Re: [PATCH v6 8/9] compiler.h, bug.h: Prevent double error messages with BUILD_BUG{,_ON}

Borislav Petkov
2012-11-22T19:07:30Z

On Tue, Nov 20, 2012 at 03:05:06PM -0600, danielfsantos@att.net wrote:
> Prior to the introduction of __attribute__((error("msg"))) in gcc 4.3,
> creating compile-time errors required a little trickery.
> BUILD_BUG{,_ON} uses this attribute when available to generate
> compile-time errors, but also uses the negative-sized array trick for
> older compilers, resulting in two error messages in some cases. The
> reason it's "some" cases is that as of gcc 4.4, the negative-sized array
> will not create an error in some situations, like inline functions.
>
> This patch replaces the negative-sized array code with the new
> __compiletime_error_fallback() macro which expands to the same thing
> unless the the error attribute is available, in which case it expands to
> do{}while(0), resulting in exactly one compile-time error on all
> versions of gcc.
>
> Note that we are not changing the negative-sized array code for the
> unoptimized version of BUILD_BUG_ON, since it has the potential to catch
> problems that would be disabled in later versions of gcc were
> __compiletime_error_fallback used. The reason is that that an
> unoptimized build can't always remove calls to an error-attributed
> function call (like we are using) that should effectively become dead
> code if it were optimized. However, using a negative-sized array with a
> similar value will not result in an false-positive (error). The only
> caveat being that it will also fail to catch valid conditions, which we
> should be expecting in an unoptimized build anyway.
>
> Signed-off-by: Daniel Santos <daniel.santos@pobox.com>

Acked-by: Borislav Petkov <bp@alien8.de>

--
Regards/Gruss,
Boris.