lib/SILAnalysis/AliasAnalysis.cpp

//===-------------- AliasAnalysis.cpp - SIL Alias Analysis ----------------===//
//
// This source file is part of the Swift.org open source project
//
// Copyright (c) 2014 - 2015 Apple Inc. and the Swift project authors
// Licensed under Apache License v2.0 with Runtime Library Exception
//
// See http://swift.org/LICENSE.txt for license information
// See http://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
//
//===----------------------------------------------------------------------===//

#define DEBUG_TYPE "sil-aa"
#include "swift/SILAnalysis/AliasAnalysis.h"
#include "swift/SILAnalysis/ValueTracking.h"
#include "swift/SILAnalysis/SideEffectAnalysis.h"
#include "swift/SILPasses/Utils/Local.h"
#include "swift/SILPasses/PassManager.h"
#include "swift/SIL/Projection.h"
#include "swift/SIL/SILValue.h"
#include "swift/SIL/SILInstruction.h"
#include "swift/SIL/SILArgument.h"
#include "swift/SIL/SILFunction.h"
#include "swift/SIL/SILModule.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/raw_ostream.h"

using namespace swift;

//===----------------------------------------------------------------------===//
//                                AA Debugging
//===----------------------------------------------------------------------===//

#ifndef NDEBUG

namespace {

enum class AAKind : unsigned {
  None=0,
  BasicAA=1,
  TypedAccessTBAA=2,
  All=3,
};

} // end anonymous namespace

static llvm::cl::opt<AAKind>
DebugAAKinds("aa", llvm::cl::desc("Alias Analysis Kinds:"),
             llvm::cl::init(AAKind::All),
             llvm::cl::values(clEnumValN(AAKind::None,
                                         "none",
                                         "Do not perform any AA"),
                              clEnumValN(AAKind::BasicAA,
                                         "basic-aa",
                                         "basic-aa"),
                              clEnumValN(AAKind::TypedAccessTBAA,
                                         "typed-access-tb-aa",
                                         "typed-access-tb-aa"),
                              clEnumValN(AAKind::All,
                                         "all",
                                         "all"),
                              clEnumValEnd));

static inline bool shouldRunAA() {
  return unsigned(AAKind(DebugAAKinds));
}

static inline bool shouldRunTypedAccessTBAA() {
  return unsigned(AAKind(DebugAAKinds)) & unsigned(AAKind::TypedAccessTBAA);
}

static inline bool shouldRunBasicAA() {
  return unsigned(AAKind(DebugAAKinds)) & unsigned(AAKind::BasicAA);
}

#endif

static llvm::cl::opt<bool>
    CacheAAResults("cache-aa-results",
                   llvm::cl::desc("Should AA results be cached"),
                   llvm::cl::init(true));

//===----------------------------------------------------------------------===//
//                                 Utilities
//===----------------------------------------------------------------------===//

llvm::raw_ostream &swift::operator<<(llvm::raw_ostream &OS,
                                     AliasAnalysis::AliasResult R) {
  switch (R) {
  case AliasAnalysis::AliasResult::NoAlias:
    return OS << "NoAlias";
  case AliasAnalysis::AliasResult::MayAlias:
    return OS << "MayAlias";
  case AliasAnalysis::AliasResult::PartialAlias:
    return OS << "PartialAlias";
  case AliasAnalysis::AliasResult::MustAlias:
    return OS << "MustAlias";
  }
}

//===----------------------------------------------------------------------===//
//                           Unequal Base Object AA
//===----------------------------------------------------------------------===//

/// Return true if the given SILArgument is an argument to the first BB of a
/// function.
static bool isFunctionArgument(SILValue V) {
  auto *Arg = dyn_cast<SILArgument>(V);
  if (!Arg)
    return false;
  return Arg->isFunctionArg();
}

/// A no alias argument is an argument that is an address type of the entry
/// basic block of a function.
static bool isNoAliasArgument(SILValue V) {
  return isFunctionArgument(V) && V.getType().isAddress();
}

/// Return true if V is an object that at compile time can be uniquely
/// identified.
static bool isIdentifiableObject(SILValue V) {
  if (isa<AllocationInst>(V) || isa<LiteralInst>(V))
    return true;
  if (isNoAliasArgument(V))
    return true;
  return false;
}

