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Date: Mon, 7 May 2007 20:39:31 +0200
From: Johannes Stezenbach <js@linuxtv.org>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Esben Nielsen <nielsen.esben@googlemail.com>, Ingo Molnar <mingo@elte.hu>,
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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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