Bug#812852: Vigra auf ppc64el -- continues a little bit

[Ullrich Koethe]

>>> * Remove all occurencies of threading::memory_order* in multi_array_chunked.hxx (or equivalently, replace them with threading::memory_order_seq_cst). This will force all atomic accesses to use sequentially consistent ordering, the most strict possibility.
>>
>> That worked out ... much thanks!
>
> - First of all, there was a little mistake, that was the wrong log file. Unfortunately, the
>    problem with test_multiarray_chunked resp. multi_array_chunked.hxx on ppc64el/le remains also
>    with changing `memory_order_release/acquire` to `memory_order_seq_cst`.

bummer!

> I'm further experimenting with VIGRA_SINGLE_THREADED but no breakthrough so far ...

Looking at the log again, it occured to me that the error might not be 
related to multi-threading at all. Possibly, all failing tests are on 
ChunkedArrayCompressed<T> with T being float and double. Can you please 
check this? If this is true, the error might go away be changing the 
compression algorithm in test_chunked.cxx line 121 from LZ4 into 
ZLIB_FAST or NO_COMPRESSION. If this works, there might be a byte order 
problem in the LZ4 algorithm. Endianess is decided in 
<vigra>/src/impex/lz4.c lines 65 to 79. Please check if the correct 
endianess for your platform is retrieved there, and possibly propose a 
fix if it isn't.

Best
Ulli