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Message-ID: <20201202191306.GX534@brightrain.aerifal.cx>
Date: Wed, 2 Dec 2020 14:13:06 -0500
From: Rich Felker <dalias@...c.org>
To: Marius Hillenbrand <mhillen@...ux.ibm.com>
Cc: musl@...ts.openwall.com
Subject: Re: [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
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