1 //===- LoopAccessAnalysis.cpp - Loop Access Analysis Implementation --------==//
2 //
3 // The LLVM Compiler Infrastructure
4 //
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
7 //
8 //===----------------------------------------------------------------------===//
9 //
10 // The implementation for the loop memory dependence that was originally
11 // developed for the loop vectorizer.
12 //
13 //===----------------------------------------------------------------------===//
14
15 #include "llvm/Analysis/LoopAccessAnalysis.h"
16 #include "llvm/ADT/APInt.h"
17 #include "llvm/ADT/DenseMap.h"
18 #include "llvm/ADT/DepthFirstIterator.h"
19 #include "llvm/ADT/EquivalenceClasses.h"
20 #include "llvm/ADT/PointerIntPair.h"
21 #include "llvm/ADT/STLExtras.h"
22 #include "llvm/ADT/SetVector.h"
23 #include "llvm/ADT/SmallPtrSet.h"
24 #include "llvm/ADT/SmallSet.h"
25 #include "llvm/ADT/SmallVector.h"
26 #include "llvm/ADT/iterator_range.h"
27 #include "llvm/Analysis/AliasAnalysis.h"
28 #include "llvm/Analysis/AliasSetTracker.h"
29 #include "llvm/Analysis/LoopAnalysisManager.h"
30 #include "llvm/Analysis/LoopInfo.h"
31 #include "llvm/Analysis/MemoryLocation.h"
32 #include "llvm/Analysis/OptimizationRemarkEmitter.h"
33 #include "llvm/Analysis/ScalarEvolution.h"
34 #include "llvm/Analysis/ScalarEvolutionExpander.h"
35 #include "llvm/Analysis/ScalarEvolutionExpressions.h"
36 #include "llvm/Analysis/TargetLibraryInfo.h"
37 #include "llvm/Analysis/ValueTracking.h"
38 #include "llvm/Analysis/VectorUtils.h"
39 #include "llvm/IR/BasicBlock.h"
40 #include "llvm/IR/Constants.h"
41 #include "llvm/IR/DataLayout.h"
42 #include "llvm/IR/DebugLoc.h"
43 #include "llvm/IR/DerivedTypes.h"
44 #include "llvm/IR/DiagnosticInfo.h"
45 #include "llvm/IR/Dominators.h"
46 #include "llvm/IR/Function.h"
47 #include "llvm/IR/IRBuilder.h"
48 #include "llvm/IR/InstrTypes.h"
49 #include "llvm/IR/Instruction.h"
50 #include "llvm/IR/Instructions.h"
51 #include "llvm/IR/Operator.h"
52 #include "llvm/IR/PassManager.h"
53 #include "llvm/IR/Type.h"
54 #include "llvm/IR/Value.h"
55 #include "llvm/IR/ValueHandle.h"
56 #include "llvm/Pass.h"
57 #include "llvm/Support/Casting.h"
58 #include "llvm/Support/CommandLine.h"
59 #include "llvm/Support/Debug.h"
60 #include "llvm/Support/ErrorHandling.h"
61 #include "llvm/Support/raw_ostream.h"
62 #include <algorithm>
63 #include <cassert>
64 #include <cstdint>
65 #include <cstdlib>
66 #include <iterator>
67 #include <utility>
68 #include <vector>
69
70 using namespace llvm;
71
72 #define DEBUG_TYPE "loop-accesses"
73
74 static cl::opt<unsigned, true>
75 VectorizationFactor("force-vector-width", cl::Hidden,
76 cl::desc("Sets the SIMD width. Zero is autoselect."),
77 cl::location(VectorizerParams::VectorizationFactor));
78 unsigned VectorizerParams::VectorizationFactor;
79
80 static cl::opt<unsigned, true>
81 VectorizationInterleave("force-vector-interleave", cl::Hidden,
82 cl::desc("Sets the vectorization interleave count. "
83 "Zero is autoselect."),
84 cl::location(
85 VectorizerParams::VectorizationInterleave));
86 unsigned VectorizerParams::VectorizationInterleave;
87
88 static cl::opt<unsigned, true> RuntimeMemoryCheckThreshold(
89 "runtime-memory-check-threshold", cl::Hidden,
90 cl::desc("When performing memory disambiguation checks at runtime do not "
91 "generate more than this number of comparisons (default = 8)."),
92 cl::location(VectorizerParams::RuntimeMemoryCheckThreshold), cl::init(8));
93 unsigned VectorizerParams::RuntimeMemoryCheckThreshold;
94
95 /// The maximum iterations used to merge memory checks
96 static cl::opt<unsigned> MemoryCheckMergeThreshold(
97 "memory-check-merge-threshold", cl::Hidden,
98 cl::desc("Maximum number of comparisons done when trying to merge "
99 "runtime memory checks. (default = 100)"),
100 cl::init(100));
101
102 /// Maximum SIMD width.
103 const unsigned VectorizerParams::MaxVectorWidth = 64;
104
105 /// We collect dependences up to this threshold.
106 static cl::opt<unsigned>
107 MaxDependences("max-dependences", cl::Hidden,
108 cl::desc("Maximum number of dependences collected by "
109 "loop-access analysis (default = 100)"),
110 cl::init(100));
111
112 /// This enables versioning on the strides of symbolically striding memory
113 /// accesses in code like the following.
114 /// for (i = 0; i < N; ++i)
115 /// A[i * Stride1] += B[i * Stride2] ...
116 ///
117 /// Will be roughly translated to
118 /// if (Stride1 == 1 && Stride2 == 1) {
119 /// for (i = 0; i < N; i+=4)
120 /// A[i:i+3] += ...
121 /// } else
122 /// ...
123 static cl::opt<bool> EnableMemAccessVersioning(
124 "enable-mem-access-versioning", cl::init(true), cl::Hidden,
125 cl::desc("Enable symbolic stride memory access versioning"));
126
127 /// Enable store-to-load forwarding conflict detection. This option can
128 /// be disabled for correctness testing.
129 static cl::opt<bool> EnableForwardingConflictDetection(
130 "store-to-load-forwarding-conflict-detection", cl::Hidden,
131 cl::desc("Enable conflict detection in loop-access analysis"),
132 cl::init(true));
133
isInterleaveForced()134 bool VectorizerParams::isInterleaveForced() {
135 return ::VectorizationInterleave.getNumOccurrences() > 0;
136 }
137
stripIntegerCast(Value * V)138 Value *llvm::stripIntegerCast(Value *V) {
139 if (auto *CI = dyn_cast<CastInst>(V))
140 if (CI->getOperand(0)->getType()->isIntegerTy())
141 return CI->getOperand(0);
142 return V;
143 }
144
replaceSymbolicStrideSCEV(PredicatedScalarEvolution & PSE,const ValueToValueMap & PtrToStride,Value * Ptr,Value * OrigPtr)145 const SCEV *llvm::replaceSymbolicStrideSCEV(PredicatedScalarEvolution &PSE,
146 const ValueToValueMap &PtrToStride,
147 Value *Ptr, Value *OrigPtr) {
148 const SCEV *OrigSCEV = PSE.getSCEV(Ptr);
149
150 // If there is an entry in the map return the SCEV of the pointer with the
151 // symbolic stride replaced by one.
152 ValueToValueMap::const_iterator SI =
153 PtrToStride.find(OrigPtr ? OrigPtr : Ptr);
154 if (SI != PtrToStride.end()) {
155 Value *StrideVal = SI->second;
156
157 // Strip casts.
158 StrideVal = stripIntegerCast(StrideVal);
159
160 ScalarEvolution *SE = PSE.getSE();
161 const auto *U = cast<SCEVUnknown>(SE->getSCEV(StrideVal));
162 const auto *CT =
163 static_cast<const SCEVConstant *>(SE->getOne(StrideVal->getType()));
164
165 PSE.addPredicate(*SE->getEqualPredicate(U, CT));
166 auto *Expr = PSE.getSCEV(Ptr);
167
168 LLVM_DEBUG(dbgs() << "LAA: Replacing SCEV: " << *OrigSCEV
169 << " by: " << *Expr << "\n");
170 return Expr;
171 }
172
173 // Otherwise, just return the SCEV of the original pointer.
174 return OrigSCEV;
175 }
176
177 /// Calculate Start and End points of memory access.
178 /// Let's assume A is the first access and B is a memory access on N-th loop
179 /// iteration. Then B is calculated as:
180 /// B = A + Step*N .
181 /// Step value may be positive or negative.
182 /// N is a calculated back-edge taken count:
183 /// N = (TripCount > 0) ? RoundDown(TripCount -1 , VF) : 0
184 /// Start and End points are calculated in the following way:
185 /// Start = UMIN(A, B) ; End = UMAX(A, B) + SizeOfElt,
186 /// where SizeOfElt is the size of single memory access in bytes.
187 ///
188 /// There is no conflict when the intervals are disjoint:
189 /// NoConflict = (P2.Start >= P1.End) || (P1.Start >= P2.End)
insert(Loop * Lp,Value * Ptr,bool WritePtr,unsigned DepSetId,unsigned ASId,const ValueToValueMap & Strides,PredicatedScalarEvolution & PSE)190 void RuntimePointerChecking::insert(Loop *Lp, Value *Ptr, bool WritePtr,
191 unsigned DepSetId, unsigned ASId,
192 const ValueToValueMap &Strides,
193 PredicatedScalarEvolution &PSE) {
194 // Get the stride replaced scev.
195 const SCEV *Sc = replaceSymbolicStrideSCEV(PSE, Strides, Ptr);
196 ScalarEvolution *SE = PSE.getSE();
197
198 const SCEV *ScStart;
199 const SCEV *ScEnd;
200
201 if (SE->isLoopInvariant(Sc, Lp))
202 ScStart = ScEnd = Sc;
203 else {
204 const SCEVAddRecExpr *AR = dyn_cast<SCEVAddRecExpr>(Sc);
205 assert(AR && "Invalid addrec expression");
206 const SCEV *Ex = PSE.getBackedgeTakenCount();
207
208 ScStart = AR->getStart();
209 ScEnd = AR->evaluateAtIteration(Ex, *SE);
210 const SCEV *Step = AR->getStepRecurrence(*SE);
211
212 // For expressions with negative step, the upper bound is ScStart and the
213 // lower bound is ScEnd.
214 if (const auto *CStep = dyn_cast<SCEVConstant>(Step)) {
215 if (CStep->getValue()->isNegative())
216 std::swap(ScStart, ScEnd);
217 } else {
218 // Fallback case: the step is not constant, but we can still
219 // get the upper and lower bounds of the interval by using min/max
220 // expressions.
221 ScStart = SE->getUMinExpr(ScStart, ScEnd);
222 ScEnd = SE->getUMaxExpr(AR->getStart(), ScEnd);
223 }
224 // Add the size of the pointed element to ScEnd.
225 unsigned EltSize =
226 Ptr->getType()->getPointerElementType()->getScalarSizeInBits() / 8;
227 const SCEV *EltSizeSCEV = SE->getConstant(ScEnd->getType(), EltSize);
228 ScEnd = SE->getAddExpr(ScEnd, EltSizeSCEV);
229 }
230
231 Pointers.emplace_back(Ptr, ScStart, ScEnd, WritePtr, DepSetId, ASId, Sc);
232 }
233
234 SmallVector<RuntimePointerChecking::PointerCheck, 4>
generateChecks() const235 RuntimePointerChecking::generateChecks() const {
236 SmallVector<PointerCheck, 4> Checks;
237
238 for (unsigned I = 0; I < CheckingGroups.size(); ++I) {
239 for (unsigned J = I + 1; J < CheckingGroups.size(); ++J) {
240 const RuntimePointerChecking::CheckingPtrGroup &CGI = CheckingGroups[I];
241 const RuntimePointerChecking::CheckingPtrGroup &CGJ = CheckingGroups[J];
242
243 if (needsChecking(CGI, CGJ))
244 Checks.push_back(std::make_pair(&CGI, &CGJ));
245 }
246 }
247 return Checks;
248 }
249
generateChecks(MemoryDepChecker::DepCandidates & DepCands,bool UseDependencies)250 void RuntimePointerChecking::generateChecks(
251 MemoryDepChecker::DepCandidates &DepCands, bool UseDependencies) {
252 assert(Checks.empty() && "Checks is not empty");
253 groupChecks(DepCands, UseDependencies);
254 Checks = generateChecks();
255 }
256
needsChecking(const CheckingPtrGroup & M,const CheckingPtrGroup & N) const257 bool RuntimePointerChecking::needsChecking(const CheckingPtrGroup &M,
258 const CheckingPtrGroup &N) const {
259 for (unsigned I = 0, EI = M.Members.size(); EI != I; ++I)
260 for (unsigned J = 0, EJ = N.Members.size(); EJ != J; ++J)
261 if (needsChecking(M.Members[I], N.Members[J]))
262 return true;
263 return false;
264 }
265
266 /// Compare \p I and \p J and return the minimum.
267 /// Return nullptr in case we couldn't find an answer.
getMinFromExprs(const SCEV * I,const SCEV * J,ScalarEvolution * SE)268 static const SCEV *getMinFromExprs(const SCEV *I, const SCEV *J,
269 ScalarEvolution *SE) {
270 const SCEV *Diff = SE->getMinusSCEV(J, I);
271 const SCEVConstant *C = dyn_cast<const SCEVConstant>(Diff);
272
273 if (!C)
274 return nullptr;
275 if (C->getValue()->isNegative())
276 return J;
277 return I;
278 }
279
addPointer(unsigned Index)280 bool RuntimePointerChecking::CheckingPtrGroup::addPointer(unsigned Index) {
281 const SCEV *Start = RtCheck.Pointers[Index].Start;
282 const SCEV *End = RtCheck.Pointers[Index].End;
283
284 // Compare the starts and ends with the known minimum and maximum
285 // of this set. We need to know how we compare against the min/max
286 // of the set in order to be able to emit memchecks.
287 const SCEV *Min0 = getMinFromExprs(Start, Low, RtCheck.SE);
288 if (!Min0)
289 return false;
290
291 const SCEV *Min1 = getMinFromExprs(End, High, RtCheck.SE);
292 if (!Min1)
293 return false;
294
295 // Update the low bound expression if we've found a new min value.
296 if (Min0 == Start)
297 Low = Start;
298
299 // Update the high bound expression if we've found a new max value.
