From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from smtp-out1.suse.de (smtp-out1.suse.de [IPv6:2001:67c:2178:6::1c]) by sourceware.org (Postfix) with ESMTPS id E98DB3858D37 for ; Wed, 17 Aug 2022 08:38:06 +0000 (GMT) DMARC-Filter: OpenDMARC Filter v1.4.1 sourceware.org E98DB3858D37 Received: from relay2.suse.de (relay2.suse.de [149.44.160.134]) by smtp-out1.suse.de (Postfix) with ESMTP id DA70C344D9; Wed, 17 Aug 2022 08:38:05 +0000 (UTC) Received: from wotan.suse.de (wotan.suse.de [10.160.0.1]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by relay2.suse.de (Postfix) with ESMTPS id D47472C178; Wed, 17 Aug 2022 08:38:05 +0000 (UTC) Date: Wed, 17 Aug 2022 08:38:05 +0000 (UTC) From: Richard Biener To: Aldy Hernandez cc: gcc-patches Subject: Re: [PATCH] Refactor back_threader_profitability In-Reply-To: Message-ID: References: <81970.122081610045900133@us-mta-26.us.mimecast.lan> User-Agent: Alpine 2.22 (LSU 394 2020-01-19) MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII X-Spam-Status: No, score=-5.0 required=5.0 tests=BAYES_00, DKIM_SIGNED, DKIM_VALID, DKIM_VALID_AU, DKIM_VALID_EF, SPF_HELO_NONE, SPF_PASS, TXREP, T_FILL_THIS_FORM_SHORT, T_SCC_BODY_TEXT_LINE autolearn=ham autolearn_force=no version=3.4.6 X-Spam-Checker-Version: SpamAssassin 3.4.6 (2021-04-09) 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, 17 Aug 2022 08:38:09 -0000 On Wed, 17 Aug 2022, Aldy Hernandez wrote: > On Wed, Aug 17, 2022 at 9:54 AM Richard Biener wrote: > > > > On Wed, 17 Aug 2022, Aldy Hernandez wrote: > > > > > I just have a few high level comments. > > > > > > On Tue, Aug 16, 2022 at 4:05 PM Richard Biener wrote: > > > > > > > > The following refactors profitable_path_p in the backward threader, > > > > splitting out parts that can be computed once the exit block is known, > > > > parts that contiguously update and that can be checked allowing > > > > for the path to be later identified as FSM with larger limits, > > > > possibly_profitable_path_p, and final checks done when the whole > > > > path is known, profitable_path_p. > > > > > > I thought we were removing references to FSM, as they were leftovers > > > from some previous incarnation. For that matter, I don't think I ever > > > understood what they are, so if we're gonna keep them, could you > > > comment what makes FSM threads different from other threads? > > > > I don't know exactly what 'FSM' stands for but the FSM threader > > specifically tried to cover > > It probably stands for finite state machine then? > > > > > for (;;) > > { > > switch (state) > > { > > case 1: > > /* sth */ > > state = 3; > > break; > > ... > > case 3: > > ... > > } > > } > > > > so optimizing state machine transitions. That is, these are all > > multiway (switch or goto, but goto support has been removed with the > > ranger rewrite it seems) and the thread path going through the > > ISTR going through the computed goto path in the old code, and it > never got triggered. I just didn't get around to removing the > references to GIMPLE_GOTO. At least, the old threader not once got a > thread we didn't get with the new code, even through a full Fedora > build. I could be wrong though, but that's my recollection. When one massages the threader to even consider gotos we eventually run into find_taken_edge not handling them because range_of_expr computes the range of 'state' as [irange] void * [1, +INF]$4 = void rather than &&E. A testcase would be unsigned g; void FSM (int start) { void *states[] = { &&A, &&B, &&C, &&E }; void *state = states[start]; do { goto *state; A: g += 1; state = g & 1 ? &&B : &&E; continue; B: g += 2; state = &&C; continue; C: g += 3; state = states[g & 3]; continue; E: break; } while (1); } where we'd expect to thread the B -> C state transition. I checked gcc 7 and it doesn't do that - not sure if it broke somewhen on the way or if it was just never working. I'll file a bugreport, OTOH &label and goto *p are both GNU extensions, so not sure if it is worth optimizing. I'll attach the "simple" enabler I have. > > loop backedge. This is why FSM has different size limits because > > it was thought those threadings are especially worthwhile and > > they tend to be larger. To make discovery cheaper a TODO item > > would be