Commit 7e24039a authored by Stefan Monnier's avatar Stefan Monnier

* src/lisp.h: Add explanation in a comment

parent 321b06b8
......@@ -866,7 +866,16 @@ union vectorlike_header
Current layout limits the pseudovectors to 63 PVEC_xxx subtypes,
4095 Lisp_Objects in GC-ed area and 4095 word-sized other slots. */
ptrdiff_t size;
/* Align the union so that there is no padding after it. */
/* Align the union so that there is no padding after it.
This is needed for the following reason:
If the alignment constraint of Lisp_Object is greater than the size of
vectorlike_header (e.g. with-wide-int), vectorlike objects which have
0 Lisp_Object fields and whose 1st field has a smaller alignment
constraint than Lisp_Object may end up with their 1st field "before
pseudovector index 0", in which case PSEUDOVECSIZE will return
a "negative" number. We could fix PSEUDOVECSIZE, but it's easier to
just force rounding up the size of vectorlike_header to the alignment
of Lisp_Object. */
Lisp_Object align;
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