300 if (Min1 != End)
301 High = End;
302
303 Members.push_back(Index);
304 return true;
305 }
306
groupChecks(MemoryDepChecker::DepCandidates & DepCands,bool UseDependencies)307 void RuntimePointerChecking::groupChecks(
308 MemoryDepChecker::DepCandidates &DepCands, bool UseDependencies) {
309 // We build the groups from dependency candidates equivalence classes
310 // because:
311 // - We know that pointers in the same equivalence class share
312 // the same underlying object and therefore there is a chance
313 // that we can compare pointers
314 // - We wouldn't be able to merge two pointers for which we need
315 // to emit a memcheck. The classes in DepCands are already
316 // conveniently built such that no two pointers in the same
317 // class need checking against each other.
318
319 // We use the following (greedy) algorithm to construct the groups
320 // For every pointer in the equivalence class:
321 // For each existing group:
322 // - if the difference between this pointer and the min/max bounds
323 // of the group is a constant, then make the pointer part of the
324 // group and update the min/max bounds of that group as required.
325
326 CheckingGroups.clear();
327
328 // If we need to check two pointers to the same underlying object
329 // with a non-constant difference, we shouldn't perform any pointer
330 // grouping with those pointers. This is because we can easily get
331 // into cases where the resulting check would return false, even when
332 // the accesses are safe.
333 //
334 // The following example shows this:
335 // for (i = 0; i < 1000; ++i)
336 // a[5000 + i * m] = a[i] + a[i + 9000]
337 //
338 // Here grouping gives a check of (5000, 5000 + 1000 * m) against
339 // (0, 10000) which is always false. However, if m is 1, there is no
340 // dependence. Not grouping the checks for a[i] and a[i + 9000] allows
341 // us to perform an accurate check in this case.
342 //
343 // The above case requires that we have an UnknownDependence between
344 // accesses to the same underlying object. This cannot happen unless
345 // FoundNonConstantDistanceDependence is set, and therefore UseDependencies
346 // is also false. In this case we will use the fallback path and create
347 // separate checking groups for all pointers.
348
349 // If we don't have the dependency partitions, construct a new
350 // checking pointer group for each pointer. This is also required
351 // for correctness, because in this case we can have checking between
352 // pointers to the same underlying object.
353 if (!UseDependencies) {
354 for (unsigned I = 0; I < Pointers.size(); ++I)
355 CheckingGroups.push_back(CheckingPtrGroup(I, *this));
356 return;
357 }
358
359 unsigned TotalComparisons = 0;
360
361 DenseMap<Value *, unsigned> PositionMap;
362 for (unsigned Index = 0; Index < Pointers.size(); ++Index)
363 PositionMap[Pointers[Index].PointerValue] = Index;
364
365 // We need to keep track of what pointers we've already seen so we
366 // don't process them twice.
367 SmallSet<unsigned, 2> Seen;
368
369 // Go through all equivalence classes, get the "pointer check groups"
370 // and add them to the overall solution. We use the order in which accesses
371 // appear in 'Pointers' to enforce determinism.
372 for (unsigned I = 0; I < Pointers.size(); ++I) {
373 // We've seen this pointer before, and therefore already processed
374 // its equivalence class.
375 if (Seen.count(I))
376 continue;
377
378 MemoryDepChecker::MemAccessInfo Access(Pointers[I].PointerValue,
379 Pointers[I].IsWritePtr);
380
381 SmallVector<CheckingPtrGroup, 2> Groups;
382 auto LeaderI = DepCands.findValue(DepCands.getLeaderValue(Access));
383
384 // Because DepCands is constructed by visiting accesses in the order in
385 // which they appear in alias sets (which is deterministic) and the
386 // iteration order within an equivalence class member is only dependent on
387 // the order in which unions and insertions are performed on the
388 // equivalence class, the iteration order is deterministic.
389 for (auto MI = DepCands.member_begin(LeaderI), ME = DepCands.member_end();
390 MI != ME; ++MI) {
391 unsigned Pointer = PositionMap[MI->getPointer()];
392 bool Merged = false;
393 // Mark this pointer as seen.
394 Seen.insert(Pointer);
395
396 // Go through all the existing sets and see if we can find one
397 // which can include this pointer.
398 for (CheckingPtrGroup &Group : Groups) {
399 // Don't perform more than a certain amount of comparisons.
400 // This should limit the cost of grouping the pointers to something
401 // reasonable. If we do end up hitting this threshold, the algorithm
402 // will create separate groups for all remaining pointers.
403 if (TotalComparisons > MemoryCheckMergeThreshold)
404 break;
405
406 TotalComparisons++;
407
408 if (Group.addPointer(Pointer)) {
409 Merged = true;
410 break;
411 }
412 }
413
414 if (!Merged)
415 // We couldn't add this pointer to any existing set or the threshold
416 // for the number of comparisons has been reached. Create a new group
417 // to hold the current pointer.
418 Groups.push_back(CheckingPtrGroup(Pointer, *this));
419 }
420
421 // We've computed the grouped checks for this partition.
422 // Save the results and continue with the next one.
423 llvm::copy(Groups, std::back_inserter(CheckingGroups));
424 }
425 }
426
arePointersInSamePartition(const SmallVectorImpl<int> & PtrToPartition,unsigned PtrIdx1,unsigned PtrIdx2)427 bool RuntimePointerChecking::arePointersInSamePartition(
428 const SmallVectorImpl<int> &PtrToPartition, unsigned PtrIdx1,
429 unsigned PtrIdx2) {
430 return (PtrToPartition[PtrIdx1] != -1 &&
431 PtrToPartition[PtrIdx1] == PtrToPartition[PtrIdx2]);
432 }
433
needsChecking(unsigned I,unsigned J) const434 bool RuntimePointerChecking::needsChecking(unsigned I, unsigned J) const {
435 const PointerInfo &PointerI = Pointers[I];
436 const PointerInfo &PointerJ = Pointers[J];
437
438 // No need to check if two readonly pointers intersect.
439 if (!PointerI.IsWritePtr && !PointerJ.IsWritePtr)
440 return false;
441
442 // Only need to check pointers between two different dependency sets.
443 if (PointerI.DependencySetId == PointerJ.DependencySetId)
444 return false;
445
446 // Only need to check pointers in the same alias set.
447 if (PointerI.AliasSetId != PointerJ.AliasSetId)
448 return false;
449
450 return true;
451 }
452
printChecks(raw_ostream & OS,const SmallVectorImpl<PointerCheck> & Checks,unsigned Depth) const453 void RuntimePointerChecking::printChecks(
454 raw_ostream &OS, const SmallVectorImpl<PointerCheck> &Checks,
455 unsigned Depth) const {
456 unsigned N = 0;
457 for (const auto &Check : Checks) {
458 const auto &First = Check.first->Members, &Second = Check.second->Members;
459
460 OS.indent(Depth) << "Check " << N++ << ":\n";
461
462 OS.indent(Depth + 2) << "Comparing group (" << Check.first << "):\n";
463 for (unsigned K = 0; K < First.size(); ++K)
464 OS.indent(Depth + 2) << *Pointers[First[K]].PointerValue << "\n";
465
466 OS.indent(Depth + 2) << "Against group (" << Check.second << "):\n";
467 for (unsigned K = 0; K < Second.size(); ++K)
468 OS.indent(Depth + 2) << *Pointers[Second[K]].PointerValue << "\n";
469 }
470 }
471
print(raw_ostream & OS,unsigned Depth) const472 void RuntimePointerChecking::print(raw_ostream &OS, unsigned Depth) const {
473
474 OS.indent(Depth) << "Run-time memory checks:\n";
475 printChecks(OS, Checks, Depth);
476
477 OS.indent(Depth) << "Grouped accesses:\n";
478 for (unsigned I = 0; I < CheckingGroups.size(); ++I) {
479 const auto &CG = CheckingGroups[I];
480
481 OS.indent(Depth + 2) << "Group " << &CG << ":\n";
482 OS.indent(Depth + 4) << "(Low: " << *CG.Low << " High: " << *CG.High
483 << ")\n";
484 for (unsigned J = 0; J < CG.Members.size(); ++J) {
485 OS.indent(Depth + 6) << "Member: " << *Pointers[CG.Members[J]].Expr
486 << "\n";
487 }
488 }
489 }
490
491 namespace {
492
493 /// Analyses memory accesses in a loop.
494 ///
495 /// Checks whether run time pointer checks are needed and builds sets for data
496 /// dependence checking.
497 class AccessAnalysis {
498 public:
499 /// Read or write access location.
500 typedef PointerIntPair<Value *, 1, bool> MemAccessInfo;
501 typedef SmallVector<MemAccessInfo, 8> MemAccessInfoList;
502
AccessAnalysis(const DataLayout & Dl,Loop * TheLoop,AliasAnalysis * AA,LoopInfo * LI,MemoryDepChecker::DepCandidates & DA,PredicatedScalarEvolution & PSE)503 AccessAnalysis(const DataLayout &Dl, Loop *TheLoop, AliasAnalysis *AA,
504 LoopInfo *LI, MemoryDepChecker::DepCandidates &DA,
505 PredicatedScalarEvolution &PSE)
506 : DL(Dl), TheLoop(TheLoop), AST(*AA), LI(LI), DepCands(DA),
507 IsRTCheckAnalysisNeeded(false), PSE(PSE) {}
508
509 /// Register a load and whether it is only read from.
addLoad(MemoryLocation & Loc,bool IsReadOnly)510 void addLoad(MemoryLocation &Loc, bool IsReadOnly) {
511 Value *Ptr = const_cast<Value*>(Loc.Ptr);
512 AST.add(Ptr, LocationSize::unknown(), Loc.AATags);
513 Accesses.insert(MemAccessInfo(Ptr, false));
514 if (IsReadOnly)
515 ReadOnlyPtr.insert(Ptr);
516 }
517
518 /// Register a store.
addStore(MemoryLocation & Loc)519 void addStore(MemoryLocation &Loc) {
520 Value *Ptr = const_cast<Value*>(Loc.Ptr);
521 AST.add(Ptr, LocationSize::unknown(), Loc.AATags);
522 Accesses.insert(MemAccessInfo(Ptr, true));
523 }
524
525 /// Check if we can emit a run-time no-alias check for \p Access.
526 ///
527 /// Returns true if we can emit a run-time no alias check for \p Access.
528 /// If we can check this access, this also adds it to a dependence set and
529 /// adds a run-time to check for it to \p RtCheck. If \p Assume is true,
530 /// we will attempt to use additional run-time checks in order to get
531 /// the bounds of the pointer.
532 bool createCheckForAccess(RuntimePointerChecking &RtCheck,
533 MemAccessInfo Access,
534 const ValueToValueMap &Strides,
535 DenseMap<Value *, unsigned> &DepSetId,
536 Loop *TheLoop, unsigned &RunningDepId,
537 unsigned ASId, bool ShouldCheckStride,
538 bool Assume);
539
540 /// Check whether we can check the pointers at runtime for
541 /// non-intersection.
542 ///
543 /// Returns true if we need no check or if we do and we can generate them
544 /// (i.e. the pointers have computable bounds).
545 bool canCheckPtrAtRT(RuntimePointerChecking &RtCheck, ScalarEvolution *SE,
546 Loop *TheLoop, const ValueToValueMap &Strides,
547 bool ShouldCheckWrap = false);
548
549 /// Goes over all memory accesses, checks whether a RT check is needed
550 /// and builds sets of dependent accesses.
buildDependenceSets()551 void buildDependenceSets() {
552 processMemAccesses();
553 }
554
555 /// Initial processing of memory accesses determined that we need to
556 /// perform dependency checking.
557 ///
558 /// Note that this can later be cleared if we retry memcheck analysis without
559 /// dependency checking (i.e. FoundNonConstantDistanceDependence).
isDependencyCheckNeeded()560 bool isDependencyCheckNeeded() { return !CheckDeps.empty(); }
561
562 /// We decided that no dependence analysis would be used. Reset the state.
resetDepChecks(MemoryDepChecker & DepChecker)563 void resetDepChecks(MemoryDepChecker &DepChecker) {
564 CheckDeps.clear();
565 DepChecker.clearDependences();
566 }
567
getDependenciesToCheck()568 MemAccessInfoList &getDependenciesToCheck() { return CheckDeps; }
569
570 private:
571 typedef SetVector<MemAccessInfo> PtrAccessSet;
572
573 /// Go over all memory access and check whether runtime pointer checks
574 /// are needed and build sets of dependency check candidates.
575 void processMemAccesses();
576
577 /// Set of all accesses.
578 PtrAccessSet Accesses;
579
580 const DataLayout &DL;
581
582 /// The loop being checked.
583 const Loop *TheLoop;
584
585 /// List of accesses that need a further dependence check.
586 MemAccessInfoList CheckDeps;
587
588 /// Set of pointers that are read only.
589 SmallPtrSet<Value*, 16> ReadOnlyPtr;
590
591 /// An alias set tracker to partition the access set by underlying object and
592 //intrinsic property (such as TBAA metadata).
593 AliasSetTracker AST;
594
595 LoopInfo *LI;
596
597 /// Sets of potentially dependent accesses - members of one set share an
598 /// underlying pointer. The set "CheckDeps" identfies which sets really need a
599 /// dependence check.
600 MemoryDepChecker::DepCandidates &DepCands;
601
602 /// Initial processing of memory accesses determined that we may need
603 /// to add memchecks. Perform the analysis to determine the necessary checks.
604 ///
605 /// Note that, this is different from isDependencyCheckNeeded. When we retry
606 /// memcheck analysis without dependency checking
607 /// (i.e. FoundNonConstantDistanceDependence), isDependencyCheckNeeded is
608 /// cleared while this remains set if we have potentially dependent accesses.
609 bool IsRTCheckAnalysisNeeded;
610
611 /// The SCEV predicate containing all the SCEV-related assumptions.
612 PredicatedScalarEvolution &PSE;
613 };
614
615 } // end anonymous namespace
616
617 /// Check whether a pointer can participate in a runtime bounds check.
618 /// If \p Assume, try harder to prove that we can compute the bounds of \p Ptr
619 /// by adding run-time checks (overflow checks) if necessary.
hasComputableBounds(PredicatedScalarEvolution & PSE,const ValueToValueMap & Strides,Value * Ptr,Loop * L,bool Assume)620 static bool hasComputableBounds(PredicatedScalarEvolution &PSE,
621 const ValueToValueMap &Strides, Value *Ptr,
622 Loop *L, bool Assume) {
623 const SCEV *PtrScev = replaceSymbolicStrideSCEV(PSE, Strides, Ptr);
624
625 // The bounds for loop-invariant pointer is trivial.
