xref: /freebsd/contrib/llvm-project/llvm/lib/Transforms/Scalar/GuardWidening.cpp (revision 5b56413d04e608379c9a306373554a8e4d321bc0)
1 //===- GuardWidening.cpp - ---- Guard widening ----------------------------===//
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 file implements the guard widening pass.  The semantics of the
10 // @llvm.experimental.guard intrinsic lets LLVM transform it so that it fails
11 // more often that it did before the transform.  This optimization is called
12 // "widening" and can be used hoist and common runtime checks in situations like
13 // these:
14 //
15 //    %cmp0 = 7 u< Length
16 //    call @llvm.experimental.guard(i1 %cmp0) [ "deopt"(...) ]
17 //    call @unknown_side_effects()
18 //    %cmp1 = 9 u< Length
19 //    call @llvm.experimental.guard(i1 %cmp1) [ "deopt"(...) ]
20 //    ...
21 //
22 // =>
23 //
24 //    %cmp0 = 9 u< Length
25 //    call @llvm.experimental.guard(i1 %cmp0) [ "deopt"(...) ]
26 //    call @unknown_side_effects()
27 //    ...
28 //
29 // If %cmp0 is false, @llvm.experimental.guard will "deoptimize" back to a
30 // generic implementation of the same function, which will have the correct
31 // semantics from that point onward.  It is always _legal_ to deoptimize (so
32 // replacing %cmp0 with false is "correct"), though it may not always be
33 // profitable to do so.
34 //
35 // NB! This pass is a work in progress.  It hasn't been tuned to be "production
36 // ready" yet.  It is known to have quadriatic running time and will not scale
37 // to large numbers of guards
38 //
39 //===----------------------------------------------------------------------===//
40 
41 #include "llvm/Transforms/Scalar/GuardWidening.h"
42 #include "llvm/ADT/DenseMap.h"
43 #include "llvm/ADT/DepthFirstIterator.h"
44 #include "llvm/ADT/Statistic.h"
45 #include "llvm/Analysis/AssumptionCache.h"
46 #include "llvm/Analysis/GuardUtils.h"
47 #include "llvm/Analysis/LoopInfo.h"
48 #include "llvm/Analysis/MemorySSAUpdater.h"
49 #include "llvm/Analysis/PostDominators.h"
50 #include "llvm/Analysis/ValueTracking.h"
51 #include "llvm/IR/ConstantRange.h"
52 #include "llvm/IR/Dominators.h"
53 #include "llvm/IR/IRBuilder.h"
54 #include "llvm/IR/IntrinsicInst.h"
55 #include "llvm/IR/PatternMatch.h"
56 #include "llvm/Support/CommandLine.h"
57 #include "llvm/Support/Debug.h"
58 #include "llvm/Support/KnownBits.h"
59 #include "llvm/Transforms/Scalar.h"
60 #include "llvm/Transforms/Utils/GuardUtils.h"
61 #include "llvm/Transforms/Utils/LoopUtils.h"
62 #include <functional>
63 
64 using namespace llvm;
65 
66 #define DEBUG_TYPE "guard-widening"
67 
68 STATISTIC(GuardsEliminated, "Number of eliminated guards");
69 STATISTIC(CondBranchEliminated, "Number of eliminated conditional branches");
70 STATISTIC(FreezeAdded, "Number of freeze instruction introduced");
71 
72 static cl::opt<bool>
73     WidenBranchGuards("guard-widening-widen-branch-guards", cl::Hidden,
74                       cl::desc("Whether or not we should widen guards  "
75                                "expressed as branches by widenable conditions"),
76                       cl::init(true));
77 
78 namespace {
79 
80 // Get the condition of \p I. It can either be a guard or a conditional branch.
81 static Value *getCondition(Instruction *I) {
82   if (IntrinsicInst *GI = dyn_cast<IntrinsicInst>(I)) {
83     assert(GI->getIntrinsicID() == Intrinsic::experimental_guard &&
84            "Bad guard intrinsic?");
85     return GI->getArgOperand(0);
86   }
87   Value *Cond, *WC;
88   BasicBlock *IfTrueBB, *IfFalseBB;
89   if (parseWidenableBranch(I, Cond, WC, IfTrueBB, IfFalseBB))
90     return Cond;
91 
92   return cast<BranchInst>(I)->getCondition();
93 }
94 
95 // Set the condition for \p I to \p NewCond. \p I can either be a guard or a
96 // conditional branch.
97 static void setCondition(Instruction *I, Value *NewCond) {
98   if (IntrinsicInst *GI = dyn_cast<IntrinsicInst>(I)) {
99     assert(GI->getIntrinsicID() == Intrinsic::experimental_guard &&
100            "Bad guard intrinsic?");
101     GI->setArgOperand(0, NewCond);
102     return;
103   }
104   cast<BranchInst>(I)->setCondition(NewCond);
105 }
106 
107 // Eliminates the guard instruction properly.
108 static void eliminateGuard(Instruction *GuardInst, MemorySSAUpdater *MSSAU) {
109   GuardInst->eraseFromParent();
110   if (MSSAU)
111     MSSAU->removeMemoryAccess(GuardInst);
112   ++GuardsEliminated;
113 }
114 
115 /// Find a point at which the widened condition of \p Guard should be inserted.
116 /// When it is represented as intrinsic call, we can do it right before the call
117 /// instruction. However, when we are dealing with widenable branch, we must
118 /// account for the following situation: widening should not turn a
119 /// loop-invariant condition into a loop-variant. It means that if
120 /// widenable.condition() call is invariant (w.r.t. any loop), the new wide
121 /// condition should stay invariant. Otherwise there can be a miscompile, like
122 /// the one described at https://github.com/llvm/llvm-project/issues/60234. The
123 /// safest way to do it is to expand the new condition at WC's block.
