= GHC Commentary: The Word =

The most important type in the runtime is {{{StgWord}}}, defined in [[GhcFile(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 {{{sizeof}}}):

 {{{StgWord}}}::
  An unsiged integral type of word size

 {{{StgInt}}}::
  A signed integral type of word size

 {{{StgPtr}}}::
  Pointer to {{{StgWord}}}

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.

The `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 [[GhcFile(includes/Cmm.h)]] to make manipulating things in units of words easier in {{{.cmm}}} files.