Handling prefetcher tag collisions while allocating registers

[Kugan Vivekanandarajah <kugan dot vivekanandarajah at linaro dot org>]

Hi All,

I am wondering if there is anyway we can prefer certain registers in
register allocations. That is, I want to have some way of recording
register allocation decisions (for loads in loop that are accessed in
steps) and use this to influence register allocation of other loads
(again that are accessed in steps).

This is for architectures (like falkor AArch64) that use hardware
perefetchers that use signatures of the loads to lock into and tune
prefetching parameters. Ideally, If the loads are from the same
stream, they should have same signature and if they are from different
stream, they should have different signature. Destination, base
register and offset are used in the signature. Therefore, selecting
different register can influence this.

In LLVM, this is implemented as a machine specific pass that runs
after register allocation. It then inserts mov instruction with
scratch registers to manage this. We can do a machine reorg pass in
gcc but detecting strided loads at that stage is not easy.

I am trying to implement this in gcc and wondering what is the
preferred and acceptable way to implement this. Any thoughts ?

Thanks,
Kugan