Re: [PATCH 06/10] siphash: Use more hygienic state initialiser

[Stefano Brivio <sbrivio@redhat.com>]

On Sat, 23 Sep 2023 00:06:26 +1000
David Gibson <david@gibson.dropbear.id.au> wrote:

> The PREAMBLE macro sets up the SipHash initial internal state.  It also
> defines that state as a variable, which isn't macro hygeinic.  With
> previous changes simplifying this premable, it's now possible to replace it
> with a macro which simply expands to a C initialisedrfor that state.
> 
> Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
> ---
>  siphash.c | 29 ++++++++++++-----------------
>  1 file changed, 12 insertions(+), 17 deletions(-)
> 
> diff --git a/siphash.c b/siphash.c
> index 6932da2..21c560d 100644
> --- a/siphash.c
> +++ b/siphash.c
> @@ -58,15 +58,12 @@
>  
>  #define ROTL(x, b) (uint64_t)(((x) << (b)) | ((x) >> (64 - (b))))
>  
> -#define PREAMBLE							  \
> -	uint64_t v[4] = { 0x736f6d6570736575ULL, 0x646f72616e646f6dULL,	  \
> -			  0x6c7967656e657261ULL, 0x7465646279746573ULL }; \
> -	int __i;							  \
> -									  \
> -	do {								  \
> -		for (__i = sizeof(v) / sizeof(v[0]) - 1; __i >= 0; __i--) \
> -			v[__i] ^= k[__i % 2];				  \
> -	} while (0)
> +#define SIPHASH_INIT(k) {						\
> +		0x736f6d6570736575ULL ^ (k)[0],				\
> +		0x646f72616e646f6dULL ^ (k)[1],				\
> +		0x6c7967656e657261ULL ^ (k)[0],				\
> +		0x7465646279746573ULL ^ (k)[1]				\

I don't think it actually matters (given the rationale for the choice
of these constants given in the paper), but earlier this was equivalent
to:

  0x736f6d6570736575ULL ^ (k)[1],
  0x646f72616e646f6dULL ^ (k)[0],
  0x6c7967656e657261ULL ^ (k)[1],
  0x7465646279746573ULL ^ (k)[0]

and it matched both reference implementations linked in the file
header. Anyway, the paper says:

  ...where k0 and k1 are the little-endian 64-bit words encoding the key
  k.

without giving an order, so I guess either interpretation is fine.

-- 
Stefano