bitfield.hh revision 4262
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 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__
346