Re: [PATCH] math: move i386 sqrtf to C

[Szabolcs Nagy <nsz@port70.net>]

* Rich Felker <dalias@libc.org> [2020-01-09 17:00:14 -0500]:
> On Thu, Jan 09, 2020 at 10:00:06PM +0100, Szabolcs Nagy wrote:
> > > Note that eval_as_float only helps if -ffloat-store is used, which is
> > > a nasty hack and also nonconforming, arguably worse than the behavior
> > > without it, so we should probably drop use of that as a fallback, and
> > > use fp_barrier[f] instead if needed.
> > 
> > i think -ffloat-store almost always drops excess precision
> > including returns and assignments, so with that no
> > annotation is needed. but yes the way the annotation is
> > defined now is not useful against broken compilers or
> > non-standard excess precision setting, in glibc the
> > annotation is defined differently (with inline asm).
> 
> I was thinking in the context of wanting to remove from configure the:
> 
> || { test "$ARCH" = i386 && tryflag CFLAGS_C99FSE -ffloat-store ; }
> 
> which is probably doing more harm than good. Do you know if there are
> things that'd break if we did that? I think eval_as_float should
> probably be defined as fp_barrierf to make it safe in your code,
> conditional on FLT_EVAL_METHOD>0 (and likewise >1 for eval_as_double).

i think -fexcess-precision=standard was introduced in
gcc 4.5 and to get reliable behaviour before that we
needed -ffloat-store. the freebsd math code had
volatile hacks to avoid -ffloat-store but those were
incomplete (there were only a few bugs e.g. see commit
c4359e01303da2755fe7e8033826b132eb3659b1 freebsd sets
x87 precision to double so for them some of these were
not issues in practice).

since we had -ffloat-store i turned off the volatile
hacks in commit 6d3f1a39c14b12026df84f386875b094e3652990
and later completely removed the annotations in commit
9b0fcb441a44456c7b071c7cdaf90403f81ec05a

on new compilers -fexcess-precision=standard is used,
but that turned out to do too many stores on the fdlibm
code (which is why glibc kept using =fast), so in commit
e216951f509b71da193da2fc63e25b998740d58b i started using
float_t and double_t to get fast code in standard mode.
(of course this made things worse for -ffloat-store).

i think we would need to add back the old annotations
to make old compilers safe without -ffloat-store.
(fdlibm often raises fenv exceptions via a final rounding
before return, those could be often handled more cleanly
by __math_oflow etc helpers, but since it was not designed
for inline errno handling some normal return paths can
raise fp exceptions too and thus need eval_as_* annotation).