[llvm] [VPlan] Implement interleaving as VPlan-to-VPlan transform. (PR #95842)

[via llvm-commits]

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@@ -1397,6 +1400,9 @@ class VPInstruction : public VPRecipeWithIRFlags {
   /// Returns true if this VPInstruction's operands are single scalars and the
   /// result is also a single scalar.
   bool isSingleScalar() const;
+
+  /// Return the interleave count from VPInstruction's last operand.
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ayalz wrote:

If interleaving took place, otherwise returns 1 indicating interleaving did not take place?
Perhaps better to use the term `unroll factor`, as in UF, to avoid confusion with interleave groups?

https://github.com/llvm/llvm-project/pull/95842