touch-packages team mailing list archive

[Niall Douglas <s_launchpad@xxxxxxxxxxx>]

Good news on this, though it took a day of head scratching to figure it
out.

It turns out that ARM clang 3.3 and earlier implements C++ exceptions
using the inefficient SJLJ exception ABI - you know, the one that calls
setjmp all the time for every place an unwind might happen. Anyway, they
finally got round to implementing the ARM EHABI exception ABI which is
zero runtime cost and they went ahead and turned it on by default in
3.4, or at least it is being turned on by default in the Debian/Ubuntu
binaries as well as the LLVM binaries.

Unfortunately, it was very broken indeed. It produces ARM binaries which
simply cannot catch non-trivial C++ exceptions, though otherwise work
fine. This caused Boost.Thread when interrupting a thread wait to enter
an infinite loop at described above, indeed if you EVER catch a type
with RTTI it infinite loops.

Fortunately, Chromium realised this shortly after the 3.4 release, and
they've been hard at work making a EHABI implementation which actually
works for 3.5. I just finished compiling 3.5 from trunk and I can
confirm that all Boost.AFIO unit test pass swimmingly with it for armhf.

You can read more about the 3.5 ARM exception handling improvements at
http://llvm.org/docs/ReleaseNotes.html#changes-to-the-arm-backend.

Can I recommend you upcall this to Debian as well and recommend that
they mark clang 3.4 as broken on ARM? Either that or have them patched
to disable EHABI exception handling?

Niall

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You received this bug notification because you are a member of Ubuntu
Touch seeded packages, which is subscribed to llvm-toolchain-3.4 in
Ubuntu.
https://bugs.launchpad.net/bugs/1349789

Title:
  clang armhf in trusty simply doesn't work

Status in “llvm-toolchain-3.4” package in Ubuntu:
  Confirmed

Bug description:
  I'm running Ubuntu 14.04 on armhf. clang is from the standard repo:

  root@tegra-ubuntu:~# apt-cache show clang
  Package: clang
  Priority: optional
  Section: universe/devel
  Installed-Size: 27
  Maintainer: Ubuntu Developers <ubuntu-devel-discuss@xxxxxxxxxxxxxxxx>
  Original-Maintainer: LLVM Packaging Team <pkg-llvm-team@xxxxxxxxxxxxxxxxxxxxxxx>
  Architecture: armhf
  Source: llvm-defaults (0.21ubuntu1)
  Version: 1:3.4-0ubuntu1
  Replaces: clang (<< 3.2-1~exp2)
  Depends: clang-3.4 (>= 3.4~rc3-1~)
  Filename: pool/universe/l/llvm-defaults/clang_3.4-0ubuntu1_armhf.deb
  Size: 2478
  MD5sum: eadb3f7c344e364480bdf7fdd0f698ab
  SHA1: ec4d0bfcad42bf536ee8d9257f8930dd32e1a261
  SHA256: f6d9a8fbfc06d93cd4c1a2437114454cc1eec805a635efc490a7e1e6eec2b3de
  Description-en: C, C++ and Objective-C compiler (LLVM based)
   Clang project is a C, C++, Objective C and Objective C++ front-end
   for the LLVM compiler. Its goal is to offer a replacement to the GNU Compiler
   Collection (GCC).
   .
   Clang implements all of the ISO C++ 1998 and 2001 standards and also provides
   a partial support of C++1y.
   .
   This is a dependency package providing the default clang compiler.
  Description-md5: ea1f164ac255f39c6ec78685f71ef19b
  Bugs: https://bugs.launchpad.net/ubuntu/+filebug
  Origin: Ubuntu

  
  Anyway, it does not produce working output. Here are some examples:

  https://ci.nedprod.com/view/Boost.AFIO/job/Boost.AFIO%20Build%20POSIX_ARM_clang%203.4/9/consoleFull

  This log shows how clang has the wrong default target CPU, so it
  outputs unsupported ARM instructions (see the end after the warnings
  spew).

  If I force the target cpu to a cortex-a15 it now at least compiles and
  links and starts to run unit tests:

  https://ci.nedprod.com/view/Boost.AFIO/job/Boost.AFIO%20Test%20POSIX_ARM_clang%203.4/7/consoleFull

  ... but hangs in the first unit test, and is timed out. Before you
  think it the unit tests, here is the exact same thing for GCC 4.8:

  https://ci.nedprod.com/view/Boost.AFIO/job/Boost.AFIO%20Test%20POSIX_ARM_GCC%204.8/7/console

  The unit tests all run and pass as expected. Everything also works
  fine on x86 and x64 with clang 3.4 on Ubuntu 14.04, this appears to be
  an armhf misconfiguration.

  Niall

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