626 if (PSE.getSE()->isLoopInvariant(PtrScev, L))
627 return true;
628
629 const SCEVAddRecExpr *AR = dyn_cast<SCEVAddRecExpr>(PtrScev);
630
631 if (!AR && Assume)
632 AR = PSE.getAsAddRec(Ptr);
633
634 if (!AR)
635 return false;
636
637 return AR->isAffine();
638 }
639
640 /// Check whether a pointer address cannot wrap.
isNoWrap(PredicatedScalarEvolution & PSE,const ValueToValueMap & Strides,Value * Ptr,Loop * L)641 static bool isNoWrap(PredicatedScalarEvolution &PSE,
642 const ValueToValueMap &Strides, Value *Ptr, Loop *L) {
643 const SCEV *PtrScev = PSE.getSCEV(Ptr);
644 if (PSE.getSE()->isLoopInvariant(PtrScev, L))
645 return true;
646
647 int64_t Stride = getPtrStride(PSE, Ptr, L, Strides);
648 if (Stride == 1 || PSE.hasNoOverflow(Ptr, SCEVWrapPredicate::IncrementNUSW))
649 return true;
650
651 return false;
652 }
653
createCheckForAccess(RuntimePointerChecking & RtCheck,MemAccessInfo Access,const ValueToValueMap & StridesMap,DenseMap<Value *,unsigned> & DepSetId,Loop * TheLoop,unsigned & RunningDepId,unsigned ASId,bool ShouldCheckWrap,bool Assume)654 bool AccessAnalysis::createCheckForAccess(RuntimePointerChecking &RtCheck,
655 MemAccessInfo Access,
656 const ValueToValueMap &StridesMap,
657 DenseMap<Value *, unsigned> &DepSetId,
658 Loop *TheLoop, unsigned &RunningDepId,
659 unsigned ASId, bool ShouldCheckWrap,
660 bool Assume) {
661 Value *Ptr = Access.getPointer();
662
663 if (!hasComputableBounds(PSE, StridesMap, Ptr, TheLoop, Assume))
664 return false;
665
666 // When we run after a failing dependency check we have to make sure
667 // we don't have wrapping pointers.
668 if (ShouldCheckWrap && !isNoWrap(PSE, StridesMap, Ptr, TheLoop)) {
669 auto *Expr = PSE.getSCEV(Ptr);
670 if (!Assume || !isa<SCEVAddRecExpr>(Expr))
671 return false;
672 PSE.setNoOverflow(Ptr, SCEVWrapPredicate::IncrementNUSW);
673 }
674
675 // The id of the dependence set.
676 unsigned DepId;
677
678 if (isDependencyCheckNeeded()) {
679 Value *Leader = DepCands.getLeaderValue(Access).getPointer();
680 unsigned &LeaderId = DepSetId[Leader];
681 if (!LeaderId)
682 LeaderId = RunningDepId++;
683 DepId = LeaderId;
684 } else
685 // Each access has its own dependence set.
686 DepId = RunningDepId++;
687
688 bool IsWrite = Access.getInt();
689 RtCheck.insert(TheLoop, Ptr, IsWrite, DepId, ASId, StridesMap, PSE);
690 LLVM_DEBUG(dbgs() << "LAA: Found a runtime check ptr:" << *Ptr << '\n');
691
692 return true;
693 }
694
canCheckPtrAtRT(RuntimePointerChecking & RtCheck,ScalarEvolution * SE,Loop * TheLoop,const ValueToValueMap & StridesMap,bool ShouldCheckWrap)695 bool AccessAnalysis::canCheckPtrAtRT(RuntimePointerChecking &RtCheck,
696 ScalarEvolution *SE, Loop *TheLoop,
697 const ValueToValueMap &StridesMap,
698 bool ShouldCheckWrap) {
699 // Find pointers with computable bounds. We are going to use this information
700 // to place a runtime bound check.
701 bool CanDoRT = true;
702
703 bool NeedRTCheck = false;
704 if (!IsRTCheckAnalysisNeeded) return true;
705
706 bool IsDepCheckNeeded = isDependencyCheckNeeded();
707
708 // We assign a consecutive id to access from different alias sets.
709 // Accesses between different groups doesn't need to be checked.
710 unsigned ASId = 1;
711 for (auto &AS : AST) {
712 int NumReadPtrChecks = 0;
713 int NumWritePtrChecks = 0;
714 bool CanDoAliasSetRT = true;
715
716 // We assign consecutive id to access from different dependence sets.
717 // Accesses within the same set don't need a runtime check.
718 unsigned RunningDepId = 1;
719 DenseMap<Value *, unsigned> DepSetId;
720
721 SmallVector<MemAccessInfo, 4> Retries;
722
723 for (auto A : AS) {
724 Value *Ptr = A.getValue();
725 bool IsWrite = Accesses.count(MemAccessInfo(Ptr, true));
726 MemAccessInfo Access(Ptr, IsWrite);
727
728 if (IsWrite)
729 ++NumWritePtrChecks;
730 else
731 ++NumReadPtrChecks;
732
733 if (!createCheckForAccess(RtCheck, Access, StridesMap, DepSetId, TheLoop,
734 RunningDepId, ASId, ShouldCheckWrap, false)) {
735 LLVM_DEBUG(dbgs() << "LAA: Can't find bounds for ptr:" << *Ptr << '\n');
736 Retries.push_back(Access);
737 CanDoAliasSetRT = false;
738 }
739 }
740
741 // If we have at least two writes or one write and a read then we need to
742 // check them. But there is no need to checks if there is only one
743 // dependence set for this alias set.
744 //
745 // Note that this function computes CanDoRT and NeedRTCheck independently.
746 // For example CanDoRT=false, NeedRTCheck=false means that we have a pointer
747 // for which we couldn't find the bounds but we don't actually need to emit
748 // any checks so it does not matter.
749 bool NeedsAliasSetRTCheck = false;
750 if (!(IsDepCheckNeeded && CanDoAliasSetRT && RunningDepId == 2))
751 NeedsAliasSetRTCheck = (NumWritePtrChecks >= 2 ||
752 (NumReadPtrChecks >= 1 && NumWritePtrChecks >= 1));
753
754 // We need to perform run-time alias checks, but some pointers had bounds
755 // that couldn't be checked.
756 if (NeedsAliasSetRTCheck && !CanDoAliasSetRT) {
757 // Reset the CanDoSetRt flag and retry all accesses that have failed.
758 // We know that we need these checks, so we can now be more aggressive
759 // and add further checks if required (overflow checks).
760 CanDoAliasSetRT = true;
761 for (auto Access : Retries)
762 if (!createCheckForAccess(RtCheck, Access, StridesMap, DepSetId,
763 TheLoop, RunningDepId, ASId,
764 ShouldCheckWrap, /*Assume=*/true)) {
765 CanDoAliasSetRT = false;
766 break;
767 }
768 }
769
770 CanDoRT &= CanDoAliasSetRT;
771 NeedRTCheck |= NeedsAliasSetRTCheck;
772 ++ASId;
773 }
774
775 // If the pointers that we would use for the bounds comparison have different
776 // address spaces, assume the values aren't directly comparable, so we can't
777 // use them for the runtime check. We also have to assume they could
778 // overlap. In the future there should be metadata for whether address spaces
779 // are disjoint.
780 unsigned NumPointers = RtCheck.Pointers.size();
781 for (unsigned i = 0; i < NumPointers; ++i) {
782 for (unsigned j = i + 1; j < NumPointers; ++j) {
783 // Only need to check pointers between two different dependency sets.
784 if (RtCheck.Pointers[i].DependencySetId ==
785 RtCheck.Pointers[j].DependencySetId)
786 continue;
787 // Only need to check pointers in the same alias set.
788 if (RtCheck.Pointers[i].AliasSetId != RtCheck.Pointers[j].AliasSetId)
789 continue;
790
791 Value *PtrI = RtCheck.Pointers[i].PointerValue;
792 Value *PtrJ = RtCheck.Pointers[j].PointerValue;
793
794 unsigned ASi = PtrI->getType()->getPointerAddressSpace();
795 unsigned ASj = PtrJ->getType()->getPointerAddressSpace();
796 if (ASi != ASj) {
797 LLVM_DEBUG(
798 dbgs() << "LAA: Runtime check would require comparison between"
799 " different address spaces\n");
800 return false;
801 }
802 }
803 }
804
805 if (NeedRTCheck && CanDoRT)
806 RtCheck.generateChecks(DepCands, IsDepCheckNeeded);
807
808 LLVM_DEBUG(dbgs() << "LAA: We need to do " << RtCheck.getNumberOfChecks()
809 << " pointer comparisons.\n");
810
811 RtCheck.Need = NeedRTCheck;
812
813 bool CanDoRTIfNeeded = !NeedRTCheck || CanDoRT;
814 if (!CanDoRTIfNeeded)
815 RtCheck.reset();
816 return CanDoRTIfNeeded;
817 }
818
processMemAccesses()819 void AccessAnalysis::processMemAccesses() {
820 // We process the set twice: first we process read-write pointers, last we
821 // process read-only pointers. This allows us to skip dependence tests for
822 // read-only pointers.
823
824 LLVM_DEBUG(dbgs() << "LAA: Processing memory accesses...\n");
825 LLVM_DEBUG(dbgs() << " AST: "; AST.dump());
826 LLVM_DEBUG(dbgs() << "LAA: Accesses(" << Accesses.size() << "):\n");
827 LLVM_DEBUG({
828 for (auto A : Accesses)
829 dbgs() << "\t" << *A.getPointer() << " (" <<
830 (A.getInt() ? "write" : (ReadOnlyPtr.count(A.getPointer()) ?
831 "read-only" : "read")) << ")\n";
832 });
833
834 // The AliasSetTracker has nicely partitioned our pointers by metadata
835 // compatibility and potential for underlying-object overlap. As a result, we
836 // only need to check for potential pointer dependencies within each alias
837 // set.
838 for (auto &AS : AST) {
839 // Note that both the alias-set tracker and the alias sets themselves used
840 // linked lists internally and so the iteration order here is deterministic
841 // (matching the original instruction order within each set).
842
843 bool SetHasWrite = false;
844
845 // Map of pointers to last access encountered.
846 typedef DenseMap<Value*, MemAccessInfo> UnderlyingObjToAccessMap;
847 UnderlyingObjToAccessMap ObjToLastAccess;
848
849 // Set of access to check after all writes have been processed.
850 PtrAccessSet DeferredAccesses;
851
852 // Iterate over each alias set twice, once to process read/write pointers,
853 // and then to process read-only pointers.
854 for (int SetIteration = 0; SetIteration < 2; ++SetIteration) {
855 bool UseDeferred = SetIteration > 0;
856 PtrAccessSet &S = UseDeferred ? DeferredAccesses : Accesses;
857
858 for (auto AV : AS) {
859 Value *Ptr = AV.getValue();
860
861 // For a single memory access in AliasSetTracker, Accesses may contain
862 // both read and write, and they both need to be handled for CheckDeps.
863 for (auto AC : S) {
864 if (AC.getPointer() != Ptr)
865 continue;
866
867 bool IsWrite = AC.getInt();
868
869 // If we're using the deferred access set, then it contains only
870 // reads.
871 bool IsReadOnlyPtr = ReadOnlyPtr.count(Ptr) && !IsWrite;
872 if (UseDeferred && !IsReadOnlyPtr)
873 continue;
874 // Otherwise, the pointer must be in the PtrAccessSet, either as a
875 // read or a write.
876 assert(((IsReadOnlyPtr && UseDeferred) || IsWrite ||
877 S.count(MemAccessInfo(Ptr, false))) &&
878 "Alias-set pointer not in the access set?");
879
880 MemAccessInfo Access(Ptr, IsWrite);
881 DepCands.insert(Access);
882
883 // Memorize read-only pointers for later processing and skip them in
884 // the first round (they need to be checked after we have seen all
885 // write pointers). Note: we also mark pointer that are not
886 // consecutive as "read-only" pointers (so that we check
887 // "a[b[i]] +="). Hence, we need the second check for "!IsWrite".
888 if (!UseDeferred && IsReadOnlyPtr) {
889 DeferredAccesses.insert(Access);
890 continue;
891 }
892
893 // If this is a write - check other reads and writes for conflicts. If
894 // this is a read only check other writes for conflicts (but only if
895 // there is no other write to the ptr - this is an optimization to
896 // catch "a[i] = a[i] + " without having to do a dependence check).
897 if ((IsWrite || IsReadOnlyPtr) && SetHasWrite) {
898 CheckDeps.push_back(Access);
899 IsRTCheckAnalysisNeeded = true;
900 }
901
902 if (IsWrite)
903 SetHasWrite = true;
904
905 // Create sets of pointers connected by a shared alias set and
906 // underlying object.
907 typedef SmallVector<Value *, 16> ValueVector;
908 ValueVector TempObjects;
909
910 GetUnderlyingObjects(Ptr, TempObjects, DL, LI);
911 LLVM_DEBUG(dbgs()
912 << "Underlying objects for pointer " << *Ptr << "\n");
913 for (Value *UnderlyingObj : TempObjects) {
914 // nullptr never alias, don't join sets for pointer that have "null"
915 // in their UnderlyingObjects list.
916 if (isa<ConstantPointerNull>(UnderlyingObj) &&
917 !NullPointerIsDefined(
918 TheLoop->getHeader()->getParent(),
919 UnderlyingObj->getType()->getPointerAddressSpace()))
920 continue;
921
922 UnderlyingObjToAccessMap::iterator Prev =
923 ObjToLastAccess.find(UnderlyingObj);
924 if (Prev != ObjToLastAccess.end())
925 DepCands.unionSets(Access, Prev->second);
926
927 ObjToLastAccess[UnderlyingObj] = Access;
928 LLVM_DEBUG(dbgs() << " " << *UnderlyingObj << "\n");
929 }
930 }
931 }
932 }
933 }
934 }
935
isInBoundsGep(Value * Ptr)936 static bool isInBoundsGep(Value *Ptr) {
937 if (GetElementPtrInst *GEP = dyn_cast<GetElementPtrInst>(Ptr))
938 return GEP->isInBounds();
939 return false;
940 }
941
942 /// Return true if an AddRec pointer \p Ptr is unsigned non-wrapping,
943 /// i.e. monotonically increasing/decreasing.
isNoWrapAddRec(Value * Ptr,const SCEVAddRecExpr * AR,PredicatedScalarEvolution & PSE,const Loop * L)944 static bool isNoWrapAddRec(Value *Ptr, const SCEVAddRecExpr *AR,
945 PredicatedScalarEvolution &PSE, const Loop *L) {
946 // FIXME: This should probably only return true for NUW.
947 if (AR->getNoWrapFlags(SCEV::NoWrapMask))
948 return true;
949
950 // Scalar evolution does not propagate the non-wrapping flags to values that
951 // are derived from a non-wrapping induction variable because non-wrapping
952 // could be flow-sensitive.
