On 3/27/20 6:07 PM, LIU Zhiwei wrote:
>
>
> On 2020/3/28 8:32, Richard Henderson wrote:
>> 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
>> = 0x0000000100000000
>>
>> Shift that right by 1 and you do indeed get 0x80000000.
>> There's no saturation involved.
>
> The minuend 0x7fffffff is INT32_MAX, and the subtrahend 0x80000000 is
> INT32_MIN.
>
> The difference between the minuend and the subtrahend should be a positive
> number. But the result here is 0x80000000.
>
> So it is overflow. However, according to the spec, it should not overflow.
Unless I'm missing something, the spec is wrong about "there can be no
overflow", the above being a counter-example.
Do you have hardware to compare against? Perhaps it is in fact "overflow is
ignored", as the 0.8 spec says for vasubu?
I wouldn't add saturation, because the spec says nothing about saturation, and
does mention truncation, at least for vasubu.
r~
