Re: [patch] CFS scheduler, -v8

[Esben Nielsen <nielsen.esben@googlemail.com>]

On Tue, 8 May 2007, Johannes Stezenbach wrote:

> On Tue, May 08, 2007, Esben Nielsen wrote:
> > This is contrary to C99 standeard annex H2.2
> > (http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1124.pdf):
> >
> > "An implementation that defines signed integer types as also being modulo
> > need
> > not detect integer overflow, in which case, only integer divide-by-zero need
> > be detected."
> >
> > So if it doesn't properly defines wrapping it has to detect integer
> > overflow, right?
>
> No. Annex H (informative!) only talks about LIA-1 conformance.
>
> C99 isn't LIA-1 conformant. H2.2 describes what an implementation
> might do to make signed integers LIA-1 compatible.

"The signed C integer types int, long int, long long int, and the 
corresponding unsigned types are compatible with LIA-1."

I read this as any C99 implementation must be compatible. I would like to 
see LIA-1 to check.


> , which is
> what gcc does with -fwarpv or -ftrapv.

Yes, either or: Either wrap or trap.

> At least that's how I understand it, the C99 standard
> seems to have been written with the "it was hard to
> write, so it should be hard to read" mindset. :-/
>
> I still don't know _why_ signed integer overflow behaviour
> isn't defined in C. It just goes against everyones expectation
> and thus causes bugs.

Because it is hard to make wrapping work on non twos complement 
architectures. Then it is easier to trap.

Esben

> Johannes
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