GHC Commentary: The Word
The most important type in the runtime is
StgWord, defined in includes/stg/Types.h. A word is defined to be the same size as a pointer on the current platform. All these types are interconvertible without losing information, and have the same size (as reported by
An unsigned integral type of word size
A signed integral type of word size
The word is the basic unit of allocation in GHC: the heap and stack are both allocated in units of a word. Throughout the runtime we often use sizes that are in units of words, so as to abstract away from the real word size of the underlying architecture.
StgWord type is also useful for storing the size of a memory object, since an
StgWord is guaranteed to at least span the range of addressable memory. It is rather like
size_t in this respect, although we prefer to use
StgWord in the RTS sources.
C-- only understands units of bytes, so we have various macros in includes/Cmm.h to make manipulating things in units of words easier in