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 3B2F03858CD1 for ; Mon, 27 Nov 2023 11:46:35 +0000 (GMT) DMARC-Filter: OpenDMARC Filter v1.4.2 sourceware.org 3B2F03858CD1 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 3B2F03858CD1 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=1701085596; cv=none; b=oi0hEr3ACN//DxCur12P6vSKAJKK7yjMqSTAAoiDwW4wD337xw2/+8coo2eUugVC02afFoeT2q/s5Ug8vefYhU5yxc+vcPDmptQLLe0lGrkqGPWJ+hPkFWe1GJ4grLY37ZOuKWE90EVR5N/QPvQoCRl/iOA7DOrvKgw8ENBIBnY= ARC-Message-Signature: i=1; a=rsa-sha256; d=sourceware.org; s=key; t=1701085596; c=relaxed/simple; bh=6DnFP6TyOOTIsaMSVa6ajYp1ougHIKCHblotDuoorT8=; h=Date:From:To:Subject:Message-ID:MIME-Version; b=AF7+rdEiHfrQvAYTl+O08HRKlfOrWWmcdqHCE8k+BsheD3XbBSEm9zNnKdQVZ41zftzQhOzsR3jUJMl1W8zWFKcpdf1sChtkojnvBtdvyhD7nVSGSdFHCTlFQztLQF5XWW21eGMgxJKpceMGHJ9KLaPl4OVcRlz/QtvANSKM6tM= 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 66C1C40F1DC3; Mon, 27 Nov 2023 11:46:33 +0000 (UTC) DKIM-Filter: OpenDKIM Filter v2.11.0 mail.ispras.ru 66C1C40F1DC3 Date: Mon, 27 Nov 2023 14:46:33 +0300 (MSK) From: Alexander Monakov To: Ralf Jung cc: Paul Eggert , Adhemerval Zanella Netto , libc-alpha Subject: Re: Support for memcpy with equal source and destination In-Reply-To: <69271612-79a5-43c6-9fc7-fb2461c5d39f@ralfj.de> Message-ID: <089bc099-39ab-1a30-eea2-ebb74e489a8d@ispras.ru> References: <1e8beece-f865-4309-a28f-6782135e2a8a@linaro.org> <9e6eb1ab-9a9d-4b69-ae49-4805ee7cdce8@cs.ucla.edu> <69271612-79a5-43c6-9fc7-fb2461c5d39f@ralfj.de> 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, 27 Nov 2023, Ralf Jung wrote: > This probably needs benchmark to determine on which side the branch is less > expensive overall? Or a dedicated memcpy variant that allows src==dest, as has > been brought up elsewhere. Please note that GCC does not use memcpy for "sufficiently small" structure copies at -O2, as it's faster to emit the necessary loads+stores inline. The threshold for "sufficiently small" varies with target and compiler version; for instance, it is "above 64 bytes" for 32-bit arm and "above 8192 bytes" for x86_64 with current trunk (it also depends on default -march/-mtune, etc.). So on x86 at least adding such a branch in memcpy is not a practical choice. Alexander