124 static Instruction *findInsertionPointForWideCondition(Instruction *WCOrGuard) {
125   if (isGuard(WCOrGuard))
126     return WCOrGuard;
127   if (auto WC = extractWidenableCondition(WCOrGuard))
128     return cast<Instruction>(WC);
129   return nullptr;
130 }
131 
132 class GuardWideningImpl {
133   DominatorTree &DT;
134   PostDominatorTree *PDT;
135   LoopInfo &LI;
136   AssumptionCache &AC;
137   MemorySSAUpdater *MSSAU;
138 
139   /// Together, these describe the region of interest.  This might be all of
140   /// the blocks within a function, or only a given loop's blocks and preheader.
141   DomTreeNode *Root;
142   std::function<bool(BasicBlock*)> BlockFilter;
143 
144   /// The set of guards and conditional branches whose conditions have been
145   /// widened into dominating guards.
146   SmallVector<Instruction *, 16> EliminatedGuardsAndBranches;
147 
148   /// The set of guards which have been widened to include conditions to other
149   /// guards.
150   DenseSet<Instruction *> WidenedGuards;
151 
152   /// Try to eliminate instruction \p Instr by widening it into an earlier
153   /// dominating guard.  \p DFSI is the DFS iterator on the dominator tree that
154   /// is currently visiting the block containing \p Guard, and \p GuardsPerBlock
155   /// maps BasicBlocks to the set of guards seen in that block.
156   bool eliminateInstrViaWidening(
157       Instruction *Instr, const df_iterator<DomTreeNode *> &DFSI,
158       const DenseMap<BasicBlock *, SmallVector<Instruction *, 8>>
159           &GuardsPerBlock);
160 
161   /// Used to keep track of which widening potential is more effective.
162   enum WideningScore {
163     /// Don't widen.
164     WS_IllegalOrNegative,
165 
166     /// Widening is performance neutral as far as the cycles spent in check
167     /// conditions goes (but can still help, e.g., code layout, having less
168     /// deopt state).
169     WS_Neutral,
170 
171     /// Widening is profitable.
172     WS_Positive,
173 
174     /// Widening is very profitable.  Not significantly different from \c
175     /// WS_Positive, except by the order.
176     WS_VeryPositive
177   };
178 
179   static StringRef scoreTypeToString(WideningScore WS);
180 
181   /// Compute the score for widening the condition in \p DominatedInstr
182   /// into \p WideningPoint.
183   WideningScore computeWideningScore(Instruction *DominatedInstr,
184                                      Instruction *ToWiden,
185                                      Instruction *WideningPoint,
186                                      SmallVectorImpl<Value *> &ChecksToHoist,
187                                      SmallVectorImpl<Value *> &ChecksToWiden);
188 
189   /// Helper to check if \p V can be hoisted to \p InsertPos.
190   bool canBeHoistedTo(const Value *V, const Instruction *InsertPos) const {
191     SmallPtrSet<const Instruction *, 8> Visited;
192     return canBeHoistedTo(V, InsertPos, Visited);
193   }
194 
195   bool canBeHoistedTo(const Value *V, const Instruction *InsertPos,
196                       SmallPtrSetImpl<const Instruction *> &Visited) const;
197 
198   bool canBeHoistedTo(const SmallVectorImpl<Value *> &Checks,
199                       const Instruction *InsertPos) const {
200     return all_of(Checks,
201                   [&](const Value *V) { return canBeHoistedTo(V, InsertPos); });
202   }
203   /// Helper to hoist \p V to \p InsertPos.  Guaranteed to succeed if \c
204   /// canBeHoistedTo returned true.
205   void makeAvailableAt(Value *V, Instruction *InsertPos) const;
206 
207   void makeAvailableAt(const SmallVectorImpl<Value *> &Checks,
208                        Instruction *InsertPos) const {
209     for (Value *V : Checks)
210       makeAvailableAt(V, InsertPos);
211   }
212 
213   /// Common helper used by \c widenGuard and \c isWideningCondProfitable.  Try
214   /// to generate an expression computing the logical AND of \p ChecksToHoist
215   /// and \p ChecksToWiden. Return true if the expression computing the AND is
216   /// only as expensive as computing one of the set of expressions. If \p
217   /// InsertPt is true then actually generate the resulting expression, make it
218   /// available at \p InsertPt and return it in \p Result (else no change to the
219   /// IR is made).
220   std::optional<Value *> mergeChecks(SmallVectorImpl<Value *> &ChecksToHoist,
221                                      SmallVectorImpl<Value *> &ChecksToWiden,
222                                      Instruction *InsertPt);
223 
224   /// Generate the logical AND of \p ChecksToHoist and \p OldCondition and make
225   /// it available at InsertPt
226   Value *hoistChecks(SmallVectorImpl<Value *> &ChecksToHoist,
227                      Value *OldCondition, Instruction *InsertPt);
228 
229   /// Adds freeze to Orig and push it as far as possible very aggressively.
230   /// Also replaces all uses of frozen instruction with frozen version.
231   Value *freezeAndPush(Value *Orig, Instruction *InsertPt);
232 
233   /// Represents a range check of the form \c Base + \c Offset u< \c Length,
234   /// with the constraint that \c Length is not negative.  \c CheckInst is the
235   /// pre-existing instruction in the IR that computes the result of this range
236   /// check.
237   class RangeCheck {
238     const Value *Base;
239     const ConstantInt *Offset;
240     const Value *Length;
241     ICmpInst *CheckInst;
242 
243   public:
244     explicit RangeCheck(const Value *Base, const ConstantInt *Offset,
245                         const Value *Length, ICmpInst *CheckInst)
246         : Base(Base), Offset(Offset), Length(Length), CheckInst(CheckInst) {}
247 
248     void setBase(const Value *NewBase) { Base = NewBase; }
249     void setOffset(const ConstantInt *NewOffset) { Offset = NewOffset; }
250 
251     const Value *getBase() const { return Base; }
252     const ConstantInt *getOffset() const { return Offset; }
253     const APInt &getOffsetValue() const { return getOffset()->getValue(); }
254     const Value *getLength() const { return Length; };
255     ICmpInst *getCheckInst() const { return CheckInst; }
256 
257     void print(raw_ostream &OS, bool PrintTypes = false) {
258       OS << "Base: ";
259       Base->printAsOperand(OS, PrintTypes);
260       OS << " Offset: ";
261       Offset->printAsOperand(OS, PrintTypes);
262       OS << " Length: ";
263       Length->printAsOperand(OS, PrintTypes);
264     }
265 
266     LLVM_DUMP_METHOD void dump() {
267       print(dbgs());
268       dbgs() << "\n";
269     }
270   };
271 
272   /// Parse \p ToParse into a conjunction (logical-and) of range checks; and
273   /// append them to \p Checks.  Returns true on success, may clobber \c Checks
274   /// on failure.
