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 14038 invoked from network); 2 Dec 2020 19:13:22 -0000 Received: from mother.openwall.net (195.42.179.200) by inbox.vuxu.org with ESMTPUTF8; 2 Dec 2020 19:13:22 -0000 Received: (qmail 24036 invoked by uid 550); 2 Dec 2020 19:13:19 -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 24010 invoked from network); 2 Dec 2020 19:13:18 -0000 Date: Wed, 2 Dec 2020 14:13:06 -0500 From: Rich Felker To: Marius Hillenbrand Cc: musl@lists.openwall.com Message-ID: <20201202191306.GX534@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> <20201202160140.GW534@brightrain.aerifal.cx> <0fda3e37-b513-91fc-f09e-cfca11a76a20@linux.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <0fda3e37-b513-91fc-f09e-cfca11a76a20@linux.ibm.com> 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 06:09:44PM +0100, Marius Hillenbrand wrote: > > > On 12/2/20 5:01 PM, Rich Felker wrote: > > 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. > > Right, that's the patch. The check only applies to a "native build", > with the assumption that the build environment is the same as the > intended target environment. > > > What happens when cross > > compiling? Do you get the old behavior unless manually setting > > --disable-s390-excess-float-precision? > > When cross compiling, we get the new behavior (the setting starts at > "auto", which is never resolved to yes or no; so the AC_DEFINE is left out). > > In any case, manually setting > --enable/disable-s390-excess-float-precision takes precedence. FWIW this means building GCC 11 for any older version of glibc or musl will give a broken configuration unless you pass --disable-s390-excess-float-precision to configure. I'm not sure if anything should be done about that; at least I might want to handle it in mcm... In any case this probably means I should include your patch in this release cycle so at least current version builds right. BTW is there a -m option to override at runtime in order to test both behaviors, so you don't have to build a new GCC from scratch to do it? Rich