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~
