Re: [musl] [PATCH] s390x: derive float_t from compiler or default to float

[Rich Felker <dalias@libc.org>]

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