Richard Biener <richard.guent...@gmail.com> writes: > 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.
OK. FWIW, that's also how we had it until very recently. I added the known/maybe stuff in a late and probably misguided attempt to make things prettier. I've pulled that part out and switched back to using U. I'll post the new 001 patch in a sec. Should I repost all the other patches that changed as well, or isn't it worth it for a change like that? Thanks, Richard