[llvm] [X86]RMW instrs not handled in pre-RA-sched=fast (PR #67281)

[Phoebe Wang via llvm-commits]

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@@ -231,6 +231,11 @@ SUnit *ScheduleDAGFast::CopyAndMoveSuccessors(SUnit *SU) {
     if (!TII->unfoldMemoryOperand(*DAG, N, NewNodes))
       return nullptr;
 
+    // unfolding an x86 RMW operation results in store, dec, load which
+    // can't be handled here so quit
+    if (NewNodes.size() == 3)
+      return nullptr;
+
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phoebewang wrote:

Why can't add it in `unfoldMemoryOperand` if it is required in each time?

https://github.com/llvm/llvm-project/pull/67281