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[WIP] Even more registers #217
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Extract as an overload of Scop::activePromotions taking a set of active statement instances and a tensor id. Overload was chosen because both functions return the same data structure and are semantically close.
Internally, we may need to modify the stored active promotions but the public functions return either a copy of or a const reference to that storage. Extract logic to find active promotions into a separate function that returns indexes into the storage, and use it to create a copy inside a public call.
If a group of references was promoted into shared memory, but it could be also promoted to registers while covering exactly the same statement instances accessing it, demote it from shared memory before promoting to registers.
These option combinations were failing with previous implementations of double promotion. Make sure they never fail again.
All other ScheduleTree node types are printed in such a way that each set(map) of the union_set(union_map) present in the node is printed on a new line. Do the same for extension nodes.
This creates a private convenience function to obtain a copy of active promotions specified by a list of their indexes in the storage. Use this function in Scop::promoteGroup to avoid retraversing the list of all promotions twice in a row.
In cases when the appoximate footprint of the reference group being promoted to registers is not a subset of any of the approximate footprints of the reference groups promoted to shared, it is still possible to promote by copying directly from global memory as long as all overlapping reference groups have only read the data. It will just create multiple copies of the data in different storages without compromising correctness.
In cases where a reference group promoted to registers covered exactly the same accesses as another group promoted to shared memory, the second group was demoted to save up shared memory space. However, this led to adverse effects since copying global->shared is performed in the beginning of the block while copying global->register deeper in the tree, which does not allow to hide latency from loads. Keep the group promoted to shared memory and perform a copy shared->register. Alternative solution would be to decrease the promotion scope depth for register promotion. This would require to ensure that loops indices of which are present in subscripts of "register" arrays are fully unrolled so that the elements of that array are effectively mapped to registers. Since unrolling is expensive in compilation time and is exposed to the autotuner, we would prefer to also expose the register promotion depth in the future.
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UNCLEAN AND NON-REBASED WORK IN PROGRESS
Do not merge or even review.