llvm.org GIT mirror llvm / c94da20 lib / Analysis / TypeBasedAliasAnalysis.cpp
c94da20

Tree @c94da20 (Download .tar.gz)

TypeBasedAliasAnalysis.cpp @c94da20raw · history · blame

  1
  2
  3
  4
  5
  6
  7
  8
  9
 10
 11
 12
 13
 14
 15
 16
 17
 18
 19
 20
 21
 22
 23
 24
 25
 26
 27
 28
 29
 30
 31
 32
 33
 34
 35
 36
 37
 38
 39
 40
 41
 42
 43
 44
 45
 46
 47
 48
 49
 50
 51
 52
 53
 54
 55
 56
 57
 58
 59
 60
 61
 62
 63
 64
 65
 66
 67
 68
 69
 70
 71
 72
 73
 74
 75
 76
 77
 78
 79
 80
 81
 82
 83
 84
 85
 86
 87
 88
 89
 90
 91
 92
 93
 94
 95
 96
 97
 98
 99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
//===- TypeBasedAliasAnalysis.cpp - Type-Based Alias Analysis -------------===//
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This file defines the TypeBasedAliasAnalysis pass, which implements
// metadata-based TBAA.
//
// In LLVM IR, memory does not have types, so LLVM's own type system is not
// suitable for doing TBAA. Instead, metadata is added to the IR to describe
// a type system of a higher level language. This can be used to implement
// typical C/C++ TBAA, but it can also be used to implement custom alias
// analysis behavior for other languages.
//
// We now support two types of metadata format: scalar TBAA and struct-path
// aware TBAA. After all testing cases are upgraded to use struct-path aware
// TBAA and we can auto-upgrade existing bc files, the support for scalar TBAA
// can be dropped.
//
// The scalar TBAA metadata format is very simple. TBAA MDNodes have up to
// three fields, e.g.:
//   !0 = metadata !{ metadata !"an example type tree" }
//   !1 = metadata !{ metadata !"int", metadata !0 }
//   !2 = metadata !{ metadata !"float", metadata !0 }
//   !3 = metadata !{ metadata !"const float", metadata !2, i64 1 }
//
// The first field is an identity field. It can be any value, usually
// an MDString, which uniquely identifies the type. The most important
// name in the tree is the name of the root node. Two trees with
// different root node names are entirely disjoint, even if they
// have leaves with common names.
//
// The second field identifies the type's parent node in the tree, or
// is null or omitted for a root node. A type is considered to alias
// all of its descendants and all of its ancestors in the tree. Also,
// a type is considered to alias all types in other trees, so that
// bitcode produced from multiple front-ends is handled conservatively.
//
// If the third field is present, it's an integer which if equal to 1
// indicates that the type is "constant" (meaning pointsToConstantMemory
// should return true; see
// http://llvm.org/docs/AliasAnalysis.html#OtherItfs).
//
// With struct-path aware TBAA, the MDNodes attached to an instruction using
// "!tbaa" are called path tag nodes.
//
// The path tag node has 4 fields with the last field being optional.
//
// The first field is the base type node, it can be a struct type node
// or a scalar type node. The second field is the access type node, it
// must be a scalar type node. The third field is the offset into the base type.
// The last field has the same meaning as the last field of our scalar TBAA:
// it's an integer which if equal to 1 indicates that the access is "constant".
//
// The struct type node has a name and a list of pairs, one pair for each member
// of the struct. The first element of each pair is a type node (a struct type
// node or a sclar type node), specifying the type of the member, the second
// element of each pair is the offset of the member.
//
// Given an example
// typedef struct {
//   short s;
// } A;
// typedef struct {
//   uint16_t s;
//   A a;
// } B;
//
// For an acess to B.a.s, we attach !5 (a path tag node) to the load/store
// instruction. The base type is !4 (struct B), the access type is !2 (scalar
// type short) and the offset is 4.
//
// !0 = metadata !{metadata !"Simple C/C++ TBAA"}
// !1 = metadata !{metadata !"omnipotent char", metadata !0} // Scalar type node
// !2 = metadata !{metadata !"short", metadata !1}           // Scalar type node
// !3 = metadata !{metadata !"A", metadata !2, i64 0}        // Struct type node
// !4 = metadata !{metadata !"B", metadata !2, i64 0, metadata !3, i64 4}
//                                                           // Struct type node
// !5 = metadata !{metadata !4, metadata !2, i64 4}          // Path tag node
//
// The struct type nodes and the scalar type nodes form a type DAG.
//         Root (!0)
//         char (!1)  -- edge to Root
//         short (!2) -- edge to char
//         A (!3) -- edge with offset 0 to short
//         B (!4) -- edge with offset 0 to short and edge with offset 4 to A
//
// To check if two tags (tagX and tagY) can alias, we start from the base type
// of tagX, follow the edge with the correct offset in the type DAG and adjust
// the offset until we reach the base type of tagY or until we reach the Root
// node.
// If we reach the base type of tagY, compare the adjusted offset with
// offset of tagY, return Alias if the offsets are the same, return NoAlias
// otherwise.
// If we reach the Root node, perform the above starting from base type of tagY
// to see if we reach base type of tagX.
//
// If they have different roots, they're part of different potentially
// unrelated type systems, so we return Alias to be conservative.
// If neither node is an ancestor of the other and they have the same root,
// then we say NoAlias.
//
// TODO: The current metadata format doesn't support struct
// fields. For example:
//   struct X {
//     double d;
//     int i;
//   };
//   void foo(struct X *x, struct X *y, double *p) {
//     *x = *y;
//     *p = 0.0;
//   }
// Struct X has a double member, so the store to *x can alias the store to *p.
// Currently it's not possible to precisely describe all the things struct X
// aliases, so struct assignments must use conservative TBAA nodes. There's
// no scheme for attaching metadata to @llvm.memcpy yet either.
//
//===----------------------------------------------------------------------===//

