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GHC Commentary: The Word
The most important type in the runtime is StgWord, defined in includes/StgTypes.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):
- An unsiged integral type of word size
- A signed integral type of word size
- 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.
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 .cmm files.