[clang] [clang-repl] Refactor locking of runtime PTU stack (NFC) (PR #84176)

[Stefan Gränitz via cfe-commits]

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@@ -137,9 +136,10 @@ class Interpreter {
 
   Expr *SynthesizeExpr(Expr *E);
 
-private:
   size_t getEffectivePTUSize() const;
+  void finalizeInitPTUStack();
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weliveindetail wrote:

We could also consider renaming this to `getNumUndoSteps()` and `setUndoBarrier()` respectively (or some thing like that).

https://github.com/llvm/llvm-project/pull/84176