On Thu, 2025-01-30 at 13:23 +0200, Dimitar Kanaliev wrote:
Hi Dimitar,
[...]
> +struct tnum tnum_scast(struct tnum a, u8 size)
> +{
> + u64 s = size * 8 - 1;
> + u64 sign_mask;
> + u64 value_mask;
> + u64 new_value, new_mask;
> + u64 sign_bit_unknown, sign_bit_value;
> + u64 mask;
> +
> + if (size >= 8) {
> + return a;
> + }
> +
> + sign_mask = 1ULL << s;
> + value_mask = (1ULL << (s + 1)) - 1;
> +
> + new_value = a.value & value_mask;
> + new_mask = a.mask & value_mask;
> +
> + sign_bit_unknown = (a.mask >> s) & 1;
> + sign_bit_value = (a.value >> s) & 1;
> +
> + mask = ~value_mask;
> + new_mask |= mask & (0 - sign_bit_unknown);
> + new_value |= mask & (0 - ((sign_bit_unknown ^ 1) & sign_bit_value));
> +
> + return TNUM(new_value, new_mask);
> +}
So, effectively what you want to achieve here:
- pick a sign bit SM from mask and set signed extended bits of mask to SM
- pick a sign bit SV from value and set signed extended bits of value to SV
right?
I think this could be done a bit simpler, e.g.:
struct tnum tnum_scast(struct tnum a, u8 size)
{
u8 s = 64 - size * 8;
u64 value, mask;
if (size >= 8)
return a;
value = ((s64)a.value << s) >> s;
mask = ((s64)a.mask << s) >> s;
return TNUM(value, mask);
}
wdyt?
