AliasAnalysisSummary.h

Go to the documentation of this file.

//=====- CFLSummary.h - Abstract stratified sets implementation. --------=====//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
/// \file
/// This file defines various utility types and functions useful to
/// summary-based alias analysis.
///
/// Summary-based analysis, also known as bottom-up analysis, is a style of
/// interprocedrual static analysis that tries to analyze the callees before the
/// callers get analyzed. The key idea of summary-based analysis is to first
/// process each function independently, outline its behavior in a condensed
/// summary, and then instantiate the summary at the callsite when the said
/// function is called elsewhere. This is often in contrast to another style
/// called top-down analysis, in which callers are always analyzed first before
/// the callees.
///
/// In a summary-based analysis, functions must be examined independently and
/// out-of-context. We have no information on the state of the memory, the
/// arguments, the global values, and anything else external to the function. To
/// carry out the analysis conservative assumptions have to be made about those
/// external states. In exchange for the potential loss of precision, the
/// summary we obtain this way is highly reusable, which makes the analysis
/// easier to scale to large programs even if carried out context-sensitively.
///
/// Currently, all CFL-based alias analyses adopt the summary-based approach
/// and therefore heavily rely on this header.
///
//===----------------------------------------------------------------------===//
 
#ifndef LLVM_ANALYSIS_ALIASANALYSISSUMMARY_H
#define LLVM_ANALYSIS_ALIASANALYSISSUMMARY_H
 
#include "llvm/ADT/DenseMapInfo.h"
#include "llvm/ADT/SmallVector.h"
#include <bitset>
#include <optional>
 
namespace llvm {
 
class CallBase;
class Value;
 
namespace cflaa {
 
//===----------------------------------------------------------------------===//
// AliasAttr related stuffs
//===----------------------------------------------------------------------===//
 
/// The number of attributes that AliasAttr should contain. Attributes are
/// described below, and 32 was an arbitrary choice because it fits nicely in 32
/// bits (because we use a bitset for AliasAttr).
static const unsigned NumAliasAttrs = 32;
 
/// These are attributes that an alias analysis can use to mark certain special
/// properties of a given pointer. Refer to the related functions below to see
/// what kinds of attributes are currently defined.
typedef std::bitset<NumAliasAttrs> AliasAttrs;
 
/// Attr represent whether the said pointer comes from an unknown source
/// (such as opaque memory or an integer cast).
AliasAttrs getAttrNone();
 
/// AttrUnknown represent whether the said pointer comes from a source not known
/// to alias analyses (such as opaque memory or an integer cast).
AliasAttrs getAttrUnknown();
bool hasUnknownAttr(AliasAttrs);
 
/// AttrCaller represent whether the said pointer comes from a source not known
/// to the current function but known to the caller. Values pointed to by the
/// arguments of the current function have this attribute set
AliasAttrs getAttrCaller();
bool hasCallerAttr(AliasAttrs);
bool hasUnknownOrCallerAttr(AliasAttrs);
 
/// AttrEscaped represent whether the said pointer comes from a known source but
/// escapes to the unknown world (e.g. casted to an integer, or passed as an
/// argument to opaque function). Unlike non-escaped pointers, escaped ones may
/// alias pointers coming from unknown sources.
AliasAttrs getAttrEscaped();
bool hasEscapedAttr(AliasAttrs);
 
/// AttrGlobal represent whether the said pointer is a global value.
/// AttrArg represent whether the said pointer is an argument, and if so, what
/// index the argument has.
AliasAttrs getGlobalOrArgAttrFromValue(const Value &);
bool isGlobalOrArgAttr(AliasAttrs);
 
/// Given an AliasAttrs, return a new AliasAttrs that only contains attributes
/// meaningful to the caller. This function is primarily used for
/// interprocedural analysis
/// Currently, externally visible AliasAttrs include AttrUnknown, AttrGlobal,
/// and AttrEscaped
AliasAttrs getExternallyVisibleAttrs(AliasAttrs);
 
//===----------------------------------------------------------------------===//
// Function summary related stuffs
//===----------------------------------------------------------------------===//
 
/// The maximum number of arguments we can put into a summary.
static const unsigned MaxSupportedArgsInSummary = 50;
 
/// We use InterfaceValue to describe parameters/return value, as well as
/// potential memory locations that are pointed to by parameters/return value,
/// of a function.
/// Index is an integer which represents a single parameter or a return value.
/// When the index is 0, it refers to the return value. Non-zero index i refers
/// to the i-th parameter.
/// DerefLevel indicates the number of dereferences one must perform on the
/// parameter/return value to get this InterfaceValue.
struct InterfaceValue {
  unsigned Index;
  unsigned DerefLevel;
};
 
inline bool operator==(InterfaceValue LHS, InterfaceValue RHS) {
  return LHS.Index == RHS.Index && LHS.DerefLevel == RHS.DerefLevel;
}
inline bool operator!=(InterfaceValue LHS, InterfaceValue RHS) {
  return !(LHS == RHS);
}
inline bool operator<(InterfaceValue LHS, InterfaceValue RHS) {
  return LHS.Index < RHS.Index ||
         (LHS.Index == RHS.Index && LHS.DerefLevel < RHS.DerefLevel);
}
inline bool operator>(InterfaceValue LHS, InterfaceValue RHS) {
  return RHS < LHS;
}
inline bool operator<=(InterfaceValue LHS, InterfaceValue RHS) {
  return !(RHS < LHS);
}
inline bool operator>=(InterfaceValue LHS, InterfaceValue RHS) {
  return !(LHS < RHS);
}
 
