[SelectionDAG] Fold (avg x, 0) -> x >> 1

Author: AZero13

llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp

@@ -5083,7 +5083,11 @@ SDValue DAGCombiner::visitAVG(SDNode *N) {
   if (N0 == N1 && Level >= AfterLegalizeTypes)
     return N0;
 
-  // TODO If we use avg for scalars anywhere, we can add (avgfl x, 0) -> x >> 1
+  if (isNullOrNullSplat(N1))
+    return DAG.getNode((Opcode == ISD::AVGFLOORS || Opcode == ISD::AVGCEILS)
+                           ? ISD::SRA
+                           : ISD::SRL,
+                       DL, VT, N0, DAG.getConstant(1, DL, VT));
 
   return SDValue();
 }

llvm/test/CodeGen/AArch64/hadd-combine.ll

@@ -464,8 +464,7 @@ define <8 x i16> @rhaddu_i_const_lhs(<8 x i16> %src1) {
 define <8 x i16> @rhaddu_i_const_zero(<8 x i16> %src1) {
 ; CHECK-LABEL: rhaddu_i_const_zero:
 ; CHECK:       // %bb.0:
-; CHECK-NEXT:    movi v1.2d, #0000000000000000
-; CHECK-NEXT:    urhadd v0.8h, v0.8h, v1.8h
+; CHECK-NEXT:    ushr v0.8h, v0.8h, #1
 ; CHECK-NEXT:    ret
   %result = call <8 x i16> @llvm.aarch64.neon.urhadd.v8i16(<8 x i16> <i16 0, i16 0, i16 0, i16 0, i16 0, i16 0, i16 0, i16 0>, <8 x i16> %src1)
   ret <8 x i16> %result
@@ -633,8 +632,7 @@ define <8 x i16> @rhadds_i_const_lhs(<8 x i16> %src1) {
 define <8 x i16> @rhadds_i_const_zero(<8 x i16> %src1) {
 ; CHECK-LABEL: rhadds_i_const_zero:
 ; CHECK:       // %bb.0:
-; CHECK-NEXT:    movi v1.2d, #0000000000000000
-; CHECK-NEXT:    srhadd v0.8h, v0.8h, v1.8h
+; CHECK-NEXT:    sshr v0.8h, v0.8h, #1
 ; CHECK-NEXT:    ret
   %result = call <8 x i16> @llvm.aarch64.neon.srhadd.v8i16(<8 x i16> <i16 0, i16 0, i16 0, i16 0, i16 0, i16 0, i16 0, i16 0>, <8 x i16> %src1)
   ret <8 x i16> %result