[llvm] [ConstantFolding] Add folding for [de]interleave2, insert and extract (PR #141301)

[Nikolay Panchenko via llvm-commits]

================
@@ -3734,6 +3738,77 @@ static Constant *ConstantFoldFixedVectorCall(
     }
     return nullptr;
   }
+  case Intrinsic::vector_extract: {
+    auto *Vec = dyn_cast<Constant>(Operands[0]);
+    auto *Idx = dyn_cast<ConstantInt>(Operands[1]);
+    if (!Vec || !Idx)
+      return nullptr;
+
+    unsigned NumElements = FVTy->getNumElements();
+    unsigned VecNumElements =
+        cast<FixedVectorType>(Vec->getType())->getNumElements();
+    unsigned StartingIndex = Idx->getZExtValue();
+    // Extracting entire vector is nop
+    if (NumElements == VecNumElements && StartingIndex == 0)
+      return Vec;
+
+    const unsigned NonPoisonNumElements =
+        std::min(StartingIndex + NumElements, VecNumElements);
+    for (unsigned I = StartingIndex; I < NonPoisonNumElements; ++I)
+      Result[I - StartingIndex] = Vec->getAggregateElement(I);
+
+    // Remaining elements are poison since they are out of bounds.
+    for (unsigned I = NonPoisonNumElements, E = StartingIndex + NumElements;
+         I < E; ++I)
+      Result[I - StartingIndex] = PoisonValue::get(FVTy->getElementType());
+
+    return ConstantVector::get(Result);
+  }
+  case Intrinsic::vector_insert: {
+    auto *Vec = dyn_cast<Constant>(Operands[0]);
+    auto *SubVec = dyn_cast<Constant>(Operands[1]);
+    auto *Idx = dyn_cast<ConstantInt>(Operands[2]);
+    if (!Vec || !SubVec || !Idx)
+      return nullptr;
+
+    unsigned SubVecNumElements =
+        cast<FixedVectorType>(SubVec->getType())->getNumElements();
+    unsigned VecNumElements =
+        cast<FixedVectorType>(Vec->getType())->getNumElements();
+    unsigned IdxN = Idx->getZExtValue();
+    // Replacing entire vector with a subvec is nop
+    if (SubVecNumElements == VecNumElements && IdxN == 0)
+      return SubVec;
+
+    // Make sure indices are in the range [0, VecNumElements), otherwise the
+    // result is a poison value.
+    if (IdxN >= VecNumElements || IdxN + SubVecNumElements > VecNumElements ||
+        (IdxN && (SubVecNumElements % IdxN) != 0))
----------------
npanchen wrote:

[langref](https://llvm.org/docs/LangRef.html#llvm-vector-insert-intrinsic) and [verifier](https://github.com/llvm/llvm-project/blob/main/llvm/lib/IR/Verifier.cpp#L6300-L6302) do require this for fixed vectors too, which IMO does not make sense: it should be possible to insert subvec into any part of the vec.

https://github.com/llvm/llvm-project/pull/141301