From mboxrd@z Thu Jan 1 00:00:00 1970 X-Spam-Checker-Version: SpamAssassin 3.4.4 (2020-01-24) on inbox.vuxu.org X-Spam-Level: X-Spam-Status: No, score=-3.3 required=5.0 tests=DKIM_SIGNED,DKIM_VALID, MAILING_LIST_MULTI,NICE_REPLY_A,RCVD_IN_DNSWL_MED,RCVD_IN_MSPIKE_H3, RCVD_IN_MSPIKE_WL autolearn=ham autolearn_force=no version=3.4.4 Received: (qmail 2819 invoked from network); 2 Dec 2020 10:45:25 -0000 Received: from mother.openwall.net (195.42.179.200) by inbox.vuxu.org with ESMTPUTF8; 2 Dec 2020 10:45:25 -0000 Received: (qmail 10119 invoked by uid 550); 2 Dec 2020 10:45:19 -0000 Mailing-List: contact musl-help@lists.openwall.com; run by ezmlm Precedence: bulk List-Post: List-Help: List-Unsubscribe: List-Subscribe: List-ID: Reply-To: musl@lists.openwall.com Received: (qmail 10096 invoked from network); 2 Dec 2020 10:45:18 -0000 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=ibm.com; h=subject : to : references : from : message-id : date : mime-version : in-reply-to : content-type : content-transfer-encoding; s=pp1; bh=+WMuPCIXrxusf3NOrJ7c9xRz5Sv9Hh9xbxaCG/2LbRg=; b=gtLAYOtzhVkvpWS/+edn4b+NnQOCLdCDiZBQLfcZqBmz1W1uPdTcn0CmETZEBLPNR5+w E7CIh2kJ+33yXaVXrV+kFYfs6afu1JK8lvHj3HjvaRUVqqofEoUuWGMvO/ri9lCQQXJJ HNSN5XNXpWrQPhgnTO1sOijyTYmjeZ9k09TMpINIX2YdR+SPFLeO8Aq1QMfKLe9DnS4H ieNEpgfoYs6BjuCFgBlqS2I04ApLVhtnXh47qJlcMWqOKeQl3OnJ2d6/Hc2r+sO2UGb8 ROGoOMQkV0sOb02whAZx2urCkMemZTnMkwQBcZrHn+zR+7fxesaodTLVFGwNE1KQlmsn XQ== To: musl@lists.openwall.com References: <20201201143634.13419-1-mhillen@linux.ibm.com> <20201201205020.GT534@brightrain.aerifal.cx> From: Marius Hillenbrand Message-ID: <3a1b3e6f-3978-4a7a-70b5-9bf18db7704b@linux.ibm.com> Date: Wed, 2 Dec 2020 11:44:59 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.4.0 MIME-Version: 1.0 In-Reply-To: <20201201205020.GT534@brightrain.aerifal.cx> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit X-TM-AS-GCONF: 00 X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:6.0.312,18.0.737 definitions=2020-12-02_04:2020-11-30,2020-12-02 signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 lowpriorityscore=0 phishscore=0 impostorscore=0 adultscore=0 priorityscore=1501 bulkscore=0 mlxscore=0 suspectscore=0 clxscore=1011 mlxlogscore=999 spamscore=0 malwarescore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2009150000 definitions=main-2012020062 Subject: Re: [musl] [PATCH] s390x: derive float_t from compiler or default to float On 12/1/20 9:50 PM, Rich Felker wrote: > On Tue, Dec 01, 2020 at 03:36:34PM +0100, Marius Hillenbrand wrote: >> Hi, >> >> float_t should represent the type that is used to evaluate float >> expressions internally. On s390(x), float_t is currently set to double. >> In contrast, the isa supports single-precision float operations and >> compilers by default evaluate float in single precision, which violates >> the C standard (sections 5.2.4.2.2 and 7.12 in C11/C17). With >> -fexcess-precision=standard, gcc evaluates float in double precision, >> which aligns with the standard yet at the cost of added conversion >> instructions. To improve standards compliance, this patch changes the >> definition of float_t to be derived from the compiler's >> __FLT_EVAL_METHOD__. >> >> The port of glibc to s390 incorrectly deferred to the generic >> definitions which, back then, tied float_t to double. Since then, this >> definition has been kept to avoid ABI changes, most recently in the >> refactoring of float_t into bits/flt-eval-method.h >> https://sourceware.org/legacy-ml/libc-alpha/2016-11/msg00903.html >> and the discussion around >> https://gcc.gnu.org/legacy-ml/gcc-patches/2016-09/msg02392.html >> musl apparently adopted the definition from glibc. >> >> Given the performance overhead and reduced standards compliance, I have >> reevaluated cleaning up the special behavior on s390x. I found only two >> packages, ImageMagick and clucene, that use float_t in their API, out of >>> 130k Debian source packages scanned. To avoid breaking ABI changes, I >> patched these packages to avoid their reliance on float_t (in >> ImageMagick since 7.0.10-39, patch in >> https://github.com/ImageMagick/ImageMagick/pull/2832 - patch for >> clucene in https://sourceforge.net/p/clucene/bugs/233). >> >> gcc-11 will drop the special case to retrofit double >> precision behavior for -fexcess-precision=standard so that >> __FLT_EVAL_METHOD__ will be 0 on s390x in any scenario. >> https://gcc.gnu.org/pipermail/gcc-patches/2020-November/560224.html >> https://gcc.gnu.org/git/?p=gcc.git;a=commitdiff;h=a5dd6b69fcbe74c02d4821ac2daf2b8c9f819f6e >> >> glibc 2.33 will most likely adopt the same behavior as in this patch, so >> that float_t will eventually be float on s390x in any scenario. >> https://sourceware.org/pipermail/libc-alpha/2020-November/120212.html >> >> Testing with libc-test showed no regressions. Failing testcases >> src/math/lgammaf[_r].exe succeed with the patch. >> >> Please review and consider merging this patch. > > Thanks for the detailed report. To be clear, all models/ISA-levels > support the single-precision ops and future GCC will always use them > even with -fexcess-precision=standard, but old ones switch to using > double precision ops with -fexcess-precision=standard to meet the > contract of evaluating in (old definition of) float_t. Is this > correct? Yes, your summary is correct -- with one exception that I omitted in my original post: future GCC compiled against current libc will still switch to using double precision ops with -fexcess-precision=standard to match the old definition of float_t. When future GCC detects a future libc at compile-time, it will always use single-precision ops. Without that switch, updating GCC while keeping your current libc would have worsened the situation wrt the C standard. Marius -- Marius Hillenbrand Linux on Z development IBM Deutschland Research & Development GmbH Vors. des Aufsichtsrats: Gregor Pillen / Geschäftsführung: Dirk Wittkopp Sitz der Gesellschaft: Böblingen / Registergericht: Amtsgericht Stuttgart, HRB 243294