[AArch64][SVE] Add SPLAT_VECTOR ISD Node

Adds a new ISD node to replicate a scalar value across all elements of
a vector. This is needed for scalable vectors, since BUILD_VECTOR cannot
be used.

Fixes up default type legalization for scalable vectors after the
new MVT type ranges were introduced.

At present I only use this node for scalable vectors. A DAGCombine has
been added to transform a BUILD_VECTOR into a SPLAT_VECTOR if all
elements are the same, but only if the default operation action of
Expand has been overridden by the target.

I've only added result promotion legalization for scalable vector
i8/i16/i32/i64 types in AArch64 for now.

Reviewers: t.p.northover, javed.absar, greened, cameron.mcinally, jmolloy

Reviewed By: jmolloy

Differential Revision: https://reviews.llvm.org/D47775

llvm-svn: 375222

1 files changed, 20 insertions, 5 deletions

diff --git a/llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp b/llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
index b522e64..3d26345 100644
--- a/llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
@@ -3538,17 +3538,32 @@ void SelectionDAGBuilder::visitExtractElement(const User &I) {
 void SelectionDAGBuilder::visitShuffleVector(const User &I) {
   SDValue Src1 = getValue(I.getOperand(0));
   SDValue Src2 = getValue(I.getOperand(1));
+  Constant *MaskV = cast<Constant>(I.getOperand(2));
   SDLoc DL = getCurSDLoc();
-
-  SmallVector<int, 8> Mask;
-  ShuffleVectorInst::getShuffleMask(cast<Constant>(I.getOperand(2)), Mask);
-  unsigned MaskNumElts = Mask.size();
-
   const TargetLowering &TLI = DAG.getTargetLoweringInfo();
   EVT VT = TLI.getValueType(DAG.getDataLayout(), I.getType());
   EVT SrcVT = Src1.getValueType();
   unsigned SrcNumElts = SrcVT.getVectorNumElements();
 
+  if (MaskV->isNullValue() && VT.isScalableVector()) {
+    // Canonical splat form of first element of first input vector.
+    SDValue FirstElt = DAG.getNode(ISD::EXTRACT_VECTOR_ELT, DL,
+                                   SrcVT.getScalarType(), Src1,
+                                   DAG.getConstant(0, DL, 
+                                   TLI.getVectorIdxTy(DAG.getDataLayout())));
+    setValue(&I, DAG.getNode(ISD::SPLAT_VECTOR, DL, VT, FirstElt));
+    return;
+  }
+
+  // For now, we only handle splats for scalable vectors.
+  // The DAGCombiner will perform a BUILD_VECTOR -> SPLAT_VECTOR transformation
+  // for targets that support a SPLAT_VECTOR for non-scalable vector types.
+  assert(!VT.isScalableVector() && "Unsupported scalable vector shuffle");
+
+  SmallVector<int, 8> Mask;
+  ShuffleVectorInst::getShuffleMask(MaskV, Mask);
+  unsigned MaskNumElts = Mask.size();
+
   if (SrcNumElts == MaskNumElts) {
     setValue(&I, DAG.getVectorShuffle(VT, DL, Src1, Src2, Mask));
     return;