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Date: Sat, 12 Jun 2010 17:35:28 +0200
From: Ulrich =?utf-8?B?U3DDtnJsZWlu?= <uqs@spoerlein.net>
To: Dag-Erling =?utf-8?B?U23DuHJncmF2?= <des@des.no>
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Cc: current@freebsd.org
Subject: Re: Cleanup for cryptographic algorithms vs. compiler optimizations
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On Fri, 11.06.2010 at 21:37:29 +0200, Dag-Erling Smørgrav wrote:
> Ulrich Spörlein <uqs@spoerlein.net> writes:
> > optimizing compilers have a tendency to remove assignments that have
> > no side effects. The code in sys/crypto/sha2/sha2.c is doing a lot of
> > zeroing variables, which is however optimized away.  [...]  Is there a
> > canonical way to zero those variables and should we use them (memset
> > perhaps? what are the performance implications?)
> 
> If you stick these variables in a struct, you can memset the struct to
> zero them; if there are many of them, it may be faster than zeroing them
> individually.
> 
> Alternatively, you can use something like this:
> 
> #define FORCE_ASSIGN(type, var, value) \
>         *(volatile type *)&(var) = (value)

Interesting trick, thanks. I'll try this ...

Uli