#include "llvm/Analysis/Passes.h"
#include "llvm/Analysis/AliasAnalysis.h"
#include "llvm/IR/Constants.h"
#include "llvm/IR/LLVMContext.h"
#include "llvm/IR/Metadata.h"
#include "llvm/IR/Module.h"
#include "llvm/Pass.h"
#include "llvm/Support/CommandLine.h"
using namespace llvm;

// A handy option for disabling TBAA functionality. The same effect can also be
// achieved by stripping the !tbaa tags from IR, but this option is sometimes
// more convenient.
static cl::opt<bool> EnableTBAA("enable-tbaa", cl::init(true));

namespace {
  /// TBAANode - This is a simple wrapper around an MDNode which provides a
  /// higher-level interface by hiding the details of how alias analysis
  /// information is encoded in its operands.
  class TBAANode {
    const MDNode *Node;

  public:
    TBAANode() : Node(nullptr) {}
    explicit TBAANode(const MDNode *N) : Node(N) {}

    /// getNode - Get the MDNode for this TBAANode.
    const MDNode *getNode() const { return Node; }

    /// getParent - Get this TBAANode's Alias tree parent.
    TBAANode getParent() const {
      if (Node->getNumOperands() < 2)
        return TBAANode();
      MDNode *P = dyn_cast_or_null<MDNode>(Node->getOperand(1));
      if (!P)
        return TBAANode();
      // Ok, this node has a valid parent. Return it.
      return TBAANode(P);
    }

    /// TypeIsImmutable - Test if this TBAANode represents a type for objects
    /// which are not modified (by any means) in the context where this
    /// AliasAnalysis is relevant.
    bool TypeIsImmutable() const {
      if (Node->getNumOperands() < 3)
        return false;
      ConstantInt *CI = mdconst::dyn_extract<ConstantInt>(Node->getOperand(2));
      if (!CI)
        return false;
      return CI->getValue()[0];
    }
  };

