[llvm] [RISCV] Extend zvqdot matching to handle reduction trees (PR #138965)

Wed May 7 13:54:25 PDT 2025

https://github.com/preames created https://github.com/llvm/llvm-project/pull/138965

Now that we have matching for vqdot in it's basic variants, we can extend the matcher to handle reduction trees instead of individual reductions.  This is important as we canonicalize reductions by performing a tree in the vector domain before the root reduction instruction.

The particular approach taken here has the unfortunate implication that non-matches visit the entire reduction tree once for each time the reduction root is visited in DAG.  While conceptually problematic for compile time, this is probably fine in practice as we should only visit the root once per pass of DAGCombine. I don't really see a better solution - suggestions welcome.

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