[PATCH][Peephole] Rewrite copies to avoid cross register banks copies.

[Quentin Colombet]

Hi,

Here is a patch to rewrite copies to avoid cross register banks copies when possible.
Thanks for your reviews.

** Context **

By definition copies across register bank are not coalescable. Still, it may be possible to get rid of such a copy when the value is available in another register of the same register file.
Consider the following example, where capital and lower letters denote different register file:
b = copy A <-- cross-bank copy
…
C = copy b <-- cross-bank copy

This could have been optimized this way:
b = copy A  <-- cross-bank copy
…
C = copy A <-- same-bank copy

Note: b and C's definitions may be in different basic blocks.


** Proposed Solution **

Add a peephole optimization that looks through a chain of copies leading to a cross-bank copy and reuses a source that is on the same register file if available.

This solution could also be used to get rid of some copies (e.g., A could have been used instead of C). However, we do not do so because:
- It may over constrain the coloring of the source register for coalescing.
- The register allocator may not be able to find a nice split point for the longer live-range, leading to more spill.


** Testcase?! **

This patch does not include a test case, because it is very difficult to reproduce this behavior with a reasonably small input.

Cheers,
-Quentin
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