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author | Eli Zaretskii <eliz@gnu.org> | 2017-02-10 12:12:49 +0200 |
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committer | Eli Zaretskii <eliz@gnu.org> | 2017-02-10 12:12:49 +0200 |
commit | c54cf8dcfd016783bc4881eb218d7b556020a18c (patch) | |
tree | 2c9ef2be10d4c6132cb3ac63e64939841fe1daed | |
parent | 8b92f8672b5ed6fe38d10654962aa373e8e7bda8 (diff) | |
download | emacs-c54cf8dcfd016783bc4881eb218d7b556020a18c.tar.gz |
Improve commentary in lisp.h
* src/lisp.h: Explain in the comment why enlarging a Lisp_Misc
object is discouraged.
-rw-r--r-- | src/lisp.h | 10 |
1 files changed, 7 insertions, 3 deletions
diff --git a/src/lisp.h b/src/lisp.h index b410ee45aed..5aee80cea11 100644 --- a/src/lisp.h +++ b/src/lisp.h @@ -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). |