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Message-ID: <20160311163851.GF29662@port70.net>
Date: Fri, 11 Mar 2016 17:38:51 +0100
From: Szabolcs Nagy <nsz@...t70.net>
To: musl@...ts.openwall.com
Subject: Re: long double on powerpc64
* Rich Felker <dalias@...c.org> [2016-03-11 10:55:30 -0500]:
> On Fri, Mar 11, 2016 at 02:04:35PM +0100, Szabolcs Nagy wrote:
> > * 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 [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.
>
> I don't follow. Are you saying we would not want quad if it requires
> soft float? I think the idea here is that the baseline binaries that
gcc-6 supports a __float128 type on powerpc64
this is hard float with isa 3.0 (power9) and soft-float earlier.
there is discussion to support __float128 in glibc, but i assumed
we would not want it (needs new math library functions for this
type, header changes e.g. tgmath.h etc)
> work on all models would need to use soft float operations for long
> double, but higher -march could use the hardware directly in the
> future (and the soft support should correctly use the fenv from
> hardware). That's the same as the situation for aarch64, no?
>
aarch64 has 128bit registers architecturally, but no 128bit
arithmetic instructions.
> > 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
>
> Support for __float128 is separate from support for targets where long
> double is IEEE quad, I think. The former is not interesting IMO.
>
yes.
however whitout __float128 and with 64bit long double musl
cannot use the fancy 128bit hw float instructions
(this was my point above).
> > 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
>
> Any idea why IEEE quad support requires bleeding-edge gcc for some
> targets when mips64 had it way back in gcc 4.2 or earlier?
>
it was not supported on powerpc64, because they used different
long double and untangling that mess is apparently non-trivial.
(they had to introduce new modes
https://gcc.gnu.org/ml/gcc-patches/2015-06/msg00229.html
and do lot of changes in gcc/config/rs6000/ to make this
work, powerpc also has decimal float stuff and converting
those to ieee128 does not work yet)
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