[PATCH] D82005: [InstCombine] Replace selects with Phis

[Max Kazantsev via Phabricator via llvm-commits]

mkazantsev added a comment.

In D82005#2100058 <https://reviews.llvm.org/D82005#2100058>, @nikic wrote:

> @mkazantsev I don't think this is about backend-specific optimizations (which indeed InstCombine should not be concerned about), phis and selects generally have different weak points in mid-level optimization. While you are right that the use of phis enables better jump threading, phis will usually not work with redundancy elimination and code motion optimizations. E.g. I don't think CSE or GVN do anything with phis right now.
>
> That said, I can definitely see the argument that //if// you already have a branch, it is beneficial to represent a select as a phi, and leave the decision on whether to turn the whole branch into a sequence of selects to EarlyCSE.


Well, my patch doesn't break any existing test. I also have tests that get improved and may be potentially improved further. If you say that it might *not* be a profitable transform, please come up with a particular example.

Actually, GVN does *a lot* about Phis (and mostly Phis) in PRE. And EarlyCSE does not walk above merge points anyways.


CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D82005/new/

https://reviews.llvm.org/D82005