Cleanup for cryptographic algorithms vs. compiler optimizations

Ulrich Spörlein uqs at
Sat Jun 12 15:35:33 UTC 2010

On Fri, 11.06.2010 at 21:37:29 +0200, Dag-Erling Smørgrav wrote:
> Ulrich Spörlein <uqs at> 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 ...


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