// We use UnknownOffset to represent pointer offsets that cannot be determined
// at compile time. Note that MemoryLocation::UnknownSize cannot be used here
// because we require a signed value.
static const int64_t UnknownOffset = INT64_MAX;
 
inline int64_t addOffset(int64_t LHS, int64_t RHS) {
  if (LHS == UnknownOffset || RHS == UnknownOffset)
    return UnknownOffset;
  // FIXME: Do we need to guard against integer overflow here?
  return LHS + RHS;
}
 
/// We use ExternalRelation to describe an externally visible aliasing relations
/// between parameters/return value of a function.
struct ExternalRelation {
  InterfaceValue From, To;
  int64_t Offset;
};
 
inline bool operator==(ExternalRelation LHS, ExternalRelation RHS) {
  return LHS.From == RHS.From && LHS.To == RHS.To && LHS.Offset == RHS.Offset;
}
inline bool operator!=(ExternalRelation LHS, ExternalRelation RHS) {
  return !(LHS == RHS);
}
inline bool operator<(ExternalRelation LHS, ExternalRelation RHS) {
  if (LHS.From < RHS.From)
    return true;
  if (LHS.From > RHS.From)
    return false;
  if (LHS.To < RHS.To)
    return true;
  if (LHS.To > RHS.To)
    return false;
  return LHS.Offset < RHS.Offset;
}
inline bool operator>(ExternalRelation LHS, ExternalRelation RHS) {
  return RHS < LHS;
}
inline bool operator<=(ExternalRelation LHS, ExternalRelation RHS) {
  return !(RHS < LHS);
}
inline bool operator>=(ExternalRelation LHS, ExternalRelation RHS) {
  return !(LHS < RHS);
}
 
/// We use ExternalAttribute to describe an externally visible AliasAttrs
/// for parameters/return value.
struct ExternalAttribute {
  InterfaceValue IValue;
  AliasAttrs Attr;
};
 
/// AliasSummary is just a collection of ExternalRelation and ExternalAttribute
struct AliasSummary {
  // RetParamRelations is a collection of ExternalRelations.
  SmallVector<ExternalRelation, 8> RetParamRelations;
 
  // RetParamAttributes is a collection of ExternalAttributes.
  SmallVector<ExternalAttribute, 8> RetParamAttributes;
};
 
/// This is the result of instantiating InterfaceValue at a particular call
struct InstantiatedValue {
  Value *Val;
  unsigned DerefLevel;
};
std::optional<InstantiatedValue>
instantiateInterfaceValue(InterfaceValue IValue, CallBase &Call);
 
inline bool operator==(InstantiatedValue LHS, InstantiatedValue RHS) {
  return LHS.Val == RHS.Val && LHS.DerefLevel == RHS.DerefLevel;
}
inline bool operator!=(InstantiatedValue LHS, InstantiatedValue RHS) {
  return !(LHS == RHS);
}
inline bool operator<(InstantiatedValue LHS, InstantiatedValue RHS) {
  return std::less<Value *>()(LHS.Val, RHS.Val) ||
         (LHS.Val == RHS.Val && LHS.DerefLevel < RHS.DerefLevel);
}
inline bool operator>(InstantiatedValue LHS, InstantiatedValue RHS) {
  return RHS < LHS;
}
inline bool operator<=(InstantiatedValue LHS, InstantiatedValue RHS) {
  return !(RHS < LHS);
}
inline bool operator>=(InstantiatedValue LHS, InstantiatedValue RHS) {
  return !(LHS < RHS);
}
 
/// This is the result of instantiating ExternalRelation at a particular
/// callsite
struct InstantiatedRelation {
  InstantiatedValue From, To;
  int64_t Offset;
};
std::optional<InstantiatedRelation>
instantiateExternalRelation(ExternalRelation ERelation, CallBase &Call);
 
/// This is the result of instantiating ExternalAttribute at a particular
/// callsite
struct InstantiatedAttr {
  InstantiatedValue IValue;
  AliasAttrs Attr;
};
std::optional<InstantiatedAttr>
instantiateExternalAttribute(ExternalAttribute EAttr, CallBase &Call);
}
 
template <> struct DenseMapInfo<cflaa::InstantiatedValue> {
  static inline cflaa::InstantiatedValue getEmptyKey() {
    return cflaa::InstantiatedValue{DenseMapInfo<Value *>::getEmptyKey(),
                                    DenseMapInfo<unsigned>::getEmptyKey()};
  }
  static inline cflaa::InstantiatedValue getTombstoneKey() {
    return cflaa::InstantiatedValue{DenseMapInfo<Value *>::getTombstoneKey(),
                                    DenseMapInfo<unsigned>::getTombstoneKey()};
  }
  static unsigned getHashValue(const cflaa::InstantiatedValue &IV) {
    return DenseMapInfo<std::pair<Value *, unsigned>>::getHashValue(
        std::make_pair(IV.Val, IV.DerefLevel));
  }
  static bool isEqual(const cflaa::InstantiatedValue &LHS,
                      const cflaa::InstantiatedValue &RHS) {
    return LHS.Val == RHS.Val && LHS.DerefLevel == RHS.DerefLevel;
  }
};
}
 
#endif