Re: Alignment of Lisp_Subr

[Miles Bader]

On Wed, Nov 12, 2003 at 11:22:33AM -0500, Stefan Monnier wrote:
> - Structures of type Lisp_Subr are statically allocated and are not
>   guaranteed to be aligned on a multiple of 8.  I currently solve this
>   by adding __attributes__ ((__aligned__ (8))), but this only works for GCC
>   AFAIK.  The only alternative I can think of is to add a padding field
>   to Lisp_Subr and when its address is not a multiple of 8, memmove it
>   by a few bytes (I think `defsubr' is the only place where we refer
>   to these statically allocated Sfoo variables).
>   Any comment on which approach is preferable ?

I think the compiler will force the structure to be aligned to the strictest
alignment of any component, so if you stick an field with 8-byte-alignment in
there _somewhere_, the compiler should do everything for you.  It should even
work to put the `forcing field' in as a member of a union with another field,
and simply never use the forcing field.  On many platforms, `double' could be
used as the type of the forcing-field, but I'm not sure how universal this
is.

Given that subrs are not the most space critical object though, it seems
simplest to just use __attributes__ when compiling with gcc (#ifdef
__GNUC__), and add the padding field when compiling with another compiler.

> - malloc does not guarantee on all systems that the returned value
>   will be a multiple of 8 (or does it?).

It guarantees `most strict' alignment, which on many platforms is 8 bytes
(because of doubles), and I think many malloc implementations are
conservative and assume 8 bytes anyway.  However I don't think you can assume
this though; a quick google shows plenty of people complaining about exactly
this issue (they expected 8-byte alignment, and got less; common culprits
seem to be MS and SCO mallocs... :-)

I guess in the common case where emacs uses its builtin malloc, or glibc's,
you'll have #defines to tell you though...

-Miles
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