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

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Message-ID: <20070508095421.GA10456@linuxtv.org>
Date:	Tue, 8 May 2007 11:54:21 +0200
From:	Johannes Stezenbach <js@...uxtv.org>
To:	Esben Nielsen <nielsen.esben@...glemail.com>
Cc:	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, 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, which is
what gcc does with -fwarpv or -ftrapv.

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.


Johannes
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