[llvm] [PassInstrumentation] Make getPassNameForClassName return optional (PR #150029)

[Nikita Popov via llvm-commits]

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@@ -165,7 +165,8 @@ class PassInstrumentationCallbacks {
   /// Add a class name to pass name mapping for use by pass instrumentation.
   LLVM_ABI void addClassToPassName(StringRef ClassName, StringRef PassName);
   /// Get the pass name for a given pass class name.
-  LLVM_ABI StringRef getPassNameForClassName(StringRef ClassName);
+  LLVM_ABI std::optional<StringRef>
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nikic wrote:

Can we keep using empty StringRef as the sentinel here, and only avoid the hash table modification?

https://github.com/llvm/llvm-project/pull/150029