From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (qmail 102351 invoked by alias); 25 Oct 2017 13:08:29 -0000 Mailing-List: contact gcc-patches-help@gcc.gnu.org; run by ezmlm Precedence: bulk List-Id: List-Archive: List-Post: List-Help: Sender: gcc-patches-owner@gcc.gnu.org Received: (qmail 101239 invoked by uid 89); 25 Oct 2017 13:08:28 -0000 Authentication-Results: sourceware.org; auth=none X-Virus-Found: No X-Spam-SWARE-Status: No, score=-1.6 required=5.0 tests=AWL,BAYES_00,FREEMAIL_FROM,RCVD_IN_DNSWL_NONE,RCVD_IN_SORBS_SPAM,SPF_PASS autolearn=no version=3.3.2 spammy= X-HELO: mail-wm0-f44.google.com Received: from mail-wm0-f44.google.com (HELO mail-wm0-f44.google.com) (74.125.82.44) by sourceware.org (qpsmtpd/0.93/v0.84-503-g423c35a) with ESMTP; Wed, 25 Oct 2017 13:08:27 +0000 Received: by mail-wm0-f44.google.com with SMTP id b189so1754890wmd.4 for ; Wed, 25 Oct 2017 06:08:26 -0700 (PDT) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:mime-version:in-reply-to:references:from:date :message-id:subject:to; bh=zOEOKeqTKhsBN/CIWX7R4V4nVJRRhjDz+P9MKJWoE4E=; b=DtOBmRBDibSOglPRWQa97/0L/OBzghdnTbKofuDDT+ZUCsAgyZofxfdrC85/oljGh5 YPq7j4EJERCapbgMw63VqZb9ipfq2eWzH66wHANa60kS4TEkpxm+LbjUugJ9pC4zTIMr 627vnJN5Fv+T08yqGecGkrQHNpRq343l3Ziii1UARfe5EydZ4aAwiFFuZv0jmfQSGt/v SlEQcWQzl0IVjGSUgV8psctClY+syKMF3gUdgyo9hLBLysBB8f4O07xERx5R22kVuhy8 xG0TU5vlu11XgCONnae3XCYPO0QYoQBmrHp5Zey2GRgQBqIF8hv/4HwZrVhStXCEHx2m aJlA== X-Gm-Message-State: AMCzsaXMclxvwJqoGHWe9M3E9VNjU17IGE9SP79A8BpzTze3YPXUCWqf I9Zdi/G4VGmk3Y+8uIjSh5VxzP9Wp8Hhl2TfrZp0Gw== X-Google-Smtp-Source: ABhQp+SGbDNgrhKzpLcLYbwZ+mydNuJrkFI59j4mbRTXnH9tns9XOktObKldVIPMeK3h6gFaSuc7axp6Sjy+K7AMkqw= X-Received: by 10.80.208.222 with SMTP id g30mr23540466edf.246.1508936904835; Wed, 25 Oct 2017 06:08:24 -0700 (PDT) MIME-Version: 1.0 Received: by 10.80.143.34 with HTTP; Wed, 25 Oct 2017 06:08:24 -0700 (PDT) In-Reply-To: <87she7tqmo.fsf@linaro.org> References: <871sltvm7r.fsf@linaro.org> <4728974.295PUQgt1k@polaris> <87o9owq35v.fsf@linaro.org> <10524871.8B8OuvVQlb@polaris> <87fua8pz6v.fsf@linaro.org> <87bmkwpv8j.fsf@linaro.org> <877evkptkr.fsf@linaro.org> <87she7tqmo.fsf@linaro.org> From: Richard Biener Date: Wed, 25 Oct 2017 13:09:00 -0000 Message-ID: Subject: Re: [000/nnn] poly_int: representation of runtime offsets and sizes To: Richard Biener , Eric Botcazou , GCC Patches , Richard Sandiford Content-Type: text/plain; charset="UTF-8" X-IsSubscribed: yes X-SW-Source: 2017-10/txt/msg01806.txt.bz2 On Wed, Oct 25, 2017 at 1:26 PM, Richard Sandiford wrote: > Richard Biener writes: >> On Tue, Oct 24, 2017 at 3:24 PM, Richard Sandiford >> wrote: >>> Richard Biener writes: >>>> On Tue, Oct 24, 2017 at 2:48 PM, Richard Sandiford >>>> wrote: >>>>> Richard Biener writes: >>>>>> On Tue, Oct 24, 2017 at 1:23 PM, Richard Sandiford >>>>>> wrote: >>>>>>> Eric Botcazou 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