  /// This is a simple wrapper around an MDNode which provides a
  /// higher-level interface by hiding the details of how alias analysis
  /// information is encoded in its operands.
  class TBAAStructTagNode {
    /// This node should be created with createTBAAStructTagNode.
    const MDNode *Node;

  public:
    explicit TBAAStructTagNode(const MDNode *N) : Node(N) {}

    /// Get the MDNode for this TBAAStructTagNode.
    const MDNode *getNode() const { return Node; }

    const MDNode *getBaseType() const {
      return dyn_cast_or_null<MDNode>(Node->getOperand(0));
    }
    const MDNode *getAccessType() const {
      return dyn_cast_or_null<MDNode>(Node->getOperand(1));
    }
    uint64_t getOffset() const {
      return mdconst::extract<ConstantInt>(Node->getOperand(2))->getZExtValue();
    }
    /// TypeIsImmutable - Test if this TBAAStructTagNode represents a type for
    /// objects which are not modified (by any means) in the context where this
    /// AliasAnalysis is relevant.
    bool TypeIsImmutable() const {
      if (Node->getNumOperands() < 4)
        return false;
      ConstantInt *CI = mdconst::dyn_extract<ConstantInt>(Node->getOperand(3));
      if (!CI)
        return false;
      return CI->getValue()[0];
    }
  };

  /// This is a simple wrapper around an MDNode which provides a
  /// higher-level interface by hiding the details of how alias analysis
  /// information is encoded in its operands.
  class TBAAStructTypeNode {
    /// This node should be created with createTBAAStructTypeNode.
    const MDNode *Node;

  public:
    TBAAStructTypeNode() : Node(nullptr) {}
    explicit TBAAStructTypeNode(const MDNode *N) : Node(N) {}

    /// Get the MDNode for this TBAAStructTypeNode.
    const MDNode *getNode() const { return Node; }

    /// Get this TBAAStructTypeNode's field in the type DAG with
    /// given offset. Update the offset to be relative to the field type.
    TBAAStructTypeNode getParent(uint64_t &Offset) const {
      // Parent can be omitted for the root node.
      if (Node->getNumOperands() < 2)
        return TBAAStructTypeNode();

      // Fast path for a scalar type node and a struct type node with a single
      // field.
      if (Node->getNumOperands() <= 3) {
        uint64_t Cur = Node->getNumOperands() == 2
                           ? 0
                           : mdconst::extract<ConstantInt>(Node->getOperand(2))
                                 ->getZExtValue();
        Offset -= Cur;
        MDNode *P = dyn_cast_or_null<MDNode>(Node->getOperand(1));
        if (!P)
          return TBAAStructTypeNode();
        return TBAAStructTypeNode(P);
      }

      // Assume the offsets are in order. We return the previous field if
      // the current offset is bigger than the given offset.
      unsigned TheIdx = 0;
      for (unsigned Idx = 1; Idx < Node->getNumOperands(); Idx += 2) {
        uint64_t Cur = mdconst::extract<ConstantInt>(Node->getOperand(Idx + 1))
                           ->getZExtValue();
        if (Cur > Offset) {
          assert(Idx >= 3 &&
                 "TBAAStructTypeNode::getParent should have an offset match!");
          TheIdx = Idx - 2;
          break;
        }
      }
      // Move along the last field.
      if (TheIdx == 0)
        TheIdx = Node->getNumOperands() - 2;
      uint64_t Cur = mdconst::extract<ConstantInt>(Node->getOperand(TheIdx + 1))
                         ->getZExtValue();
      Offset -= Cur;
      MDNode *P = dyn_cast_or_null<MDNode>(Node->getOperand(TheIdx));
      if (!P)
        return TBAAStructTypeNode();
      return TBAAStructTypeNode(P);
    }
  };
}

namespace {
  /// TypeBasedAliasAnalysis - This is a simple alias analysis
  /// implementation that uses TypeBased to answer queries.
  class TypeBasedAliasAnalysis : public ImmutablePass,
                                 public AliasAnalysis {
  public:
    static char ID; // Class identification, replacement for typeinfo
    TypeBasedAliasAnalysis() : ImmutablePass(ID) {
      initializeTypeBasedAliasAnalysisPass(*PassRegistry::getPassRegistry());
    }

    bool doInitialization(Module &M) override;

