From: Szabolcs Nagy <nsz@port70.net>
To: musl@lists.openwall.com
Subject: Re: long double on powerpc64
Date: Fri, 11 Mar 2016 14:04:35 +0100 [thread overview]
Message-ID: <20160311130434.GE29662@port70.net> (raw)
In-Reply-To: <CAK4o1WzoNh_C7gQhfGQReZmX86WxwhJPd7SWz11pnU7nOKGnTw@mail.gmail.com>
* Justin Cormack <justin@specialbusservice.com> [2016-03-11 11:19:22 +0000]:
> 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.
>
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
next prev parent reply other threads:[~2016-03-11 13:04 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-03-11 3:16 Bobby Bingham
2016-03-11 4:17 ` Rich Felker
2016-03-11 11:19 ` Justin Cormack
2016-03-11 13:04 ` Szabolcs Nagy [this message]
2016-03-11 15:55 ` Rich Felker
2016-03-11 16:38 ` Szabolcs Nagy
2016-03-14 20:13 ` Bobby Bingham
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