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

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Date:	Mon, 7 May 2007 20:39:31 +0200
From:	Johannes Stezenbach <js@...uxtv.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Esben Nielsen <nielsen.esben@...glemail.com>,
	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 Mon, May 07, 2007, Linus Torvalds wrote:
> On Mon, 7 May 2007, Esben Nielsen wrote:
> > 
> > What is (long)(a-b) ? I have tried to look it up in the C99 standeard but I
> > can't find it. Maybe it is in the referred LIA-1 standeard, which I can't find
> > with google.

C99 defines unsigned overflow semantics, but it doesn't say anything
about signed overflow, thus it's undefined -- and you have a hard
time finding it out.

However, I have no clue *why* it's undefined and not
implementation defined. Does someone know?

> I don't worry about non-2's-complement machines (they don't exist, and 
> likely won't exist in the future either).

I think DSPs can do saturated arithmetics (clamp to min/max
values instead of wrap around). Not that it matters for Linux...

> So I worry about compilers rewriting my code.

gcc has -fwrapv and -ftrapv to change signed integer overflow
behaviour.

One baffling example where gcc rewrites code is when
conditionals depend on signed integer overflow:

$ cat xx.c
#include <assert.h>

int foo(int a)
{
	assert(a + 100 > a);
	return a;
}

int bar(int a)
{
	if (a + 100 > a)
		a += 100;
	return a;
}
$ gcc -Wall -Wextra -fomit-frame-pointer -c xx.c
$ objdump -dr xx.o

xx.o:     file format elf32-i386

Disassembly of section .text:

00000000 <foo>:
   0:   8b 44 24 04             mov    0x4(%esp),%eax
   4:   c3                      ret

00000005 <bar>:
   5:   83 44 24 04 64          addl   $0x64,0x4(%esp)
   a:   8b 44 24 04             mov    0x4(%esp),%eax
   e:   c3                      ret


The assert and the condition were just dropped
by gcc -- without any warning.

gcc-4.2 will add -fstrict-overflow and -Wstrict-overflow.
http://gcc.gnu.org/gcc-4.2/changes.html


Johannes
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