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 596763851C1A for ; Wed, 25 Nov 2020 19:58:28 +0000 (GMT) DMARC-Filter: OpenDMARC Filter v1.3.2 sourceware.org 596763851C1A 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-508-Gr-htsqSMFu8hZT_th5F3Q-1; Wed, 25 Nov 2020 14:58:26 -0500 X-MC-Unique: Gr-htsqSMFu8hZT_th5F3Q-1 Received: from smtp.corp.redhat.com (int-mx04.intmail.prod.int.phx2.redhat.com [10.5.11.14]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 0DE41803F48; Wed, 25 Nov 2020 19:58:25 +0000 (UTC) Received: from localhost.localdomain (ovpn-113-191.phx2.redhat.com [10.3.113.191]) by smtp.corp.redhat.com (Postfix) with ESMTP id 98FFE5D9C6; Wed, 25 Nov 2020 19:58:24 +0000 (UTC) Subject: Re: [00/23] Make fwprop use an on-the-side RTL SSA representation To: gcc-patches@gcc.gnu.org, richard.sandiford@arm.com References: From: Jeff Law Message-ID: Date: Wed, 25 Nov 2020 12:58:23 -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 10.5.11.14 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=-5.5 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_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: Wed, 25 Nov 2020 19:58:30 -0000 On 11/13/20 1:10 AM, Richard Sandiford via Gcc-patches wrote: > Just after GCC 10 stage 1 closed (oops), I posted a patch to add a new > combine pass. One of its main aims was to allow instructions to move > around where necessary in order to make a combination possible. > It also tried to parallelise instructions that use the same resource. > > That pass contained its own code for maintaining limited def-use chains. > When I posted the patch, Segher asked why we wanted yet another piece > of pass-specific code to do that. Although I had specific reasons > (which I explained at the time) I've gradually come round to agreeing > that that was a flaw. > > This series of patches is the result of a Covid-time project to add > a more general, pass-agnostic framework. There are two parts: > adding the framework itself, and using it to make fwprop.c faster. > > The framework part > ------------------ > > The framework provides an optional, on-the-side SSA view of existing > RTL instructions. Each instruction gets a list of definitions and a > list of uses, with each use having a single definition. Phi nodes > handle cases in which there are multiple possible definitions of a > register on entry to a basic block. There are also routines for > updating instructions while keeping the SSA representation intact. > > The aim is only to provide a different view of existing RTL instructions. > Unlike gimple, and unlike (IIRC) the old RTL SSA project from way back, > the new framework isn't a “native” SSA representation. This means that > all inputs to a phi node for a register R are also definitions of > register R; no move operation is “hidden” in the phi node. Hmm, I'm trying to parse what the last phrase means.  Does it mean that the "hidden copy" problem for out-of-ssa is avoided?  And if so, how is that maintained over time.  Things like copy-prop will tend to introduce those issues even if they didn't originally exist. > > Like gimple, the framework treats memory as a single unified resource. > > A more in-depth summary is contained in the doc patch, but some > other random notes: > > * At the moment, the SSA information is local to one pass, but it might > be good to maintain it between passes in future. Right.  I think we can look at the passes near fwprop as good targets for extending the lifetime over which we have an SSA framework.   I note CSE is just before the first fwprop and CSE is a hell of a lot easier in an SSA world :-)  It's unfortunately that there's no DCE passes abutting fwprop as DCE is really easy in an SSA world. > > * The SSA code groups blocks into extended basic blocks, with the > EBBs rather than individual blocks having phi nodes. So I haven't looked at the patch, but the usual place to put PHIs is at the dominance frontier.  But extra PHIs just increase time/memory and shouldn't affect correctness. > > * The framework also provides live range information for registers > within an extended basic block and allows instructions to move within > their EBB. It might be useful to allow further movement in future; > I just don't have a use case for it yet. Yup.   You could do something like Click's algorithm to schedule the instructions in a block to maximize CSE opportunities on top of this. > > * One advantage of the new infrastructure is that it gives > recog_for_combine-like behaviour: if recog wants to add clobbers > of things like the flags register, the SSA code will make sure > that the flags register is free. I look more at the intersection between combine and SSA as an opportunity to combine on extended blocks, simplify the "does dataflow allow this combination" logic, drop the need to build/maintain LOG_LINKS and more generally simplify note distribution. > * I've tried to optimise the code for both memory footprint and > compile time. The first part involves quite a bit of overloading > of pointers and various other kinds of reuse, so most of the new data > structures use private member variables and public accessor functions. > I know that style isn't universally popular, but I think it's > justified here. Things could easily go wrong if passes tried > to operate directly on the underlying data structures. ACK. > > * Debug instructions get SSA information too, on a best-effort basis. > Providing complete information would be significantly more expensive. > > * I wasn't sure for new C++ code whether to stick to the old C /* … */ > comments, or whether to switch to //. In the end I went for //, > on the basis that: > > - The ranger code already does this. > > - // is certainly more idiomatic in C++. > > - // is in the lisp tradition of per-line comments and it matches the > ;; used in .md files. I feel sure that GCC would have been written > using // from the outset if that had been possible. I think we're allowing both and realistically /* */ vs // shouldn't be something we spend a lot of time arguing about :-) > > The patches only do this for new files. The aim is to ensure that > each file is at least self-consistent. ACK. Anyway, given this posted before end of stage1, it deserves consideration of gcc-11.  It's (by far) the largest set in my gcc-11 queue. jeff