely on BPI since
899 // Loop predication is usually run in an LPM and BPI is only preserved
900 // lossily within loop pass managers, while BPI has an inherent notion of
901 // being complete for an entire function.
902
903 // If the latch exits into a deoptimize or an unreachable block, do not
904 // predicate on that latch check.
905 auto *LatchExitBlock = LatchTerm->getSuccessor(Idx: LatchBrExitIdx);
906 if (isa<UnreachableInst>(Val: LatchTerm) ||
907 LatchExitBlock->getTerminatingDeoptimizeCall())
908 return false;
909
910 // Latch terminator has no valid profile data, so nothing to check
911 // profitability on.
912 if (!hasValidBranchWeightMD(I: *LatchTerm))
913 return true;
914
915 auto ComputeBranchProbability =
916 [&](const BasicBlock *ExitingBlock,
917 const BasicBlock *ExitBlock) -> BranchProbability {
918 auto *Term = ExitingBlock->getTerminator();
919 unsigned NumSucc = Term->getNumSuccessors();
920 if (MDNode *ProfileData = getValidBranchWeightMDNode(I: *Term)) {
921 SmallVector<uint32_t> Weights;
922 extractBranchWeights(ProfileData, Weights);
923 uint64_t Numerator = 0, Denominator = 0;
924 for (auto [i, Weight] : llvm::enumerate(First&: Weights)) {
925 if (Term->getSuccessor(Idx: i) == ExitBlock)
926 Numerator += Weight;
927 Denominator += Weight;
928 }
929 // If all weights are zero act as if there was no profile data
930 if (Denominator == 0)
931 return BranchProbability::getBranchProbability(Numerator: 1, Denominator: NumSucc);
932 return BranchProbability::getBranchProbability(Numerator, Denominator);
933 } else {
934 assert(LatchBlock != ExitingBlock &&
935 "Latch term should always have profile data!");
936 // No profile data, so we choose the weight as 1/num_of_succ(Src)
937 return BranchProbability::getBranchProbability(Numerator: 1, Denominator: NumSucc);
938 }
939 };
940
941 BranchProbability LatchExitProbability =
942 ComputeBranchProbability(LatchBlock, LatchExitBlock);
943
944 // Protect against degenerate inputs provided by the user. Providing a value
945 // less than one, can invert the definition of profitable loop predication.
946 float ScaleFactor = LatchExitProbabilityScale;
947 if (ScaleFactor < 1) {
948 LLVM_DEBUG(
949 dbgs()
950 << "Ignored user setting for loop-predication-latch-probability-scale: "
951 << LatchExitProbabilityScale << "\n");
952 LLVM_DEBUG(dbgs() << "The value is set to 1.0\n");
953 ScaleFactor = 1.0;
954 }
955 const auto LatchProbabilityThreshold = LatchExitProbability * ScaleFactor;
956
957 for (const auto &ExitEdge : ExitEdges) {
958 BranchProbability ExitingBlockProbability =
959 ComputeBranchProbability(ExitEdge.first, ExitEdge.second);
960 // Some exiting edge has higher probability than the latch exiting edge.
961 // No longer profitable to predicate.
962 if (ExitingBlockProbability > LatchProbabilityThreshold)
963 return false;
964 }
965
966 // We have concluded that the most probable way to exit from the
967 // loop is through the latch (or there's no profile information and all
968 // exits are equally likely).
969 return true;
970}
971
972/// If we can (cheaply) find a widenable branch which controls entry into the
973/// loop, return it.
974static BranchInst *FindWidenableTerminatorAboveLoop(Loop *L, LoopInfo &LI) {
975 // Walk back through any unconditional executed blocks and see if we can find
976 // a widenable condition which seems to control execution of this loop. Note
977 // that we predict that maythrow calls are likely untaken and thus that it's
978 // profitable to widen a branch before a maythrow call with a condition
979 // afterwards even though that may cause the slow path to run in a case where
980 // it wouldn't have otherwise.
