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

[via llvm-commits]

================
@@ -3825,6 +3862,41 @@ inline bool isUniformAfterVectorization(const VPValue *VPV) {
 
 /// Return true if \p V is a header mask in \p Plan.
 bool isHeaderMask(const VPValue *V, VPlan &Plan);
+
+/// Checks if \p C is uniform across all VF lanes and UF parts. It is considered
+/// as such if it is either loop invariant (defined outside the vector region)
+/// or its operand is known to be uniform across all VFs and UFs (e.g.
+/// VPDerivedIV or VPCanonicalIVPHI).
+inline bool isUniformAcrossVFsAndUFs(VPValue *V) {
+  // Loop invariants are uniform:
+  if (V->isDefinedOutsideVectorRegions())
+    return true;
+
+  auto *R = V->getDefiningRecipe();
+  // Canonical IV chain is uniform:
+  auto *CanonicalIV = R->getParent()->getPlan()->getCanonicalIV();
+  if (R == CanonicalIV || V == CanonicalIV->getBackedgeValue())
+    return true;
+
+  // DerivedIV is uniform:
+  if (isa<VPDerivedIVRecipe>(R))
+    return true;
+
+  // Loads and stores that are uniform across VF lanes are handled by
+  // VPReplicateRecipe.IsUniform. They are also uniform across UF parts if all
+  // their operands are invariant:
+  if (isa<VPReplicateRecipe>(V) && cast<VPReplicateRecipe>(V)->isUniform() &&
+      (isa<LoadInst, StoreInst>(V->getUnderlyingValue())) &&
+      all_of(R->operands(),
+             [](VPValue *Op) { return Op->isDefinedOutsideVectorRegions(); }))
+    return true;
+
+  return isa<VPScalarCastRecipe, VPWidenCastRecipe>(R) &&
+         (R->getOperand(0)->isLiveIn() ||
+          isa<VPDerivedIVRecipe>(R->getOperand(0)) ||
+          isa<VPCanonicalIVPHIRecipe>(R->getOperand(0)));
----------------
ayalz wrote:

```suggestion
  if (isa<VPScalarCastRecipe, VPWidenCastRecipe>(R))
      return isUniformAcrossVFsAndUFs(R->getOperand(0));
  // A values is considered non-uniform unless proven otherwise.
  return false;
```

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