Cleanup for cryptographic algorithms vs. compiler optimizations
Tijl Coosemans
tijl at coosemans.org
Fri Jun 11 21:04:15 UTC 2010
On Friday 11 June 2010 21:37:29 Dag-Erling Smørgrav wrote:
> Ulrich Spörlein <uqs at 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)
memset can be optimised away as well. The only way is to declare those
variables volatile. None of the assignments below are removed for
instance:
volatile int a = 0; a = 1; a = 2;
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