From patchwork Fri Sep 9 12:20:11 2022 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Patchwork-Submitter: Tobias Burnus X-Patchwork-Id: 57521 Return-Path: X-Original-To: patchwork@sourceware.org Delivered-To: patchwork@sourceware.org Received: from server2.sourceware.org (localhost [IPv6:::1]) by sourceware.org (Postfix) with ESMTP id 60FC93857014 for ; Fri, 9 Sep 2022 12:20:45 +0000 (GMT) X-Original-To: gcc-patches@gcc.gnu.org Delivered-To: gcc-patches@gcc.gnu.org Received: from esa4.mentor.iphmx.com (esa4.mentor.iphmx.com [68.232.137.252]) by sourceware.org (Postfix) with ESMTPS id 08E2B38582BF for ; Fri, 9 Sep 2022 12:20:26 +0000 (GMT) DMARC-Filter: OpenDMARC Filter v1.4.1 sourceware.org 08E2B38582BF Authentication-Results: sourceware.org; dmarc=none (p=none dis=none) header.from=codesourcery.com Authentication-Results: sourceware.org; spf=pass smtp.mailfrom=mentor.com X-IronPort-AV: E=Sophos;i="5.93,303,1654588800"; d="diff'?scan'208";a="82715123" Received: from orw-gwy-02-in.mentorg.com ([192.94.38.167]) by esa4.mentor.iphmx.com with ESMTP; 09 Sep 2022 04:20:19 -0800 IronPort-SDR: 62tu7zWTgka5uvV4piMKs0Gk5fFK2fMgralgYWFxgpxUq+OWvHb3Dqnvd6t2mlk+5fyb5zk92k SGsuCZw54CvENtihziDQh4UG7++QyA/VO4FgZSak+kqMJhjwysylAQeZWGlDgeuIOJzSkwLGpB uShCAZOAEvPWus7hm0oGMLxXFHImp30ALrZrHorFqmHZEAIWdP/GO7NkFiz81/UfgzPvmf6R3q qp44oS/uoEssr1MYptdTMaJ46DCo+NFoKr70PfQVC+YeusMER/oqQI+QqZyDNjkpye7047G95m XWI= Message-ID: <5d7ca95c-8a61-6d07-dd5b-e7c2d03072ff@codesourcery.com> Date: Fri, 9 Sep 2022 14:20:11 +0200 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.2.1 Subject: GCN: Add -mlow-precision-sqrt for double-precision sqrt [PR105246] (was: Re: [PATCH] amdgcn: Add support for additional natively supported floating-point operations) Content-Language: en-US To: Andrew Stubbs References: <08966068-719a-30d1-5b71-7cf839e507e7@codesourcery.com> From: Tobias Burnus In-Reply-To: X-Originating-IP: [137.202.0.90] X-ClientProxiedBy: svr-ies-mbx-13.mgc.mentorg.com (139.181.222.13) To svr-ies-mbx-12.mgc.mentorg.com (139.181.222.12) X-Spam-Status: No, score=-11.4 required=5.0 tests=BAYES_00, GIT_PATCH_0, HEADER_FROM_DIFFERENT_DOMAINS, KAM_DMARC_STATUS, KAM_SHORT, SPF_HELO_PASS, 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 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: , Cc: Richard Biener , Kwok Cheung Yeung , gcc-patches Errors-To: gcc-patches-bounces+patchwork=sourceware.org@gcc.gnu.org Sender: "Gcc-patches" On 09.09.22 12:16, Richard Biener wrote: > On Fri, 9 Sep 2022, Tobias Burnus wrote: >> -funsafe-math-optimizations implies -fno-signed-zeros, -fno-trapping-math, >> -fassociative-math, >> and -freciprocal-math. All of them reduce precision and my violate IEEE or >> ISO/language standards. >> >> However, I think it is rather surprising to have all of the sudden only a >> precision of the order of 100,000,000 ULP instead of ~4 ULP as to be expected. >> That's a precision loss of the order of 10^8 or 2^29 which is huge! >> ... > I agree - for example powerpc has -mrecip= to control which instructions > to use (float/double rsqrt or inverse) and -mrecip-precision to > specify whether further iteration is done or not. > [...] > Your suggested huge reduction in precision isn't usually acceptable > and should be always explicitely enabled. First, I have to correct myself – Kwok's -munsafe-math-optimizations is only about single-precision functions, which do not have this problem. However, the pre-existing 'sqrt' problem still is real. It also applies to reverse sqrt ("v_rsq"), but that's for whatever reason not used for GCN. This patch now adds a commandline flag - off by default - to choose whether this behavior is wanted. I did use the same name as aarch64, https://gcc.gnu.org/onlinedocs/gcc/AArch64-Options.html#index-mlow-precision-sqrt (the latter also has -mlow-precision-recip-sqrt, which is not (yet) sensible for GCN.) This patch was manually tested for all combinations and I also looked at insn-recog.cc, given that it is my first .md patch – it it seems to work fine. OK for mainline – or are there comments or more suggestions? I also included some word for the release notes. Tobias ----------------- Siemens Electronic Design Automation GmbH; Anschrift: Arnulfstraße 201, 80634 München; Gesellschaft mit beschränkter Haftung; Geschäftsführer: Thomas Heurung, Frank Thürauf; Sitz der Gesellschaft: München; Registergericht München, HRB 106955 gcc-13/changes.html - GCN: document -mlow-precision-sqrt diff --git a/htdocs/gcc-13/changes.html b/htdocs/gcc-13/changes.html index 390193ca..d335eab3 100644 --- a/htdocs/gcc-13/changes.html +++ b/htdocs/gcc-13/changes.html @@ -179,6 +179,12 @@ a work-in-progress.

  • Support for the Instinct MI200 series devices ( gfx90a) has been added.
    • +
  • The -mlow-precision-sqrt option (disabled by default) + has been added to use the hardware sqrt also for + double-precision floating point arguments; note that the result + only has much a reduced accurary of 229 ULP. This + option requires -funsafe-math-optimizations + (implied by -ffast-math) in addition.