[PATCH 2/3] mpih-const-time: avoid branches in _gcry_mpih_cmp_ui

[Jussi Kivilinna]

Hello,

On 13.2.2025 8.20, NIIBE Yutaka wrote:
> Hello, again for _gcry_mpih_cmp_ui,
> 
> Jussi Kivilinna <jussi.kivilinna at iki.fi> wrote:
>> * mpi/mpih-const-time.c (_gcry_mpih_cmp_ui): Avoid conditional
>> branches for return value selection.
> 
> After the commit of this, I realize that there is a thinko.
> 
> When IS_ALL_ZERO (meaning all the limbs sans the least one are zero)
> is false, the function should return 1 (instead of 0).

Ah, that's right. Here's patch attach with new tests to catch this. I'll push this to master soon.

-Jussi

> 
> Something like this:
> 
> diff --git a/mpi/mpih-const-time.c b/mpi/mpih-const-time.c
> index d8b66c46..a6314932 100644
> --- a/mpi/mpih-const-time.c
> +++ b/mpi/mpih-const-time.c
> @@ -232,5 +232,5 @@ _gcry_mpih_cmp_ui (mpi_ptr_t up, mpi_size_t usize, unsigned long v)
>     for (i = 1; i < usize; i++)
>       is_all_zero &= ct_ulong_gen_mask(mpih_limb_is_zero (up[i]));
>   
> -  return cmp0 & (int)is_all_zero;
> +  return (cmp0 & (int)is_all_zero) | (~is_all_zero & 1);
>   }
> 
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