275   bool parseRangeChecks(SmallVectorImpl<Value *> &ToParse,
276                         SmallVectorImpl<RangeCheck> &Checks) {
277     for (auto CheckCond : ToParse) {
278       if (!parseRangeChecks(CheckCond, Checks))
279         return false;
280     }
281     return true;
282   }
283 
284   bool parseRangeChecks(Value *CheckCond, SmallVectorImpl<RangeCheck> &Checks);
285 
286   /// Combine the checks in \p Checks into a smaller set of checks and append
287   /// them into \p CombinedChecks.  Return true on success (i.e. all of checks
288   /// in \p Checks were combined into \p CombinedChecks).  Clobbers \p Checks
289   /// and \p CombinedChecks on success and on failure.
290   bool combineRangeChecks(SmallVectorImpl<RangeCheck> &Checks,
291                           SmallVectorImpl<RangeCheck> &CombinedChecks) const;
292 
293   /// Can we compute the logical AND of \p ChecksToHoist and \p ChecksToWiden
294   /// for the price of computing only one of the set of expressions?
295   bool isWideningCondProfitable(SmallVectorImpl<Value *> &ChecksToHoist,
296                                 SmallVectorImpl<Value *> &ChecksToWiden) {
297     return mergeChecks(ChecksToHoist, ChecksToWiden, /*InsertPt=*/nullptr)
298         .has_value();
299   }
300 
301   /// Widen \p ChecksToWiden to fail if any of \p ChecksToHoist is false
302   void widenGuard(SmallVectorImpl<Value *> &ChecksToHoist,
303                   SmallVectorImpl<Value *> &ChecksToWiden,
304                   Instruction *ToWiden) {
305     Instruction *InsertPt = findInsertionPointForWideCondition(ToWiden);
306     auto MergedCheck = mergeChecks(ChecksToHoist, ChecksToWiden, InsertPt);
307     Value *Result = MergedCheck ? *MergedCheck
308                                 : hoistChecks(ChecksToHoist,
309                                               getCondition(ToWiden), InsertPt);
310 
311     if (isGuardAsWidenableBranch(ToWiden)) {
312       setWidenableBranchCond(cast<BranchInst>(ToWiden), Result);
313       return;
314     }
315     setCondition(ToWiden, Result);
316   }
317 
318 public:
319   explicit GuardWideningImpl(DominatorTree &DT, PostDominatorTree *PDT,
320                              LoopInfo &LI, AssumptionCache &AC,
321                              MemorySSAUpdater *MSSAU, DomTreeNode *Root,
322                              std::function<bool(BasicBlock *)> BlockFilter)
323       : DT(DT), PDT(PDT), LI(LI), AC(AC), MSSAU(MSSAU), Root(Root),
324         BlockFilter(BlockFilter) {}
325 
326   /// The entry point for this pass.
327   bool run();
328 };
329 }
330 
331 static bool isSupportedGuardInstruction(const Instruction *Insn) {
332   if (isGuard(Insn))
333     return true;
334   if (WidenBranchGuards && isGuardAsWidenableBranch(Insn))
335     return true;
336   return false;
337 }
338 
339 bool GuardWideningImpl::run() {
340   DenseMap<BasicBlock *, SmallVector<Instruction *, 8>> GuardsInBlock;
341   bool Changed = false;
342   for (auto DFI = df_begin(Root), DFE = df_end(Root);
343        DFI != DFE; ++DFI) {
344     auto *BB = (*DFI)->getBlock();
345     if (!BlockFilter(BB))
346       continue;
347 
348     auto &CurrentList = GuardsInBlock[BB];
349 
350     for (auto &I : *BB)
351       if (isSupportedGuardInstruction(&I))
352         CurrentList.push_back(cast<Instruction>(&I));
353 
354     for (auto *II : CurrentList)
355       Changed |= eliminateInstrViaWidening(II, DFI, GuardsInBlock);
356   }
357 
358   assert(EliminatedGuardsAndBranches.empty() || Changed);
359   for (auto *I : EliminatedGuardsAndBranches)
360     if (!WidenedGuards.count(I)) {
361       assert(isa<ConstantInt>(getCondition(I)) && "Should be!");
362       if (isSupportedGuardInstruction(I))
363         eliminateGuard(I, MSSAU);
364       else {
365         assert(isa<BranchInst>(I) &&
366                "Eliminated something other than guard or branch?");
367         ++CondBranchEliminated;
368       }
369     }
370 
371   return Changed;
372 }
373 
374 bool GuardWideningImpl::eliminateInstrViaWidening(
375     Instruction *Instr, const df_iterator<DomTreeNode *> &DFSI,
376     const DenseMap<BasicBlock *, SmallVector<Instruction *, 8>>
377         &GuardsInBlock) {
378   SmallVector<Value *> ChecksToHoist;
379   parseWidenableGuard(Instr, ChecksToHoist);
380   // Ignore trivial true or false conditions. These instructions will be
381   // trivially eliminated by any cleanup pass. Do not erase them because other
382   // guards can possibly be widened into them.
383   if (ChecksToHoist.empty() ||
384       (ChecksToHoist.size() == 1 && isa<ConstantInt>(ChecksToHoist.front())))
385     return false;
386 
387   Instruction *BestSoFar = nullptr;
388   auto BestScoreSoFar = WS_IllegalOrNegative;
389 
390   // In the set of dominating guards, find the one we can merge GuardInst with
391   // for the most profit.
