Bug#869691: gmp: please add support for arm64ilp32

[Wookey]

Source: gmp
Version: 6.1.2+dfsg-1
Severity: wishlist
Tags: upstream patch

Attached is a patched providing very basic initial suport for
arm64ilp32, using the generic C support. To some degreee it is a
placeholder until a less hacky patch is developed, although it does
work and builds the packages.

ILP32 is a new ABI for arm64(aarch64) exactly equivalent to x32 on x86
(32-bit ints, longs and pointers in the ABI, but a 64-bit instruction set).
https://wiki.linaro.org/Platform/arm64-ilp32

The patching of configure.ac is crude and almost certainly wrong, as
I'm really not sure exacltly how it all fits together, but I hope it
makes fairly clear the sort of thing that is needed. It might actually
make sense to restructure the arm/arm64 support a little as it
currently seems to list a lot of possibilities then exclude some of
them.

aarch64 can support ILP32, and it should use the arm64 dirs, but I
couldn't get that to work. Because the ABI is 32-bit, the
epilogues/prologues of this code probably need munging to deal with
32-bit paramaters. That is a little bit beyond my area of expertise,
so I stuck with the generic code for now, especially as a compiler
person tells me it's quite likely to be faster anyway.

The triplet is aarch64-linux-gnu_ilp32 (much like x32), which should
default to that ABI (which is the active bit of code in the current
patch, and should work without the -mabi=ilp32 option - that is only
needed on a native arm64 system (aarch64-linux-gnu) aka 'LP64' ABI,
building for the non-default ILP32 ABI.

I'd be very happy to work with someone who understands the details to
make a proper non-hacky patch for this. Perhaps I should just talk to
upstream?

A dh_autoreconf is needed to get the configure.ac changes into the
configure that gets run. This gives some warnings, so some more work
there might be a good idea, although it seems to work OK. I just
realised that this patch doesn't include a build-dep for that
(dh-autoreconf). That needs fixing.

There are also a set of symbols changes for libgmp10. Most match with
the arm64 exceptions, but 3 are extra. I'm not sure if this is due to
using 'generic' rather than 'arm64' functions?

So apologies for this being a bit rough, but I thought it best to file
before I forgot what I did.

-- System Information:
Debian Release: 8.9
  APT prefers oldstable-updates
  APT policy: (500, 'oldstable-updates'), (500, 'oldstable')
Architecture: amd64 (x86_64)

Kernel: Linux 3.16.0-4-amd64 (SMP w/8 CPU cores)
Locale: LANG=en_GB.UTF-8, LC_CTYPE=en_GB.UTF-8 (charmap=UTF-8)
Shell: /bin/sh linked to /bin/dash
Init: systemd (via /run/systemd/system)
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