Messages in this thread | | | Date | Wed, 4 May 2016 10:40:20 -0400 | Subject | Re: [PATCH 1/3] random: replace non-blocking pool with a Chacha20-based CRNG | From | Jeffrey Walton <> |
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> +static inline u32 rotl32(u32 v, u8 n) > +{ > + return (v << n) | (v >> (sizeof(v) * 8 - n)); > +}
That's undefined behavior when n=0.
I think the portable way to do a rotate that avoids UB is the following. GCC, Clang and ICC recognize the pattern, and emit a rotate instruction.
static const unsigned int MASK=31; return (v<<n)|(v>>(-n&MASK));
You should also avoid the following because its not constant time due to the branch:
return n == 0 ? v : (v << n) | (v >> (sizeof(v) * 8 - n));
Jeff
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