[llvm] [SDAG] Fix llvm.modf for ppc_fp128 (attempt two) (PR #127976)

[Benjamin Maxwell via llvm-commits]

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@@ -2644,8 +2644,10 @@ bool SelectionDAG::expandMultipleResultFPLibCall(
     // optimized out. This prevents an FP stack pop from being emitted for it.
     // Setting the root like this ensures there will be a use of the
     // `CopyFromReg` chain, and ensures the FP pop will be emitted.
+    SDValue OldRoot = getRoot();
     SDValue NewRoot =
-        getNode(ISD::TokenFactor, DL, MVT::Other, getRoot(), CallChain);
+        OldRoot ? getNode(ISD::TokenFactor, DL, MVT::Other, OldRoot, CallChain)
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MacDue wrote:

Well, whether or not it's okay to ignore the new root created by the expansion workround depends on if the ignored result is returned via some sort of stack, that needs to be popped regardless of if that value is used or not. 

https://github.com/llvm/llvm-project/pull/127976