/// Return true if V1 and V2 are distinct objects that can be uniquely
/// identified at compile time.
static bool areDistinctIdentifyableObjects(SILValue V1, SILValue V2) {
  // Do both values refer to the same global variable?
  if (auto *GA1 = dyn_cast<GlobalAddrInst>(V1)) {
    if (auto *GA2 = dyn_cast<GlobalAddrInst>(V2)) {
      return GA1->getReferencedGlobal() != GA2->getReferencedGlobal();
    }
  }
  if (isIdentifiableObject(V1) && isIdentifiableObject(V2))
    return V1 != V2;
  return false;
}

/// Returns true if both values are equal or yield the address of the same
/// global variable.
static bool isSameValueOrGlobal(SILValue V1, SILValue V2) {
  if (V1 == V2)
    return true;
  // Do both values refer to the same global variable?
  if (auto *GA1 = dyn_cast<GlobalAddrInst>(V1)) {
    if (auto *GA2 = dyn_cast<GlobalAddrInst>(V2)) {
      return GA1->getReferencedGlobal() == GA2->getReferencedGlobal();
    }
  }
  return false;
}

/// Is this a literal which we know can not refer to a global object?
///
/// FIXME: function_ref?
static bool isLocalLiteral(SILValue V) {
  switch (V->getKind()) {
  case ValueKind::IntegerLiteralInst:
  case ValueKind::FloatLiteralInst:
  case ValueKind::StringLiteralInst:
    return true;
  default:
    return false;
  }
}

/// Is this a value that can be unambiguously identified as being defined at the
/// function level.
static bool isIdentifiedFunctionLocal(SILValue V) {
  return isa<AllocationInst>(*V) || isNoAliasArgument(V) || isLocalLiteral(V);
}

/// Returns true if V is a function argument that is not an address implying
/// that we do not have the gaurantee that it will not alias anything inside the
/// function.
static bool isAliasingFunctionArgument(SILValue V) {
  return isFunctionArgument(V) && !V.getType().isAddress();
}

/// Returns true if V is an apply inst that may read or write to memory.
static bool isReadWriteApplyInst(SILValue V) {
  // See if this is a normal function application.
  if (auto *AI = dyn_cast<ApplyInst>(V)) {
    return AI->mayReadOrWriteMemory();
  }
  
  // Next, see if this is a builtin.
  if (auto *BI = dyn_cast<BuiltinInst>(V)) {
    return BI->mayReadOrWriteMemory();
  }

  // If we fail, bail...
  return false;
}

/// Return true if the pointer is one which would have been considered an escape
/// by isNonEscapingLocalObject.
static bool isEscapeSource(SILValue V) {
  if (isReadWriteApplyInst(V))
    return true;

  if (isAliasingFunctionArgument(V))
    return true;

  // The LoadInst case works since valueMayBeCaptured always assumes stores are
  // escapes.
  if (isa<LoadInst>(*V))
    return true;

  // We could not prove anything, be conservative and return false.
  return false;
}

/// Returns true if we can prove that the two input SILValues which do not equal
/// can not alias.
static bool aliasUnequalObjects(SILValue O1, SILValue O2) {
  assert(O1 != O2 && "This function should only be called on unequal values.");

  // If O1 and O2 do not equal and they are both values that can be statically
  // and uniquely identified, they can not alias.
  if (areDistinctIdentifyableObjects(O1, O2)) {
    DEBUG(llvm::dbgs() << "            Found two unequal identified "
          "objects.\n");
    return true;
  }

  // Function arguments can't alias with things that are known to be
  // unambigously identified at the function level.
  //
  // Note that both function arguments must be identified. For example, an @in
  // argument may be an interior pointer into a box that is passed separately as
  // @owned. We must consider uses on the @in argument as potential uses of the
  // @owned object.
  if ((isFunctionArgument(O1) && isIdentifiedFunctionLocal(O2)) ||
      (isFunctionArgument(O2) && isIdentifiedFunctionLocal(O1))) {
    DEBUG(llvm::dbgs() << "            Found unequal function arg and "
          "identified function local!\n");
    return true;
  }

