bitfield.hh revision 4274
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 */
40inline uint64_t
41mask(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 */
52template <class T>
53inline
54T
55bits(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 */
65template <class T>
66inline
67T
68mbits(T val, int first, int last)
69{
70    return val & (mask(first+1) & ~mask(last));
71}
72
73inline uint64_t
74mask(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 */
82template <int N>
83inline
84int64_t
85sext(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 */
94template <class T, class B>
95inline
96T
97insertBits(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 */
107template <class T, class B>
108inline
109void
110replaceBits(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 */
118inline
119int
120findMsbSet(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).
144namespace 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 uint64_t () const
186            {
187                return this->getBits(first, last);
188            }
189
190            uint64_t
191            operator=(const uint64_t _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            uint64_t
208            operator=(const uint64_t _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 uint64_t () const;
217
218          public:
219            using Bitfield<first, last>::operator=;
220        };
221    };
222
223    //This class contains all the "regular" bitfield classes. It is inherited
224    //by all BitUnions which give them access to those types.
225    template<class Type>
226    class SignedBitfieldTypes
227    {
228      protected:
229        //This class implements ordinary bitfields, that is a span of bits
230        //who's msb is "first", and who's lsb is "last".
231        template<int first, int last=first>
232        class SignedBitfield : public BitfieldBase<Type>
233        {
234          public:
235            operator int64_t () const
236            {
237                return sext<first - last + 1>(this->getBits(first, last));
238            }
239
240            int64_t
241            operator=(const int64_t _data)
242            {
243                this->setBits(first, last, _data);
244                return _data;
245            }
246        };
247
248        //A class which specializes the above so that it can only be read
249        //from. This is accomplished explicitly making sure the assignment
250        //operator is blocked. The conversion operator is carried through
251        //inheritance. This will unfortunately need to be copied into each
252        //bitfield type due to limitations with how templates work
253        template<int first, int last=first>
254        class SignedBitfieldRO : public SignedBitfield<first, last>
255        {
256          private:
257            int64_t
258            operator=(const int64_t _data);
259        };
260
261        //Similar to the above, but only allows writing.
262        template<int first, int last=first>
263        class SignedBitfieldWO : public SignedBitfield<first, last>
264        {
265          private:
266            operator int64_t () const;
267
268          public:
269            int64_t operator=(const int64_t _data)
270            {
271                *((SignedBitfield<first, last> *)this) = _data;
272                return _data;
273            }
274        };
275    };
276
277    template<class Type>
278    class BitfieldTypes : public RegularBitfieldTypes<Type>,
279                          public SignedBitfieldTypes<Type>
280    {};
281
282    //When a BitUnion is set up, an underlying class is created which holds
283    //the actual union. This class then inherits from it, and provids the
284    //implementations for various operators. Setting things up this way
285    //prevents having to redefine these functions in every different BitUnion
286    //type. More operators could be implemented in the future, as the need
287    //arises.
288    template <class Type, class Base>
289    class BitUnionOperators : public Base
290    {
291      public:
292        operator Type () const
293        {
294            return Base::__data;
295        }
296
297        Type
298        operator=(const Type & _data)
299        {
300            Base::__data = _data;
301            return _data;
302        }
303
304        bool
305        operator<(const Base & base) const
306        {
307            return Base::__data < base.__data;
308        }
309
310        bool
311        operator==(const Base & base) const
312        {
313            return Base::__data == base.__data;
314        }
315    };
316}
317
318//This macro is a backend for other macros that specialize it slightly.
319//First, it creates/extends a namespace "BitfieldUnderlyingClasses" and
320//sticks the class which has the actual union in it, which
321//BitfieldOperators above inherits from. Putting these classes in a special
322//namespace ensures that there will be no collisions with other names as long
323//as the BitUnion names themselves are all distinct and nothing else uses
324//the BitfieldUnderlyingClasses namespace, which is unlikely. The class itself
325//creates a typedef of the "type" parameter called __DataType. This allows
326//the type to propagate outside of the macro itself in a controlled way.
327//Finally, the base storage is defined which BitfieldOperators will refer to
328//in the operators it defines. This macro is intended to be followed by
329//bitfield definitions which will end up inside it's union. As explained
330//above, these is overlayed the __data member in its entirety by each of the
331//bitfields which are defined in the union, creating shared storage with no
332//overhead.
333#define __BitUnion(type, name) \
334    namespace BitfieldUnderlyingClasses \
335    { \
336        class name; \
337    } \
338    class BitfieldUnderlyingClasses::name : \
339        public BitfieldBackend::BitfieldTypes<type> \
340    { \
341      public: \
342        typedef type __DataType; \
343        union { \
344            type __data;\
345
346//This closes off the class and union started by the above macro. It is
347//followed by a typedef which makes "name" refer to a BitfieldOperator
348//class inheriting from the class and union just defined, which completes
349//building up the type for the user.
350#define EndBitUnion(name) \
351        }; \
352    }; \
353    typedef BitfieldBackend::BitUnionOperators< \
354        BitfieldUnderlyingClasses::name::__DataType, \
355        BitfieldUnderlyingClasses::name> name;
356
357//This sets up a bitfield which has other bitfields nested inside of it. The
358//__data member functions like the "underlying storage" of the top level
359//BitUnion. Like everything else, it overlays with the top level storage, so
360//making it a regular bitfield type makes the entire thing function as a
361//regular bitfield when referred to by itself.
362#define __SubBitUnion(fieldType, first, last, name) \
363    class : public BitfieldBackend::BitfieldTypes<__DataType> \
364    { \
365      public: \
366        union { \
367            fieldType<first, last> __data;
368
369//This closes off the union created above and gives it a name. Unlike the top
370//level BitUnion, we're interested in creating an object instead of a type.
371//The operators are defined in the macro itself instead of a class for
372//technical reasons. If someone determines a way to move them to one, please
373//do so.
374#define EndSubBitUnion(name) \
375        }; \
376        inline operator const __DataType () \
377        { return __data; } \
378        \
379        inline const __DataType operator = (const __DataType & _data) \
380        { __data = _data; } \
381    } name;
382
383//Regular bitfields
384//These define macros for read/write regular bitfield based subbitfields.
385#define SubBitUnion(name, first, last) \
386    __SubBitUnion(Bitfield, first, last, name)
387
388//Regular bitfields
389//These define macros for read/write regular bitfield based subbitfields.
390#define SignedSubBitUnion(name, first, last) \
391    __SubBitUnion(SignedBitfield, first, last, name)
392
393//Use this to define an arbitrary type overlayed with bitfields.
394#define BitUnion(type, name) __BitUnion(type, name)
395
396//Use this to define conveniently sized values overlayed with bitfields.
397#define BitUnion64(name) __BitUnion(uint64_t, name)
398#define BitUnion32(name) __BitUnion(uint32_t, name)
399#define BitUnion16(name) __BitUnion(uint16_t, name)
400#define BitUnion8(name) __BitUnion(uint8_t, name)
401
402#endif // __BASE_BITFIELD_HH__
403