[PATCH] D50977: [TableGen] Prefer user-defined subregister compositions over inferred ones

[Bjorn Pettersson via Phabricator via llvm-commits]

bjope added a reviewer: bjope.
bjope added a comment.

I think a difference is that the ll composition clash with a user-defined subreg, while the hh composition clashes with a tablegen synthesized subreg.
And we do not get the extra synthesized subreg index for hh when there is a user defined composition.

Maybe we can give a warning/error if the user tries to define a subreg like `ComposedSubRegIndex<X, Y>` and the result is an alias/duplicate for Y.
Those are the subregs that I think we want to avoid, because they result in duplicate subregs, and additional lane masks.
What do you think about that?

Btw, the ideas about adding support for aliases was just to plant a seed. I did not see it as a quick fix here, but maybe a future feature.


Repository:
  rL LLVM

https://reviews.llvm.org/D50977