linux-kernel - Re: [PATCH] utilize _Static_assert() for BUILD_BUG

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Date:	Thu, 13 Dec 2012 15:20:13 -0600
From:	Daniel Santos <danielfsantos@....net>
To:	Jan Beulich <JBeulich@...e.com>
CC:	Daniel Santos <daniel.santos@...ox.com>,
	Rusty Russell <rusty@...tcorp.com.au>, david.daney@...ium.com,
	kosaki.motohiro@...fujitsu.com,
	Andrew Morton <akpm@...ux-foundation.org>,
	Arjan van de Ven <arjan@...ux.intel.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] utilize _Static_assert() for BUILD_BUG_ON() when the
 compiler supports it

On 12/13/2012 03:43 AM, Jan Beulich wrote:
>>>> On 13.12.12 at 01:29, Daniel Santos<danielfsantos@....net>  wrote:
>> Wow, it's really easy to miss parallel development on the same issue.
>> Sorry for my late response to this thread.  I started another thread
>> addressing these issues (as well as a few others) back in September
>> (https://lkml.org/lkml/2012/9/28/1136).  I've finally gotten ACKs from
>> maintainers with v6 of the patches (here
>> https://lkml.org/lkml/2012/11/20/621) and I'm just waiting for 3.8-rc1
>> to re-submit them.  I actually submitted these patches back in June as
>> part of a larger patch set, but broke it apart in September (I had way
>> to many changes for one patch set)
> Since yours is apparently ready to go in, but doesn't use
> _Static_assert, I guess I'll wait for it to appear until I re-work
> whatever might be left to actually make use of _Static_assert.
>
> Jan
>
Interesting! They've enabled it by default (I suppose as an extension?) 
in every standard (except -pedantic of course). One minor draw-back is 
that it appears to enjoy escaping tickmarks in the error message.  I've 
opened a bug for it (http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55678)

But realistically, the "compiletime_assert" macro I wrote in compiler.h 
can be renamed to "static_assert", analogous to C11's static_assert from 
assert.h (§7.2 of C11) and it can expand to the _Static_assert keyword, 
when that is available.

Something else that I didn't consider too much before was support for 
compiling -O0 or -O1, which will cause many expressions that are 
otherwise evaluated as compile-time constants to become non-constant and 
result in failed assertions.  This isn't anything new however, building 
-O0 has been broken for quite some time, but I presume it could help 
some development of out of tree modules.

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