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Date: Tue, 11 Feb 2003 11:24:38 +0100
From: Helge Hafting <helgehaf@aitel.hist.no>
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To: Horst von Brand <brand@jupiter.cs.uni-dortmund.de>
CC: linux-kernel@vger.kernel.org, Art Haas <ahaas@airmail.net>
Subject: Re: Is -fno-strict-aliasing still needed?
References: <200302110714.h1B7EA3A006209@eeyore.valparaiso.cl>
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Horst von Brand wrote:
> 
> Art Haas <ahaas@airmail.net> said:
> > I ask because I've just built a kernel without using that flag -
> > linus-2.5 BK from this morning, probably missing the 2.5.60 release by
> > a few hours.
> 
> The problem with strict aliasing is that it allows the compiler to assume
> that in:
> 
>      void somefunc(int *foo, int *bar)
> 
> foo and bar will _*never*_ point to the same memory area (at the same
> struct, or into the same array, etc). There is no way to check for this in
> the compiler in general (the function and the call might be in different
> files, many functions are being called via pointers, ...).

I though pointers to the same type, such as int *, could alias still.

But pointers to different types, such as int* and short* is assumed
to never clash with strict aliasing.  And this bites linux
because it sometimes choose to see "two adjacent shorts as one int" for
performance reasons.  

I remember the flag was introduced because some IP or TCP
code do exactly this, and converting it all to unions would
render that code unreadable.

Helge Hafting
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