From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-ed1-x531.google.com (mail-ed1-x531.google.com [IPv6:2a00:1450:4864:20::531]) by sourceware.org (Postfix) with ESMTPS id 9C1C2385840B for ; Tue, 19 Oct 2021 08:40:56 +0000 (GMT) DMARC-Filter: OpenDMARC Filter v1.4.1 sourceware.org 9C1C2385840B Received: by mail-ed1-x531.google.com with SMTP id r4so3357245edi.5 for ; Tue, 19 Oct 2021 01:40:56 -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=IHoJAm1eh1v03SDJy2FhfUxtMUyvXTTD6//5bNIoEZg=; b=7QENlnWqmsCeNqfhxZKn+biQMZli3Bbqt9I9S9m6voIHhDzkjiWLdI0zw9CqWtNfb9 c9ZPGgw5t5W8fD4eeiA7K4Tb7d5gGX2Ry8K0Nshz23p0llz/XEaSEIbYE0dp0HQO8Psc 4WgxFUP6lKBy3QIrDNdHQJltuQgxpU/zm/iVDp0DJRcFKd/BBGTaUOCwh5hw3hJaYlJ9 OqMsQinvr/YQKdbTgz9/RV6ltYuphXoEnSwPK42QeLYoSf/LLzyVATTlYd6jjNyrOnX/ Uqqt6BYAjVLAa8G41UkJK8JcRqmRRnreu9N3oZCfI3I/OQYbqBESdkMNWSHBKqXXCch+ ptGw== X-Gm-Message-State: AOAM531bfZ6KkltJnbjnX7m5FoVA6b2YZfPmEoGSgQpvFvJHD0jyeMw+ mAllRbwi1cz+wiXj9ikOC7pX0T/cJGxk9WF+a6I= X-Google-Smtp-Source: ABdhPJxv+7MP+yJv6QU9lrQVJIMeTIsYfcVS2z97Dqfh9oe6PDZxfrclV/MrwrxUrXzpWavYPe50oyeeSrEUZKr2jRQ= X-Received: by 2002:a05:6402:27d3:: with SMTP id c19mr50940626ede.70.1634632855251; Tue, 19 Oct 2021 01:40:55 -0700 (PDT) MIME-Version: 1.0 References: <20211018134117.410106-1-aldyh@redhat.com> <69d78e89-c086-d10c-901f-72f504aeb93f@redhat.com> In-Reply-To: From: Richard Biener Date: Tue, 19 Oct 2021 10:40:44 +0200 Message-ID: Subject: Re: [RFC] Remove VRP threader passes in exchange for better threading pre-VRP. To: Aldy Hernandez Cc: Jeff Law , GCC patches , Andrew MacLeod Content-Type: text/plain; charset="UTF-8" X-Spam-Status: No, score=-2.6 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: Tue, 19 Oct 2021 08:40:58 -0000 On Tue, Oct 19, 2021 at 9:33 AM Aldy Hernandez wrote: > > On Tue, Oct 19, 2021 at 8:52 AM Richard Biener > wrote: > > > > On Mon, Oct 18, 2021 at 4:03 PM Aldy Hernandez wrote: > > > > > > > > > > > > On 10/18/21 3:41 PM, Aldy Hernandez wrote: > > > > > > > I've been experimenting with reducing the total number of threading > > > > passes, and I'd like to see if there's consensus/stomach for altering > > > > the pipeline. Note, that the goal is to remove forward threader clients, > > > > not the other way around. So, we should prefer to remove a VRP threader > > > > instance over a *.thread one immediately before VRP. > > > > > > > > After some playing, it looks like if we enable fully-resolving mode in > > > > the *.thread passes immediately preceeding VRP, we can remove the VRP > > > > threading passes altogether, thus removing 2 threading passes (and > > > > forward threading passes at that!). > > > > > > It occurs to me that we could also remove the threading before VRP > > > passes, and enable a fully-resolving backward threader after VRP. I > > > haven't played with this scenario, but it should be just as good. That > > > being said, I don't know the intricacies of why we had both pre and post > > > VRP threading passes, and if one is ideally better than the other. > > > > It was done because they were different threaders. Since the new threader > > uses built-in VRP it shouldn't really matter whether it's before or after > > VRP _for the threading_, but it might be that if threading runs before VRP > > then VRP itself can do a better job on cleaning up the IL. > > Good point. > > FWIW, earlier this season I played with replacing the VRPs with evrp > instances (which fold far more conditionals) and I found that the > threaders can actually find LESS opportunities after *vrp fold away > things. I don't know if this is a good or a bad thing. Probably a sign that either threading theads stuff that's pointless (does not consider conditions on the path that always evaluate false?) or that after VRP and removing redundant conditions blocks are now bigger and we run into some --param limit (that would suggest the limits behave odd if it would thread when we'd artificially split blocks). Examples might be interesting to look at to understand what's going "wrong". > Perhaps we > should benchmark three alternatives: > > 1. Mainline > 2. Fully resolving threader -> VRP -> No threading. > 3. No threading -> VRP -> Full resolving threader. > > ...and see what the actual effect is, regardless of number of threaded paths. As said, only 2. makes "sense" to me unless examples show why we really have the usual pass ordering issue. As said, I think threading exposes new VRP (esp. constant/copy prop) opportunities but VRP shouldn't expose new threading opportunities. > Speak of which, what's the blessed way of benchmarking performance > nowadays? I've seen some PRs fly that measure some more lightweight > benchmarks (open source?) than a full blown SPEC. The answer is SPEC. The other answer would be GCC bootstrap time and resulting code size. > > > > + /* ?? Is this still needed. ?? */ > > /* Threading can leave many const/copy propagations in the IL. > > Clean them up. Instead of just copy_prop, we use ccp to > > compute alignment and nonzero bits. */ > > > > Yes, it's still needed but not for the stated reason - the VRP > > substitution and folding stage should deal with copy/constant propagation > > but we replaced the former copy propagation with CCP to re-compute > > nonzero bits & alignment so I'd change the comment to > > > > /* Run CCP to compute alignment and nonzero bits. */ > > Ahh.. > > There's another similar comment after DOM. Is this comment still relevant? Yes, since DOM still does threading the threaded paths lack const/copy propagation. > NEXT_PASS (pass_dominator, true /* may_peel_loop_headers_p */); > /* Threading can leave many const/copy propagations in the IL. > Clean them up. Failure to do so well can lead to false > positives from warnings for erroneous code. */ > NEXT_PASS (pass_copy_prop); > /* Identify paths that should never be executed in a conforming > program and isolate those paths. */ > > Thanks. > Aldy >