953 //
954 // Look through the potentially overflowing instruction to try to prove
955 // non-wrapping for the *specific* value of Ptr.
956
957 // The arithmetic implied by an inbounds GEP can't overflow.
958 auto *GEP = dyn_cast<GetElementPtrInst>(Ptr);
959 if (!GEP || !GEP->isInBounds())
960 return false;
961
962 // Make sure there is only one non-const index and analyze that.
963 Value *NonConstIndex = nullptr;
964 for (Value *Index : make_range(GEP->idx_begin(), GEP->idx_end()))
965 if (!isa<ConstantInt>(Index)) {
966 if (NonConstIndex)
967 return false;
968 NonConstIndex = Index;
969 }
970 if (!NonConstIndex)
971 // The recurrence is on the pointer, ignore for now.
972 return false;
973
974 // The index in GEP is signed. It is non-wrapping if it's derived from a NSW
975 // AddRec using a NSW operation.
976 if (auto *OBO = dyn_cast<OverflowingBinaryOperator>(NonConstIndex))
977 if (OBO->hasNoSignedWrap() &&
978 // Assume constant for other the operand so that the AddRec can be
979 // easily found.
980 isa<ConstantInt>(OBO->getOperand(1))) {
981 auto *OpScev = PSE.getSCEV(OBO->getOperand(0));
982
983 if (auto *OpAR = dyn_cast<SCEVAddRecExpr>(OpScev))
984 return OpAR->getLoop() == L && OpAR->getNoWrapFlags(SCEV::FlagNSW);
985 }
986
987 return false;
988 }
989
990 /// Check whether the access through \p Ptr has a constant stride.
getPtrStride(PredicatedScalarEvolution & PSE,Value * Ptr,const Loop * Lp,const ValueToValueMap & StridesMap,bool Assume,bool ShouldCheckWrap)991 int64_t llvm::getPtrStride(PredicatedScalarEvolution &PSE, Value *Ptr,
992 const Loop *Lp, const ValueToValueMap &StridesMap,
993 bool Assume, bool ShouldCheckWrap) {
994 Type *Ty = Ptr->getType();
995 assert(Ty->isPointerTy() && "Unexpected non-ptr");
996
997 // Make sure that the pointer does not point to aggregate types.
998 auto *PtrTy = cast<PointerType>(Ty);
999 if (PtrTy->getElementType()->isAggregateType()) {
1000 LLVM_DEBUG(dbgs() << "LAA: Bad stride - Not a pointer to a scalar type"
1001 << *Ptr << "\n");
1002 return 0;
1003 }
1004
1005 const SCEV *PtrScev = replaceSymbolicStrideSCEV(PSE, StridesMap, Ptr);
1006
1007 const SCEVAddRecExpr *AR = dyn_cast<SCEVAddRecExpr>(PtrScev);
1008 if (Assume && !AR)
1009 AR = PSE.getAsAddRec(Ptr);
1010
1011 if (!AR) {
1012 LLVM_DEBUG(dbgs() << "LAA: Bad stride - Not an AddRecExpr pointer " << *Ptr
1013 << " SCEV: " << *PtrScev << "\n");
1014 return 0;
1015 }
1016
1017 // The accesss function must stride over the innermost loop.
1018 if (Lp != AR->getLoop()) {
1019 LLVM_DEBUG(dbgs() << "LAA: Bad stride - Not striding over innermost loop "
1020 << *Ptr << " SCEV: " << *AR << "\n");
1021 return 0;
1022 }
1023
1024 // The address calculation must not wrap. Otherwise, a dependence could be
1025 // inverted.
1026 // An inbounds getelementptr that is a AddRec with a unit stride
1027 // cannot wrap per definition. The unit stride requirement is checked later.
1028 // An getelementptr without an inbounds attribute and unit stride would have
1029 // to access the pointer value "0" which is undefined behavior in address
1030 // space 0, therefore we can also vectorize this case.
1031 bool IsInBoundsGEP = isInBoundsGep(Ptr);
1032 bool IsNoWrapAddRec = !ShouldCheckWrap ||
1033 PSE.hasNoOverflow(Ptr, SCEVWrapPredicate::IncrementNUSW) ||
1034 isNoWrapAddRec(Ptr, AR, PSE, Lp);
1035 if (!IsNoWrapAddRec && !IsInBoundsGEP &&
1036 NullPointerIsDefined(Lp->getHeader()->getParent(),
1037 PtrTy->getAddressSpace())) {
1038 if (Assume) {
1039 PSE.setNoOverflow(Ptr, SCEVWrapPredicate::IncrementNUSW);
1040 IsNoWrapAddRec = true;
1041 LLVM_DEBUG(dbgs() << "LAA: Pointer may wrap in the address space:\n"
1042 << "LAA: Pointer: " << *Ptr << "\n"
1043 << "LAA: SCEV: " << *AR << "\n"
1044 << "LAA: Added an overflow assumption\n");
1045 } else {
1046 LLVM_DEBUG(
1047 dbgs() << "LAA: Bad stride - Pointer may wrap in the address space "
1048 << *Ptr << " SCEV: " << *AR << "\n");
1049 return 0;
1050 }
1051 }
1052
1053 // Check the step is constant.
1054 const SCEV *Step = AR->getStepRecurrence(*PSE.getSE());
1055
1056 // Calculate the pointer stride and check if it is constant.
1057 const SCEVConstant *C = dyn_cast<SCEVConstant>(Step);
1058 if (!C) {
1059 LLVM_DEBUG(dbgs() << "LAA: Bad stride - Not a constant strided " << *Ptr
1060 << " SCEV: " << *AR << "\n");
1061 return 0;
1062 }
1063
1064 auto &DL = Lp->getHeader()->getModule()->getDataLayout();
1065 int64_t Size = DL.getTypeAllocSize(PtrTy->getElementType());
1066 const APInt &APStepVal = C->getAPInt();
1067
1068 // Huge step value - give up.
1069 if (APStepVal.getBitWidth() > 64)
1070 return 0;
1071
1072 int64_t StepVal = APStepVal.getSExtValue();
1073
1074 // Strided access.
1075 int64_t Stride = StepVal / Size;
1076 int64_t Rem = StepVal % Size;
1077 if (Rem)
1078 return 0;
1079
1080 // If the SCEV could wrap but we have an inbounds gep with a unit stride we
1081 // know we can't "wrap around the address space". In case of address space
1082 // zero we know that this won't happen without triggering undefined behavior.
1083 if (!IsNoWrapAddRec && Stride != 1 && Stride != -1 &&
1084 (IsInBoundsGEP || !NullPointerIsDefined(Lp->getHeader()->getParent(),
1085 PtrTy->getAddressSpace()))) {
1086 if (Assume) {
1087 // We can avoid this case by adding a run-time check.
1088 LLVM_DEBUG(dbgs() << "LAA: Non unit strided pointer which is not either "
1089 << "inbouds or in address space 0 may wrap:\n"
1090 << "LAA: Pointer: " << *Ptr << "\n"
1091 << "LAA: SCEV: " << *AR << "\n"
1092 << "LAA: Added an overflow assumption\n");
1093 PSE.setNoOverflow(Ptr, SCEVWrapPredicate::IncrementNUSW);
1094 } else
1095 return 0;
1096 }
1097
1098 return Stride;
1099 }
1100
sortPtrAccesses(ArrayRef<Value * > VL,const DataLayout & DL,ScalarEvolution & SE,SmallVectorImpl<unsigned> & SortedIndices)1101 bool llvm::sortPtrAccesses(ArrayRef<Value *> VL, const DataLayout &DL,
1102 ScalarEvolution &SE,
1103 SmallVectorImpl<unsigned> &SortedIndices) {
1104 assert(llvm::all_of(
1105 VL, [](const Value *V) { return V->getType()->isPointerTy(); }) &&
1106 "Expected list of pointer operands.");
1107 SmallVector<std::pair<int64_t, Value *>, 4> OffValPairs;
1108 OffValPairs.reserve(VL.size());
1109
1110 // Walk over the pointers, and map each of them to an offset relative to
1111 // first pointer in the array.
1112 Value *Ptr0 = VL[0];
1113 const SCEV *Scev0 = SE.getSCEV(Ptr0);
1114 Value *Obj0 = GetUnderlyingObject(Ptr0, DL);
1115
1116 llvm::SmallSet<int64_t, 4> Offsets;
1117 for (auto *Ptr : VL) {
1118 // TODO: Outline this code as a special, more time consuming, version of
1119 // computeConstantDifference() function.
1120 if (Ptr->getType()->getPointerAddressSpace() !=
1121 Ptr0->getType()->getPointerAddressSpace())
1122 return false;
1123 // If a pointer refers to a different underlying object, bail - the
1124 // pointers are by definition incomparable.
1125 Value *CurrObj = GetUnderlyingObject(Ptr, DL);
1126 if (CurrObj != Obj0)
1127 return false;
1128
1129 const SCEV *Scev = SE.getSCEV(Ptr);
1130 const auto *Diff = dyn_cast<SCEVConstant>(SE.getMinusSCEV(Scev, Scev0));
1131 // The pointers may not have a constant offset from each other, or SCEV
1132 // may just not be smart enough to figure out they do. Regardless,
1133 // there's nothing we can do.
1134 if (!Diff)
1135 return false;
1136
1137 // Check if the pointer with the same offset is found.
1138 int64_t Offset = Diff->getAPInt().getSExtValue();
1139 if (!Offsets.insert(Offset).second)
1140 return false;
1141 OffValPairs.emplace_back(Offset, Ptr);
1142 }
1143 SortedIndices.clear();
1144 SortedIndices.resize(VL.size());
1145 std::iota(SortedIndices.begin(), SortedIndices.end(), 0);
1146
1147 // Sort the memory accesses and keep the order of their uses in UseOrder.
1148 std::stable_sort(SortedIndices.begin(), SortedIndices.end(),
1149 [&OffValPairs](unsigned Left, unsigned Right) {
1150 return OffValPairs[Left].first < OffValPairs[Right].first;
1151 });
1152
1153 // Check if the order is consecutive already.
1154 if (llvm::all_of(SortedIndices, [&SortedIndices](const unsigned I) {
1155 return I == SortedIndices[I];
1156 }))
1157 SortedIndices.clear();
1158
1159 return true;
1160 }
1161
1162 /// Take the address space operand from the Load/Store instruction.
1163 /// Returns -1 if this is not a valid Load/Store instruction.
getAddressSpaceOperand(Value * I)1164 static unsigned getAddressSpaceOperand(Value *I) {
1165 if (LoadInst *L = dyn_cast<LoadInst>(I))
1166 return L->getPointerAddressSpace();
1167 if (StoreInst *S = dyn_cast<StoreInst>(I))
1168 return S->getPointerAddressSpace();
1169 return -1;
1170 }
1171
1172 /// Returns true if the memory operations \p A and \p B are consecutive.
isConsecutiveAccess(Value * A,Value * B,const DataLayout & DL,ScalarEvolution & SE,bool CheckType)1173 bool llvm::isConsecutiveAccess(Value *A, Value *B, const DataLayout &DL,
1174 ScalarEvolution &SE, bool CheckType) {
1175 Value *PtrA = getLoadStorePointerOperand(A);
1176 Value *PtrB = getLoadStorePointerOperand(B);
1177 unsigned ASA = getAddressSpaceOperand(A);
1178 unsigned ASB = getAddressSpaceOperand(B);
1179
1180 // Check that the address spaces match and that the pointers are valid.
1181 if (!PtrA || !PtrB || (ASA != ASB))
1182 return false;
1183
1184 // Make sure that A and B are different pointers.
1185 if (PtrA == PtrB)
1186 return false;
1187
1188 // Make sure that A and B have the same type if required.
1189 if (CheckType && PtrA->getType() != PtrB->getType())
1190 return false;
1191
1192 unsigned IdxWidth = DL.getIndexSizeInBits(ASA);
1193 Type *Ty = cast<PointerType>(PtrA->getType())->getElementType();
1194 APInt Size(IdxWidth, DL.getTypeStoreSize(Ty));
1195
1196 APInt OffsetA(IdxWidth, 0), OffsetB(IdxWidth, 0);
1197 PtrA = PtrA->stripAndAccumulateInBoundsConstantOffsets(DL, OffsetA);
1198 PtrB = PtrB->stripAndAccumulateInBoundsConstantOffsets(DL, OffsetB);
1199
1200 // OffsetDelta = OffsetB - OffsetA;
1201 const SCEV *OffsetSCEVA = SE.getConstant(OffsetA);
1202 const SCEV *OffsetSCEVB = SE.getConstant(OffsetB);
1203 const SCEV *OffsetDeltaSCEV = SE.getMinusSCEV(OffsetSCEVB, OffsetSCEVA);
1204 const SCEVConstant *OffsetDeltaC = dyn_cast<SCEVConstant>(OffsetDeltaSCEV);
1205 const APInt &OffsetDelta = OffsetDeltaC->getAPInt();
1206 // Check if they are based on the same pointer. That makes the offsets
1207 // sufficient.
1208 if (PtrA == PtrB)
1209 return OffsetDelta == Size;
1210
1211 // Compute the necessary base pointer delta to have the necessary final delta
1212 // equal to the size.
1213 // BaseDelta = Size - OffsetDelta;
1214 const SCEV *SizeSCEV = SE.getConstant(Size);
1215 const SCEV *BaseDelta = SE.getMinusSCEV(SizeSCEV, OffsetDeltaSCEV);
1216
1217 // Otherwise compute the distance with SCEV between the base pointers.
