long double on powerpc64

[Bobby Bingham <koorogi@koorogi.info>]

I've been working on a PPC64 port of musl lately.  I've made some good
progress, and it's time to decide what to do about the long double type.

The PPC64 ELFv2 ABI [1] calls for a 128 bit long double.  It allows an
implementation to choose to use either IEEE quad, or IBM double double,
with IEEE quad being preferred.

On the compiler side, it looks like things are a bit of a mess.

Clang only supports IBM double double on PPC64, AFAICS, and therefore
won't work for us currently.

GCC support is more complicated.  It supports both 128 bit variants, as
well as supporting (and defaulting to) a 64 bit long double.  To get a
128 bit long double, you must build gcc with --with-long-double-128 or
pass -mlong-double-128, and even then you get IBM double double.  To get
IEEE quad, you must additionally pass -mlong-double-128, though there
are whispers that the default may change in gcc 7 [2].

The final piece of bad news is that gcc can't successfully build musl on
PPC64 with IEEE quad long double.  It chokes on even trivial code using
long double complex [3].  So only 64 bit long double is usable for now.

The good news is that gcc's predefined macros are sufficient to detect
which long double variant is in use.  My current thinking is that we can
support both 64 bit long and IEEE quad as two powerpc64 subarchs, even
if we can only implement 64 bit for now.  Because it looks like the
future direction is for IEEE quad to become the default, I think that
should be the suffix-less subarch, and the 64 bit long double subarch
should have a -ld64 suffix or similar.

  1. https://members.openpowerfoundation.org/document/dl/576
  2. https://gcc.gnu.org/ml/gcc-patches/2015-10/msg02507.html
  3. https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70179

--
Bobby