libquadmath, glibc, and the Q format flag

[Florian Weimer <fweimer@redhat.com>]

I recently discovered that libquadmath registers custom printf callbacks
on load.  As far as I can tell, this is done so that the Q format flag
can be used to print floating point numbers, using format strings such
as "%Qf".  To enable Q flag processing, libquadmath has to register
replacements for all floating point format specifiers (aAeEfFgG).

Unfortunately, this has the side effect that even with out the Q flag,
printing any floating point number enters a generic, slow path in glibc,
prior to the number formatting itself.  The effect is quite pronounced.
For example this admittedly silly benchmarking script

    for i=1,5000000 do
        print(i, i * math.pi)
    end

runs for 5.8 seconds without libquadmath.so.0 loaded on my x86-64
system.  With libquadmath.so.0 from GCC 12 loaded, it runs for 6.3
seconds.

This impacts most (all?) Fortran code on GNU/Linux because libgfortran
depends on libquadmath.

Would it make sense to transplant the implementation of the Q specifier
from libquadmath to glibc, and disable the registration code in
libquadmath if glibc is recent enough?  At least for the targets that
today have float128 support in glibc?

Thanks,
Florian