Lots and lots of comments.
--HG-- extra : convert_revision : 0de510464e2e002775ccd79e2922ccb0055845e5
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@ -130,20 +130,38 @@ findMsbSet(uint64_t val) {
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return msb;
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}
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// The following implements the BitUnion system of defining bitfields
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//on top of an underlying class. This is done through the extensive use of
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//both named and unnamed unions which all contain the same actual storage.
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//Since they're unioned with each other, all of these storage locations
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//overlap. This allows all of the bitfields to manipulate the same data
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//without having to know about each other. More details are provided with the
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//individual components.
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//This namespace is for classes which implement the backend of the BitUnion
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//stuff. Don't use any of this directly! Use the macros at the end instead.
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namespace BitfieldBackend
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{
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//A base class for all bitfields. It instantiates the actual storage,
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//and provides getBits and setBits functions for manipulating it. The
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//Data template parameter is type of the underlying storage.
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template<class Data>
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class BitfieldBase
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{
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protected:
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Data __data;
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//This function returns a range of bits from the underlying storage.
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//It relies on the "bits" function above. It's the user's
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//responsibility to make sure that there is a properly overloaded
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//version of this function for whatever type they want to overlay.
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inline uint64_t
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getBits(int first, int last)
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{
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return bits(__data, first, last);
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}
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//Similar to the above, but for settings bits with replaceBits.
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inline void
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setBits(int first, int last, uint64_t val)
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{
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@ -151,6 +169,9 @@ namespace BitfieldBackend
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}
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};
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//A class which specializes a given base so that it can only be read
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//from. This is accomplished by only passing through the conversion
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//operator.
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template<class Type, class Base>
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class _BitfieldRO : public Base
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{
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@ -161,6 +182,7 @@ namespace BitfieldBackend
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}
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};
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//Similar to the above, but only allows writing.
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template<class Type, class Base>
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class _BitfieldWO : public Base
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{
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@ -172,6 +194,8 @@ namespace BitfieldBackend
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}
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};
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//This class implements ordinary bitfields, that is a span of bits
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//who's msb is "first", and who's lsb is "last".
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template<class Data, int first, int last=first>
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class _Bitfield : public BitfieldBase<Data>
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{
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@ -189,6 +213,12 @@ namespace BitfieldBackend
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}
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};
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//When a BitUnion is set up, an underlying class is created which holds
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//the actual union. This class then inherits from it, and provids the
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//implementations for various operators. Setting things up this way
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//prevents having to redefine these functions in every different BitUnion
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//type. More operators could be implemented in the future, as the need
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//arises.
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template <class Type, class Base>
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class BitUnionOperators : public Base
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{
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@ -218,6 +248,21 @@ namespace BitfieldBackend
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};
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}
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//This macro is a backend for other macros that specialize it slightly.
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//First, it creates/extends a namespace "BitfieldUnderlyingClasses" and
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//sticks the class which has the actual union in it, which
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//BitfieldOperators above inherits from. Putting these classes in a special
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//namespace ensures that there will be no collisions with other names as long
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//as the BitUnion names themselves are all distinct and nothing else uses
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//the BitfieldUnderlyingClasses namespace, which is unlikely. The class itself
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//creates a typedef of the "type" parameter called __DataType. This allows
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//the type to propagate outside of the macro itself in a controlled way.
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//Finally, the base storage is defined which BitfieldOperators will refer to
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//in the operators it defines. This macro is intended to be followed by
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//bitfield definitions which will end up inside it's union. As explained
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//above, these is overlayed the __data member in its entirety by each of the
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//bitfields which are defined in the union, creating shared storage with no
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//overhead.
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#define __BitUnion(type, name) \
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namespace BitfieldUnderlyingClasses \
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{ \
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@ -229,6 +274,10 @@ namespace BitfieldBackend
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union { \
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type __data;\
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//This closes off the class and union started by the above macro. It is
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//followed by a typedef which makes "name" refer to a BitfieldOperator
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//class inheriting from the class and union just defined, which completes
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//building up the type for the user.
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#define EndBitUnion(name) \
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}; \
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}; \
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@ -236,6 +285,13 @@ namespace BitfieldBackend
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BitfieldUnderlyingClasses::name::__DataType, \
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BitfieldUnderlyingClasses::name> name;
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//This sets up a bitfield which has other bitfields nested inside of it. The
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//__data member functions like the "underlying storage" of the top level
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//BitUnion. Like everything else, it overlays with the top level storage, so
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//making it a regular bitfield type makes the entire thing function as a
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//regular bitfield when referred to by itself. The operators are defined in
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//the macro itself instead of a class for technical reasons. If someone
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//determines a way to move them to one, please do so.
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#define __SubBitUnion(type, name) \
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union { \
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type __data; \
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@ -245,24 +301,36 @@ namespace BitfieldBackend
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inline const __DataType operator = (const __DataType & _data) \
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{ __data = _data; }
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//This closes off the union created above and gives it a name. Unlike the top
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//level BitUnion, we're interested in creating an object instead of a type.
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#define EndSubBitUnion(name) } name;
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//This is so we can send in parameters with commas
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//The preprocessor will treat everything inside of parenthesis as a single
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//argument even if it has commas in it. This is used to pass in templated
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//classes which typically have commas to seperate their parameters.
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#define wrap(guts) guts
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//Read only bitfields
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//This wraps another bitfield class inside a _BitfieldRO class using
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//inheritance. As explained above, the _BitfieldRO class only passes through
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//the conversion operator, so the underlying bitfield can then only be read
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//from.
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#define __BitfieldRO(base) \
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BitfieldBackend::_BitfieldRO<__DataType, base>
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#define __SubBitUnionRO(name, base) \
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__SubBitUnion(wrap(_BitfieldRO<__DataType, base>), name)
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//Write only bitfields
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//Similar to above, but for making write only versions of bitfields with
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//_BitfieldWO.
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#define __BitfieldWO(base) \
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BitfieldBackend::_BitfieldWO<__DataType, base>
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#define __SubBitUnionWO(name, base) \
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__SubBitUnion(wrap(_BitfieldWO<__DataType, base>), name)
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//Regular bitfields
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//This uses all of the above to define macros for read/write, read only, and
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//write only versions of regular bitfields.
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#define Bitfield(first, last) \
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BitfieldBackend::_Bitfield<__DataType, first, last>
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#define SubBitUnion(name, first, last) \
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#define SubBitUnionWO(name, first, last) \
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__SubBitUnionWO(Bitfield(first, last), name)
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//Use this to define an arbitrary type overlayed with bitfields.
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#define BitUnion(type, name) __BitUnion(type, name)
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//Use this to define conveniently sized values overlayed with bitfields.
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#define BitUnion64(name) __BitUnion(uint64_t, name)
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#define BitUnion32(name) __BitUnion(uint32_t, name)
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#define BitUnion16(name) __BitUnion(uint16_t, name)
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