From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail.ispras.ru (mail.ispras.ru [83.149.199.84]) by sourceware.org (Postfix) with ESMTPS id ACDDF3829BCF for ; Mon, 20 Nov 2023 13:52:17 +0000 (GMT) DMARC-Filter: OpenDMARC Filter v1.4.2 sourceware.org ACDDF3829BCF Authentication-Results: sourceware.org; dmarc=pass (p=none dis=none) header.from=ispras.ru Authentication-Results: sourceware.org; spf=pass smtp.mailfrom=ispras.ru ARC-Filter: OpenARC Filter v1.0.0 sourceware.org ACDDF3829BCF Authentication-Results: server2.sourceware.org; arc=none smtp.remote-ip=83.149.199.84 ARC-Seal: i=1; a=rsa-sha256; d=sourceware.org; s=key; t=1700488339; cv=none; b=n+Ygrn6c3GKwssk4I36XtOW3NjjuMfRaYAUhalD04wAuKe/XA+yKwxU879IeWMZefwmJcAs6q461A9sjZIEO3cVbfpPyIaUYFW/+B4m5qcegajGL2rZBdFkUv1tNLfNitZh6UXUVVtoqHykuZtf1NX68VGv3nhHgPrm51owj1Nk= ARC-Message-Signature: i=1; a=rsa-sha256; d=sourceware.org; s=key; t=1700488339; c=relaxed/simple; bh=WQq4kUAq7OoTqP7kkahnEL+LxqnYOb725ojFRt4LCpI=; h=Date:From:To:Subject:Message-ID:MIME-Version; b=bNXwILDq4D3KOaIOXOyB/jFSwcWegzN8eHagMxj9b6CH8LJ7MUNv2J6vhID6bZDoDKofpP0dTGljkhl73gXg3VAyWqaqamQWyoK2hJC2gR4TekBaFsJIpfAUbILQg6dxnscSvDFOqSMcymABFIKP0XIfayAMrWzin8+1wTm3yCo= ARC-Authentication-Results: i=1; server2.sourceware.org Received: from [10.10.3.121] (unknown [10.10.3.121]) by mail.ispras.ru (Postfix) with ESMTPS id E237340F1DD2; Mon, 20 Nov 2023 13:52:15 +0000 (UTC) DKIM-Filter: OpenDKIM Filter v2.11.0 mail.ispras.ru E237340F1DD2 Date: Mon, 20 Nov 2023 16:52:15 +0300 (MSK) From: Alexander Monakov To: Maxim Kuvyrkov cc: gcc-patches@gcc.gnu.org, Bernd Schmidt , Vladimir Makarov , Jeff Law Subject: Re: [PATCH 1/1] sched-deps.cc (find_modifiable_mems): Avoid exponential behavior In-Reply-To: <20231120120649.672893-2-maxim.kuvyrkov@linaro.org> Message-ID: <8587bddb-e348-a637-2fe2-8318bb09b3be@ispras.ru> References: <20231120120649.672893-1-maxim.kuvyrkov@linaro.org> <20231120120649.672893-2-maxim.kuvyrkov@linaro.org> MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII X-Spam-Status: No, score=-3.1 required=5.0 tests=BAYES_00,KAM_DMARC_STATUS,SPF_HELO_NONE,SPF_PASS,TXREP,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 List-Id: On Mon, 20 Nov 2023, Maxim Kuvyrkov wrote: > This patch avoids sched-deps.cc:find_inc() creating exponential number > of dependencies, which become memory and compilation time hogs. > Consider example (simplified from PR96388) ... > === > sp=sp-4 // sp_insnA > mem_insnA1[sp+A1] > ... > mem_insnAN[sp+AN] > sp=sp-4 // sp_insnB > mem_insnB1[sp+B1] > ... > mem_insnBM[sp+BM] > === > ... in this example find_modifiable_mems() will arrange for mem_insnA* > to be able to pass sp_insnA, and, while doing this, will create > dependencies between all mem_insnA*s and sp_insnB -- because sp_insnB > is a consumer of sp_insnA. After this sp_insnB will have N new > backward dependencies. > Then find_modifiable_mems() gets to mem_insnB*s and starts to create > N new dependencies for _every_ mem_insnB*. This gets us N*M new > dependencies. It's a bit hard to read this without knowing which value of 'backwards' is assumed. Say 'backwards' is true and we are inspecting producer sp_insnB of mem_insnB1. This is a true dependency. We know we can break it by adjusting B1 by -4, but we need to be careful not to move such modified mem_insnB1 above sp_insnA, so we need to iterate over *incoming true dependencies* of sp_insnB and add them. But the code seems to be iterating over *all incoming dependencies*, so it will e.g. take anti-dependency mem_insnA1 -> sp_insnB and create a true dependency mem_insnA1 -> mem_insnB1'. This seems utterly inefficient, if my understanding is correct. Alexander