On 2020/3/29 0:13, LIU Zhiwei wrote:
>
>
> On 2020/3/28 23:47, Richard Henderson wrote:
>> On 3/28/20 8:17 AM, LIU Zhiwei wrote:
>>>> Missed the improvement here. See tcg_gen_mulsu2_i64.
>>> Though I have not gotten the principle, the code in
>>> tcg_gen_mulsu2_i64 is much
>>> tidier.
>> Let A = signed operand,
>> B = unsigned operand
>> P = unsigned product
>>
>> If the sign bit A is set, then P is too large.
>> In that case we subtract 2**64 * B to fix that:
>>
>> HI_P -= (A < 0 ? B : 0)
>>
>> where the conditional is computed as (A >> 63) & B.
>
> I think I get it.
>
> LET A = 2 ** 64 - X
>
> THEN
>
> X = 2 ** 64 - A
> SIGNED_P = -X * B
>
> if (A * B == P) then
>
> (2 ** 64 - X) * B == P
> 2 **64 * B - X * B == P
>
> -X *B == P - 2**64*B
>
> HI_P -= (A < 0 ? B ：0）
>
It's confusing here. I paste the clearer code.
/*
* Let A = signed operand,
* B = unsigned operand
* P = mulu64(A, B), unsigned product
*
* LET X = 2 ** 64 - A, 2's complement of A
* SP = signed product
* THEN
* IF A < 0
* SP = -X * B
* = -(2 ** 64 - A) * B
* = A * B - 2 ** 64 * B
* = P - 2 ** 64 * B
* ELSE
* SP = P
* THEN
* HI_P -= (A < 0 ? B : 0）
*/
static int64_t do_mulhsu_d(int64_t s2, uint64_t s1)
{
uint64_t hi_64, lo_64;
mulu64(&lo_64, &hi_64, s2, s1);
hi_64 -= s2 < 0 ? s1 : 0;
return hi_64;
}
Zhiwei
> Zhiwei
>>
>> r~
>