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Re: [RFC, PATCH] shrink struct vectorlike_header #2
From: |
Andreas Rottmann |
Subject: |
Re: [RFC, PATCH] shrink struct vectorlike_header #2 |
Date: |
Fri, 09 Nov 2012 19:04:42 +0100 |
User-agent: |
Gnus/5.13 (Gnus v5.13) Emacs/24.3.50 (gnu/linux) |
Nix <address@hidden> writes:
> On 8 Nov 2012, Paul Eggert spake thusly:
>> But better yet would be to omit the casts entirely.
>> This might let GCC do more optimizations safely. That is,
>> change struct Lisp_Vector to be something like this:
>>
>> struct Lisp_Vector
>> {
>> struct vectorlike_header header;
>> union
>> {
>> Lisp_Object contents[1];
>> struct Lisp_Vector *next;
>> } u;
>> };
>>
>> Replace all current uses of 'contents'
>> with 'u.contents', and then use u.next
>> when you want to use the memory as a next field.
>
> It's a shame we can't use an unnamed union here:
>
> struct Lisp_Vector
> {
> struct vectorlike_header header;
> union
> {
> Lisp_Object contents[1];
> struct Lisp_Vector *next;
> };
> };
>
> No changes to source necessary.
>
> Downside: GCC extension :( which probably rules it out unless wrapped in
> compiler-specific macro trickery, and if you're doing that you might as
> well just unconditionally use a named union.
>
At least according to Wikipedia[0], anonymous unions and structs are
part of C11, so they're not compiler-specific any longer. However,
given the newness of C11, similiar considerations regarding portability
to non-GCC compilers may still apply.
[0] http://en.wikipedia.org/wiki/C11_(C_standard_revision)
Regards, Rotty
--
Andreas Rottmann -- <http://rotty.xx.vu/>
- Re: [RFC, PATCH] shrink struct vectorlike_header #2, (continued)
- Re: [RFC, PATCH] shrink struct vectorlike_header #2, Dmitry Antipov, 2012/11/07
- Re: [RFC, PATCH] shrink struct vectorlike_header #2, Stefan Monnier, 2012/11/07
- Re: [RFC, PATCH] shrink struct vectorlike_header #2, Paul Eggert, 2012/11/08
- Re: [RFC, PATCH] shrink struct vectorlike_header #2, Dmitry Antipov, 2012/11/08
- Re: [RFC, PATCH] shrink struct vectorlike_header #2, Stefan Monnier, 2012/11/08
- Re: [RFC, PATCH] shrink struct vectorlike_header #2, Dmitry Antipov, 2012/11/08
- Re: [RFC, PATCH] shrink struct vectorlike_header #2, Dmitry Antipov, 2012/11/08
- Re: [RFC, PATCH] shrink struct vectorlike_header #2, Paul Eggert, 2012/11/08
- Re: [RFC, PATCH] shrink struct vectorlike_header #2, Andreas Schwab, 2012/11/08
- Re: [RFC, PATCH] shrink struct vectorlike_header #2, Nix, 2012/11/08
- Re: [RFC, PATCH] shrink struct vectorlike_header #2,
Andreas Rottmann <=
- Re: [RFC, PATCH] shrink struct vectorlike_header #2, Stephen J. Turnbull, 2012/11/08
Re: [RFC, PATCH] shrink struct vectorlike_header #2, Paul Eggert, 2012/11/06