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      1 //===- llvm/ADT/PointerEmbeddedInt.h ----------------------------*- C++ -*-===//
      2 //
      3 //                     The LLVM Compiler Infrastructure
      4 //
      5 // This file is distributed under the University of Illinois Open Source
      6 // License. See LICENSE.TXT for details.
      7 //
      8 //===----------------------------------------------------------------------===//
      9 
     10 #ifndef LLVM_ADT_POINTEREMBEDDEDINT_H
     11 #define LLVM_ADT_POINTEREMBEDDEDINT_H
     12 
     13 #include "llvm/ADT/DenseMapInfo.h"
     14 #include "llvm/Support/MathExtras.h"
     15 #include "llvm/Support/PointerLikeTypeTraits.h"
     16 #include <climits>
     17 
     18 namespace llvm {
     19 
     20 /// Utility to embed an integer into a pointer-like type. This is specifically
     21 /// intended to allow embedding integers where fewer bits are required than
     22 /// exist in a pointer, and the integer can participate in abstractions along
     23 /// side other pointer-like types. For example it can be placed into a \c
     24 /// PointerSumType or \c PointerUnion.
     25 ///
     26 /// Note that much like pointers, an integer value of zero has special utility
     27 /// due to boolean conversions. For example, a non-null value can be tested for
     28 /// in the above abstractions without testing the particular active member.
     29 /// Also, the default constructed value zero initializes the integer.
     30 template <typename IntT, int Bits = sizeof(IntT) * CHAR_BIT>
     31 class PointerEmbeddedInt {
     32   uintptr_t Value;
     33 
     34   // Note: This '<' is correct; using '<=' would result in some shifts
     35   // overflowing their storage types.
     36   static_assert(Bits < sizeof(uintptr_t) * CHAR_BIT,
     37                 "Cannot embed more bits than we have in a pointer!");
     38 
     39   enum : uintptr_t {
     40     // We shift as many zeros into the value as we can while preserving the
     41     // number of bits desired for the integer.
     42     Shift = sizeof(uintptr_t) * CHAR_BIT - Bits,
     43 
     44     // We also want to be able to mask out the preserved bits for asserts.
     45     Mask = static_cast<uintptr_t>(-1) << Bits
     46   };
     47 
     48   struct RawValueTag {
     49     explicit RawValueTag() = default;
     50   };
     51 
     52   friend class PointerLikeTypeTraits<PointerEmbeddedInt>;
     53 
     54   explicit PointerEmbeddedInt(uintptr_t Value, RawValueTag) : Value(Value) {}
     55 
     56 public:
     57   PointerEmbeddedInt() : Value(0) {}
     58 
     59   PointerEmbeddedInt(IntT I) {
     60     *this = I;
     61   }
     62 
     63   PointerEmbeddedInt &operator=(IntT I) {
     64     assert((std::is_signed<IntT>::value ? llvm::isInt<Bits>(I)
     65                                         : llvm::isUInt<Bits>(I)) &&
     66            "Integer has bits outside those preserved!");
     67     Value = static_cast<uintptr_t>(I) << Shift;
     68     return *this;
     69   }
     70 
     71   // Note that this implicit conversion additionally allows all of the basic
     72   // comparison operators to work transparently, etc.
     73   operator IntT() const {
     74     if (std::is_signed<IntT>::value)
     75       return static_cast<IntT>(static_cast<intptr_t>(Value) >> Shift);
     76     return static_cast<IntT>(Value >> Shift);
     77   }
     78 };
     79 
     80 // Provide pointer like traits to support use with pointer unions and sum
     81 // types.
     82 template <typename IntT, int Bits>
     83 class PointerLikeTypeTraits<PointerEmbeddedInt<IntT, Bits>> {
     84   typedef PointerEmbeddedInt<IntT, Bits> T;
     85 
     86 public:
     87   static inline void *getAsVoidPointer(const T &P) {
     88     return reinterpret_cast<void *>(P.Value);
     89   }
     90   static inline T getFromVoidPointer(void *P) {
     91     return T(reinterpret_cast<uintptr_t>(P), typename T::RawValueTag());
     92   }
     93   static inline T getFromVoidPointer(const void *P) {
     94     return T(reinterpret_cast<uintptr_t>(P), typename T::RawValueTag());
     95   }
     96 
     97   enum { NumLowBitsAvailable = T::Shift };
     98 };
     99 
    100 // Teach DenseMap how to use PointerEmbeddedInt objects as keys if the Int type
    101 // itself can be a key.
    102 template <typename IntT, int Bits>
    103 struct DenseMapInfo<PointerEmbeddedInt<IntT, Bits>> {
    104   typedef PointerEmbeddedInt<IntT, Bits> T;
    105 
    106   typedef DenseMapInfo<IntT> IntInfo;
    107 
    108   static inline T getEmptyKey() { return IntInfo::getEmptyKey(); }
    109   static inline T getTombstoneKey() { return IntInfo::getTombstoneKey(); }
    110   static unsigned getHashValue(const T &Arg) {
    111     return IntInfo::getHashValue(Arg);
    112   }
    113   static bool isEqual(const T &LHS, const T &RHS) { return LHS == RHS; }
    114 };
    115 }
    116 
    117 #endif
    118