981 BasicBlock *BB = L->getLoopPreheader();
982 if (!BB)
983 return nullptr;
984 do {
985 if (BasicBlock *Pred = BB->getSinglePredecessor())
986 if (BB == Pred->getSingleSuccessor()) {
987 BB = Pred;
988 continue;
989 }
990 break;
991 } while (true);
992
993 if (BasicBlock *Pred = BB->getSinglePredecessor()) {
994 if (auto *BI = dyn_cast<BranchInst>(Val: Pred->getTerminator()))
995 if (BI->getSuccessor(i: 0) == BB && isWidenableBranch(U: BI))
996 return BI;
997 }
998 return nullptr;
999}
1000
1001/// Return the minimum of all analyzeable exit counts. This is an upper bound
1002/// on the actual exit count. If there are not at least two analyzeable exits,
1003/// returns SCEVCouldNotCompute.
1004static const SCEV *getMinAnalyzeableBackedgeTakenCount(ScalarEvolution &SE,
1005 DominatorTree &DT,
1006 Loop *L) {
1007 SmallVector<BasicBlock *, 16> ExitingBlocks;
1008 L->getExitingBlocks(ExitingBlocks);
1009
1010 SmallVector<const SCEV *, 4> ExitCounts;
1011 for (BasicBlock *ExitingBB : ExitingBlocks) {
1012 const SCEV *ExitCount = SE.getExitCount(L, ExitingBlock: ExitingBB);
1013 if (isa<SCEVCouldNotCompute>(Val: ExitCount))
1014 continue;
1015 assert(DT.dominates(ExitingBB, L->getLoopLatch()) &&
1016 "We should only have known counts for exiting blocks that "
1017 "dominate latch!");
1018 ExitCounts.push_back(Elt: ExitCount);
1019 }
1020 if (ExitCounts.size() < 2)
1021 return SE.getCouldNotCompute();
1022 return SE.getUMinFromMismatchedTypes(Ops&: ExitCounts);
1023}
1024
1025/// This implements an analogous, but entirely distinct transform from the main
1026/// loop predication transform. This one is phrased in terms of using a
1027/// widenable branch *outside* the loop to allow us to simplify loop exits in a
1028/// following loop. This is close in spirit to the IndVarSimplify transform
1029/// of the same name, but is materially different widening loosens legality
1030/// sharply.
1031bool LoopPredication::predicateLoopExits(Loop *L, SCEVExpander &Rewriter) {
1032 // The transformation performed here aims to widen a widenable condition
1033 // above the loop such that all analyzeable exit leading to deopt are dead.
1034 // It assumes that the latch is the dominant exit for profitability and that
1035 // exits branching to deoptimizing blocks are rarely taken. It relies on the
1036 // semantics of widenable expressions for legality. (i.e. being able to fall
1037 // down the widenable path spuriously allows us to ignore exit order,
1038 // unanalyzeable exits, side effects, exceptional exits, and other challenges
1039 // which restrict the applicability of the non-WC based version of this
1040 // transform in IndVarSimplify.)
1041 //
1042 // NOTE ON POISON/UNDEF - We're hoisting an expression above guards which may
1043 // imply flags on the expression being hoisted and inserting new uses (flags
1044 // are only correct for current uses). The result is that we may be
1045 // inserting a branch on the value which can be either poison or undef. In
1046 // this case, the branch can legally go either way; we just need to avoid
1047 // introducing UB. This is achieved through the use of the freeze
1048 // instruction.
1049
1050 SmallVector<BasicBlock *, 16> ExitingBlocks;
1051 L->getExitingBlocks(ExitingBlocks);
1052
1053 if (ExitingBlocks.empty())
1054 return false; // Nothing to do.
1055
1056 auto *Latch = L->getLoopLatch();
1057 if (!Latch)
1058 return false;
1059
1060 auto *WidenableBR = FindWidenableTerminatorAboveLoop(L, LI&: *LI);
1061 if (!WidenableBR)
1062 return false;
1063
1064 const SCEV *LatchEC = SE->getExitCount(L, ExitingBlock: Latch);
1065 if (isa<SCEVCouldNotCompute>(Val: LatchEC))
1066 return false; // profitability - want hot exit in analyzeable set
1067
1068 // At this point, we have found an analyzeable latch, and a widenable
1069 // condition above the loop. If we have a widenable exit within the loop
1070 // (for which we can't compute exit counts), drop the ability to further
1071 // widen so that we gain ability to analyze it's exit count and perform this
1072 // transform. TODO: It'd be nice to know for sure the exit became
1073 // analyzeable after dropping widenability.
