[musl] Re: ppc64le and 32-bit LE userland compatibility

[Daniel Kolesa]

On Tue, Jun 2, 2020, at 04:36, Segher Boessenkool wrote:
> On Tue, Jun 02, 2020 at 04:12:26AM +0200, Daniel Kolesa wrote:
> > On Tue, Jun 2, 2020, at 03:58, Segher Boessenkool wrote:
> > > I recommend new ports that cannot jump to IEEE QP float directly to use
> > > long double == double for the time being, avoiding the extra
> > > complications that IBM double double would bring.  But you'll still have
> > > a transition to IEEE 128 if you ever want to go there.
> > > 
> > > But if you already use double-double, I don't know if the cost changing
> > > away from that is worth it now.
> > 
> > The transition cost is relatively low, which is why I'm thinking about this in the first place. For one, relatively few things use long double in the first place.
> 
> Then your cost switching to QP float later will be low as well.  I envy
> you :-)
> 
> > For two, on ppc*, at least the bfd linker (which we use always in Void) always tags ELFs with an FP ABI tag, and things not using long double (or using 64-bit long double) don't receive this tag. It even tags *which* ABI is used. See:
> 
> That works for statically linked stuff, sure.  That is the easy case :-/

It works for dynamically linked stuff too, since anything either using or exposing any API from anywhere that contains a long double signature will result in the tag being emitted. The difference becomes fuzzier once you switch to -mlong-double-64, as then they effectively become one type, and you can no longer check for it. But when it's still on 128-bit (any format) it's reasonably trivial (mostly just check the whole repo for binaries/libraries, dump the tags everywhere, collect them...)

> 
> > I went through this once already (I had the 64-bit ldbl transition nearly done) and the number of packages to rebuild in the whole repo was about 200-300 out of ~12000.
> 
> Cool!  Do you perchance have info you can share about which packages?
> Offline, if you want.

Sure. Here's the list of stuff I had to bump in our repo when I attempted the transition ~half a year ago: https://gist.github.com/q66/08720863a3aec12a6612356cd4c0110f

Of course, we've since gained packages, so it might be slightly different now, and our repos are smaller than something like Debian's, but otherwise this list is not super difficult to compile. I'll be compiling a new one for the ieee754 binary128 transition on glibc when the time comes :)

> 
> > > > There is also one more thing while we're at this. The 64-bit big endian Void port uses the ELFv2 ABI, even on glibc. This is not officially supported on glibc as far as I can tell, but it does work out of box, without any patching (things in general match little endian then, i.e. ld64.so.2 etc, but they're big endian). Is there any chance of making that support official?
> > > 
> > > (I don't talk for glibc).
> > > 
> > > The first thing needed is for "us" to have faith in it.  That starts
> > > with seeing test results for the testsuites!
> > > 
> > > (Something similar goes for the GCC port -- there is no official support
> > > for BE ELFv2, but of course it does work, and if we get test results we
> > > may keep it that way, hint hint :-) )
> > 
> > Well, FreeBSD defaults to it since 13; OpenBSD's new powerpc64 port (which is supposedly dual-endian) defaults to it; musl defaults to it on LE and BE.
> 
> ... and no one ever has sent us (GCC) test results (nothing I have seen
> anyway).  All we "officially" know is that Power7 BE ELFv2 was a
> bring-up vehicle for the current powerpc64le-linux.  Everyone tries not
> to break things without reason to, of course, and things are a little
> bit tested anyway because it is convenient to build ELFv2 stuff on BE
> systems as well (if only to figure out effortlessly if some bug is due
> to the ABI or due to the endianness), but if we do not know something is
> used and we never officially supported it, we might just want to take it
> away if it is inconvenient.

Void keeps track of recent versions fairly closely, so if something does break, you can be sure you'll hear about it :) If there is a breakage in something common on ppc BE configurations, we're usually the first to come across it these days (the Adélie folks are doing a good job but they're usually on LTS versions, and Gentoo musl folks tend to be helpful, but still)

Unfortunately, the maintainer team is stretched relatively thin, and on ppc-related maintenance in the distro, it's mostly just me when it comes to major work, so deaing with all upstreams can be tricky. Still, I try to make sure we don't lag behind too much.

> 
> > FreeBSD and OpenBSD have to, since they primarily target LLVM system toolchain (with GCC in ports) and ld.lld doesn't support ELFv1 (at all). Void's port was new (and any precompiled binaries would generally be enterprisey stuff which doesn't concern us enough - people can just make a chroot/container with say, Debian, if they really need to),
> 
> Yeah, enterprisey enough, then just rebuild :-)
> 
> > so I felt like it didn't make sense to go with the legacy ABI (besides, function descriptors are gross ;)).
> 
> Descriptors are Great, you just do not understand the True Way!
> 
> > The situation in the overall userland has been improving too, so the patch burden is actually very low nowadays.
> 
> Is that because long double just isn't used a lot?  Or are there more
> reasons?

I meant ELFv2+BE. Recently support was gained in OpenSSL, for example (though it was a fairly trivial patch... but then, it's pretty much always a fairly trivial patch; the most common pitfalls tend to be mixing of __LITTLE_ENDIAN__ and _CALL_ELF == 2, like, in 90% of cases when there's assembly code hidden behind __LITTLE_ENDIAN__ you can be almost sure it's actually just ELFv2, but those cases have been slowly disappearing)

> 
> 
> Segher
>