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~
>