392   for (unsigned i = 0, e = DFSI.getPathLength(); i != e; ++i) {
393     auto *CurBB = DFSI.getPath(i)->getBlock();
394     if (!BlockFilter(CurBB))
395       break;
396     assert(GuardsInBlock.count(CurBB) && "Must have been populated by now!");
397     const auto &GuardsInCurBB = GuardsInBlock.find(CurBB)->second;
398 
399     auto I = GuardsInCurBB.begin();
400     auto E = Instr->getParent() == CurBB ? find(GuardsInCurBB, Instr)
401                                          : GuardsInCurBB.end();
402 
403 #ifndef NDEBUG
404     {
405       unsigned Index = 0;
406       for (auto &I : *CurBB) {
407         if (Index == GuardsInCurBB.size())
408           break;
409         if (GuardsInCurBB[Index] == &I)
410           Index++;
411       }
412       assert(Index == GuardsInCurBB.size() &&
413              "Guards expected to be in order!");
414     }
415 #endif
416 
417     assert((i == (e - 1)) == (Instr->getParent() == CurBB) && "Bad DFS?");
418 
419     for (auto *Candidate : make_range(I, E)) {
420       auto *WideningPoint = findInsertionPointForWideCondition(Candidate);
421       if (!WideningPoint)
422         continue;
423       SmallVector<Value *> CandidateChecks;
424       parseWidenableGuard(Candidate, CandidateChecks);
425       auto Score = computeWideningScore(Instr, Candidate, WideningPoint,
426                                         ChecksToHoist, CandidateChecks);
427       LLVM_DEBUG(dbgs() << "Score between " << *Instr << " and " << *Candidate
428                         << " is " << scoreTypeToString(Score) << "\n");
429       if (Score > BestScoreSoFar) {
430         BestScoreSoFar = Score;
431         BestSoFar = Candidate;
432       }
433     }
434   }
435 
436   if (BestScoreSoFar == WS_IllegalOrNegative) {
437     LLVM_DEBUG(dbgs() << "Did not eliminate guard " << *Instr << "\n");
438     return false;
439   }
440 
441   assert(BestSoFar != Instr && "Should have never visited same guard!");
442   assert(DT.dominates(BestSoFar, Instr) && "Should be!");
443 
444   LLVM_DEBUG(dbgs() << "Widening " << *Instr << " into " << *BestSoFar
445                     << " with score " << scoreTypeToString(BestScoreSoFar)
446                     << "\n");
447   SmallVector<Value *> ChecksToWiden;
448   parseWidenableGuard(BestSoFar, ChecksToWiden);
449   widenGuard(ChecksToHoist, ChecksToWiden, BestSoFar);
450   auto NewGuardCondition = ConstantInt::getTrue(Instr->getContext());
451   setCondition(Instr, NewGuardCondition);
452   EliminatedGuardsAndBranches.push_back(Instr);
453   WidenedGuards.insert(BestSoFar);
454   return true;
455 }
456 
457 GuardWideningImpl::WideningScore GuardWideningImpl::computeWideningScore(
458     Instruction *DominatedInstr, Instruction *ToWiden,
459     Instruction *WideningPoint, SmallVectorImpl<Value *> &ChecksToHoist,
460     SmallVectorImpl<Value *> &ChecksToWiden) {
461   Loop *DominatedInstrLoop = LI.getLoopFor(DominatedInstr->getParent());
462   Loop *DominatingGuardLoop = LI.getLoopFor(WideningPoint->getParent());
463   bool HoistingOutOfLoop = false;
464 
465   if (DominatingGuardLoop != DominatedInstrLoop) {
466     // Be conservative and don't widen into a sibling loop.  TODO: If the
467     // sibling is colder, we should consider allowing this.
468     if (DominatingGuardLoop &&
469         !DominatingGuardLoop->contains(DominatedInstrLoop))
470       return WS_IllegalOrNegative;
471 
472     HoistingOutOfLoop = true;
473   }
474 
475   if (!canBeHoistedTo(ChecksToHoist, WideningPoint))
476     return WS_IllegalOrNegative;
477   // Further in the GuardWideningImpl::hoistChecks the entire condition might be
478   // widened, not the parsed list of checks. So we need to check the possibility
479   // of that condition hoisting.
480   if (!canBeHoistedTo(getCondition(ToWiden), WideningPoint))
481     return WS_IllegalOrNegative;
482 
483   // If the guard was conditional executed, it may never be reached
484   // dynamically.  There are two potential downsides to hoisting it out of the
485   // conditionally executed region: 1) we may spuriously deopt without need and
486   // 2) we have the extra cost of computing the guard condition in the common
487   // case.  At the moment, we really only consider the second in our heuristic
488   // here.  TODO: evaluate cost model for spurious deopt
489   // NOTE: As written, this also lets us hoist right over another guard which
490   // is essentially just another spelling for control flow.
491   if (isWideningCondProfitable(ChecksToHoist, ChecksToWiden))
492     return HoistingOutOfLoop ? WS_VeryPositive : WS_Positive;
493 
494   if (HoistingOutOfLoop)
495     return WS_Positive;
496 
497   // For a given basic block \p BB, return its successor which is guaranteed or
498   // highly likely will be taken as its successor.
499   auto GetLikelySuccessor = [](const BasicBlock * BB)->const BasicBlock * {
500     if (auto *UniqueSucc = BB->getUniqueSuccessor())
501       return UniqueSucc;
502     auto *Term = BB->getTerminator();
503     Value *Cond = nullptr;
504     const BasicBlock *IfTrue = nullptr, *IfFalse = nullptr;
505     using namespace PatternMatch;
506     if (!match(Term, m_Br(m_Value(Cond), m_BasicBlock(IfTrue),
507                           m_BasicBlock(IfFalse))))
508       return nullptr;
509     // For constant conditions, only one dynamical successor is possible
510     if (auto *ConstCond = dyn_cast<ConstantInt>(Cond))
511       return ConstCond->isAllOnesValue() ? IfTrue : IfFalse;
512     // If one of successors ends with deopt, another one is likely.
