Re: [patch] CFS scheduler, -v8

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

On Mon, 7 May 2007, Johannes Stezenbach wrote:

> 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

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?

gcc does niether with that optimization :-(

Esben
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