From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-vs1-xe34.google.com (mail-vs1-xe34.google.com [IPv6:2607:f8b0:4864:20::e34]) by sourceware.org (Postfix) with ESMTPS id 4D6033857817 for ; Thu, 9 Sep 2021 01:26:11 +0000 (GMT) DMARC-Filter: OpenDMARC Filter v1.4.1 sourceware.org 4D6033857817 Received: by mail-vs1-xe34.google.com with SMTP id a25so199142vso.5 for ; Wed, 08 Sep 2021 18:26:11 -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=rSZUBJTojyfTdQY6uTHIs5YnY0337wpCcJCMsPX21Ww=; b=ymB+h/YSu8roxMQ1xm5kzGmLUybSQqKLmK5aZX9YoQBcsoUoUxuclOZZ1/LRKvOhdL O5bVxnL/TjbZid+v9uDTN40ebHLJrR+s22QfIsHYGjb6Xn6HpX74gN9M1YbpxsvPT5Hi oQvVWILNvcsyE1EetZuBzBQMeOPkAd1cFqTlS8zG4EKzz7BguocuxvamrqyQe26jJUF9 tuaSpbziIHkYQctw/u6sKSiefiqACExNFw2vmP+bjlow8vHBYQeQfDTNBfPWmUef/5QO 7r2uqJ36Qruo0fKsbNFQIcfUDr7FJ/uX1EHbnXc+0XcNYi5hwYi32zmSZO9emRYUJsPH TNcg== X-Gm-Message-State: AOAM533t9QD79mCqyZA7DVLU4lzepPNCX+4vess6EbSr14Cm7qR6+wRd z3pNBZAuKQNak52subPHhYiaCZ4w6oUEW38jBVY= X-Google-Smtp-Source: ABdhPJy86n5oNkYkqQ0UrKuhVSdJVe4iisYmsZyK7C+pZG0ApP4XmoDBDoNet7IgXqa/w4qGcs4gJBBeKsXMu8xrf8g= X-Received: by 2002:a67:d583:: with SMTP id m3mr76474vsj.59.1631150770830; Wed, 08 Sep 2021 18:26:10 -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> In-Reply-To: <20210908191602.GQ1583@gate.crashing.org> From: Hongtao Liu Date: Thu, 9 Sep 2021 09:31:46 +0800 Message-ID: Subject: Re: [PATCH] Fix SFmode subreg of DImode and TImode To: Segher Boessenkool Cc: Richard Biener , Michael Meissner , Peter Bergner , Bill Schmidt , GCC Patches , David Edelsohn Content-Type: text/plain; charset="UTF-8" X-Spam-Status: No, score=-3.5 required=5.0 tests=BAYES_00, DKIM_SIGNED, DKIM_VALID, DKIM_VALID_AU, DKIM_VALID_EF, FREEMAIL_FROM, KAM_SHORT, 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: Thu, 09 Sep 2021 01:26:12 -0000 On Thu, Sep 9, 2021 at 3:17 AM Segher Boessenkool wrote: > > On Wed, Sep 08, 2021 at 08:39:31PM +0200, Richard Biener wrote: > > On September 8, 2021 7:08:09 PM GMT+02:00, Segher Boessenkool wrote: > > >It is not a good idea to do allow all those things. Most backends can > > >only support a few combinations of them, and everything else results in > > >*worse* machine code, best case, and more and more complicated (and more > > >buggy!) backend code. > > > > > >But that is a code quality issue. The current problem is that we have > > >at least PR102211 and PR102154 (as well as reports elsewhere of bugs on > > >other targets). Code that used before doesn't anymore, and we have no > > >clear way out, no recommendation how to fix this and a) keep the same > > >functionality without huge changes, and b) keep the same machine code > > >quality. > > > > > >I do not think something like that can be done. That is why I am asking > > >for the patch to be reverted until all of the groundwork for it has been > > >done. That includes making generic testcases that show how such subregs > > >behave, so that we can see in testresults what changes do to existing > > >targets. > > > > Heh, I understood your earlier reply that you supported the change in principle based on the fact that nested subregs are invalid. > > Ah. No. I meant to lament the fact that we use subregs for multiple > things, so that doing a bit_cast of a real subreg has to be expressed as > just one subreg, which is clearly sub-optimal for most backends. > > I say *has to* because a subreg of a subreg is not valid RTL; it has to > be written as just one subreg. Which makes thing more confusing and > confused than this already non-trivial thing has to be. > > > Now, I don't think that validate_subreg is supposed to be the decision maker on what a target allows. > > Right, some target hook or macro or whatnot should. > > > For subregs of hardregs we seem to have a good way of validating, but > > what do we have for subregs of pseudos? Is it the passes generating > > the new unsupported subregs that should do different things? Should > > validate_subreg use a target hook to allow those special casings we > > removed which all were necessary just for specific targets but > > appearantly did not do any harm for other targets? > > Currently, we can disallow things in predicates and/or the instruction > conditions. > > > Can you give advice as to how to address the needs of the HFmode subregs on x86 if not by adding another (generic) narrow exception in validate_subreg? > > If I knew what the problem was, perhaps? Is this explained in some mail > somewhere? it's at [1], target_hook may not be the best solution, if other targets support HFmode, they may encounter the same problem, and revisit it again. [1] https://gcc.gnu.org/pipermail/gcc-patches/2021-August/576827.html > > > That said, I fail to see a good way forward after now two appearantly failed attempts. > > > > Btw, I'm fine reverting the patch but then what's the solution here? > > 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 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. > > > Segher > > > p.s. Very unrelated... Should we have __builtin_bit_cast for C as well? > Is there any reason this could not work? -- BR, Hongtao