From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-ed1-x529.google.com (mail-ed1-x529.google.com [IPv6:2a00:1450:4864:20::529]) by sourceware.org (Postfix) with ESMTPS id 1B6283858C39 for ; Fri, 10 Sep 2021 10:53:50 +0000 (GMT) DMARC-Filter: OpenDMARC Filter v1.4.1 sourceware.org 1B6283858C39 Received: by mail-ed1-x529.google.com with SMTP id q3so1876639edt.5 for ; Fri, 10 Sep 2021 03:53:50 -0700 (PDT) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=x-gm-message-state:mime-version:references:in-reply-to:from:date :message-id:subject:to:cc; bh=a4p7chsPIBZ7tHRU5ecmVKgSWW1q2MJzYT9WGoN0NcI=; b=IdCDXPkhhNWm4U40ZFCszD5UkQITUShXIEULaf39wrV0MPKGqYIfgEkzH9dLdBwdpa FFVfbSyVQMutQJ+8haevCSgc/dkrpAco9RdrQ0gwU43b1ySrv2R+YYAJ/yONadfHT7/l RfLVqeO5hlpONWEXUbR7DC/tTKXreXlA4ZA3+o/mtJYW1lSeBezbiZ9XGCK5iDYVlejH gXYI88EPhTQ78buc06rEgbtXJh0N8b2tV4JAc083m1iVM/Dd7S9X6EwYX7cRAzakie3a f/yaazrKWA7tgj7CDHV58V3NUMjaTXmm0fQ7LMhYSUiEJz41QLz1RkEmcWNYb7OQPMYa NoCw== X-Gm-Message-State: AOAM531H7Idr6iofhetXOVv+ceC8zUrfAf1SqSONWVwsFD+DR7VvBg/V HwuxFYDab1NBbS5efV0rpJwCVdgydo+yZlGJzcM= X-Google-Smtp-Source: ABdhPJzbCVB93Dt8TqthuUrVysRVg1nZyjoQhB144meF+HPfVohV2zmd1sdCjoanFWpJFN8jDmGXtIl0ihuM8zt/4wA= X-Received: by 2002:a05:6402:40cf:: with SMTP id z15mr8188296edb.70.1631271228503; Fri, 10 Sep 2021 03:53:48 -0700 (PDT) MIME-Version: 1.0 References: <20210907230730.GM1583@gate.crashing.org> <20210908170809.GP1583@gate.crashing.org> <5DBC1101-9DD6-48F8-BC25-F4DD354B4D74@gmail.com> <20210908191602.GQ1583@gate.crashing.org> <20210909234832.GU1583@gate.crashing.org> In-Reply-To: <20210909234832.GU1583@gate.crashing.org> From: Richard Biener Date: Fri, 10 Sep 2021 12:53:37 +0200 Message-ID: Subject: Re: [PATCH] Fix SFmode subreg of DImode and TImode To: Segher Boessenkool Cc: Michael Meissner , GCC Patches , David Edelsohn , Bill Schmidt , Peter Bergner , Will Schmidt Content-Type: text/plain; charset="UTF-8" X-Spam-Status: No, score=-2.8 required=5.0 tests=BAYES_00, DKIM_SIGNED, DKIM_VALID, DKIM_VALID_AU, DKIM_VALID_EF, FREEMAIL_FROM, RCVD_IN_DNSWL_NONE, SPF_HELO_NONE, SPF_PASS, TXREP autolearn=ham autolearn_force=no version=3.4.4 X-Spam-Checker-Version: SpamAssassin 3.4.4 (2020-01-24) on server2.sourceware.org X-BeenThere: gcc-patches@gcc.gnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Gcc-patches mailing list List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Fri, 10 Sep 2021 10:53:51 -0000 On Fri, Sep 10, 2021 at 1:50 AM Segher Boessenkool wrote: > > On Thu, Sep 09, 2021 at 08:16:16AM +0200, Richard Biener wrote: > > > I think we should (longer term) get rid of the overloaded meanings and > > > uses of subregs. One fairly simple thing is to make a new rtx code > > > "bit_cast" (or is there a nice short more traditional name for it?) > > > > But subreg _is_ bit_cast. > > It is not. (subreg:M (reg:N) O) for O>0, little-endian, is not a > bit_cast. It is taking a part of a register, or a single register from > a multi-register thing. Paradoxicals are not bit-casts either. > > Subregs from or to (but not both) integer modes are generally bit_cast, > yeah. > > > What is odd to me is that a "disallowed" subreg > > like (subreg:SF (reg:TI ..) 0) magically becomes valid (in terms of > > validate_subreg) if you rewrite it as (subreg:SF (subreg:SI (reg:TI ..) 0) 0). > > Of course that's nested and invalid but just push the inner subreg to a > > new pseudo and the thing becomes valid. > > Bingo. > > And many targets have strange rules for bit-strings in which modes can > be used as bit-strings in which other modes, and at what offsets in > which registers. Now perhaps none of that is optimal (I bet it isn't), > but changing this without a transition plan simply does not work. But we _do_ already allow some of them :/ Like /* ??? Similarly, e.g. with (subreg:DF (reg:TI)). Though store_bit_field is the culprit here, and not the backends. */ else if (known_ge (osize, regsize) && known_ge (isize, osize)) ; so for the special case where 'regsize' matches osize it would be a bit-cast of a full register from int to float. But as written it also allows (subreg:XF (reg:TI)) which will likely wreck havoc? Similar for the omode == word_mode check which allows (subreg:DI (reg:TF ..)). That is, the existing special-cases look too broad to me - and they probably exist because when validate_subreg rejects sth then we can't put it together later when expand split it into two subregs and a pseudo ... > > > But that is not the core problem we had here. The behaviour of the > > > generic parts of the compiler was changed, without testing if that > > > works on other targets but x86. That is an understandable mistake, it > > > takes some experience to know where the morasses are. But this change > > > should have been accompanied by testcases exercising the changed code. > > > We would have clearly seen there are issues then, simply by watching > > > gcc-testresults@ (and/or maintainers are on top of the test results > > > anyway). Also, if there were testcases for this, we could have some > > > confidence that a change in this area is robust. > > > > Well, that only works if some maintainers that are familiar enough > > with all this chime in ;) > > Not really. It works always. And it works way better than the > pandemonium we now have with broken targets left and right. > > With testcases anyone can see if any specific target is broken here. > > > It's stage1 so it's understandable that some > > people (like me ...) are tyring to help people making progress even > > if that involves trying to decipher 30 years of GCC history in this > > area (without much success in the end as we see) ;) > > Yeah :-) And my thanks to you and everyone involved for tackling this > problematic part of GCC, which has been neglected and patched over for > way too long. But from that same history it follows that anything you > do not super carefully (with testing everywhere) will cause some serious > problems. And nonse of these are easy to fix at all -- there is a > *reason* targets did this nastiness. > > > > p.s. Very unrelated... Should we have __builtin_bit_cast for C as well? > > > Is there any reason this could not work? > > Still interested in this btw :-) (And still very unrelated.) Sure, why not ... Richard. > > Segher