513     if (IfFalse->getPostdominatingDeoptimizeCall())
514       return IfTrue;
515     if (IfTrue->getPostdominatingDeoptimizeCall())
516       return IfFalse;
517     // TODO: Use branch frequency metatada to allow hoisting through non-deopt
518     // branches?
519     return nullptr;
520   };
521 
522   // Returns true if we might be hoisting above explicit control flow into a
523   // considerably hotter block.  Note that this completely ignores implicit
524   // control flow (guards, calls which throw, etc...).  That choice appears
525   // arbitrary (we assume that implicit control flow exits are all rare).
526   auto MaybeHoistingToHotterBlock = [&]() {
527     const auto *DominatingBlock = WideningPoint->getParent();
528     const auto *DominatedBlock = DominatedInstr->getParent();
529 
530     // Descend as low as we can, always taking the likely successor.
531     assert(DT.isReachableFromEntry(DominatingBlock) && "Unreached code");
532     assert(DT.isReachableFromEntry(DominatedBlock) && "Unreached code");
533     assert(DT.dominates(DominatingBlock, DominatedBlock) && "No dominance");
534     while (DominatedBlock != DominatingBlock) {
535       auto *LikelySucc = GetLikelySuccessor(DominatingBlock);
536       // No likely successor?
537       if (!LikelySucc)
538         break;
539       // Only go down the dominator tree.
540       if (!DT.properlyDominates(DominatingBlock, LikelySucc))
541         break;
542       DominatingBlock = LikelySucc;
543     }
544 
545     // Found?
546     if (DominatedBlock == DominatingBlock)
547       return false;
548     // We followed the likely successor chain and went past the dominated
549     // block. It means that the dominated guard is in dead/very cold code.
550     if (!DT.dominates(DominatingBlock, DominatedBlock))
551       return true;
552     // TODO: diamond, triangle cases
553     if (!PDT)
554       return true;
555     return !PDT->dominates(DominatedBlock, DominatingBlock);
556   };
557 
558   return MaybeHoistingToHotterBlock() ? WS_IllegalOrNegative : WS_Neutral;
559 }
560 
561 bool GuardWideningImpl::canBeHoistedTo(
562     const Value *V, const Instruction *Loc,
563     SmallPtrSetImpl<const Instruction *> &Visited) const {
564   auto *Inst = dyn_cast<Instruction>(V);
565   if (!Inst || DT.dominates(Inst, Loc) || Visited.count(Inst))
566     return true;
567 
568   if (!isSafeToSpeculativelyExecute(Inst, Loc, &AC, &DT) ||
569       Inst->mayReadFromMemory())
570     return false;
571 
572   Visited.insert(Inst);
573 
574   // We only want to go _up_ the dominance chain when recursing.
575   assert(!isa<PHINode>(Loc) &&
576          "PHIs should return false for isSafeToSpeculativelyExecute");
577   assert(DT.isReachableFromEntry(Inst->getParent()) &&
578          "We did a DFS from the block entry!");
579   return all_of(Inst->operands(),
580                 [&](Value *Op) { return canBeHoistedTo(Op, Loc, Visited); });
581 }
582 
583 void GuardWideningImpl::makeAvailableAt(Value *V, Instruction *Loc) const {
584   auto *Inst = dyn_cast<Instruction>(V);
585   if (!Inst || DT.dominates(Inst, Loc))
586     return;
587 
588   assert(isSafeToSpeculativelyExecute(Inst, Loc, &AC, &DT) &&
589          !Inst->mayReadFromMemory() &&
590          "Should've checked with canBeHoistedTo!");
591 
592   for (Value *Op : Inst->operands())
593     makeAvailableAt(Op, Loc);
594 
595   Inst->moveBefore(Loc);
596 }
597 
598 // Return Instruction before which we can insert freeze for the value V as close
599 // to def as possible. If there is no place to add freeze, return empty.
600 static std::optional<BasicBlock::iterator>
601 getFreezeInsertPt(Value *V, const DominatorTree &DT) {
602   auto *I = dyn_cast<Instruction>(V);
603   if (!I)
604     return DT.getRoot()->getFirstNonPHIOrDbgOrAlloca()->getIterator();
605 
606   std::optional<BasicBlock::iterator> Res = I->getInsertionPointAfterDef();
607   // If there is no place to add freeze - return nullptr.
608   if (!Res || !DT.dominates(I, &**Res))
609     return std::nullopt;
610 
611   Instruction *ResInst = &**Res;
612 
613   // If there is a User dominated by original I, then it should be dominated
614   // by Freeze instruction as well.
615   if (any_of(I->users(), [&](User *U) {
616         Instruction *User = cast<Instruction>(U);
617         return ResInst != User && DT.dominates(I, User) &&
618                !DT.dominates(ResInst, User);
619       }))
620     return std::nullopt;
621   return Res;
622 }
623 
624 Value *GuardWideningImpl::freezeAndPush(Value *Orig, Instruction *InsertPt) {
625   if (isGuaranteedNotToBePoison(Orig, nullptr, InsertPt, &DT))
626     return Orig;
627   std::optional<BasicBlock::iterator> InsertPtAtDef =
628       getFreezeInsertPt(Orig, DT);
629   if (!InsertPtAtDef) {
630     FreezeInst *FI = new FreezeInst(Orig, "gw.freeze");
631     FI->insertBefore(InsertPt);
632     return FI;
633   }
634   if (isa<Constant>(Orig) || isa<GlobalValue>(Orig)) {
635     BasicBlock::iterator InsertPt = *InsertPtAtDef;
636     FreezeInst *FI = new FreezeInst(Orig, "gw.freeze");
637     FI->insertBefore(*InsertPt->getParent(), InsertPt);
638     return FI;
639   }
640 
641   SmallSet<Value *, 16> Visited;
642   SmallVector<Value *, 16> Worklist;
643   SmallSet<Instruction *, 16> DropPoisonFlags;
644   SmallVector<Value *, 16> NeedFreeze;
645   DenseMap<Value *, FreezeInst *> CacheOfFreezes;
646 
647   // A bit overloaded data structures. Visited contains constant/GV
648   // if we already met it. In this case CacheOfFreezes has a freeze if it is
649   // required.