  // If one pointer is the result of an apply or load and the other is a
  // non-escaping local object within the same function, then we know the object
  // couldn't escape to a point where the call could return it.
  if ((isEscapeSource(O1) && isNonEscapingLocalObject(O2)) ||
      (isEscapeSource(O2) && isNonEscapingLocalObject(O1))) {
    DEBUG(llvm::dbgs() << "            Found unequal escape source and non "
          "escaping local object!\n");
    return true;
  }

  // We failed to prove that the two objects are different.
  return false;
}

//===----------------------------------------------------------------------===//
//                           Projection Address AA
//===----------------------------------------------------------------------===//

/// Uses a bunch of ad-hoc rules to disambiguate a GEP instruction against
/// another pointer. We know that V1 is a GEP, but we don't know anything about
/// V2. O1, O2 are getUnderlyingObject of V1, V2 respectively.
static
AliasAnalysis::AliasResult
aliasAddressProjection(AliasAnalysis &AA, SILValue V1, SILValue V2, SILValue O1,
                       SILValue O2) {

  // If V2 is also a gep instruction with a must-alias or not-aliasing base
  // pointer, figure out if the indices of the GEPs tell us anything about the
  // derived pointers.
  if (!Projection::isAddrProjection(V2)) {
    // Ok, V2 is not an address projection. See if V2 after stripping casts
    // aliases O1. If so, then we know that V2 must partially alias V1 via a
    // must alias relation on O1. This ensures that given an alloc_stack and a
    // gep from that alloc_stack, we say that they partially alias.
    if (isSameValueOrGlobal(O1, V2.stripCasts()))
      return AliasAnalysis::AliasResult::PartialAlias;

    return AliasAnalysis::AliasResult::MayAlias;
  }
  
  assert(!Projection::isAddrProjection(O1) &&
         "underlying object may not be a projection");
  assert(!Projection::isAddrProjection(O2) &&
         "underlying object may not be a projection");

  // Do the base pointers alias?
  AliasAnalysis::AliasResult BaseAlias = AA.alias(O1, O2);

  // If the underlying objects are not aliased, the projected values are also
  // not aliased.
  if (BaseAlias == AliasAnalysis::AliasResult::NoAlias)
    return AliasAnalysis::AliasResult::NoAlias;

  // Let's do alias checking based on projections.
  auto V1Path = ProjectionPath::getAddrProjectionPath(O1, V1, true);
  auto V2Path = ProjectionPath::getAddrProjectionPath(O2, V2, true);

  // getUnderlyingPath and findAddressProjectionPathBetweenValues disagree on
  // what the base pointer of the two values are. Be conservative and return
  // MayAlias.
  //
  // FIXME: The only way this should happen realistically is if there are
  // casts in between two projection instructions. getUnderlyingObject will
  // ignore that, while findAddressProjectionPathBetweenValues wont. The two
  // solutions are to make address projections variadic (something on the wee
  // horizon) or enable Projection to represent a cast as a special sort of
  // projection.
  if (!V1Path || !V2Path)
    return AliasAnalysis::AliasResult::MayAlias;

  auto R = V1Path->computeSubSeqRelation(*V2Path);

  // If all of the projections are equal (and they have the same base pointer),
  // the two GEPs must be the same.
  if (BaseAlias == AliasAnalysis::AliasResult::MustAlias &&
      R == SubSeqRelation_t::Equal)
    return AliasAnalysis::AliasResult::MustAlias;

  // The two GEPs do not alias if they are accessing different fields, even if
  // we don't know the base pointers. Different fields should not overlap.
  //
  // TODO: Replace this with a check on the computed subseq relation. See the
  // TODO in computeSubSeqRelation.
  if (V1Path->hasNonEmptySymmetricDifference(V2Path.getValue()))
    return AliasAnalysis::AliasResult::NoAlias;

  // If one of the GEPs is a super path of the other then they partially
  // alias. W
  if (BaseAlias == AliasAnalysis::AliasResult::MustAlias &&
      isStrictSubSeqRelation(R))
    return AliasAnalysis::AliasResult::PartialAlias;

  // We failed to prove anything. Be conservative and return MayAlias.
  return AliasAnalysis::AliasResult::MayAlias;
}


