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Improve commentary in lisp.h

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* src/lisp.h: Explain in the comment why enlarging a Lisp_Misc
object is discouraged.
This commit is contained in:
Eli Zaretskii 2017-02-10 12:12:49 +02:00
parent 8b92f8672b
commit c54cf8dcfd
1 changed files with 7 additions and 3 deletions
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src/lisp.h
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@ -511,10 +511,14 @@ enum Lisp_Fwd_Type
to add a new Lisp_Misc, extend the Lisp_Misc_Type enumeration.
For a Lisp_Misc, you will also need to add your entry to union
Lisp_Misc (but make sure the first word has the same structure as
Lisp_Misc, but make sure the first word has the same structure as
the others, starting with a 16-bit member of the Lisp_Misc_Type
enumeration and a 1-bit GC markbit) and make sure the overall size
of the union is not increased by your addition.
enumeration and a 1-bit GC markbit. Also make sure the overall
size of the union is not increased by your addition. The latter
requirement is to keep Lisp_Misc objects small enough, so they
are handled faster: since all Lisp_Misc types use the same space,
enlarging any of them will affect all the rest. If you really
need a larger object, it is best to use Lisp_Vectorlike instead.
For a new pseudovector, it's highly desirable to limit the size
of your data type by VBLOCK_BYTES_MAX bytes (defined in alloc.c).