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(gmp.info)Assembler Software Pipelining

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Software Pipelining
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   Software pipelining consists of scheduling instructions around the
branch point in a loop.  For example a loop taking a checksum of an
array of limbs might have a load and an add, but the load wouldn't be
for that add, rather for the one next time around the loop.  Each load
then is effectively scheduled back in the previous iteration, allowing
latency to be hidden.

   Naturally this is wanted only when doing things like loads or
multiplies that take a few cycles to complete, and only where a CPU has
multiple functional units so that other work can be done while waiting.

   A pipeline with several stages will have a data value in progress at
each stage and each loop iteration moves them along one stage.  This is
like juggling.

   Within the loop some moves between registers may be necessary to
have the right values in the right places for each iteration.  Loop
unrolling can help this, with each unrolled block able to use different
registers for different values, even if some shuffling is still needed
just before going back to the top of the loop.