    /// getAdjustedAnalysisPointer - This method is used when a pass implements
    /// an analysis interface through multiple inheritance.  If needed, it
    /// should override this to adjust the this pointer as needed for the
    /// specified pass info.
    void *getAdjustedAnalysisPointer(const void *PI) override {
      if (PI == &AliasAnalysis::ID)
        return (AliasAnalysis*)this;
      return this;
    }

    bool Aliases(const MDNode *A, const MDNode *B) const;
    bool PathAliases(const MDNode *A, const MDNode *B) const;

  private:
    void getAnalysisUsage(AnalysisUsage &AU) const override;
    AliasResult alias(const Location &LocA, const Location &LocB) override;
    bool pointsToConstantMemory(const Location &Loc, bool OrLocal) override;
    ModRefBehavior getModRefBehavior(ImmutableCallSite CS) override;
    ModRefBehavior getModRefBehavior(const Function *F) override;
    ModRefResult getModRefInfo(ImmutableCallSite CS,
                               const Location &Loc) override;
    ModRefResult getModRefInfo(ImmutableCallSite CS1,
                               ImmutableCallSite CS2) override;
  };
}  // End of anonymous namespace

// Register this pass...
char TypeBasedAliasAnalysis::ID = 0;
INITIALIZE_AG_PASS(TypeBasedAliasAnalysis, AliasAnalysis, "tbaa",
                   "Type-Based Alias Analysis", false, true, false)

ImmutablePass *llvm::createTypeBasedAliasAnalysisPass() {
  return new TypeBasedAliasAnalysis();
}

bool TypeBasedAliasAnalysis::doInitialization(Module &M) {
  InitializeAliasAnalysis(this, &M.getDataLayout());
  return true;
}

void
TypeBasedAliasAnalysis::getAnalysisUsage(AnalysisUsage &AU) const {
  AU.setPreservesAll();
  AliasAnalysis::getAnalysisUsage(AU);
}

/// Check the first operand of the tbaa tag node, if it is a MDNode, we treat
/// it as struct-path aware TBAA format, otherwise, we treat it as scalar TBAA
/// format.
static bool isStructPathTBAA(const MDNode *MD) {
  // Anonymous TBAA root starts with a MDNode and dragonegg uses it as
  // a TBAA tag.
  return isa<MDNode>(MD->getOperand(0)) && MD->getNumOperands() >= 3;
}

/// Aliases - Test whether the type represented by A may alias the
/// type represented by B.
bool
TypeBasedAliasAnalysis::Aliases(const MDNode *A,
                                const MDNode *B) const {
  // Make sure that both MDNodes are struct-path aware.
  if (isStructPathTBAA(A) && isStructPathTBAA(B))
    return PathAliases(A, B);

  // Keep track of the root node for A and B.
  TBAANode RootA, RootB;

  // Climb the tree from A to see if we reach B.
  for (TBAANode T(A); ; ) {
    if (T.getNode() == B)
      // B is an ancestor of A.
      return true;

    RootA = T;
    T = T.getParent();
    if (!T.getNode())
      break;
  }

  // Climb the tree from B to see if we reach A.
  for (TBAANode T(B); ; ) {
    if (T.getNode() == A)
      // A is an ancestor of B.
      return true;

    RootB = T;
    T = T.getParent();
    if (!T.getNode())
      break;
  }

  // Neither node is an ancestor of the other.
  
  // If they have different roots, they're part of different potentially
  // unrelated type systems, so we must be conservative.
  if (RootA.getNode() != RootB.getNode())
    return true;

  // If they have the same root, then we've proved there's no alias.
  return false;
}

/// Test whether the struct-path tag represented by A may alias the
/// struct-path tag represented by B.
bool
TypeBasedAliasAnalysis::PathAliases(const MDNode *A,
                                    const MDNode *B) const {
  // Verify that both input nodes are struct-path aware.
  assert(isStructPathTBAA(A) && "MDNode A is not struct-path aware.");
  assert(isStructPathTBAA(B) && "MDNode B is not struct-path aware.");

  // Keep track of the root node for A and B.
  TBAAStructTypeNode RootA, RootB;
  TBAAStructTagNode TagA(A), TagB(B);

  // TODO: We need to check if AccessType of TagA encloses AccessType of
  // TagB to support aggregate AccessType. If yes, return true.

  // Start from the base type of A, follow the edge with the correct offset in
  // the type DAG and adjust the offset until we reach the base type of B or
  // until we reach the Root node.
  // Compare the adjusted offset once we have the same base.

