On 3/18/20 8:46 PM, LIU Zhiwei wrote:
+static inline int32_t asub32(CPURISCVState *env, int vxrm, int32_t a, int32_t b)
+ int64_t res = (int64_t)a - b;
+ uint8_t round = get_round(vxrm, res, 1);
+ return (res >> 1) + round;
I find a corner case here. As the spec said in Section 13.2
"There can be no overflow in the result".
If the a is 0x7fffffff, b is 0x80000000, and the round mode is round to up(rnu),
then the result is (0x7fffffff - 0x80000000 + 1) >> 1, equals 0x80000000,
according the v0.7.1
That's why we used int64_t as the intermediate type:
0x000000007fffffff - 0xffffffff80000000 + 1
= 0x000000007fffffff + 0x0000000080000000 + 1
= 0x00000000ffffffff + 1
Shift that right by 1 and you do indeed get 0x80000000.
There's no saturation involved.