[Bug libquadmath/105101] incorrect rounding for sqrtq

["joseph at codesourcery dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105101

--- Comment #22 from joseph at codesourcery dot com <joseph at codesourcery dot com> ---
On Mon, 13 Jun 2022, already5chosen at yahoo dot com via Gcc-bugs wrote:

> > The function should be sqrtf128 (present in glibc 2.26 and later on 
> > x86_64, x86, powerpc64le, ia64).  I don't know about support in non-glibc 
> > C libraries.
> 
> x86-64 gcc on Godbolt does not appear to know about it.
> I think, Godbolt uses rather standard Linux with quite new glibc and headers.
> https://godbolt.org/z/Y4YecvxK6

Make sure to define _GNU_SOURCE or __STDC_WANT_IEC_60559_TYPES_EXT__ to 
get these declarations.

> May be. I don't know how to get soft-fp version.

For long double it's sysdeps/ieee754/soft-fp/s_fmal.c in glibc - some 
adjustments would be needed to be able to use that as a version for 
_Float128 (where sysdeps/ieee754/float128/s_fmaf128.c currently always 
uses the ldbl-128 version), in appropriate cases.

> It seems, you didn't pay attention that in my later posts I am giving
> implementations of binary128 *division* rather than sqrtq().

Ah - binary128 division is nothing to do with libquadmath at all (the 
basic arithmetic operations go in libgcc, not libquadmath).  Using a PR 
about one issue as an umbrella discussion of various vaguely related 
things is generally confusing and unhelpful to tracking the status of what 
is or is not fixed.

In general, working out how to optimize the format-generic code in soft-fp 
if possible would be preferred to writing format-specific implementations.  
Note that for multiplication and division there are already various 
choices of implementation approaches that can be selected via macros 
defined in sfp-machine.h.

> BTW, I see no mentioning of rounding control or of any sort of exceptions in
> GCC libquadmath docs. No APIs with names resembling fesetround() or
> mpfr_set_default_rounding_mode().

The underlying arithmetic (in libgcc, not libquadmath) uses the hardware 
rounding mode and exceptions (if the x87 and SSE rounding modes disagree, 
things are liable to go wrong), via various macros defined in 
sfp-machine.h.