Loop unrolling.
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This pass performs loop unrolling. We only perform this
optimization on innermost loops (with single exception) because
the impact on performance is greatest here, and we want to avoid
unnecessary code size growth. The gain is caused by greater sequentiality
of code, better code to optimize for further passes and in some cases
by fewer testings of exit conditions. The main problem is code growth,
that impacts performance negatively due to effect of caches.
What we do:
-- unrolling of loops that roll constant times; this is almost always
win, as we get rid of exit condition tests.
-- unrolling of loops that roll number of times that we can compute
in runtime; we also get rid of exit condition tests here, but there
is the extra expense for calculating the number of iterations
-- simple unrolling of remaining loops; this is performed only if we
are asked to, as the gain is questionable in this case and often
it may even slow down the code
For more detailed descriptions of each of those, see comments at
appropriate function below.
There is a lot of parameters (defined and described in params.def) that
control how much we unroll.
??? A great problem is that we don't have a good way how to determine
how many times we should unroll the loop; the experiments I have made
showed that this choice may affect performance in order of several %.
Information about induction variables to split.
