[llvm-branch-commits] [llvm] [LV] Use VPReductionRecipe for partial reductions (PR #146073)

[Sander de Smalen via llvm-branch-commits]

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@@ -2744,6 +2702,12 @@ class VPSingleDefBundleRecipe : public VPSingleDefRecipe {
     /// vector operands, performing a reduction.add on the result, and adding
     /// the scalar result to a chain.
     MulAccumulateReduction,
+    /// Represent an inloop multiply-accumulate reduction, multiplying the
+    /// extended vector operands, negating the multiplication, performing a
+    /// reduction.add
+    /// on the result, and adding
+    /// the scalar result to a chain.
+    ExtNegatedMulAccumulateReduction,
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sdesmalen-arm wrote:

This looks like something that is currently not yet supported for InLoop reductions. Because you're trying to align the PartialReductions with other reductions, we need to add support for `sub` reductions as well to InLoop reductions. I'd suggest pulling that out into a separate PR (and rebasing this one on top of that).

This will require a few changes:
* `RecurrenceDescriptor::getReductionOpChain` does not recognise sub-reductions at the moment, which will need fixing. The reason for that is described in one of the comments:
```
// [..] Subs are also currently not allowed (which are usually
// treated as part of a add reduction) as they are expected to generally be
// more expensive than out-of-loop reductions, and need to be costed more
// carefully.
```
Because you're adding a cost-model for those here, we can now handle sub-reductions as well.

* When the `VPReductionRecipe` is created in `LoopVectorizationPlanner::adjustRecipesForReductions`, for in-loop sub-reductions we'd need to add a `sub(0, VecOp)` similar to how we do this for partial reductions.

When we finally properly merge the two implementations, the special code we currently have for partial reductions to insert the `sub` can be removed, in favour of the code that we'd need to add for in-loop reductions. (FWIW, that is not something I'm suggesting to do as part of this PR, but rather as a future follow-up)

https://github.com/llvm/llvm-project/pull/146073