On Wed, Nov 8, 2017 at 10:39 AM, Richard Sandiford <richard.sandif...@linaro.org> wrote: > 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?
Not worth re-posting IMHO. Richard. > Thanks, > Richard