//===----------------------------------------------------------------------===//
//                                TBAA
//===----------------------------------------------------------------------===//

static bool typedAccessTBAABuiltinTypesMayAlias(SILType LTy, SILType RTy,
                                                SILModule &Mod) {
  assert(LTy != RTy && "LTy should have already been shown to not equal RTy to "
         "call this function.");

  // If either of our types are raw pointers, they may alias any builtin.
  if (LTy.is<BuiltinRawPointerType>() || RTy.is<BuiltinRawPointerType>())
    return true;

  // At this point, we have 3 possibilities:
  //
  // 1. (Pointer, Scalar): A pointer to a pointer can never alias a scalar.
  //
  // 2. (Pointer, Pointer): If we have two pointers to pointers, since we know
  // that the two values do not equal due to previous AA calculations, one must
  // be a native object and the other is an unknown object type (i.e. an objc
  // object) which can not alias.
  //
  // 3. (Scalar, Scalar): If we have two scalar pointers, since we know that the
  // types are already not equal, we know that they can not alias. For those
  // unfamiliar even though BuiltinIntegerType/BuiltinFloatType are single
  // classes, the AST represents each integer/float of different bit widths as
  // different types, so equality of SILTypes allows us to know that they have
  // different bit widths.
  //
  // Thus we can just return false since in none of the aforementioned cases we
  // can not alias, so return false.
  return false;
}

/// Return true if LTyRef and RTyRef have a parent-child relationship, i.e.
/// the objects with the given types mayalias.
static bool referenceTypeTBAAMayAlias(SILType LTy, SILType RTy) {
  // Walk the type hiearchy to see whether LTy and RTy have a parent-child
  // relationship.
  SILType C = SILType();
  // Walk from LTy to RTy.
  C = LTy;
  do {
    // There is a parent-child relationship.
    if (C == RTy)
      return true;
    C = C.getSuperclass(nullptr);
  } while(C);

  // Walk from RTy to LTy.
  C = RTy;
  do {
    // There is a parent-child relationship.
    if (C == LTy)
      return true;
    C = C.getSuperclass(nullptr);
  } while(C);

  return false;
}

/// \brief return True if the types \p LTy and \p RTy may alias.
///
/// Currently this only implements typed access based TBAA. See the TBAA section
/// in the SIL reference manual.
static bool typedAccessTBAAMayAlias(SILType LTy, SILType RTy, SILModule &Mod) {
#ifndef NDEBUG
  if (!shouldRunTypedAccessTBAA())
    return true;
#endif

  // If the two types are the same they may alias.
  if (LTy == RTy)
    return true;

  // If 2 reference types have no parent-child relationship, then they can
  // not alias.
  if (!LTy.isAddress() && !RTy.isAddress()) {
    if (LTy.isHeapObjectReferenceType() && RTy.isHeapObjectReferenceType())
      return referenceTypeTBAAMayAlias(LTy, RTy);
  }
 
  // Typed access based TBAA only occurs on pointers. If we reach this point and
  // do not have a pointer, be conservative and return that the two types may
  // alias. *NOTE* This ensures we return may alias for local_storage.
  if(!LTy.isAddress() || !RTy.isAddress())
    return true;

  // If the types have unbound generic arguments then we don't know
  // the possible range of the type. A type such as $Array<Int> may
  // alias $Array<T>.  Right now we are conservative and we assume
  // that $UnsafeMutablePointer<T> and $Int may alias.
  if (LTy.hasArchetype() || RTy.hasArchetype())
    return true;

  // If either type is a protocol type, we don't know the underlying type so
  // return may alias.
  //
  // FIXME: We could be significantly smarter here by using the protocol
  // hierarchy.
  if (LTy.isAnyExistentialType() || RTy.isAnyExistentialType())
    return true;

  // If either type is an address only type, bail so we are conservative.
  if (LTy.isAddressOnly(Mod) || RTy.isAddressOnly(Mod))
    return true;

  // If both types are builtin types, handle them separately.
  if (LTy.is<BuiltinType>() && RTy.is<BuiltinType>())
    return typedAccessTBAABuiltinTypesMayAlias(LTy, RTy, Mod);

  // Otherwise, we know that at least one of our types is not a builtin
  // type. If we have a builtin type, canonicalize it on the right.
  if (LTy.is<BuiltinType>())
    std::swap(LTy, RTy);