1218 const SCEV *PtrSCEVA = SE.getSCEV(PtrA);
1219 const SCEV *PtrSCEVB = SE.getSCEV(PtrB);
1220 const SCEV *X = SE.getAddExpr(PtrSCEVA, BaseDelta);
1221 return X == PtrSCEVB;
1222 }
1223
1224 MemoryDepChecker::VectorizationSafetyStatus
isSafeForVectorization(DepType Type)1225 MemoryDepChecker::Dependence::isSafeForVectorization(DepType Type) {
1226 switch (Type) {
1227 case NoDep:
1228 case Forward:
1229 case BackwardVectorizable:
1230 return VectorizationSafetyStatus::Safe;
1231
1232 case Unknown:
1233 return VectorizationSafetyStatus::PossiblySafeWithRtChecks;
1234 case ForwardButPreventsForwarding:
1235 case Backward:
1236 case BackwardVectorizableButPreventsForwarding:
1237 return VectorizationSafetyStatus::Unsafe;
1238 }
1239 llvm_unreachable("unexpected DepType!");
1240 }
1241
isBackward() const1242 bool MemoryDepChecker::Dependence::isBackward() const {
1243 switch (Type) {
1244 case NoDep:
1245 case Forward:
1246 case ForwardButPreventsForwarding:
1247 case Unknown:
1248 return false;
1249
1250 case BackwardVectorizable:
1251 case Backward:
1252 case BackwardVectorizableButPreventsForwarding:
1253 return true;
1254 }
1255 llvm_unreachable("unexpected DepType!");
1256 }
1257
isPossiblyBackward() const1258 bool MemoryDepChecker::Dependence::isPossiblyBackward() const {
1259 return isBackward() || Type == Unknown;
1260 }
1261
isForward() const1262 bool MemoryDepChecker::Dependence::isForward() const {
1263 switch (Type) {
1264 case Forward:
1265 case ForwardButPreventsForwarding:
1266 return true;
1267
1268 case NoDep:
1269 case Unknown:
1270 case BackwardVectorizable:
1271 case Backward:
1272 case BackwardVectorizableButPreventsForwarding:
1273 return false;
1274 }
1275 llvm_unreachable("unexpected DepType!");
1276 }
1277
couldPreventStoreLoadForward(uint64_t Distance,uint64_t TypeByteSize)1278 bool MemoryDepChecker::couldPreventStoreLoadForward(uint64_t Distance,
1279 uint64_t TypeByteSize) {
1280 // If loads occur at a distance that is not a multiple of a feasible vector
1281 // factor store-load forwarding does not take place.
1282 // Positive dependences might cause troubles because vectorizing them might
1283 // prevent store-load forwarding making vectorized code run a lot slower.
1284 // a[i] = a[i-3] ^ a[i-8];
1285 // The stores to a[i:i+1] don't align with the stores to a[i-3:i-2] and
1286 // hence on your typical architecture store-load forwarding does not take
1287 // place. Vectorizing in such cases does not make sense.
1288 // Store-load forwarding distance.
1289
1290 // After this many iterations store-to-load forwarding conflicts should not
1291 // cause any slowdowns.
1292 const uint64_t NumItersForStoreLoadThroughMemory = 8 * TypeByteSize;
1293 // Maximum vector factor.
1294 uint64_t MaxVFWithoutSLForwardIssues = std::min(
1295 VectorizerParams::MaxVectorWidth * TypeByteSize, MaxSafeDepDistBytes);
1296
1297 // Compute the smallest VF at which the store and load would be misaligned.
1298 for (uint64_t VF = 2 * TypeByteSize; VF <= MaxVFWithoutSLForwardIssues;
1299 VF *= 2) {
1300 // If the number of vector iteration between the store and the load are
1301 // small we could incur conflicts.
1302 if (Distance % VF && Distance / VF < NumItersForStoreLoadThroughMemory) {
1303 MaxVFWithoutSLForwardIssues = (VF >>= 1);
1304 break;
1305 }
1306 }
1307
1308 if (MaxVFWithoutSLForwardIssues < 2 * TypeByteSize) {
1309 LLVM_DEBUG(
1310 dbgs() << "LAA: Distance " << Distance
1311 << " that could cause a store-load forwarding conflict\n");
1312 return true;
1313 }
1314
1315 if (MaxVFWithoutSLForwardIssues < MaxSafeDepDistBytes &&
1316 MaxVFWithoutSLForwardIssues !=
1317 VectorizerParams::MaxVectorWidth * TypeByteSize)
1318 MaxSafeDepDistBytes = MaxVFWithoutSLForwardIssues;
1319 return false;
1320 }
1321
mergeInStatus(VectorizationSafetyStatus S)1322 void MemoryDepChecker::mergeInStatus(VectorizationSafetyStatus S) {
1323 if (Status < S)
1324 Status = S;
1325 }
1326
1327 /// Given a non-constant (unknown) dependence-distance \p Dist between two
1328 /// memory accesses, that have the same stride whose absolute value is given
1329 /// in \p Stride, and that have the same type size \p TypeByteSize,
1330 /// in a loop whose takenCount is \p BackedgeTakenCount, check if it is
1331 /// possible to prove statically that the dependence distance is larger
1332 /// than the range that the accesses will travel through the execution of
1333 /// the loop. If so, return true; false otherwise. This is useful for
1334 /// example in loops such as the following (PR31098):
1335 /// for (i = 0; i < D; ++i) {
1336 /// = out[i];
1337 /// out[i+D] =
1338 /// }
isSafeDependenceDistance(const DataLayout & DL,ScalarEvolution & SE,const SCEV & BackedgeTakenCount,const SCEV & Dist,uint64_t Stride,uint64_t TypeByteSize)1339 static bool isSafeDependenceDistance(const DataLayout &DL, ScalarEvolution &SE,
1340 const SCEV &BackedgeTakenCount,
1341 const SCEV &Dist, uint64_t Stride,
1342 uint64_t TypeByteSize) {
1343
1344 // If we can prove that
1345 // (**) |Dist| > BackedgeTakenCount * Step
1346 // where Step is the absolute stride of the memory accesses in bytes,
1347 // then there is no dependence.
1348 //
1349 // Ratioanle:
1350 // We basically want to check if the absolute distance (|Dist/Step|)
1351 // is >= the loop iteration count (or > BackedgeTakenCount).
1352 // This is equivalent to the Strong SIV Test (Practical Dependence Testing,
1353 // Section 4.2.1); Note, that for vectorization it is sufficient to prove
1354 // that the dependence distance is >= VF; This is checked elsewhere.
1355 // But in some cases we can prune unknown dependence distances early, and
1356 // even before selecting the VF, and without a runtime test, by comparing
1357 // the distance against the loop iteration count. Since the vectorized code
1358 // will be executed only if LoopCount >= VF, proving distance >= LoopCount
1359 // also guarantees that distance >= VF.
1360 //
1361 const uint64_t ByteStride = Stride * TypeByteSize;
1362 const SCEV *Step = SE.getConstant(BackedgeTakenCount.getType(), ByteStride);
1363 const SCEV *Product = SE.getMulExpr(&BackedgeTakenCount, Step);
1364
1365 const SCEV *CastedDist = &Dist;
1366 const SCEV *CastedProduct = Product;
1367 uint64_t DistTypeSize = DL.getTypeAllocSize(Dist.getType());
1368 uint64_t ProductTypeSize = DL.getTypeAllocSize(Product->getType());
1369
1370 // The dependence distance can be positive/negative, so we sign extend Dist;
1371 // The multiplication of the absolute stride in bytes and the
1372 // backdgeTakenCount is non-negative, so we zero extend Product.
1373 if (DistTypeSize > ProductTypeSize)
1374 CastedProduct = SE.getZeroExtendExpr(Product, Dist.getType());
1375 else
1376 CastedDist = SE.getNoopOrSignExtend(&Dist, Product->getType());
1377
1378 // Is Dist - (BackedgeTakenCount * Step) > 0 ?
1379 // (If so, then we have proven (**) because |Dist| >= Dist)
1380 const SCEV *Minus = SE.getMinusSCEV(CastedDist, CastedProduct);
1381 if (SE.isKnownPositive(Minus))
1382 return true;
1383
1384 // Second try: Is -Dist - (BackedgeTakenCount * Step) > 0 ?
1385 // (If so, then we have proven (**) because |Dist| >= -1*Dist)
1386 const SCEV *NegDist = SE.getNegativeSCEV(CastedDist);
1387 Minus = SE.getMinusSCEV(NegDist, CastedProduct);
1388 if (SE.isKnownPositive(Minus))
1389 return true;
1390
1391 return false;
1392 }
1393
1394 /// Check the dependence for two accesses with the same stride \p Stride.
1395 /// \p Distance is the positive distance and \p TypeByteSize is type size in
1396 /// bytes.
1397 ///
1398 /// \returns true if they are independent.
areStridedAccessesIndependent(uint64_t Distance,uint64_t Stride,uint64_t TypeByteSize)1399 static bool areStridedAccessesIndependent(uint64_t Distance, uint64_t Stride,
1400 uint64_t TypeByteSize) {
1401 assert(Stride > 1 && "The stride must be greater than 1");
1402 assert(TypeByteSize > 0 && "The type size in byte must be non-zero");
1403 assert(Distance > 0 && "The distance must be non-zero");
1404
1405 // Skip if the distance is not multiple of type byte size.
1406 if (Distance % TypeByteSize)
1407 return false;
1408
1409 uint64_t ScaledDist = Distance / TypeByteSize;
1410
1411 // No dependence if the scaled distance is not multiple of the stride.
1412 // E.g.
1413 // for (i = 0; i < 1024 ; i += 4)
1414 // A[i+2] = A[i] + 1;
1415 //
1416 // Two accesses in memory (scaled distance is 2, stride is 4):
1417 // | A[0] | | | | A[4] | | | |
1418 // | | | A[2] | | | | A[6] | |
1419 //
1420 // E.g.
1421 // for (i = 0; i < 1024 ; i += 3)
1422 // A[i+4] = A[i] + 1;
1423 //
1424 // Two accesses in memory (scaled distance is 4, stride is 3):
1425 // | A[0] | | | A[3] | | | A[6] | | |
1426 // | | | | | A[4] | | | A[7] | |
1427 return ScaledDist % Stride;
1428 }
1429
1430 MemoryDepChecker::Dependence::DepType
isDependent(const MemAccessInfo & A,unsigned AIdx,const MemAccessInfo & B,unsigned BIdx,const ValueToValueMap & Strides)1431 MemoryDepChecker::isDependent(const MemAccessInfo &A, unsigned AIdx,
1432 const MemAccessInfo &B, unsigned BIdx,
1433 const ValueToValueMap &Strides) {
1434 assert (AIdx < BIdx && "Must pass arguments in program order");
1435
1436 Value *APtr = A.getPointer();
1437 Value *BPtr = B.getPointer();
1438 bool AIsWrite = A.getInt();
1439 bool BIsWrite = B.getInt();
1440
1441 // Two reads are independent.
1442 if (!AIsWrite && !BIsWrite)
1443 return Dependence::NoDep;
1444
1445 // We cannot check pointers in different address spaces.
1446 if (APtr->getType()->getPointerAddressSpace() !=
1447 BPtr->getType()->getPointerAddressSpace())
1448 return Dependence::Unknown;
1449
1450 int64_t StrideAPtr = getPtrStride(PSE, APtr, InnermostLoop, Strides, true);
1451 int64_t StrideBPtr = getPtrStride(PSE, BPtr, InnermostLoop, Strides, true);
1452
1453 const SCEV *Src = PSE.getSCEV(APtr);
1454 const SCEV *Sink = PSE.getSCEV(BPtr);
1455
1456 // If the induction step is negative we have to invert source and sink of the
1457 // dependence.
1458 if (StrideAPtr < 0) {
1459 std::swap(APtr, BPtr);
1460 std::swap(Src, Sink);
1461 std::swap(AIsWrite, BIsWrite);
1462 std::swap(AIdx, BIdx);
1463 std::swap(StrideAPtr, StrideBPtr);
1464 }
1465
1466 const SCEV *Dist = PSE.getSE()->getMinusSCEV(Sink, Src);
1467
1468 LLVM_DEBUG(dbgs() << "LAA: Src Scev: " << *Src << "Sink Scev: " << *Sink
1469 << "(Induction step: " << StrideAPtr << ")\n");
1470 LLVM_DEBUG(dbgs() << "LAA: Distance for " << *InstMap[AIdx] << " to "
1471 << *InstMap[BIdx] << ": " << *Dist << "\n");
1472
1473 // Need accesses with constant stride. We don't want to vectorize
1474 // "A[B[i]] += ..." and similar code or pointer arithmetic that could wrap in
1475 // the address space.
1476 if (!StrideAPtr || !StrideBPtr || StrideAPtr != StrideBPtr){
1477 LLVM_DEBUG(dbgs() << "Pointer access with non-constant stride\n");
1478 return Dependence::Unknown;
1479 }
1480
1481 Type *ATy = APtr->getType()->getPointerElementType();
1482 Type *BTy = BPtr->getType()->getPointerElementType();
1483 auto &DL = InnermostLoop->getHeader()->getModule()->getDataLayout();
1484 uint64_t TypeByteSize = DL.getTypeAllocSize(ATy);
1485 uint64_t Stride = std::abs(StrideAPtr);
1486 const SCEVConstant *C = dyn_cast<SCEVConstant>(Dist);
1487 if (!C) {
1488 if (TypeByteSize == DL.getTypeAllocSize(BTy) &&
1489 isSafeDependenceDistance(DL, *(PSE.getSE()),
1490 *(PSE.getBackedgeTakenCount()), *Dist, Stride,
1491 TypeByteSize))
1492 return Dependence::NoDep;
1493
1494 LLVM_DEBUG(dbgs() << "LAA: Dependence because of non-constant distance\n");
1495 FoundNonConstantDistanceDependence = true;
1496 return Dependence::Unknown;
1497 }
1498
1499 const APInt &Val = C->getAPInt();
1500 int64_t Distance = Val.getSExtValue();
1501
1502 // Attempt to prove strided accesses independent.
1503 if (std::abs(Distance) > 0 && Stride > 1 && ATy == BTy &&
1504 areStridedAccessesIndependent(std::abs(Distance), Stride, TypeByteSize)) {
1505 LLVM_DEBUG(dbgs() << "LAA: Strided accesses are independent\n");
1506 return Dependence::NoDep;
1507 }
1508
1509 // Negative distances are not plausible dependencies.
1510 if (Val.isNegative()) {
1511 bool IsTrueDataDependence = (AIsWrite && !BIsWrite);
1512 if (IsTrueDataDependence && EnableForwardingConflictDetection &&
1513 (couldPreventStoreLoadForward(Val.abs().getZExtValue(), TypeByteSize) ||
1514 ATy != BTy)) {
1515 LLVM_DEBUG(dbgs() << "LAA: Forward but may prevent st->ld forwarding\n");
1516 return Dependence::ForwardButPreventsForwarding;
1517 }
1518
1519 LLVM_DEBUG(dbgs() << "LAA: Dependence is negative\n");
1520 return Dependence::Forward;
1521 }
1522
1523 // Write to the same location with the same size.
1524 // Could be improved to assert type sizes are the same (i32 == float, etc).
1525 if (Val == 0) {
1526 if (ATy == BTy)
1527 return Dependence::Forward;
1528 LLVM_DEBUG(
1529 dbgs() << "LAA: Zero dependence difference but different types\n");
1530 return Dependence::Unknown;
1531 }
1532
1533 assert(Val.isStrictlyPositive() && "Expect a positive value");
1534
1535 if (ATy != BTy) {
1536 LLVM_DEBUG(
1537 dbgs()
1538 << "LAA: ReadWrite-Write positive dependency with different types\n");
1539 return Dependence::Unknown;
1540 }
1541
1542 // Bail out early if passed-in parameters make vectorization not feasible.
