[llvm] [X86] Insert CALLSEQ when lowering GlobalTLSAddress for ELF targets (PR #113706)

[Matt Arsenault via llvm-commits]

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@@ -18841,8 +18842,25 @@ GetTLSADDR(SelectionDAG &DAG, SDValue Chain, GlobalAddressSDNode *GA,
     TGA = DAG.getTargetExternalSymbol("_TLS_MODULE_BASE_", PtrVT, OperandFlags);
     auto UI = TGA->use_begin();
     // Reuse existing GetTLSADDR node if we can find it.
-    if (UI != TGA->use_end())
-      return SDValue(*UI->use_begin()->use_begin(), 0);
+    if (UI != TGA->use_end()) {
+      // TLSDESC uses TGA.
+      auto TLSDescOp = UI;
+      assert(TLSDescOp->getOpcode() == X86ISD::TLSDESC &&
+             "Unexpected TLSDESC DAG");
+      // CALLSEQ_END uses TGA via a chain and glue.
+      auto CallSeqEndOp = TLSDescOp->use_begin();
+      assert(CallSeqEndOp->getOpcode() == ISD::CALLSEQ_END &&
+             "Unexpected TLSDESC DAG");
+      // CopyFromReg uses CALLSEQ_END via a chain and glue.
+      auto CopyFromRegOp = CallSeqEndOp->use_begin();
+      assert(CopyFromRegOp->getOpcode() == ISD::CopyFromReg &&
+             "Unexpected TLSDESC DAG");
+      // The Add generated at the final return of this function uses
+      // CopyFromReg.
+      auto AddOp = CopyFromRegOp->use_begin();
+      assert(AddOp->getOpcode() == ISD::ADD && "Unexpected TLSDESC DAG");
+      return SDValue(*AddOp, 0);
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arsenm wrote:

Assuming this will be the only use doesn't feel great. Can you just find the root node, and let the auto-CSE property of the DAG take care of this by simply trying to recreate the add at the end? 

https://github.com/llvm/llvm-project/pull/113706