Re: [Xen-devel] [PATCH 4/4] xen/arm: correctly handle an empty array of platform descs.

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 14.05.13 at 11:32, Ian Campbell <Ian.Campbell@xxxxxxxxxx> wrote:
> Sadly this turns out to be a bit more complicated :-( CCing Jan and Keir
> since this was wider implications (this construct is used elsewhere) and
> Ian J because he pointed this out.
> 
> TL;DR: There is probably a problem here, but I'm not sure it is one
> worth worrying about, at least not yet/for 4.3.
> 
> The problem is that the compiler is allowed to assume that:
>         extern const struct platform_desc _splatform[], _eplatform[];
> Defines two *distinct* arrays, which therefore can never be equal. Hence
> it optimises 
>     for ( platform = _splatform; platform != _eplatform; platform++ )
> with the assumption that there must always be at least one iteration.

Did you actually observe gcc doing this? I agree that the abstract
C specification allows for such an assumption, but with the gcc
itself providing ways to create symbols aliases I would be pretty
surprised if it actually then produces broken code here.

> FWIW this seems to work:
>     for ( platform = &_splatform[0]; platform < &_eplatform[0]; platform++ )
> But I'm not sure it is strictly any less undefined than the original
> code and it might just be obscuring things enough that today's optimiser
> misses a trick.

This surely is as (un)defined as the original code.

Jan


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