[llvm] [VPlan] Detect and create partial reductions in VPlan. (NFCI) (PR #167851)

[Sam Tebbs via llvm-commits]

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@@ -991,3 +996,381 @@ bool VPlanTransforms::handleMaxMinNumReductions(VPlan &Plan) {
   MiddleTerm->setOperand(0, NewCond);
   return true;
 }
+
+namespace {
+/// A VPlan-based chain of recipes that form a partial reduction.
+/// Designed to match either:
+///   reduction_bin_op (extend (A), accumulator), or
+///   reduction_bin_op (bin_op (extend (A), (extend (B))), accumulator).
+struct VPPartialReductionChain {
+  VPPartialReductionChain(VPWidenRecipe *Reduction, VPWidenCastRecipe *ExtendA,
+                          VPWidenCastRecipe *ExtendB, VPWidenRecipe *ExtendUser)
+      : Reduction(Reduction), ExtendA(ExtendA), ExtendB(ExtendB),
+        ExtendUser(ExtendUser) {}
+  /// The top-level binary operation that forms the reduction to a scalar
+  /// after the loop body.
+  VPWidenRecipe *Reduction;
+  /// The extension of each of the inner binary operation's operands.
+  VPWidenCastRecipe *ExtendA;
+  VPWidenCastRecipe *ExtendB;
+
+  /// The user of the extends that is then reduced.
+  VPWidenRecipe *ExtendUser;
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SamTebbs33 wrote:

Would it be a good idea to use optionals for these two variables, to make it easy to handle the null case?

https://github.com/llvm/llvm-project/pull/167851