Date: Fri, 11 Mar 2016 14:04:35 +0100 From: Szabolcs Nagy <nsz@...t70.net> To: musl@...ts.openwall.com Subject: Re: long double on powerpc64 * Justin Cormack <justin@...cialbusservice.com> [2016-03-11 11:19:22 +0000]: > 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. > if long double is 64bit then the 128bit hw floats cannot be used with musl, because we don't want library support for __float128. note that the glibc position is that for __float128 support the minimum required gcc version has to change to gcc-7 because it is abi and libc needs complex support (which will not be in gcc-6 yet). https://gcc.gnu.org/ml/gcc-patches/2015-12/msg02222.html https://sourceware.org/ml/libc-alpha/2016-03/msg00193.html the problem with -ld64 suffix is that gcc-6 already has hardcoded dynamic linker names (i don't think we can change that now, before the gcc-6 release). so i'd leave the dynlinker name as is, use 64bit ld for now and rediscuss the issue when ieee128 long double works in gcc-7
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