diff options
| -rw-r--r-- | src/base/bitfield.hh | 72 |
1 files changed, 71 insertions, 1 deletions
diff --git a/src/base/bitfield.hh b/src/base/bitfield.hh index 5af453a76..8731608ef 100644 --- a/src/base/bitfield.hh +++ b/src/base/bitfield.hh @@ -130,20 +130,38 @@ findMsbSet(uint64_t val) { return msb; } +// The following implements the BitUnion system of defining bitfields +//on top of an underlying class. This is done through the extensive use of +//both named and unnamed unions which all contain the same actual storage. +//Since they're unioned with each other, all of these storage locations +//overlap. This allows all of the bitfields to manipulate the same data +//without having to know about each other. More details are provided with the +//individual components. + +//This namespace is for classes which implement the backend of the BitUnion +//stuff. Don't use any of this directly! Use the macros at the end instead. namespace BitfieldBackend { + //A base class for all bitfields. It instantiates the actual storage, + //and provides getBits and setBits functions for manipulating it. The + //Data template parameter is type of the underlying storage. template<class Data> class BitfieldBase { protected: Data __data; + //This function returns a range of bits from the underlying storage. + //It relies on the "bits" function above. It's the user's + //responsibility to make sure that there is a properly overloaded + //version of this function for whatever type they want to overlay. inline uint64_t getBits(int first, int last) { return bits(__data, first, last); } + //Similar to the above, but for settings bits with replaceBits. inline void setBits(int first, int last, uint64_t val) { @@ -151,6 +169,9 @@ namespace BitfieldBackend } }; + //A class which specializes a given base so that it can only be read + //from. This is accomplished by only passing through the conversion + //operator. template<class Type, class Base> class _BitfieldRO : public Base { @@ -161,6 +182,7 @@ namespace BitfieldBackend } }; + //Similar to the above, but only allows writing. template<class Type, class Base> class _BitfieldWO : public Base { @@ -172,6 +194,8 @@ namespace BitfieldBackend } }; + //This class implements ordinary bitfields, that is a span of bits + //who's msb is "first", and who's lsb is "last". template<class Data, int first, int last=first> class _Bitfield : public BitfieldBase<Data> { @@ -189,6 +213,12 @@ namespace BitfieldBackend } }; + //When a BitUnion is set up, an underlying class is created which holds + //the actual union. This class then inherits from it, and provids the + //implementations for various operators. Setting things up this way + //prevents having to redefine these functions in every different BitUnion + //type. More operators could be implemented in the future, as the need + //arises. template <class Type, class Base> class BitUnionOperators : public Base { @@ -218,6 +248,21 @@ namespace BitfieldBackend }; } +//This macro is a backend for other macros that specialize it slightly. +//First, it creates/extends a namespace "BitfieldUnderlyingClasses" and +//sticks the class which has the actual union in it, which +//BitfieldOperators above inherits from. Putting these classes in a special +//namespace ensures that there will be no collisions with other names as long +//as the BitUnion names themselves are all distinct and nothing else uses +//the BitfieldUnderlyingClasses namespace, which is unlikely. The class itself +//creates a typedef of the "type" parameter called __DataType. This allows +//the type to propagate outside of the macro itself in a controlled way. +//Finally, the base storage is defined which BitfieldOperators will refer to +//in the operators it defines. This macro is intended to be followed by +//bitfield definitions which will end up inside it's union. As explained +//above, these is overlayed the __data member in its entirety by each of the +//bitfields which are defined in the union, creating shared storage with no +//overhead. #define __BitUnion(type, name) \ namespace BitfieldUnderlyingClasses \ { \ @@ -229,6 +274,10 @@ namespace BitfieldBackend union { \ type __data;\ +//This closes off the class and union started by the above macro. It is +//followed by a typedef which makes "name" refer to a BitfieldOperator +//class inheriting from the class and union just defined, which completes +//building up the type for the user. #define EndBitUnion(name) \ }; \ }; \ @@ -236,6 +285,13 @@ namespace BitfieldBackend BitfieldUnderlyingClasses::name::__DataType, \ BitfieldUnderlyingClasses::name> name; +//This sets up a bitfield which has other bitfields nested inside of it. The +//__data member functions like the "underlying storage" of the top level +//BitUnion. Like everything else, it overlays with the top level storage, so +//making it a regular bitfield type makes the entire thing function as a +//regular bitfield when referred to by itself. The operators are defined in +//the macro itself instead of a class for technical reasons. If someone +//determines a way to move them to one, please do so. #define __SubBitUnion(type, name) \ union { \ type __data; \ @@ -245,24 +301,36 @@ namespace BitfieldBackend inline const __DataType operator = (const __DataType & _data) \ { __data = _data; } +//This closes off the union created above and gives it a name. Unlike the top +//level BitUnion, we're interested in creating an object instead of a type. #define EndSubBitUnion(name) } name; -//This is so we can send in parameters with commas +//The preprocessor will treat everything inside of parenthesis as a single +//argument even if it has commas in it. This is used to pass in templated +//classes which typically have commas to seperate their parameters. #define wrap(guts) guts //Read only bitfields +//This wraps another bitfield class inside a _BitfieldRO class using +//inheritance. As explained above, the _BitfieldRO class only passes through +//the conversion operator, so the underlying bitfield can then only be read +//from. #define __BitfieldRO(base) \ BitfieldBackend::_BitfieldRO<__DataType, base> #define __SubBitUnionRO(name, base) \ __SubBitUnion(wrap(_BitfieldRO<__DataType, base>), name) //Write only bitfields +//Similar to above, but for making write only versions of bitfields with +//_BitfieldWO. #define __BitfieldWO(base) \ BitfieldBackend::_BitfieldWO<__DataType, base> #define __SubBitUnionWO(name, base) \ __SubBitUnion(wrap(_BitfieldWO<__DataType, base>), name) //Regular bitfields +//This uses all of the above to define macros for read/write, read only, and +//write only versions of regular bitfields. #define Bitfield(first, last) \ BitfieldBackend::_Bitfield<__DataType, first, last> #define SubBitUnion(name, first, last) \ @@ -274,8 +342,10 @@ namespace BitfieldBackend #define SubBitUnionWO(name, first, last) \ __SubBitUnionWO(Bitfield(first, last), name) +//Use this to define an arbitrary type overlayed with bitfields. #define BitUnion(type, name) __BitUnion(type, name) +//Use this to define conveniently sized values overlayed with bitfields. #define BitUnion64(name) __BitUnion(uint64_t, name) #define BitUnion32(name) __BitUnion(uint32_t, name) #define BitUnion16(name) __BitUnion(uint16_t, name) |