Re: long double on powerpc64

[Rich Felker <dalias@libc.org>]

On Thu, Mar 10, 2016 at 09:16:36PM -0600, Bobby Bingham wrote:
> 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.

My leaning would be to just go with ld64 if nobody has their act
together for quad support, but let's see what people who want to use
powerpc64 think about it. The only option that's not on the table is
IBM double-double (because it's incompatible with musl's assumption of
IEEE semantics; math-savvy people in the musl community already know
this of course but I'm repeating it for the sake of possible
newcomers).

Thanks for digging up this information.

Rich