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=MAILING_LIST_MULTI, RCVD_IN_DNSWL_MED,RCVD_IN_MSPIKE_H3,RCVD_IN_MSPIKE_WL autolearn=ham autolearn_force=no version=3.4.4 Received: (qmail 17476 invoked from network); 2 Dec 2020 16:01:55 -0000 Received: from mother.openwall.net (195.42.179.200) by inbox.vuxu.org with ESMTPUTF8; 2 Dec 2020 16:01:55 -0000 Received: (qmail 21932 invoked by uid 550); 2 Dec 2020 16:01:53 -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 21914 invoked from network); 2 Dec 2020 16:01:52 -0000 Date: Wed, 2 Dec 2020 11:01:40 -0500 From: Rich Felker To: Marius Hillenbrand Cc: musl@lists.openwall.com Message-ID: <20201202160140.GW534@brightrain.aerifal.cx> References: <20201201143634.13419-1-mhillen@linux.ibm.com> <20201201205020.GT534@brightrain.aerifal.cx> <3a1b3e6f-3978-4a7a-70b5-9bf18db7704b@linux.ibm.com> <20201202142504.GV534@brightrain.aerifal.cx> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20201202142504.GV534@brightrain.aerifal.cx> User-Agent: Mutt/1.5.21 (2010-09-15) Subject: Re: [musl] [PATCH] s390x: derive float_t from compiler or default to float On Wed, Dec 02, 2020 at 09:25:04AM -0500, Rich Felker wrote: > On Wed, Dec 02, 2020 at 11:44:59AM +0100, Marius Hillenbrand wrote: > > 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. > > How does this "detecting an updated libc" take place? That sounds like > it could be really problematic... I'm looking at https://gcc.gnu.org/pipermail/gcc-patches/2020-November/560225.html which seems to be what you're talking about, and don't understand how it's intended to work. It looks like it's running a test for target behavior on the host compiler (there is no target compiler at the point this test is run). Looking again, I guess that's why it's under a condition for build==host==target. What happens when cross compiling? Do you get the old behavior unless manually setting --disable-s390-excess-float-precision? Also I guess this mildly breaks use of a libc older than the one the compiler was built for, but that's probably the case in general with GCC for various other reasons too. Rich