Date: Fri, 11 Mar 2016 11:19:22 +0000 From: Justin Cormack <justin@...cialbusservice.com> To: musl@...ts.openwall.com Subject: Re: long double on powerpc64 On 11 March 2016 at 04:17, Rich Felker <dalias@...c.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  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 . >> >> 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 . 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
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