Re: test-bitrotate.c missing test cases

[Bruno Haible]

Hi Jeffrey,

> It looks like test-bitrotate.c is missing test cases. It is missing
> the 32-bit rotl and rotr of 0-bits.
> 
> The 0-bit rotate should tickle undefined behavior.
> 
> If you want to clear the undefined behavior, then use this code. ...

The functions are specified in bitrotate.h, e.g. like this:

/* Given an unsigned 64-bit argument X, return the value corresponding
   to rotating the bits N steps to the left.  N must be between 1 and
   63 inclusive. */
BITROTATE_INLINE uint64_t
rotl64 (uint64_t x, int n)

I think it is on purpose that N = 0 and N = 64 are not allowed. Namely,
when N = 0 or N = 64, you would have a different, more efficient code
branch anyway.

> It will be compiled down to a single instruction on platforms like IA-32.

Yes, this is the intent. And we should help the compiler produce good
code, for example by adding statements like
   assume (n > 0 && n < 64);
Allowing N = 0 or N = 64 goes backwards, because on some platforms it
will prevent the compiler from choosing the best possible instruction.

Bruno