[PATCH] D107711: [DAGCombiner] Add target hook function to decide folding (mul (add x, c1), c2)

[Jessica Clarke via Phabricator via llvm-commits]

jrtc27 added inline comments.


================
Comment at: llvm/include/llvm/CodeGen/TargetLowering.h:2083
 
+  /// Return true if it may be profitable to fold
+  /// (mul (add x, c1), c2) -> (add (mul x, c2), c1*c2), and return false
----------------
The comment's still not great, putting grammatical issues aside. A lot of it is just explaining the basics of TLI hooks, but also is overly prescriptive with what backends should do to evaluate it (and I also don't like "to avoid definite worse code generated", often TLI hooks end up being best-effort heuristics, unable to give a definitive answer, because that might require extremely expensive whole-function checks that depend on knowing what other transformations are going to be made).

I'd just go with something like (borrowing style from surrounding examples):

> Return true if it may be profitable to transform
> (mul (add x, c1), c2) -> (add (mul x, c2), c1*c2).
> This may not be true if c1 and c2 can be represented as immediates but c1*c2 cannot, for example.


CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D107711/new/

https://reviews.llvm.org/D107711