  // Climb the type DAG from base type of A to see if we reach base type of B.
  const MDNode *BaseA = TagA.getBaseType();
  const MDNode *BaseB = TagB.getBaseType();
  uint64_t OffsetA = TagA.getOffset(), OffsetB = TagB.getOffset();
  for (TBAAStructTypeNode T(BaseA); ; ) {
    if (T.getNode() == BaseB)
      // Base type of A encloses base type of B, check if the offsets match.
      return OffsetA == OffsetB;

    RootA = T;
    // Follow the edge with the correct offset, OffsetA will be adjusted to
    // be relative to the field type.
    T = T.getParent(OffsetA);
    if (!T.getNode())
      break;
  }

  // Reset OffsetA and climb the type DAG from base type of B to see if we reach
  // base type of A.
  OffsetA = TagA.getOffset();
  for (TBAAStructTypeNode T(BaseB); ; ) {
    if (T.getNode() == BaseA)
      // Base type of B encloses base type of A, check if the offsets match.
      return OffsetA == OffsetB;

    RootB = T;
    // Follow the edge with the correct offset, OffsetB will be adjusted to
    // be relative to the field type.
    T = T.getParent(OffsetB);
    if (!T.getNode())
      break;
  }

  // Neither node is an ancestor of the other.

  // If they have different roots, they're part of different potentially
  // unrelated type systems, so we must be conservative.
  if (RootA.getNode() != RootB.getNode())
    return true;

  // If they have the same root, then we've proved there's no alias.
  return false;
}

AliasAnalysis::AliasResult
TypeBasedAliasAnalysis::alias(const Location &LocA,
                              const Location &LocB) {
  if (!EnableTBAA)
    return AliasAnalysis::alias(LocA, LocB);

  // Get the attached MDNodes. If either value lacks a tbaa MDNode, we must
  // be conservative.
  const MDNode *AM = LocA.AATags.TBAA;
  if (!AM) return AliasAnalysis::alias(LocA, LocB);
  const MDNode *BM = LocB.AATags.TBAA;
  if (!BM) return AliasAnalysis::alias(LocA, LocB);

  // If they may alias, chain to the next AliasAnalysis.
  if (Aliases(AM, BM))
    return AliasAnalysis::alias(LocA, LocB);

  // Otherwise return a definitive result.
  return NoAlias;
}

bool TypeBasedAliasAnalysis::pointsToConstantMemory(const Location &Loc,
                                                    bool OrLocal) {
  if (!EnableTBAA)
    return AliasAnalysis::pointsToConstantMemory(Loc, OrLocal);

  const MDNode *M = Loc.AATags.TBAA;
  if (!M) return AliasAnalysis::pointsToConstantMemory(Loc, OrLocal);

  // If this is an "immutable" type, we can assume the pointer is pointing
  // to constant memory.
  if ((!isStructPathTBAA(M) && TBAANode(M).TypeIsImmutable()) ||
      (isStructPathTBAA(M) && TBAAStructTagNode(M).TypeIsImmutable()))
    return true;

  return AliasAnalysis::pointsToConstantMemory(Loc, OrLocal);
}

AliasAnalysis::ModRefBehavior
TypeBasedAliasAnalysis::getModRefBehavior(ImmutableCallSite CS) {
  if (!EnableTBAA)
    return AliasAnalysis::getModRefBehavior(CS);

  ModRefBehavior Min = UnknownModRefBehavior;

  // If this is an "immutable" type, we can assume the call doesn't write
  // to memory.
  if (const MDNode *M = CS.getInstruction()->getMetadata(LLVMContext::MD_tbaa))
    if ((!isStructPathTBAA(M) && TBAANode(M).TypeIsImmutable()) ||
        (isStructPathTBAA(M) && TBAAStructTagNode(M).TypeIsImmutable()))
      Min = OnlyReadsMemory;

  return ModRefBehavior(AliasAnalysis::getModRefBehavior(CS) & Min);
}

AliasAnalysis::ModRefBehavior
TypeBasedAliasAnalysis::getModRefBehavior(const Function *F) {
  // Functions don't have metadata. Just chain to the next implementation.
  return AliasAnalysis::getModRefBehavior(F);
}

