Emacs 25.1 build failures on Ubuntu arm64

[Barry Warsaw]

I'm not sure how much of this email will be actionable, but I wanted to at
least let y'all know about a problem we've seen building Emacs 25.1 on Ubuntu
17.04 arm64.

Quick background: In Ubuntu we have a few minor changes above the package in
Debian, which we mostly inherit.  One of those changes is that we build with
-j1 on arm64 only.  There are a couple of other changes that shouldn't have an
impact on the problem I'll describe, but the changelogs can be easily
inspected.

In any case, for several weeks now we've seen Emacs segfault on arm64 builders
consistently in what I think are the CEDET tests.  The full traceback, when
we've been able to capture it, is here:

https://launchpadlibrarian.net/302409391/gdb-bt-full.txt

The full build log is here:

https://launchpadlibrarian.net/310046964/buildlog_ubuntu-zesty-arm64.emacs25_25.1+1-3ubuntu3~ppa0_BUILDING.txt.gz

(scroll to the bottom to see the crash)

and the bug that tracks this is here:

https://bugs.launchpad.net/ubuntu/+source/emacs25/+bug/1656474

We're using gcc based on 6.2.1/6.3.0 (depending on the build).

All the other architectures have been building fine, but arm64 crashes
consistently on our build machines.  Attempts to reproduce the crash on
development machines, even on arm64 chroots and bare metal have not been
successful, so debugging the problem isn't easy.  Based on some research into
other crashes, various other attempts to work around the problem, and some
discussions among Ubuntu developers on IRC, we wondered whether turning off
optimization would help.  Indeed, adding -O0 to CFLAGS only on arm64 solved
the problem in a test build.

I've now uploaded a new build to the archive with -O0 on arm64 and I expect
this to build and get published once it's approved (Ubuntu is currently in
final beta freeze).

I'll watch this thread via Gmane and try to answer any additional questions
you might have.  As I said, I'm not sure what the Emacs developers can do
about it, but I wanted you to know about the problem and how we think we've
solved it.

Cheers,
-Barry

pgpqBPPg_8o6V.pgp
Description: OpenPGP digital signature