[PATCH] D23601: [TII] add new target hook isAdd

[Sjoerd Meijer via Phabricator via llvm-commits]

SjoerdMeijer added a comment.

The context of this work was to recognise loop induction statements in Machine Instructions/Loops and to allow calculation of loop iteration counts. And isAdd provides more semantic information about a MI and helps recognising induction variables. Thus, this was the ground work for https://reviews.llvm.org/D27193, which was the next step to recognise loop induction. This patch was abandoned because the Quentin's comment was that the analysis was too ad-hoc and missing some formalism.  The reason this is used only in the Hexagon backend, is because I refactored it and took it from the Hexagon backend. The Hexagon backend has a lot of MachineLoop/MI analysis and other goodness (sw pipeliner) that is mostly target independent and could be made available, but a few target specific hooks are required to do this.
Now, I really would like to pick up this work again because I still think it will be valuable, but need to rethink strategy after abandoning https://reviews.llvm.org/D27193. Probably that will be a some SVEC-like approach, but hopefully that can be a light-weight implementation to recognise simple loops (which is what we're doing anyway). This will probably be quite some work, so in the mean time, I really don't mind removing or moving isAdd back to the Hexagon backend. I could also dig up the exact requirements and document that if it's okay to keep it.


Repository:
  rL LLVM

https://reviews.llvm.org/D23601