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PointerTagging

v10	v11
7	7	In GHC we "tag" pointers to heap objects with information about the object they point to. The tag goes in the low 2 bits (3 bits on a 64-bit platform) of the pointer, which would normally be zero since heap objects are always [wiki:Commentary/Rts/Word word]-aligned.
8	8
	9	== Meaning of the tag bits ==
	10
9	11	The way the tag bits are used depends on the type of object pointed to:
10	12
11		* If the object is a ~~constructor~~, the tag bits contain the ''constructor tag'', if the number of
	13	* If the object is a '''constructor''', the tag bits contain the ''constructor tag'', if the number of
12	14	constructors in the datatype is less than 4 (less than 8 on a 64-bit platform). If the number of
13	15	constructors in the datatype is more than 4 (resp 8), then the tag bits have the value 1, and the constructor tag
14	16	is extracted from the constructor's info table instead.
15	17
16		* If the object is a ~~function~~, the tag bits contain the ''arity'' of the function, if the arity fits
	18	* If the object is a '''function''', the tag bits contain the ''arity'' of the function, if the arity fits
17	19	in the tag bits.
18	20
19	21	* For a pointer to any other object, the tag bits are always zero.
	22
	23	== Optimisations enabled by tag bits ==
20	24
21	25	The presence of tag bits enables certain optimisations: