[Analysis] Refine matching and merging of TBAA tags

[kosarev]

Summary

This patch combines the code that matches and merges TBAA access tags. The aim is to simplify future changes and making sure that these operations produce consistent results.

[hfinkel]

This looks pretty good.

Please follow LLVM's coding conventions, especially for new functions, function names should start with a lower-case letter. findAccessType, not FindAccessType (see http://llvm.org/docs/CodingStandards.html#name-types-functions-variables-and-enumerators-properly).

lib/Analysis/TypeBasedAliasAnalysis.cpp

565

I don't like the terminology "type group". It's not finding a type group (we have no such things), it's finding a type. Can't we just call this: getLeastCommonType?

[kosarev]

• Fixed names of new functions.
• The "type group" term replaced with just "type".

[hfinkel]

LGTM

lib/Analysis/TypeBasedAliasAnalysis.cpp

static bool isStructPathTBAA(const MDNode *MD) {
   return isa<MDNode>(MD->getOperand(0)) && MD->getNumOperands() >= 3;
 }
 
 AliasResult TypeBasedAAResult::alias(const MemoryLocation &LocA,
                                      const MemoryLocation &LocB) {
   if (!EnableTBAA)
     return AAResultBase::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 AAResultBase::alias(LocA, LocB);
-  const MDNode *BM = LocB.AATags.TBAA;
-  if (!BM)
-    return AAResultBase::alias(LocA, LocB);
-
-  // If they may alias, chain to the next AliasAnalysis.
-  if (Aliases(AM, BM))
+  // If accesses may alias, chain to the next AliasAnalysis.
+  if (Aliases(LocA.AATags.TBAA, LocB.AATags.TBAA))
     return AAResultBase::alias(LocA, LocB);
 
   // Otherwise return a definitive result.
   return NoAlias;
 }
 
 bool TypeBasedAAResult::pointsToConstantMemory(const MemoryLocation &Loc,
                                                bool OrLocal) {
if (!Tag)
     return false;
   if (MDString *Tag1 = dyn_cast<MDString>(Tag->getOperand(0))) {
     if (Tag1->getString() == "vtable pointer")
       return true;
   }
   return false;
 }
 
+static bool matchAccessTags(const MDNode *A, const MDNode *B,
+                            const MDNode **GenericTag = nullptr);
+
 MDNode *MDNode::getMostGenericTBAA(MDNode *A, MDNode *B) {
+  const MDNode *GenericTag;
+  matchAccessTags(A, B, &GenericTag);
+  return const_cast<MDNode*>(GenericTag);
+}
+
+static const MDNode *getLeastCommonType(const MDNode *A, const 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.
-  assert(isStructPathTBAA(A) && isStructPathTBAA(B) &&
-         "Auto upgrade should have taken care of this!");
-  A = cast_or_null<MDNode>(MutableTBAAStructTagNode(A).getAccessType());
-  if (!A)
-    return nullptr;
-  B = cast_or_null<MDNode>(MutableTBAAStructTagNode(B).getAccessType());
-  if (!B)
-    return nullptr;
-
-  SmallSetVector<MDNode *, 4> PathA;
-  MutableTBAANode TA(A);
+  SmallSetVector<const MDNode *, 4> PathA;
+  TBAANode TA(A);
   while (TA.getNode()) {
     if (PathA.count(TA.getNode()))
       report_fatal_error("Cycle found in TBAA metadata.");
     PathA.insert(TA.getNode());
     TA = TA.getParent();
   }
 
-  SmallSetVector<MDNode *, 4> PathB;
-  MutableTBAANode TB(B);
+  SmallSetVector<const MDNode *, 4> PathB;
+  TBAANode TB(B);
   while (TB.getNode()) {
     if (PathB.count(TB.getNode()))
       report_fatal_error("Cycle found in TBAA metadata.");
     PathB.insert(TB.getNode());
     TB = TB.getParent();
   }
 
   int IA = PathA.size() - 1;
   int IB = PathB.size() - 1;
 
-  MDNode *Ret = nullptr;
+  const MDNode *Ret = nullptr;
   while (IA >= 0 && IB >= 0) {
     if (PathA[IA] == PathB[IB])
       Ret = PathA[IA];
     else
       break;
     --IA;
     --IB;
   }
 
-  // We either did not find a match, or the only common base "type" is
-  // the root node.  In either case, we don't have any useful TBAA
-  // metadata to attach.
-  if (!Ret || Ret->getNumOperands() < 2)
-    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);
+  return Ret;
 }
 
