https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105101
--- Comment #7 from Thomas Koenig <tkoenig at gcc dot gnu.org> ---
(In reply to Jakub Jelinek from comment #6)
> (In reply to Thomas Koenig from comment #5)
> > There is another, much worse, problem, reported and analyzed by "Michael S"
> > on comp.arch. The code has
> >
> > #ifdef HAVE_SQRTL
> > {
> > long double xl = (long double) x;
> > if (xl <= LDBL_MAX && xl >= LDBL_MIN)
> > {
> > /* Use long double result as starting point. */
> > y = (__float128) sqrtl (xl);
> >
> > /* One Newton iteration. */
> > y -= 0.5q * (y - x / y);
> > return y;
> > }
> > }
> > #endif
> >
> > which assumes that long double has a higher precision than
> > normal double. On x86_64, this depends o the settings of the
> > FPU flags, so a number like 0x1.06bc82f7b9d71dfcbddf2358a0eap-1024
> > is corrected with 32 ULP of error because there is only a single
> > round of Newton iterations if the FPU flags are set to normal precision.
>
> That is only a problem on OSes that do that, I think mainly BSDs, no?
Correct.
> On Linux it should be fine (well, still not 0.5ulp precise, but not as bad
> as when sqrtl is just double precision precise).
In this case, it was discovered on some version of WSL.
