[DAG] Fold trunc(avg(x,y)) for avgceil/floor u/s nodes if they have sufficient leading zero/sign bits

[RKSimon]

Similar to #147683

If a ISD::AVG* node has sufficient leading zero/sign bits then it should still work in a truncated type.

• Create alive2 links to prove when all 4 avg patterns can be safely truncated
• Add test coverage (using aarch64 hadd intrinsics is probably easiest as that will always start from the correct nodes)
• Add suitable folds in DAGCombiner::visitTRUNCATE - including legality checks

[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:

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[llvmbot]

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

Author: Simon Pilgrim (RKSimon)

Similar to #147683

If a ISD::AVG* node has sufficient leading zero/sign bits then it should still work in a truncated type.

• Create alive2 links to prove when all 4 avg patterns can be safely truncated
• Add test coverage (using aarch64 hadd intrinsics is probably easiest as that will always start from the correct nodes)
• Add suitable folds in DAGCombiner::visitTRUNCATE - including legality checks

[RKSimon]

avgfloor cases: https://alive2.llvm.org/ce/z/8uBXB9

[houngkoungting]

Hi @RKSimon , I’d like to take on this issue .