[Bug libstdc++/78851] Resolve DR 550 in cmath and continue using __builtin_powil() even in C++11

["glisse at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78851

--- Comment #3 from Marc Glisse <glisse at gcc dot gnu.org> ---
On linux with glibc, both versions output the same number, so your libm is to
blame. Normally, the int version, implemented with multiplications, may be less
accurate than the general version. And gcc does generate multiplications for
you if you pass it the flag -ffast-math (probably something a little weaker is
sufficient), meaning that you are ok with the slight loss of precision as long
as the program is faster. Depending on the flags (-fwhole-program or -flto for
instance), you may even get f inlined and the result of pow is computed at
compile-time by MPFR ;-)

The template version right below in cmath catches the int case, uses
__builtin_powl, which gcc's optimizers know they can replace with
__builtin_powil if the second argument is converted from an integer and the
circumstances are right (command-line flags). I don't see what in the comment
in cmath makes it look like a temporary patch, we tend to add those DR markers
as documentation of past changes, not as FIXME. Actually, I guess we could
remove those overloads completely, even in C++03, since we have the template
version. But it might confuse users who were relying on the performance benefit
(and relaxed accuracy) in C++03 even without suitable command-line flags.
Everyone should move to C++14 anyway...