path: root/lib/Analysis/TypeBasedAliasAnalysis.cpp

//===- 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.
//
// The current 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).
//
// 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));
static cl::opt<bool> EnableStructPathTBAA("struct-path-tbaa", cl::init(false));

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(0) {}
    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 = dyn_cast<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:
    TBAAStructTagNode() : Node(0) {}
    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 cast<ConstantInt>(Node->getOperand(2))->getZExtValue();
    }
  };

  /// 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(0) {}
    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 {
      if (Node->getNumOperands() < 2)
        return TBAAStructTypeNode();

      // 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 = cast<ConstantInt>(Node->getOperand(Idx))->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 = cast<ConstantInt>(Node->getOperand(TheIdx))->
                       getZExtValue();
      Offset -= Cur;
      MDNode *P = dyn_cast_or_null<MDNode>(Node->getOperand(TheIdx + 1));
      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());
    }

    virtual void initializePass() {
      InitializeAliasAnalysis(this);
    }

    /// 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.
    virtual void *getAdjustedAnalysisPointer(const void *PI) {
      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:
    virtual void getAnalysisUsage(AnalysisUsage &AU) const;
    virtual AliasResult alias(const Location &LocA, const Location &LocB);
    virtual bool pointsToConstantMemory(const Location &Loc, bool OrLocal);
    virtual ModRefBehavior getModRefBehavior(ImmutableCallSite CS);
    virtual ModRefBehavior getModRefBehavior(const Function *F);
    virtual ModRefResult getModRefInfo(ImmutableCallSite CS,
                                       const Location &Loc);
    virtual ModRefResult getModRefInfo(ImmutableCallSite CS1,
                                       ImmutableCallSite CS2);
  };
}  // 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();
}

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

/// 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 {
  if (EnableStructPathTBAA)
    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 {
  // 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.TBAATag;
  if (!AM) return AliasAnalysis::alias(LocA, LocB);
  const MDNode *BM = LocB.TBAATag;
  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.TBAATag;
  if (!M) return AliasAnalysis::pointsToConstantMemory(Loc, OrLocal);

  // If this is an "immutable" type, we can assume the pointer is pointing
  // to constant memory.
  if (!EnableStructPathTBAA && TBAANode(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 (!EnableStructPathTBAA && TBAANode(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.TBAATag)
    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);
}

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

  if (A == B)
    return A;

  // For struct-path aware TBAA, we use the access type of the tag.
  if (EnableStructPathTBAA) {
    A = cast_or_null<MDNode>(A->getOperand(1));
    if (!A) return 0;
    B = cast_or_null<MDNode>(B->getOperand(1));
    if (!B) return 0;
  }

  SmallVector<MDNode *, 4> PathA;
  MDNode *T = A;
  while (T) {
    PathA.push_back(T);
    if (EnableStructPathTBAA)
      T = T->getNumOperands() >= 3 ? cast_or_null<MDNode>(T->getOperand(2)) : 0;
    else
      T = T->getNumOperands() >= 2 ? cast_or_null<MDNode>(T->getOperand(1)) : 0;
  }

  SmallVector<MDNode *, 4> PathB;
  T = B;
  while (T) {
    PathB.push_back(T);
    if (EnableStructPathTBAA)
      T = T->getNumOperands() >= 3 ? cast_or_null<MDNode>(T->getOperand(2)) : 0;
    else
      T = T->getNumOperands() >= 2 ? cast_or_null<MDNode>(T->getOperand(1)) : 0;
  }

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

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

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