Sometimes we need to be able to signal one thread among a mask, without
caring much about which bit will be picked. At the moment we use ffsl()
for this but this sometimes results in imbalance at certain specific
places where the same first thread in a set is always the same one that
is selected.
Another approach would consist in using the rank finding function but it
requires a popcount and a setup phase, and possibly a modulo operation
depending on the popcount, which starts to be very expensive.
Here we take a different approach. The idea is an input bit value is
passed, from 0 to LONGBITS-1, and that as much as possible we try to
pick the bit matching it if it is set. Otherwise we look at a mirror
position based on a decreasing power of two, and jump to the side
that still has bits left. In 6 iterations it ends up spotting one bit
among 64 and the operations are very cheap and optimizable. This method
has the benefit that we don't care where the holes are located in the
mask, thus it shows a good distribution of output bits based on the
input ones. A long-time test shows an average of 16 cycles, or ~4ns
per lookup at 3.8 GHz, which is about twice as fast as using the rank
finding function.
Just like for that one, the code was stored into tools.c since we don't
have a C file for intops.
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