[DAG] Fold AVGU(ZEXT(X),ZEXT(Y)) -> ZEXT(AVGU(X,Y))

[RKSimon]

This works for AVGCEILU and AVGFLOORU: https://alive2.llvm.org/ce/z/ea-m5Y

If the pre-extended AVGCEILU/AVGFLOORU node is legal, then we should perform the operation with the shorter width and then extend afterward.

[llvmbot]

Hi!

This issue may be a good introductory issue for people new to working on LLVM. If you would like to work on this issue, your first steps are:

1. In the comments of the issue, request for it to be assigned to you.
2. Fix the issue locally.
3. Run the test suite locally. Remember that the subdirectories under test/ create fine-grained testing targets, so you can e.g. use make check-clang-ast to only run Clang's AST tests.
4. Create a Git commit.
5. Run git clang-format HEAD~1 to format your changes.
6. Open a pull request to the upstream repository on GitHub. Detailed instructions can be found in GitHub's documentation.

If you have any further questions about this issue, don't hesitate to ask via a comment in the thread below.

[llvmbot]

@llvm/issue-subscribers-good-first-issue

Author: Simon Pilgrim (RKSimon)

This works for AVGCEILU and AVGFLOORU: https://alive2.llvm.org/ce/z/ea-m5Y

If the pre-extended AVGCEILU/AVGFLOORU node is legal, then we should perform the operation with the shorter width and then extend afterward.

[houndlord]

I would like to work on this as well

[RKSimon]

@houndlord any progress? #92096 is likely to create more examples of this pattern

[houndlord]

Thank you for the new context.
I think I do not have to touch TargetLowering yet and should implement the changes in a similar way to foldAddToAvg i.e. pattern matching followed by DAG transformation?

[RKSimon]

You should be able to handle this in DAGCombiner::visitAVG

[RKSimon]

@houndlord ping?

[c8ef]

I'd be happy to work on this if @houndlord is unavailable.

[houndlord]

@houndlord ping?

Sorry for the delay. Well, it seems @c8ef already picked it up.