On Wed, Oct 25, 2017 at 1:26 PM, Richard Sandiford
<richard.sandif...@linaro.org> wrote:
> Richard Biener <richard.guent...@gmail.com> writes:
>> On Tue, Oct 24, 2017 at 3:24 PM, Richard Sandiford
>> <richard.sandif...@linaro.org> wrote:
>>> Richard Biener <richard.guent...@gmail.com> writes:
>>>> On Tue, Oct 24, 2017 at 2:48 PM, Richard Sandiford
>>>> <richard.sandif...@linaro.org> wrote:
>>>>> Richard Biener <richard.guent...@gmail.com> writes:
>>>>>> On Tue, Oct 24, 2017 at 1:23 PM, Richard Sandiford
>>>>>> <richard.sandif...@linaro.org> wrote:
>>>>>>> Eric Botcazou <ebotca...@adacore.com> writes:
>>>>>>>>> Yeah. E.g. for ==, the two options would be:
>>>>>>>>>
>>>>>>>>> a) must_eq (a, b) -> a == b
>>>>>>>>> must_ne (a, b) -> a != b
>>>>>>>>>
>>>>>>>>> which has the weird property that (a == b) != (!(a != b))
>>>>>>>>>
>>>>>>>>> b) must_eq (a, b) -> a == b
>>>>>>>>> may_ne (a, b) -> a != b
>>>>>>>>>
>>>>>>>>> which has the weird property that a can be equal to b when a != b
>>>>>>>>
>>>>>>>> Yes, a) was the one I had in mind, i.e. the traditional operators are
>>>>>>>> the must
>>>>>>>> variants and you use an outer ! in order to express the may. Of
>>>>>>>> course this
>>>>>>>> would require a bit of discipline but, on the other hand, if most of
>>>>>>>> the cases
>>>>>>>> fall in the must category, that could be less ugly.
>>>>>>>
>>>>>>> I just think that discipline is going to be hard to maintain in
>>>>>>> practice,
>>>>>>> since it's so natural to assume (a == b || a != b) == true. With the
>>>>>>> may/must approach, static type checking forces the issue.
>>>>>>>
>>>>>>>>> Sorry about that. It's the best I could come up with without losing
>>>>>>>>> the may/must distinction.
>>>>>>>>
>>>>>>>> Which variant is known_zero though? Must or may?
>>>>>>>
>>>>>>> must. maybe_nonzero is the may version.
>>>>>>
>>>>>> Can you rename known_zero to must_be_zero then?
>>>>>
>>>>> That'd be OK with me.
>>>>>
>>>>> Another alternative I wondered about was must_eq_0 / may_ne_0.
>>>>>
>>>>>> What's wrong with must_eq (X, 0) / may_eq (X, 0) btw?
>>>>>
>>>>> must_eq (X, 0) generated a warning if X is unsigned, so sometimes you'd
>>>>> need must_eq (X, 0) and sometimes must_eq (X, 0U).
>>>>
>>>> Is that because they are templates? Maybe providing a partial
>>>> specialization
>>>> would help?
>>>
>>> I don't think it's templates specifically. We end up with something like:
>>>
>>> int f (unsigned int x, const int y)
>>> {
>>> return x != y;
>>> }
>>>
>>> int g (unsigned int x) { return f (x, 0); }
>>>
>>> which generates a warning too.
>>>
>>>> I'd be fine with must_eq_p and may_eq_0.
>>>
>>> OK, I'll switch to that if there are no objections.
>>
>> Hum. But then we still warn for must_eq_p (x, 1), no?
>
> Yeah. The patch also had a known_one and known_all_ones for
> those two (fairly) common cases. For other values the patches
> just add "U" where necessary.
>
> If you think it would be better to use U consistently and not
> have the helpers, then I'm happy to do that instead.
>
>> So why does
>>
>> int f (unsigned int x)
>> {
>> return x != 0;
>> }
>>
>> not warn? Probably because of promotion of the arg.
>
> [Jakub's already answered this part.]
>
>> Shouldn't we then simply never have a may/must_*_p (T1, T2)
>> with T1 and T2 being not compatible? That is, force promotion
>> rules on them with template magic?
>
> This was what I meant by:
>
> Or we could suppress warnings by forcibly converting the input.
> Sometimes the warnings are useful though.
>
> We already do this kind of conversion for arithmetic, to ensure
> that poly_uint16 + poly_uint16 -> poly_int64 promotes before the
> addition rather than after it. But it defeats the point of the
> comparison warning, which is that you're potentially redefining
> the sign bit.
>
> I think the warning's just as valuable for may/must comparison of
> non-literals as it is for normal comparison operators. It's just
> unfortunate that we no longer get the special handling of literals.
Ok, I see.
I think I have a slight preference for using 0U consistently but I haven't
looked at too many patches yet to see how common/ugly that would be.
Richard.
> Thanks,
> Richard
