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To: Paul Koning <pkoning@equallogic.com>
Cc: ebotcazou@libertysurf.fr, gcc@gcc.gnu.org
Subject: Re: generalized lvalues -- patch outline
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From: Andreas Schwab <schwab@suse.de>
X-Yow: ..  I think I'd better go back to my DESK and toy with
 a few common MISAPPREHENSIONS...
Date: Tue, 23 Nov 2004 17:16:00 -0000
In-Reply-To: <16803.26864.624255.231502@gargle.gargle.HOWL> (Paul Koning's
 message of "Tue, 23 Nov 2004 11:44:32 -0500")
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Paul Koning <pkoning@equallogic.com> writes:

>>>>>> "Eric" == Eric Botcazou <ebotcazou@libertysurf.fr> writes:
>
>  >> The average C programmer would not have thought to use this
>  >> extension unless they just decided one day to try it or they
>  >> actually read the gcc documentation (which I know almost nobody
>  >> does).
>
>  Eric> I disagree: *((int *)p)++ was accepted by virtually all the
>  Eric> good old compilers: GCC 2.x, Microsoft, Borland, Watcom.  This
>  Eric> is a natural idiom when you're manipulating images with
>  Eric> different color depths ...
>
> ... or doing alignment-optimized copying.  I've seen this notation in
> our code, and I'm pretty sure the author didn't get it from the GCC
> docs.  It's a natural thing to write -- unless you ARE a language
> lawyer and understand the hairy details of precisely what is an lvalue
> and what is not.

... and what strict aliasing can do to ruin your code. :-)

Andreas.

-- 
Andreas Schwab, SuSE Labs, schwab@suse.de
SuSE Linux Products GmbH, Maxfeldstraße 5, 90409 Nürnberg, Germany
Key fingerprint = 58CA 54C7 6D53 942B 1756  01D3 44D5 214B 8276 4ED5
"And now for something completely different."