Spill code and unrolling

Hi,


I've a question about loop unrolling. Let's assume that you compile
your program without loop unrolling in advance to check if the loops
which are candidates for loop unrolling contain any spill code. If so,
it is very likely that after loop unrolling the contained spill code
is duplicated by the number of how often this loop is unrolled.


Now, I was wondering if it would be a good heuristic for a loop
unroller to skip loops containing spill code. What do you think? Is it
still useful to unroll such loops (since the positive effects of
unrolling will overbalance) or will the additional spill code probably
decrease the total program execution time (or just negligibly improve
it), thus unrolling of these loops should be omitted?


It is also an interesting question, how this scenario will effect the
program execution time for systems with and without caches.
Intuitively, I would assume that in a system with data caches, the
additional spill code will have in total a negative influence on the
unrolled code. Without data caches, I don't really know what could be
expected.


Regards,
Stephan

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