Date: Wed, 2 Dec 2020 11:44:59 +0100 From: Marius Hillenbrand <mhillen@...ux.ibm.com> To: musl@...ts.openwall.com Subject: Re: [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 126.96.36.199.2 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
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