linux-kernel - Re: [patch] CFS scheduler, -v8

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Date:	Tue, 8 May 2007 12:27:28 +0200 (CEST)
From:	Esben Nielsen <nielsen.esben@...glemail.com>
To:	Johannes Stezenbach <js@...uxtv.org>
cc:	Esben Nielsen <nielsen.esben@...glemail.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>,
	Balbir Singh <balbir@...ux.vnet.ibm.com>,
	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Con Kolivas <kernel@...ivas.org>,
	Nick Piggin <npiggin@...e.de>, Mike Galbraith <efault@....de>,
	Arjan van de Ven <arjan@...radead.org>,
	Peter Williams <pwil3058@...pond.net.au>,
	Thomas Gleixner <tglx@...utronix.de>, caglar@...dus.org.tr,
	Willy Tarreau <w@....eu>,
	Gene Heskett <gene.heskett@...il.com>, Mark Lord <lkml@....ca>,
	Zach Carter <linux@...hcarter.com>,
	buddabrod <buddabrod@...il.com>
Subject: Re: [patch] CFS scheduler, -v8



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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