[DAG] Fold insert_subvector (splat X), (splat X), N2 - > splat X (#147380)

If we're inserting a splat into a splat of the same value, then
regardless of the index, the result is simply a splat of that value.

Author: preames

llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp

@@ -27620,6 +27620,11 @@ SDValue DAGCombiner::visitINSERT_SUBVECTOR(SDNode *N) {
     if (DAG.isConstantValueOfAnyType(N1.getOperand(0)) || N1.hasOneUse())
       return DAG.getNode(ISD::SPLAT_VECTOR, SDLoc(N), VT, N1.getOperand(0));
 
+  // insert_subvector (splat X), (splat X), N2 -> splat X
+  if (N0.getOpcode() == ISD::SPLAT_VECTOR && N0.getOpcode() == N1.getOpcode() &&
+      N0.getOperand(0) == N1.getOperand(0))
+    return N0;
+
   // If we are inserting a bitcast value into an undef, with the same
   // number of elements, just use the bitcast input of the extract.
   // i.e. INSERT_SUBVECTOR UNDEF (BITCAST N1) N2 ->

llvm/test/CodeGen/RISCV/rvv/insert-subvector.ll

@@ -620,11 +620,8 @@ define <vscale x 8 x i32> @insert_splat_to_undef() {
 define <vscale x 8 x i32> @insert_splat_to_splat() {
 ; CHECK-LABEL: insert_splat_to_splat:
 ; CHECK:       # %bb.0:
-; CHECK-NEXT:    vsetvli a0, zero, e32, m2, ta, ma
-; CHECK-NEXT:    vmv.v.i v12, 1
 ; CHECK-NEXT:    vsetvli a0, zero, e32, m4, ta, ma
 ; CHECK-NEXT:    vmv.v.i v8, 1
-; CHECK-NEXT:    vmv2r.v v8, v12
 ; CHECK-NEXT:    ret
   %v = call <vscale x 8 x i32> @llvm.vector.insert.nxv4i32.nxv8i32(<vscale x 8 x i32> splat (i32 1), <vscale x 4 x i32> splat (i32 1), i64 0)
   ret <vscale x 8 x i32> %v