 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);
 }
 
-/// Aliases - Test whether the type represented by A may alias the
-/// type represented by B.
-bool TypeBasedAAResult::Aliases(const MDNode *A, const MDNode *B) const {
+static bool findAccessType(TBAAStructTagNode BaseTag,
+                           const MDNode *AccessTypeNode,
+                           uint64_t &OffsetInBase) {
+  // Start from the base type, follow the edge with the correct offset in
+  // the type DAG and adjust the offset until we reach the access type or
+  // until we reach a root node.
+  TBAAStructTypeNode BaseType(BaseTag.getBaseType());
+  OffsetInBase = BaseTag.getOffset();
+
+  while (const MDNode *BaseTypeNode = BaseType.getNode()) {
+    if (BaseTypeNode == AccessTypeNode)
+      return true;
+
+    // Follow the edge with the correct offset, Offset will be adjusted to
+    // be relative to the field type.
+    BaseType = BaseType.getParent(OffsetInBase);
+  }
+  return false;
+}
+
+static const MDNode *createAccessTag(const MDNode *AccessType) {
+  Type *Int64 = IntegerType::get(AccessType->getContext(), 64);
+  auto *ImmutabilityFlag = ConstantAsMetadata::get(ConstantInt::get(Int64, 0));
+  Metadata *Ops[] = {const_cast<MDNode*>(AccessType),
+                     const_cast<MDNode*>(AccessType), ImmutabilityFlag};
+  return MDNode::get(AccessType->getContext(), Ops);
+}
+
+/// matchTags - Return true if the given couple of accesses are allowed to
+/// overlap. If \arg GenericTag is not null, then on return it points to the
+/// most generic access descriptor for the given two.
+static bool matchAccessTags(const MDNode *A, const MDNode *B,
+                            const MDNode **GenericTag) {
+  if (A == B) {
+    if (GenericTag)
+      *GenericTag = A;
+    return true;
+  }
+
+  // Accesses with no TBAA information may alias with any other accesses.
+  if (!A || !B) {
+    if (GenericTag)
+      *GenericTag = nullptr;
+    return true;
+  }
+
   // Verify that both input nodes are struct-path aware.  Auto-upgrade should
   // have taken care of this.
-  assert(isStructPathTBAA(A) && "MDNode A is not struct-path aware.");
-  assert(isStructPathTBAA(B) && "MDNode B is not struct-path aware.");
+  assert(isStructPathTBAA(A) && "Access A is not struct-path aware!");
+  assert(isStructPathTBAA(B) && "Access 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.
+  // Climb the type DAG from base type of A to see if we reach base type of B.
+  uint64_t OffsetA;
+  if (findAccessType(TagA, BaseB, OffsetA)) {
+    if (GenericTag)
+      *GenericTag = createAccessTag(TagB.getAccessType());
+    return OffsetA == TagB.getOffset();
+  }
+
+  // Climb the type DAG from base type of B to see if we reach base type of A.
+  uint64_t OffsetB;
+  if (findAccessType(TagB, BaseA, OffsetB)) {
+    if (GenericTag)
+      *GenericTag = createAccessTag(TagA.getAccessType());
+    return OffsetB == TagA.getOffset();
+  }
+
+  // If neither node is an ancestor of the other, then try to find the type
+  // that is common to both the final access types.
+  const MDNode *CommonType = getLeastCommonType(TagA.getAccessType(),
+                                                TagB.getAccessType());
+
+  // If there is no common type or the only common type is the root node, then
+  // we don't have any useful generic access tag to return.
+  if (GenericTag)
+    *GenericTag = !CommonType || CommonType->getNumOperands() < 2 ?
+        nullptr : createAccessTag(CommonType);
 
   // If they have different roots, they're part of different potentially
   // unrelated type systems, so we must be conservative.
-  if (RootA.getNode() != RootB.getNode())
+  if (!CommonType)
     return true;
 
   // If they have the same root, then we've proved there's no alias.
   return false;
 }
 
+/// Aliases - Test whether the access represented by tag A may alias the
+/// access represented by tag B.
+bool TypeBasedAAResult::Aliases(const MDNode *A, const MDNode *B) const {
+  return matchAccessTags(A, B);
+}
+
 AnalysisKey TypeBasedAA::Key;
 
 TypeBasedAAResult TypeBasedAA::run(Function &F, FunctionAnalysisManager &AM) {
   return TypeBasedAAResult();
 }
 
 char TypeBasedAAWrapperPass::ID = 0;
 INITIALIZE_PASS(TypeBasedAAWrapperPass, "tbaa", "Type-Based Alias Analysis",