I wrote a very small code that mimics this problem, it is at:

  https://github.com/leitao/altivec

I found that the information we are looking for (i.e, the bitmap) is at
the second half of the vector, i.e,  vec_extract(VSR, 1) will return the
correct value of the output of the perm value, and vec_extract(VSR, 0)
will always return zero, independent of the Vector input and the bit
mask.

That said, it seems that the change makes sense on current compilers,
but, I am wondering why it changed from GCC 5.4 to current GCC.

I will test the same code on old compilers and see if I get a different
behavior. I suspect that there was a change on GCC (Maybe the one
provided by 84033?) that caused this element order change.

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https://bugs.launchpad.net/bugs/1758118

Title:
  src/mongo/db/fts/unicode is not optimised on ppc64el

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