Could you go into more detail about this choice of cost calculation?
It looks like we first calculate per-group flags, which are true only if
the unrolled offsets are valid for all uses in the group. Then we create
per-candidate flags when associating candidates with groups.
Instead, couldn't we take this into account in get_address_cost,
which calculates the cost of an address use for a given candidate?
E.g. after the main if-else at the start of the function,
perhaps it would make sense to add the worst-case offset to
the address in “parts”, check whether that too is a valid address,
and if not, increase var_cost by the cost of one add instruction.
I guess there are two main sources of inexactness if we do that:
(1) It might underestimate the cost because it assumes that vuse[0]
stands for all vuses in the group.
(2) It might overestimates the cost because it treats all unrolled
iterations as having the cost of the final unrolled iteration.
(1) could perhaps be avoided by adding a flag to the iv_use to say
whether it wants this treatment. I think the flag approach suffers
from (2) too, and I'd be surprised if it makes a difference in practice.
Thanks,
Richard