to skip to the loop backedge when we reach the regular > > thread limit and only continue the real search there (and > > pick the "smallest" ways through any diamonds on the way). > > Ah, thanks. If you could, add some similar blurb, it'd be great. I'm > sure we'll all forget it at some time (well, I will :)). Sure ;) Richard. > Aldy > > > > > > In your possibly_profitable_path_p function, could you document a bit > > > better what's the difference between profitable_path_p and > > > possibly_profitable_path_p? > > > > Sure. > > > > > > > > > > I've removed the back_threader_profitability instance from the > > > > back_threader class and instead instantiate it once per path > > > > discovery. I've kept the size compute non-incremental to simplify > > > > the patch and not worry about unwinding. > > > > > > > > There's key changes to previous behavior - namely we apply > > > > the param_max_jump_thread_duplication_stmts early only when > > > > we know the path cannot become an FSM one (multiway + thread through > > > > latch) but make sure to elide the path query when we we didn't > > > > yet discover that but are over this limit. Similarly the > > > > speed limit is now used even when we did not yet discover a > > > > hot BB on the path. Basically the idea is to only stop path > > > > discovery when we know the path will never become profitable > > > > but avoid the expensive path range query when we know it's > > > > currently not. > > > > > > > > I've done a few cleanups, merging functions, on the way. > > > > > > > > Bootstrapped and tested on x86_64-unknown-linux-gnu. > > > > > > > > Statistics show an overall slight increase in threading but > > > > looking at different files theres noise up and down. That's > > > > somewhat expected since we now are applying the "more correct" > > > > limits in the end. Unless I made big mistakes of course. > > > > > > > > The next thing cost-wise would be to raise the backwards > > > > threading limit to the limit of DOM so we don't get > > > > artificial high counts for that. > > > > > > The DOM threader has limits? I thought most of those limits were just > > > due to the fact that it couldn't determine long enough paths? Either > > > way, I like that we're merging the necessary forward threader bits > > > here, in preparation for its demise ;-). > > > > Both use param_max_jump_thread_duplication_stmts, but the backwards > > threader applies this limit to non-FSM threads with > > > > /* The generic copier used by the backthreader does not re-use an > > existing threading path to reduce code duplication. So for that > > case, drastically reduce the number of statements we are allowed > > to copy. */ > > if (!(threaded_through_latch && threaded_multiway_branch) > > && (n_insns * param_fsm_scale_path_stmts > > >= param_max_jump_thread_duplication_stmts)) > > > > and param_fsm_scale_path_stmts happens to be two. I have no idea > > why we apply this scaling, the scaling is otherwise used in > > > > /* We avoid creating irreducible inner loops unless we thread through > > a multiway branch, in which case we have deemed it worth losing > > other loop optimizations later. > > > > We also consider it worth creating an irreducible inner loop if > > the number of copied statement is low relative to the length of > > the path -- in that case there's little the traditional loop > > optimizer would have done anyway, so an irreducible loop is not > > so bad. */ > > if (!threaded_multiway_branch > > && creates_irreducible_loop > > && *creates_irreducible_loop > > && (n_insns * (unsigned) param_fsm_scale_path_stmts > > > (m_path.length () * > > (unsigned) param_fsm_scale_path_blocks))) > > > > so my idea is to drop the scaling from applying the > > param_max_jump_thread_duplication_stmts limit which raises the > > effective default limit from 15 / 2 to 15, just like what DOM > > applies (where DOM also estimates some optimization on the path, > > reducing the stmt count). > > > > > Looks good. > > > > Thanks - I'll adjust the commentary and push. > > > > Richard. > > > > -- Richard Biener SUSE Software Solutions Germany GmbH, Frankenstrasse 146, 90461 Nuernberg, Germany; GF: Ivo Totev, Andrew Myers, Andrew McDonald, Boudien Moerman; HRB 36809 (AG Nuernberg)