Re: [Tarantool-patches] [PATCH 2/2] uuid: fix unaligned memory access

[Aleksandr Lyapunov <alyapunov@tarantool.org>]

Thanks for the patch! see my comment below.

On 5/16/20 2:03 AM, Vladislav Shpilevoy wrote:
>   tt_uuid_is_nil(const struct tt_uuid *uu)
>   {
> -	const uint64_t *p = (const uint64_t *) uu;
> -	return !p[0] && !p[1];
> +	const uint32_t *p = (const uint32_t *) uu;
> +	return p[0] == 0 && p[1] == 0 && p[2] == 0 && p[3] == 0;
>   }
>   
>   /**
> @@ -172,9 +172,10 @@ tt_uuid_is_nil(const struct tt_uuid *uu)
>   inline bool
>   tt_uuid_is_equal(const struct tt_uuid *lhs, const struct tt_uuid *rhs)
>   {
> -	const uint64_t *lp = (const uint64_t *) lhs;
> -	const uint64_t *rp = (const uint64_t *) rhs;
> -	return lp[0] == rp[0] && lp[1] == rp[1];
> +	const uint32_t *lp = (const uint32_t *) lhs;
> +	const uint32_t *rp = (const uint32_t *) rhs;
> +	return lp[0] == rp[0] && lp[1] == rp[1] && lp[2] == rp[2] &&
> +	       lp[3] == rp[3];

It seems that we degrade performance just for clang to be happy..
I would suggest to use memcmp in this case.
It's portable and allows a compiler to generate the best possible code.
I've measured it (gcc) and memcmp version is twice faster than your 
solution.
Even for _is_nil method it's better to use memcmp with statically 
allocated zero buffer.