[llvm] [LV]: Teach LV to recursively (de)interleave. (PR #89018)

[Paul Walker via llvm-commits]

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@@ -2126,10 +2127,39 @@ static Value *interleaveVectors(IRBuilderBase &Builder, ArrayRef<Value *> Vals,
   // Scalable vectors cannot use arbitrary shufflevectors (only splats), so
   // must use intrinsics to interleave.
   if (VecTy->isScalableTy()) {
-    VectorType *WideVecTy = VectorType::getDoubleElementsVectorType(VecTy);
-    return Builder.CreateIntrinsic(WideVecTy, Intrinsic::vector_interleave2,
-                                   Vals,
-                                   /*FMFSource=*/nullptr, Name);
+    unsigned InterleaveFactor = Vals.size();
+    SmallVector<Value *> InterleavingValues;
+    unsigned InterleavingValuesCount =
+        InterleaveFactor + (InterleaveFactor - 2);
+    InterleavingValues.resize(InterleaveFactor);
+    // Place the values to be interleaved in the correct order for the
+    // interleaving
+    for (unsigned I = 0, J = InterleaveFactor / 2, K = 0; K < InterleaveFactor;
+         K++) {
+      if (K % 2 == 0) {
+        InterleavingValues[K] = Vals[I];
+        I++;
+      } else {
+        InterleavingValues[K] = Vals[J];
+        J++;
+      }
+    }
+#ifndef NDEBUG
+    for (Value *Val : InterleavingValues)
+      assert(Val && "NULL Interleaving Value");
+#endif
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paulwalker-arm wrote:

Does this assert add any value?

>From the code it can be seen that `InterleavingValues` has `InterleaveFactor` elements, which itself is the size of `Vals`, and the loop goes from 0:InterleaveFactor.  This means the only way `InterleavingValues` can have a NULL entry is if it came from `Vals`, which cannot happen because there's already an assert above where the type of each element of `Vals` is checked (i.e. all the Value* have been dereference by this point anyway).

https://github.com/llvm/llvm-project/pull/89018