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 [216.205.24.124]) by sourceware.org (Postfix) with ESMTP id CCB1E3857C6F for ; Mon, 30 Nov 2020 01:28:27 +0000 (GMT) DMARC-Filter: OpenDMARC Filter v1.3.2 sourceware.org CCB1E3857C6F Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-269-UGznUQqyMHGwVYjEI9ExYg-1; Sun, 29 Nov 2020 20:28:25 -0500 X-MC-Unique: UGznUQqyMHGwVYjEI9ExYg-1 Received: from smtp.corp.redhat.com (int-mx06.intmail.prod.int.phx2.redhat.com [10.5.11.16]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 951D010059A4; Mon, 30 Nov 2020 01:28:24 +0000 (UTC) Received: from localhost.localdomain (ovpn-112-145.phx2.redhat.com [10.3.112.145]) by smtp.corp.redhat.com (Postfix) with ESMTP id 6A9B35C1BB; Mon, 30 Nov 2020 01:28:24 +0000 (UTC) Subject: Re: [07/23] Add a class that multiplexes two pointer types To: gcc-patches@gcc.gnu.org, richard.sandiford@arm.com References: <25e7106c-bd83-647f-1247-d9ab474a8528@redhat.com> From: Jeff Law Message-ID: <12a11043-2bf2-8cf5-8e21-60fcd4eab031@redhat.com> Date: Sun, 29 Nov 2020 18:28:24 -0700 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.4.0 MIME-Version: 1.0 In-Reply-To: X-Scanned-By: MIMEDefang 2.79 on 10.5.11.16 X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 8bit Content-Language: en-US X-Spam-Status: No, score=-6.2 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_NONE, RCVD_IN_MSPIKE_H3, 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, 30 Nov 2020 01:28:28 -0000 On 11/26/20 9:15 AM, Richard Sandiford wrote: > Jeff Law writes: >> On 11/13/20 1:14 AM, Richard Sandiford via Gcc-patches wrote: >>> This patch adds a pointer_mux class that provides similar >>> functionality to: >>> >>> union { T1 *a; T2 *b; }; >>> ... >>> bool is_b_rather_than_a; >>> >>> except that the is_b_rather_than_a tag is stored in the low bit >>> of the pointer. See the comments in the patch for a comparison >>> between the two approaches and why this one can be more efficient. >>> >>> I've tried to microoptimise the class a fair bit, since a later >>> patch uses it extensively in order to keep the sizes of data >>> structures down. >>> >>> gcc/ >>> * mux-utils.h: New file. >> Do we have any potentially bootstrappable targets where we can't >> guarantee pointer alignment of at least 16 bits?  I see what look like >> suitable asserts, and presumably if we trigger them, then we're going to >> need to rethink this and fall back to a separate bit? > Like you say, the asserts should trap that if it happens. I think the > fix would be to increase the alignment of the data types involved (which > we can now do in a standard way -- probably wouldn't have been possible > before the switch to C++11, at least not without using extensions). Yea, I keep forgetting about some of the facilities C++ can provide us now. Seems like we've got a good backup plan.  So this is OK by me. jeff