src/base/bitfield.hh

   1 /*
   2  * Copyright (c) 2003-2005 The Regents of The University of Michigan
   3  * All rights reserved.
   4  *
   5  * Redistribution and use in source and binary forms, with or without
   6  * modification, are permitted provided that the following conditions are
   7  * met: redistributions of source code must retain the above copyright
   8  * notice, this list of conditions and the following disclaimer;
   9  * redistributions in binary form must reproduce the above copyright
  10  * notice, this list of conditions and the following disclaimer in the
  11  * documentation and/or other materials provided with the distribution;
  12  * neither the name of the copyright holders nor the names of its
  13  * contributors may be used to endorse or promote products derived from
  14  * this software without specific prior written permission.
  15  *
  16  * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
  17  * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
  18  * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
  19  * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
  20  * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
  21  * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
  22  * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
  23  * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
  24  * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
  25  * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
  26  * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  27  *
  28  * Authors: Steve Reinhardt
  29  *          Nathan Binkert
  30  */
  31
  32 #ifndef __BASE_BITFIELD_HH__
  33 #define __BASE_BITFIELD_HH__
  34
  35 #include <inttypes.h>
  36
  37 /**
  38  * Generate a 64-bit mask of 'nbits' 1s, right justified.
  39  */
  40 inline uint64_t
  41 mask(int nbits)
  42 {
  43     return (nbits == 64) ? (uint64_t)-1LL : (1ULL << nbits) - 1;
  44 }
  45
  46
  47
  48 /**
  49  * Extract the bitfield from position 'first' to 'last' (inclusive)
  50  * from 'val' and right justify it.  MSB is numbered 63, LSB is 0.
  51  */
  52 template <class T>
  53 inline
  54 T
  55 bits(T val, int first, int last)
  56 {
  57     int nbits = first - last + 1;
  58     return (val >> last) & mask(nbits);
  59 }
  60
  61 /**
  62  * Mask off the given bits in place like bits() but without shifting.
  63  * msb = 63, lsb = 0
  64  */
  65 template <class T>
  66 inline
  67 T
  68 mbits(T val, int first, int last)
  69 {
  70     return val & (mask(first+1) & ~mask(last));
  71 }
  72
  73 inline uint64_t
  74 mask(int first, int last)
  75 {
  76     return mbits((uint64_t)-1LL, first, last);
  77 }
  78
  79 /**
  80  * Sign-extend an N-bit value to 64 bits.
  81  */
  82 template <int N>
  83 inline
  84 int64_t
  85 sext(uint64_t val)
  86 {
  87     int sign_bit = bits(val, N-1, N-1);
  88     return sign_bit ? (val | ~mask(N)) : val;
  89 }
  90
  91 /**
  92  * Return val with bits first to last set to bit_val
  93  */
  94 template <class T, class B>
  95 inline
  96 T
  97 insertBits(T val, int first, int last, B bit_val)
  98 {
  99     T bmask = mask(first - last + 1) << last;
 100     return ((bit_val << last) & bmask) | (val & ~bmask);
 101 }
 102
 103 /**
 104  * A convenience function to replace bits first to last of val with bit_val
 105  * in place.
 106  */
 107 template <class T, class B>
 108 inline
 109 void
 110 replaceBits(T& val, int first, int last, B bit_val)
 111 {
 112     val = insertBits(val, first, last, bit_val);
 113 }
 114
 115 /**
 116  * Returns the bit position of the MSB that is set in the input
 117  */
 118 inline
 119 int
 120 findMsbSet(uint64_t val) {
 121     int msb = 0;
 122     if (!val)
 123         return 0;
 124     if (bits(val, 63,32)) { msb += 32; val >>= 32; }
 125     if (bits(val, 31,16)) { msb += 16; val >>= 16; }
 126     if (bits(val, 15,8))  { msb += 8;  val >>= 8;  }
 127     if (bits(val, 7,4))   { msb += 4;  val >>= 4;  }
 128     if (bits(val, 3,2))   { msb += 2;  val >>= 2;  }
 129     if (bits(val, 1,1))   { msb += 1; }
 130     return msb;
 131 }
 132
 133 //      The following implements the BitUnion system of defining bitfields
 134 //on top of an underlying class. This is done through the pervasive use of
 135 //both named and unnamed unions which all contain the same actual storage.
