From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [63.128.21.124]) by sourceware.org (Postfix) with ESMTP id 907143851C34 for ; Mon, 4 Jan 2021 21:01:30 +0000 (GMT) DMARC-Filter: OpenDMARC Filter v1.3.2 sourceware.org 907143851C34 Received: from mail-qv1-f70.google.com (mail-qv1-f70.google.com [209.85.219.70]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-374-r5ZPc7hsOZSJYAJis020Rg-1; Mon, 04 Jan 2021 16:01:28 -0500 X-MC-Unique: r5ZPc7hsOZSJYAJis020Rg-1 Received: by mail-qv1-f70.google.com with SMTP id bp20so26170940qvb.20 for ; Mon, 04 Jan 2021 13:01:28 -0800 (PST) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:subject:to:cc:references:from:message-id:date :user-agent:mime-version:in-reply-to:content-language :content-transfer-encoding; bh=XaoHBOaekrZSq/Yx4lAErXgMJELNC4XElyuB39fBFAg=; b=PomCuMzLR9Oc/3cdnTvBLI2R1MwtQR2gb367ZdLqYl4Wtgjrg1ZYyB09dj/Co9xznv pt+f40TmM92GqrA/hXy2vY6857uGkTOwVt0lJWSvC3viHKIYDFvzwPdAWaPwiywoO1K/ 876tqwegUW2G0nCGqj3ExG2UvdEttk9hf+kWwdlgihpQaO8GweHRpEKx2reP4riCdxxt 6C8L9j0OKwLvCAPMs6+hcyN0rpXC2C7iX808NkD+9WAhH9uyLw45pjaSWeikTBWULfVG 4ZHF7yrKoJfPVNEbTN1LS1KvW2T28eqzhwySOnkXO0Vi1y4SxBrcL+iiXHO//dK1vdcJ lR8A== X-Gm-Message-State: AOAM531aJm11tJXO+foZmmgNQnBNZtfNdV9l50vH85AhzAgByyoR3Ij7 kZ+Qzyk39BVuY759x2De8DZMoqbIPAtZuhPfkvyASfxWJTnKhE8bnmSvJFy9tmEkJu/8Os1otUd QLPx5iqmcoX8+fUjnYF83ON0ar1ffvRHrpA2H86i1y8tDLWiEHfwDQqULyRgmBU7tkQ== X-Received: by 2002:a0c:b21e:: with SMTP id x30mr77221606qvd.21.1609794087627; Mon, 04 Jan 2021 13:01:27 -0800 (PST) X-Google-Smtp-Source: ABdhPJzL77vJl2VngwbUyJR8kpuoa1Ek68kDXBDXean8buRpWJiLoU3ZAbpYZCjb2pIIpQhHFUUz8w== X-Received: by 2002:a0c:b21e:: with SMTP id x30mr77221584qvd.21.1609794087371; Mon, 04 Jan 2021 13:01:27 -0800 (PST) Received: from [192.168.1.148] (209-6-216-142.s141.c3-0.smr-cbr1.sbo-smr.ma.cable.rcncustomer.com. [209.6.216.142]) by smtp.gmail.com with ESMTPSA id b6sm37142602qkc.128.2021.01.04.13.01.26 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 04 Jan 2021 13:01:26 -0800 (PST) Subject: Re: [PATCH] c++: Fix ICE with __builtin_bit_cast [PR98469] To: Jakub Jelinek Cc: gcc-patches@gcc.gnu.org References: <20201230091343.GU3788@tucnak> <20210104204820.GB725145@tucnak> From: Jason Merrill Message-ID: <989b7d53-25fe-dc11-63a1-deaf18df615c@redhat.com> Date: Mon, 4 Jan 2021 16:01:25 -0500 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.4.3 MIME-Version: 1.0 In-Reply-To: <20210104204820.GB725145@tucnak> X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit X-Spam-Status: No, score=-10.0 required=5.0 tests=BAYES_00, DKIMWL_WL_HIGH, DKIM_SIGNED, DKIM_VALID, DKIM_VALID_AU, DKIM_VALID_EF, NICE_REPLY_A, RCVD_IN_DNSWL_LOW, RCVD_IN_MSPIKE_H4, RCVD_IN_MSPIKE_WL, SPF_HELO_NONE, SPF_PASS, TXREP autolearn=ham autolearn_force=no version=3.4.2 X-Spam-Checker-Version: SpamAssassin 3.4.2 (2018-09-13) 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: Mon, 04 Jan 2021 21:01:31 -0000 On 1/4/21 3:48 PM, Jakub Jelinek wrote: > On Mon, Jan 04, 2021 at 03:44:46PM -0500, Jason Merrill wrote: >> This change is OK, but part of the problem is that we're trying to do >> overload resolution for an S copy/move constructor, which we shouldn't be >> because bit_cast is a prvalue, so in C++17 and up we should use it to >> directly initialize the target without any implied constructor call. >> >> It seems we're mishandling this because the code in >> build_special_member_call specifically looks for TARGET_EXPR or CONSTRUCTOR, >> and BIT_CAST_EXPR is neither of those. >> >> Wrapping a BIT_CAST_EXPR of aggregate type in a TARGET_EXPR would address >> this, and any other places that expect a class prvalue to come in the form >> of a TARGET_EXPR. > > I can try that tomorrow. Won't that cause copying through extra temporary > in some cases though, or is that guaranteed to be optimized? It won't cause any extra copying when it's used to initialize another object (like the return value of std::bit_cast). Class prvalues are always expressed with a TARGET_EXPR in the front end; the TARGET_EXPR melts away when used as an initializer, it only creates a temporary when it's used in another way. Jason