AliasAnalysis::ModRefResult
TypeBasedAliasAnalysis::getModRefInfo(ImmutableCallSite CS,
                                      const Location &Loc) {
  if (!EnableTBAA)
    return AliasAnalysis::getModRefInfo(CS, Loc);

  if (const MDNode *L = Loc.AATags.TBAA)
    if (const MDNode *M =
            CS.getInstruction()->getMetadata(LLVMContext::MD_tbaa))
      if (!Aliases(L, M))
        return NoModRef;

  return AliasAnalysis::getModRefInfo(CS, Loc);
}

AliasAnalysis::ModRefResult
TypeBasedAliasAnalysis::getModRefInfo(ImmutableCallSite CS1,
                                      ImmutableCallSite CS2) {
  if (!EnableTBAA)
    return AliasAnalysis::getModRefInfo(CS1, CS2);

  if (const MDNode *M1 =
          CS1.getInstruction()->getMetadata(LLVMContext::MD_tbaa))
    if (const MDNode *M2 =
            CS2.getInstruction()->getMetadata(LLVMContext::MD_tbaa))
      if (!Aliases(M1, M2))
        return NoModRef;

  return AliasAnalysis::getModRefInfo(CS1, CS2);
}

bool MDNode::isTBAAVtableAccess() const {
  if (!isStructPathTBAA(this)) {
    if (getNumOperands() < 1) return false;
    if (MDString *Tag1 = dyn_cast<MDString>(getOperand(0))) {
      if (Tag1->getString() == "vtable pointer") return true;
    }
    return false;
  }

  // For struct-path aware TBAA, we use the access type of the tag.
  if (getNumOperands() < 2) return false;
  MDNode *Tag = cast_or_null<MDNode>(getOperand(1));
  if (!Tag) return false;
  if (MDString *Tag1 = dyn_cast<MDString>(Tag->getOperand(0))) {
    if (Tag1->getString() == "vtable pointer") return true;
  }
  return false;  
}

MDNode *MDNode::getMostGenericTBAA(MDNode *A, MDNode *B) {
  if (!A || !B)
    return nullptr;

  if (A == B)
    return A;

  // For struct-path aware TBAA, we use the access type of the tag.
  bool StructPath = isStructPathTBAA(A) && isStructPathTBAA(B);
  if (StructPath) {
    A = cast_or_null<MDNode>(A->getOperand(1));
    if (!A) return nullptr;
    B = cast_or_null<MDNode>(B->getOperand(1));
    if (!B) return nullptr;
  }

  SmallVector<MDNode *, 4> PathA;
  MDNode *T = A;
  while (T) {
    PathA.push_back(T);
    T = T->getNumOperands() >= 2 ? cast_or_null<MDNode>(T->getOperand(1))
                                 : nullptr;
  }

  SmallVector<MDNode *, 4> PathB;
  T = B;
  while (T) {
    PathB.push_back(T);
    T = T->getNumOperands() >= 2 ? cast_or_null<MDNode>(T->getOperand(1))
                                 : nullptr;
  }

  int IA = PathA.size() - 1;
  int IB = PathB.size() - 1;

  MDNode *Ret = nullptr;
  while (IA >= 0 && IB >=0) {
    if (PathA[IA] == PathB[IB])
      Ret = PathA[IA];
    else
      break;
    --IA;
    --IB;
  }
  if (!StructPath)
    return Ret;

  if (!Ret)
    return nullptr;
  // We need to convert from a type node to a tag node.
  Type *Int64 = IntegerType::get(A->getContext(), 64);
  Metadata *Ops[3] = {Ret, Ret,
                      ConstantAsMetadata::get(ConstantInt::get(Int64, 0))};
  return MDNode::get(A->getContext(), Ops);
}

void Instruction::getAAMetadata(AAMDNodes &N, bool Merge) const {
  if (Merge)
    N.TBAA =
        MDNode::getMostGenericTBAA(N.TBAA, getMetadata(LLVMContext::MD_tbaa));
  else
    N.TBAA = getMetadata(LLVMContext::MD_tbaa);

  if (Merge)
    N.Scope = MDNode::getMostGenericAliasScope(
        N.Scope, getMetadata(LLVMContext::MD_alias_scope));
  else
    N.Scope = getMetadata(LLVMContext::MD_alias_scope);

  if (Merge)
    N.NoAlias =
        MDNode::intersect(N.NoAlias, getMetadata(LLVMContext::MD_noalias));
  else
    N.NoAlias = getMetadata(LLVMContext::MD_noalias);
}