Hi,

The aim of this RFC is to explore a way of cleaning up the codegen 
around data_references.  To be specific, I'd like to reuse the 
main-loop's updated data_reference as the base_address for the 
epilogue's corresponding data_reference, rather than use the niters.  We 
have found this leads to better codegen in the vectorized epilogue loops.

The approach in this RFC creates a map if iv_updates which always 
contain an updated pointer that is caputed in vectorizable_{load,store}, 
an iv_update may also contain a skip_edge in case we decide the 
vectorization can be skipped in 'vect_do_peeling'. During the epilogue 
update this map of iv_updates is then checked to see if it contains an 
entry for a data_reference and it is used accordingly and if not it 
reverts back to the old behavior of using the niters to advance the 
data_reference.

The motivation for this work is to improve codegen for the option 
`--param vect-partial-vector-usage=1` for SVE. We found that one of the 
main problems for the codegen here was coming from unnecessary 
conversions caused by the way we update the data_references in the epilogue.

This patch passes regression tests in aarch64-linux-gnu, but the codegen 
is still not optimal in some cases. Specifically those where we have a 
scalar epilogue, as this does not use the data_reference's and will rely 
on the gimple scalar code, thus constructing again a memory access using 
the niters.  This is a limitation for which I haven't quite worked out a 
solution yet and does cause some minor regressions due to unfortunate 
spills.

Let me know what you think and if you have ideas of how we can better 
achieve this.

Kind regards,
Andre Vieira