Re: [PATCH v6 18/61] target/riscv: vector single-width integer multiply instructions

[LIU Zhiwei]

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）

Zhiwei
> r~