650   auto handleConstantOrGlobal = [&](Use &U) {
651     Value *Def = U.get();
652     if (!isa<Constant>(Def) && !isa<GlobalValue>(Def))
653       return false;
654 
655     if (Visited.insert(Def).second) {
656       if (isGuaranteedNotToBePoison(Def, nullptr, InsertPt, &DT))
657         return true;
658       BasicBlock::iterator InsertPt = *getFreezeInsertPt(Def, DT);
659       FreezeInst *FI = new FreezeInst(Def, Def->getName() + ".gw.fr");
660       FI->insertBefore(*InsertPt->getParent(), InsertPt);
661       CacheOfFreezes[Def] = FI;
662     }
663 
664     if (CacheOfFreezes.count(Def))
665       U.set(CacheOfFreezes[Def]);
666     return true;
667   };
668 
669   Worklist.push_back(Orig);
670   while (!Worklist.empty()) {
671     Value *V = Worklist.pop_back_val();
672     if (!Visited.insert(V).second)
673       continue;
674 
675     if (isGuaranteedNotToBePoison(V, nullptr, InsertPt, &DT))
676       continue;
677 
678     Instruction *I = dyn_cast<Instruction>(V);
679     if (!I || canCreateUndefOrPoison(cast<Operator>(I),
680                                      /*ConsiderFlagsAndMetadata*/ false)) {
681       NeedFreeze.push_back(V);
682       continue;
683     }
684     // Check all operands. If for any of them we cannot insert Freeze,
685     // stop here. Otherwise, iterate.
686     if (any_of(I->operands(), [&](Value *Op) {
687           return isa<Instruction>(Op) && !getFreezeInsertPt(Op, DT);
688         })) {
689       NeedFreeze.push_back(I);
690       continue;
691     }
692     DropPoisonFlags.insert(I);
693     for (Use &U : I->operands())
694       if (!handleConstantOrGlobal(U))
695         Worklist.push_back(U.get());
696   }
697   for (Instruction *I : DropPoisonFlags)
698     I->dropPoisonGeneratingFlagsAndMetadata();
699 
700   Value *Result = Orig;
701   for (Value *V : NeedFreeze) {
702     BasicBlock::iterator FreezeInsertPt = *getFreezeInsertPt(V, DT);
703     FreezeInst *FI = new FreezeInst(V, V->getName() + ".gw.fr");
704     FI->insertBefore(*FreezeInsertPt->getParent(), FreezeInsertPt);
705     ++FreezeAdded;
706     if (V == Orig)
707       Result = FI;
708     V->replaceUsesWithIf(
709         FI, [&](const Use & U)->bool { return U.getUser() != FI; });
710   }
711 
712   return Result;
713 }
714 
715 std::optional<Value *>
716 GuardWideningImpl::mergeChecks(SmallVectorImpl<Value *> &ChecksToHoist,
717                                SmallVectorImpl<Value *> &ChecksToWiden,
718                                Instruction *InsertPt) {
719   using namespace llvm::PatternMatch;
720 
721   Value *Result = nullptr;
722   {
723     // L >u C0 && L >u C1  ->  L >u max(C0, C1)
724     ConstantInt *RHS0, *RHS1;
725     Value *LHS;
726     ICmpInst::Predicate Pred0, Pred1;
727     // TODO: Support searching for pairs to merge from both whole lists of
728     // ChecksToHoist and ChecksToWiden.
729     if (ChecksToWiden.size() == 1 && ChecksToHoist.size() == 1 &&
730         match(ChecksToWiden.front(),
731               m_ICmp(Pred0, m_Value(LHS), m_ConstantInt(RHS0))) &&
732         match(ChecksToHoist.front(),
733               m_ICmp(Pred1, m_Specific(LHS), m_ConstantInt(RHS1)))) {
734 
735       ConstantRange CR0 =
736           ConstantRange::makeExactICmpRegion(Pred0, RHS0->getValue());
737       ConstantRange CR1 =
738           ConstantRange::makeExactICmpRegion(Pred1, RHS1->getValue());
739 
740       // Given what we're doing here and the semantics of guards, it would
741       // be correct to use a subset intersection, but that may be too
742       // aggressive in cases we care about.
743       if (std::optional<ConstantRange> Intersect =
744               CR0.exactIntersectWith(CR1)) {
745         APInt NewRHSAP;
746         CmpInst::Predicate Pred;
747         if (Intersect->getEquivalentICmp(Pred, NewRHSAP)) {
748           if (InsertPt) {
749             ConstantInt *NewRHS =
750                 ConstantInt::get(InsertPt->getContext(), NewRHSAP);
751             assert(canBeHoistedTo(LHS, InsertPt) && "must be");
752             makeAvailableAt(LHS, InsertPt);
753             Result = new ICmpInst(InsertPt, Pred, LHS, NewRHS, "wide.chk");
754           }
755           return Result;
756         }
757       }
758     }
759   }
760 
761   {
762     SmallVector<GuardWideningImpl::RangeCheck, 4> Checks, CombinedChecks;
763     if (parseRangeChecks(ChecksToWiden, Checks) &&
764         parseRangeChecks(ChecksToHoist, Checks) &&
765         combineRangeChecks(Checks, CombinedChecks)) {
766       if (InsertPt) {
767         for (auto &RC : CombinedChecks) {
768           makeAvailableAt(RC.getCheckInst(), InsertPt);
769           if (Result)
770             Result = BinaryOperator::CreateAnd(RC.getCheckInst(), Result, "",
771                                                InsertPt);
772           else
773             Result = RC.getCheckInst();
774         }
775         assert(Result && "Failed to find result value");
776         Result->setName("wide.chk");
777         Result = freezeAndPush(Result, InsertPt);
778       }
779       return Result;
780     }
781   }
782   // We were not able to compute ChecksToHoist AND ChecksToWiden for the price
783   // of one.