  // If RTy is a builtin raw pointer type, it can alias anything.
  if (RTy.is<BuiltinRawPointerType>())
    return true;

  ClassDecl *LTyClass = LTy.getClassOrBoundGenericClass();

  // The Builtin reference types can alias any class instance.
  if (RTy.is<BuiltinUnknownObjectType>() && LTyClass)
    return true;
  if (RTy.is<BuiltinNativeObjectType>() && LTyClass)
    return true;
  if (RTy.is<BuiltinBridgeObjectType>() && LTyClass)
    return true;
  
  // If one type is an aggregate and it contains the other type then the record
  // reference may alias the aggregate reference.
  if (LTy.aggregateContainsRecord(RTy, Mod) ||
      RTy.aggregateContainsRecord(LTy, Mod))
    return true;

  // FIXME: All the code following could be made significantly more aggressive
  // by saying that aggregates of the same type that do not contain each other
  // can not alias.

  // Tuples do not alias non-tuples.
  bool LTyTT = LTy.is<TupleType>();
  bool RTyTT = RTy.is<TupleType>();
  if ((LTyTT && !RTyTT) || (!LTyTT && RTyTT))
    return false;

  // Structs do not alias non-structs.
  StructDecl *LTyStruct = LTy.getStructOrBoundGenericStruct();
  StructDecl *RTyStruct = RTy.getStructOrBoundGenericStruct();
  if ((LTyStruct && !RTyStruct) || (!LTyStruct && RTyStruct))
    return false;

  // Enums do not alias non-enums.
  EnumDecl *LTyEnum = LTy.getEnumOrBoundGenericEnum();
  EnumDecl *RTyEnum = RTy.getEnumOrBoundGenericEnum();
  if ((LTyEnum && !RTyEnum) || (!LTyEnum && RTyEnum))
    return false;

  // Classes do not alias non-classes.
  ClassDecl *RTyClass = RTy.getClassOrBoundGenericClass();
  if ((LTyClass && !RTyClass) || (!LTyClass && RTyClass))
    return false;

  // Classes with separate class hierarchies do not alias.
  if (!LTy.isSuperclassOf(RTy) && !RTy.isSuperclassOf(LTy))
    return false;

  // Otherwise be conservative and return that the two types may alias.
  return true;
}

static bool typesMayAlias(SILType T1, SILType T2, SILType TBAA1Ty,
                          SILType TBAA2Ty, SILModule &Mod) {
  // Perform type access based TBAA if we have TBAA info.
  if (TBAA1Ty && TBAA2Ty)
    return typedAccessTBAAMayAlias(TBAA1Ty, TBAA2Ty, Mod);

  // Otherwise perform class based TBAA on the passed in refs.
  //
  // FIXME: Implement class based TBAA.
  return true;
}

//===----------------------------------------------------------------------===//
//                                Entry Points
//===----------------------------------------------------------------------===//

/// The main AA entry point. Performs various analyses on V1, V2 in an attempt
/// to disambiguate the two values.
AliasAnalysis::AliasResult AliasAnalysis::alias(SILValue V1, SILValue V2,
                                                SILType TBAAType1,
                                                SILType TBAAType2) {
#ifndef NDEBUG
  // If alias analysis is disabled, always return may alias.
  if (!shouldRunAA())
    return AliasResult::MayAlias;
#endif

  // If the two values equal, quickly return must alias.
  if (isSameValueOrGlobal(V1, V2))
    return AliasResult::MustAlias;

  DEBUG(llvm::dbgs() << "ALIAS ANALYSIS:\n    V1: " << *V1.getDef()
        << "    V2: " << *V2.getDef());

  // Pass in both the TBAA types so we can perform typed access TBAA and the
  // actual types of V1, V2 so we can perform class based TBAA.
  if (!typesMayAlias(V1.getType(), V2.getType(), TBAAType1, TBAAType2, *Mod))
    return AliasResult::NoAlias;

#ifndef NDEBUG
  if (!shouldRunBasicAA())
    return AliasResult::MayAlias;
#endif

  // Strip off any casts on V1, V2.
  V1 = V1.stripCasts();
  V2 = V2.stripCasts();
  DEBUG(llvm::dbgs() << "        After Cast Stripping V1:" << *V1.getDef());
  DEBUG(llvm::dbgs() << "        After Cast Stripping V2:" << *V2.getDef());

