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Message-ID: <0fda3e37-b513-91fc-f09e-cfca11a76a20@linux.ibm.com>
Date: Wed, 2 Dec 2020 18:09:44 +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 12/2/20 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.
> 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.
In this specific case, you would get a mismatch between GCC's behavior
and the definition of float_t. Selectively updating GCC while keeping an
older libc is fine when you build GCC in the target environment, or add
the --enable-s390-excess-float-precision flag.
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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