Re: long double on powerpc64

[Justin Cormack <justin@specialbusservice.com>]

On 11 March 2016 at 04:17, Rich Felker <dalias@libc.org> wrote:
> 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).

I think it would be a mistake to only support ld64, I think Bobby's approach
of two architectures is probably better, and maybe look to retire ld64
eventually.

Justin