1074 bool ChangedLoop = false;
1075
1076 for (auto *ExitingBB : ExitingBlocks) {
1077 if (LI->getLoopFor(BB: ExitingBB) != L)
1078 continue;
1079
1080 auto *BI = dyn_cast<BranchInst>(Val: ExitingBB->getTerminator());
1081 if (!BI)
1082 continue;
1083
1084 if (auto WC = extractWidenableCondition(U: BI))
1085 if (L->contains(BB: BI->getSuccessor(i: 0))) {
1086 assert(WC->hasOneUse() && "Not appropriate widenable branch!");
1087 WC->user_back()->replaceUsesOfWith(
1088 From: WC, To: ConstantInt::getTrue(Context&: BI->getContext()));
1089 ChangedLoop = true;
1090 }
1091 }
1092 if (ChangedLoop)
1093 SE->forgetLoop(L);
1094
1095 // The insertion point for the widening should be at the widenably call, not
1096 // at the WidenableBR. If we do this at the widenableBR, we can incorrectly
1097 // change a loop-invariant condition to a loop-varying one.
1098 auto *IP = cast<Instruction>(Val: WidenableBR->getCondition());
1099
1100 // The use of umin(all analyzeable exits) instead of latch is subtle, but
1101 // important for profitability. We may have a loop which hasn't been fully
1102 // canonicalized just yet. If the exit we chose to widen is provably never
1103 // taken, we want the widened form to *also* be provably never taken. We
1104 // can't guarantee this as a current unanalyzeable exit may later become
1105 // analyzeable, but we can at least avoid the obvious cases.
1106 const SCEV *MinEC = getMinAnalyzeableBackedgeTakenCount(SE&: *SE, DT&: *DT, L);
1107 if (isa<SCEVCouldNotCompute>(Val: MinEC) || MinEC->getType()->isPointerTy() ||
1108 !SE->isLoopInvariant(S: MinEC, L) ||
1109 !Rewriter.isSafeToExpandAt(S: MinEC, InsertionPoint: IP))
1110 return ChangedLoop;
1111
1112 Rewriter.setInsertPoint(IP);
1113 IRBuilder<> B(IP);
1114
1115 bool InvalidateLoop = false;
1116 Value *MinECV = nullptr; // lazily generated if needed
1117 for (BasicBlock *ExitingBB : ExitingBlocks) {
1118 // If our exiting block exits multiple loops, we can only rewrite the
1119 // innermost one. Otherwise, we're changing how many times the innermost
1120 // loop runs before it exits.
1121 if (LI->getLoopFor(BB: ExitingBB) != L)
1122 continue;
1123
1124 // Can't rewrite non-branch yet.
1125 auto *BI = dyn_cast<BranchInst>(Val: ExitingBB->getTerminator());
1126 if (!BI)
1127 continue;
1128
1129 // If already constant, nothing to do.
1130 if (isa<Constant>(Val: BI->getCondition()))
1131 continue;
1132
1133 const SCEV *ExitCount = SE->getExitCount(L, ExitingBlock: ExitingBB);
1134 if (isa<SCEVCouldNotCompute>(Val: ExitCount) ||
1135 ExitCount->getType()->isPointerTy() ||
1136 !Rewriter.isSafeToExpandAt(S: ExitCount, InsertionPoint: WidenableBR))
1137 continue;
1138
1139 const bool ExitIfTrue = !L->contains(BB: *succ_begin(BB: ExitingBB));
1140 BasicBlock *ExitBB = BI->getSuccessor(i: ExitIfTrue ? 0 : 1);
1141 if (!ExitBB->getPostdominatingDeoptimizeCall())
1142 continue;
1143
1144 /// Here we can be fairly sure that executing this exit will most likely
1145 /// lead to executing llvm.experimental.deoptimize.
1146 /// This is a profitability heuristic, not a legality constraint.
1147
1148 // If we found a widenable exit condition, do two things:
1149 // 1) fold the widened exit test into the widenable condition
1150 // 2) fold the branch to untaken - avoids infinite looping
1151
1152 Value *ECV = Rewriter.expandCodeFor(SH: ExitCount);
1153 if (!MinECV)
1154 MinECV = Rewriter.expandCodeFor(SH: MinEC);
1155 Value *RHS = MinECV;
1156 if (ECV->getType() != RHS->getType()) {
1157 Type *WiderTy = SE->getWiderType(Ty1: ECV->getType(), Ty2: RHS->getType());
1158 ECV = B.CreateZExt(V: ECV, DestTy: WiderTy);
1159 RHS = B.CreateZExt(V: RHS, DestTy: WiderTy);
1160 }
1161 assert(!Latch || DT->dominates(ExitingBB, Latch));
1162 Value *NewCond = B.CreateICmp(P: ICmpInst::ICMP_UGT, LHS: ECV, RHS);
1163 // Freeze poison or undef to an arbitrary bit pattern to ensure we can
1164 // branch without introducing UB. See NOTE ON POISON/UNDEF above for
1165 // context.