 136 //Since they're unioned with each other, all of these storage locations
 137 //overlap. This allows all of the bitfields to manipulate the same data
 138 //without having to have access to each other. More details are provided with the
 139 //individual components.
 140
 141 //This namespace is for classes which implement the backend of the BitUnion
 142 //stuff. Don't use any of these directly, except for the Bitfield classes in
 143 //the *BitfieldTypes class(es).
 144 namespace BitfieldBackend
 145 {
 146     //A base class for all bitfields. It instantiates the actual storage,
 147     //and provides getBits and setBits functions for manipulating it. The
 148     //Data template parameter is type of the underlying storage.
 149     template<class Data>
 150     class BitfieldBase
 151     {
 152       protected:
 153         Data __data;
 154
 155         //This function returns a range of bits from the underlying storage.
 156         //It relies on the "bits" function above. It's the user's
 157         //responsibility to make sure that there is a properly overloaded
 158         //version of this function for whatever type they want to overlay.
 159         inline uint64_t
 160         getBits(int first, int last)
 161         {
 162             return bits(__data, first, last);
 163         }
 164
 165         //Similar to the above, but for settings bits with replaceBits.
 166         inline void
 167         setBits(int first, int last, uint64_t val)
 168         {
 169             replaceBits(__data, first, last, val);
 170         }
 171     };
 172
 173     //This class contains all the "regular" bitfield classes. It is inherited
 174     //by all BitUnions which give them access to those types.
 175     template<class Type>
 176     class RegularBitfieldTypes
 177     {
 178       protected:
 179         //This class implements ordinary bitfields, that is a span of bits
 180         //who's msb is "first", and who's lsb is "last".
 181         template<int first, int last=first>
 182         class Bitfield : public BitfieldBase<Type>
 183         {
 184           public:
 185             operator const Type ()
 186             {
 187                 return this->getBits(first, last);
 188             }
 189
 190             const Type
 191             operator=(const Type & _data)
 192             {
 193                 this->setBits(first, last, _data);
 194                 return _data;
 195             }
 196         };
 197
 198         //A class which specializes the above so that it can only be read
 199         //from. This is accomplished explicitly making sure the assignment
 200         //operator is blocked. The conversion operator is carried through
 201         //inheritance. This will unfortunately need to be copied into each
 202         //bitfield type due to limitations with how templates work
 203         template<int first, int last=first>
 204         class BitfieldRO : public Bitfield<first, last>
 205         {
 206           private:
 207             const Type
 208             operator=(const Type & _data);
 209         };
 210
 211         //Similar to the above, but only allows writing.
 212         template<int first, int last=first>
 213         class BitfieldWO : public Bitfield<first, last>
 214         {
 215           private:
 216             operator const Type ();
 217
 218           public:
 219             const Type operator=(const Type & _data)
 220             {
 221                 *((Bitfield<first, last> *)this) = _data;
 222                 return _data;
 223             }
 224         };
 225     };
 226
 227     template<class Type>
 228     class BitfieldTypes : public RegularBitfieldTypes<Type>
 229     {};
 230
 231     //When a BitUnion is set up, an underlying class is created which holds
 232     //the actual union. This class then inherits from it, and provids the
 233     //implementations for various operators. Setting things up this way
 234     //prevents having to redefine these functions in every different BitUnion
 235     //type. More operators could be implemented in the future, as the need
 236     //arises.
 237     template <class Type, class Base>
 238     class BitUnionOperators : public Base
 239     {
 240       public:
 241         operator const Type ()
 242         {
 243             return Base::__data;
 244         }
 245
 246         const Type
 247         operator=(const Type & _data)
 248         {
 249             Base::__data = _data;
 250         }
 251
 252         bool
 253         operator<(const Base & base)
 254         {
 255             return Base::__data < base.__data;
 256         }
 257
 258         bool
 259         operator==(const Base & base)
 260         {
 261             return Base::__data == base.__data;
 262         }
 263     };
 264 }
 265
 266 //This macro is a backend for other macros that specialize it slightly.