  // Create a key to lookup if we have already computed an alias result for V1,
  // V2. Canonicalize our cache keys so that the pointer with the lower address
  // is always the first element of the pair. This ensures we do not pollute our
  // cache with two entries with the same key, albeit with the key's fields
  // swapped.
  auto Key = V1 < V2? std::make_pair(V1, V2) : std::make_pair(V2, V1);

  // If we find our key in the cache, just return the alias result.
  if (CacheAAResults) {
    auto Pair = AliasCache.find(Key);
    if (Pair != AliasCache.end()) {
      DEBUG(llvm::dbgs() << "      Found value in the cache: " << Pair->second
                         << "\n");

      return Pair->second;
    }
  }

  // Ok, we need to actually compute an Alias Analysis result for V1, V2. Begin
  // by finding the "base" of V1, V2 by stripping off all casts and GEPs.
  SILValue O1 = getUnderlyingObject(V1);
  SILValue O2 = getUnderlyingObject(V2);
  DEBUG(llvm::dbgs() << "        Underlying V1:" << *O1.getDef());
  DEBUG(llvm::dbgs() << "        Underlying V2:" << *O2.getDef());

  // If O1 and O2 do not equal, see if we can prove that they can not be the
  // same object. If we can, return No Alias.
  if (O1 != O2 && aliasUnequalObjects(O1, O2))
    return cacheValue(Key, AliasResult::NoAlias);

  // Ok, either O1, O2 are the same or we could not prove anything based off of
  // their inequality. Now we climb up use-def chains and attempt to do tricks
  // based off of GEPs.

  // First if one instruction is a gep and the other is not, canonicalize our
  // inputs so that V1 always is the instruction containing the GEP.
  if (!Projection::isAddrProjection(V1) && Projection::isAddrProjection(V2)) {
    std::swap(V1, V2);
    std::swap(O1, O2);
  }

  // If V1 is an address projection, attempt to use information from the
  // aggregate type tree to disambiguate it from V2.
  if (Projection::isAddrProjection(V1)) {
    AliasResult Result = aliasAddressProjection(*this, V1, V2, O1, O2);
    if (Result != AliasResult::MayAlias)
      return cacheValue(Key, Result);
  }


  // We could not prove anything. Be conservative and return that V1, V2 may
  // alias.
  return AliasResult::MayAlias;
}

/// Check if this is the address of a "let" member.
/// Nobody can write into let members.
bool swift::isLetPointer(SILValue V) {
  // Traverse the "access" path for V and check that starts with "let"
  // and everything along this path is a value-type (i.e. struct or tuple).

  // Is this an address of a "let" class member?
  if (auto *REA = dyn_cast<RefElementAddrInst>(V))
    return REA->getField()->isLet();

  // Is this an address of a global "let"?
  if (auto *GAI = dyn_cast<GlobalAddrInst>(V)) {
    auto *GlobalDecl = GAI->getReferencedGlobal()->getDecl();
    return GlobalDecl && GlobalDecl->isLet();
  }

  // Is this an address of a struct "let" member?
  if (auto *SEA = dyn_cast<StructElementAddrInst>(V))
    // Check if it is a "let" in the parent struct.
    // Check if its parent is a "let".
    return isLetPointer(SEA->getOperand());


  // Check if a parent of a tuple is a "let"
  if (TupleElementAddrInst *TEA = dyn_cast<TupleElementAddrInst>(V))
    return isLetPointer(TEA->getOperand());

  return false;
}

AliasAnalysis::AliasResult AliasAnalysis::cacheValue(AliasCacheKey Key,
                                                     AliasResult Result) {
  if (!CacheAAResults)
    return Result;
  DEBUG(llvm::dbgs() << "Caching Alias Result: " << Result << "\n" << Key.first
                     << Key.second << "\n");
  return AliasCache[Key] = Result;
}

void AliasAnalysis::initialize(SILPassManager *PM) {
  SEA = PM->getAnalysis<SideEffectAnalysis>();
}

SILAnalysis *swift::createAliasAnalysis(SILModule *M) {
  return new AliasAnalysis(M);
}