Subject: Re: ppc64le and 32-bit LE userland compatibility

From: Will Springer <[email-masked]>
Date: Sun, 31 May 2020 00:57:12 +0000

On Saturday, May 30, 2020 12:22:12 PM PDT Segher Boessenkool wrote:
> The original sysv PowerPC supplement
> http://refspecs.linux-foundation.org/elf/elfspec_ppc.pdf
> supports LE as well, and most powerpcle ports use that.  But, the
> big-endian Linux ABI differs in quite a few places, and it of course
> makes a lot better sense if powerpcle-linux follows that.

Right, I should have clarified I was talking about Linux ABIs 
specifically.

> What patches did you need?  I regularly build >30 cross compilers (on
> both BE and LE hosts; I haven't used 32-bit hosts for a long time, but
> in the past those worked fine as well).  I also cross-built
> powerpcle-linux-gcc quite a few times (from powerpc64le, from powerpc64,
> from various x86).

There was just an assumption that LE == powerpc64le in libgo, spotted by 
q66 (daniel@ on the CC). I just pushed the patch to [1].

> Almost no project that used 32-bit PowerPC in LE mode has sent patches
> to the upstreams.

Right, but I have heard concerns from at least one person familiar with 
the ppc kernel about breaking existing users of this arch-endianness 
combo, if any. It seems likely that none of those use upstream, though ^^;

> The ABI says long longs are passed in the same order in registers as it
> would be in memory; so the high part and the low part are swapped
> between BE and LE.  Which registers make up a pair is exactly the same
> between the two.  (You can verify this with an existing powerpcle-*
> compiler, too; I did, and we implement it correctly as far as I can
> see).

I'll give it a closer look. This is my first time poking at this sort of 
thing in depth, so excuse my unfamiliarity!

> A huge factor in having good GCC support for powerpcle-linux (or
> anything else) is someone needs to regularly test it, and share test
> results with us (via gcc-testresults@).  Hint hint hint :-)
> 
> That way we know it is in good shape, know when we are regressing it,
> know there is interest in it.

Once I have more of a bootstrapped userland than a barely-functional 
cross chroot, I'll get back to you on that :)
 
> gl;hf,
> Segher

Thanks,
Will [she/her]

[1]: https://github.com/Skirmisher/void-packages/blob/master/srcpkgs/gcc/patches/libgo-ppcle.patch