784   return std::nullopt;
785 }
786 
787 Value *GuardWideningImpl::hoistChecks(SmallVectorImpl<Value *> &ChecksToHoist,
788                                       Value *OldCondition,
789                                       Instruction *InsertPt) {
790   assert(!ChecksToHoist.empty());
791   IRBuilder<> Builder(InsertPt);
792   makeAvailableAt(ChecksToHoist, InsertPt);
793   makeAvailableAt(OldCondition, InsertPt);
794   Value *Result = Builder.CreateAnd(ChecksToHoist);
795   Result = freezeAndPush(Result, InsertPt);
796   Result = Builder.CreateAnd(OldCondition, Result);
797   Result->setName("wide.chk");
798   return Result;
799 }
800 
801 bool GuardWideningImpl::parseRangeChecks(
802     Value *CheckCond, SmallVectorImpl<GuardWideningImpl::RangeCheck> &Checks) {
803   using namespace llvm::PatternMatch;
804 
805   auto *IC = dyn_cast<ICmpInst>(CheckCond);
806   if (!IC || !IC->getOperand(0)->getType()->isIntegerTy() ||
807       (IC->getPredicate() != ICmpInst::ICMP_ULT &&
808        IC->getPredicate() != ICmpInst::ICMP_UGT))
809     return false;
810 
811   const Value *CmpLHS = IC->getOperand(0), *CmpRHS = IC->getOperand(1);
812   if (IC->getPredicate() == ICmpInst::ICMP_UGT)
813     std::swap(CmpLHS, CmpRHS);
814 
815   auto &DL = IC->getModule()->getDataLayout();
816 
817   GuardWideningImpl::RangeCheck Check(
818       CmpLHS, cast<ConstantInt>(ConstantInt::getNullValue(CmpRHS->getType())),
819       CmpRHS, IC);
820 
821   if (!isKnownNonNegative(Check.getLength(), DL))
822     return false;
823 
824   // What we have in \c Check now is a correct interpretation of \p CheckCond.
825   // Try to see if we can move some constant offsets into the \c Offset field.
826 
827   bool Changed;
828   auto &Ctx = CheckCond->getContext();
829 
830   do {
831     Value *OpLHS;
832     ConstantInt *OpRHS;
833     Changed = false;
834 
835 #ifndef NDEBUG
836     auto *BaseInst = dyn_cast<Instruction>(Check.getBase());
837     assert((!BaseInst || DT.isReachableFromEntry(BaseInst->getParent())) &&
838            "Unreachable instruction?");
839 #endif
840 
841     if (match(Check.getBase(), m_Add(m_Value(OpLHS), m_ConstantInt(OpRHS)))) {
842       Check.setBase(OpLHS);
843       APInt NewOffset = Check.getOffsetValue() + OpRHS->getValue();
844       Check.setOffset(ConstantInt::get(Ctx, NewOffset));
845       Changed = true;
846     } else if (match(Check.getBase(),
847                      m_Or(m_Value(OpLHS), m_ConstantInt(OpRHS)))) {
848       KnownBits Known = computeKnownBits(OpLHS, DL);
849       if ((OpRHS->getValue() & Known.Zero) == OpRHS->getValue()) {
850         Check.setBase(OpLHS);
851         APInt NewOffset = Check.getOffsetValue() + OpRHS->getValue();
852         Check.setOffset(ConstantInt::get(Ctx, NewOffset));
853         Changed = true;
854       }
855     }
856   } while (Changed);
857 
858   Checks.push_back(Check);
859   return true;
860 }
861 
862 bool GuardWideningImpl::combineRangeChecks(
863     SmallVectorImpl<GuardWideningImpl::RangeCheck> &Checks,
864     SmallVectorImpl<GuardWideningImpl::RangeCheck> &RangeChecksOut) const {
865   unsigned OldCount = Checks.size();
866   while (!Checks.empty()) {
867     // Pick all of the range checks with a specific base and length, and try to
868     // merge them.
869     const Value *CurrentBase = Checks.front().getBase();
870     const Value *CurrentLength = Checks.front().getLength();
871 
872     SmallVector<GuardWideningImpl::RangeCheck, 3> CurrentChecks;
873 
874     auto IsCurrentCheck = [&](GuardWideningImpl::RangeCheck &RC) {
875       return RC.getBase() == CurrentBase && RC.getLength() == CurrentLength;
876     };
877 
878     copy_if(Checks, std::back_inserter(CurrentChecks), IsCurrentCheck);
879     erase_if(Checks, IsCurrentCheck);
880 
881     assert(CurrentChecks.size() != 0 && "We know we have at least one!");
882 
883     if (CurrentChecks.size() < 3) {
884       llvm::append_range(RangeChecksOut, CurrentChecks);
885       continue;
886     }
887 
888     // CurrentChecks.size() will typically be 3 here, but so far there has been
889     // no need to hard-code that fact.
890 
891     llvm::sort(CurrentChecks, [&](const GuardWideningImpl::RangeCheck &LHS,
892                                   const GuardWideningImpl::RangeCheck &RHS) {
893       return LHS.getOffsetValue().slt(RHS.getOffsetValue());
894     });
895 
896     // Note: std::sort should not invalidate the ChecksStart iterator.
