FYI I am one of the Boost core dev team, and this problem emerged during
our prerelease testing of Boost 1.56.

I thought a reasonable test of who is at fault would be if I purged all
trace of clang and llvm from the system, and then download these
prebuilt binaries from llvm:

http://llvm.org/releases/3.4.2/clang+llvm-3.4.2-armv7a-linux-
gnueabihf.tar.xz

I unpack these into /usr/local and ensure perms and ownership are reset
to correct values.

If I now repeat the earlier test I see:

1. I no longer need to manually override -mcpu, because the correct
local CPU is now chosen and the atomics compile instead of the assembler
not recognising instructions. I would assume the default triple of
armv7a-linux-gnueabihf helps a lot here.

2. All the unit tests still hang in the same place as with your clang.

This suggests to me that something is very wrong with Boost on clang on
armhf.

I'll investigate more this end, but fixing the default cpu such that it
supports atomics would be a great help, so I'll leave this issue open
for now. I'll come back if I figure out the cause of the hangs and it's
related to packaging.

Niall

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https://bugs.launchpad.net/bugs/1349789

Title:
  clang armhf in trusty simply doesn't work

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