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From: Simon Wright <simon@pogner.demon.co.uk>
Subject: Re: Not intended for use in medical,
Date: 1997/05/03
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Date: 1997-05-03T00:00:00+00:00
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kaz@vision.crest.nt.com (Kaz Kylheku) writes:

> int foo(int x, int y)
> 
> {
> 	return ((x < y) && x++) && ((x < y) && x++);
> }
[...]
> /*
>  * GCC seems to illegaly optimize foo() by factoring the (x < y)
>  * as a common subexpression ignoring the x++ side effect, e.g:
>  *
>  *     return ((x < y) && x++) && x++;
>  *
>  * A more complex version of this expression has caused a serious
>  * failure in some of my code.
>  */
[...]
> I readily discovered this problem when my program was behaving oddly, and
> a thorough code inspection failed to turn up any error in the source.

Hmm. I believe that writing eg

  a[i] = ++i;

invokes undefined behaviour (is the pre- or post- value of i used for
the index operation?).

While inspecting your program, I would be fairly sure of the meaning
you intended but quite unsure without checking the standard as to
whether it had entered the realm of the undefined. And I see that your
original code was _more_ complex! Crikey!

I would also wonder why the second ++ was there anyway, it has no
effect (unless of course x was a reference, but we're talking C here,
and then the effect of the code would be amazingly hard to understand)

-- 
Simon Wright                        Work Email: simon.j.wright@gecm.com
GEC-Marconi Radar & Defence Systems            Voice: +44(0)1705-701778
Command & Information Systems Divsion            FAX: +44(0)1705-701800