[llvm] SystemZ: Stop casting fp typed atomic loads in the IR (PR #90768)

[Matt Arsenault via llvm-commits]

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@@ -6249,6 +6249,41 @@ SDValue SystemZTargetLowering::LowerOperation(SDValue Op,
   }
 }
 
+// Manually lower a bitcast to avoid introducing illegal types after type
+// legalization.
+static SDValue expandBitCastI128ToF128(SelectionDAG &DAG, SDValue Src,
+                                       SDValue Chain, const SDLoc &SL) {
+  MachineFunction &MF = DAG.getMachineFunction();
+  const DataLayout &DL = DAG.getDataLayout();
+
+  assert(DL.isBigEndian());
+
+  Align F128Align = DL.getPrefTypeAlign(Type::getFP128Ty(*DAG.getContext()));
+  SDValue StackTemp = DAG.CreateStackTemporary(MVT::f128, F128Align.value());
+  int FI = cast<FrameIndexSDNode>(StackTemp)->getIndex();
+  Align A = MF.getFrameInfo().getObjectAlign(FI);
+
+  SDValue Hi =
+      DAG.getTargetExtractSubreg(SystemZ::subreg_h64, SL, MVT::i64, Src);
+  SDValue Lo =
+      DAG.getTargetExtractSubreg(SystemZ::subreg_l64, SL, MVT::i64, Src);
+
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arsenm wrote:

If I do that I ultimately get this from copyPhysReg: 
$f0q = $r0q
Impossible reg-to-reg copy

https://github.com/llvm/llvm-project/pull/90768