[clang] [llvm] [SystemZ] Support fp16 vector ABI and basic codegen. (PR #171066)

[Matt Arsenault via cfe-commits]

================
@@ -6345,6 +6386,39 @@ bool SystemZTargetLowering::isVectorElementLoad(SDValue Op) const {
   return false;
 }
 
+static SDValue mergeHighParts(SelectionDAG &DAG, const SDLoc &DL,
+                              unsigned MergedBits, EVT VT, SDValue Op0,
+                              SDValue Op1) {
+  MVT IntVecVT = MVT::getVectorVT(MVT::getIntegerVT(MergedBits),
+                                  SystemZ::VectorBits / MergedBits);
+  assert(VT.getSizeInBits() == 128 && IntVecVT.getSizeInBits() == 128 &&
+         "Handling full vectors only.");
+  Op0 = DAG.getNode(ISD::BITCAST, DL, IntVecVT, Op0);
+  Op1 = DAG.getNode(ISD::BITCAST, DL, IntVecVT, Op1);
+  SDValue Op = DAG.getNode(SystemZISD::MERGE_HIGH, DL, IntVecVT, Op0, Op1);
+  return DAG.getNode(ISD::BITCAST, DL, VT, Op);
+}
+
+static SDValue buildFPVecFromScalars4(SelectionDAG &DAG, const SDLoc &DL,
+                                      EVT VT, SmallVectorImpl<SDValue> &Elems,
+                                      unsigned Pos) {
+  SDValue Op01 = buildMergeScalars(DAG, DL, VT, Elems[Pos + 0], Elems[Pos + 1]);
+  SDValue Op23 = buildMergeScalars(DAG, DL, VT, Elems[Pos + 2], Elems[Pos + 3]);
+  // Avoid unnecessary undefs by reusing the other operand.
+  if (Op01.isUndef()) {
+    if (Op23.isUndef())
+      return Op01;
+    Op01 = Op23;
+  }
+  else if (Op23.isUndef())
----------------
arsenm wrote:

```suggestion
  } else if (Op23.isUndef())
```

https://github.com/llvm/llvm-project/pull/171066