diff --git a/llvm/include/llvm/CodeGen/SelectionDAG.h b/llvm/include/llvm/CodeGen/SelectionDAG.h
--- a/llvm/include/llvm/CodeGen/SelectionDAG.h
+++ b/llvm/include/llvm/CodeGen/SelectionDAG.h
@@ -280,6 +280,11 @@
 
   DenseMap<const SDNode *, CallSiteDbgInfo> SDCallSiteDbgInfo;
 
+  /// PersistentId counter to be used when inserting the next
+  /// SDNode to this SelectionDAG. We do not place that under
+  /// `#if LLVM_ENABLE_ABI_BREAKING_CHECKS` intentionally because
+  /// it adds unneeded complexity without noticeable
+  /// benefits (see discussion with @thakis in D120714).
   uint16_t NextPersistentId = 0;
 
   /// Are instruction referencing variable locations desired for this function?
diff --git a/llvm/include/llvm/CodeGen/SelectionDAGNodes.h b/llvm/include/llvm/CodeGen/SelectionDAGNodes.h
--- a/llvm/include/llvm/CodeGen/SelectionDAGNodes.h
+++ b/llvm/include/llvm/CodeGen/SelectionDAGNodes.h
@@ -615,8 +615,10 @@
   SDNodeFlags Flags;
 
 public:
-  /// Unique and persistent id per SDNode in the DAG.
-  /// Used for debug printing.
+  /// Unique and persistent id per SDNode in the DAG. Used for debug printing.
+  /// We do not place that under `#if LLVM_ENABLE_ABI_BREAKING_CHECKS`
+  /// intentionally because it adds unneeded complexity without noticeable
+  /// benefits (see discussion with @thakis in D120714).
   uint16_t PersistentId;
 
   //===--------------------------------------------------------------------===//