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From: =?utf-8?Q?Dag-Erling_Sm=C3=B8rgrav?= <des@des.no>
To: current@freebsd.org
References: <20100611162118.GR39829@acme.spoerlein.net>
Date: Fri, 11 Jun 2010 21:37:29 +0200
In-Reply-To: <20100611162118.GR39829@acme.spoerlein.net> ("Ulrich
	=?utf-8?Q?Sp=C3=B6rlein=22's?=
	message of "Fri, 11 Jun 2010 18:21:18 +0200")
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Cc: 
Subject: Re: Cleanup for cryptographic algorithms vs. compiler optimizations
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Ulrich Sp=C3=B6rlein <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) =3D (value)

DES
--=20
Dag-Erling Sm=C3=B8rgrav - des@des.no