897 
898     const ConstantInt *MinOffset = CurrentChecks.front().getOffset();
899     const ConstantInt *MaxOffset = CurrentChecks.back().getOffset();
900 
901     unsigned BitWidth = MaxOffset->getValue().getBitWidth();
902     if ((MaxOffset->getValue() - MinOffset->getValue())
903             .ugt(APInt::getSignedMinValue(BitWidth)))
904       return false;
905 
906     APInt MaxDiff = MaxOffset->getValue() - MinOffset->getValue();
907     const APInt &HighOffset = MaxOffset->getValue();
908     auto OffsetOK = [&](const GuardWideningImpl::RangeCheck &RC) {
909       return (HighOffset - RC.getOffsetValue()).ult(MaxDiff);
910     };
911 
912     if (MaxDiff.isMinValue() || !all_of(drop_begin(CurrentChecks), OffsetOK))
913       return false;
914 
915     // We have a series of f+1 checks as:
916     //
917     //   I+k_0 u< L   ... Chk_0
918     //   I+k_1 u< L   ... Chk_1
919     //   ...
920     //   I+k_f u< L   ... Chk_f
921     //
922     //     with forall i in [0,f]: k_f-k_i u< k_f-k_0  ... Precond_0
923     //          k_f-k_0 u< INT_MIN+k_f                 ... Precond_1
924     //          k_f != k_0                             ... Precond_2
925     //
926     // Claim:
927     //   Chk_0 AND Chk_f  implies all the other checks
928     //
929     // Informal proof sketch:
930     //
931     // We will show that the integer range [I+k_0,I+k_f] does not unsigned-wrap
932     // (i.e. going from I+k_0 to I+k_f does not cross the -1,0 boundary) and
933     // thus I+k_f is the greatest unsigned value in that range.
934     //
935     // This combined with Ckh_(f+1) shows that everything in that range is u< L.
936     // Via Precond_0 we know that all of the indices in Chk_0 through Chk_(f+1)
937     // lie in [I+k_0,I+k_f], this proving our claim.
938     //
939     // To see that [I+k_0,I+k_f] is not a wrapping range, note that there are
940     // two possibilities: I+k_0 u< I+k_f or I+k_0 >u I+k_f (they can't be equal
941     // since k_0 != k_f).  In the former case, [I+k_0,I+k_f] is not a wrapping
942     // range by definition, and the latter case is impossible:
943     //
944     //   0-----I+k_f---I+k_0----L---INT_MAX,INT_MIN------------------(-1)
945     //   xxxxxx             xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
946     //
947     // For Chk_0 to succeed, we'd have to have k_f-k_0 (the range highlighted
948     // with 'x' above) to be at least >u INT_MIN.
949 
950     RangeChecksOut.emplace_back(CurrentChecks.front());
951     RangeChecksOut.emplace_back(CurrentChecks.back());
952   }
953 
954   assert(RangeChecksOut.size() <= OldCount && "We pessimized!");
955   return RangeChecksOut.size() != OldCount;
956 }
957 
958 #ifndef NDEBUG
959 StringRef GuardWideningImpl::scoreTypeToString(WideningScore WS) {
960   switch (WS) {
961   case WS_IllegalOrNegative:
962     return "IllegalOrNegative";
963   case WS_Neutral:
964     return "Neutral";
965   case WS_Positive:
966     return "Positive";
967   case WS_VeryPositive:
968     return "VeryPositive";
969   }
970 
971   llvm_unreachable("Fully covered switch above!");
972 }
973 #endif
974 
975 PreservedAnalyses GuardWideningPass::run(Function &F,
976                                          FunctionAnalysisManager &AM) {
977   // Avoid requesting analyses if there are no guards or widenable conditions.
978   auto *GuardDecl = F.getParent()->getFunction(
979       Intrinsic::getName(Intrinsic::experimental_guard));
980   bool HasIntrinsicGuards = GuardDecl && !GuardDecl->use_empty();
981   auto *WCDecl = F.getParent()->getFunction(
982       Intrinsic::getName(Intrinsic::experimental_widenable_condition));
983   bool HasWidenableConditions = WCDecl && !WCDecl->use_empty();
984   if (!HasIntrinsicGuards && !HasWidenableConditions)
985     return PreservedAnalyses::all();
986   auto &DT = AM.getResult<DominatorTreeAnalysis>(F);
987   auto &LI = AM.getResult<LoopAnalysis>(F);
988   auto &PDT = AM.getResult<PostDominatorTreeAnalysis>(F);
989   auto &AC = AM.getResult<AssumptionAnalysis>(F);
990   auto *MSSAA = AM.getCachedResult<MemorySSAAnalysis>(F);
991   std::unique_ptr<MemorySSAUpdater> MSSAU;
992   if (MSSAA)
993     MSSAU = std::make_unique<MemorySSAUpdater>(&MSSAA->getMSSA());
994   if (!GuardWideningImpl(DT, &PDT, LI, AC, MSSAU ? MSSAU.get() : nullptr,
995                          DT.getRootNode(), [](BasicBlock *) { return true; })
996            .run())
997     return PreservedAnalyses::all();
998 
999   PreservedAnalyses PA;
1000   PA.preserveSet<CFGAnalyses>();
1001   PA.preserve<MemorySSAAnalysis>();
1002   return PA;
1003 }
1004 
1005 PreservedAnalyses GuardWideningPass::run(Loop &L, LoopAnalysisManager &AM,
1006                                          LoopStandardAnalysisResults &AR,
1007                                          LPMUpdater &U) {
1008   BasicBlock *RootBB = L.getLoopPredecessor();
1009   if (!RootBB)
1010     RootBB = L.getHeader();
1011   auto BlockFilter = [&](BasicBlock *BB) {
1012     return BB == RootBB || L.contains(BB);
1013   };
1014   std::unique_ptr<MemorySSAUpdater> MSSAU;
1015   if (AR.MSSA)
1016     MSSAU = std::make_unique<MemorySSAUpdater>(AR.MSSA);
1017   if (!GuardWideningImpl(AR.DT, nullptr, AR.LI, AR.AC,
1018                          MSSAU ? MSSAU.get() : nullptr, AR.DT.getNode(RootBB),
1019                          BlockFilter)
1020            .run())
1021     return PreservedAnalyses::all();
1022 
1023   auto PA = getLoopPassPreservedAnalyses();
1024   if (AR.MSSA)
1025     PA.preserve<MemorySSAAnalysis>();
1026   return PA;
1027 }
1028