1543 unsigned ForcedFactor = (VectorizerParams::VectorizationFactor ?
1544 VectorizerParams::VectorizationFactor : 1);
1545 unsigned ForcedUnroll = (VectorizerParams::VectorizationInterleave ?
1546 VectorizerParams::VectorizationInterleave : 1);
1547 // The minimum number of iterations for a vectorized/unrolled version.
1548 unsigned MinNumIter = std::max(ForcedFactor * ForcedUnroll, 2U);
1549
1550 // It's not vectorizable if the distance is smaller than the minimum distance
1551 // needed for a vectroized/unrolled version. Vectorizing one iteration in
1552 // front needs TypeByteSize * Stride. Vectorizing the last iteration needs
1553 // TypeByteSize (No need to plus the last gap distance).
1554 //
1555 // E.g. Assume one char is 1 byte in memory and one int is 4 bytes.
1556 // foo(int *A) {
1557 // int *B = (int *)((char *)A + 14);
1558 // for (i = 0 ; i < 1024 ; i += 2)
1559 // B[i] = A[i] + 1;
1560 // }
1561 //
1562 // Two accesses in memory (stride is 2):
1563 // | A[0] | | A[2] | | A[4] | | A[6] | |
1564 // | B[0] | | B[2] | | B[4] |
1565 //
1566 // Distance needs for vectorizing iterations except the last iteration:
1567 // 4 * 2 * (MinNumIter - 1). Distance needs for the last iteration: 4.
1568 // So the minimum distance needed is: 4 * 2 * (MinNumIter - 1) + 4.
1569 //
1570 // If MinNumIter is 2, it is vectorizable as the minimum distance needed is
1571 // 12, which is less than distance.
1572 //
1573 // If MinNumIter is 4 (Say if a user forces the vectorization factor to be 4),
1574 // the minimum distance needed is 28, which is greater than distance. It is
1575 // not safe to do vectorization.
1576 uint64_t MinDistanceNeeded =
1577 TypeByteSize * Stride * (MinNumIter - 1) + TypeByteSize;
1578 if (MinDistanceNeeded > static_cast<uint64_t>(Distance)) {
1579 LLVM_DEBUG(dbgs() << "LAA: Failure because of positive distance "
1580 << Distance << '\n');
1581 return Dependence::Backward;
1582 }
1583
1584 // Unsafe if the minimum distance needed is greater than max safe distance.
1585 if (MinDistanceNeeded > MaxSafeDepDistBytes) {
1586 LLVM_DEBUG(dbgs() << "LAA: Failure because it needs at least "
1587 << MinDistanceNeeded << " size in bytes");
1588 return Dependence::Backward;
1589 }
1590
1591 // Positive distance bigger than max vectorization factor.
1592 // FIXME: Should use max factor instead of max distance in bytes, which could
1593 // not handle different types.
1594 // E.g. Assume one char is 1 byte in memory and one int is 4 bytes.
1595 // void foo (int *A, char *B) {
1596 // for (unsigned i = 0; i < 1024; i++) {
1597 // A[i+2] = A[i] + 1;
1598 // B[i+2] = B[i] + 1;
1599 // }
1600 // }
1601 //
1602 // This case is currently unsafe according to the max safe distance. If we
1603 // analyze the two accesses on array B, the max safe dependence distance
1604 // is 2. Then we analyze the accesses on array A, the minimum distance needed
1605 // is 8, which is less than 2 and forbidden vectorization, But actually
1606 // both A and B could be vectorized by 2 iterations.
1607 MaxSafeDepDistBytes =
1608 std::min(static_cast<uint64_t>(Distance), MaxSafeDepDistBytes);
1609
1610 bool IsTrueDataDependence = (!AIsWrite && BIsWrite);
1611 if (IsTrueDataDependence && EnableForwardingConflictDetection &&
1612 couldPreventStoreLoadForward(Distance, TypeByteSize))
1613 return Dependence::BackwardVectorizableButPreventsForwarding;
1614
1615 uint64_t MaxVF = MaxSafeDepDistBytes / (TypeByteSize * Stride);
1616 LLVM_DEBUG(dbgs() << "LAA: Positive distance " << Val.getSExtValue()
1617 << " with max VF = " << MaxVF << '\n');
1618 uint64_t MaxVFInBits = MaxVF * TypeByteSize * 8;
1619 MaxSafeRegisterWidth = std::min(MaxSafeRegisterWidth, MaxVFInBits);
1620 return Dependence::BackwardVectorizable;
1621 }
1622
areDepsSafe(DepCandidates & AccessSets,MemAccessInfoList & CheckDeps,const ValueToValueMap & Strides)1623 bool MemoryDepChecker::areDepsSafe(DepCandidates &AccessSets,
1624 MemAccessInfoList &CheckDeps,
1625 const ValueToValueMap &Strides) {
1626
1627 MaxSafeDepDistBytes = -1;
1628 SmallPtrSet<MemAccessInfo, 8> Visited;
1629 for (MemAccessInfo CurAccess : CheckDeps) {
1630 if (Visited.count(CurAccess))
1631 continue;
1632
1633 // Get the relevant memory access set.
1634 EquivalenceClasses<MemAccessInfo>::iterator I =
1635 AccessSets.findValue(AccessSets.getLeaderValue(CurAccess));
1636
1637 // Check accesses within this set.
1638 EquivalenceClasses<MemAccessInfo>::member_iterator AI =
1639 AccessSets.member_begin(I);
1640 EquivalenceClasses<MemAccessInfo>::member_iterator AE =
1641 AccessSets.member_end();
1642
1643 // Check every access pair.
1644 while (AI != AE) {
1645 Visited.insert(*AI);
1646 EquivalenceClasses<MemAccessInfo>::member_iterator OI = std::next(AI);
1647 while (OI != AE) {
1648 // Check every accessing instruction pair in program order.
1649 for (std::vector<unsigned>::iterator I1 = Accesses[*AI].begin(),
1650 I1E = Accesses[*AI].end(); I1 != I1E; ++I1)
1651 for (std::vector<unsigned>::iterator I2 = Accesses[*OI].begin(),
1652 I2E = Accesses[*OI].end(); I2 != I2E; ++I2) {
1653 auto A = std::make_pair(&*AI, *I1);
1654 auto B = std::make_pair(&*OI, *I2);
1655
1656 assert(*I1 != *I2);
1657 if (*I1 > *I2)
1658 std::swap(A, B);
1659
1660 Dependence::DepType Type =
1661 isDependent(*A.first, A.second, *B.first, B.second, Strides);
1662 mergeInStatus(Dependence::isSafeForVectorization(Type));
1663
1664 // Gather dependences unless we accumulated MaxDependences
1665 // dependences. In that case return as soon as we find the first
1666 // unsafe dependence. This puts a limit on this quadratic
1667 // algorithm.
1668 if (RecordDependences) {
1669 if (Type != Dependence::NoDep)
1670 Dependences.push_back(Dependence(A.second, B.second, Type));
1671
1672 if (Dependences.size() >= MaxDependences) {
1673 RecordDependences = false;
1674 Dependences.clear();
1675 LLVM_DEBUG(dbgs()
1676 << "Too many dependences, stopped recording\n");
1677 }
1678 }
1679 if (!RecordDependences && !isSafeForVectorization())
1680 return false;
1681 }
1682 ++OI;
1683 }
1684 AI++;
1685 }
1686 }
1687
1688 LLVM_DEBUG(dbgs() << "Total Dependences: " << Dependences.size() << "\n");
1689 return isSafeForVectorization();
1690 }
1691
1692 SmallVector<Instruction *, 4>
getInstructionsForAccess(Value * Ptr,bool isWrite) const1693 MemoryDepChecker::getInstructionsForAccess(Value *Ptr, bool isWrite) const {
1694 MemAccessInfo Access(Ptr, isWrite);
1695 auto &IndexVector = Accesses.find(Access)->second;
1696
1697 SmallVector<Instruction *, 4> Insts;
1698 transform(IndexVector,
1699 std::back_inserter(Insts),
1700 [&](unsigned Idx) { return this->InstMap[Idx]; });
1701 return Insts;
1702 }
1703
1704 const char *MemoryDepChecker::Dependence::DepName[] = {
1705 "NoDep", "Unknown", "Forward", "ForwardButPreventsForwarding", "Backward",
1706 "BackwardVectorizable", "BackwardVectorizableButPreventsForwarding"};
1707
print(raw_ostream & OS,unsigned Depth,const SmallVectorImpl<Instruction * > & Instrs) const1708 void MemoryDepChecker::Dependence::print(
1709 raw_ostream &OS, unsigned Depth,
1710 const SmallVectorImpl<Instruction *> &Instrs) const {
1711 OS.indent(Depth) << DepName[Type] << ":\n";
1712 OS.indent(Depth + 2) << *Instrs[Source] << " -> \n";
1713 OS.indent(Depth + 2) << *Instrs[Destination] << "\n";
1714 }
1715
canAnalyzeLoop()1716 bool LoopAccessInfo::canAnalyzeLoop() {
1717 // We need to have a loop header.
1718 LLVM_DEBUG(dbgs() << "LAA: Found a loop in "
1719 << TheLoop->getHeader()->getParent()->getName() << ": "
1720 << TheLoop->getHeader()->getName() << '\n');
1721
1722 // We can only analyze innermost loops.
1723 if (!TheLoop->empty()) {
1724 LLVM_DEBUG(dbgs() << "LAA: loop is not the innermost loop\n");
1725 recordAnalysis("NotInnerMostLoop") << "loop is not the innermost loop";
1726 return false;
1727 }
1728
1729 // We must have a single backedge.
1730 if (TheLoop->getNumBackEdges() != 1) {
1731 LLVM_DEBUG(
1732 dbgs() << "LAA: loop control flow is not understood by analyzer\n");
1733 recordAnalysis("CFGNotUnderstood")
1734 << "loop control flow is not understood by analyzer";
1735 return false;
1736 }
1737
1738 // We must have a single exiting block.
1739 if (!TheLoop->getExitingBlock()) {
1740 LLVM_DEBUG(
1741 dbgs() << "LAA: loop control flow is not understood by analyzer\n");
1742 recordAnalysis("CFGNotUnderstood")
1743 << "loop control flow is not understood by analyzer";
1744 return false;
1745 }
1746
1747 // We only handle bottom-tested loops, i.e. loop in which the condition is
1748 // checked at the end of each iteration. With that we can assume that all
1749 // instructions in the loop are executed the same number of times.
1750 if (TheLoop->getExitingBlock() != TheLoop->getLoopLatch()) {
1751 LLVM_DEBUG(
1752 dbgs() << "LAA: loop control flow is not understood by analyzer\n");
1753 recordAnalysis("CFGNotUnderstood")
1754 << "loop control flow is not understood by analyzer";
1755 return false;
1756 }
1757
1758 // ScalarEvolution needs to be able to find the exit count.
1759 const SCEV *ExitCount = PSE->getBackedgeTakenCount();
1760 if (ExitCount == PSE->getSE()->getCouldNotCompute()) {
1761 recordAnalysis("CantComputeNumberOfIterations")
1762 << "could not determine number of loop iterations";
1763 LLVM_DEBUG(dbgs() << "LAA: SCEV could not compute the loop exit count.\n");
1764 return false;
1765 }
1766
1767 return true;
1768 }
1769
analyzeLoop(AliasAnalysis * AA,LoopInfo * LI,const TargetLibraryInfo * TLI,DominatorTree * DT)1770 void LoopAccessInfo::analyzeLoop(AliasAnalysis *AA, LoopInfo *LI,
1771 const TargetLibraryInfo *TLI,
1772 DominatorTree *DT) {
1773 typedef SmallPtrSet<Value*, 16> ValueSet;
1774
1775 // Holds the Load and Store instructions.
1776 SmallVector<LoadInst *, 16> Loads;
1777 SmallVector<StoreInst *, 16> Stores;
1778
1779 // Holds all the different accesses in the loop.
1780 unsigned NumReads = 0;
1781 unsigned NumReadWrites = 0;
1782
1783 PtrRtChecking->Pointers.clear();
1784 PtrRtChecking->Need = false;
1785
1786 const bool IsAnnotatedParallel = TheLoop->isAnnotatedParallel();
1787
1788 // For each block.
1789 for (BasicBlock *BB : TheLoop->blocks()) {
1790 // Scan the BB and collect legal loads and stores.
1791 for (Instruction &I : *BB) {
1792 // If this is a load, save it. If this instruction can read from memory
1793 // but is not a load, then we quit. Notice that we don't handle function
1794 // calls that read or write.
1795 if (I.mayReadFromMemory()) {
1796 // Many math library functions read the rounding mode. We will only
1797 // vectorize a loop if it contains known function calls that don't set
1798 // the flag. Therefore, it is safe to ignore this read from memory.
1799 auto *Call = dyn_cast<CallInst>(&I);
1800 if (Call && getVectorIntrinsicIDForCall(Call, TLI))
1801 continue;
1802
1803 // If the function has an explicit vectorized counterpart, we can safely
1804 // assume that it can be vectorized.
1805 if (Call && !Call->isNoBuiltin() && Call->getCalledFunction() &&
1806 TLI->isFunctionVectorizable(Call->getCalledFunction()->getName()))
1807 continue;
1808
1809 auto *Ld = dyn_cast<LoadInst>(&I);
1810 if (!Ld || (!Ld->isSimple() && !IsAnnotatedParallel)) {
1811 recordAnalysis("NonSimpleLoad", Ld)
1812 << "read with atomic ordering or volatile read";
1813 LLVM_DEBUG(dbgs() << "LAA: Found a non-simple load.\n");
1814 CanVecMem = false;
1815 return;
1816 }
1817 NumLoads++;
1818 Loads.push_back(Ld);
1819 DepChecker->addAccess(Ld);
1820 if (EnableMemAccessVersioning)
1821 collectStridedAccess(Ld);
1822 continue;
1823 }
1824
1825 // Save 'store' instructions. Abort if other instructions write to memory.
1826 if (I.mayWriteToMemory()) {
1827 auto *St = dyn_cast<StoreInst>(&I);
1828 if (!St) {
1829 recordAnalysis("CantVectorizeInstruction", St)
1830 << "instruction cannot be vectorized";
1831 CanVecMem = false;
1832 return;
1833 }
1834 if (!St->isSimple() && !IsAnnotatedParallel) {
1835 recordAnalysis("NonSimpleStore", St)
1836 << "write with atomic ordering or volatile write";
1837 LLVM_DEBUG(dbgs() << "LAA: Found a non-simple store.\n");
1838 CanVecMem = false;
1839 return;
1840 }
1841 NumStores++;
1842 Stores.push_back(St);
1843 DepChecker->addAccess(St);
1844 if (EnableMemAccessVersioning)
1845 collectStridedAccess(St);
1846 }
1847 } // Next instr.
