From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (qmail 3189 invoked by alias); 13 Oct 2014 16:30:22 -0000 Mailing-List: contact gcc-patches-help@gcc.gnu.org; run by ezmlm Precedence: bulk List-Id: List-Archive: List-Post: List-Help: Sender: gcc-patches-owner@gcc.gnu.org Received: (qmail 3176 invoked by uid 89); 13 Oct 2014 16:30:21 -0000 Authentication-Results: sourceware.org; auth=none X-Virus-Found: No X-Spam-SWARE-Status: No, score=-1.5 required=5.0 tests=AWL,BAYES_00,FREEMAIL_ENVFROM_END_DIGIT,FREEMAIL_FROM,FREEMAIL_REPLY,RCVD_IN_DNSWL_LOW,SPF_PASS autolearn=no version=3.3.2 X-HELO: mail-oi0-f45.google.com Received: from mail-oi0-f45.google.com (HELO mail-oi0-f45.google.com) (209.85.218.45) by sourceware.org (qpsmtpd/0.93/v0.84-503-g423c35a) with (AES128-SHA encrypted) ESMTPS; Mon, 13 Oct 2014 16:30:19 +0000 Received: by mail-oi0-f45.google.com with SMTP id i138so13673491oig.4 for ; Mon, 13 Oct 2014 09:30:18 -0700 (PDT) MIME-Version: 1.0 X-Received: by 10.202.172.72 with SMTP id v69mr1361764oie.107.1413217818006; Mon, 13 Oct 2014 09:30:18 -0700 (PDT) Received: by 10.60.125.98 with HTTP; Mon, 13 Oct 2014 09:30:17 -0700 (PDT) In-Reply-To: References: Date: Mon, 13 Oct 2014 16:30:00 -0000 Message-ID: Subject: Re: [PATCH] Add zero-overhead looping for xtensa backend From: Felix Yang To: "augustine.sterling@gmail.com" Cc: "Yangfei (Felix)" , "gcc-patches@gcc.gnu.org" Content-Type: text/plain; charset=UTF-8 X-SW-Source: 2014-10/txt/msg01158.txt.bz2 Thanks for the comments. The patch checked the usage of teh trip count register, making sure that it is not used in the loop body other than the doloop_end or lives past the doloop_end instruction, as the following code snippet shows: + /* Scan all the blocks to make sure they don't use iter_reg. */ + if (loop->iter_reg_used || loop->iter_reg_used_outside) + { + if (dump_file) + fprintf (dump_file, ";; loop %d uses iterator\n", + loop->loop_no); + return false; + } For the spill issue, I think we need to handle it. The reason is that currently we are not telling GCC about the existence of the LCOUNT register. Instead, we keep the trip count in a general register and it's possible that this register can be spilled when register pressure is high. It's a good idea to post another patch to describe the LCOUNT register in GCC in order to free this general register. But I want this patch applied as a first step, OK? Cheers, Felix On Tue, Oct 14, 2014 at 12:09 AM, augustine.sterling@gmail.com wrote: > On Fri, Oct 10, 2014 at 6:59 AM, Felix Yang wrote: >> Hi Sterling, >> >> I made some improvement to the patch. Two changes: >> 1. TARGET_LOOPS is now used as a condition of the doloop related >> patterns, which is more elegant. > > Fine. > >> 2. As the trip count register of the zero-cost loop maybe >> potentially spilled, we need to change the patterns in order to handle >> this issue. > > Actually, for xtensa you don't. The trip count is copied into LCOUNT > at the execution of the loop instruction, and therefore a spill or > whatever doesn't matter--it won't affect the result. So as long as you > have the trip count at the start of the loop, you are fine. > > This does bring up an issue of whether or not the trip count can be > modified during the loop. (note that this is different than early > exit.) If it can, you can't use a zero-overhead loop. Does your patch > address this case. > > The solution is similar to that adapted by c6x backend. >> Just turn the zero-cost loop into a regular loop when that happens >> when reload is completed. >> Attached please find version 4 of the patch. Make check regression >> tested with xtensa-elf-gcc/simulator. >> OK for trunk?