 267 //First, it creates/extends a namespace "BitfieldUnderlyingClasses" and
 268 //sticks the class which has the actual union in it, which
 269 //BitfieldOperators above inherits from. Putting these classes in a special
 270 //namespace ensures that there will be no collisions with other names as long
 271 //as the BitUnion names themselves are all distinct and nothing else uses
 272 //the BitfieldUnderlyingClasses namespace, which is unlikely. The class itself
 273 //creates a typedef of the "type" parameter called __DataType. This allows
 274 //the type to propagate outside of the macro itself in a controlled way.
 275 //Finally, the base storage is defined which BitfieldOperators will refer to
 276 //in the operators it defines. This macro is intended to be followed by
 277 //bitfield definitions which will end up inside it's union. As explained
 278 //above, these is overlayed the __data member in its entirety by each of the
 279 //bitfields which are defined in the union, creating shared storage with no
 280 //overhead.
 281 #define __BitUnion(type, name) \
 282     namespace BitfieldUnderlyingClasses \
 283     { \
 284         class name; \
 285     } \
 286     class BitfieldUnderlyingClasses::name : \
 287         public BitfieldBackend::BitfieldTypes<type> \
 288     { \
 289       public: \
 290         typedef type __DataType; \
 291         union { \
 292             type __data;\
 293
 294 //This closes off the class and union started by the above macro. It is
 295 //followed by a typedef which makes "name" refer to a BitfieldOperator
 296 //class inheriting from the class and union just defined, which completes
 297 //building up the type for the user.
 298 #define EndBitUnion(name) \
 299         }; \
 300     }; \
 301     typedef BitfieldBackend::BitUnionOperators< \
 302         BitfieldUnderlyingClasses::name::__DataType, \
 303         BitfieldUnderlyingClasses::name> name;
 304
 305 //This sets up a bitfield which has other bitfields nested inside of it. The
 306 //__data member functions like the "underlying storage" of the top level
 307 //BitUnion. Like everything else, it overlays with the top level storage, so
 308 //making it a regular bitfield type makes the entire thing function as a
 309 //regular bitfield when referred to by itself.
 310 #define __SubBitUnion(fieldType, first, last, name) \
 311     class : public BitfieldBackend::BitfieldTypes<__DataType> \
 312     { \
 313       public: \
 314         union { \
 315             fieldType<first, last> __data;
 316
 317 //This closes off the union created above and gives it a name. Unlike the top
 318 //level BitUnion, we're interested in creating an object instead of a type.
 319 //The operators are defined in the macro itself instead of a class for
 320 //technical reasons. If someone determines a way to move them to one, please
 321 //do so.
 322 #define EndSubBitUnion(name) \
 323         }; \
 324         inline operator const __DataType () \
 325         { return __data; } \
 326         \
 327         inline const __DataType operator = (const __DataType & _data) \
 328         { __data = _data; } \
 329     } name;
 330
 331 //Regular bitfields
 332 //These define macros for read/write regular bitfield based subbitfields.
 333 #define SubBitUnion(name, first, last) \
 334     __SubBitUnion(Bitfield, first, last, name)
 335
 336 //Use this to define an arbitrary type overlayed with bitfields.
 337 #define BitUnion(type, name) __BitUnion(type, name)
 338
 339 //Use this to define conveniently sized values overlayed with bitfields.
 340 #define BitUnion64(name) __BitUnion(uint64_t, name)
 341 #define BitUnion32(name) __BitUnion(uint32_t, name)
 342 #define BitUnion16(name) __BitUnion(uint16_t, name)
 343 #define BitUnion8(name) __BitUnion(uint8_t, name)
 344
 345 #endif // __BASE_BITFIELD_HH__