1166 NewCond = B.CreateFreeze(V: NewCond);
1167
1168 widenWidenableBranch(WidenableBR, NewCond);
1169
1170 Value *OldCond = BI->getCondition();
1171 BI->setCondition(ConstantInt::get(Ty: OldCond->getType(), V: !ExitIfTrue));
1172 InvalidateLoop = true;
1173 }
1174
1175 if (InvalidateLoop)
1176 // We just mutated a bunch of loop exits changing there exit counts
1177 // widely. We need to force recomputation of the exit counts given these
1178 // changes. Note that all of the inserted exits are never taken, and
1179 // should be removed next time the CFG is modified.
1180 SE->forgetLoop(L);
1181
1182 // Always return `true` since we have moved the WidenableBR's condition.
1183 return true;
1184}
1185
1186bool LoopPredication::runOnLoop(Loop *Loop) {
1187 L = Loop;
1188
1189 LLVM_DEBUG(dbgs() << "Analyzing ");
1190 LLVM_DEBUG(L->dump());
1191
1192 Module *M = L->getHeader()->getModule();
1193
1194 // There is nothing to do if the module doesn't use guards
1195 auto *GuardDecl =
1196 M->getFunction(Intrinsic::getName(Intrinsic::experimental_guard));
1197 bool HasIntrinsicGuards = GuardDecl && !GuardDecl->use_empty();
1198 auto *WCDecl = M->getFunction(
1199 Intrinsic::getName(Intrinsic::experimental_widenable_condition));
1200 bool HasWidenableConditions =
1201 PredicateWidenableBranchGuards && WCDecl && !WCDecl->use_empty();
1202 if (!HasIntrinsicGuards && !HasWidenableConditions)
1203 return false;
1204
1205 DL = &M->getDataLayout();
1206
1207 Preheader = L->getLoopPreheader();
1208 if (!Preheader)
1209 return false;
1210
1211 auto LatchCheckOpt = parseLoopLatchICmp();
1212 if (!LatchCheckOpt)
1213 return false;
1214 LatchCheck = *LatchCheckOpt;
1215
1216 LLVM_DEBUG(dbgs() << "Latch check:\n");
1217 LLVM_DEBUG(LatchCheck.dump());
1218
1219 if (!isLoopProfitableToPredicate()) {
1220 LLVM_DEBUG(dbgs() << "Loop not profitable to predicate!\n");
1221 return false;
1222 }
1223 // Collect all the guards into a vector and process later, so as not
1224 // to invalidate the instruction iterator.
1225 SmallVector<IntrinsicInst *, 4> Guards;
1226 SmallVector<BranchInst *, 4> GuardsAsWidenableBranches;
1227 for (const auto BB : L->blocks()) {
1228 for (auto &I : *BB)
1229 if (isGuard(U: &I))
1230 Guards.push_back(Elt: cast<IntrinsicInst>(Val: &I));
1231 if (PredicateWidenableBranchGuards &&
1232 isGuardAsWidenableBranch(U: BB->getTerminator()))
1233 GuardsAsWidenableBranches.push_back(
1234 Elt: cast<BranchInst>(Val: BB->getTerminator()));
1235 }
1236
1237 SCEVExpander Expander(*SE, *DL, "loop-predication");
1238 bool Changed = false;
1239 for (auto *Guard : Guards)
1240 Changed |= widenGuardConditions(Guard, Expander);
1241 for (auto *Guard : GuardsAsWidenableBranches)
1242 Changed |= widenWidenableBranchGuardConditions(BI: Guard, Expander);
1243 Changed |= predicateLoopExits(L, Rewriter&: Expander);
1244
1245 if (MSSAU && VerifyMemorySSA)
1246 MSSAU->getMemorySSA()->verifyMemorySSA();
1247 return Changed;
1248}
1249

source code of llvm/lib/Transforms/Scalar/LoopPredication.cpp