1848 } // Next block.
1849
1850 // Now we have two lists that hold the loads and the stores.
1851 // Next, we find the pointers that they use.
1852
1853 // Check if we see any stores. If there are no stores, then we don't
1854 // care if the pointers are *restrict*.
1855 if (!Stores.size()) {
1856 LLVM_DEBUG(dbgs() << "LAA: Found a read-only loop!\n");
1857 CanVecMem = true;
1858 return;
1859 }
1860
1861 MemoryDepChecker::DepCandidates DependentAccesses;
1862 AccessAnalysis Accesses(TheLoop->getHeader()->getModule()->getDataLayout(),
1863 TheLoop, AA, LI, DependentAccesses, *PSE);
1864
1865 // Holds the analyzed pointers. We don't want to call GetUnderlyingObjects
1866 // multiple times on the same object. If the ptr is accessed twice, once
1867 // for read and once for write, it will only appear once (on the write
1868 // list). This is okay, since we are going to check for conflicts between
1869 // writes and between reads and writes, but not between reads and reads.
1870 ValueSet Seen;
1871
1872 // Record uniform store addresses to identify if we have multiple stores
1873 // to the same address.
1874 ValueSet UniformStores;
1875
1876 for (StoreInst *ST : Stores) {
1877 Value *Ptr = ST->getPointerOperand();
1878
1879 if (isUniform(Ptr))
1880 HasDependenceInvolvingLoopInvariantAddress |=
1881 !UniformStores.insert(Ptr).second;
1882
1883 // If we did *not* see this pointer before, insert it to the read-write
1884 // list. At this phase it is only a 'write' list.
1885 if (Seen.insert(Ptr).second) {
1886 ++NumReadWrites;
1887
1888 MemoryLocation Loc = MemoryLocation::get(ST);
1889 // The TBAA metadata could have a control dependency on the predication
1890 // condition, so we cannot rely on it when determining whether or not we
1891 // need runtime pointer checks.
1892 if (blockNeedsPredication(ST->getParent(), TheLoop, DT))
1893 Loc.AATags.TBAA = nullptr;
1894
1895 Accesses.addStore(Loc);
1896 }
1897 }
1898
1899 if (IsAnnotatedParallel) {
1900 LLVM_DEBUG(
1901 dbgs() << "LAA: A loop annotated parallel, ignore memory dependency "
1902 << "checks.\n");
1903 CanVecMem = true;
1904 return;
1905 }
1906
1907 for (LoadInst *LD : Loads) {
1908 Value *Ptr = LD->getPointerOperand();
1909 // If we did *not* see this pointer before, insert it to the
1910 // read list. If we *did* see it before, then it is already in
1911 // the read-write list. This allows us to vectorize expressions
1912 // such as A[i] += x; Because the address of A[i] is a read-write
1913 // pointer. This only works if the index of A[i] is consecutive.
1914 // If the address of i is unknown (for example A[B[i]]) then we may
1915 // read a few words, modify, and write a few words, and some of the
1916 // words may be written to the same address.
1917 bool IsReadOnlyPtr = false;
1918 if (Seen.insert(Ptr).second ||
1919 !getPtrStride(*PSE, Ptr, TheLoop, SymbolicStrides)) {
1920 ++NumReads;
1921 IsReadOnlyPtr = true;
1922 }
1923
1924 // See if there is an unsafe dependency between a load to a uniform address and
1925 // store to the same uniform address.
1926 if (UniformStores.count(Ptr)) {
1927 LLVM_DEBUG(dbgs() << "LAA: Found an unsafe dependency between a uniform "
1928 "load and uniform store to the same address!\n");
1929 HasDependenceInvolvingLoopInvariantAddress = true;
1930 }
1931
1932 MemoryLocation Loc = MemoryLocation::get(LD);
1933 // The TBAA metadata could have a control dependency on the predication
1934 // condition, so we cannot rely on it when determining whether or not we
1935 // need runtime pointer checks.
1936 if (blockNeedsPredication(LD->getParent(), TheLoop, DT))
1937 Loc.AATags.TBAA = nullptr;
1938
1939 Accesses.addLoad(Loc, IsReadOnlyPtr);
1940 }
1941
1942 // If we write (or read-write) to a single destination and there are no
1943 // other reads in this loop then is it safe to vectorize.
1944 if (NumReadWrites == 1 && NumReads == 0) {
1945 LLVM_DEBUG(dbgs() << "LAA: Found a write-only loop!\n");
1946 CanVecMem = true;
1947 return;
1948 }
1949
1950 // Build dependence sets and check whether we need a runtime pointer bounds
1951 // check.
1952 Accesses.buildDependenceSets();
1953
1954 // Find pointers with computable bounds. We are going to use this information
1955 // to place a runtime bound check.
1956 bool CanDoRTIfNeeded = Accesses.canCheckPtrAtRT(*PtrRtChecking, PSE->getSE(),
1957 TheLoop, SymbolicStrides);
1958 if (!CanDoRTIfNeeded) {
1959 recordAnalysis("CantIdentifyArrayBounds") << "cannot identify array bounds";
1960 LLVM_DEBUG(dbgs() << "LAA: We can't vectorize because we can't find "
1961 << "the array bounds.\n");
1962 CanVecMem = false;
1963 return;
1964 }
1965
1966 LLVM_DEBUG(
1967 dbgs() << "LAA: We can perform a memory runtime check if needed.\n");
1968
1969 CanVecMem = true;
1970 if (Accesses.isDependencyCheckNeeded()) {
1971 LLVM_DEBUG(dbgs() << "LAA: Checking memory dependencies\n");
1972 CanVecMem = DepChecker->areDepsSafe(
1973 DependentAccesses, Accesses.getDependenciesToCheck(), SymbolicStrides);
1974 MaxSafeDepDistBytes = DepChecker->getMaxSafeDepDistBytes();
1975
1976 if (!CanVecMem && DepChecker->shouldRetryWithRuntimeCheck()) {
1977 LLVM_DEBUG(dbgs() << "LAA: Retrying with memory checks\n");
1978
1979 // Clear the dependency checks. We assume they are not needed.
1980 Accesses.resetDepChecks(*DepChecker);
1981
1982 PtrRtChecking->reset();
1983 PtrRtChecking->Need = true;
1984
1985 auto *SE = PSE->getSE();
1986 CanDoRTIfNeeded = Accesses.canCheckPtrAtRT(*PtrRtChecking, SE, TheLoop,
1987 SymbolicStrides, true);
1988
1989 // Check that we found the bounds for the pointer.
1990 if (!CanDoRTIfNeeded) {
1991 recordAnalysis("CantCheckMemDepsAtRunTime")
1992 << "cannot check memory dependencies at runtime";
1993 LLVM_DEBUG(dbgs() << "LAA: Can't vectorize with memory checks\n");
1994 CanVecMem = false;
1995 return;
1996 }
1997
1998 CanVecMem = true;
1999 }
2000 }
2001
2002 if (CanVecMem)
2003 LLVM_DEBUG(
2004 dbgs() << "LAA: No unsafe dependent memory operations in loop. We"
2005 << (PtrRtChecking->Need ? "" : " don't")
2006 << " need runtime memory checks.\n");
2007 else {
2008 recordAnalysis("UnsafeMemDep")
2009 << "unsafe dependent memory operations in loop. Use "
2010 "#pragma loop distribute(enable) to allow loop distribution "
2011 "to attempt to isolate the offending operations into a separate "
2012 "loop";
2013 LLVM_DEBUG(dbgs() << "LAA: unsafe dependent memory operations in loop\n");
2014 }
2015 }
2016
blockNeedsPredication(BasicBlock * BB,Loop * TheLoop,DominatorTree * DT)2017 bool LoopAccessInfo::blockNeedsPredication(BasicBlock *BB, Loop *TheLoop,
2018 DominatorTree *DT) {
2019 assert(TheLoop->contains(BB) && "Unknown block used");
2020
2021 // Blocks that do not dominate the latch need predication.
2022 BasicBlock* Latch = TheLoop->getLoopLatch();
2023 return !DT->dominates(BB, Latch);
2024 }
2025
recordAnalysis(StringRef RemarkName,Instruction * I)2026 OptimizationRemarkAnalysis &LoopAccessInfo::recordAnalysis(StringRef RemarkName,
2027 Instruction *I) {
2028 assert(!Report && "Multiple reports generated");
2029
2030 Value *CodeRegion = TheLoop->getHeader();
2031 DebugLoc DL = TheLoop->getStartLoc();
2032
2033 if (I) {
2034 CodeRegion = I->getParent();
2035 // If there is no debug location attached to the instruction, revert back to
2036 // using the loop's.
2037 if (I->getDebugLoc())
2038 DL = I->getDebugLoc();
2039 }
2040
2041 Report = make_unique<OptimizationRemarkAnalysis>(DEBUG_TYPE, RemarkName, DL,
2042 CodeRegion);
2043 return *Report;
2044 }
2045
isUniform(Value * V) const2046 bool LoopAccessInfo::isUniform(Value *V) const {
2047 auto *SE = PSE->getSE();
2048 // Since we rely on SCEV for uniformity, if the type is not SCEVable, it is
2049 // never considered uniform.
2050 // TODO: Is this really what we want? Even without FP SCEV, we may want some
2051 // trivially loop-invariant FP values to be considered uniform.
2052 if (!SE->isSCEVable(V->getType()))
2053 return false;
2054 return (SE->isLoopInvariant(SE->getSCEV(V), TheLoop));
2055 }
2056
2057 // FIXME: this function is currently a duplicate of the one in
2058 // LoopVectorize.cpp.
getFirstInst(Instruction * FirstInst,Value * V,Instruction * Loc)2059 static Instruction *getFirstInst(Instruction *FirstInst, Value *V,
2060 Instruction *Loc) {
2061 if (FirstInst)
2062 return FirstInst;
2063 if (Instruction *I = dyn_cast<Instruction>(V))
2064 return I->getParent() == Loc->getParent() ? I : nullptr;
2065 return nullptr;
2066 }
2067
2068 namespace {
2069
2070 /// IR Values for the lower and upper bounds of a pointer evolution. We
2071 /// need to use value-handles because SCEV expansion can invalidate previously
2072 /// expanded values. Thus expansion of a pointer can invalidate the bounds for
2073 /// a previous one.
2074 struct PointerBounds {
2075 TrackingVH<Value> Start;
2076 TrackingVH<Value> End;
2077 };
2078
2079 } // end anonymous namespace
2080
2081 /// Expand code for the lower and upper bound of the pointer group \p CG
2082 /// in \p TheLoop. \return the values for the bounds.
2083 static PointerBounds
expandBounds(const RuntimePointerChecking::CheckingPtrGroup * CG,Loop * TheLoop,Instruction * Loc,SCEVExpander & Exp,ScalarEvolution * SE,const RuntimePointerChecking & PtrRtChecking)2084 expandBounds(const RuntimePointerChecking::CheckingPtrGroup *CG, Loop *TheLoop,
2085 Instruction *Loc, SCEVExpander &Exp, ScalarEvolution *SE,
2086 const RuntimePointerChecking &PtrRtChecking) {
2087 Value *Ptr = PtrRtChecking.Pointers[CG->Members[0]].PointerValue;
2088 const SCEV *Sc = SE->getSCEV(Ptr);
2089
2090 unsigned AS = Ptr->getType()->getPointerAddressSpace();
2091 LLVMContext &Ctx = Loc->getContext();
2092
2093 // Use this type for pointer arithmetic.
2094 Type *PtrArithTy = Type::getInt8PtrTy(Ctx, AS);
2095
2096 if (SE->isLoopInvariant(Sc, TheLoop)) {
2097 LLVM_DEBUG(dbgs() << "LAA: Adding RT check for a loop invariant ptr:"
2098 << *Ptr << "\n");
2099 // Ptr could be in the loop body. If so, expand a new one at the correct
2100 // location.
2101 Instruction *Inst = dyn_cast<Instruction>(Ptr);
2102 Value *NewPtr = (Inst && TheLoop->contains(Inst))
2103 ? Exp.expandCodeFor(Sc, PtrArithTy, Loc)
2104 : Ptr;
2105 // We must return a half-open range, which means incrementing Sc.
2106 const SCEV *ScPlusOne = SE->getAddExpr(Sc, SE->getOne(PtrArithTy));
2107 Value *NewPtrPlusOne = Exp.expandCodeFor(ScPlusOne, PtrArithTy, Loc);
2108 return {NewPtr, NewPtrPlusOne};
2109 } else {
2110 Value *Start = nullptr, *End = nullptr;
2111 LLVM_DEBUG(dbgs() << "LAA: Adding RT check for range:\n");
2112 Start = Exp.expandCodeFor(CG->Low, PtrArithTy, Loc);
2113 End = Exp.expandCodeFor(CG->High, PtrArithTy, Loc);
2114 LLVM_DEBUG(dbgs() << "Start: " << *CG->Low << " End: " << *CG->High
2115 << "\n");
2116 return {Start, End};
2117 }
2118 }
2119
2120 /// Turns a collection of checks into a collection of expanded upper and
2121 /// lower bounds for both pointers in the check.
expandBounds(const SmallVectorImpl<RuntimePointerChecking::PointerCheck> & PointerChecks,Loop * L,Instruction * Loc,ScalarEvolution * SE,SCEVExpander & Exp,const RuntimePointerChecking & PtrRtChecking)2122 static SmallVector<std::pair<PointerBounds, PointerBounds>, 4> expandBounds(
2123 const SmallVectorImpl<RuntimePointerChecking::PointerCheck> &PointerChecks,
2124 Loop *L, Instruction *Loc, ScalarEvolution *SE, SCEVExpander &Exp,
2125 const RuntimePointerChecking &PtrRtChecking) {
2126 SmallVector<std::pair<PointerBounds, PointerBounds>, 4> ChecksWithBounds;
2127
2128 // Here we're relying on the SCEV Expander's cache to only emit code for the
2129 // same bounds once.
2130 transform(
2131 PointerChecks, std::back_inserter(ChecksWithBounds),
2132 [&](const RuntimePointerChecking::PointerCheck &Check) {
2133 PointerBounds
2134 First = expandBounds(Check.first, L, Loc, Exp, SE, PtrRtChecking),
2135 Second = expandBounds(Check.second, L, Loc, Exp, SE, PtrRtChecking);
2136 return std::make_pair(First, Second);
2137 });
2138
2139 return ChecksWithBounds;
2140 }
2141
addRuntimeChecks(Instruction * Loc,const SmallVectorImpl<RuntimePointerChecking::PointerCheck> & PointerChecks) const2142 std::pair<Instruction *, Instruction *> LoopAccessInfo::addRuntimeChecks(
2143 Instruction *Loc,
2144 const SmallVectorImpl<RuntimePointerChecking::PointerCheck> &PointerChecks)
2145 const {
2146 const DataLayout &DL = TheLoop->getHeader()->getModule()->getDataLayout();
2147 auto *SE = PSE->getSE();
2148 SCEVExpander Exp(*SE, DL, "induction");
2149 auto ExpandedChecks =
2150 expandBounds(PointerChecks, TheLoop, Loc, SE, Exp, *PtrRtChecking);
2151
2152 LLVMContext &Ctx = Loc->getContext();
2153 Instruction *FirstInst = nullptr;
2154 IRBuilder<> ChkBuilder(Loc);
2155 // Our instructions might fold to a constant.
2156 Value *MemoryRuntimeCheck = nullptr;
2157
2158 for (const auto &Check : ExpandedChecks) {
2159 const PointerBounds &A = Check.first, &B = Check.second;
2160 // Check if two pointers (A and B) conflict where conflict is computed as:
2161 // start(A) <= end(B) && start(B) <= end(A)
2162 unsigned AS0 = A.Start->getType()->getPointerAddressSpace();
2163 unsigned AS1 = B.Start->getType()->getPointerAddressSpace();
2164
2165 assert((AS0 == B.End->getType()->getPointerAddressSpace()) &&
2166 (AS1 == A.End->getType()->getPointerAddressSpace()) &&
2167 "Trying to bounds check pointers with different address spaces");
2168
2169 Type *PtrArithTy0 = Type::getInt8PtrTy(Ctx, AS0);
2170 Type *PtrArithTy1 = Type::getInt8PtrTy(Ctx, AS1);
2171
2172 Value *Start0 = ChkBuilder.CreateBitCast(A.Start, PtrArithTy0, "bc");
2173 Value *Start1 = ChkBuilder.CreateBitCast(B.Start, PtrArithTy1, "bc");
2174 Value *End0 = ChkBuilder.CreateBitCast(A.End, PtrArithTy1, "bc");
2175 Value *End1 = ChkBuilder.CreateBitCast(B.End, PtrArithTy0, "bc");
2176
2177 // [A|B].Start points to the first accessed byte under base [A|B].
2178 // [A|B].End points to the last accessed byte, plus one.
2179 // There is no conflict when the intervals are disjoint:
2180 // NoConflict = (B.Start >= A.End) || (A.Start >= B.End)
2181 //
2182 // bound0 = (B.Start < A.End)
2183 // bound1 = (A.Start < B.End)
2184 // IsConflict = bound0 & bound1
2185 Value *Cmp0 = ChkBuilder.CreateICmpULT(Start0, End1, "bound0");
2186 FirstInst = getFirstInst(FirstInst, Cmp0, Loc);
2187 Value *Cmp1 = ChkBuilder.CreateICmpULT(Start1, End0, "bound1");
2188 FirstInst = getFirstInst(FirstInst, Cmp1, Loc);
2189 Value *IsConflict = ChkBuilder.CreateAnd(Cmp0, Cmp1, "found.conflict");
2190 FirstInst = getFirstInst(FirstInst, IsConflict, Loc);
2191 if (MemoryRuntimeCheck) {
2192 IsConflict =
2193 ChkBuilder.CreateOr(MemoryRuntimeCheck, IsConflict, "conflict.rdx");
2194 FirstInst = getFirstInst(FirstInst, IsConflict, Loc);
2195 }
2196 MemoryRuntimeCheck = IsConflict;
2197 }
2198
2199 if (!MemoryRuntimeCheck)
2200 return std::make_pair(nullptr, nullptr);
2201
2202 // We have to do this trickery because the IRBuilder might fold the check to a
2203 // constant expression in which case there is no Instruction anchored in a
2204 // the block.
2205 Instruction *Check = BinaryOperator::CreateAnd(MemoryRuntimeCheck,
2206 ConstantInt::getTrue(Ctx));
2207 ChkBuilder.Insert(Check, "memcheck.conflict");
2208 FirstInst = getFirstInst(FirstInst, Check, Loc);
2209 return std::make_pair(FirstInst, Check);
2210 }
2211
2212 std::pair<Instruction *, Instruction *>
addRuntimeChecks(Instruction * Loc) const2213 LoopAccessInfo::addRuntimeChecks(Instruction *Loc) const {
2214 if (!PtrRtChecking->Need)
2215 return std::make_pair(nullptr, nullptr);
2216
2217 return addRuntimeChecks(Loc, PtrRtChecking->getChecks());
2218 }
2219
collectStridedAccess(Value * MemAccess)2220 void LoopAccessInfo::collectStridedAccess(Value *MemAccess) {
2221 Value *Ptr = nullptr;
2222 if (LoadInst *LI = dyn_cast<LoadInst>(MemAccess))
2223 Ptr = LI->getPointerOperand();
2224 else if (StoreInst *SI = dyn_cast<StoreInst>(MemAccess))
2225 Ptr = SI->getPointerOperand();
2226 else
2227 return;
2228
2229 Value *Stride = getStrideFromPointer(Ptr, PSE->getSE(), TheLoop);
2230 if (!Stride)
2231 return;
2232
2233 LLVM_DEBUG(dbgs() << "LAA: Found a strided access that is a candidate for "
2234 "versioning:");
2235 LLVM_DEBUG(dbgs() << " Ptr: " << *Ptr << " Stride: " << *Stride << "\n");
2236
2237 // Avoid adding the "Stride == 1" predicate when we know that
2238 // Stride >= Trip-Count. Such a predicate will effectively optimize a single
2239 // or zero iteration loop, as Trip-Count <= Stride == 1.
2240 //
2241 // TODO: We are currently not making a very informed decision on when it is
2242 // beneficial to apply stride versioning. It might make more sense that the
2243 // users of this analysis (such as the vectorizer) will trigger it, based on
2244 // their specific cost considerations; For example, in cases where stride
2245 // versioning does not help resolving memory accesses/dependences, the
2246 // vectorizer should evaluate the cost of the runtime test, and the benefit
2247 // of various possible stride specializations, considering the alternatives
2248 // of using gather/scatters (if available).
2249
2250 const SCEV *StrideExpr = PSE->getSCEV(Stride);
2251 const SCEV *BETakenCount = PSE->getBackedgeTakenCount();
2252
2253 // Match the types so we can compare the stride and the BETakenCount.
2254 // The Stride can be positive/negative, so we sign extend Stride;
2255 // The backdgeTakenCount is non-negative, so we zero extend BETakenCount.
2256 const DataLayout &DL = TheLoop->getHeader()->getModule()->getDataLayout();
2257 uint64_t StrideTypeSize = DL.getTypeAllocSize(StrideExpr->getType());
2258 uint64_t BETypeSize = DL.getTypeAllocSize(BETakenCount->getType());
2259 const SCEV *CastedStride = StrideExpr;
2260 const SCEV *CastedBECount = BETakenCount;
2261 ScalarEvolution *SE = PSE->getSE();
2262 if (BETypeSize >= StrideTypeSize)
2263 CastedStride = SE->getNoopOrSignExtend(StrideExpr, BETakenCount->getType());
2264 else
2265 CastedBECount = SE->getZeroExtendExpr(BETakenCount, StrideExpr->getType());
2266 const SCEV *StrideMinusBETaken = SE->getMinusSCEV(CastedStride, CastedBECount);
2267 // Since TripCount == BackEdgeTakenCount + 1, checking:
2268 // "Stride >= TripCount" is equivalent to checking:
2269 // Stride - BETakenCount > 0
2270 if (SE->isKnownPositive(StrideMinusBETaken)) {
2271 LLVM_DEBUG(
2272 dbgs() << "LAA: Stride>=TripCount; No point in versioning as the "
2273 "Stride==1 predicate will imply that the loop executes "
2274 "at most once.\n");
2275 return;
2276 }
2277 LLVM_DEBUG(dbgs() << "LAA: Found a strided access that we can version.");
2278
2279 SymbolicStrides[Ptr] = Stride;
2280 StrideSet.insert(Stride);
2281 }
2282
LoopAccessInfo(Loop * L,ScalarEvolution * SE,const TargetLibraryInfo * TLI,AliasAnalysis * AA,DominatorTree * DT,LoopInfo * LI)2283 LoopAccessInfo::LoopAccessInfo(Loop *L, ScalarEvolution *SE,
2284 const TargetLibraryInfo *TLI, AliasAnalysis *AA,
2285 DominatorTree *DT, LoopInfo *LI)
2286 : PSE(llvm::make_unique<PredicatedScalarEvolution>(*SE, *L)),
2287 PtrRtChecking(llvm::make_unique<RuntimePointerChecking>(SE)),
2288 DepChecker(llvm::make_unique<MemoryDepChecker>(*PSE, L)), TheLoop(L),
2289 NumLoads(0), NumStores(0), MaxSafeDepDistBytes(-1), CanVecMem(false),
2290 HasDependenceInvolvingLoopInvariantAddress(false) {
2291 if (canAnalyzeLoop())
2292 analyzeLoop(AA, LI, TLI, DT);
2293 }
2294
print(raw_ostream & OS,unsigned Depth) const2295 void LoopAccessInfo::print(raw_ostream &OS, unsigned Depth) const {
2296 if (CanVecMem) {
2297 OS.indent(Depth) << "Memory dependences are safe";
2298 if (MaxSafeDepDistBytes != -1ULL)
2299 OS << " with a maximum dependence distance of " << MaxSafeDepDistBytes
2300 << " bytes";
2301 if (PtrRtChecking->Need)
2302 OS << " with run-time checks";
2303 OS << "\n";
2304 }
2305
2306 if (Report)
2307 OS.indent(Depth) << "Report: " << Report->getMsg() << "\n";
2308
2309 if (auto *Dependences = DepChecker->getDependences()) {
2310 OS.indent(Depth) << "Dependences:\n";
2311 for (auto &Dep : *Dependences) {
2312 Dep.print(OS, Depth + 2, DepChecker->getMemoryInstructions());
2313 OS << "\n";
2314 }
2315 } else
2316 OS.indent(Depth) << "Too many dependences, not recorded\n";
2317
2318 // List the pair of accesses need run-time checks to prove independence.
2319 PtrRtChecking->print(OS, Depth);
2320 OS << "\n";
2321
2322 OS.indent(Depth) << "Non vectorizable stores to invariant address were "
2323 << (HasDependenceInvolvingLoopInvariantAddress ? "" : "not ")
2324 << "found in loop.\n";
2325
2326 OS.indent(Depth) << "SCEV assumptions:\n";
2327 PSE->getUnionPredicate().print(OS, Depth);
2328
2329 OS << "\n";
2330
2331 OS.indent(Depth) << "Expressions re-written:\n";
2332 PSE->print(OS, Depth);
2333 }
2334
getInfo(Loop * L)2335 const LoopAccessInfo &LoopAccessLegacyAnalysis::getInfo(Loop *L) {
2336 auto &LAI = LoopAccessInfoMap[L];
2337
2338 if (!LAI)
2339 LAI = llvm::make_unique<LoopAccessInfo>(L, SE, TLI, AA, DT, LI);
2340
2341 return *LAI.get();
2342 }
2343
print(raw_ostream & OS,const Module * M) const2344 void LoopAccessLegacyAnalysis::print(raw_ostream &OS, const Module *M) const {
2345 LoopAccessLegacyAnalysis &LAA = *const_cast<LoopAccessLegacyAnalysis *>(this);
2346
2347 for (Loop *TopLevelLoop : *LI)
2348 for (Loop *L : depth_first(TopLevelLoop)) {
2349 OS.indent(2) << L->getHeader()->getName() << ":\n";
2350 auto &LAI = LAA.getInfo(L);
2351 LAI.print(OS, 4);
2352 }
2353 }
2354
runOnFunction(Function & F)2355 bool LoopAccessLegacyAnalysis::runOnFunction(Function &F) {
2356 SE = &getAnalysis<ScalarEvolutionWrapperPass>().getSE();
2357 auto *TLIP = getAnalysisIfAvailable<TargetLibraryInfoWrapperPass>();
2358 TLI = TLIP ? &TLIP->getTLI() : nullptr;
2359 AA = &getAnalysis<AAResultsWrapperPass>().getAAResults();
2360 DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
2361 LI = &getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
2362
2363 return false;
2364 }
2365
getAnalysisUsage(AnalysisUsage & AU) const2366 void LoopAccessLegacyAnalysis::getAnalysisUsage(AnalysisUsage &AU) const {
2367 AU.addRequired<ScalarEvolutionWrapperPass>();
2368 AU.addRequired<AAResultsWrapperPass>();
2369 AU.addRequired<DominatorTreeWrapperPass>();
2370 AU.addRequired<LoopInfoWrapperPass>();
2371
2372 AU.setPreservesAll();
2373 }
2374
2375 char LoopAccessLegacyAnalysis::ID = 0;
2376 static const char laa_name[] = "Loop Access Analysis";
2377 #define LAA_NAME "loop-accesses"
2378
2379 INITIALIZE_PASS_BEGIN(LoopAccessLegacyAnalysis, LAA_NAME, laa_name, false, true)
2380 INITIALIZE_PASS_DEPENDENCY(AAResultsWrapperPass)
2381 INITIALIZE_PASS_DEPENDENCY(ScalarEvolutionWrapperPass)
2382 INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
2383 INITIALIZE_PASS_DEPENDENCY(LoopInfoWrapperPass)
2384 INITIALIZE_PASS_END(LoopAccessLegacyAnalysis, LAA_NAME, laa_name, false, true)
2385
2386 AnalysisKey LoopAccessAnalysis::Key;
2387
run(Loop & L,LoopAnalysisManager & AM,LoopStandardAnalysisResults & AR)2388 LoopAccessInfo LoopAccessAnalysis::run(Loop &L, LoopAnalysisManager &AM,
2389 LoopStandardAnalysisResults &AR) {
2390 return LoopAccessInfo(&L, &AR.SE, &AR.TLI, &AR.AA, &AR.DT, &AR.LI);
2391 }
2392
2393 namespace llvm {
2394
createLAAPass()2395 Pass *createLAAPass() {
2396 return new LoopAccessLegacyAnalysis();
2397 }
2398
2399 } // end namespace llvm
2400